Chap9atomabsspec
- 1. Chapter 9: Atomic Absorption SpectrometryChapter 9: Atomic Absorption Spectrometry Read: pp. 230 – 249 Problems: 9-1,3,5,6,8 Figure 9-13a A(λ) = ε(λ)bC = log Po/P
- 2. FlameFlame--BurnerBurner In AAS, the flame functions as (i) sample holder, (ii) desolvation source, and (iii) volatilization source. Figure 9-5
- 4. Flame StructureFlame Structure Primary zone: C2, CH, and other radical emission Secondary zone: oxygen present so stable molecular oxides are formed for some metals Interzonal regions: hot region, most widely used for analysis Figure 9-2 hν
- 5. Optimum analysis position in the flame depends on the particular element and its chemistry: Figure 9-4
- 6. Line SourceLine Source –– Hollow Cathode LampHollow Cathode Lamp Figure 9-11 Cathode material made of the element of interest, e.g. Na HCL for the analysis of Na. An individual lamp is needed for each element. So AAS is a one-element-at-a-time measurement!
- 7. Desired line of source is selected with monochromator: Figure 9-10
- 8. MonochromatorMonochromator –– Wavelength SeparatorWavelength Separator Figure 7-21 nλ = d(sin i + sin r) R = λ/∆λ = nN N = grooves/mm Properties: light gathering power, stray light rejection, resolution, and linear dispersion
- 9. Optical DetectorsOptical Detectors Single vs. multichannel detectors! S = kP + kd Figure 7-31b Photomultiplier tubePhotomultiplier tube
- 13. Typical Figures of Merit for AASTypical Figures of Merit for AAS • Detection limits: ng/mL (ppb) for flame pg/mL (ppt) for electrothermal • Linear range: 103 – 104 for flame 102 for electrothermal • Precision: 1 – 2% RSD for flame 5 – 10% RSD for electrothermal • Accuracy: 1 – 2% for flame % for flame Remember: mg/L = ppm ug/L = ppb ng/L = ppt
- 14. Chemical ProblemChemical Problem Typical data for the determination of lead (Pb2+) by graphite furnace AAS in standards was 0.05 µg/mL (0.09 AU), 0.1 µg/mL (0.16 AU), 0.2 µg/mL (0.31 AU) and in a sample of canned orange juice was 0.10 AU. Assume that these absorbance data were obtained for 2 µL aliquots of standards and sample. Calculate the concentration of lead in the orange juice sample. Figure 9-7
- 15. Calibration Curve for PbCalibration Curve for Pb2+2+ y = 1.4714x + 0.015 R 2 = 0.9997 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.00 0.05 0.10 0.15 0.20 0.25 CONCENTRATION (µg/mL) ABSORBANCE
- 16. Calibration equation: A = 1.4714 C + 0.015 Orange juice sample: A = 0.10 + µ= − = − = 2 PbmL/g058.0 4714.1 015.010.0 4714.1 015.0A C
- 17. Spectral and Chemical InterferencesSpectral and Chemical Interferences Remember: Goal is neutral atoms in the gas phase!Remember: Goal is neutral atoms in the gas phase! • Absorption or emission of an interfering species overlaps or lies so close to the analyte absorption or emission that resolution is not possible. Rare with HCLs. • Presence of combustion products that exhibit broadband absorption or particulates that scatter radiation. Both diminish power of transmitted beam and lead to positive errors. If caused by fuel/oxidant mixture, then correction is possible by running a blank and performing background subtraction. More troublesome problem when absorption or scattering results from the sample matrix.
- 18. • Interference by anions that form low volatility complexes with the analyte, and thus reduce the atoms formed. Lead to negative errors. Can be corrected by – Releasing agents (cations added to preferentially react) – Protecting agents (e.g., EDTA added to protect analyte cation) • Dissociation equilibria MO ↔ M + O M(OH)2 ↔ M + 2 OH • Ionization equilibria M ↔ M+ + e– In both cases, analyte atoms are not all in the proper form to absorb or emit at desired wavelength. Lead to negative errors.
- 19. Terms to Know!!! ablation chemical interference matrix aerosol detection limit nebulization atomic absorption Doppler effect releasing agent atomic emission graphite furnace self-absorption atomic fluorescence hollow cathode lamp spectral interference atomization ionization interference ionization suppressor background correction Boltzman distribution