Atomic Absorption Spectroscopy (AAS) See also: Fundamental reviews in Analytical Chemistry e.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C. Anal. Chem.

Atomic Absorption Spectroscopy (AAS)Atomic Absorption Spectroscopy (AAS)

See also: Fundamental reviews in See also: Fundamental reviews in Analytical ChemistryAnalytical Chemistry e.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C. e.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C. Anal. Anal. Chem. Chem. 20022002, , 7474, 2691-2712 (“Atomic Spectroscopy”), 2691-2712 (“Atomic Spectroscopy”)

•1802 Wollaston observes absorption lines in solar spectrum1802 Wollaston observes absorption lines in solar spectrum

•1914 Hollow cathode lamp1914 Hollow cathode lamp

•1955 Walsh describes analytical AAS1955 Walsh describes analytical AAS

•1959 11959 1stst Commercial Flame AAS Commercial Flame AAS

•1960s L’vov and Massman describe graphite furnace 1960s L’vov and Massman describe graphite furnace (commercial in 1970s)(commercial in 1970s)

Recall: A = -logRecall: A = -log1010(T)(T)

Hollow Cathode LampHollow Cathode Lamp

Kellner et al., Kellner et al., Analytical ChemistryAnalytical Chemistry

•Typical primary source of radiation: Hollow cathode lampTypical primary source of radiation: Hollow cathode lamp* Typically one lamp per element* Typically one lamp per element* Different intensities for different elements* Different intensities for different elements* Multielement lamps for multielement analysis* Multielement lamps for multielement analysis

•Continuum sources (e.g. Xe arc lamp) only for multielement analysisContinuum sources (e.g. Xe arc lamp) only for multielement analysis

Ideal AtomizerIdeal Atomizer

1.1. Provide complete atomization of the element of interestProvide complete atomization of the element of interest2.2. Atomic vapor should not be highly diluted by the Atomic vapor should not be highly diluted by the

atomizer gasatomizer gas3.3. Excitation of the analyte (and other species) should be Excitation of the analyte (and other species) should be

minimalminimal

Flame AASFlame AAS

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical AnalysisKellner et al., Kellner et al., Analytical ChemistryAnalytical Chemistry

At <5000 K most atoms are At <5000 K most atoms are predominantly in their electronic predominantly in their electronic ground state.ground state.

Slot burners with 5-10 cm path Slot burners with 5-10 cm path lengths.lengths.

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Electrothermal AtomizationElectrothermal Atomization

•Heating current of Heating current of several hundred Aseveral hundred A

•Heating rates of up to Heating rates of up to 1000 °C/s1000 °C/s

•LOD 100 times lower LOD 100 times lower than flame AASthan flame AAS

Heated in three stages:Heated in three stages:

•Typical furnace material: GraphiteTypical furnace material: Graphite

Graphite Furnace AASGraphite Furnace AAS*Graphite tube 18-28 mm Graphite tube 18-28 mm *Samples 5-100 uLSamples 5-100 uL*200 to 1000 cycles200 to 1000 cycles*Temperature up to 3000 °C to avoid graphite decompositionTemperature up to 3000 °C to avoid graphite decomposition*Carbon may be reducing agent for metal ionsCarbon may be reducing agent for metal ions*Argon flow avoids oxidationArgon flow avoids oxidation

•Other furnace materials: Ta, W, PtOther furnace materials: Ta, W, Pt*High melting point requiredHigh melting point required*Should not emit brightly at high temperature (disadvantage for WShould not emit brightly at high temperature (disadvantage for W and Ta) and Ta)

Electrothermal AtomizationElectrothermal Atomization

Are you getting the concept?Are you getting the concept?

Is ICP a good source for AAS?Is ICP a good source for AAS?

Double-Beam AASDouble-Beam AAS

Skoog Skoog Principles of Instrumental AnalysisPrinciples of Instrumental Analysis

Single-BeamSingle-Beam

Double-BeamDouble-Beam

IIsamplesample

II00

What is the problem with just measuring IWhat is the problem with just measuring Isamplesample/I/I00??

BackgroundBackground

Sources of Background: scattering or molecular emissionSources of Background: scattering or molecular emission

Background Correction:Background Correction:

•With blank sampleWith blank sample AAcc = A = Att - A - Ablankblank

•Deuterium lamp (arc in deuterium atmosphere; Deuterium lamp (arc in deuterium atmosphere; continuum 200-380 nm)continuum 200-380 nm) absorption of deuterium lamp represents Aabsorption of deuterium lamp represents Abackgroundbackground

absorption of HCL radiation represents Aabsorption of HCL radiation represents A tt

Advantage over blank sample: observe fluctuations in flameAdvantage over blank sample: observe fluctuations in flame

Culver et al, Anal. Chem., 47, 920, 1975.Culver et al, Anal. Chem., 47, 920, 1975.

Background Correction with Zeeman EffectBackground Correction with Zeeman Effect

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Lines differ by ~0.01 nmLines differ by ~0.01 nm

Background Correction with Zeeman EffectBackground Correction with Zeeman Effect

Skoog, Skoog, Principles of Instrumental AnalysisPrinciples of Instrumental Analysis

1. Unpolarized light from HCL (A) passes through the rotating polarizer (B)2. Light is separated into perpendicular and parallel components (C)3. The light enters the furnace with an applied magnetic field, producing

3 absorption peaks (D)4. Either analyte or analyte + matrix absorb light (E)5. A cyclical absorbance pattern results (F)6. Subtract absorbance during perpendicular half of cycle from absorbance

during parallel half of cycle to get the background corrected value

Background Correction with Zeeman EffectBackground Correction with Zeeman Effect

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

DC on atomizerDC on atomizer

AC on atomizerAC on atomizer

DC on sourceDC on source

AAS: Figures of MeritAAS: Figures of Merit

•Linearity over 2 to 3 concentration decades (can be problem forLinearity over 2 to 3 concentration decades (can be problem for multielement analysis)multielement analysis)

•Probability for line overlap is small Probability for line overlap is small Resolution not as critical Resolution not as critical as for AES as for AES

•Precision: Typically a few % for graphite furnacePrecision: Typically a few % for graphite furnace 0.3 to 1.0% for flame0.3 to 1.0% for flame

•Accuracy: Largely determined by calibration with standardsAccuracy: Largely determined by calibration with standards

•Applicability: Limited for certain elements for which flame or Applicability: Limited for certain elements for which flame or furnace is not hot enough (e.g. W, Ta, Nb).furnace is not hot enough (e.g. W, Ta, Nb).Flow rates of flame are compatible with HPLC flow rates.Flow rates of flame are compatible with HPLC flow rates.

•Speed: Multielement analysis with multiple HCLs may require lamp Speed: Multielement analysis with multiple HCLs may require lamp exchanges to select desired elements. This is tedious and also costs light exchanges to select desired elements. This is tedious and also costs light because of beam splitters.because of beam splitters.

Method of Standard AdditionsMethod of Standard Additions

In order to quantitate the element of interest in a sample, it is In order to quantitate the element of interest in a sample, it is necessary to calibrate with the necessary to calibrate with the method of standard method of standard additionsadditions..

1.1. The analytical signal for the sample, SThe analytical signal for the sample, Sxx, is obtained (after , is obtained (after measuring the blank signal).measuring the blank signal).

2.2. A small volume, VA small volume, Vss, of a concentrated standard solution of , of a concentrated standard solution of known concentration, cknown concentration, css, is added to a relatively large , is added to a relatively large volume, Vvolume, Vxx of the analytical sample. of the analytical sample.

3.3. The analytical signal for the standard addition solution, SThe analytical signal for the standard addition solution, Sx+sx+s, , is obtained.is obtained.

ccxx = (S = (SxxVVssccss)/[S)/[Sx+sx+s(V(Vxx+V+Vss) – S) – SxxVVxx]]

ccxx = (S = (SxxVVssccss)/[(S)/[(Sx+sx+s – S – Sxx)V)Vxx]]

if Vif Vss << V << Vxx

Are you getting the concept?Are you getting the concept?

The determination of Pb in a brass sample is done with AAS. The 50 mL The determination of Pb in a brass sample is done with AAS. The 50 mL original sample was introduced into the instrument and an absorbance of original sample was introduced into the instrument and an absorbance of 0.420 was obtained. To the original solution, 20 0.420 was obtained. To the original solution, 20 L of a 10.0 L of a 10.0 g/mL Pb g/mL Pb standard was then added. The absorbance of this solution was 0.580. Find standard was then added. The absorbance of this solution was 0.580. Find the concentration of Pb in the original sample. What assumption has been the concentration of Pb in the original sample. What assumption has been made in order to use a single standard addition?made in order to use a single standard addition?

AAS: Figures of MeritAAS: Figures of Merit

•Detection limits:Detection limits: * Generally lower LOD for very volatile elements* Generally lower LOD for very volatile elements * Higher LOD for carbide-forming elements (e.g. Ba, * Higher LOD for carbide-forming elements (e.g. Ba,

B, Ca, Mo, W, V, Zr) B, Ca, Mo, W, V, Zr) * Concentration in GF up to 1000 times higher than * Concentration in GF up to 1000 times higher than

in flame; much lower LOD for GF. in flame; much lower LOD for GF. * Lower LOD for GF-AAS than ICP-AES unless* Lower LOD for GF-AAS than ICP-AES unless

atomization requires high temperatureatomization requires high temperature * Generally similar LOD for flame-AAS and ICP-AES* Generally similar LOD for flame-AAS and ICP-AES * Generally higher LOD for very volatile elements* Generally higher LOD for very volatile elements

•Chemical Chemical * HCl often avoided as acid in GF-AAS because * HCl often avoided as acid in GF-AAS becauseinterferences:interferences: metal chlorides are more volatile than sulfates ormetal chlorides are more volatile than sulfates or

phosphates.phosphates. * Addition of Cs salt to sample suppresses ionization.* Addition of Cs salt to sample suppresses ionization. * La precipitates phosphate, facilitating Ca analysis.* La precipitates phosphate, facilitating Ca analysis. * Proteins may clog burners and are precipitated with* Proteins may clog burners and are precipitated with trichloroacetic acid.trichloroacetic acid.

mmAA: Concentration giving rise to 1% absorption.: Concentration giving rise to 1% absorption.