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 problem solution
characterization representative sampling chemical sample object of investigation information
preparation measurement evaluation data diminution analysis
smaller sample for measurement analytical sample measurement data result Backup Slide The Analytical Process
strategy problem interpretation
characterization representative sampling chemical sample object of investigation information
preparation measurement evaluation data diminution analysis
smaller sample for measurement analytical sample measurement data result Backup Slide Analytical Problems in Science, Industry, Law…
Science Industry Law example: Determination by X- example: manufacturer of iron - example: Is the accused guilty ray Absorption Spectroscopy of Shall I by this iron ore or not ? or not guilty ? the Fe-Fe Separation in the Oxidized Form of the Hydroxylase of Methane Monooxygenase Alone and in the Presence of MMOD Rudd D. J., Sazinsky M. H., Merkx M., et al. Inorg. Chem. 2004, 43 (15), 4579-4589, 2004 Motivation Motivation fundamental interest Motivation human fate money future developments murder conviction precision (e.g.) genetic fingerprint fits – lifelong 53 ± 5wt% vs. 53 ± 1wt% prison genetic fingerprint does not fit - accuracy (e.g.) acquittal 53 ± 1wt% vs. 51 ± 1wt% 1. Introduction 1.1 Concentration Ranges
concentration ranges in elemental analysis
trace components side main components components
1 ppt 1 ppb 1 ppm 1 ‰ 1 % 100 % 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 1. Introduction 1.1 Concentration Ranges
mass ratio
10-2 10-3 10-6 10-9 10-12 1 % 1 ‰ 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 Ø describes in a positive (negative) consistency between the mean manner the influence of random of an analytic result and the errors on an analytic result value held as the true value Ø describing quantities: Ø has to be verified before each analysis by means of standards 2 å (xi - x) Ø describing quantities: s = i Gaussian n - 1 distributed
errorx = ± x - xˆ' errors! 2 ± x - xˆ' wrong å (xi - x) 2 i relative errorx = ×100 v = s = xˆ' results n -1 main and trace and side ultra trace component analysis 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 limit guaranties the quality of a quantitative result (user specific)
t × s t × s y = y + f ,a B Þ y = y + bx Þ x = f ,a B k B n k B NG NG b× n for a = b x = 2× x EG NG DIN 32645 2. Methods for Quantitative Elemental Analysis
2.2 Electroanalytical 2.1 Chemical Methods Methods 2.3 Spectroscopic Methods
Ø atomic emission Ø atomic absorption Ø volumetric methods Ø potentiometry Ø inductively coupled plasma mass Ø gravimetric methods Ø polarography 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
100 p = VB × k × N × in % me
p = percentage of the analyte in the sample
VB = volume in ml dropped in from the burette k = stoichiometric factor (mg analyte/ml) A + B ® C + D N = correction factor for the theoretical value k e = weighted sample (in mg)
2 2 æ s ö æ s ö 2 s P me VB æ s N ö = ç ÷ + ç ÷ + ç ÷ Ø fast, complete and stoichiometric well p è me ø è VB ø è N ø defined reaction inserting typical values Ø the extend of the reaction can be monitored (e.g. pH, electric potential) s 0.1% < P < 1% Ø the point of equivalence can be p detected precisely (e.g. sudden color change, steep pH or potential change) advantage: high precision!!! 2.1 Chemical Methods 2.1.1 Volumetric Methods – Acid Base Titration
n- + acid – base reaction: HnA + nBOH V A + nB + nH2O (zA=n, zB=1)
zB ×CB ×VB Ka VS +V æ K w - pH ö + - titration curve: t = = - pH + ×ç - pH - 10 ÷ K w = [H ][OH ] zA ×C A ×VA 10 + Ka C S ×VS è 10 ø
strong acid – strong base weak acid – strong base different Ka – strong base
- H+
+ H+
pH = pK a,Ind ± 1 Ø choice of the indicator depends on Ka 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
m+ m+ complex formation: nL + M V MLn most used ligand - EDTA
n- m+ (m-4) + H4-nY + M V [MY] + (4-n)H (zA=1, zB=1)
[MY (m-4) ] [M m+ ] = K f ×[EDTA]total ×aY 4-
complex formation only with Y4- Þ 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)
- 2+ + 2+ 3+ examples: MnO4 + 5Fe + 8H V Mn + 5Fe + 4H2O Ce4+ + Fe2+ V Ce3+ + Fe3+ titration curve
z ×C ×V t = A A A zB ×CB ×VB
Q RT t 0 < t < 1 U = U A = U A + × ln nB F 1-t
Q Q nAU B + nBU A t = 1 Ueq = nA + nB
Q RT t > 1 U = U B = U B + × ln(t - 1) nAF
- 2+ indication: self indication (violet MnO4 ® colorless Mn ) - redox indicator (InRed F InOx + ne ) potentiometric endpoint detection 2.1 Chemical Methods 2.1.2 Gravimetric Methods
precipitation reaction: n M z+ (aq) +n Az- (aq) « M A (s) + - n + n - 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
2+ 2- 2+ 2- -10 2 2 Ba + SO4 F BaSO4 (KL=[Ba ]×[SO4 ]=1×10 mol /l ) as BaSO4 3+ - 3+ - 3 -38 4 4 Fe + 3OH F Fe(OH)3 (KL=[Fe ]×[OH ] =1×10 mol /l ) as Fe2O3
n - n + z+ æn + ö z- æn - ö n + +n - ç ÷ n + +n - ç ÷ [M ] = K L ç ÷ [A ] = K L ç ÷ èn - ø èn + ø [M z+ ]×V Ø for analytical purposes: a p = 1- z+ ³ 0.997 [M ]o ×V0
2 2 Ø advantage æ s ö æ s ö s m me ma s m = ç ÷ + ç ÷ 0.01% < < 0.1% Þ one of the most precise analytical methods m è me ø è ma ø m 2.2 Electroanalytical Methods
electrode reaction, IFaraday=0 (potentiometry) Ø ion-selective electrodes Ø potentiometric titration
electrode reaction, DC, no electrode I ¹0 (voltametry) Faraday electroanalytical reaction, AC, high Ø polarography methods frequency Ø coulometry Ø conductometry
electrode reaction, AC,
IFaraday¹0 Ø AC polarography 2.2 Electroanalytical Methods 2.2.1 potentiometry, potentiometric titration
Ø principle Þ 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
0.05916V E = E 0' - log[Cl - ]2 2
Ø electrolyte = saturated KCl solution (3.8mol/l at 25°C) Þ [Cl-] = constant Þ 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) -10 2 2 KL=1.8×10 mol /l
0 2.303RT 1 E Ag = E Ag / Ag+ - log 1× F a Ag +
0 - E Ag = E Ag / Ag+ + 0.059log K L - 0.059log[Cl ]
+ + Ag/AgCl/KClsat/H in/H out/KClsat/AgCl/Ag RT E = Dj + Dj + ln10×( pH - pH ) As Diff F in out 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 -6 -3 (working range 1×10 < cA < 1×10 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 (i /2) proportional to the d -4 2+ concentration of the analyte A: 5×10 mol Cd in 1 mol/l HCl as supporting electrolyte Ø working potential range -2.2V < E < 0.3V appl B: pure supporting electrolyte Ø applicable to reducible or oxidizable metal ions or organic compounds 2.2 Electroanalytical Methods 2.2.2 Polarography
polarographic oligo-element determination
-4 + + 2+ 2+ 2+ A: 1×10 mol/l Ag , Tl , Cd , Ni , Zn 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
æ h× c × g × A× N ö æ - E ö I = Fç m ÷ × expç m ÷ è 4p × l × Z ø è kT ø
Ø intensity of a spectral line ~N, i.e. ~C Ø population of excited states requires high temperatures (typical 2000 K-7000 K) h× c l = Ø relative method, calibration required E 2.3 Spectroscopic Methods 2.3.1 Atomic Emission Spectroscopy
analytical performance instrumentation 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, 1- 10ppm with spark, 1ppb-10ppm with flame, 10ppt-5ppb for ICP
application: radiation sources (classification) Ø flame AES – low cost system for alkali and earth alkaline metals inductively coupled Ø ICP AES – suited for any sample plasma (ICP) liquid that can be brought into solution samples flame Ø arc AES – with photographic plate for qualitative overview analysis spark Ø spark AES – unsurpassable for arc solid metal analysis (steel, alloys) samples glow discharge Ø laser ablation – for direct analysis of solids (transient signals) laser 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
4 æ I0 ö gm l 1 Aabs = logç ÷ = 0.434× A× × × × n0 × l è I ø g0 8pc Dleff
AAS instrumentation 2.3 Spectroscopic Methods 2.3.2 Atomic Absorption Spectroscopy
radiation source Ø atomic absorption lines are very working principle of hollow cathode lamps small (Dl£0.005nm) Þ narrow excitation lines required Ø hollow cathode lamp filled with Ar (~5 Torr) Þ discharge in cathode cup Ø cup-shaped cathode made from the element to be determined Þ emission of sharp de-excitation lines
source of free atoms flame graphite furnace Ø pneumatic nebulization of the Ø electrically heated graphite tube liquid sample into the flame (T =3000°C, under Ar) or hydrid generation max (e.g. for As, Bi, Ge) Ø T vs. t program Þ drying, ashing, atomization, cleaning Ø air/C2H2 for nonrefractory elements (e.g. Ca, Cr, Fe) Ø transient signals
Ø N2O/C2H2 for refractory Ø liquid and solid samples elements (e.g. Al, Si, Ta) 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 Þ i.e. matrix constituents (anything but the analyte) influence the analyte signal Ø there are two types of interferences Þ chemical and spectral interferences chemical interferences spectral interferences
Ø limited temperature of the flame does Ø two spectral lines overlap within the not ensure full dissociation and band pass of the dispersive element atomization of thermally stable (e.g. Dld » 0.1nm, Cd 228.802nm and compounds (e.g. Ca phosphates) Þ As 228.812nm) Þ select another line hotter N2O flame (La buffer) Ø nonspecific absorptions from solid or Ø large amounts of easily ionized liquid particles in the atomizer (e.g. elements (alkali metals) modifies the light scatterring by liquid or solid equilibrium between atoms and ions NaCl particles) Þ adding excess of Cs Ø emission of molecular bands by Ø loss of volatile analyte species during molecules and radicals present in the the ashing process in GF-AAS (e.g. atomizer ZnCl2) Þ adding of matrix modifiers (metal halides from 200 - 400nm)
NH4NO3 + NaCl ® NH4Cl + NaNO3 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 working principle
Ø simple correction method Ø requirements: smoothly varying background, l < 360nm ing background!!! Ø bad performancey for strongly varying backgrounds problem: var 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 Þ 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 starting an ICP
a) plasma off Ø ICP works under atmospheric pressure, MS under high vacuum Þ b) RF voltage applied differentially pumped interface required c) ignition by HT spark Ø typical RF frequencies 27 or 40 MHz d) plasma formation Ø power consumption 1 – 5 kW 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 Þ Saha equation (law of mass action for ionization equilibria)
3 a i (1 + ae ) (i+1) (i+1) gi le æ Ii+1 ö = pK p (T ) K p (T ) = expç ÷ ai+1ae ge gi+1 T è T ø
temperature profile
0 1 + 2 Ø example: Ar ( S0) ® Ar ( P3/2) I+=15.76 eV, 1 bar 0 2 + 1 Na ( S1/2) ® Na ( S0) 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
40Ar+ 40Ca+ 40Ar16O+ 56Fe+ 138Ba2+ 69Ga+
Ø spectral interferences cause a high background Þ 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
Vm p dk = 4s × S = k ×T × ln S 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 concentration ion 64 u 66 u 67 u 68 u in seawater Zn2+ 48.6% n.a. 27.9% n.a. 4.1% n.a. 18.8% n.a. ~1 ng/ml Cl- 35Cl16O16O+ 35Cl16O17O+ ca. 16 mg/l 36S16O16O+ 34S16O16O+ 33S34S+ 32S36S+ 32S34S+ 32S17O18O+ 34S17O17O+ 32S16O16O+ 33S33S+ SO 2- 33S16O18O+ 34S16O18O+ ca. 2 mg/l 4 32S32S+ 32S17O17O+ 33S17O17O+ 34S34S+ 32S16O18O+ 34S16O17O+ 33S17O18O+ 33S16O17O+ 32S18O18O+ 24Mg40Ar+ Mg2+ 26Mg40Ar+ ca. 1 mg/ml 26Mg38Ar+ 2.3 Spectroscopic Methods 2.3.3 ETV – ICP – MS
application to the Zn in seawater problem
13000 64amu, 48,6% n.H. 12000 Ø analyte – matrix separation by ETV 66amu, 27.9% n.H. 11000 67amu, 4.1% n.H. 68amu, 18.8% n.H. 10000
Ø sample matrix can be vaporized prior the 9000 Zn signal, correct isotopic ratio analyte or in reversed fashion 8000 Zn vaporizes before the matrix components 7000 Ø application of modifiers (ETV as 6000 thermochemical reactor, cp. atomic 5000 4000 intensity in counts/s absorption spectroscopy) 3000 2000
1000
0 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000
Ø resulting temperature – time program 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 2.3 Spectroscopic Methods 2.3.3 ETV – ICP – MS
Ø if matrix matched calibration samples are not available Þ 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 Þ 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 Þ 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) Þ 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 ¶y Ø …is based upon the concept of partial sensitivities I SIJ = ¶xJ
vector of partial sensitivities
s A = (S AA S AB ...S AZ )
S AA × x A spec(A/ B,..., Z ) = Z å S AK × xK K = A
0 £ spec(A/B,…,Z) £ 1 for practical work spec(A/B,…,Z)>0.9 sufficient 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 ¶yI SIJ = ¶xJ matrix of partial sensitivities æ S S S S S ö ç AA AB L AN AN +1 L AZ ÷ ç SBA SBB L SBN SBN +1 L SBZ ÷ S = ç ÷ ç M M O M M L M ÷ ç ÷ è SNA SNB L SNN SNN +1 LS NZ ø N å SII × xI I = A sel(A, B,..., N / N + 1,..., Z ) = N Z åå SIJ × xI J = A I = A 0 £ sel(A,B,…,N/N+1,…,Z) £ 1 for practical work sel(A,B,…,N/N+1,…,Z) >0.9 sufficient Backup Slide Specifity and Selectivity of Analytical Methods
Ø application of the selectivity/specificity concept to compare PN-ICP-MS and ETV-ICP- MS for the determination of Zn in seawater
æ DI DI DI DI ö ç 64amu 66amu 67amu 68amu ÷ DC DC DC DC ç Zn2+ Zn 2+ Zn2+ Zn2+ ÷
ç DI64amu DI66amu DI67amu DI68amu ÷ ç ÷ specificity matrix for the DC - DC - DC - DC - S = ç Cl Cl Cl Cl ÷ sensors (masses) 64,66,67,68 u ç DI64amu DI66amu DI67amu DI68amu ÷ ç ÷ DC 2- DC 2- DC 2- DC 2- ç SO4 SO4 SO4 SO4 ÷ ç DI64amu DI66amu DI67amu DI68amu ÷ ç DC DC DC DC ÷ è Mg2+ Mg2+ Mg2+ Mg2+ ø
2+ - 2- 2+ sample [Zn ] in [Cl ] in [SO4 ] in [Mg ] in ng/ml mg/ml mg/ml mg/ml 1 50 10 1 0.5 test samples to 2 100 10 1 0.5 determine S 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)
æ 64amu 66amu 67amu 68amu ö æ 64amu 66amu 67amu 68amu ö ç ÷ ç ÷ 2+ 2+ Zn 5669 3557 524 2438 ç Zn 616 378 63.9 265 ÷ ç ÷ - -3 -3 -3 -3 ç - -3 -4 ÷ ç ÷ S = Cl 1.22×10 - 3.48×10 0 0 SETV = Cl -10.2 ×10 - 5.67×10 -1.01×10 - 4.13×10 PN ç ÷ ç ÷ 2- -3 ç SO2- 0 - 23.0 ×10-3 0 -17.3×10-3 ÷ ç SO4 18.6×10 0 0 0 ÷ 4 ç 2+ -3 -3 -3 -3 ÷ ç 2+ -3 -3 ÷ è Mg 28.5×10 14.3×10 2.52×10 8.97 ×10 ø èMg 0 - 40.4 ×10 0 - 33.4 ×10 ø
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
- 2+ - Ø influence of Cl : Zn + 2Cl ® ZnCl2(s) ® ZnCl2(g) - 2+ (the less Cl in solution the less Zn vaporizes as ZnCl2) Þ non-spectral interference due to sample transport mechanism
2- 2+ 2- Ø influence of SO4 on the drying step: Zn + SO4 ® ZnSO4(s) above 770°C ZnSO4(s) ® ZnO (mp = 1974°C) + SO2+ ½ O2 2- 2+ Þ as the vaporization step is done at 800°C SO4 ions cause a loss of Zn Backup Slide Specifity and Selectivity of Analytical Methods
Ø specificities for a typical seawater sample
S S S S S S S S C prakt. prakt. prakt. prakt. prakt. prakt. prakt. prakt. ion 64amu 66amu 67amu 68amu 64amu 66amu 67amu 68amu in ng/ml PN PN PN PN ETV ETV ETV ETV Zn2+ 75 Cl- 15×106 0.33 0.59 0.66 0.69 0.74 0.59 0.72 0.57 2- 6 SO4 2.5×10 Mg2+ 1.0×106
1.0 no external calibration 0.9 (i.e. standards) possible 0.8 ETV for ng/ml concentrations 0.7 , Mg) 4 0.6 PN 0.5 616 x 2+ 2+ - 2- 2+ × Zn specPN ,64amu (Zn / Cl , SO4 , Mg ) = 0.4 616 x 2+ 93300 × Zn + (Zn / Cl , SO 0.3 0.2 spec 0.1 5669× x 2+ spec (Zn2+ / Cl - , SO 2- , Mg 2+ ) = Zn 0.0 ETV ,64amu 4 0 100 200 300 400 500 600 700 800 900 1000 5669 x 2+ 153000 × Zn + 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 energetic relaxation by X-ray copper) by absorption of a X-ray photon fluorescence 2.3 Spectroscopic Methods 2.3.4 X-Ray Fluorescence Spectroscopy
Ø transitions are governed by quantum mechanical selection rules (Dn>1, Dl=±1, Dj=0,±1) atomic energy levels and allowed transitions 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 a1 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) 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 Be- Rh window thickness anode Ø only ~1% of the electric power is 45kV converted to X-rays, rest is heat Þ effective water or air cooling required Ø quantitative work Þ stable filament heating and accelerating voltage required 2.3 Spectroscopic Methods 2.3.4 X-Ray Fluorescence Spectroscopy
instrumentation – wavelength dispersive instruments Ø irradiated sample emits geometric arrangement polychromatic X-rays
detector Ø crystal as dispersive element Ø wavelength selection according to flow proportional Bragg’s law counter for low energetic X-rays n× l = 2× d ×sinq
Ø wavelength scan by varying q Þ scintillation rotation of the crystal with respect to counter for high the incoming beam 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 Þ semiconductor crystal example (Si(Li), hyperpure Ge) at liquid nitrogen Ø an Fe K-L3,2 X-ray photon (E=6.400keV) temperature (-196°C) hits a Si detector Ø energy to create 1 electron – hole pair = 3.85eV Þ creation of 1662 ehp’s Ø the preamplifier converts this current in voltage pulse of e.g. 32mV Ø a connected ADC translates the amplified pulse height into a digital Ø incoming X-ray photons create number, e.g. 320 Þ channel 320 in a electron hole pairs Þ current Þ voltage memory is increased by 1 pulse Ø common memories employ 1024 or 2048 Ø the number of electron – hole pairs is channels (0-20 keV and 0 – 40 keV proportional to the energy respectively) Þ resolution ~20 eV Ø best suited for energies > 2 keV 2.3 Spectroscopic Methods 2.3.4 X-Ray Fluorescence Spectroscopy
Ø application of XRF: qualitative and quantitative analysis of elements with Z ³ 9 (F) Þ multi-element method Ø relatively simple sample preparation (bulk solids, pressed powder pellets, fused discs, liquids) Ø information depth in the µm range Þ homogeneous and smoothed surface required Ø qualitative analysis by recording the entire fluorescence spectrum Þ 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 a) no matrix effect I = I × exp(-m × r × x) Ø observed Fe – K line intensity x 0 b) attenuation = sum of primary and secondary c) enhancement fluorescence msample = åCi mi fundamental parameters Ø Cr – K line intensity = sum of i approach (implemented in primary, secondary and tertiary software) 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 SEM instrumentation combination of Scanning Electron Microscopy and X-Ray Fluorescence Analysis
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)
information depths, spatial resolution X – ray generation
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 Ni either sophisticated mathematical models (e.g. ZAF correction) or suitable calibration samples 3. Specification of an Analytical Result
Ø any quantitative analysis relies on a relationship between the measurement value y and the analyte concentration x Þ 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 reference methods definite methods direct RM indirect RM y = b×x Y = f(x) Y = a+bx b = F b = k×F B = yr/xr ICP-MS, AES, AAS gravimetry photometry XPS I F S(E )s (E )l E cosq × n n M A B B A X k k ( k ) A A m = q p × m = = Aq B p A I R FX S(Ek )s (Ek )l(Ek )cosq × nR nR q × M A I A = e × d × c I = R × n l l A A n A B R 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 ± Dx
x = mean of np parallel determinations Dx = prediction interval of x
Ø verbal: with a probability of P is the concentration x of the analyte A in the concentration range x ± Dx 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 Þ 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%
2 å (xi - x) 1 i mean : x = å xi =38.76% standard deviation : s = = 0.1381% np i n p -1
degrees of freedom : f = np -1 = 4 confidence level : P = 95% t-table
s × t(0.95,4) 0.1381%× 2.78 result : x ± Dx = x ± = 38.76% ± = 38.76% ± 0.17% n p 5 3. Specification of an Analytical Result
example 2: Zn determination by AAS
Ø calibration: m samples (xi, yi) Þ f = m-2
Ø xi taken as error free xi/ppm 2 4 6 8 10 Ø least squares fit yi (E=log(I0/I)) 0.266 0.598 0.856 1.222 1.467 y = aˆ + bxˆ
å (xi - x)(yi - y) ˆ i b = 2 å (xi - x) i
å yi - bå xi aˆ = i i m ˆ 2 å (yi - yi ) s2 = i 0 m - 2
é 2 ù ê 1 (x - x) ú Dyˆ = t(P, f ) s2 + k k 0 êm 2 ú ê å (xi - x) ú ë i û 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 y = 0.979 yi (E=log(I0/I)) 1.064 0.930 0.942
y - aˆ 0.979 - (- 0.026) Ø calculation of x = = = 6.656 ppm bˆ 0.151
s 1 1 (x - x)2 Ø calculation of Dx = t(P, f )× 0 × + + bˆ m n 2 p å (xi - x) i
0.0314 ppm 1 1 (6.656 - 6)2 ppm 2 Dx = 3.18× × + + = 0.488 ppm 0.151 5 3 40 ppm 2
x ± Dx = 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 instrumental methods methods advantages advantages cheap, direct, easy to cover concentrations from 100% understand, precise to sub ppt, solid and liquid samples, multi-element disadvantages character cumbersome, restricted to main disadvantages components, danger of systematic errors, liquid expensive, “black box” samples only 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. Wiley- VCH, 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.; Smeyers- Verbeke J.; Handbook of Chemometrics and Qualimetrics Part A and B. Elsevier, Amsterdam, 1997