Download The Influence of Droplets with Net Charge in Atomic Spectroscopy

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
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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
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W. H. Gunter, K. Visser and P. B. Zeeman, Spectrochim. Acta 408,617 (1 985).
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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).
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CoupZedP l m Emission SpechoscoW>', Part 2,
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53B,593 (1998).
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13,55 (1997).
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E. E.Dodd, J: Appl. Phys. 24,73 (1953).
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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).
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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).
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J. A. K o r o p c u M. Veber and J. Hemes, Spectrochim. Acta 47B,825 (1992).
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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. Spectrom. 8,359
(1993).
G. R Agnes and G. Horlick, Appl. Spectrosc. 46,401 (1992).
G. R Agnes and G. Horlick,Appl. spechosc. 49,324 (1995).
D. C. Taflin, T.L. Ward, and E. J. Davis, Langmuir 5,376 (1989).
L. B. Loeb, "Sratic Electrifcation (Springer-Verlag, Berlin, 1%8), pp. 58- 124.
"
B. L. Sharp, J. Anal. At. S p e c m . 3,939 (1988).
E. E. Dodd, 1 Appl. Phys. 24,73 (1953).
A. Hirabayashi, M.Sakairi, and H . Koinimi, Anal. Chem. 66,4557 ( 1 994).
D.H. Tracy, S. A. Myers, and B. G. Baliste, Spectrochim. Acta 37B, 739 (1982).
P. A. Michslik and R Stephens, Talmîa 28,37 (1981).
P. Kebarle and L. TangyAnal. Chem. 65,972A (1 993).
J. W. Olesik and G. M . Hieftje, Anal. Chem. 57,2049 (1985).
B. L. Sharp, 1 Anal. A t Spectrom. 3 , 6 13 (1988).
R F. Browner and A. W. Boom, Anal. Chem. 56,786A (1984).
J. W. Olesik,L. J. Srnia and E. J. Williamsen, A d . Chem. 61,2002 (1989).
S. E. Hobbs and J. W. Olesik,Spectrochim. Acta 48B, 8 17 (1 993).
J. W. 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).
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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.
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