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1.1 Atomic Absorption Spectrometry (AAS) determination of elements not compounds needs radiation source high temperature for atomization Atomization a.

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Presentation on theme: "1.1 Atomic Absorption Spectrometry (AAS) determination of elements not compounds needs radiation source high temperature for atomization Atomization a."— Presentation transcript:

1 1.1 Atomic Absorption Spectrometry (AAS) determination of elements not compounds needs radiation source high temperature for atomization Atomization a. Flame b. Electrothermal

2 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}

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

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

5 Flame structure a. Primary combustion zone: blue luminescence from emission of C 2, 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 FuelOxidanttemperature (  C) Natural gasAir1700 ~ 1900 H 2 O ~ 2700 AcetyleneO ~ 3000

6 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

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

8 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

9 1.3Electrothermal 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(  C, 60s), removal of volatile hydroxides, sulfates, carbonates - atomize of remaining analyte at  C (ms~s) High sensitivity  less sample and longer residence time in optical path ( g analyte, uL sample, 2x x10 -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)

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

11 2.1 Radiation source Each element has narrow absorption lines ( nm), 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 < nm, difficult! Solutions: - LINE source at discrete wavelength, resonance line, using 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

12 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

13 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

14 Fig (p.238) Absorption of a resonance line by atoms

15 2.2 AA Spectrophotometers - Single beam design - Double beam design and lock-in amplifier

16 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 P ) - 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,

17 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 -

18 1.Quantitative determination of > 60 metals or metalloids flameelectrothermal detection limit pm2x 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)


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