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The Influence of Droplets with Net Charge in Atomic Spectroscopy: Toward an Improved Understanding of the Chernical Matrix Effect by Qiang Xu B.Sc,Yunnan Normal University, Kunming, Yunnan, China, 1986 M.Sc,Xiamen University, Xiamen,Fujian, China, 1989 THESIS S ~ M I T T l 3 D IN PARTIAL F'ULF'ILLMENT OF TEE REQUIREMENT FOR THE DEGREE OF in the Department of O Qiang Xu SIMON FRASER IIWVERSm September 1998 AM rights reserved. This work may not be reproduced in whole or in part, by photocopy or other means, without the permission of the anthor. National Library Bibliothèque nationale du Canada Acquisitions and Bibliographie Services Acquisitions et services bibliographiques 395 Wellington Street OtmwaON K l A O N 4 395. rue Weliington OttawaON K1AON4 Canada Canada The author has granted a nonexclusive licence dowing the National Library of Canada to reproduce, loan, distribute or sell copies of this thesis in microfonn, paper or electronic formats- L'auteur a accordé une licence non exclusive permettant à la Bibliothèque nationale du Canada de reproduire, prêter, distribuer ou vendre des copies de cette thèse sous la fome de microfiche/^ de reproduction sur papier ou sur format électronique. The author retains ownership of the copyright in this thesis. Neither the thesis nor substântial extracts fiom it may be p h t e d or otberwise reproduced without the author's permission. L'auteur conserve la propriété du droit d'auteur qui protège cette thèse. Ni la thése ni des extraits substantiels de celle-ci ne doivent être imprimés ou autrement reproduits sans son autorisation. The presence of a chemicai rnatrix in a sample will cause a spatial shiff in the emission profile of an analyte species in the Inductively Coupled Plasma UCP). This historic problem has been extensively investîgated for 25 years with little advancement in the understanding of it. In this work, a new hypothesis, that of droplets with net charge existing in the aerosol as being responsible for causing what is cornmonly referred to as the chemical matrix effect, has been investigated According to this theory, as a droplet with a net charge desolvates, it will eventually reach the Rayleigh fission Iimî,and break spart. The result of a fission would be to introduce more s d e r droplets into the aerosol than would otherwise be present had droplets of zero net charge been produced. To test this hypothesis, a mesh (size 1O) was positioned inside the spray chamber. When a DC potential is applied to the mesh, either positive or negative with respect to ground, the d y t e emission characteristics in the plasma are dtered. The biased mesh affects the secondary aerosol. The marner in which the emission profiles are dtered is usually a decreaçe in the spatial signal intensity maximum,but with a signincantly reduced chemical ma& effect. The results are interpreted with respect to the removal of droplets with net charge fiom the aerosol because of the biased mesh. Furthemore, by monitoring the quaternary aerosol by way of laser light scattering, it was found that this signal is dependent on the chemical matrix concentration. It is demonstrated that the relative intensity of the laser light scattering signal can be used in a feedback loop to control the potential applied to the mesh, v W y eIiminsting the chernical matrix effect DEDICATION To m y wife, Lynn Luchun Lin and son, BiUy Yuxïang Xn ACKNOWLEDGMENTS The work presented in this thesis could not have been done without the guidance and encouragement of Dr. George Agnes, who was my graduate study senior supervisor. I wish to express my gratitude to him for many interesthg discussions about this research. I would like to thank Dr. Gary Leach and Dr. Louis Peterson for their assistance, suggestion,and encouragement during the course of this thesis work. 1also have to thank Dr. Jean-Claude Brodovitch for his kindly lending me several electronic measurement instruments and the engineers in the glassware, electronic, and machine shop for their help in modming the spray chamber and making the ICP generator operational. Furthemore, 1 would like to thank all my group members: Hongjun Wang, Yuping Chen, and Xiao Feng for their help and fiiendship. I especialiy appreciated the insightfid discussions with Xiao about my research project I also wish to thank the Department of Chemistry and my many fiiends in the department for their assistance. Lastly, 1wish thank my M e , Lynn and my son, B a y for their patience and support throughout m y studies. TABLE OF CONTENTS m APPROVAL ABSTRACT DEDICATION ACKNOWIXDGMENTS TABLE OF CONTENTS LIST OF F'IGURES LIST OF TULIES ABIBRE'CTLATIONS CHAPTER 1: INTRODUCTXON 1.1 Chemical Matrut Effects 1.2 The Inductively Coupled Plasma (ICP) 1.2.1 Converting Liquid Samples into Signals 1.2.2 Plasma Processes 1-2.3 LTE or non-LTE 1.2.4 Plasma Dynamics 1.3 Plasma Processes in the Presence of a Chernical Ma& 1.4 Aerosol Generation and TransportationProcesses in the Presence of a Chemical Matrix 1.5 Motivation for this Research References CHAPTER 2: MECHANISMS OF FORlMATION OF DROPLETS 'WLTH NET CHARGE AND TBEIR EVAPORATION BEEAVIOR 19 Nebukation 2.1.1 The Concentrïc Pneumatic Nebuber 2.1.2 The Rimary Aerosol Droplet Size Distribution Droplets with Net Charge in the Aerosol 2.2.1 Evidence of Droplets with Net Charge 2.2.2 Electncal Double Layers 2.2.3 Statistical Fluctuations The Function of the Spray Chamber Sample Calculations of DropIets with Net Charge 2.4.1 Rayleigh Fission Instability 2.4.2 Solid Residue Size Rayleigh Fission versus Matrix Effect Solvent Evaporation References m m R 3: 3.1 rnSTRUMENTAL Inductively Coupled Plasma - Monochromatic Imaging Spectrometer 3.2 Laser Light Scatter Measurements 3.3 The Concentnc Pneumatic Nebulizer 3.4 Electrode Positioning inside the Spray Chamber CHAPTlER 4: THE INFLUENCE OF DROPLETS WITE? lVET CHARGE IN PLASMA SPECTROSCOPY AND IMPLICATIONS FOR T'HE CEEMTCAL MATRIX EFFECT 38 4.1 Ixlîroduction 38 4.2 Experimental 41 4.3 Results and Discussion 43 4.4 Conclusion 55 References 55 CHARGED DROPLETS PRODUCTION BY CONCEZYTRIC CELAPTER 5: NEBULIZATTON AND ITS EFFECT IN ATOMrC SPECTROCHEMICAL MEASUREMENTS 5.1 Introduction 5.2 Experimentd 5.3 Results and Discussion C d Emission Contours as a Function of Nebulant Gas Flow Rate CaII Emission Contours in the Presence of NaCl, KC1, CsC1, and SDS Chernical Matrices The EEect of a DC Bias on a Mesh Positioned inside the Spray Chamber on Ca11 Emission Contours in the Absence and Presence of Alkali-metd Chloride Salt Matrices CaII Emission Contours as a Function ofDC Voltage Appiied in the Presence of NaCl Matrix CaIZ Contours in the Presence of Nitric Acid and NaCI Revisitation of Local Thermodynamic Equilibrium (LTE)of the ICP in the Presence of a Chernical Matrix 5.4 Conclusion References CHAPlXR 6: USE OF LASER LIGHT S C A W R AS A FEEDBACK SIGNAL TO Ml[MMaZE CHEMICAL MATRIX EFFEC"P'IN ICP-OES 6.1 Introduction 6.2 Experimental 63 Results 6.3.1 Primary Aerosol Laser Light Scatter Signais 6.3.2 Quaternary Aerosol Laser Light Scatter Signals 6.3.3 Use of an Electnc Field to Control Laser Light Scatter Signals 97 6.4 Discussion 99 6.5 Conclusion 104 References CHAPTER 7: CONCLUSION Future Directions 104 LIST OF FIGURES Figure Page Schematic diagram of ICP discharge and torch structure 3 Schematic diagram of a glass concentric nebulizer (type C) 22 Schematic diagram of the gas-liquid interaction with a concentric nebulizer 21 A postulated elecùical double layer at the liquid-gas interface 24 The aerosol modifying processes of the Scott-type spray chamber 26 Net statistical charge on droplets of 1,5, and 10 micrometers in radius as a fiuiction of NaCl concentration 27 Rayleigh limit charge as a finiction of charged droplet size in radius 29 Cornparison of Rayleigh limit radius R)with solid residue radius (4) as a function of matrix concentration for droplets that were initidy of 1,5, and 10 micrometers in radius Cartoon diagram of ICP - Monochromatic Imaging Spectrometer Schematic diagram of the mesh positioned inside the spray chamber Lateral emission profiles for Ca11 (393.4 nrn) in an ICP The effect of a 1 x IO-'M NaCl chemical matrix on the lateral emission profiles for CaII (393.4 nm) in an ICP The effect of a 1x IO-' M NaCl chemical matrix on the lateral emission profiles for CaII (393.4 nm) in an ICP The effect of nebulant gas flow rate (FJ on the lateral emission contours for CalI (393.4 nm) in an ICP Lateral emission contours for CaII (393.4 nm)in an ICP The eflect of a DC potentiai applied to the mesh on the lateral emission contours for CaII (393-4 nm) in an ICP 69 The eEect of a DC potential applied to the mesh on the lateral emission contours for Ca11 (393.4 nm) in an ICP The effect of a DC potentid applied to the mesh on the lateral emission contours for CaII (3 93 -4 nm) in an ICP The effect of a DC potential applied to the mesh on the lateral emission contours for CaIi (393.4 nm) in an ICP The effect of a DC potential applied to the mesh on the lateral emission contours for CaII (393-4nm) in an ICP The effect of a DC potential applied to the mesh on the laterd emission contours for CaII (393.4 nm) in an ICP The effect of a DC potential applied to the mesh on the lateral emission contours for Ca11 (393 -4 nm)in an ICP The effect of a DC potential applied to the mesh on the lateral emission contours for Cari (3 93-4 nm)in an ICP The effect of a DC potential applied to the mesh on the lateral emission contours for CaII (393 -4 nm) in an ICP The effect of a DC potential applied to the mesh on the lateral emission contours for Ca11 (393.4 nm) in an ICP The primary aerosol laser Iight scattering signal as a funciion of NaCl chemical ma& concentration in molarity The quaternary aerosol laser light scattering signal as a function of NaCl chemical ma& concentration in molaxity The quatemary aerosol laser light scattering signal a s a firnction of NaCl chemical ma& concentration in molarity The effect of a DC potential applied to the mesh on the lateral emission contours for Ca11 (393 -4 nm) in an ICP LIST OF TABLES Page Table 6.1 Physical Properties of Two Aqueous Solutions at 200C 101 6.2 Sauter Mean Diameter (4, pm) at Different Gas Flow Rates 101 ABBREVIATIONS AIR Aerosol Ionic Redistribution ALC Above Load Cod cm Charged Coupled Detector EIEs Easily Ionized Elements 1.WHlbl Full With at Half Maximum ICP Inductively Coupled Plasma LTE Local Thermal Equilibrium MS Mass Spectrometry NAZ Nonnd Mytical Zone non-LTE Non-local Thermal Equilibrium OES Optical Emission Spectroscopy PMT Photomultiplier Tube RF Radio Frequency xii Chapter 1: Introduction Inductively coupied plasma (ICP) spectrometry has become the dominant technique to determine, quafltitatively, multiple elements in samples that are extremely diverse nature, in routine analysis [l, 21. The ICP is used to analyse samples of interest in earth science, environmental, biomedical, chernical, and forensic science, among many others. The detection M t for most elements is at the part per billion level @pb) [3]. The attributes of modem ICP spectroscopies, either emission or mass spectral, are described as having fast analysis time, mufielement capability, low detection lirnits, wide h e a r dynamic range, high precision, and are applicable for analysis of gases, Iiquids, or solids. The e-g problems of this technology include spectral interferences, matrix effects fiom concomitant species and solvenf difnculty in analyzing solids without dissolution, inefficient sample introduction, detection b i t s too hi& drift and insu£ficient precision for some applications [4]. To gain a better understanding of the ICP as an excitation source, much research has been done over the past three decades because it is widely recognized that results fkom fundamental research ars needed to improve analysis accuracy, precision, deveiop practicai automated instrument diagnostics, and reduce operator skill requirements [5]. 1.1 Chernical Matrix Effects Although ICP-OES(opticai emission spectroscopy) and ICP-MS(mass spectrometry) are used widely in today's routine multielement analysis, the ICP still &ers fiom sample-related matrk effects. The te= rnatrix is meant to include aU reagents in the sample such as acids, organic solvents, and dissoIved solids. When uncorrected for, condtuents in the sample can cause large emrs in the measued analyte concentration. An improved understanding of the processes that occur in the plasma and the sample introduction step could yield methods for reducing, or perhaps eliminating errors of determination caused by the sample matrix. This is the subject of this thesis. 1.2 The Inductively Coupled Piasma (ICP) The ICP is a partïally ionized gas that is e l e c t n d y sustained by a radio hquency power supply (27 MHz, 1-3 kW). The RF power is coupled into the plasma with a two or three tum load coil. Argon is the most commonly used working gas, and it is roughly 1% ionized under normal conditions. Fig. 1.1 shows a cartoon of the ICP discharge and torch structure [6,7J. In temis of the temperatures in a typical ICP source, Fasse1 [8 ] has reporteci that the induction zone is - 10,000 JX, and the normal analytical zone ( N U ) ranges nom - 4000 to 8000 K depending on the spatial position within the NAZ and plasma operating conditions. 13.1 Converting Liquid Sarnples into S i s Liquid sample introduction is by fa the most important sample introduction route because most solid samples are dissolved @or to analysis. Liquids are always converted to - Fig. 1.1 Schematic diagram of ICP discharge and torch structure: 1 Nebulant M e r gas, 2 - Auxiliary gas, 3 - Plasma gas, 4 Sample injection tube, 5 Intermediate tube, 6 - Outer tube, 7 - Load CO& 8 - Normal analytical zone (NAZ),9 - Plasma plume, 10 - Initial radiative zone 11 Induction zone, 12 - Central channel, 13 - Pre-heating zone(PHZ). - - w), - an aerosol before introduction to the ICP. A polydisperse aerosol passes through a spray chamber where the droplet size distribution is modined, such that only droplets having a - radius < 30 pn are delivered to the plasma Each droplet undergoes several processes as it travels up through the center of the plasma As the droplet is heated, the solvent evaporates and leaves behind a solid residue. Vaporization of the solid residue is believed to occur only aRer desolvation, not concurrentiy. Molecules or atoms are produced as gaseous species, and then atomkmtion or ionization takes place. A small hction of the atoms and ions become excited and emit light, or altematively a fiaction of the d y t e ions reach the detector of a mass spectrometer. This sequence of desolvation through to is kineticdy controIled [9]. ionization~excitatio~i The processes of aerosol generation and transport, production of atoms or ions, and excitation detemiines the analytical signal for OES. For example, when a liquid sample is introduced to the system, the gaseous analyte density (n) (m-3)in the ICP can be estimatecl with eqn. 1.1 [IO]: = N~Qi&n~~YTRChMQ& Avogadro constant Sample solution uptake rate (m3/s) Nebubation efficiency Local fkaction of desolvation Local fiaction of vaporization Ambient temperature (K) Analyte concentration in solution (g/m') Mole expansion coefficient Molar mass of anaiyte Nebulant carrier gas flow rate (m3/s) Plasma tempera- (K) eqn. 1.1 q can be denned as the part ofthe solution uptake leaving the spray chamber as aerosol d e r n e b u t i o n . E, is the ratio of mass of anaiyte in the desolvated aerosol (solid residue) versus the undesolvated aerosol, and E,is the ratio of the amount of analyte in the plasma (gas phase) relative to a particle (solid phase). Both E, and E, are dependent on the spatial position in the plasma hoplets of different size will undergo desolvation and vaporization at different observation heights in the plasma. Thus, the signal intensity will be af3ected signincantly by local plasma conditions. 1.2.2 PIasma Processes With the assumption of dissociation and ionization equiïbrium for the gaseous analyte species in the ICP, the gaseous d y t e density (n) equals to the sum of densities of fiee atoms (u, ions (0 and , molecules ( n d . I(d is the dissociation constant of MX, and is the ionization constant of element M. The necessary equations are: MX=M+X &=n,nx/n& eqn. 1.2 M =w+e &=&kJnd eqn. 1.3 n=n,+%'+n, eqn. 1.4 Therefore, the dissociation degree (P) and ionkation degree (X)can be descnbed as: P = &/(nm+ n& = KJ& + nx) X=rk;/(n,+h+)=KJ(y+nJ eqn. 1.5 eqn. 1.6 Then, the population of fiee atoms (nM), fiee ions (%+),and fiee molecules (4 are given by : % = (1-X)P nm-(l-~)XJ =X nm eqn. 1.7 p n/[l -(i-P)q = (1-X) eqn, 1.8 eqn. 1.9 (1-8) ~ - ( ~ - P ) X I For a system in thermal equilibnum, at temperature T, the population density of atomic level p or ionic level j follows a Boltzmann distnbution [IO, 111: = %A k,4Z,(T>lex~(-EdkT) eqn. 1.10 eqn. 1.11 n(j) = nM+[gj/Gf Cr)] exp(-E;/kT) where $ and gj are the statistical weights of the level p and level j, m,&+Oare the partition fünctions of atom M and ion W, E,,and E , ' are the excitation energies of the level p and j, T is denned as excitation temperature,,T and k is the Boltzxnan constant. When the radiation source is optically thin,as is the ICP,the observed atomic emission intensity of a transition fkom a higher level p to lower level q and ionic emission intensity 4,of a tmnsition fiom a higher levei j to lower level k are expressed as: I , = (Y47G A, hc n W A , = W4n) Wh)A, n~ CQGO')I exp(-EdkT) = ( ~ 4 n ) ( h c / â , ) 4 ( 1 - ~ ) ~ n / r l - ( 1 - r~g) d ~1 z , m ~ e x ~ ( - ~ d eq= k ~ ) 1-12 Iji= (V4x) 41 hc nÜ)/kp = V4n) (hc/Q = 4, nM+ki/L,,'O]eq(-Ejt/kT) ml @c/Q ~ I PXnV-(l-P)XI [gt/%+('Q] e~(-Ej'/kT) eqn. 1.13 where 1is the path length of the source, A, and 4, are the transition probabiïites for spontaneous emission, h is Plank's constant, c is the velocity of light, h,and S,are the wavelengths of the emission line fkom the atom and ion respectively. Combining equation 1.1 with eqn. 1.12 or eqn. 1.13 results in a quantitative analysis criterion for atomic emiçsion intensities: b= V4N A, W A Q A E ~ U P Y M Q ~ T(1-X) P ) P/[W-P)XI ex~(-EdkT) 1%= ( v 4 ~ ) kdZ,OI eqn. 1.14 Ajk WAQFAEVTRWMQ~TP) X P/Cl-(I-P)XI [gj&+(T)I exp(-E;/kT) eqn. 1.15 Eqn. 1-14and eqn. 1-15describe the factors that atomic and ionic emission intensities are based. If the ICP operational parameters are fked (RF power, Q,, Q, for example), any change of the nebukation efficiency en,local fiaction of desolvation s, local kction of vaporization î, dissociation degree P, ionization degree X, or excitation temperatme T will a£kct the atomic ancüor ionic emission signal intensity. Because the ICP is spatially heterogeneous, the simple equifibrium-based models introduced in section 1.2.2 are often inadequate to qutitatively describe the ICP. Instead, more complex kinetic models are needed [SI. Development of a mode1 to describe the ICP has been a long and interesthg topic 112, 13, 141. Adequate models could be used to predict emission or ionization behavior as a function of experimental conditions such as RF power and carrier gas flow rate. In the ICP, the energy is coupled mainly into the induction zone near the load coil (refer to Fig. 1.1). Heaf and chernicd species such as argon atoms and ions are iransported fiom this energy addition region to the central channe1 of the plasma, leading to large temperature and concentration gradients (Le. , T < Tdm -cTiMm c T ,, < T ,id-m ). Hence, local thermal equilibrium @,TE) is not o h met, and the ICP is considered a non-local thermal equiiibrium (non-LTE) system. Olesik has reported that the effect of sample on the plasma must also be considered [SI. Although less than 50 W of power is typicdy needed to convert the sample into fkee atoms and ions, the sample can significantly affect the properties of the plasma Inaccuracy in a determination can occur because the composition of the sampb solvent (acid identity and concentration or the presence of trace organic solvents for example) and the sample itself (the matrix) aEect plasma processes. Gunter e t aL showed that the energy transfer process fiom the energetic plasma particles (fke electron, ions and atoms) to the cold nebulizer gas particles takes place at a rate that is insmcient to ensure collisional domination of the populations of the sample particles' energy [15]. The authors concluded that in order to achieve equilibrium of m f e n e d energy from the plasma gases to the sample particles requires either an increase in the energy tramfer rate, a s would be accomplished by an increase of the RF power causing the electron density to increase, or a longer residence t h e , achieved by Lowering the d e r gas flow velociv. It WU be very beneficial if the ICP fundamental processes can be fWy understood. Recent research results fÎom Olesik's group indicate that when the sample aerosol contains droplets with a wide range of skes, local plasma cooling in the normal arralytical zone due to desolvating droplets and vaporizing particles is very important 193. The desolvation and vaporization processes are dependent on the local plasma characteristics near the droplet or particle, which may be quite different fiom bulk pIasma properties. Some liquid droplets are present in the analyticai region of the ICP produce and they cause surprbingly large changes in local plasma conditions that affect sensitivity, precision, noise, accuracy, and molecular ion formation. Incompletely desolvated drop1ets and vaporizing particles extensively cool a region of the plasma (radius 1 to 2 mm) by 1000 K or more [16, 17J. 1.4 Plasma Dynamics The ICP fundamental processes are c r i t i d y dependent on plasma parameters such as gas kinetic temperature (T& electron temperature (ïJ,electron number densiq (a, analyte atom and ion number density (n,or nM*), spatial position of analyte atoms and ions, and energy transport rates in the plasma In an Ar plasma, the energy to create argon ions and electrons is derived fÎom the RF power being coupled into the plasma, and the energy is coupled mainly into electrons because the much more massive ions (argon ions) respond more slowly to the oscillahg magnetic field. Therefore, the Ar atom gas kinetic energy (TJ, is transferred primarily by collisions with electrons. Tgis likely the most important parameter for droplet desolvation and vaporization of the solid residue. Most analyte atoms and ions are excited by collision with electrons, and the probabiiity of producing an excited atom or ion is mainly dependent on the electron number density n, and the electron temperature Te. Due to the high heterogeneity of the ICP,measurements of T , Te and q must be spatidy and tempordy resolved. In order to permit p l m a modeling, laser light scattering has been found to be a powerful tool. Hieftje et. al. reported that Thomson scattering can yield temporally and spatially resolved maps of electron energy distributions (TJ and number densities (q) [18]. Rayleigh scattering cau also be perforxned simdtaneously with the same apparatus and offers tempord and spatial values for Ar gas kinetic ternperahne (Ta. Thomson scattering is the quasi-elastic scattering of light fiom fiee electrons in the plasma. Since the electrons are moving with respect to the incident laser beam as well as with respect to the detector, the scattered radiation is doubly Doppler-broadened. By measuring the shape and integrated amplitude of the scattering spectnim at various Dopplershifted wavelengths, the electron energy distribution and electron number density can be detennined [19]. Furthemore, because a pulsed laser is utilized, temporal resolution is realized with each measurement. Rayleigh scattering is the scattering of light by particles that are much malier than the incident wavelength of electromagnetic radiation. Because Rayleigh scattering in the ICP is primariy fiom Ar atoms, it is not noticeably Doppler shifted and thus appears as an extrernely large peak, centred at the laser wavelength ,on top of the Thomson-scattering spectnun. Rayleigh scattering is linearIy related to the conceniration of the scatterers, so by combining the Ideal Gas Law with the Rayleigh scattering equation, the scattering intensity can be shown to be related inversely to the gas-kinetic temperature [203. Unfortuaately, the true measurement of Tc, n, and Tgusing Thomson and Rayleigh scatterhg is very convoluted by Debye scattering h m intact particles in the observation zone of the plasna 1.3 Plasma Processes in the Presence of a Chernical Matrix In t e m of understanding the chernical m& effects, most of the work has been devoted to the processes occurring in the plasma Easily ionized elements (EIEs) have been the most commody investigated elements, because it was initially thought that only the presence of E E s in the sample matrix Sected analyte emission signals [21,22,23]. In actuality, a l l matrix types affect meanired analyte intensities [24,25,26]. The ma& c m cause spatiai and temporal changes in analyte emission intensity [27l. The spatial shift is generally an analyte emission intensiv enhancement, off axis, Iow in the plasma, and a suppression, on axis, hi& in the plasma A number of mechanisms have been postulated to explain the matrix effect. Sorne of them are based on hypothesized changes in the local electron number density q. For example, a shif? of the analyte ionkation equilibrium [21,22] would change the electrical conductivity of the plasma causing a change in the energy-coupling of the RF power to the plasma Variation in the thermal conductivity of the plasma alten the energy w o r t rate fiom the induction region to the central channel D8]. Mas-dependent ambipolar dinusion affects the transport of charged particles between differentregions in the plasma [29], and changes in the collisional excitation-ionïzation efficiency [25,30] have been descnbed. However, Olesik [4] has provided equilibrium calculations that indicate the number of electrons produced from the ionization of a 1,000 ppm solution of sodium has Little effect on the equilibrium shift in the plasma, because the electron number density (10i3/cm3)due to the matrix is much smaller than the electron number density (Le. 101S/cm3) fiom the ionized Ar in the ICP. Caughlin and Blades [3 11also reported the presence of an EIE does not Bit the fiee electron number density. Other mechanisms are bas4 on assumed variations in the electron temperature T , including themaikation of electron energies or the generation of 11 high-energy electrons during ionization of a rnatrix [25]. Sesi et. al. has suggested the &eEect is the result of at least three major processes operating simultaneously; lateral- diffusion (i.e. particle-volatilization) changes, shifts in analyte-ioniidon equilibna, and differences in collisional excitation efficiency [3 21. AU.published work has shown that the ma* eEect in the ICP is rather complex and that no clear evidence has been presented so far on its origin [33]. Mermet reported that the magnitude of the ma& eEect depends on the ICP operating conditions. Under so-cailed robust conditions (i.e. high RF power and low carrier gas flow rate), the ma& effect resulting fiom changes in the plasma conditions such as different temperatures, electron number density and spatial distribution of the varbus species, is minimirred Simüar1y, the much earlier research results fiom Gunter et. al. also showed that the severe ionization interference of Cs, and deviation fiom thermal equilibrium can be minimized by decreasing the nebulizer gas flow rate to increase the d y t e residence time in the plasma ClSI. These authon fomd that the increase of RF power could decrease the ionization interference. 1.4 Aerosol Generation and Transportation Processes in the Presence of a Chernical Matrix Liquid sample must be converted into aerosol droplets for introduction to the plasma Recently, Olesik has argued that matrix effect appears to originate primarily in the aerosoi generation and transport processes [SI. If a pneumatic nebulizer double pass spray chamber introduction systern is used, - 106polydisperse aerosol droplets of sample are delivered to the plasma per second. Droplets of different sizes reach complete desolvatation at different heights in the plasma Atoms and ions generated from small droplets, together with vaporizing particles and desolvating droplets, are all in the observation volume simdtaneously. Large droplets si@cantly affect the local plasma characteristics because their desolvation is ongoing in the NAZ observation zone. Fister and Olesik [34] have reported that incompletely desolvated droplets are a key factor in controlling vertical emission profiles as a function of power and nebulizer (central) gas flow rate. Rezaaiyaan e t al. [35] also reported thatthe sample introduction system has a dramatic effect on both the magnitude and nature of the Ca ion emission signal in the presence of 1,400 &ml Na interferent. Borowiec et. al. have proposed that an aerosol ionic redistribution (AIR) process occm during the generation of aerosol by pneumatic nebukation 1361. The authors reported that if one ion, the matrix, is present at a much higher concentration than the other, then the concentration ratios of these species ions found in the collected aerosoi leaving the ICP torch differ fiom those found in the original solution. Two explanations were used to illustrate the AIR mechanism. One is based on the Gibbs adsorption isotherm, and other on an electrïcal double layer effect. However, such explanations are still largely conjectural [37lRecently, Mermet et. al. found that the presence of Na did not modiS, the characteristics of the primary aerosol, but the teaiary aerosol was signincantly modined [38]. Finer droplets were obtained at the exit of the spray chamber when Na (10 g/l) was present Similar observations have &O been made when the acids of HN0,(0.9 M) or acetic acid (10%) wwe nebulized [39,10]. The authors confirmed that the sample introduction system is an important source of matrix effect and concluded that the spray chamber plays the key role in the matrix effect However, the fact that the tertiary aerosol was fïner in the presence of sodium, could not be explained [3 81. 1.5 Motivation for this Research The motivation for initiahg studies of the chernical ma& effect is derived fkom work involving Electrospray Ionization sources for mass spectrornetry [4l,421. Specifidy, the possibility that droplets with net charge are produced in abundance in the aerosol generation step could dramatically alter droplet transport charactenstics, and tertiary and quaternary aerosol size distributions if Rayleigh Fission processes [43], initiated by solvent evaporation, occur inside the spray chamber, or immediately pnor to entering the plasma Lndeed, it is a weU known fact that aerosol generation can lead to spray electdication [44,45,46].In this respecS the recent work of Hirabayashi et. al. supports this statement most strongly [47]. These authoe have reported the use of a pneumatic nebulization system, operated at high gas velocity, as a source of ions for mass spectrometry. They have termed this atmospheric pressure ionization technique "sonic spray". Tracy et. alreported large signal changes as a result of placing a grounded electrode in the spray chamber [48]. They attributed their observations to electrodc effects in the nebdizerkpray chamber. Michalik and Stephens made use of eledxostaîic trapping of aerosol droplets as a method of solution preconcentration [49]. They presumed that individual droplets carrïed a net positive or negative charge. The phenornenon of droplet charging during the pneumatic nebukation process has been overlooked by atomic spectroscopists in the past two decades. The effects of charged droplets in the presence of chemicai matrices have never been assessed in the analytical atomic spectroscopy field. It was the objective of this thesis to deheate the role of droplets with net charge in atomic spectroscopy, especially in t e m of the chemical matrix effects. References S. Greenfield, 1. L. 1. Jones and C. T. Berry, Analyst 89,713 (1964). R H.Wendt and V. A. Fassel, Anal. Chem. 37,920 (1965). G. M. Hieftje, P. J. Gdey, M. Glick and D. S. Hanselman, J. Anal. At. Spectrom 7,69 (1992). J. W. Olesik, Anal. Chem. 63, 12A (1991). J. W. Olesik, A d Chem. 68,469A (1996). S. R Koirtyohanq J. S. Jones and D. A. Yates, Anal. Chem 52, 1965 (1980). S. R K~irtyohann~ J. S. Jones, C. P. Jester and D. A. Yates, Spectrochim. Acta 36B, 49 (1981). V. A. Fassel, Science 202, 187 (1978). J. W.Olesik, Appl. Spectres. 551,158A (1997). X.Chen, "ICP Spectrome~:Principle arzd Applicationy', Nankai University, Tianjin, China,pl33 (1987). T. Hasegawa and H. Haraguchi, ccInd~~tively Coupled Plasmas in Ana&tical Atomic Speciromeiry", edited by A. Montaser and D. W. Golightly, VCH Publishers, Inc. p268 (1987). J. W. Oksik, Spectrochim Acta 44B,625 (1989). P. W. J. M. Boumans and F. J. de Boer, Spectochim Acta 32B, 365 (1977). B. L. Caughlin and M. W. Blades, SpectrochimActa 39B, 1583 (1984). W. H. Gunter, K. Visser and P. B. Zeeman, Spectrochim Acta 37B, 571 (1982). J. W. Olesik and J. C. Fister III,Spectrochim Actu 46B, 85 1 (1991). S. E. Hobbs and J. W. Olesik, Spectrochim Acta 48B, 817 (1993). G. M. Hiefije, P. J. Galley, M. Glick and D. S. Hanselman, J Anal. At. Spectrom. 7,69 (1992). M. Huang and G. M. Hieftje, Spectrochim. Acta 40By1387 (1985). K. A. Marshall and G. M. Hieftje, J A d . At. Spectrom. 2,567 (1987). M. W. Blades and G.Horlick, Spectrochim Acta 36B, 861 (198 1). M. W. Blades and G. Horlick, Spectrochim. Acta 36B,881 (198 1). V. A. Fasse1 and R N. Kniseley, A d . Chem. 46, 111A (1974). J. W. Olesik and E. J. Williamsen, Appl. Spectrosc. 43,1223 (1989). D. Sun, 2. Zhang, H.Qian and M. Cai, Spectrochim. Acta 43B, 391 (1988). D. S. Hansehan, N. N. Sesi, M. Huang and G. M. Hieftje, Spectmchim Acta 49B, 495 (1994). J. W. Olesik, L.S. Srnith and E, J. williamsen, Anal. Chern. 61,2002 (1989). 1. D. HoicIajtner-Antunovic and M. R. Tripkovic, J: AnaZ.At. Spectrom., 8,359 (1 993). W. H. Gunter, K. Visser and P. B. Zeeman, Spectrochim. Acta 408,617 (1 985). J. Xao, Q. Li, W. Li, H-Qian, I.Tan and 2.Zhang, L Anal. At. Spectrom. 7 , I 3 1 (1992). B. L.Caughlin and M. W. Blades, Spectrochim Acta 40B,987 (1985). N. N. Sesi and G. M. Hieftje, Speci'rochim. Acta 51B, 1601 (1996). J. M. Mermet, J Anal. At. Speci'rom. 13,419 (1998). J. C.III. Fister and J. W. Olesik, Spectrochim. Acta 45B,869 (1991). R R e d y a a n , J. W. Olesik and G. M. Hieftje, Spectrochim Acta 40B,73 (1985). J. A. Borowiec, A. W. Boom, J. H.Dfiard, M. S. Cresser, R. F. Browner and M. J. Matteson, And. Chem. 52, 1054 (1980). R F. Browner, ccInd~~tiveZy CoupZedP l m Emission SpechoscoW>', Part 2, Wiley Interscience, New York (1987). C. Dubuisson, E. Poussel, J. L. Todoli and J. M. Mermet, Spectrochim Acta 53B,593 (1998). J. L. Todoli, J. M. Mexmet, A. Canals and V.Hemmdis, J. AM[- At. Spectrom. 13,55 (1997). C. Dubuisson, E. Poussel, J. M. Mermet and J. L. Todoli, J A n d At. Specmm. 13, 63 (1997). G. R Agnes and G. Horlick, AppL Spectrosc. 46,401 (1992). G. R Agnes and G.Horlick, A@ Spectrosc. 49,324 (1995). D. C . Taflin, T. L.Ward, and E.J. Davis,Langmuir 5,376(1989). L.B.Loeb, "Staîic Electnfiatioon", pp. 58-124,Springer-Verlag, Berlin, (1 958). B. L. Sharp, J. Anal. At. Spectrorn, 3,939 ( 1 988). E. E.Dodd, J: Appl. Phys. 24,73 (1953). A. Hirabayashi, M.Sakaüi, and H . Koizumi, Anal. Chem. 66,4557(1994). D. H. Tracy, S. A. Myers, and B. G. Baliste,Spectrochim Acta 373,739 (1982). P. A. Miclialik and R Stephens, TaZanta 28,3 7 (1 981). Chapter 2: Mechanisms of Formation of Droplets with Net Charge and their Evaporation Behavior The most commonly used nebulizer, a pneumatic concentric design, and spray chamber, a Scott-type, are introduced in this chapter as they were used in this research. A short review of static electrification via bubble-bursting and spraying to form droplets with net charge will also be described in this text. Then the two known mechanisms of formation of droplets with net charge are descnbed. These mechanisms are discussed with reference to the pneumatic concentric nebulizer, illustrating how known research results fkom other groups can be better rationalized with our mode1 of the chemical matrix eEect. Sample calculations of the evaporation behavior of droplets with net charge, with respect to Rayleigh fission instability are presented. Liquid sample is converted to aerosol by the nebulizer. As such, the nebulizer is a key step in obtaining accurate and reliable remit in atomic spectroscopy [Ml. Sharp has provided a very comprehensive review of the theory of operation, and operating characteristics of pneumatic nebdizers [l]. In recent years, numerou nebukm developments and modincations have been reported [ 2 4 . Three types of pneumatic nebulizers commonly used are the concentric ffow, the cross flow, and the Babington. Droplets are produced as a result of the effect of mechanicd shear force on the liquid by a nebulant gas. Other nebuken like ultrasonic [5] and thennospray [6] nebdkers have also been applied to atomic spectroscopy. French et. al. designed a monodisperse dried microparticulate injector WMI)[4]. This device can - produce droplets on demand with diameters of 60 pm, and connected with a laminar-fiow desolvation fumace, the droplet size entering the plasma can be controlled. Olesik and Hobbs have used the MDMI to conduct fundamental studies of plasma processes 171. 2.1.1 The Concentric Pneumatic Nebnlizer Fig. 2.1 shows the schematic diagram of the concentric pneumatic nebulizer. The shearing force exerted by the nebdant gas disperses the Iiquid jet into a primary aerosol. The nebulizer consists of a n o d e where the gas interacts with liquid sample. The gas velocity can range fiom sonic to supersonic at the nozzle. Noele design parameters that are crucial in the determination of the primary aerosol droplet size distribution are the annular separation at nozzle, the sample capillary tip recession distance, and the inside diameter of the sample capillary, because these parameters will decide the volurnetric gas and liquid sample flow rates and the gas-liquid interaction area. The processes of nebuhation at the nozzle are depicted in Fig. 2.2. Capiiiary AMular separation Gas Fig. 2.1 Schematic diagram of a glas concentric nebuIizer (type C). The diameter of capillary is 200 p,m u l a .separation distance is 10-30 pm,gas pressure is 40 psi, and uncontrolled solution uptake rate is 0.7-3-5 mi/min.. Diagram is not to scale. - - Spray plume ~ 1 1 m e n t I Recombination Fragmente.bulk solutio Interaction zone Gas Gas Fig. 2.2 Schernatic diagram of the gas-liquid interaction wïth a concentric nebulizer. The liquid jet emerging from the inner glass capillary is sheared by the nebulant gas. 2.13 The Primary Aerosol Droplet Size Distribution The primary aerosol droplet size distribution strongly influences the performance of the atomic spectroscopie me- The Sauter mean diameter (4of the primary aerosol is defined as the diameter of the droplet whose volume-to-surface area ratio is the mean of the distribution. Nukiyama and Tanasawa [S] have developed an equation that can predict primary aerosol size distribution trends for anaiyticd nebulizers (eqn 2.1). Unfortuiately, this equation has been used by some researchers to explain their results in tenns of the aerosol droplet that enters an atomizer Mer aerosol r n o w g steps [9]. As a consequeme, many conficting reports exist in the fiterature. To date, no successfui mode1 has been developed to predict the aerosol size distribution emerging fiom a nebulizer-spray chamber combination. This is a curious anondy because whüe the Eiuiction of a spray chamber is obvious, modehg the processes that occur h i d e it remains elusive. 4 = 5 8 5 ~ ( o / p ) ~+' 597(rl/(op)05)0"5 x (103Q,/QJ'' 4 - the Sauter mean diameter (p) V - the velocity ciifference between gas and liquid flow (mk) - o the sUTface tension of the Iiquid (dyn.cm") p - the liquid density (g/cm3) q - the liquid viscosity (dynes.s.cni2) QI-the volume fiow rate of Iiquid (Vmin) Q,-the volume flow rate of gas (Ym2n) 2.2 Droplets with Net Charge in the Aerosol 22.1 Evidence of Droplets with Net Charge Droplets generated by the bursting of a bubble at a sea-water siaface carry net charge [lO]. When a bubble rises to the d a c e , a thin film forms that then ruptures to produce micrometer sized droplets. After the nImruptures, the bubble collapses and produces a jet, that in tirrn breaks up into drops that each carry a s m d net charge. It is believed that the droplet charging phenomenon originates because the electrïcal double layer at the liquid-air interface is randomly ruptured. Loeb has described the processes of aerosol charging in Liquid sprays by mechanicd disnption [Il]. This phenomenon presents an argument whereby the electcical double layer is rupture& or statistical fluctuations of the ionic population in the b d k occurs, leaving a net charge on the droplet. 2.23 Electrical Double Layers The charge found on individual droplets can resuit because of asymmetric disruption of a liquid at the liquid-air interface. Fig.2.3 shows a postulated electrical double layer in a solution containing NaCl. The surface is composed of a layer of water molecules that have their time averaged dipole pointing in [12]. Fig. 2.3 thus represents a "snapshot"in time ofthe motion of molecules in solution. This weak dipole electrical field will cause an elecirical double Iayer to form. When the nebulant gas shears the fiquidjet, the electrical double layer is asymmetrically dismpted, resulting in an aerosol that has discrete size distributions associated with a certain charge distribution of one, or both, polarities. Ifthis mechanism of droplet charging is actingythe characteristics of the aerosol will depend on solution physical properties such as ion concentration, ion mobility, ion size, and the pH of bulk solution. Importantly, it is expected that the droplet net charge would be bimodai. Cl- Cl- Gas Bdk Solution Cl- ClNa+ ClDiffuse Layer Fig. 2.3 ; Cl- ;>O: 1 I I 1 Interface A postulated electncal double Iayer at the liquid-gas interface. See text for detail. Michelson has described a slight excess of the d a c e atoms having an orientaîion in which the hydrogen atoms of the water molecules face the air [13]. In terms of the achlal time averaged water molecule orientation at the d a c e , there rem& much controversy. Regardless of the actual microscopie state of the interface, it is accepted that the physical propeaies of a liquid surface (Le. the interface) are dinerent from the bulk. For a liquid passing through a glas nebulizer, the liquid-glas interface will establish an electrical double layer. Provided the liquid is nebulized before this double layer collapses in the liquid jet extending fiom the nebuüzer, the net droplet charge characteristics of the aerosol will be dependent on the liquid-glas interface. Dodd et. al. reported that the charge distribution among clean mercury droplets in the diameter range 1-6p sprayed fiom a giass sprayer is ssymmetric because of residual contact effects at the mercq-glas interface. The average droplet charge is positive, relatively large, and increases with droplet diameter [Ml. Another possibility of imparting net charge onto droplets can be described as time dependent fluctuations of an ion population over a s m d volume of Bquid. Disruption of the iiquid jet emerging fkom the nebulizer, in a time shorter than the period of ionic fluctuation, causes the production of droplets with a net excess of either cations or anions. With this charging mechanisxn, a charge distribution would be expected over the entire droplet size distribution. This mechanism is examineci in detail later in this chapter. 23 The Function of the Spray Chamber The primary aerosol droplets produced by a pneumatic nebuüzer are of poor quality with respect to droplet size. A spray chamber is necessary to remove large droplets prior to delivery to the pl- Overaü, droplet nimiber densities are reduced in the spray chamber by coagulation, impaction onto the spray chamber walls, and gravitational setrling processes. Various spray chamber designs have been used, such as bead impaction, mixer paddle, and double pass [12]. The Scott - type double p a s spray chamber (Fig. 2.4) is most often encountered in plasma atomic spectroscopy. As depicted in Fig. 2.4, the primary aerosol produced by the nebulizer is modifïed during passage through the ber tube of the spray chamber. The aerosol is now temed secondary. The aerosol droplets emerging fioom the spray chamber are called the tertiary aerosol. The aerosol delivered by an injection tube to the ICP has been temed quaternary by us. PIasma te- Quatemq Drain Fig. 2.4 The aerosol rnodifyiog processes of the Scott-type spray chamber. 2.4 Sample Calculations of Droplets with Net Charge Referring back to the mechanisms of formation of droplets with net charge, a predictive mode1 to obiah the net charge on a droplet, f?om the point of view of the electncal double layer mechanism, does not exist. Hence, the droplet net charge prediction described beiow is based on simple statistical fluctuations of the ionic populations [15].The equation used is: eqn. 2.2 qi is the root rnean square charge (in elementary units) for a droplet volume V at the instant of droplet formation. In these calculatiom, we assume a monovalent chernical matrix (ie. NaCl) of concentration C in molarity. V is the droplet volume (V = 4&/3), r is droplet radius in decimeter, No4.023 x 1O= rno~'. qi is plotîed as a function of the concentration of NaCl inFig. 2.5, for droplets of radius = 1 pm, 5 ~ l mand , 10 p. The average net charge estimated by eqn. 2.2 can be quite large. 1-00 Z O O 5.00 1.00 200 5.00 1.00 2.00 5.00 1.00 E44 E-04 E-04 E 4 3 E-03 E-03 E-02 E-M €42 E-01 Matrix concentration (M) Fig. 2.5 Net statistical charge on droplets of 1,5, and 10 micrometers in radius as a function of NaCl concentration. 2.4.1 Rayleigh Fission Instability The fate of charged droplets during desolvation with respect to Rayleigh fission has been described [13, 14,151. The phenornenon of Rayleigh fission is summarized below. During the evaporation of a charged droplet, the dropiet cm undergo one or more Rayleigh fission events. When the radius of the droplet reaches the instability limit, the repulsive electrostatic force of the net charge equals the surface tension of the droplet solution. The - breakup of the parent droplet initiated with M e r desolvation, and a cloud of 20 smaller - droplets that carry away 2% of the m a s of the parent's droplet and - 15% of the parent's - charge is ejected. The radius of the ejected droplets are one-tenth that of the parent droplet. Droplet stability is predicted with the Rayleigh equation (eqn. 2.3). In eqn. 2.3, the elementary charge (Gis defined as the number of charges that cause Rayleigh fission in a droplet of radius F& This equation has been rewriaen as eqn. 2.4 for aqueous solutions III-' -Je',e = 1.60 x l ~ - 'C. ~ From this where y = 0.07275 ~ . m - 'E,, = 8.854 x 10-12c2- equation, we are able to calculate the net elementary charge number required that will cause the droplet of a specined radius to undergo a Rayleigh fission event %*e2 = 6 4 2 &3 ~~ %= 1.26 x 1014 eqn. 2.3 @+d si rneter) eqn. 2.4 The net charge (Qis plotted as a h c t i o n of dropIet size (r in p)in Fig. 2.6. Comparing the data in this figure to the results in Fig. 2.5, indicates that all droplets produced by pneumatic nebuiization, assuming the statistical charging model, are stable at the moment of formation. 1 2 3 4 5 6 7 8 9 1 0 DropIet size (micron) Fig. 2.6 Rayleigh lunit charge as a function of charged droplet size in radius. 2.4.2 Solid Residne Size Let us now consider the evaporation of the charged droplets. As the solvent evaporates, the droplet radius decreases to the Rayleigh instability six. The droplet then ruptures, sending out s m d progeny droplets and leaving behind a stable residual chplet. This description has assumed that the size of the residuai solid particle d e r desolvation is unable to contain the initial net charge. This situation is valid when the radius of the solid particle remaining after desolvation @J is less than R, To compare the & with R, we have plotted the graph shown in the Fig. 2.7. It is clear that assurning a statistical droplet charging rnechanism, the Rayleigh limit radius (RJ is larger than the solid residue radius (W. A E 3-00- - O - 2.50 g 2.00 -. -- Cr, d - , :-A- R s ( 5 u m ) tO 0 - - - -+--Rs(l - - - R r ( l uum) rn) J Y h--I 1.50 -- Rr(5um) Rs (10 urn) Rr (1O um) Matrix concentration (M) Fig. 2.7 Cornparison of Rayleigh limit radius (RJ with solid residue radius (R3 as a function of matrix concentration for droplets that were initially 1,5, and 10 micrometers in radius. Based on these calculations, we can conclude that the quantity of charge on each aerosol droplet is proportional to the size of the droplet and ma& concentration, with iarger droplets carrying more elementary charges when the mat& concentration is high. Hence at hi& concentration of dissolved solid, this phenornenon of Rayleigh fission of charged droplets will dramatically alter the properties of the aerosol delivered to the plasma, again assuming the statistical fluctuation model. 2.5 Rayleigh Fission versus Matrix Effect Let us examine the matrix effect error caused because of Rayleigh fission using the calculations fiom section 2.4. Rayleigh fission, if occurring, will take place diiring the aerosol transport process. The net result of nebulizing a solution with a high concentration of matrix is that more small droplets, and a slightiy reduced size of the large droplets that undago Rayleigh fission,are delivered to the plasma relative to when a solution is nebulized that has no ma&. The smaller droplets produced by Rayleigh fission, will evaporate quickly because the surface area to volume ratio is much greater. The contents of the droplet, once in the gas phase, will be ionized and excited, and subsequently release characteristic emission relaîively low in the plasma- Thus, according to this mechanism, analyte emission intemity will be greater at lower observation heights, off-ais, and reduced emission intensity on-axis, hi& in the plasma where the large droplets are eventually desolvated in the ICP or flame due to the presence of a rnatrix. 2.6 Solvent Evaporation The initial droplet charge qi is not d c i e n t to cause a Rayleigh fission event (refer to Fig. 2.5 & Fig. 2.6), so solvent evaporation must take place before the Rayleigh instability limit has been reached. For the volatile solvents nomally used such as rnethanol and water, the solvent evaporation rate wiu be under surface control rather than dinusion control [lq. This means that the evaporation rate will be controlled by the rate of liquid to vapor conversion at the d a c e . This assumes that recondensation of evaporated solvent molecules is negligible relative to the d a c e evaporation. The rate of solvent evaporation under surface control is expressed by eqn. 2.6 [17J d d d t = (ac4nR2/4)(POM/R,T) eqn. 2.6 d d d t is the rate of change of droplet mas, a is the condensation coefficient equd to the fkction of solvent vapor molecules which, on collision with the droplet d a c e , condense on the droplet. a = 0.04 for water and ethanol[18]. c is the average thermal velocity of the vapor molecules of the solvent (ds), PO is the vapor pressure of solvent (Pa), M is the molar mass of the solvent @g!rnol), $ is the gas constant (JKmol), and T is temperature (K). Using the relationship between mass m, density p, and volume of the drople~m = (4/3n~~) p, eqn. 2.6 c m be rewritien as: dWdt = - (ad4p) (POM/R,T) eqn. 2.7 For water, c c m be cdculated by using the equation of c = (8kT/nm)IR= 437 m/s [19], PO = 3 167 Pa, M = 0.01 8 kghol, p = 1O' k9/m3, a = 0.04, and for T = 298 K, eqn. 2.7 simplifies to eqn. 2.8 R=&-1.0x104t where the % is the radius of droplet at t = 0, and R is the radius of droplet after time t in seconds. R, and R are in meters. We use the example of a 10 pm radius droplet generated by the nebukation of a NaCl (lx1O-' M) solution. According to the calcdations fiom section 2.4, solvent must evaporate to reduce the droplet radius to - 3.2 Fm. If we substitute the l&, = 1O-' m, R, = 3.2 - x 104 m, the time for desolvation will be 68 ms. The droplet residence tirne in a double - pass spray chamber of the dimensions used is 200 ms [Il. Thus the desolvation time is shorter than the aerosol residence t h e in the spray chamber. Hence, these calculations indicate that Rayleigh fission events could occur during aerosol transport to the plasma Our hypothesis that droplets with net charge are responsible for causing the chemical mat& effect is examined in the next three cbapters. References 1. B.L. Sharp, J. Anal Ar. Spectrom. 3,613 (1988). 2. D. E.Nixon,Spectrochim. Acta 48B,447 (1993). 3. P. J. Gally and G. M. Hieftje, Appl. Spectrosc. 46,1460 (1992). 4. J. B. French, B. Etkin and R Jong, Anal. Chem. 66,685 (1994). 5. M. A. Tan,G. Zhu and R F. Browner, Anal- C'hem. 65,1689 (1993). 6. J. A. K o r o p c u M. Veber and J. Hemes, Spectrochim. Acta 47B,825 (1992). 7. J. W. Oelsik and S. E. Hobbs, Anal. Chem 66,3371 (1994). 8. S. Nukiyama and R. Tamsawa, c c ~ e r i m e i lon l s the AtomUation of Liguids in Air Stream", (E. Hope, Transi.), Defeiise Research Board, Department of National Defense, Ottawa, Canada (1950). 9. M. R Tripkovic and 1. D. Holclajtner-Antunovic, J. Anal. At. Spectrom. 8,349 (1 993). 1O. D. C. Blanchard, "The Electrification of the Atmosphere by Particles fkom Bubbles in the Sea" Progress in Oceanogrqhy, The Macmillan Company, New York (1963). Il. L. B. Loeb, "Static Electrzjkztion ' Springer-Verlag,Berlin (1958). 12. B. L. Sharp, J. A d At. Spectrom. 3,939 (1988). 13. D. Michelson, "'ElectrostatcAtomization", Adam Hilger, Bristol and New York (1990). 14. E. E. Dodd, J Appl. PPhys. 24,73 (1953). 25. J. V. fibarne and B. A. Thomson, J. Chem Phys. 64,2287(1976). 16. P. Kebarle and L. Tang, Anal. Chem. 65,972A(1993). 17. C. N. Davies, "'Fundanzentalof uerosol science D.T. Shaw,- John Wïley & Som, " New York, pl55 (1978). 18. L. Tang and P.Kebarle, AML Chem. 65,3654(1993). 19. D.P. Shoemaker, C. W. Garland and J. W. Nibler, "EqverimentaZ in PlysicaZ Chemistryy',6h edition, The Mcgraw-Hill Companies, Inc., 1996. Chapter 3: Instrumental The imûuments used to acquire the data presented in this thesis are describecl briefly in this chapter. Detailed information regarding the equipment, and its operation, is provided accordingiy in subsequent chapters. Inductively Coupled Plasma - Monochromatic Imaging Spectrometer. Fig. 3.1 is a cartoon diagram of the hductively Coupled Plasma (ICP) Monochromatic Imaging Spectrometer apparatus. A Charged CoupIed Device (CCD) camera was used as a detector to obtain contour pronles of Ca11emission in the plasma. AU contour profles in this thesis have the same scale, O to 260 unie with contour h e s drawn every 20 units. Lens Monochromator Fig. 3.1 Cartoon diagram of ICP - Monochromaîic Imaging Spectrometer. 3.2 Laser Light Scatter Measurements Laser light scattering signals fiom the primary and quatemary aerosol were performed using the same optical system as in Fig. 3.1, but with the plasma off. The laser beam was scattered by the droplets in the quatemary aerosol that would have otherwise entered the plasma. A 4 mW He--Neon laser (Uniphase, Mode1 1676,h = 543.5 nm) was used as the incident light source. The incident laser beam was focused to a thin and narrow line ushg a cylindrical lem. In these experiments, the CCD was replaced by a photomultiplier tube (PMT) as the detector, and the concave lem located in Iiont of the CCD for imaging, was removed. Photon-counting was employed. Thus,in this experimental set-up, the laser scatter signal was coilected with an inefficient opticd system. This fact is however unimportant as only relative laser scatter signals were sought. Absolute droplet number demities were not measured 3.3 The Concentric Pneumatic Nebuker Two types of concen~cpneumatic glass nebulizer were used to convert liquid samples into aerosol in this thesis. A type C nebulizer (TR-30-Cl, Meinhard) has a £ire- polished noide, and 0.5 mm recession of sample capflary tip relative to the outer tube. The nominal water uptake rate is 1.2 ml/&. at the nominal nebulant argon gas pressure of 35 psi. The data in the Chapter 4 and Chapter 6 were coIlected by uskg this nebulizer. A type A nebulizer (TR-3043, Meinhard) has a lapped and coplanar nozzle. The nominal water uptake rate is 3.0 d .at. the nominal nebulant argon gas pressure of 33 psi. The data in the Chapter 5 were coilected using this nebulizer. Electrode Positioning inside the Spray Chamber 3.4 A mesh electrode (size 10) was positioned in the inner tube of the Scott-type spray chamber (see Fig. 3.2). The mesh was biased with a DC power supply to either positive or negative potentials relative to ground. The mesh electrode used was actualiy thimble shaped- The sample solution was always held at ground potential with a graphite electrode. Plasma 4 Drain L Fig- 3 -2 Schematic diagram of the mesh positioned inside the spray chamber. 37 Chapter 4: The Influence of Droplets with Net Charge in Plasma Spectroscopy and Implications for the Chemical Matrix Effect 4.1 Introduction The perturbation by an easily ionized element (EIE)on the spatial emission characteristics of an aaalyte in the Inductively Coupled Plasma (ICP) has been well documented [l, 23. Now, it is lmown that not oniy a .EIE, but M in sufficient concenrntion will cause spatial s y any chernical matrix e in the analyte emission profile in the plasma [3,4]. Furthermore, the spatial shift is generally an enhancement off axis low in the plasma, and a suppression on axis high in the plasma [5,6]. The impetus to develop an a l l encompassing mechanism to rationally explain the general chemical ma& effect, and hencefoah develop methods to minimize the effect, is that enors of determination are the n o m rather than the exception [6J. The determination error arises because the chemical ma& codd alter the plasma environment [7J, the efficiency of nebulization, andfor aerosol transportation processes [8]. In attempts to m v e I the origin(s) of the chemical matrix effect, many fundamental investigations have been perfomed. For example, Borowiec et. al. [9]and Mermet [IO] have reported that the chemical matrix effect originates in the aerosol generation and transportation steps. The aerosol transport efficiency has been investigated in the presence of chemical matzices by different groups, and results show that the tertiary aerosol droplet mean diameter is infiuenced by the presence of a chemical matrix [Il-131. Olesik has studied droplet fate in the plasma and has provided extensive evidence that plasma conditions are strongly influenced locally by evaporating droplets and vaporinng particles [8,14, 151. Therefore, the physical properties of each individual droplet delivered to the plarma is likely to detemillie the fate of that droplet in the plasma. Others have measured time independent gas phase plasma parameters such as the number density of electrons, atoms, and ions and plasma temperature via three different probes (electrons, gas kinetic, and electronic excitation) [5, 16-20]. These plasma parameter studies were directed toward the elucidation of one, or more, of the following mechanistic hypotheses to explain the chemical matrix effect as a result of lateral-diaision changes, shifts in dyte-ionization equilibnum, changes in the collisional excitation rate, and ambipolar diffusion in the plasma. Sesi et- al. recently reported that the inter-element matrix effect is the combined effect of three plasma processes; i) expanded lateral diffusion, ii) ionization equilibrium shift and iii) enhanced coilisional excitation [2]. Unfortunately, the s u m of these studies has not lead to a clear, robust explanation of the chemicai ma& effect. The only consensus we draw from the existing fiterature base is that there are likely several factors acting simultaneously, each acting individually to cause either a suppression or enhancement ofthe analyte ion emission signal in the presence of a chernid ma&. Our motivation for initiating studies of the chernicd matrix effect is derived fiom work involving Electrospray Ioni7sition sources for mass spectrometry [21,221. Specifically, the possibility that dropIets with net charge are produced in abundance in the aerosol generation step could dramaticaily alter droplet transport characteristics and tertiary aerosol size distributions if Rayleigh Fission processes [23], initiated by solvent evaporation, occur inside the spray chamber or immediately pnor to entering the plasma Indeed, it is a well known fact that aerosol generation c m lead to spray electrifïcation [24-261. In this respeck the recent work of Hirabayashi e t al. supports this statement most strongly 1271. These authors have reported the use of a pneumatic nebulization system, operated at hi& gas velocity, as a source of ions for mass spectrometry. These authors have temed thk atmospheric pressure ionization technique "sonic sprayn. Tracy et. al. reported large signal changes as a result of placing a grounded electrode into the spray chamber [28]. They attnbuted their observations to electrostatic effects in the nebukm/spray chamber. MichaIik and Stephens made use of electrostatic trapping of aerosol droplets as a method of solution preconcentration [29]. They presumed that individual droplets canied a net positive or negative charge. The effect of the net charge on a desolvating tiroplet wiu be to induce a Rayleigh fission even&a process whereby the destabüipng electric charge is ejected D3]. The ejected charge is carrieci away in smaller droplets that are temed progeny droplets 13O]. Based on mass conservation considerations, an estirnated 20 progeny droplets, with radii of the parent droplet, are generated. But, these progeny droplets cary away - 10 % that - 15% of the parent droplet's net charge. With respect to sample introduction into a plasma, Rayleigh fission events would alter the size distribution of the aerosol, with subsequent repercussions on the spatial emission behavior of an d y t e . Interestingiy however, this phenornenon has not been investigated fiom the perspective that this codd be the dominant source of the 40 chemical matrix effect error. In this paper, we present recent observations of calcium ion emission behavior in the plasma as a fiinction of added EIE (NaCl) when a DC biased mesh was placed inside the spray chamber. These observations are rationalized fiom the perspective that the size distribution of droglets produced by nebukation have associated with i~ a net charge distribution. Experimental A monochromatic imaging spectrorneter has been assembled to permit Iated emission profiles of an emitting species in the plasma [3 11. A 0.5 m monochromator (CV?, model DK480) equipped with a 1,200 g h grating blazed for 200 MI, was used throughout The spectrometer slit widîhs were set to 2 mm. The CaII emission line at 393.4 nm was monitored. Two 2" diameter planoconvex Ienses were used (focal lengths of 50.8 mm and 130.8 mm) to produce an object rnagnification of 0.39 at the CCD detector (Santa Barbara Instnunent Group, model ST6). An integration time of 50 msec. was used throughout The CCD camera output was binned for output resolution of 250 x 121 pixels (each binned region was 34.4 and 54 pn respectively). The region of the plasma viewed was fiom 7 to 24 mm above the load coi1 (ALC). The horizontal view was 21 mm wide. Lateral emission profües of the analyte, in 1 mm high slices, were acquired staaing at 7 mm above the reference position, the top of the load coil. In each figure presented, the viewing height indicated is the top of a 1 mm slice. In aIl figues, the profle labeled a is 10 mm ALC. Similarly, b, c, d, and e are 13, 16,19, and 22 mm ALC, and f, g, 4 i, and j are 10, 13,16, 19,22 mm ALC respectively. The hductively Coupled Plasma (Plasma-Therm Inc., Type 2500D,27.12 MHz) was operated at 1.3 kW forward power. The sample introduction system consisted of a concentric g l a s nebulizer (Meinhard, TR-30-Cl) and a double pass spray chamber (Scottwe). The flow rate of nebulant gas (Ar) was controlled with a mass flow controuer (MKS, mode1 1159B-02000SV). The fkee so1ution uptake rate was 1.2 ml/&., and was invariant to the range of central gas flow rates used. The sealed glass end of the spray chamber was replaced with a ground glass joint This facilitated rapid and simple positionhg of a stainless steel mesh inside the spray chamber. S i z e 10 mesh was used because there was no ciifference in the spatial emission characteristics of CaII before and after the grounded mesh was positioned inside the spray chamber. A larger mesh number adversely perturbed aerosol transport as monitored indkecdy by CaII emission intensity. The mesh was thimble shaped and fit snugly inside the end of the inner tube of the spray chamber (in this work, the mesh was positioned 10 cm fkom the tip of the nebulizer). The screen was maintallied either at ground potential or at + 200 V using a DC power supply (Stadord Research Systems, mode1 PS350/5OQO). A negative DC bias on the mesh had the same effect as a positively biased mesh. Electrical comection to this mesh was via the drain tube. The sample solution being nebulùed was always maintained at ground potentiai with a graphite electrode. The reference soluîion for this work was 1x 104 M Ca2+,prepared from CaCIi2H20. The NaCl chernical matrix solution consisted of 1 x IO4 M Ca2+,prepared fiom CaCIi2H2O plus 1x1O*' M NaCl. Solutions were prepared k s h weekly using distilled deionized water. The background signal intensity has been subtracted fiom the lateral Ca K i emission signal intensÏties presented here using matrix matched blanks. Results and Discussion The pneumatic n e b h t i o n of a liquid produces a non-uniform distribution of droplet size that is classined as the primary aerosol. In addition to the nebdker-design dependent distribution of droplet size 1321, there is a distribution of net charge being carried by individual droplets within the aerosolL241. The net charge distribution on individual droplets is less well characterized than primary aerosol size distributions. Laterat emission profiles of calcium atomic ions in the plasma are used here to illustrate the effect of droplet size and droplet net charge on Calcium atomic ion emission signal intensities. First, the characteristics of aerosol generation, transpo& and subsequent fate in a plasma are examined to provide the reference for subsequent discussion within this article. The primary aerosol is typicdy generated directly inside a spray charnber whose furiction is to remove excessively large droplets fkom the gas Stream that would otherwise simply p a s through the excitation source without being completely desolvated and vaporized 1251. Aerosol inside the spray chamber is termed secondary, and aerosol exiting the spray chamber is termed tertiary [33]. We add another level of aerosol classincation, calling the aerosol exiting the central tube of the torch the quatemary aerosol because, as will be discussed, the influence of solvent evaporation nom the droplets can markedly change the aerosol. A welI designed spray charnber will control the extent of gravitational settling, inertial losses, and aerosol coaguiation rates in the secondary aerosol. Also, large droplets locally cool the plasma, as measured by a decreased electron density [8], so their removal by the spray chamber is beneficial for improved noise characteristics of the analyte emission intensity [34,351. Olesik et. al. have proven that in the plasma, the time for droplet desolvation is much longer than the time required for particle vaporization [14,34]. This implies that perhaps the greatest influence of a chemical matrix could in fact be caused by diBerences in aerosol characterïstics, rather than alteration of plasma processes. The range of droplet sizes in the quatemq aerosol being introduced to the ICP can be appreciated through consideration of the calcium atornic ion laterai emission profiles as a function of viewing height above the load coil (Fig. 4. l .a*). In Fig. 4.1 .a, a low CaII - emission signal is evident. The Smallest droplets in the quatemq aerosol, < 0.1 pm in radius, are responsible for this signal. The central channel is heated fkom the outside to the inside, hence the Ca11 emission intensity peaks around the edge of the centrai channel. With increased viewing height and longer residence times in the plasma, larger droplets have desolvated and are subsequently vaporized (Fig. 4.1.b-d), ' f i h g in' the Ca11 emission si@ across the central channel. In continuhg up to 22 mm, the signal intensity doubles, with still yet larger droplets being desolvated and vaporized. The FWHM (full width at half maximum) of the CaII lateral emission profile in the centrai channel does not increase in moving from 19 mm to 22 mm viewing height above the load coil (ALC), Fig. 4.1.d and Fig. 4. l .e respectively. This indicates that the remains of the larges droplets (- < 35 pn diameter) fiom the quaternary aerosol were still king desolvated at the viewing height of 19 JO- =- Lateral Viewing Position (mm) Fig. 4.1 Lateral emission profiles for CaII (3 93-4 nm)in an ICP. The sample soluti011 1 x 104 M CaC1,. The nebulant gas flow rate was 900 d m i e In the series of laterd profile a-e, the mesh was maintained at ground potential and in the series f-j,the mesh was rnaintained at + 200 V DC. In a i l cases, the solution was held at ground potential using a graphite electrode. The DC biased mesh removes droplets with net charge h m the aerosol stream (f-j), caushg the Ca11 profile to be altered. See text for discussion. Note the vertical scale. mm and th& the residual salt paaicle vaporized in the region between 19 and 22 mm ALC. Had d droplets been desolvated and vaporized at the viewing height of 19 mm, the lateral emission pronle at 22 mm would have been much wider due to diffusion of atomdions. During the acquisition of this series of lateral emission profiles, the nebulant gas flow rate was relatively high (900 rnhin.), causing the central charnel of the plasma to be wmparatively cool, meaning that the sequential steps of desolvation, vaporization, ionization, and excitation are delayed relative to optimized central gas £lowrates. In addition to these effects, Olesik has measured slightly irnproved aerosol transportatioa efficiency at higher centrai gas fiow rates, further cooling the central channe1 due to the additional solvent load [36]. This was done deliberately to ailow observation of the first CalI emission in the plasma (Le. Fig. 4.1 .a). Notice that the enhanced off-axis emission in Fig. 4. l .a at +/- 1.6 mm is analogous to the effect caused by a chemical matrix. This irnplied to us that the manifestation of the chemical matrix effect is simply an enhancement of a process already present, rather than a new process emerging because of the presence of a ma&. A process that exists whenever a solution is nebulized is the deposition of net charge onto individual droplets, and this process, as w i l l be discussed, is ion concentration dependent We believe that net charge on individual droplets is responsible for causing the chemical rnatrix-induced perturbation of analyte emission profiles in the plasma The premise that individual droplets in the primâry aerosol distribution carry a net charge, implies that an electric field could be used to remove çuch droplets fiom the aerosol. A SUR 10 mesh was positioned inside the spray chamber and biased to + 200 V DC. The effect of the biased mesh on the analyte emission characteristics of CaII is exemplrfied with the lateral emission profiies plotted in Fig. 4. l .f-j. The biased mesh causes several dramatic 46 changes in the emission profile charactenstics. First, the signal intemity rn;Urimum at 22 - mm viewing height (Fig. 4.1 .j) is 0.5 that when no voltage bias is applied (Fig. 4.1 .e). Second, the signal htensity maximum in this set of pronles (Fig- 4.1 .fto Fig. 4.1 .j)is reached at - 19 mm ALC, rather than - 22 mm ALC. This behavior is indicative of no, or very few, droplets/particles remaining above 19 mm because I a t d dinusion was the most noticeable change between these two profiles. We specdate that the droplets that have the largest charge-to-mass ratio have been removed fiom the aerosol because of the DC biased mesh. In so doing, the solvent load in the plasma has been reduced, meaning that less cooling of the central channel by the aerosol has also resulted. Lastly, the peaks of CaII emission around the periphery of the central channel are no longer apparent at the viewing height of 10 mm ALC, though the on-axis signal intensity is relatively unchanged (compare Fig. 4.1 .f to Fig. 4.1 .a). We take these observations to imply that one, there is an appreciable hction of droplets that do cary a net charge in the aerosol, and two, that the removal of droplets with net charge fiom the aerosol is correlated with the off-axis enhancement low in the plasma As was briefly o v e ~ e w e din the introduction, a Rayleigh fission event in the tertiary or quateniary aerosol would dter the size distribution of droplets entering the plasma In the series of CaII profiles without applied potential (Fig. 4.1.a -e), the droplets with net charge could be undergoing one, or multiple stages of Rayleigh fission, brought-on by solvent evaporation in the spray chamber or by the sIightIy elevated temperature in the central channel of the torch. If so, an increased abundance of s m d droplets wodd be present relative to a case where the droplets carrïed a net charge of zero. These smaller progeny droplets contribute to the off-axis emission low in the plasma With the application of a potential to the mesh (Fig. 4. l .f-j),the droplets with greatest charge-to-mass ratio are removed e o m the aerosol Ieaving only predomhantly droplets of neutral droplets, or ones with relatively low charge-to-mas ratio, in the secondary aerosol Stream. These droplets do not undergo Rayleigh fission prior to entering the plasma, and hence the enhanced off-axis emission low in the plasma is signincantly reduced. Quantitative assessments of the relative dihbution of droplet size that carries what distribution of net charge should not be estimateci fiom this data set. Alteration of the plasma central channel temperature, because of the reduced solvent load, would make such an estimation tentative at best Direct measurement of dropIet size distribution and net charge distribution is however, presently undenvay [371. Two mechanisms to explain the acquisition of charge on individual droplets during the nebukation step exist in the fiteratme. One mode4 a d a c e charging mechanism, based on random ruphire of the electrïcal double layer during the instant of droplet formation, wiii lave a net charge imbalance on individual droplets [24]. The other model describes local ionic population fluctuations on time scdes greater than the time required to create an individual droplet [26,38]. Either of these two models is plausible for depositing a net charge onto a droplet, and depending on solution physicd properties, both rnechanisms could act simdtaneously. Overall, the aerosol should remain neutral in the absence of surface active species, but individual droplets c a . carry a net positive or negative charge. The action of the biased mesh inside the spray chamber on the coIloid is to perturb the trajectories of droplets that carry a net positive or negative charge, c a h g those droplets with the greatest charge-to-mas ratio to be removed. An additional complication could be that the dominant droplet charging mechanism favors a particular droplet size. For example, if samples contain varied concentrations of a d a c e active species, dramatic changes in droplet net charge (and size) distribution would resdt. It is straghtforward to illusttate that a chernicd matrix will dTect the droplet net charge distribution using the statiçtical charge model, though at present, such a cornparison should be viewed as crude [2q. In eqn. 4.1, the net charge d e d by a droplet is dependent on the ionic concentration of the buik solution (C) and the droplet size (V). q is the RMS (root mean square) number of elementary charges on the droplet and No is the Avogadro Constant. For example, at 1 x 104 M CaCI, the mean statistical elementary charge on a - droplet with a radius of 10 pm is 2.3 x IO4, yet a similarly sized droplet generated nom a - solution containing 1 x 1O-' M NaCl would carry 7.1 x 1OS charges. Clearly, if this equation reflects the real situation there wili be a dramatic effect on net droplet charge when a chernical matrix is present in a sample, causing more s m d droplets to be introduced to the plasma relative to the aerosol size distribution htroduced to the plasma by a calibration set that has no matrix. Let us now examine this possibility by viewing CaII lateral emission pronles as a fiinction of matrix and bias applied to the mesh. (q2)ln = (ZCVN,,)'~ eqn. 4.1 The lateral emission pronles for CaIl in the absence and presence of NaCl acquired at 700 d m i n . centrai gas £iow rate are provided in Fig. 4.2.a-e and Fig. 4.2.f-j respectively. Because the central chamel is cooled less than with 900 mVmin, the overall profile of the CaII emission is shifted downward, toward the load coil in Fig. 4.2.a-erelative to Fig. 4.1.a- e. For example, the maximum CaII emission signal occurs at 19 mm ALC (Fig. 4.2.d) at 700 d m i n . , whereas at 900 mVmin., the maximum is not reached until22 mm ALC (Fig. Lateral Viewing Position (mm) The effect of a 1x10'' M NaCl chemical matrix on the laterai emission ~rofilesfor CaII (393.4 MI)in an ICP. The nebulant gas flow rate was 700 d m i n . The me& was maintained at ground potential and the sample solution also at ground potential. In the series a-e there was no NaCl present, and in the series f-j the NaCl chemical matrix was present. The weU lmown rnatrix eEect is exemplifïed in this series of Ca11 emission profiles, showing the off-axis e h c e m e n t of the analyte emission low in the plasma, and the on-axis suppression high in the plasma Note the vertical scale. 4.1 .e). Additional evidence of the entire emission profile shifang lower in the pl- is that at 22 mm ALC in Fig. 42.e, the FWHM of the CaII profile has grown wider due to lateral diffiision of the gaseous d y t e species, yet the on-axis emission intensity is unchanged relative to 19 mm ALC (Fig. 2d). With the addition of 0.1 M NaCl, the CaII pronle changes in two noticeable ways (Fig. 4.2.f -j). Fisi, there are peaks of CaII emission at 10 mm ALC (Fig. 4.2.0 and the profile is wider at ail viewing heights. A visual cornparison of the laterd profile width is easily made between Fig. 4.2.c where no mtrix was present, and Fig. 4.2.h. We attribute both of these observable effects to a droplet charging phenornenon as described below. An initidy 10 prn radius droplet would have a mean charge of - 7.1 x 1Os for a chernical matrix concentration of 1 x 10'' M NaC1 @lus 1 x 104 M CaClJ. As this droplet passes through the spray chamber and up the central tube of the torch, this droplet will desolvate and undergo one, or possibly several, Rayleigh fission events. This initidy 10 pradius droplet WU fïrst undergo fission when it has desolvated to a radius of - 3.2 pm. The progeny droplets, each - 0.3 p m in radius, desolvate and vaporize much lower in the plasma. It is these additional droplets that are present in the tertiary and quaternary aerosol that account for the off-axis CaII enhancement low in the plasma (Fig. 4.2.0. Also, if Rayleigh fission is occurrïng, and because the larger droplets fiom the secondrily or tertiary aerosol contribute to the Ca11signal at high viewing heights, then these droplets will be reduced in size in the quaternary aerosol by an amount equivalent to the mass of material lost due to Rayleigh fission. This interpretation can account for the increased anaiyte emission intemity, off-axis, low in the plasma, and decreased aualyte emission intensity, on- axis, high in the plasma in the presence of a matrk Xt is very interesthg to note that the inte-ted signal for CaII at a viewing height of 22 mm ALC in the absence of matrix (fig. 4.2.e) is 6.878 x 106au.,and in the presence of 1 x IO-' MNaCl (Fig. 4.24) is 6.502x 106 au., representing a signal intensity difference of only 5-5 %. This is not out of Line with - normal experimental signal relative standard deviations of 5 % and it suggests that the total CaII delivered to the plasma is largely d e c t e d by the presence of a chemical matrix such as NaCl (Le. sinface tension and viscosity are not grossly altered). In contrast to the argument presented above for a droplet produced in the presence of a matrix, the initially IO pradius droplets produced fkom a solution containing only CaCl, at 1 x lo4 M would not undergo Rayleigh fission until their radii decreased to - 0.32 p.If this critical size is not reached until the droplet has entered the plasma, then the effects of the initial charge fkom the nebulktion step would be much less innuential on the spatial emission profile. If our mode1 that Rayleigh fission events are occmhg is correcf then the application of a potential to the mesh positioned inside the spray chamber to discriminate against droplets with hi& charge-to-mass ratio should reduce the spatial emission discrepancies of an analyte in the plasma, regardless of the chemical matrix and its concentration. With the mesh DC biased to + 200 V, the CaII emission profiles are plotted for Ca alone (no matrix) in Fig. 4.3.a-e and in the presence of 1 x 10" M NaCl in Fig. 4.3.f-j. The effect of the biased mesh on the CaII pronles in Fig. 4.3 c m be compared to those in Fig. 4.2 where the mesh was maintained at ground potential. It is immediately apparent that the profiles in Fig. 4.3 have lower signal intensity maximum,and that for the case when maîrix was present, the off-axis enhancement of CaII emission at 10 m m ALC in Fig. 4.2.f is Lateral Viewing Position (mm) Fig. 4.3 The effect of a 1x1 O-' M NaCl chernical rnatrUr on the lateral emission profiles for CaII (393 -4 nm) in an ICP. The nebulant gas flow rats was 700 &min, The mesh was maintaineci at + 200 V DC and the sample solution at ground potential. In the series ae, there was no NaCl present, and in the series f-j, the NaCl chemicai matrix was present The mtrk effêct-induced spatial shift of the d y t e in the presence of the matrix is significantiy reduced because of the DC biased mesh positioned in the spray chamber. See text for discussion* Note the vertical sale. now absent (Fig. 4.3.0. These observations can be rationalized with respect to the arguments presented above in that Rayleigh fission occurrences determine the shape of the lated emission profles presented in Fig. 4.2. Yet, with the mesh biased to 200 V, the majorîty of droplets with net charge are removed fiom the aerosol, leaving only droplets with zero, or low net charge in the secondary aerosol. The effect of Rayleigh fission processes in the tertiary or quaternary aerosol is m h h k e c t , and the enhanced off-axis emission low in the plasma is nearly eliminated. The effect of the biased mesh on maximum signal intensity in the set of CaII profiles for Calcium, no ma& present (Fig. 4.3.a-e) and the CaII profiles in the presence of a ma& (Fig. 4.3 .f-j)caused a surprisingly similar reduction of 1 10 % and 115 % respectively. This is indicative that the biased mesh does sufnciently perturb the trajectories of droplets carrying a net charge to cause an appreciable fiaction of them to be removed fiom the aerosol fiom both solution types. (Note that this observation canot be used to address the question of charge magnitude on similarly sized droplets for the two cases of no rnaîrix present versus ma& present.) An intnguing possibility that we are evaluating, is that it should be possible to tune the potential applied to the mesh to vimially eliminate the chernical matruc effect error [39,40]. As a preIiminary illustration of this experimentaf possibiiïty, compare the pronles in Fig. 4.2.a-eto Fig. 4.3 .f-j. Though the absolute intensities are different, the lateral profile of Ca11 emission in the absence of matrix as a fimction ofviewing height ALC with the mesh grounded in Fig. 4.2.a-e,are very similar in appearance to the pronles in Fig. 4.3.f-j where a matrix is present, but the mesh was biased to + 200 V. The difference in net signal intensity is expected because charged droplets were allowed to pass to the plasma in Fig. 4.2.a-e, but not in Fig. 4.3.f-j, and as discussed above, there is a large population of droplets with net charge in the aerosol. It was argued that the spatial emission profile of an d y t e in the presence of a chemical matrix (Le. increased off-axis emission low in the plasma and decreased on-axis emission high in the plasma) is causeci by net charge imparted onto individual droplets during nebukation. Evidence of dropleîs with net positive andlor negative charge in an aerosol created by pneumatic nebukation has been venfied. Application of a DC voltage to a mesh positioned in the spray chamber reçulted in observable changes in the spatial emission profile of an analyte in the plasma. The biased mesh removes droplets with high charge-to-mass ratio fkom the aerosol Stream. Most irnportantly, application of + 200 V DC to the mesh in the presence of a chernicd matrix (1 x 10" M NaCl) caused the spatial emission profiles for CaII to appear similar to the profiles for CaII profiles in the absence of a chemical maîrk (O V on the mesh). It is speculated that with suitable application of DC voltage to the mesh, the aerosol characteristics can be rnod5ed to miriimize the chemical matrix effect. More experiments are now being conducted in o u ,laboratory on this interesting hypothesis, that net charge on individual aerosol droplets is a.dominant factor in determining the spatial emission characteristics of an analyte in the plasna M. W. Blades and G. Horlick, Spechochim. Acta 36B, 861 (1981). M. W. Blades and G. Horlick, Specirochim. Acta 3 6B,881 (1981). J. W. Olesik and E. J. WiIliamsen, Appl. Spectrosc. 43, 1223 ( 1 989). J. W. Olesik, Anal. Chenz. 68,469A (1996)N. N. Sesi and G. M. Hiefije, Spectrochim. Acta 51B, 1601 (1996). G. M. Hieftje, Spectrochim. Acta 47B, 3 (1992). M. W. Blades and D. G. W e i -S p e c h o s c o ~9,14 (1994). J. W. Olesik, Appl. Spec~osc.51, 158A (1997). J. A. Borowiec, A. W. Boom, J. H. Diliard, M. S. Cresser, and R F . Browner, Anal. Chem. 52,1054 (1980). J. M. Memet, J. Anal. At. Spectrom. l 3 , 4 19 (1998). R K Skogerboe and S. J. Freeland, A& Spectrosc. 39, 925 (1985). R. K. Skogerboe and G. B. Butcher, Spectrochim Acta 40B,1631 (1985). K. O'Hanlon, L. Ebdon, and M. Foulkes, J Anal. At. Spectrom. 12,329 (1997). J. W. Olesik and J. C . Fister, III, Spectrochim Acta 468,851 (1991). S. E. Hobbs and J. W . Olesik,Anal. Chem. 64,274 (1992). D. S. Hanselman, N. N. Sesi, M.Huang, and G.M . Hieftje, Spectrochim Acta 49B, 495 (1996). P. J. Galley, M. Glick, and G. M. Hieftje, Spectrochim Acta 48B,769 (1993). M. Wu and G. M. Hieftje,Spectrochim Acta 49B, 149 (1994). M. R Tripkovic and 1. D.Holclatjtner-Antunovic, 1 A d At. Spectrom. 8,349 (1993). 1. D.Holclajtner-Antunovicand M.R Tripkovic, J. Anal. At. 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Olesik and L.C.Bates, Specîmchim Acta 50B, 285 (1995). Q. Xu and G. R. Agnes, Appl. Spectrosc. in preparation (1 998). J. V. Iribarne and B. A. Thomson, J Chem Phys. 64,2287 (1976). Q. Xu and G. R Agnes, Anal. Chem. in preparation, (1 998). Q. Xu and G. R Agnes,Appl. Spectrosc. in preparation (1998). Chapter 5: Charged Droplet Production by Concentric Nebukation and its Effect in Atomic Spectrochemical Measurements The ICP is widely used as an excitation source for emission or ioni7ation source for mass spectral multielement analysis in today's routine service laboratory. However, the one major caveat ofthis technique is the chemical ma& effect Consequently, the chemical matrix effect in the inductively coupled plasma has received tremendous research attention [l-101. The chernical matrix effect is known to have spatial and temporal dependence. W y t e partitionhg in the plasma is a h c t i o n of concentration, matrix concentration, local environment and spatial position. As such, informative investigations have provided spatially and radially resolved element partition coefficients fiom emission and fluorescence/absorptionmeasurements [l,2, 121. Reports that derive conclusions fiom single point experiments (i.e. k e d observation position, PM?' detection) must be viewed with caution. Wïth respect to temporal analyte partitioning, Li et. al.asnimed that under normal plasma operathg conditions, the droplets injected into the plasma were compietely desolvated AND vaporized [13]. However, Olesik et. al. have since proven this widely accepted assumption as being false [I4,151. In fact, the presence of a desolvathg droplet or a vaporizing particle dramatically alters analyte intensities. Near a droplet, emission is suppressed, due to localized cooling of the plasma The droplet acts as a local heat sink. But near a particle, emission is enhanced due to a locally higher concentration of d y t e being boiled off the particle. Olesik has also reported that droplet desolvation requires much longer time than does the particle vaporization [16,17. Thus it is plausible that the chemical ma& effect could be caused by ciifferences in the physical properties of the aerosol. In the previous chapter, we proposed that a charged droplet phenornenon during the aerosol generation step c m be used to account for the chemical mat& effect- In the proposed model charged droplets undergo Rayleigh fission during the aerosol transport process, because desolvation has taken place thereby generaîing more progeny droplets that are transferred to the plasma The srnaIl progeny droplets desolvate and vaporize faster, resulting in an enhancement of anaIyte signal low in the plasma According to the mechanism descnbed above, the droplet size distribution of the tertiary aerosol will have been m o ~ e d In . support of this prediction, Skogerboe et al.have reported that reduced aerosol size distributions are associated with high total solute concentrations [18, 191. These authors noted that these results are con- to the effect expected, because hi& solute concentrations are expected to suppress solvent evaporaton. However, if Rayleigh fission events are ongoing to generate smaller droplets, the shift toward a smder droplet s k e distribution in the teaiary aerosol measurements can be explained. In the preceding chapter, we presented the effect of a DC potential (+ 200 V) on a size 10 mesh positioned inside the spray chamber. Also the Ca ion l a t e d emission profile in the plasma was compared in the absence and presence of a NaCl ma& [20]. From these studies, we concluded the tertiary aerosol droplet size distribution modified by the presence of a chemical matruc, can be M e r modified by the DC potential. However, it is unclear whether changes in the primary aerosol or aerosol transport is responsible for this behavior. An investigation to m e r this question has not yet been mounted because the necessary equipment is not available in our laboratory. In this study, various alkali metal chloride salts and a surfactaa sodium dodecyl sulfate (SDS), were used as the chemical matrices. Nitric acid was also used as a rnatrix itself, or combined with NaCl. Nebulant gas flow rate (FJ were varied, as was the DC bias applied to the mesh. The results of this investigation are presented as contour maps of the Ca11 lated emission intensity. Experimental The instrumental set-up used for this work was described in Chapter 3. Here, calcium ion emission contour maps are reportd - The Inductively Coupled Plasma (Plasma The& Inc., Type HFP 2500D,27.12 MHz) was operated at 1.2 kW forward power. Liquid sample was nebulized with a Meinhard TR-3043concentzic nebulizer. The fiee solution uptake rate of this nebulizer was 3-0rnU& A DC potentid fÎom O V to + 3000 V was applied to the mesh positioned inside the spray chamber. All solutions contain 1 x 104 M Ca2+,prepared Eom CaC1;2H20. The various chemical matrices were present at the indicated condition (1 x 1O-' M NaCI, 1 x 10" M KCI, 1x 1O-' M CsCl, 1 x 1O-' M sodium dodecyl sulfate (SDS), 0.1% HNO,, and 0.1% HNO, plus 1 x 10.' M NaCl). Distilled deionized water was used in the preparation of aII solutions. Background signal has been subtracted fkom the CaII e k s i o n contours, presented here using matrix matched blanks- 5.3 Results and Discussion Incorrect conclusions are ofien made because researchers have studied chernid maûk effect using a fixed,singled point detector. In t h i s work, lateral contour maps of Calcium ion emission intensities in the plasma present much detailed information about the effects of a chemical matsix. It has been recognized that the nature and extent of chemical rnatrix effects are very dependent on the operating conditions of the ICP 1111. It has also been reported that the r ~ t r i effects x are less severe under the "robust condition'' of relatively low nebuiizer gas flow rate and high RF power [2 1,221. Under robust conditions, the anaIyîe aerosol reaching the normal analytical zone (NU) Ui the plasma is completely desolvated and vaporized. The drawback of the robust condition is a reduced sensitivity due to high background signals, and lower analyte transport efficiency to the plasma 53.1 CaII Emission Contours as a Fnnction of Nebulant Gas FIow Rate The two test solutions were 1 x 104 M CaC1,.2H,O, and 1 x 104 M CaC1,-2H20 plus 1 x IO-'M NaCl. Three nebulant gas fiow rates (FJ were chosen for this experiment (400, 500, and 600 dmin.). For F, = 400 drnin., the operating conditions approximated the robust condition. Fig. 5.1 shows the effect of nebulant gas flow rate (FJ on the lateral emission contours for CaII in the ICP . In comparing the Fig. 5. l .a to Fig. 5.1 .d for no ma& v e m matrix present respectively, the off-axisenhancement of CaII emission low in the plasma (7 mm ALC) is minimized even though there is 1 x IO-' M NaCl present. This confimis that the spatial dependence of the matrix effect is less severe at lower gas flow rate (FJ is utilized. However, the matrix effect is s t i l l present in ternis of the net CaII emission intensity, so simply operating under conditions where all aerosol is desolvated is not the answer in miriiminng detennination errors, nor is it aiding in understanding the mechanistic origin of the problem. In raising the F, to 500 ml/'min_, the Ca11 emission contours are shifted upward relative to the reference point (top of the load coil) by comparing Fig. 5. l .b with Fig. 5.1 .a, and Fig. 5. l .e with Fig. 5.1 .d. Very obvious changes in the CaII emission have resulted because of the increased F, (compare Fig. 5.l.b with Fig. 5.1 .e). The CaII emission contour map is broadened in the presence of the m a t a and the onset of CaTI emission is 2 mm lower. The effect of a chernical mat* on analyte emission to cause off-axis - enhancement low in the plasma, and on-axis suppression high in the plasma, has been reported in the literaîure [l, 31. Increasing the F, M e r to 600 ml/&. caused more extensive cooling of the plasma due to the increased amount of aerosol delivered to the plasma The maximum CaII emission is shifted to a higher position ALC as a result (Fig. 5. l .c and Fig. 5.1 .f). Yet there is CaII emission off-axis low in the plasma (Fig. 5. l .c). Therefore, a h c t i o n of the aerosol must have been desolvated and vaporized low in the plasma even though the ICP environment is relatively cool. The enhanced CaII emission, off-mis, low in the plasma (Fig. 5-1.c and Fig. 5.1 .f) indicates that at high F, there appears to be more smalIer droplets in the aerosol delivered to the plasma From fluid dynamic principles [23], it is known that s m d droplets that have desolvated and vaporized are the source of the CaII emission low in the plasma Comparing Ca11 emission contours without matrix present for different nebulizer gas flow rates (Fig. 5. 1.a, b, and c), shows an upward shift trend (higher ALC) in the location of the CaII emission signal intensity maximum with higher Fr This observation can be attributed to the increase of aerosol mass transport rate and also cooling of the plasma due to the increased solvent load at high F, [24]. A high mass transport rate, a relatively cool plasma, and short aerosol residence time all act to cause the maximum emission intensity to shift higher in the plasma. Long and Browner also observed that the maximum peak analyte intensiv occurred higher in the plasma when sample was introduced as an aqueous aerosoI relative to when a dry,e l e c t r o t h e d y vaporized sample was used D5]. These authos concluded that the location of the maxirnumpeak emission intensity depended on the distance droplets travel before desolvation is complete. Lateral Viewing Position (mm) Fig. 5.1 The effect of nebulant gas flow rate (FJ on the lateml emission contours for CaII (393.4 nm) in an ICP. The sample solution was 1 x 104 M CaC12 for a-c. The sample solution was 1 x 104 M CaCl, plus 1x10-'M NaCl for d-f. The nebulant gas £lowrate was 400 nzVmin. for a and ci, 500 d m i n . for b and e, 600 d m l l i . for c and f. The difference in the C d emission contours in Fig. 5.1 .c v e m Fig. 5.1 .f illustrates the chemical ma& effect With respect to low viewing heights ALC, the enhanced CaII emission in the presence of a NaCl matrix codd be due to several factors. First, there could be difference in the degree of perturbation of the plasma caused by the two different samples. However, numerous investigations into plasma diagnostics have not generated conclusive evidence of sample induced plasma alterations. Second, there could be changes in the aerosol transportation efficiency, such as increased droplet coagulation in the spray chamber. Third, the nebukation efficiency rnay have changed as a result of the physical characteristics of the solution. 5.3.2 CaII Emission Contours in the Presence of NaCl, KCI, CsCI, and SDS Chemical Matrices Ail CaII emission contours shown in Fig. 5.2 were acquired using a nebulizer gas flow rate = 550 d m i n . In cornparhg the CalI emission contour with no matrix present (Fig. 5.2.a), to that obtaia.ed in the presence of an alkali-metal chloride salt (Fig. 5 2 b-e), these matrices each have a similar eEect on the CaII emission contour map. In the presence of a matrix, the EI1SUCiIIïumCaII emission is consistently shifted downward by plasma to - 3 mm in the - 19 mm ALC, and there is enhanced CalI emission, off-axis, low in the plasma at the viewing heights fkom 10 to 16 m m ALC. A distinguishable trend of perturbing the CaII emission as a function of nrst ionkition potential in going fiom Na to Cs is not observable in the contour plots in Fig. 5.2 h. This is in contrast to observations presented in the literature where the off-axis enhancement of CaII low in the plasma correlated with 65 Lateral Viewing Position (mm) Fig. 5.2 Lateral emission contours for CaII (3 93.4 m)in an ICP. The sample solution was 1 x IO4 M CaCI, for a, and plus 1 x IO-'M NaCl, KCL, CsCl, SDS for b, c, d, and e respectively. The nebulant gas fIow rate was 550 mlhin. decreasing first ionization potentid [L ,3,4]. This discrepancy is very likeIy to be sîmply because of temporal delay in acquuing this set of data Because of this, it is our contention that the contours presented in Fig. 5.2 indicate a consistent perturbation of the aerosol production and transport steps that act to change the quaternary aerosol, and that the matrix is not just the cation, but the total dissolved solid. This view is markedly different than that which contends that the matrix pemirbs plasma conditions. Hence, fkorn o u perspective, it should be expected that because ciifferences in the physical properties of the allcali-metal chloride sait solutions is very small, the effect of these different matrices on the CaII contours shouid be consistent Several reports describe that anions are also likely to cause matrix effects [l, 261. The effect of dodecyl d a t e (Fig. 52.e) as the anion versus chloride (Fig. 5.2.b), is to shift the CalI emission maximum even bwer in the plasma (the common cation was sodium). The effect of this surfactant is most probably an increased abmdance of very s m d droplets in the quateniary aerosol that are responsible for earlier desolvation leading to the enhanced CaU emission intemity off axis, low in the plasma. However, the maximum C d signal intensity in the presence of the sodium dodecyl sulfate (SDS) is noticeably reduced as compared to both Ca with no m a e (Fig. 5.2.a), and Ca with NaCl matrix @ig. 5.2.b). The consensus reached fiom the sum of these observations in the presence of the SDS, is that there is a decreased analyte transport rate to the plasma The actual cause of this overd effect could be the production of an aerosol chaacterized by a smaller average primary droplet size, changes in the aerosol transportation due to increased net charge on the droplets in the presence of a surface active agent, changes in the aerosol coagulation rates, or a combination of these effects. Kodama and Miyagawa reported that the addition of 0.05 M SDS can produce &er aerosol droplets than in the absence of SDS solution. The authors explained the observation due to the lower d a c e tension of solution in the presence of SDS 127. Olesik and Moore observed a downward shift in the location of the peak intensity when organic solvent was added 1281. These authors introduced the argument that the location of analyte emission intemity maximum is related to the point where desolvation is complete. They concluded that either the droplets entering the plasma are d l e r , or the rate of desolvation is increased in the presence of organic. R e f e h g back to the Rayleigh instability equation described in Chapter 2, a relatively low sudace tension favors the occurrence of a Rayleigh fission event nom larger droplets. Hence, the presence of SDS codd cause an increased abundance of small droplets in the quatemary aerosol. 5.33 The Effect of a DC Bias on a Mesh Positioned inside the Spray Chamber on CaIl Emission Contours in the Absence and Presence of &Ii-metai Chloride Salt Matrices In this study, the nebulizer gas flow rate (Fg) was set to 600 mVmin_ For the concentric nebulizer used, this is a high flow rate of nebulant gas that is in excess of the optimum. As will be discussed, this a h w s us to manipulate the vertical position of maximum Ca11 emission within the range covered by our optical system by changing the DC potential applied to the me& The effect of a positive DC bias potential applied to the mesh on the Ca11 emission contours is presented in Fig. 5.3. In Fig. 5 - 3 4there is no bias potential applied. The Ca11 68 signal intensity is relatively low in this case. As previously described, with the hi& nebulizer gas flow rate the central channel of the plasma is c o m p d v e l y cooled, causing a decrease in the measlaable rate of droplet desolvation. As a result, the peak CaII ernission is Lateral Position (mm) Fig. 5.3 The eEect of a DC potential applied to the mesh on the lateral emission contours for CaII (393.4 nm) in an ICP. The sample solution was 1 x IO4 M CaCI, no ma& present. The mesh was maintained at O V, + 100 V,+ 200 V, and + 500 V for a, b, c, d respectively and the sample solution at ground potential. The nebulant gas flow rate was 600 mVrnin. high ALC and in fact is above the viewing height of our optical setup ( > 25 mm ALC). However, it has been measured that the sample transport efficiency is improved at higher nebulizer flow rates. This fact is borne out in viewing the CaII emission contour shown in Fig. 5.3 .b where a + 100 V DC potential was applied to the me&. There are several noticeable effects of the DC biased mesh on the CaII signal intensity. Firc the signal intensity maximumhas shifted spatially lower to - 22 mm ALC. Second, the signal intensity has more than doubled in going fiom O V (Fig. 5.3.a) to + 100 V DC on the mesh (Fig. 5.3.b). Thirdly, the Ca11 emission low in the plasma has been shifted upward to the - extent that the nrst CaII emission low in the plasma is now at 8 mm ALC. At first glance, these observations may seem counter-intuitive because though the position of maximum CaII emission shifted lower toward the load coil, indicating the central channel is at a higher temperatuse in Fig. 5.3.b than in Fig.5.3 .a, there is simultaneously no Call emission very low at 7 mm ALC in the 'hotter' plasma However, these observations are not contsadictory because rather than interpreting the net plasma and plasma signals, we will rationalize these obsemations based on changes in the aerosol transpoa efficiency. At + 100 V, the biased mesh is thought to perîurb the aerosol tramportation efficiency the mosf and alteration of the nebuiization step by the electnc field between the mesh and the nebulizer is thought to be minimal. The droplets that were removed fkom the aerosol Stream by the electric field on the mesh c&ed the highest charge-to-mass ratio. The mallest droplets removed fkom the aerosol stream most ceaainly were of high charge-tomass ratio because the CaII emission low in the plasma (Le. 7 mm ALC) has been eliminated. Based on the large downward shift of the position of CaII nxix.imum signal intensity, the solvent load deïivered to the plasma has decreased. The greatest reduction in solvent load wodd be realized if the larger droplets in the quatemsily aerosol Stream carried a high charge-to-mas ratio, and these droplets were also removed nom the aerosol stream. 70 In any event, the removd of the smallest droplets, and the spatial lowerhg of the CaII emission maximum as a result of the imposed el&c field, are correlated. We have presented an argument in Chapter 4 that in addition to the size and net charge distribution of the primary aerosol, durhg the aerosol transport, the large droplets that carry net charge undergo Rayleigh fission, creating more smdler droplets in the quaternary aerosol relative to had ail droplets camed zero, or low, net charge. In the presence of an electric field in the spray chamber, the droplets with net charge are removed ,and only droplets with zero, or low charge-to-mass ratio are transmitted through the mesh to the plasma, thereby reducing the chernical matrix effect. Upon increasing the potential to + 200 V DC, the Ca11 signai intensity maximum is shifted lower again, though this time by only - 1 mm (Fig. 5.3.c). There is little change in the net CdI signai intensïty. There is however, a slight increase in the spatial position ALC where the nrst CaII emission appears. At this slightly higher potential, there is likely more efficient removal of droplets carrying net charge fiom the aerosol stream, so the argument presented above for the effect of the + 100 V DC bias is reinforced by the data at + 200 V. hcreasing the appiied potentiai to + 500 V DC results in similar shifts to the CaII emission intensity, and profile appearance, but we believe the explmation now is not as - simple. The position of maximum CaII emission is now 20 mm ALC, but now, this C d emission m;Uamum is at a lower net intensity. Also notice that the position of fkst CaII emission appears to have anomalously shifted lower toward the load coil. The sum of these three observations indicate that now modification of the aerosol transport und nebubation steps are taking place. Because the solution is grounded, the electric field fkorn the nebukm to the mesh is positive, meaning that the primary charge distribution is being shifted toward 71 the generation of more negative droplets. Concurrent with this is a discrimination against droplets with net charge remaining in the aerosol Stream because of the DC potential on the mesh. The overall r e d t wodd be reduced aerosol tramport to the plasma because there are less droplets with zero, or low net charge being produced in the primary aerosol. The CaII emission pronles when a NaCl (Fig. 5.4), KCI (Fig. 5.9, RbCl (Fig 5 3 , and a CsCl (Fig. 5.7) mat& are presented as a funciion of the DC potential applied to the rnesh. The CalI contours presented in these four figures are very similar, and the contours also behave very sunilarly to the applied potential. In all cases, the trends described for the CaII contours fkom Fig. 5.3 above are mirrored in this set of Figures, but the net changes as a fiuiction of the DC potentid are less dramatic. The most noticeable effect of the DC potential is in going fkom O to + 100 V in each of the figures where the spatial position of the maximum CaII drops fiom - 19 mm to - 18 mm ALC. In ternis of removing the off-axis enhancement ofthe Ca11 emission (i.e. one aspect of the chernical matrk effect), the applied potential of + 200 V does minimize this effect, but certainly does not remove it under the ICP operating conditions used. We suspect that at a lower, closer to optimum, nebulizer gas flow rate, there wodd be much more noticeable ef5ects of the DC potential on these CaII contours (i.e. r e c d the data presented in Chapter 4). There is also noticeable narrow contour profiles by increasing potential on the mesh. Lateral Position (mm) Fig. 5.4 The effect of a DC potentid applied to the mesh on the lateral emission contours for CaII (393.4 nm) in an ICP. The sample solution was 1 x 1O4 M CaCI, plus 1x 10-1M NaCl. The me& was maintainecl at O V,+ 100 V,+ 200 V, and + 500 V for a, b, c, d respectively and the sample solution at ground potential. The nebulant gas ffow rate was 600 mi/&- Lateral Position (mm) Fig. 5.5 The effect of a DC potentid applied to the mesh on the lateral emission contours for CaII (3 93-4 nm) in an ICP. The sample solution was 1 x I O4 M CaC12plus 1 x 1O-' M KCl. The mesh was rnaintained at O V,+ 100 V, + 200 V, and + 500 V for a, b, c, d respectively and the sample solution at ground potential. The nebulant gas flow rate was 600 &min. Lateral Position (mm) Fig. 5.6 The effect of a DC potential applied to the mesh on the lateral emission contours for CaII (393.4 nm) in an ICP. The sample solution was 1 x IO4 M CaCl, plus 1 x IO-'M RbCl. The mesh was maintained at O V, + 100 V,+ 200 V,and + 500 V for a, b, c, d respectively and the sample solution at ground potential. The nebulant gas flow rate was 600 mi/min. Lateral Position (mm) Fig. 5.7 The effect of a DC potentid applied to the mesh on the lateral emission contours for CaII (393.4 nm) in an ICP. The sample solution was 1 x 104 M CaCl, plus 1 x 10-1M CsCl chemical matrix. The mesh was maintallied at O V,+ 100 V, + 200 V,and + 500 V for a, b, c, d respectively and the sample solution at ground potentid. The nebulant gas fiow rate was 600 mVmin. 53.4 CaII Ernission Contours as a Fnnction of DC Voltages Appiied in the Presence of NaCl Matrix This next set of experiments explores the effect of relatively high voltages applied to the mesh positioned inside the spray chamber. In this experimen~a nebulizer gas flow rate of 500 d m i n . was u s d Fig. 5.8 qb,c show CaII emission contours as a funetion of DC voltages applied (O V, 100 V, 200 V respectively). In Fig. 5.8.4efthe analogous contours are displayed for Ca in the presence of a NaCl matrix. In comparing Fig. 5.8.a to Fig. 5.8.4 the chernical ma& effect is clear. There is a reduced CaII signal intensity in the presence of the NaCl m a t e as Xndicated by the number of contour lines and the characteristic off-axis enhancement low in the plasma and the on-axis suppression high in the plasma in Fig. 5.8.d relative to when no ma& is present (Fig. 5.8.a). The contour pronle in Fig. 5.8.d is also wider. The application of + 100 V DC to the mesh causes quite different behavior for the two test solutions. For the set of contours Fig. 5.8.a to Fig. 5.8.b where there is no NaCl ma&, the entire contour pronle drops to a lower height ALC so that the spatial position of the maximum CalI intensity drops nom - 17mm to - 16 mm ALC. The intensity of the Ca11 emission was not noticeably affected. In contra$ the Ca11 contour in the presence of NaCl a c t u d y increased in intemity in going Eom O V (Fig. 5.8.d) to + 100 V (Fig. 5.8.e) DC potential on the mesh. However, the intensity of the CaII emission in Fig. 5.8.e is SU less than in Fig. 5.8.b. Also the entire profile also shifted lower towards the load coi1 by - Imm in going f?om Fig. 5.8.d at O V to Fig. 5.8.e at + 100 V. The main ciifference in going to + 200 V is that the CaII emission intensity has decreased, and the CaII emission prome in the absence of mat& (Fig. 5.8 .c) is looking more similar to that jn the presence of NaCl matrix (Fig. 5.8.f). Lateral Position (mm) Fig. 5.8 The effect of a DC potential applied to the mesh on the lateral emission contours for CaII (393.4 nm) in an ICP.The sample solution was 1 x 1 0 M ~ &CI, for a, b, c. The sample solution was 1 x 10'" M CaCI, plus 1 x IO-' M NaCl for d, e, f. The nebulant gas flow rate was 500 ml/min. The mesh was maintained at O V for a andd,+100Vforbande,+200Vforcandf. For the solution where no matrix was present, the + 100 V potential on the mesh resulted in a slight reduction in the net CaII signal. This is likely due to a siight reduction in the aerosol transport efficiency, and is iikely indicative of a low net charge distribution on the droplets produced nom this solution of 1 x 104 M CaCI,. However, for the solution that contained 1 x 10" M NaCl + 1 x 1 0 M ~ CaC1, the effect of a DC bias on the rnesh was to increase the CaII signal intensi~.We speculate that the electric field efficiently removes droplets with high net charge-to-mass ratio, decreasing the aerosol number density. This causes a decrease in the degree of aerosol coagulation, allowing the droplets with low charge-to-mass ratio to be more efficiently transported to the plasma, and hence an increase in the CaII emission intensity. At + 200 V, the average droplet size in the aerosol trmqorted to the plasma is necessarily smder than at O V, because the spatial position of the CaII emission maximum shifted lower toward the load coil, and that these droplets are of zero, or low net charge. Notice that the off-axis enhancement of CaII emission low in the plasma is reduced in going fiom O V (Fig. 5.8.d) to + 100 V (Fig. 5.8.e), indicating that fewer of the small droplets, that we believe cary a high net charge, were delivered to the plasma These same two solutions used for acquiring the data in Fig. 5.8 were dso used when acquKing CaII emission contours at high DC potentials (400,800, and 1600 V) applied to the mesh. The contours at high potentials are shown in Fig. 5.9. Here an interesting trend is Uustrated. The CaII signal intensity for the solution with no matrix decreased over the range fiom O to 400 V. In going to higher potentials, the CaII emission signal contour lines are largely unchanged fkom 400 to 1,600 V (Fig. 5.9.a,b,c), but the entire profile is shifted Lateral Position (mm) Fig. 5.9 The effect of a DC potential applied to the mesh on the lateral emission contours for CaII (393.4 nm)in an ICP. The sample solution was 1 x 1O4 M CaCI, for a, b, c. The sample solution was I x IO4 M CaCI, plus 1 x 10-'M NaCI for d, e, f.The nebulant gas fiow rate was 500 mVmin, The me& was maintained at + 400 V for a and d, + 800 V for b and e, + 1600 V for c and f. @dy lower toward the load c d . A diBerent trend exïsts for the solution with NaCl matrk Over the range nom 400 to l,6OO, a decrease in the emission intensi~ of CaII can be seen from 800 to 1600 V (Fig. 5.9.e and f). These contour profiles also show a small shift toward the load coil, as evidenced by the position of first CaII emission. The interpretation of what is o c c ~ isgonly evident by c o m p a ~ the g effect of the DC biased mesh on solutions of no matrix versus ma&. From O to 200 V, the DC biased mesh influences aerosol tmnsport, but at higher potentids, both the aerosol transpoa and the nebuliïdon step itçelf are being affecteci. In the O to 200 V regime, the density of droplets with net charge in solutions of no matrix is small and their presence does not seriously affect transport efficiency. The presence of an elecbic field then removes droplets with the highest charge-to-mass ratio, decreasing the aerosol transport efficiency, and a lower CaII signal is obsenred (See Fig. 5.8.a-c). For solutions with matrix present, the density of droplets with net charge is much pater, and droplet coagulation in the aerosol spray chamber when the mesh is at O V is appreciable, leading to a strongiy decreased signal intensity. With the application of 200 V to the mesh, droplets with the highest charge-to-mass ratio are removed fiom the aerosol and aerosol transport increases (See Fig. 5.8.e-f). In the high potential regime, in addition to the processing of droplets with net charge, the electric field is now influencing the nebuIization step. The electrk field changes the surface characteristics of the liquid jet emerging fiom the sarnple injection tube of the nebulizer and the droplet charge distribution is shifted toward negative droplets. Because the sample with NaCI has a higher conductivity, this solution is affected to a Iarger degree and the shift toward more negative droplets is more pronounced (compare Fig. 5.9.b and c relative Fig. 5.9.e and Fig. 5 . 9 4 . In addition, the mesh is still acting to remove droplets with net charge, the net result is a reduced aerosol transport efficiency as observed by the reduced CaII emission signal intensiv in Fig. 5.9.f. 5.3.5 CaII Contours in the Presence of Nitric Acid and NaCl Nitric acid is often added to stock solufions to minimize adsorption of the analyte ions onto container wails and is often used for dissolution of solid samples. The response of the CaII emission signal to applied potentials on the mesh was rnonitored for two different - acid containing solutions. The fïrst solution contained 0.1% HNO,( 1.6 x loS M) and 1 x 1o4 M CaC1,. The second solution contained 0.1% KNO and 1 x 10-1M NaCl, in addition to 1 x 1 0 M ~ C~CI,. The CaII emission contour maps are shown in Fig. 5.10 and 5.1 1 for low and high potentials applied to the mesh. The behavior of these two soIutions (the acid solution and the acid plus NaCl solution) to the applied potentid are very similar to the behavior to the two solutions used in quiring the data for Fig. 5.8 and 5.9 (one solution had no NaCl matrix and the second had 1 x 10-' M NaCl). For example, in applying 100 V to the mesh, the acid solution without NaCl experienced a net decrease in signal Intensity (Fig. 5.1 O.a and 5.10.b), whereas the acid solution with NaCl experienced a net increase in signal ùitensity (Fig. 5.10.d and 5.10.e). The one noticeable merence occurs at high potentials. The solutions with acid present, because of the increased conductivity, shows a net loss in CaII signal intensity in going fiom 800 V to 1,600 V (Fig. 5.1 1.b and 5.1 1.c). Yet, the analogous solution in Fig. 5.9.b and c did not experience a signai intensity decrease. Notice that the Lateral Position (mm) Fig. 5.10 The effect of a DC potential appiied to the mesh on the lateral emission contours for CaII (3 93 -4 nrn) in an ICP. The sample solution was 1 x 1o4 M CaCl, plus 0.1% HNO,for a, b, c. The sample solution was 1 x IO4 M CaCI, plus 0.1% HNO, and 1x 10-' MNaCl for cl, e, f. The nebulant gas ff ow rate was 500 d m i n . The mesh was rnain+ainedat O V for a and d, + 100 V for b and e, + 200 V for c and f. L Q) 5 O 6.6 O 6.6 Lateral Position (mm) Fig. 5.1 1 The effect of a DC potentid appiied to the mesh on the laterd emission contours for CaII (393.4nm) in an ICP. The sample solution was 1 x 104 M CaCI, plus 0.1 % HNO, for a, b, c. The sample solution was 1 x 1O4 M CaCI, plus 0.1% HNO, and 1x IO-' M NaCl for d, e, f. The nebulant gas flow rate was 500 mi/min_ The mesh was maintriined at + 400 V for a and d, + 800 V for b and e, + 1600 V for c and f- entire contour pronle in Fig. 5.1 1.e is lower, closer to the load coil, than the contour profile in Fig. 5.9.e. This trend is more evident with the solution containing acid plus NaCl (Fig. 5.1I .d-0, where there is a noticeable decrease in signal intensity over the 400 to 1600 V range of applied potential. The CaII emission contour map in Fig. 5.1 1 .fis noticeably wider than the contour in Fig. 5.1 1.d and e. The increased laterai d i h i o n means that droplets have desolvated and the particles have vaporized at lower observation heights ALC, indicaihg that the aerosol droplets reaching the plasma are being shifted to a smaller size distribution, and or, the solvent loading of the plasma is being decreased so that the tend chamiel of the plasma is less cooled. At a potential of 3,000 V, the CaII emission contour map appears quite similar to that at O V (compare Fig. 5.12.a to Fig. 5.10.a). This signal would fluctuate occasionally, ranging from very low signal intensities with contour plots that are centered at Iow vie- heights ALC,to that shown in Fig. 5.12.a The omet of this behavior is dependent on solution conductivi~.because this behavior was not observed with the solution containing only 1 x 104 M CaCl at potentials up to 3,000 V (data not shown). Yet, for the solution containing 0.1 % HN03 and 0.1 M NaCl (plus 1 x 104 M CaClJ, the CaII emission was erratic at only 1,600V (Fig. 5.12.b-d). The conductivity of this solution was the highest of those tested in this study. For this solution, the CaII contour over a 50 msec. integration thne varied signiscantly in terms of net intensity and spatial position. The nature of the quaternary aerosol delivered to the plasma is likely quite different for each of the pronles in Fig. 12.b-d. This is likely because of discharging between the nebulizer and the biased mesh at these high potentials with the highly conductive solution. The question of analyticai Lateral Position (mm) Fig. 5.12 The effect of a DC potentid applied to the mesh on the lateral emission contours for CaII (393.4 m) in an ICP. The sample solution was 1 x 104 M CaCl? plus 0.1% HNO, for a The sample solution was 1 x 104 M CaC12plus 0.1% HNû, and 1x IO-'M NaCl for b, c, d The nebulant gas flow rate was 500 ml/&. The mesh was maintaineci at + 3000 V for a, + 1800 V for b, c, and d. u t i ï i t y at high potentials, in terms of raw signal intensity, aerosol tmnsport efficiency, and chernical matrix effect needs to be examined in more detail. 53.6 Revisitation of Local Thermodynamic Eqnüibrium &TE) of the ICP in the Presence of a Chernicd Matrix The ICP discharge has been recognized as a non local thermodynamic equilibrium (non-LTE) because electron temperatures (TJ are u p to 2000 K higher than gas temperatures (TJ [29]. The extent of non-LTE depends on the energy transfer in the plasma [30]. By increasing the rate of energy tramfer and the anaiyte residence time, the non-LTE plasma tends to be closer to L E . Recently, Mermet reported that under so-cded robust conditions (high power and low carrier gas flow rate), the matrix effects resulting fiom a change in the plasma conditions (various temperatures and electron number density in the ICP) are rninimized to the same extent [22]. According to o u observations, we strongly support this statement. If the plasma is operated in a robust condition, the energy transfer efficiency attains the highest level. In terms of chemical ma& effects, ail aerosol droplets will expenence complete desolvation and the resultant particles will be vaporized low in the plasma, regardless of the aerosol ske distribution in the presence or absence of matrices because the droplets have s 6 c i e n t time to undergo these processes prior to rising to the NAZ. As we illustrated and discussed above, the nebulant gas flow rate plays an important role in the extent of chemical matrix effects. The spatial position of complete desolvation of a droplet in the plasma dramatidy afTects the andyte spatial emission signal. Operating the plasma under robust conditions demonstrates that the matrix effects originate in the aerosol generation and transport steps. It also strongly supports o u hypothesis that aerosol aBêcts the quaternary droplet size distribrrtion because of Rayleigh fission processes. AU of which occurs because the nebuiization step generated dropiets with net charge. I Data has been presented showing that the effect of an electric field in the spmy chamber on Ca11 emission signals in the plasma is simdtaneously complex and informative. Low electnc field strengths appear to affect only aerosol transport efficiency by way of removhg droplets with hi& charge-to-mass ratio. High electric fields act to innuence the nebukation step itself, in addition to removing droplets with net charge from the aerosol. From our work, it is very clear that the chernical matrix effect originates in the Merences in the physical properties of the sample solution. There were large differences in solution conductivity, yet by cornparison, solution density, viscosity, and d a c e tension (excluding the SDS ma&) were relatively invariant We have presented an argument in a previous chapter for population of droplets with net charge, and higher net charge, as a function of dissolved soMs (Le. conductivity) in the sample. The influence of droplets with net charge on evaporation, gravitational setting, inertial impaction, and in pdcular, droplet coalescence inside the spray chamber needs to be evaluated in d e t . . The resultant quatemary aerosol droplet size distribution, that c m be modified by an electric field within the spray chamber, must be attributed to other solution physical properties such as ion mobility and conductivity during the droplet fornation step. This also warrants detailed investigation. References 1. J. W. Olesik and E. J. W'iafllsen, A@ Spectrosc. 43, 1223 (1989). N. N. Sesi and G. M. Hieftje, Spectrochim Acra SlB, 1601 (1996). M. W. Blades and G. Horlick, Spectrochim Acta 36B, 881 (198 1). L. M. Faires, C. T.Apel and T. M. Niemcqk, Appl. Spec~osc.37,558 (1983). P. W. J. M. Boumans and F. J. De Boer, Spectrochim. Acta 3 lB, 355 (1976). G. F. Larson, V. A. Fassel,R H.Scott and R N. Kniseley, Anal. Chem. 47,238 (1975). M. Thompson and M . H. Ramsey, AmZyst 10,1413 (1985). D. C.Gregoire, Spechochim. Acta. 42B, 895 (1987). S. H.Tan and G. Horlick, J. Anal. At. Spec~om.2,745 (1987). S. D. Tanner, L. M. Cousins and D. J. Douglas, Appl. Spectrosc. 48, 1367 (1994). R Rezaaiyaan, I. W. Olesik and G. M. Hieftje, Spectrochim Acta 40B,73 (1985). P. J. Galley, M. Glick and G. M. Hieftje, Spectrochim. Acta 48B, 769 (1993). K. P. Li, M. Dowling, T.Fogg, T. Yu, K. S.Yeah, J. D. Hwang and J. D. %kefordner,Am~. C h .60,1590 (1988). J. W. Olesik and J. C. Fister III, Spectrochim Acta 46B,851 (199 1). J. C. Fister III and J. W. Olesik, Spectrochim Acta 46B,869 ( 1 991). J. W. Olesik,Anal. Chem. 68,469A (1996). J. W. Olesik Appl. Spectrosc. 5 1,158A (1997). K. G. Kronhoh and R. K. Skogerboe, Appl. Spec~osc.40,116 1 (1986). R K. Skogerboe, S. J. Freeland and K. G. Kronholm, Appl.Spectrosc. 40,1166 (1986). Q.Xu,G. Maîîu and G. R Agnes, AppL Spechosc. Submitted (1998). E. Pousse1 and J. M.Mermet, Spectrochim Acta 48B,743 (1993). J. M. Mermec J Anal. At. Spectmrn. 13,419 (1 998). B. V. L'vov, D. k Katskov, L. P. KPUglikova and L. K. Polzik, Specfrochim. Acta 3 IB, 49 (1 976). A. C d s ,V. Hemandis and R F. Browner, Spectrochim Acta 45B,59 1 (1 990). S. E. Long and R F. Browner, Spectrochim Acta 43B,1461 (1988). M. Marichy, M.Mermet and J. M. Mermet, Spectrochim Acta 45B, 1195 (1990). M. Kodama and S . Miyagawa, Anal. Chem. 52,2358 ( 1 980). J. W.Olesik and A. W.Moore, Jr., Anal. Chem. 62,840 (1990). M. Huang, D. S. Hansehan, P,Yang and G. M . Hieftje, Spectrochim Acta 47l3, 765 (1992). W. H. Gunter, K. Visser and P . 2.Zeeman, Spsctmchim Acta 37B,571 (1982). Chapter 6:Use of Laser Light Scatter as Feedback Signal to Minimize Chemical Matrix Effect in ICP-OES 6.1 Introduction Chemical matrix eEects can cause large enors in the determination of an analyte by ICP-OES or ICP-MS. Much work has been done to reduce the influence of chemical matrices, but it is stili often diBcult to recognize a problem due to a ma* let alone correct for i t The operating conditions of the ICP can be adjusted to m h i m k the magnitude of the matrix effect [l-41,but at the detriment of reduced sensitivity. lntemal standards can be used to improve d y t i c a l precision [S-7. Matrix matching of calibration standards or standard addition methods are presently the best way to minimiif?chemical rnatrix effects, but detailed knowledge of the ma& must be known [8]. In addition this method is laborintensive. On-line rnatrix removai has been utihed to counteract the ma& effect [9], but this approach requires a separation step and is sample dependent. Clearly, better ICP-OES and ICP-MS performance, in te- of reliability, accuracy, and ease of use is needed. It is hoped that research of fundamental processes in the sample introduction system and in the plasma will allow these goals to be met [IO]. Our contribution to this subject area is exposing the presence of droplets with net charge in the aerosol, and illustrating how the analyte ernission signals in the plasma are thereby affécted. In the previous chapters we proposed that a charged droplet phenornenon diiring the aerosol generation step can be used to account for the chemicai matrix eEects in the presence of a chemical matrix. Data was provided in these chapters to support the hypothesis that dropleis with high net charge are present in the aerosol, and these droplets are undergoing desolvation leading to Rayleigh fission during tramport to the plasma The result of this would be more smaller droplets in the quatemary aerosol in the presence of a matrix relative to the absence of a matrix. Due to the higher d a c e area of the progeny droplet system, faster desolvation, vaporization, ionkation and excitation, leading to an enhancement of analyte signal low in the plasma According to the above speculative rnechanism, the droplet size distribution of a quatemary aerosol has been shifted to a smaller droplet size because of the progeny droplets. Skogerboe et. al. have reported that small droplets are in relative abundance in tertiary aerosols when solutions of high dissolved solids content are nebulized [Il, 121. The authors reported that this observation is contrary to the effect expected f?om evaporation Grom droplets of higher solute concentration, because the higher solute concentrations are expected to suppress evaporation the most due to reduced vapor pressure. Therefore, we argue that the Rayleigh fission events by highly charged droplets occur in the aerosol tramport step, produchg the smder droplets and this resuits in a shift toward a smaller droplet size distribution in the quatemary aerosol. Recail that for each droplet imdergoing a Rayleigh fission event, - 20 progeny droplets are generated. In an expeximent designed to eliminate the droplets with net charge fiom the aerosol, we have positioned a size 10 mesh screen inside spray chamber and applied a DC potential. The electric field rernoved droplets with net charge fiom the aerosol Stream. The effect of the DC biased mesh on CaII lateral emission profiles was however convoluted by changes in the aerosol transport efficiency. In this study, another method of probing the effect of a DC biased mesh on quaternary aerosols was sought to support our emission work Here, laser light scattering by the quatemary aerosol was used to monitor the droplet number density as a function of matrïx concentration and DC potentiai applied to the mesh. Resuits are presented that show the quaternazy aerosol laser light scatter signal increases with increasing concentrations of a chernicd matrix and how the ninnber density of this aerosol can be manipulated through application of a DC potentid on the mesh. It is also shown that the quatemary aerosol laser light scatter signal c m be used as a feedback signal to control the potential applied to the rnesh, thereby strongly minimizing the chernical ma& effect 6.2 Experimental The monochromatic imaging spectrometer has been described in Chapter 3. The laser light scatter experiments were acquired with the same instrument, apart fiom the following changes. The CCD camera and refocusing lem were replaced with a PMT and photon counting electronics (Hamamatsu,C3866). An injector tube was fabncated to be 25 mm longer than the injector tube in a standard MAK torch to permit laser scatter measurements at a fïxed height of IS mm ALC (plasma off). The laser light scatter signal was integrated for one second. Due to the poor collection, and transmission of the scattered radiation through the spectrometer, relative quateniary aerosol number densities were measured. Absolute droplet number densities were not measured in this experiment. The Iaser beam (Uniphase HeNe mode1 1676,4mW, 532 nm)was passed through a cylindrical lens prior to intersecting the quatemary aerosol stream so as to measure droplets firom across the entire width of the 2 mm channe1 of the injector tube. The laser scatter signal nom the p- aerosol was measilred 5 mm fiom the tip of the concentric nebulizer. The quatemary aerosol scatter signal was measured 5 mm fiom the tip of the sample injector tube. The laser light scatter value reported is the average of twelve integration events, each one second long. A Meinhard TR-30-Cl concentric nebulizer was employed in this study. Vaned nebulizer gas flow rates were used The average fiee solution uptake rate for the range of nebulizer flow rates used was 1.2 mYmin_ Though the sample uptake rate was not controlled, very s d a r laser scatter signal trends were obtained at aU nebulizer flow rates. A size 10 mesh, shaped to resemble a thi;nble, was positioned inside the end of the inner tube of the spray chamber. The DC potential applied to this screen ranged nom O to 1,000 v. AU s o l ~ o nwere s prepared by dissolving ACS grade sahs in distilled deionized water. The sdt composition of each solution used is indicated in each figure. The background signal intensity fiom the contour plots showing CaTI ernission has been subtracted using matrk matched blanks. 6.3 Results 63.1 Primary Aerosol Laser Light Scatter Signais The laser light scatter signals fiom the prinary aerosol was invariant to the NaCI matrix concentration (Eg. 6.1). The least squares linear regression fit for each set of nebulizer flow rates was essentidy zero. Higher nebulant gas fIow rates result in higher laser scatter s i g d s - --1 900 mlpm Co w aNa a \ n 8 z 500 mlpm 300 mlpm 0.0 4 I I 1 I I 5 4 3 2 1 i -log [NaCI] Fig. 6. 11laser Iight scattering signal as a fiinction ofNaCl chemical ma& concentration in molarity. 6.3.2 Quaternary Aerosol Laser Scatter Signais The primary aerosol was passed through a double pass spray chamber before being directed up to the injector tube. The size 10 mesh was positioned inside the spray chamber and connected to ground potentid for these measurements. The laser scatter signal fkom the quaternary aerosol is very dependent on the NaCl ~llittrixconcentration (Fig. 6.2). AU solid Lines are exponentid fits to the experimental data for regression coefncients. The higher laser light scatter si@ fiom the primary aerosol at higher flow rates is càmed over to the quatemary aerosol, indicative of a slightiy higher aerosol tramport efficiency at higher nebulizer gas flow rates. The quaternary aerosol scatter signal is roughly 100 t h e s less than the primary aerosol scatter signal. f 900 mlpm C 500 mlpm / L Fig. 6.2 The quate~liiryaerosol laser light scattering signal as a fiinction of NaCl chemical matrix concentration in mtiIarity. The exponential fit to this data supports o u hypothesis that droplets of high net charge produced in the primary aerosol, undergo desolvation and RayIeigh fission events diiring transport to the plasna (laser beam). This accounts for the exponentid fit because for every prbmuy droplet undergohg one fission event, a total of - 21 droplets are produced. The increasing value of the exponential fit t e m for higher nebulant gas £iow rates is due either to more droplets with net charge in the primary aerosol and or higher transport efficiency of droplets with net charge. This agrees with previous observations fiom Olesik and CO-workers[13]. The exponential behavior of the quatemary aerosol has not been observed by other researchers performing similar shidies [14]. A hear fit was always obtallned in these otber studies. The différence, we believe, is that a laser line was used rather than a circular beam. either focused or not. The laser line was able to intersect a l l droplets emerging from the 2 mm wide injector tube with d o m light intensity. If in the quatemary aerosol emerging fkom the injector tube, the smaller droplets in this Stream are pushed to the penphery of the flow profile by flow dynamic and eleceostatic considerations, then a single focused laser beam will not have detected the smaller droplets. 6.33 Use of an Electric Field to Control Laser Light Scatter Signais The laser light scatter signals for two solutions representatwe of no matrix, and matrix as a function of DC bias applied to the mesh is presented in Fig. 6.3. As expected fiom the data in Fig. 6.2, the laser scatter signal for the solution with matrix is much higher than the case for no matrix (le. only CaC1, at 1 x lu4 M). With the application of a DC potential to the mesh, the relative scatter signals fkom both solutions drop rapidly. The scatter signal fiom the solution that has 1 x IO-'M NaCl appears to plateau at potentids above 200 V, whereas the solution without matrix does not lead to potential invariant laser light scatter signal u n d - 500 V. The plateau of the light scatter signal at a higher level for - the solution with the XaC1 matrix (200 1,000 V) relative to the solution without matrix (500 - 1,000V) could be due to ciifferences in the primary aerosol. O 200 400 600 800 1000 DC Potential applied tu Mesh Electrode (V) Laser light scatter signal for two different solutions as a bction of applied potential to the me& electrode positioned inside the spray chamber. 6.4 Discussion With respect to understanding the origin of the chernical ma& effect, it is clear fiom the laser light scatter measurements reported here that the quaternary aerosol number densities are correlated with the concentration of the ma&. Because there is not a simdar correlation with the prlmary aerosol, there must be extensive processing of the aerosol as it is passed through the spray chamber. However, it will be argued in the following discussion the ongin of the chernical matrix effect, as observed in the spatid emission profile of an analyte in the ICP, is in the nebubation step, and that the processing in the spray chamber is consequential to the nebulization step. The exponential behavior of the laser scatter signal by the quaternary aerosol as a function of ma& concentration has not been previously reported. What processes could be acting to increase the quaternary aerosol -ber density? Re-nebukation of the liquid off the walls of the spray chamber is a process of low probabiüty in a double pass spray chamber. Borowiec et. al. proposed an aerosol ionic redistribution mechanisrn [15] in an attempt to explain the matrix effect However, this hypothesis has b e n examined by others with little success 116,171. In short, it is difncult to postdate any process, other than Rayleigh fission of droplets with net charge in the spray chamber, that could lead to dramatic increases in the quatemary aerosol nimiber densiw. The occurrence of Rayleigh fission events in the spray chamber would certainly lead to an exponentid increase in the quaternary aerosol number density as monitored by laser scatkring. Note that these laser scatter experiments were conducted with the plasma off. Therefore, the fission events are occurring as a result of the dry nebulant gas being ullsaturated with respect to solvent vapor, and pre-heating of the aerosol in the injection hibe by radiative heating fkom the plasma is not important for this phenornenon. Necessary for our hypothesis to be proven correct is that the aerosol denved fiom a solution of high matrix concentrationmust produce droplets of high net charge, as compared to droplets of lower net charge produced nom a solution of no ma&. A statistical argument was presented in Chapter 2 to show that the ne: charge on an individual droplet will increase as a b c t i o n of the total dissolved solids concentration. Likewise, an argument could be presented to argue that as the ionic strength inmeases, asymmetric shearing ofthe electrical double layer, would lead to an increased average net charge deposited onto individual droplets. Also critical to our hypothesis is that there is sufncient t h e for desolvation to occw to the extent that the Rayleigh fission limit is reached while the droplet is inside the spray chamber. The time required for desolvation was also cdcdated in Chapter 2, and this time is shorter than a droplet residence time in the spray chamber. Hence, we return to the primary aerosol in search of an explmation for the origin of the chernical matrix effect. In comparing a 1 x lo4 M solution of CaCl to a solution that is 1 x IO-'M in NaCl, the density, viscosity, and droplet size distribution are nearly identical (Table 6.1). The primary aerosol size Sauter mean diameter (dJ, as estimated with eqn. 2.1, shows the trend of no difference in Sauter mean for these two solutions (Table 6.2), regardless of nebulizer gas flow rate. It is important to note that though the Sauter meaq as caldated by eqn. 2.1 does not change for these two solutions, it is only a crude estimate of the reaI behavior, because the actual distribution of prïmary droplets codd be changing quite dramatically without changing the Sauter mean. Table 6.1 Physical Properties of Two Aqueous Solutions at 200C Solution 1 lo4 M cac12 1 x 10" M NaC1 Density Viscosi~ Surface Tension (91cm3) (dynes.s/cm5 (dynedcm) 1 ,0000 1 .O02 72-60 1 .O025 1,014 72-92 Ref. Table 6.2 Sauter Mean Diameter (4, p)at DBerent Gas Flow Rates Flow rate O/min.) 0.5 0.7 0.9 Again, we retum to the net charge distribution that wiiI be superimposed on the primary droplet size distribution. This known phenornenon can easily be used to explain how the quaternary aerosol numbet density increases in the presence of a matrix. We have explored this hypothesis with favorable results in Chapters 4 and 5 of this thesis. In those chapters, a mesh positioned in the spray chamber was used to m a t e a DC electric field between the mesh and the nebulizer (solution was grounded). The emission intensity of CaII was monitored as a function of DC potential applied to the mesh, and we interpreted the results tu mean that droplets with net charge were responsible for causing the chemical matrix effect. In this work, the exponential inmease in the quate- aerosol number density was speculated to be correlated with increases in the concentration of the matruc. Hence, appiication of a DC potential to the mesh should remove droplets with net charge from the quatemaxy aerosol, thereby mhimkhg the chernical matrk effect in the plasma The results of this experiment were provided in Fig. 6.3. In this set of data, it is clear that the droplet number density deiivered to the plasma can be manipulated by the potential applied to the mesh. The results of an experiment to minimize the chemical matrix effect with respect to emission are s h o w in Fig. 6.4. With the mesh held at ground potential, there is a large clifference in the CaII spatial emission intensity in the plasma (Fig. 6.4). Yeq with 200 V applied to the mesh while the solution containing 1 x 104 M Ca was nebubxi, and 600 V on the rnesh when the solution containhg the same concenfration of Ca plus 1 x IO*'M NaCl was nebuIized, a signiscaflt decrease in the chemical ma& effect was obtained. The data from Fig. 6.3 fiom laser light scatter experiments determined the potentid to be applied to the mesh. For this experiment, the laser scatter and the emission experiments were pe&ormed sequentiaiiy, one day apart. Therefore, it is entirely feasible that laser light scattering by the quate- aerosol in the injector tube of the torch could be used as a r d time feedback monitor to control the potentid applied to the mesh. It does remain t o be verined ifthis approach is sufficiently robust to be applied to any chemical matrix. CaCl, CaCl, + NaCl Difference Fig. 6.4 The effect of a DC potential applied to the mesh on the lateral emission contours for Ca11 (393.4 nm)in an ICP. O V was applied for two test sample solutions of 1 x 1o4 M CaCl, and 1 x IO4 M CaCI, + 1 x 10.' M NaCl. In (a) the potential applied is 200 V for the sample solution of CaCI, at 1 x 10' M. In (b) the potential applied is 600 V for the solution 1 x 1 O4 M CaCI, + 1 x 10.' M NaCl. Notice that the different potential applied in each case, as per the laser light scatter signal (refer to Fig. 6.3), effectively minimized the presence of the NaCl matrix. 6.5 Conclusion The relative laser scatter signal fiom the quaternary aerosol increases exponentially with matrk concentration. In con- the signai fkom the prhary aerosol was independent of matrix concenaation. This strongly indicates that processes occur in the spray chamber so as to increase the quaternary aerosoi droplet number density. Rayleigh fission of a droplet with net charge that has undergone desolvation in the spray chamber has been used to explain this data In addition, the laser light scatter signal by the quaternary aerosol has been demonstrated tu be a capable indicator for control over the potential applied to the mesh positioned inside the spray chamber to signiscantly minimize the NaCl chernical rnatrix effect. References J. M. Mermet, J. A d At. Spectrom. 13,419(1998). S. H.Tan and G. Horlick, J. A d A t Spectrom. 2,745 (1987). D. C.Greoire, Spectrochim.Acta 42B,895 (1987). Q.Xu,G. Mathi and G. R. Agnes, Appl- Spectres. subrnitted. J. W . Olesik, AmII Chem. 63,12A (1991). M.H.Ramsey and M. Thompson, AnaZyst 110,s 19 (1 985). G.Xiao and D. Beaushemin, 1 Anal. At. Spectmm. 9,509(1 994). J. H.Kalivas and B. R Kowalski, Anal. C h .53,2207 (1 98 1). M. J. Bloxham, S. J. Hill and P. J. Wordod, J A d At. S p e c m . 9,935 (1994). J. W.Olesik, Anal. Chem. 68,469A (1996). K.G.Kronholm and R K. Skogerboe, Appl Spectrosc.40, 1161 (1986). R K.Skogerboe and G. B. Butcher, Spectrochim Acta 40B,1631 (1985). J. W.Olesik, A n d C h .68,469A (1996). R H.Clifford, H.Tan, H.Liu, A. Montaser, F. Zamn and P. B. Keady, Specîrochim Acta 48B,122.1(1993). J. A. Borowiec, A. W. Boom, J. H. Dillard, M. S. Creeser, R F. Browner and M J. Matteson, Anal. Chem. 52,1054(1980). R K Skogerboe and S. J. Freeland, A d . Spectrosc. 39,925 (1 985). C.Dubuisson, E. Poussel, J. L. Todoli and J. M. Mermet, Spectrochim. Acta 53B, 593 (1998). C . Robles, J. Mora and A. Canals, Appl. Spect~o.46,669 (1992). CRC Handbook of Chemistry and Physics, 63d,CRC Press, Inc., Boca Raton, Florida Chapter 7:Conclusion In this thesis, the presence of droplets with net charge in the aerosol has k e n indirectly verified, and our interpretation of the role of these droplets on the chemical matrix effect has been presented. The motivation to study the chexnical ma& effect was based on charged droplet evaporation behavior learned fiom pnor experimentation with the Electrospray Ionïzation Source for mass spectromem. This initial research e x e m p m g the role of charged droplets in causing the rnatrix effect is expected to lead to renewed efforts towards improving sample introduction efnciency in atomic spectroscopy. In this work, the well h o w n phenornenon of spray electrifïcation was used to illustrate the ongin of droplets with net charge fiom pneumatic nebubation. A net charge on individual droplets, either positive or negative, is expected during the aerosol generation step whenever there are dissolved solids present. Two models that can accuunt for the droplet net charge exist in the fiteranire. Surface layer rupturing or statistical fluctuations of the ion population during droplet formation can result in a net charge imbdance on individual droplets. The statisticd fluctuation mode1 was used to estimate the average net charge on individual droplets. The calculated results showed that Rayleigh fission events can occur dining the aerosol transport stage. One or more Rayleigh fission events are possible because the Rayleigh instability Iimit (RJ is much larger than the solid residue radius . Moreover, cdculation of the rate of solvent desolvation during the transport step indicates that the Rayleigh fission events can occur because the residence time of the droplets in the spray chamber is longer than the t h e required for desolvation to the Rayleigh instability limit. For example, a 10 pdroplet generated nom a solution of 1 x 10-lM NaCl wilI undergo solvent desolvation to reach RayIeigh instability limit in a time of - 68 ms, yet the estimated aerosol residence time is - 200 ms in the spray chamber. The proposed manifestation of the chexnical matrix effect has been described in this research and is sUmmanzed below. The effect of a high concentration of matrix is that droplets will be produced that have a higher net charge. This increases the likelihood of Rayleigh fission events occurrhg at some point during the aerosol transport step. The srnaIl droplets produced by each fission event will evaporate much more quickly than the larger droplets in the plasma because of the increased d a c e area to volume ratio of the systern (Le. the aerosol). Also, the time required for particle vaporization will be decreased. According to this mechanism, the aerosol entering the plasma in the presence of a matrix will have a higher percentage of srnail droplets, and a slightly reduced size of the larger droplets, relative to had there been no matrix present. This will result in enhanced arialyte emission at lower observation heights, off-axis in the ICP due to the presence of the smaller droplets. At higher observation height, the signal intensi~is reduced, on-axis by an amornt equivalent to the m a s ejected by Rayleigh fission events fiom the larger droplets (with net charge) in the aerosol. Data indicating the presence of s d e r droplets that are responsible for the chernicd matrix effect has been presented. Application of a DC voltage to a mesh positioned in the spray chamber resulted in observable changes in the spatial emission characteristics of Ca11 emission in the plasma nie biased mesh acts to remove droplets that have a hi& charge-to-volume ratio fiom the secondary aerosol stream. Most importantly, it was found that with suitable application of DC voltage to the mesh, the aerosol characteristics c m be modified to minimize the chemical ma& effect. It was also shown that laser light scatter by the quatemary aerosoi was dependent on chemical rnatrix concentration, yet the light scatter by the primary aerosol was independent of rnatrix concentration, This is in accordance with the prediction by the Nukryama- Tanasam equation, that shows the chernical ma& does not &ect the aerosol size distribution in our sample solutions. However, the quate- aerosol is affecteci in the presence of chemical matrix, meaning that though the primary aerosol size distribution is not affecte& the quaternary aerosol is. We have presented the argument that quaternary aerosol modifications by a ma& are because of droplets with net charge. Hence, Rayleigh fission events during the aerosol transport processes in the spray chamber account for more srnalier droplets entering the plasma in the presence of a matrix. In support of this theory, the application of a DC potential to the mesh demonstrated an efficient way to m o w the aerosol by way of removing droplets with high charge-to- m a s ratio, and thereby m h h i z e the chemical ma& effect. Lastly, the relative intensity of the laser light scattering signal was used in a feedback loop to control the potential applied to the mesh, minimi7jng the chernicd maPix effect This feedback loop control over the chemical matrk effect couid see widespread use in the field of atomic spectroscopy. Future Directions In conclusion, some general remarks cm be made regarding continuation of this research: We have only started to characterize CaII emission in the presence of EIE matrices. Extending this work to other matrices such as surfactants is necessary to improve our understanding of the manifestation of the chemical ma& effect Furthemore, because an element is partitioned between three primary fonns in the plasma (molecular, atomic, and ionic), this work must be extended to other elements as anaiytes and mairices, to ensure our expianation of the chemical ma& effect is robust. CaX and MgX matrices are problematic real samples and these matrices will be potential candidates to be investigated in o m laboratory. To ver@ the proposed chemical matrix effect mechanism, measurement of the primary and quaternary aerosol size distributions will be indispensable. A varie@ of chemical mahces such as NaCI PIE), ZnC1, (non-EIE),acid (FINO,, HCI, &PO4) and surfactants shodd be investigated Argon atom and ion emission intensities as a function of applied DC potential will be monitored in future experiments to study the change of the plasma conditions due to solvent Ioading. Understand the detailed origin of dropiets with net charge by using pneumatic nebulization. MeasUrexnent of the droplet net charge and size distribution within an aerosol are needed to establish whether one, or both of the proposed charging mechanisms are acting. Interfach the nebulizer and spray chamber combination with mass spectrometry could be helpful in this respect. The equation (qi2)IR= (2CVNJIR needs to be v d e d For example, should C be replaced with activity. Also the solution conductivity, ion mobility, d a c e tension, and the valence of ions needs to be considered. Use of a pneumatically assiçted electrospray nebulizer where control over the potential of the siirface that the solution passes over could be used to directly control droplet net charge. This eEect on the aerosol transport processes and droplet fate could be an intereshg and idonnative study. Ultrasonic nebulizers are used for small volume sample introduction in atomic spectroscopy. The matrix effect is also severe with this sample inQoduction method. If our mode1 of the chemical matrix effect cm be used to iilustrate the origin of the matrix effect with this nebulizer, it could be mechanidcaily informative as to the origin of droplet net charge. Accurding to the above research results, the spray chamber plays a key role in mod@ng the aerosol s ù e distribution. Ultimately, the aerosol passing through the spray chamber is responsible for the chemical matrix effect We need to understand spatially where Rayleigh fission is occurring, and determine optimum methods of minhkhg the transport of droplets with net charge to the plasma. In addition, we need to develop an understanding for aerosol transport efficiency as a h c t i o n of droplet size and simultaneously droplet net charge in the spray chamber. Develop new sample introduction systems to improve aerosol transport efficiency and mhimize the chemicai matrk effect. IMAGE EVALUATION TEST TARGET (QA-3) lnc - IMAGE.Street = -. --- 6i28û-5989 6/42-0300 ---- APPLIED 1653 East Main Rochester, NY 14609 USA Phone: 71 F m 71 o 1993. Appiii Image. Inc. All Rights Resewed