Frequency 2 pressure broadening atomic collisions

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Frequency 2. Pressure broadening (atomic collisions increase width of lines) • G.S. energy level perturbed Low interferences, so simultaneous detection of many elements is possible
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However, AAS is a destructive analysis method: ð The sample is “burned” at very high temperatures (2000˚ - 3000˚) in order to atomize the elements. Instrumentation Overview The flame converts the sample to atomic vapor and the atoms absorb light produced from the Hollow cathode lamp. The sample cell can be either the burner flame or a graphite furnace! The monochromator allows the selection of the wavelength of light that reaches the detector. There is no reference cell, the resonance line light is modulated using a mechanical chopper which alternately blocks the light beam reaching the detector. Thus the burner flame light reaching the detector can be compensated. Two methods used in atomic spectroscopy. i) Atomic absorption spectroscopy (AAS) AA Instrumentation: 1) hollow-cathode lamp 2) monochromator (wavelength selection ~ di ff raction grating) 3) flame or graphite furnace(sample cell) 4) Beam chopper {not shown (modulation)} 5) detector (typically a PMT) 6) read out display A working calibration curve involving standards is used to determine analyte concentrations.
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The AAS light source: Hollow cathode Lamp: Since atomic absorption is a Quantized process, we need light at specific narrow line wavelength. Therefore, the HC lamp must contain the element we are trying to determine. The lamp consists of inert gas, an anode and a cathode sealed in quartz glass. The gas is ionized, which collide with the metal and produce a sputtering e ff ect. These excited metal atoms give the light energy, which the sample analyte can absorb. AA sample cells: 1) Flame burner head cell path— A liquid sample is introduced into an atomizer whose temperature is 2000–3000K. Sample, oxidant, and fuel are combined and nebulized –broken into small droplets Droplets entering the flame lose their water through evaporation; then the remaining sample vaporizes and decomposes into atoms The most common fuel-oxidizer combination is acetylene and air (flame temp: 2400– 2700K) Oxidant Fuel Air O 2 N 2 O
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