10Lines are broadened by two effects: DopplerCollisionalOperating conditions for lamp are chosen so that the Doppler broadening in the lamp (low P, few collisions) is less than the Doppler and collisional broadening in the flame or furnace.
14Absorption lines are narrow The selected bandwidth of light from source must be narrower than chosen absorption lineIf a monochromator was able to select a narrow enough bandwidth from the output of a deuterium lamp, the power of the light would be negligibleTherefore lamps that emit spectral lines are used
17HCL Inert gas is ionized by discharge Is accelerated to cathode Causes some element to dislodge and form atomic cloud (sputtering)Some are excited (in collisions with ions) and emit line spectra.Usually lamps are for one element – but can be for as many as six.
19Electrodeless Discharge Lamp Microwave excited discharge tubesIntensities x greater than from HCLSmall amount of element or halide of an element in a small sealed tube containing a few torr of inert gasPlaced in microwave cavity (2450 MHz)Argon is ionized, the ions are accelerated and excite the metal atomsLess stable than HCL, but more intense.Not available for all elements
20High-resolution continuum source AAS: the better way to perform atomic absorption spectrometry
21Single xenon arc lampToday, multiple hollow cathode lamps are no longer used.With the use of a single xenon arc lamp, all the elements can be measured from nm.This takes AAS into a true multi-element technique with the analysis of 10 elements per minute.
22CCD technology - For the first time in an AAS CCD chips are now available with 200 pixels which act as independent detectors.Simultaneous background correction - Background is now measured simultaneously compared to sequential background on conventional AAS.Better detection limits - Due to the high intensity of the Xenon Lamp there is better signal/noise ratio thus giving better detection limits. In some cases it is up to 10 times better than conventional AAS.
26Flame Stable Safe Cheap to maintain High temperature Reducing Atmosphere - many metals form stable oxides, not easily atomized just by flame temperatures
27Flame Typical system – spray chamber and burner Sample is aspirated into spray chamber using nebulizer (sucked in by Venturi effect)Produces aerosol.Aerosol strikes obstruction – spoiler – to break it into smaller dropsOnly smallest drops proceed to flameLarger drops go down drain
29Sequence of Events in Flame Evaporation of Solvent (leaving fine salt particles suspended in flame)Loss of water of hydrationVaporization of solid particles to free atoms (due to heat and chemical reaction)ExcitationIonization (not always desirable)
30Nebulization Controls fraction of sample to reach flame Drop size is governed by viscosity, surface tension, gas flow, density, design of nebulizerOrganic solvents have lower viscosity and lower surface tension than water ( x) They also allow preconcentrationBut change flame conditions – not always so beneficialSalt increases viscosity, decreasing efficiencyThe smaller the drop, the more easily it is desolvated and vaporized
31Ultrasonic breakup of drops High frequency vibrationsUniform and controllable drop sizeBUTDrops are larger and equipment more expensive
32Desolvation Critical to number of free atoms Usually occurs at base of flameSolvent then water of crystallizationDepends on droplet size and solvent
33Vaporization Atomization to free atoms Ideally – want high temperature and long residence time (slow burn rate of the gases) - lots of time for atomizationDepends on nature of molecules and atomsAl2O3 atomizes more slowly than NaCl particle of the same sizeImportant if analyzing mixtures – different conditions are needed for different atoms
34Ionization Ions undergo different transitions than atoms Want one or the otherIons not desirable in flame methodIn ICP, ions are the desired speciesFor alkali and alkaline earths, ions form above 2000K
35Ionization Increases at low sample concentration With increasing flame temperatureWith decreasing ionization potentialPrevent by:Low flame temperatureExcess of easily ionizable metal eg LiCalled a SUPPRESSOREg: add lots of Li to solution to be analyzed for K
36Premix (Laminar flow) Burner Most commonGases premixed before entering burnerStable flameUse long narrow flames – long path length for light absorptionUse at right angles for emissionSmall (narrow) flame keeps atom concentration high
37Width of slot depends on gases Narrow slots prevent flame backing into mixing chamber and causing explosionBut must allow enough gas through to support rate of burningToo narrow:Cooling by adjacent airSalt deposition clogs burnerUse different burners for different gases
38Gases Gas mixtures with high-burning velocities are less safe Also want long residence timesC2H2-N2O (220cm/s) is better than C2H2-O2 (1130 cm/s). They have similar flame temperatures.
45SpectralMg nmNa nmNot usually much of a problem – can change to another wavelengthProblem worse in emission because more lines – High T – lots of excitationChoice of line dictates concentration rangeable to be analyzed
47Vaporization Interferences When one component of a sample influences the rate of vaporization of the species of interestPhysical – changes matrix it vaporizes fromChemical – changes the species to be vaporized
48Chemical Vaporization Interferences Metal oxides formMetal ions form thermally stable complexes with anionsThe effects usually occur during formation of the solid particleCaPO4 formation – a well known example.CaPO4 is harder to vaporize than Ca2+
49CaPO4 - Interference Prevention Put light path higher in flame to allow a longer residence timeAdd releasing agent – La2+ or Sr2+ (added in excess) will preferentially combine with PO43- and leave Ca2+ free to be analyzedProtective agent – add EDTA. Ca-EDTA complex is easily destroyed in flameGlucose – burns easily and helps droplets shatter apartHotter flame – then need ionization suppressor