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Modern Methods in Heterogeneous Catalysis Research 29.10.2004 Chemical (Elemental) Analysis Raimund Horn Dep. of Inorganic Chemistry, Group Functional Characterization, Fritz-Haber-Institute of the Max-Planck-Society Outline 1. Introduction 1.1 Concentration Ranges 1.2 Accuracy and Precision 1.3 Decision Limit, Detection Limit, Determination Limit 2. Methods for Quantitative Elemental Analysis 2.1 Chemical Methods 2.1.1 Volumetric Methods 2.1.2 Gravimetric Methods 2.2 Electroanalytical Methods 2.2.1 Potentiometry 2.2.2 Polarography Outline 2.6 Spectroscopic Methods 2.6.1 Atomic Emission Spectroscopy 2.6.2 Atomic Absorption Spectroscopy 2.6.3 Inductively Coupled Plasma Mass Spectrometry 2.6.4 X-Ray Fluorescence Spectroscopy 2.6.5 Electron Probe X-Ray Microanalysis 3 Specification of an Analytical Result 4 Summary 5 Literature Backup Slide The Analytical Process strategy solution problem characterization smaller sample measurement sample for measurement evaluation measurement data analysis preparation chemical information data diminution representative sampling object of investigation sample analytical result Backup Slide The Analytical Process strategy interpretation problem characterization smaller sample measurement sample for measurement evaluation measurement data analysis preparation chemical information data diminution representative sampling object of investigation sample analytical result Backup Slide Analytical Problems in Science, Industry, Law… Science Industry Law example: Determination by Xray Absorption Spectroscopy of the Fe-Fe Separation in the Oxidized Form of the Hydroxylase of Methane Monooxygenase Alone and in the Presence of MMOD example: manufacturer of iron Shall I by this iron ore or not ? example: Is the accused guilty or not guilty ? Rudd D. J., Sazinsky M. H., Merkx M., et al. Inorg. Chem. 2004, 43 (15), 4579-4589, 2004 Motivation fundamental interest future developments Motivation money precision (e.g.) 53 ± 5wt% vs. 53 ± 1wt% accuracy (e.g.) 53 ± 1wt% vs. 51 ± 1wt% Motivation human fate murder conviction genetic fingerprint fits – lifelong prison genetic fingerprint does not fit acquittal 1. Introduction 1.1 Concentration Ranges concentration ranges in elemental analysis side main components components trace components 1 ppt 1 ppb 1 ppm 1‰ 1% ppt (w/w) ppp (w/w) ppm (w/w) ‰ (w/w) % (w/w) 10-12 g/g 10-9 g/g 10-6 g/g 10-3 g/g 10-2 g/g pg/g ng/g µg/g mg/g 0.01 g/g ng/kg µg/kg mg/kg g/kg 10 mg/kg 100 % 1. Introduction 1.1 Concentration Ranges mass ratio 10-2 10-3 1% 10-6 1‰ 10-9 10-12 1 ppb 1 ppt 0.27 L 2.7 L 2 700 000 L 2 700 L 1 ppm 2 700 000 000 L concentration ranges in trace analysis 2.7 g 1. Introduction 1.2 Accuracy and Precision accuracy: precision (uncertainty): Ø can be defined as the consistency between the mean of an analytic result and the value held as the true value Ø describes in a positive (negative) manner the influence of random errors on an analytic result Ø describing quantities: Ø has to be verified before each analysis by means of standards s= Ø describing quantities: ∑ (x main and side component analysis wrong results Gaussian distributed errors! i errorx = ± x − x̂' ± x − xˆ ' relative errorx = ⋅ 100 xˆ ' − x) 2 i v = s2 = n−1 ∑ (x − x) 2 i i n−1 trace and ultra trace analysis 1. Introduction 1.3 Decision Limit, Detection Limit, Determination Limit Ø the lower the concentration of an analyte, the more difficult becomes its qualitative detection and its quantification Ø stochastic errors become more and more pronounced Ø the decision limit and the detection limit are lower limits characterizing the performance of a method to discriminate a true value from a blank value Ø the determination guaranties the quality quantitative result specific) yk = y B + t f ,α ⋅ s B n limit of a (user ⇒ y k = y B + bx NG ⇒ x NG = for α = β x EG = 2 ⋅ x NG t f ,α ⋅ s B b⋅ n DIN 32645 2. Methods for Quantitative Elemental Analysis 2.1 Chemical Methods 2.2 Electroanalytical Methods 2.3 Spectroscopic Methods Ø atomic emission Ø atomic absorption Ø volumetric methods Ø potentiometry Ø gravimetric methods Ø polarography Ø inductively coupled plasma mass spectrometry Ø X-ray fluorescence Ø electron probe X-ray microanalysis 2.1 Chemical Methods 2.1.1 Volumetric Methods - Principle principle of volumetric methods evaluation of the result p = VB ⋅ k ⋅ N ⋅ 100 in % me p = percentage of the analyte in the sample VB = volume in ml dropped in from the burette A+B→C+D k = stoichiometric factor (mg analyte/ml) N = correction factor for the theoretical value k e = weighted sample (in mg) 2 Ø fast, complete and stoichiometric well defined reaction Ø the extend of the reaction can be monitored (e.g. pH, electric potential) Ø the point of equivalence can be detected precisely (e.g. sudden color change, steep pH or potential change) 2 2 σ m e σ VB σ N σP + + = p m V N e B inserting typical values 0.1% < σP < 1% p advantage: high precision!!! 2.1 Chemical Methods 2.1.1 Volumetric Methods – Acid Base Titration acid – base reaction: titration curve: τ= HnA + nBOH V An- + nB+ + nH2O (zA=n, zB=1) z B ⋅ C B ⋅ VB K V +V Kw = − pH a + S ⋅ − pH − 10− pH K w = [ H + ][OH − ] z A ⋅ C A ⋅ V A 10 + K a C S ⋅ VS 10 strong acid – strong base weak acid – strong base different Ka – strong base - H+ + H+ Ø choice of the indicator depends on Ka pH = pK a , Ind ± 1 Ø very weak acids (bases) can be titrated in non-aqueous solvents as liquid ammonia or 100% acetic acid 2.3 Chemical Methods 2.1.1 Volumetric Methods - Complexometry complex formation: nL + Mm+ V MLnm+ most used ligand - EDTA H4-nYn- + Mm+ V [MY](m-4) + (4-n)H+ (zA=1, zB=1) [M m+ [ MY ( m − 4 ) ] ]= K f ⋅ [ EDTA]total ⋅ α Y 4− complex formation only with Y4⇒ buffer required indication: HI2- + Mm+ V MI(m-3) + H+ e.g. Eriochromschwarz T applicable for the determination of: Mg2+, Ca2+, Sr2+, Ba2+ at pH 8-11 Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Al3+, Pb2+, VO2+ at pH 4-7 Bi3+, Co3+, Cr3+, Fe3+, Ga3+, In3+, Sc3+, Ti3+, V3+, Th4+ at pH 1-4 2.1 Chemical Methods 2.1.1 Volumetric Methods – Redox Titration redox reaction: nBOxB + nARedA V nBRedB + nAOxA (zA=nB, zB=nA) MnO4- + 5Fe2+ + 8H+ V Mn2+ + 5Fe3+ + 4H2O examples: Ce4+ + Fe2+ V Ce3+ + Fe3+ τ= z A ⋅ C A ⋅VA z B ⋅ C B ⋅ VB 0 < τ < 1 U = U A = U AΘ + RT τ ⋅ ln nB F 1 −τ n AU BΘ + nBU AΘ τ = 1 U eq = n A + nB τ > 1 U = U B = U BΘ + indication: RT ⋅ ln(τ − 1) nA F self indication (violet MnO4- → colorless Mn2+) redox indicator (InRed F InOx + ne-) potentiometric endpoint detection titration curve 2.1 Chemical Methods 2.1.2 Gravimetric Methods precipitation reaction: ν + M z + ( aq ) + ν − A z − (aq ) ↔ Mν + Aν − ( s ) Ø precipitation of the ion to be determined by a suitable precipitating agent Ø filtering and washing of the precipitate Ø transformation into a compound of well defined stoichiometry (drying/calcination) → weighing examples Ba2+ + SO42- F BaSO4 (KL=[Ba2+]⋅[SO42-]=1⋅10-10mol2/l2) as BaSO4 Fe3+ + 3OH- F Fe(OH)3 (KL=[Fe3+]⋅[OH-]3=1⋅10-38mol4/l4) as Fe2O3 [ M z + ] = ν + +ν − Ø for analytical purposes: ν K L + ν − ν− [ A z − ] = ν + +ν − ν K L − ν + ν+ [ M z+ ]⋅V α p = 1− ≥ 0.997 z+ [ M ]o ⋅ V0 Ø advantage ⇒ one of the most precise analytical methods 2 σm σm σm = e + a m me ma 2 0.01% < σm < 0.1% m 2.2 Electroanalytical Methods electrode reaction, IFaraday=0 (potentiometry) Ø ion-selective electrodes Ø potentiometric titration electrode reaction, DC, IFaraday≠0 (voltametry) Ø polarography electroanalytical methods no electrode reaction, AC, high frequency Ø conductometry Ø coulometry electrode reaction, AC, IFaraday≠0 Ø AC polarography 2.2 Electroanalytical Methods 2.2.1 potentiometry, potentiometric titration Ø principle ⇒ measurement of the cell potential (potential between two electrodes at zero current) Ø relation of the cell potential and the analyte concentration typical reference electrode calomel electrode Hg2Cl2(s) + 2e- V 2Hg(l) + 2Cl- E = E 0' − 0.05916V log[Cl − ]2 2 Ø electrolyte = saturated KCl solution (3.8mol/l at 25°C) ⇒ [Cl-] = constant ⇒ ref. pot. E = const. 2.2 Electroanalytical Methods 2.2.1 Potentiometry, Potentiometric Titration pH measurement with the glass electrode Cl- determination with a silver electrode Ag+ + Cl- V AgCl (s) KL=1.8⋅10-10mol2/l2 0 E Ag = E Ag − / Ag + 2.303 RT 1 log 1⋅ F a Ag + 0 E Ag = E Ag + 0.059 log K L − 0.059 log[Cl − ] / Ag + Ag/AgCl/KCl sat/H+in/H+out/KClsat/AgCl/Ag E = ∆ϕ As + ∆ϕ Diff + RT ln 10 ⋅ ( pH in − pH out ) F 2.2 Electroanalytical Methods 2.2.2 Polarography Ø measurement of current – voltage curves (voltametry) gives information of the chemical composition and the concentration of the analytes in the sample (working range 1⋅10-6 < cA < 1⋅10-3 mol/l) Ø polarography was invented in 1922 by Jaroslav Heyrovsky (Nobelprice 1959) electrode arrangement for polarographic measurements Ø a polarographic cell contains: a polarizable working electrode (dropping mercury electrode) a non-polarizable counter electrode (e.g. Pt wire) a reference electrode (e.g. calomel electrode) a supporting electrolyte (e.g. Li+ Cl- in H2O) 2.2 Electroanalytical Methods 2.2.2 Polarography example: Cd determination Ø half step potential (E1/2) charcteristic for the analyte Ø hight of the step (id/2) proportional to the concentration of the analyte Ø working potential range -2.2V < Eappl < 0.3V Ø applicable to reducible or oxidizable metal ions or organic compounds A: 5⋅10-4 mol Cd2+ in 1 mol/l HCl as supporting electrolyte B: pure supporting electrolyte 2.2 Electroanalytical Methods 2.2.2 Polarography polarographic oligo-element determination A: 1⋅10-4 mol/l Ag+, Tl+, Cd2+, Ni2+, Zn2+ in 1 mol/l NH3 and 1 mol/l NH4Cl supporting electrolyte B: pure supporting electrolyte 2.3 Spectroscopic Methods 2.3.1 Atomic Emission Spectroscopy Ø history: one of the oldest methods for elemental analysis (flame emission - Bunsen, Kirchhoff 1860) Ø principle: recording of line spectra emitted by excited atoms or ions during radiative de-excitation (valence electrons) optical transitions and emission spectrum of H intensity of an emission line h ⋅ c ⋅ gm ⋅ A ⋅ N − Em I = Φ ⋅ exp 4 π ⋅ λ ⋅ Z kT Ø intensity of a spectral line ~N, i.e. ~C λ= h⋅ c E Ø population of excited states requires high temperatures (typical 2000 K-7000 K) Ø relative method, calibration required 2.3 Spectroscopic Methods 2.3.1 Atomic Emission Spectroscopy instrumentation analytical performance Ø multielement method (with arc 60-70 elements) Ø presiscion: ≤ 1% RSD for spark, flame and plasma; arc 5-10% RSD Ø decision limits: 0.1-1 ppm with arc, 110ppm with spark, 1ppb-10ppm with flame, 10ppt-5ppb for ICP application: radiation sources (classification) inductively coupled plasma (ICP) flame liquid samples spark arc glow discharge laser solid samples Ø flame AES – low cost system for alkali and earth alkaline metals Ø ICP AES – suited for any sample that can be brought into solution Ø arc AES – with photographic plate for qualitative overview analysis Ø spark AES – unsurpassable for metal analysis (steel, alloys) Ø laser ablation – for direct analysis of solids (transient signals) 2.3 Spectroscopic Methods 2.3.2 Atomic Absorption Spectroscopy Ø principle: measurement of the absorption of light by free atoms in their electronic ground state ground and excited states of Al and optical transitions used for AAS linear relation between absorbance and concentration Aabs g m λ4 1 I0 = log = 0.434 ⋅ A ⋅ ⋅ ⋅ ⋅ n0 ⋅ l I g 8 π c ∆ λ 0 eff AAS instrumentation 2.3 Spectroscopic Methods 2.3.2 Atomic Absorption Spectroscopy radiation source Ø atomic absorption lines are very small (∆λ≤0.005nm) ⇒ narrow excitation lines required working principle of hollow cathode lamps Ø hollow cathode lamp filled with Ar (~5 Torr) ⇒ discharge in cathode cup Ø cup-shaped cathode made from the element to be determined ⇒ emission of sharp de-excitation lines source of free atoms flame Ø pneumatic nebulization of the liquid sample into the flame or hydrid generation (e.g. for As, Bi, Ge) Ø air/C2H2 for nonrefractory elements (e.g. Ca, Cr, Fe) Ø N2O/C2H2 for refractory elements (e.g. Al, Si, Ta) graphite furnace Ø electrically heated graphite tube (Tmax=3000°C, under Ar) Ø T vs. t program ⇒ drying, ashing, atomization, cleaning Ø transient signals Ø liquid and solid samples 2.3 Spectroscopic Methods 2.3.2 Atomic Absorption Spectroscopy interferences in atomic absorption spectroscopy Ø flame and furnace AAS are sensitive to matrix interferences ⇒ i.e. matrix constituents (anything but the analyte) influence the analyte signal Ø there are two types of interferences ⇒ chemical and spectral interferences chemical interferences Ø limited temperature of the flame does not ensure full dissociation and atomization of thermally stable compounds (e.g. Ca phosphates) ⇒ hotter N2O flame (La buffer) Ø large amounts of easily ionized elements (alkali metals) modifies the equilibrium between atoms and ions ⇒ adding excess of Cs Ø loss of volatile analyte species during the ashing process in GF-AAS (e.g. ZnCl2) ⇒ adding of matrix modifiers NH4NO3 + NaCl → NH4Cl + NaNO3 spectral interferences Ø two spectral lines overlap within the band pass of the dispersive element (e.g. ∆λd ≈ 0.1nm, Cd 228.802nm and As 228.812nm) ⇒ select another line Ø nonspecific absorptions from solid or liquid particles in the atomizer (e.g. light scatterring by liquid or solid NaCl particles) Ø emission of molecular bands by molecules and radicals present in the atomizer (metal halides from 200 - 400nm) Ø background correction necessary 2.3 Spectroscopic Methods 2.3.2 Atomic Absorption Spectroscopy deuterium background correction Ø deuterium lamp emits a continuous spectrum in the range 200-380nm Ø alternating measurement of deuterium and hollow cathode lamp absorption setup sketch Ø simple correction method !!! d n Ø requirements: smoothly rvarying ou g k c background, λ < 360nm a gb n i ry for strongly Ø bad performance a v : mbackgrounds varying e l b pro working principle 2.3 Spectroscopic Methods 2.3.2 Atomic Absorption Spectroscopy Zeeman background correction Ø Zeeman effect: splitting of atomic energy levels in a magnetic field Ø applied in AAS for correction of strong varying backgrounds setup sketch normal Zeeman effect (S=0) (e.g. Cd atoms) working principle 2.3 Spectroscopic Methods 2.3.2 Atomic Absorption Spectroscopy analytical performance Ø AAS is most popular among the instrumental methods for elemental analysis flame-AAS: C relatively good limits decision limits (ng/ml) C working range mg/l (liner range) C easy to use, stable, comparably cheap C few and well known spectral interferences D single element method (up to 6 in parallel) D high sample consumption graphite furnace – AAS: C superior decision limits (pg/ml) (trace and ultra-trace analysis) C very low sample consumption C analysis of solids and liquids C analyte matrix separation possible D demanding operation 2.3 Spectroscopic Methods 2.3.3 Inductively Coupled Plasma Mass Spectrometry Ø a mass spectrometer separates a stream of gaseous ions into ions with different mass to charge ratio m/z (mass range in inorganic mass spectrometry from 1 – 300 u) Ø in combination with an ion source (ICP, spark, glow discharge, laser ablation) as analytical method for elemental analysis Ø most popular combination ⇒ Inductively Coupled Plasma + quadrupol Mass Spectrometer = ICP - MS setup scheme sample introduction Ø pneumatic nebulizer (liquid samples) Ø electro thermal vaporization (graphite furnace) liquid and solid samples Ø laser ablation solid samples 2.3 Spectroscopic Methods 2.3.3 Inductively Coupled Plasma Mass Spectrometry ICP as ion source Ø plasma generation by inductively heating of a gas (e.g. argon) using a high frequency field ICP assembly Ø ICP works under atmospheric pressure, MS under high vacuum ⇒ differentially pumped interface required Ø typical RF frequencies 27 or 40 MHz Ø power consumption 1 – 5 kW starting an ICP a) plasma off b) RF voltage applied c) ignition by HT spark d) plasma formation e) steady state 2.3 Spectroscopic Methods 2.3.3 Inductively Coupled Plasma Mass Spectrometry Ø sample droplets + carrier gas (inner tube connected to nebulizer) punch a channel through the toroidal plasma Ø sample transformations in the plasma: desolvation → vaporization → atomization → ionization Ø ionization behavior depends on the temperature ⇒ Saha equation (law of mass action for ionization equilibria) α i (1 + α e ) = pK (pi +1) (T ) α i +1α e K ( i +1) p g i λ3e I (T ) = exp i +1 ge g i + 1 T T temperature profile Ø example: Ar0 (1S0) → Ar+ (2P3/2) Na0 (2S1/2) → Na+ (1S0) I+=15.76 eV, 1 bar I+=5.14 eV 2.3 Spectroscopic Methods 2.3.3 Inductively Coupled Plasma Mass Spectrometry Ø determination of elements Z < 80 u by ICP - MS suffers from spectral interferences caused by the plasma gas, solvent, sample matrix etc. types of spectral interferences Ø isobaric overlaps Ø oxide-, hydroxide species Ø doubly charged ions interference analyte 40 Ar+ 40Ca+ 40 Ar16O+ 56Fe+ 138Ba2+ 69Ga+ Ø spectral interferences cause a high background ⇒ higher decision limit for disturbed analytes (e.g. xd(Fe)≈10 µg/l, xd(Ag)≈0.03 µg/l) 2.3 Spectroscopic Methods 2.3.3 ETV – ICP – MS Ø analyte – matrix separation by coupling of an ElectroThermal Vaporization to the ICP-MS → ETV-ICP-MS setup picture scheme of the ETV unit operation principle Ø quenching of the sample vapor by mixing it with cold carrier gas d k = 4σ ⋅ Vm k ⋅ T ⋅ ln S S= p peq (T ) Ø sample transport as condensed particles if the condensation nuclei exceed the critical diameter 2.3 Spectroscopic Methods 2.3.3 ETV – ICP – MS Ø one touchstone in ICP-MS analytics is the determination of light elements in saline solutions, e.g. seawater) Ø example: Zn determination in sea water relevant spectral interferences ion 64 u 66 u 67 u 68 u concentration in seawater Zn2+ 48.6% n.a. 27.9% n.a. 4.1% n.a. 18.8% n.a. ~1 ng/ml 35Cl16O16O+ 35Cl16O17O+ ca. 16 mg/l Cl34S16O16O+ 32S34S+ SO42- 32S16O16O+ 33S33S+ 32S32S+ 32S17O17O+ 32S16O18O+ 33S16O17O+ Mg2+ 24Mg40Ar+ 26Mg38Ar+ 26Mg40Ar+ 36S16O16O+ 33S34S+ 32S36S+ 32S17O18O+ 34S17O17O+ 33S16O18O+ 34S16O18O+ 33S17O17O+ 34S34S+ 34S16O17O+ 33S17O18O+ ca. 2 mg/l 32S18O18O+ ca. 1 mg/ml 2.3 Spectroscopic Methods 2.3.3 ETV – ICP – MS application to the Zn in seawater problem 13000 Ø analyte – matrix separation by ETV 12000 Ø sample matrix can be vaporized prior the analyte or in reversed fashion 10000 9000 intensity in counts/s Ø application of modifiers (ETV as thermochemical reactor, cp. atomic absorption spectroscopy) 11000 64amu, 48,6% n.H. 66amu, 27.9% n.H. 67amu, 4.1% n.H. 68amu, 18.8% n.H. 8000 7000 Zn signal, correct isotopic ratio Zn vaporizes before the matrix components 6000 5000 4000 3000 2000 1000 0 temperature in °C step T / °C ramp time / s hold time / s function 1 150 10 60 drying 2 150 0 30 baseline 3 800 1 59 Zn vaporization (prob. as ZnCl2 BP=732°C) 4 2500 5 5 cleaning 3000 2900 2800 2700 2600 2500 2400 2300 2200 2100 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 Ø resulting temperature – time program 2.3 Spectroscopic Methods 2.3.3 ETV – ICP – MS Ø if matrix matched calibration samples are not available ⇒ standard addition Ø principle of the standard addition method: - measurement of the analysis sample - stepwise addition of analyte (x+=xA…4xA) - evaluation of xA by extrapolation of the calibration function to the intercept point with the x-axis (y=f(x+)=0 ⇒ xA) - prerequisite: no blank value!!! result: 1 ng/ml < CZn < 3 ng/ml certified value: 1.24 ng/ml 2.3 Spectroscopic Methods 2.3.3 Inductively Coupled Plasma Mass Spectrometry analytical performance of ICP-MS Ø multi-element method (more than 20 analytes in parallel) Ø good performance in precision (a few percent), accuracy, number of determinable elements, decision limit (depending on the instrument 0.01-100 ng/l) and sample throughput Ø linear dynamic range ⇒ 3-5 orders of magnitude Ø difficult for elements with m/z < 80 u (spectral interferences) Ø mostly applied for liquid sample, ETV and laser vaporization enable direct analysis of solid samples Ø supplies information about the isotopic ratio of an element - isotop dilution analysis for ultra-trace analysis (sub ppt range) - age determination of biological and geological samples Ø relatively young analysis method (1980) ⇒ high innovative potential Backup Slide Specifity and Selectivity of Analytical Methods specificity… Ø …describes the ability of a method to detect 1 particular analyte on 1 sensor undisturbed from all other components present in the sample Ø …refers to a single component analysis Ø …is based upon the concept of partial sensitivities vector of partial sensitivities s A = ( S AA S AB ... S AZ ) spec( A / B,..., Z ) = S AA ⋅ x A Z ∑S K=A AK ⋅ xK 0 ≤ spec(A/B,…,Z) ≤ 1 for practical work spec(A/B,…,Z)>0.9 sufficient S IJ = ∂y I ∂x J Backup Slide Specifity and Selectivity of Analytical Methods selectivity… Ø …describes the ability of a method to determine n analytes on n sensors undisturbed and independent from each other and from other components present in the sample Ø …refers to a multi-component analysis Ø …is based upon the concept of partial sensitivities S IJ = matrix of partial sensitivities S AA S BA S= M S NA S AB L S AN S AN +1 L S AZ S BB L S BN S BN +1 L S BZ M O M M L M S NB L S NN S NN +1 L S NZ N sel ( A, B ,..., N / N + 1,..., Z ) = ∑S I=A N Z II ∑∑S J=AI=A ⋅ xI IJ ⋅ xI 0 ≤ sel(A,B,…,N/N+1,…,Z) ≤ 1 for practical work sel(A,B,…,N/N+1,…,Z) >0.9 sufficient ∂y I ∂x J Backup Slide Specifity and Selectivity of Analytical Methods Ø application of the selectivity/specificity concept to compare PN-ICP-MS and ETV-ICPMS for the determination of Zn in seawater ∆I 64amu ∆C Zn 2 + ∆I 64amu ∆C − S = ∆I Cl 64amu ∆C SO 42 − ∆I 64amu ∆C Mg 2 + specificity matrix for the sensors (masses) 64,66,67,68 u test samples to determine S ∆I 66amu ∆C Zn 2+ ∆I 66amu ∆C Cl− ∆I 66amu ∆C SO 2 − 4 ∆I 66amu ∆C Mg2 + ∆I 67amu ∆C Zn 2 + ∆I 67amu ∆C Cl− ∆I 67amu ∆C SO 2 − 4 ∆I 67amu ∆C Mg 2 + ∆I 68amu ∆C Zn 2 + ∆I 68amu ∆C Cl− ∆I 68amu ∆C SO 2 − 4 ∆I 68amu ∆C Mg2 + sample [Zn2+] in ng/ml [Cl-] in mg/ml [SO42-] in mg/ml [Mg2+] in mg/ml 1 50 10 1 0.5 2 100 10 1 0.5 3 50 20 1 0.5 4 50 10 5 0.5 5 50 10 1 1.5 Backup Slide Specifity and Selectivity of Analytical Methods Ø experimental specificity matrices after t-test (comparison of two mean values) SPN 2+ Zn = Cl − SO42 − 2+ Mg 64amu 66amu 67amu 616 1.22 ⋅ 10 − 3 378 − 3.48 ⋅ 10 − 4 63.9 0 18.6 ⋅ 10 − 3 0 0 28.5 ⋅ 10 − 3 14.3 ⋅ 10 − 3 2.52 ⋅ 10 − 3 68amu 2+ Zn 265 S ETV = Cl − 0 2− SO 4 0 Mg 2+ 8.97 ⋅ 10 − 3 64amu 66amu 67amu 5669 − 10.2 ⋅ 10−3 3557 − 5.67 ⋅ 10−3 524 − 1.01 ⋅ 10−3 0 0 − 23.0 ⋅ 10−3 − 40.4 ⋅ 10−3 0 0 2438 − 4.13 ⋅ 10−3 − 17.3 ⋅ 10−3 − 33.4 ⋅ 10−3 68amu Ø ETV-ICP-MS about 10 times more sensitive than PN-ICP-MS Ø positive partial sensitivities in SPN reflect spectral interferences (e.g. interference of 24Mg40Ar+ and 26Mg38Ar+ on 64Zn+) Ø negative partial sensitivities in SETV reflect non-spectral interferences Ø influence of Cl- : Zn2+ + 2Cl- → ZnCl2(s) → ZnCl2(g) (the less Cl- in solution the less Zn2+ vaporizes as ZnCl2) ⇒ non-spectral interference due to sample transport mechanism Ø influence of SO42- on the drying step: Zn2+ + SO42- → ZnSO4(s) above 770°C ZnSO4(s) → ZnO (mp = 1974°C) + SO2+ ½ O2 ⇒ as the vaporization step is done at 800°C SO42- ions cause a loss of Zn2+ Backup Slide Specifity and Selectivity of Analytical Methods Ø specificities for a typical seawater sample ion C in ng/ml Zn2+ 75 Cl- 15⋅106 SO42- 2.5⋅106 Mg2+ 1.0⋅106 Sprakt. 64amu PN Sprakt. 66amu PN Sprakt. 67amu PN Sprakt. 68amu PN Sprakt. 64amu ETV Sprakt. 66amu ETV Sprakt. 67amu ETV Sprakt. 68amu ETV 0.33 0.59 0.66 0.69 0.74 0.59 0.72 0.57 1.0 spec PN , 64amu ( Zn 2+ / Cl − , SO42− , Mg 2 + ) = 2+ − 2− 4 2+ 616 ⋅ x Zn2+ 616 ⋅ x Zn2+ + 93300 spec ETV ,64amu ( Zn / Cl , SO , Mg ) = 5669 ⋅ x Zn2+ 5669 ⋅ x Zn2+ + 153000 0.9 spec(Zn / Cl , SO4 , Mg) no external calibration (i.e. standards) possible for ng/ml concentrations 0.8 ETV 0.7 0.6 PN 0.5 0.4 0.3 0.2 0.1 0.0 0 100 200 300 400 500 600 700 800 900 1000 xZn in ng/ml 2.3 Spectroscopic Methods 2.3.4 X-Ray Fluorescence Spectroscopy Ø instrumental analytical technique for the elemental analysis of solids and liquids Ø wide concentration ranges (ppm - %), minimal sample preparation Ø principle: detection of element specific X – rays (characteristic X-rays) emitted from a sample under X-ray excitation (X-ray fluorescence) interaction of X-rays with matter emission of a core level electron (here from copper) by absorption of a X-ray photon energetic relaxation by X-ray fluorescence 2.3 Spectroscopic Methods 2.3.4 X-Ray Fluorescence Spectroscopy Ø transitions are governed by quantum mechanical selection rules (∆n>1, ∆l=±1, ∆j=0,±1) Ø K – lines originate from vacancies in the K – shell, L – lines from vacancies in the L – shell … Ø denotation of lines: i) IUPAC notation – Fe KL3 ii) Siegbahn notation – Fe α1 Ø line intensities depend on: i) the energetic difference of the levels (the higher the energetic difference the lower the transition probability) ii) the degeneracy of the levels iii) the fluorescence yield Ø K – lines best suited for analysis of elements Z < 45 (Rh) (mostly K-L3,2) Ø L – lines for analysis of elements Z > 45 (mostly L3-M5,4) atomic energy levels and allowed transitions 2.3 Spectroscopic Methods 2.3.4 X-Ray Fluorescence Spectroscopy instrumentation – X-ray source Ø generation of X-rays by collision of high energetic electrons with a pure metal target (Rh, Mo, Cr, Ag, W) Ø continuous X-rays by decelerating collisions with the target atoms (Bremsstrahlung) Ø characteristic X-rays by refilling of core level vacancies in the target atoms created by the impinging electrons Ø the short wavelength limit of the source depends on the accelerating voltage, the long wavelength tail depends on the Bewindow thickness Ø only ~1% of the electric power is converted to X-rays, rest is heat ⇒ effective water or air cooling required Ø quantitative work ⇒ stable filament heating and accelerating voltage required Rh anode 45kV 2.3 Spectroscopic Methods 2.3.4 X-Ray Fluorescence Spectroscopy instrumentation – wavelength dispersive instruments geometric arrangement Ø irradiated sample emits polychromatic X-rays detector Ø crystal as dispersive element Ø wavelength selection according to Bragg’s law n ⋅ λ = 2 ⋅ d ⋅ sin θ Ø wavelength scan by varying θ ⇒ rotation of the crystal with respect to the incoming beam flow proportional counter for low energetic X-rays scintillation counter for high energetic X-rays 2.3 Spectroscopic Methods 2.3.4 X-Ray Fluorescence Spectroscopy instrumentation – energy dispersive instruments Ø energy dispersion and X-ray photon counting in one step Ø detector ⇒ semiconductor crystal (Si(Li), hyperpure Ge) at liquid nitrogen temperature (-196°C) example Ø an Fe K-L3,2 X-ray photon (E=6.400keV) hits a Si detector Ø energy to create 1 electron – hole pair = 3.85eV ⇒ creation of 1662 ehp’s Ø the preamplifier converts this current in voltage pulse of e.g. 32mV Ø incoming X-ray photons create electron hole pairs ⇒ current ⇒ voltage pulse Ø the number of electron – hole pairs is proportional to the energy Ø best suited for energies > 2 keV Ø a connected ADC translates the amplified pulse height into a digital number, e.g. 320 ⇒ channel 320 in a memory is increased by 1 Ø common memories employ 1024 or 2048 channels (0-20 keV and 0 – 40 keV respectively) ⇒ resolution ~20 eV 2.3 Spectroscopic Methods 2.3.4 X-Ray Fluorescence Spectroscopy Ø application of XRF: qualitative and quantitative analysis of elements with Z ≥ 9 (F) ⇒ multi-element method Ø relatively simple sample preparation (bulk solids, pressed powder pellets, fused discs, liquids) Ø information depth in the µm range ⇒ homogeneous and smoothed surface required Ø qualitative analysis by recording the entire fluorescence spectrum ⇒ software assisted element identification by assigning the element specific K,L,M – lines qualitative analysis by XRF WD-XRF ED-XRF 2.3 Spectroscopic Methods 2.3.4 X-Ray Fluorescence Spectroscopy Ø quantitative analysis requires relation between line intensity and element concentration Ø 2 step process: i) determination of the net peak intensity (background subtraction) ii) conversion of the net peak intensity to concentration quantitative analysis complicated by x-ray attenuation x-ray enhancement Ø excitation of the Fe - K lines by the two Ni – K lines I x = I 0 ⋅ exp(− µ ⋅ ρ ⋅ x ) µ sample = ∑ C i µ i i a) no matrix effect b) attenuation c) enhancement fundamental parameters approach (implemented in software) Ø observed Fe – K line intensity = sum of primary and secondary fluorescence Ø Cr – K line intensity = sum of primary, secondary and tertiary fluorescence 2.3 Spectroscopic Methods 2.3.5 Electron Probe X-ray Microanalysis (EPXMA) EPXMA at the Fritz-Haber-Institute (Inorganic Chemistry Department, Building L) 2.3 Spectroscopic Methods 2.3.5 Electron Probe X-ray Microanalysis (EPXMA) Ø 2D elemental analysis by combination of Scanning Electron Microscopy and X-Ray Fluorescence Analysis SEM instrumentation electron sample interaction Ø topographic sample image by synchronizing the TV – scanner with the scanning coils Ø X-ray detection by a solid – state detector (Si single crystal) as in XRF (not shown above) 2.3 Spectroscopic Methods 2.3.5 Electron Probe X-ray Microanalysis (EPXMA) information depths, spatial resolution Ø in most cases, the SEM image is created using secondary electrons Ø secondary electrons are of low energy (<50eV), surface sensitive and supply an topographic image of the sample surface Ø spatial resolution < 0.1 • possible secondary electron image synthetic graphite (25keV, ×3000) 2.3 Spectroscopic Methods 2.3.5 Electron Probe X-ray Microanalysis (EPXMA) X – ray generation information depths, spatial resolution Ø X-ray generation process similar to XRF Ø X-ray spatial resolution much lower than by secondary electrons 2.3 Spectroscopic Methods 2.3.5 Electron Probe X-ray Microanalysis (EPXMA) example analytical performance (pinhole for molecular beam setup) Ø qualitative elemental analysis from Z > 11 by identification of the characteristic KLM peaks (software assisted) Ø detection limit ~1% Ø minimal sample preparation and sample damage Pt Ø quantitative analysis can be performed from element concentrations of 1 – 100% with a relative precision of 1-5% Ø similar to XAF, relating net peak intensity with element concentration requires either sophisticated mathematical models (e.g. ZAF correction) or suitable calibration samples Ni 3. Specification of an Analytical Result Ø any quantitative analysis relies on a relationship between the measurement value y and the analyte concentration x ⇒ y = f(x) Ø if y = f(x) is known, x can be calculated by x = f-1(y) Ø the mathematical form of f(x) and f-1(y) depends on the analytical method analytical methods relative methods absolute methods definite methods reference methods direct RM indirect RM B = yr/xr Y = f(x) Y = a+bx y = b⋅x b=F b = k⋅F gravimetry m Aq B p = M Aq B p q⋅ MA B ⋅ mA ICP-MS, AES, AAS photometry XPS B I A FX S ( E k )σ ( E k )λ (E k )cos θ ⋅ n A n A = = I R FX S ( E k )σ ( E k )λ (E k )cos θ ⋅ nR nR Aλ = ε λ ⋅ d ⋅ c A IA = IR ⋅ nA nR B 3. Specification of an Analytical Result Ø if the accuracy of the method is assured (e.g. by certified reference materials), the analytical result can be specified as follows x = x ± ∆x x = mean of n p parallel determinations ∆x = prediction interval of x Ø verbal: with a probability of P is the concentration x of the analyte A in the concentration range x ± ∆x Ø results from measurements with absolute, definite and direct reference methods can be evaluated using np parallel determinations (entire sample preparation process) and the uncertainties of all values in B using the rules of error propagation ⇒ x = B-1⋅y Ø specification of results from measurements with an indirect reference method (experimental calibration) requires to evaluate both the uncertainty of the analysis and of the calibration 3. Specification of an Analytical Result example 1: gravimetric determination of Fe2O3 in an iron ore Ø results of np=5 parallel determinations 38,71% 38,99% 38,62% 38,74% 38,73% mean : x = 1 np ∑x i = 38.76% standard deviation : s = i degrees of freedom : ∑ (x − x) 2 i i np − 1 = 0.1381% confidence level : P = 95% f = np − 1 = 4 t-table result : x ± ∆x = x ± s ⋅ t ( 0.95,4) 0.1381% ⋅ 2.78 = 38.76% ± = 38.76% ± 0.17% np 5 3. Specification of an Analytical Result example 2: Zn determination by AAS Ø calibration: m samples (xi, yi) ⇒ f = m-2 Ø xi taken as error free Ø least squares fit y = aˆ + bxˆ bˆ = ∑ (x i ∑ (x i aˆ = ∑y s02 = − x )( yi − y ) i i i − x) 2 i − b∑ x i i m ∑ (y i 2 − yˆ i ) i m−2 ∆yˆ k = t ( P , f ) 2 ( ) xk − x 1 s02 + m ∑ ( xi − x )2 i xi/ppm 2 4 6 8 10 yi (E=log(I0/I)) 0.266 0.598 0.856 1.222 1.467 3. Specification of an Analytical Result example 2: Zn determination by AAS Ø analysis: np measurement samples (e.g. np=3 parallel determinations) np 1 2 3 yi (E=log(I0/I)) 1.064 0.930 0.942 Ø calculation of Ø calculation of x= y = 0.979 y − aˆ 0.979 − (− 0.026) = = 6.656 ppm 0.151 bˆ s 1 1 ∆x = t ( P , f ) ⋅ 0 ⋅ + + m np bˆ (x − x ) ∑ (x − x ) 2 2 i i 0.0314 ppm 1 1 (6.656 − 6) ppm 2 ∆x = 3.18 ⋅ ⋅ + + = 0.488 ppm 0.151 5 3 40 ppm 2 2 x ± ∆x = 6.656 ± 0.488 ppm 4. Summary Ø a wide variety of methods are available to determine quantitatively the elemental composition of inorganic samples methods for elemental analysis chemical methods instrumental methods advantages advantages cheap, direct, easy to understand, precise cover concentrations from 100% to sub ppt, solid and liquid samples, multi-element character disadvantages cumbersome, restricted to main components, danger of systematic errors, liquid samples only disadvantages expensive, “black box” character, operation requires expertise Ø the choice of the right method depends on numerous parameters ( liquid or solid sample, available amount of sample, which and how many elements, expected concentrations, accuracy and precision, decision limit…) 5. Literature Ø General Analytical Chemistry Kellner R.; Mermet J.-M.; Otto M.; Valcárcel M.; Widmer H. M.; Analytical Chemistry. WileyVCH, Weinheim, 2nd edition, 2004. Ø Chemical Methods for Elemental Analysis Ackermann G.; Jugelt W.; Möbius H.-H.; Suschke H. D.; Werner G.; Elektrolytgleichgewichte und Elektrochemie. Lehrwerk Chemie, Vol. 5, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 5th edition, 1987 Ø Electroanalytical Methods Scholz F.; Electroanalytical Methods. Springer Verlag, Berlin, 1st edition, 2002 Ø Analytical Atomic Spectrometry Broekaert J. A.; Analytical Atomic Spectrometry with Plasmas and Flames. Wiley-VCH, Weinheim, 1st edition, 2001 Evans E. H.; Fisher A.; An Introduction to Analytical Atomic Spectrometry. Jon Wiley and Sons Ltd, 2nd edition, 2002 Welz B.; Sperling M.; Atomic Absorption Spectroscopy. Wiley-VCH, 3rd edition, 1998 Montaser A.; Inductively Coupled Plasma Mass Spectrometry, Wiley-VCH, 1998 5. Literature Ø X-Ray Fluorescence Spectroscopy Lachance G. R.; Claisse F.; Quantitative X-Ray Fluorescence Analysis. John Wiley & Sons Ltd, 1995 Ø Energy Dispersive X-Ray Microanalysis Goldstein J.; Scanning Electron Microscopy and X-Ray Microanalysis. Plenum Publishing Corp., New York, 3rd edition, 2002 Ø Statistics and Chemometrics Doerffel K.; Statistik in der Analytischen Chemie. Deutscher Verlag für Grundstoffindustrie, Leipzig, 5th edition, 1990 Massart D. L.; Vandeginste B. G. M.; Buydens L. M. C.; De Jong S.; Lewi P. J.; SmeyersVerbeke J.; Handbook of Chemometrics and Qualimetrics Part A and B. Elsevier, Amsterdam, 1997