Understanding Atomic Absorption Spectrometry (AAS)

1. HW # 6 Chapter 8 # 4, 8 Chapter 9 # 1, 3, 14 (for Na, use average of 5893 Å for 3s3p; for Mg+, use average of 2800Å for 3s  3p), 17, 22 (review example 1-1, use eqs. a1-31, a1-32, a1-36 for Sxx, Syy, Sy)

2. Chapter 9 Atomic Absorption Spectrometry (AAS) 1 Sample Atomization 1.1 Atomic Absorption Spectrometry (AAS) • determination of elements not compounds • needs radiation source • high temperature for atomization Atomization • Flame • Electrothermal

3. 1.2 Flame atomizer for solutions 1. Desolvation: solvent evaporates to produce solid aerosol 2. Volatilization: form the gas molecules 3. Dissociation: produce atomic gas 4. {Ionization: ionize to form cations + electrons} 5. {Excitation: excited by heat of flame, emission}

4. Fig. 9-1 (p.231) Processes occurring during atomization Fig. 8-9 (p.225) Samples are introduced into flames by a nebulizer

5. Fig. 9-3 (p.232) Temperature (c) profile for a natural gas-air flame Fig. 9-2 (p.231) Regions in a flame

6. Flame structure a. Primary combustion zone: blue luminescence from emission of C2, CH cool {thermal equilibrium not achieved) initial decomposition, molecular fragments b. Interzonal region: hottest (several cm) most free atoms, wildly used part c. Secondary combustion zone: cooler conversion of atoms to molecular oxides {then disperse to the surroundings} Flame temperatures Fuel Oxidant temperature (C) Natural gas Air 1700 ~ 1900 H2 O2 2550 ~ 2700 Acetylene O2 3050 ~ 3000

7. Sensitive part of flame for AAS varies with analyte • Sensitivity varies with element Element rapidly oxides – near burner Element poorly oxidizes – away from burner • Optimize burner position for each element • Difficult for multielement detection

8. Fig. 9-5 (p.233) A laminar-flow burner

9. Laminar flow burner • Stable and quite flame • Long path length for absorption • Disadvantages: short residence time in the flame (0.1 ms) low sensitivity (a large fraction of sample flows down the drain) Flashback Flame atomization • Simplest atomization, needs preliminary sample treatment. • Best for reproducibility (relative error <1%) • Relatively intensive – incomplete volatilization, short time in beam

10. 1.3 Electrothermal atomization(Method of choice when flame atomization fails) • Analyis of solutions as well as solids • Three stages: - dry at low temperature (120C, 20s) - ash at higher temperature(500-1000C, 60s), removal of volatile hydroxides, sulfates, carbonates - atomize of remaining analyte at 2000-3000 C (ms~s) • High sensitivity  less sample and longer residence time in optical path (10-10 -10-13 g analyte, 0.5-10uL sample, 2x10-6 -1x10-5 ppm) • Less reproducible (relative precision 5-10%) • Slow (several minutes for each element) • Narrow dynamic range Two inert gas stream are provided • External Ar gas prevents outside air from entering/incinerating tube • Internal Ar gas circulate the gaseous analyte Output signals from graphite furnace • Drying • Ashing (both from volatile absorbing species, smoke scattering) • Atomize (used for analysis)

11. Fig. 9-6 (p.234) Graphite furnace electrothermal atomizer Fig. 9-7 (p.235) Typical output from electrothermal atomizer

12. 2 Instrumentation 2.1 Radiation source • Each element has narrow absorption lines (0.002-0.005nm), very selective. • For a linear calibration curve (Beer’s law), source bandwidth should be narrower than the width of an absorption line. - continuum radiation source requests a monochromator with eff < 10-4 nm, difficult! • Solutions: - LINE source at discrete wavelength, resonance line, using 589.6 nm emission line of sodium as a source to probe Na in analyte - operate line source with bandwidth narrower than the absorption line width minimize the Doppler broadening lower temperature and pressure than atomizer

13. Hollow cathode lamp • Electric discharge (300V) of Ar between tungsten anode and a cylindrical metal cathode in a sealed glass tube filled with Ar (1-5 ) • Ar+ bombard cathode and sputter cathode atoms • Fraction of sputtered atoms excited, then emit characteristic radiation • Cathode made of metal of interest (Na, Ca, K, Fe,.. or mixture of several metals)  give intense narrow line source of cathode material Hollow cathode design: Concentrate radiation in limited region; Enhance the probability of redeposition on cathode

14. Electrodeless discharge lamps A few  of Ar and small quantity of metal of interest Energized by an internal radio-frequency or microwave radiation Discharged Ar+ excite the atoms of metal whose spectrum is sought Higher intensities than hollow cathode lamp, but less relaiable

15. Fig. 9-10 (p.238) Absorption of a resonance line by atoms

16. 2.2 AA Spectrophotometers - Single beam design - Double beam design and lock-in amplifier

17. 3 Interferences in AAS 3.1 Spectral interference • Absorption of interferant overlaps with that of analyte • Absorption or scattering by fuel/oxidant or sample matrix background should be corrected for (reading assignment P241-244) • Emission of radiation from flame at the same wavelength of AA lock in amplifier, modulate the real atomic absorption at known frequency using a lock-in amplifier,

18. 3.2 Chemical interference (more common) 1) Reactions of anions with analytes to form low volatile compound releasing agent: cations that react preferentially with interferant e.g.,Sr minimizes interference of phosphate with determination of Ca protective agent: form stable but volatile compounds with analyte e.g., EDTA-metal formation supresses the interference of Al, Si, phosphate, sulfate in determination of Ca 2) Reverse atomization MO  M + O M(OH)2  M + 2OH 3) Ionization M  M+ + e- ionization suppressor: B  B+ + e-

19. 4 Quantitative Application 1. Quantitative determination of > 60 metals or metalloids flame electrothermal detection limit 0.001-0.002 pm 2x10-6 -1 x10-5 ppm relative error 1-2% 5-10% 2. Less suitable for weaker absorbers (forbidden transitions) non-metals (absorb in VUV) metal in low IP (alkali metals)