Atomic Absorption Spectroscopy (AAS) See also: Fundamental reviews in Analytical Chemistry

1. Atomic Absorption Spectroscopy (AAS) See also: Fundamental reviews in Analytical Chemistry e.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C. Anal. Chem.2002, 74, 2691-2712 (“Atomic Spectroscopy”) • 1802 Wollaston observes absorption lines in solar spectrum • 1914 Hollow cathode lamp • 1955 Walsh describes analytical AAS • 1959 1st Commercial Flame AAS • 1960s L’vov and Massman describe graphite furnace (commercial in 1970s) Recall: A = -log10(T)

2. Hollow Cathode Lamp • Typical primary source of radiation: Hollow cathode lamp • * Typically one lamp per element • * Different intensities for different elements • * Multielement lamps for multielement analysis • Continuum sources (e.g. Xe arc lamp) only for multielement analysis Kellner et al., Analytical Chemistry

3. Ideal Atomizer • Provide complete atomization of the element of interest • Atomic vapor should not be highly diluted by the atomizer gas • Excitation of the analyte (and other species) should be minimal

4. Flame AAS At <5000 K most atoms are predominantly in their electronic ground state. Slot burners with 5-10 cm path lengths. Kellner et al., Analytical Chemistry Ingle and Crouch, Spectrochemical Analysis

5. Electrothermal Atomization • Heating current of several hundred A • Heating rates of up to 1000 °C/s • LOD 100 times lower than flame AAS Heated in three stages: Ingle and Crouch, Spectrochemical Analysis

6. Electrothermal Atomization • Typical furnace material: Graphite •  Graphite Furnace AAS • Graphite tube 18-28 mm • Samples 5-100 uL • 200 to 1000 cycles • Temperature up to 3000 °C to avoid graphite decomposition • Carbon may be reducing agent for metal ions • Argon flow avoids oxidation • Other furnace materials: Ta, W, Pt • High melting point required • Should not emit brightly at high temperature (disadvantage for W and Ta)

7. Are you getting the concept? Is ICP a good source for AAS?

8. Double-Beam AAS Single-Beam I0 Double-Beam Isample Skoog Principles of Instrumental Analysis What is the problem with just measuring Isample/I0?

9. Background • Sources of Background: scattering or molecular emission • Background Correction: • With blank sample • Ac = At - Ablank • Deuterium lamp (arc in deuterium atmosphere; • continuum 200-380 nm) •  absorption of deuterium lamp represents Abackground •  absorption of HCL radiation represents At • Advantage over blank sample: observe fluctuations in flame Culver et al, Anal. Chem., 47, 920, 1975.

10. Background Correction with Zeeman Effect Lines differ by ~0.01 nm Ingle and Crouch, Spectrochemical Analysis

11. Background Correction with Zeeman Effect • Unpolarized light from HCL (A) passes through the rotating polarizer (B) • Light is separated into perpendicular and parallel components (C) • The light enters the furnace with an applied magnetic field, producing • 3 absorption peaks (D) • Either analyte or analyte + matrix absorb light (E) • A cyclical absorbance pattern results (F) • Subtract absorbance during perpendicular half of cycle from absorbance • during parallel half of cycle to get the background corrected value Skoog, Principles of Instrumental Analysis

12. Background Correction with Zeeman Effect DC on atomizer AC on atomizer DC on source Ingle and Crouch, Spectrochemical Analysis

13. AAS: Figures of Merit • Linearity over 2 to 3 concentration decades (can be problem for • multielement analysis) • Probability for line overlap is small  Resolution not as critical • as for AES • Precision: Typically a few % for graphite furnace • 0.3 to 1.0% for flame • Accuracy: Largely determined by calibration with standards • Applicability: Limited for certain elements for which flame or • furnace is not hot enough (e.g. W, Ta, Nb). • Flow rates of flame are compatible with HPLC flow rates. • Speed: Multielement analysis with multiple HCLs may require lamp exchanges to select desired elements. This is tedious and also costs light because of beam splitters.

14. Method of Standard Additions • In order to quantitate the element of interest in a sample, it is necessary to calibrate with the method of standard additions. • The analytical signal for the sample, Sx, is obtained (after measuring the blank signal). • A small volume, Vs, of a concentrated standard solution of known concentration, cs, is added to a relatively large volume, Vx of the analytical sample. • The analytical signal for the standard addition solution, Sx+s, is obtained. cx = (SxVscs)/[Sx+s(Vx+Vs) – SxVx] if Vs << Vx cx = (SxVscs)/[(Sx+s – Sx)Vx]

15. Are you getting the concept? 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 0.420 was obtained. To the original solution, 20 mL of a 10.0 mg/mL Pb 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 made in order to use a single standard addition?

16. AAS: Figures of Merit • Detection limits:* Generally lower LOD for very volatile elements • * Higher LOD for carbide-forming elements (e.g. Ba, B, Ca, Mo, W, V, Zr) • * Concentration in GF up to 1000 times higher than in flame; much lower LOD for GF. • * Lower LOD for GF-AAS than ICP-AES unless • atomization requires high temperature • * Generally similar LOD for flame-AAS and ICP-AES • * Generally higher LOD for very volatile elements • Chemical * HCl often avoided as acid in GF-AAS because • interferences:metal chlorides are more volatile than sulfates or • phosphates. • * Addition of Cs salt to sample suppresses ionization. • * La precipitates phosphate, facilitating Ca analysis. • * Proteins may clog burners and are precipitated with • trichloroacetic acid.

17. mA: Concentration giving rise to 1% absorption.