1.1 Atomic Absorption Spectrometry (AAS)determination of elements not compoundsneeds radiation sourcehigh temperature for atomization

AtomizationFlameElectrothermal

1.2 Flame atomizer for solutions

1. Desolvation: solvent evaporates to produce solid aerosol2. Volatilization: form the gas molecules3. Dissociation: produce atomic gas4. {Ionization: ionize to form cations + electrons}5. {Excitation: excited by heat of flame, emission}

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

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

Flame structurea. Primary combustion zone:blue luminescence from emission of C2, CHcool {thermal equilibrium not achieved)initial decomposition, molecular fragments b. Interzonal region:hottest (several cm) most free atoms, wildly used partc. Secondary combustion zone:cooler conversion of atoms to molecular oxides {then disperse to the surroundings}

Flame temperaturesFuelOxidanttemperature (C)Natural gasAir1700 ~ 1900H2O22550 ~ 2700AcetyleneO23050 ~ 3000

Sensitive part of flame for AAS varies with analyte

Sensitivity varies with element Element rapidly oxides near burnerElement poorly oxidizes away from burnerOptimize burner position for each elementDifficult for multielement detection

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

Laminar flow burnerStable and quite flameLong path length for absorptionDisadvantages: short residence time in the flame (0.1 ms)low sensitivity (a large fraction of sample flows down the drain)Flashback

Flame atomizationSimplest atomization, needs preliminary sample treatment.Best for reproducibility (relative error

1.3Electrothermal atomization (Method of choice when flame atomization fails)Analyis of solutions as well as solidsThree stages: - dry at low temperature (120C, 20s)- ash at higher temperature(500-1000C, 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 providedExternal Ar gas prevents outside air from entering/incinerating tubeInternal Ar gas circulate the gaseous analyte

Output signals from graphite furnaceDrying Ashing (both from volatile absorbing species, smoke scattering)Atomize (used for analysis)

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

2.1 Radiation sourceEach element has narrow absorption lines (0.002-0.005nm), very selective.For a linear calibration curve (Beers 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

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 atomsFraction of sputtered atoms excited, then emit characteristic radiationCathode 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

Electrodeless discharge lamps

A few of Ar and small quantity of metal of interestEnergized by an internal radio-frequency or microwave radiationDischarged Ar+ excite the atoms of metal whose spectrum is soughtHigher intensities than hollow cathode lamp, but less relaiable

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

2.2 AA Spectrophotometers

- Single beam design

- Double beam design and lock-inamplifier

3.1 Spectral interference Absorption of interferant overlaps with that of analyteAbsorption or scattering by fuel/oxidant or sample matrixbackground 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,

3.2 Chemical interference (more common)1) Reactions of anions with analytes to form low volatile compoundreleasing agent: cations that react preferentially with interferant e.g.,Sr minimizes interference of phosphate with determination of Caprotective agent: form stable but volatile compounds with analytee.g., EDTA-metal formation supresses the interference of Al, Si, phosphate, sulfate in determination of Ca

2)Reverse atomization MO M + OM(OH)2 M + 2OH3) IonizationM M+ + e- ionization suppressor: B B+ + e-

1.Quantitative determination of > 60 metals or metalloids flameelectrothermaldetection limit0.001-0.002 pm2x10-6 -1 x10-5 ppmrelative 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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