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Unformatted text preview: Chapter 8: An Introduction to Optical Atomic Spectrometry Optical Atomic Spectra Atomization Methods Sample Introduction Methods - Optical Spectrometry as opposed to MS - Optics are similar to Molecular Absorption and Fluorescence Spectrometry Basics - Generally solution samples (sometimes solids) - Aspirate sample into flame or plasma - Or heat sample to atomize - Observe emission from excited state (AE) - Or use light source to measure atomic absorption (AA) - Or use light source to generate atomic fluorescence (AF) Atomic spectra are line spectra Some prominent lines in the atomic spectrum of mercury (Hg) Other atomic spectra many lines per spectra, lines are very narrow in Absorption & emission lines come from discrete transitions in Absorption & emission lines come from discrete transitions Resonance fluorescence lines Others involve the ground involve a two state step process Atomic line widths are important when considering spectral resolution, absorption experiments, etc. Doppler broadening occurs due to the observed compression and expansion of light waves as an atom moves toward or away from the radiation source Other broadening effects include - pressure or collisional broadening (10-4 ) - broadening from uncertainty effects - electric & magnetic field effects Temperature effects from Boltzmann equation Pj Ej Nj ----- = ------ exp - -----Po kT No
N = population of state P = statistical factor Ej = energy difference k = Boltzmann constant T = temp o & j signify ground & excited states Molecular emission in flames & plasmas is generally unwanted may obscure part of the spectrum & cause interference Note similarity of spectra Hollow Cathode Lamp (HCL) and Electrodeless Discharge Lamp (EDL) These lamps produce only the spectral output from the element of interest and therefore are ideally suited to atomic absorption experiments The sample must be converted to its atomic state which can be done at high temp. in a flame, furnace, plasma, arc or spark Introducing the sample to the high temperature source can be tricky Pneumatic nebulizers a) concentric tube b) cross-flow c) fritted disk d) Babington Sample introduction for solutions: 1) Pneumatic nebulizers 2) Ultrasonic nebulizers 3) Electrothermal vaporizers 4) Hydride generation 3 BH4- + 3 H+ + 4 H3AsO3 3 H3BO3 + 4 AsH3 + 3 H2O 5) Cold vapor generation Hgo + Sn4+ Hg2+ + Sn2+ Solid sample introduction: 1) Laser ablation zap sample with laser and sweep atoms into flame/plasma 2) Direct sample insertion place sample directly in atomizer (e.g. furnace) 3) Electrothermal atomizers electrically heat graphite or tantalum boat 4) Arc/Spark ablation coat sample on electrode or place in electrode well or cup 5) Glow discharge technique see p 204 Chapter 9: Atomic Absorption & Atomic Fluorescence Spectrometry Sample Atomization Atomic Absorption (AA) Atomic Fluorescence (AF) - Both AA and AF require a light source - Like Molecular Absorption & Fluorescence, in AA high intensity is NOT required, in AF high intensity results in greater sensitivity occurs in flame, plasma, etc. Molecular emission is generally unwanted & can cause interference AA AE AE They're HOT ! AA Slot Burner and Flame ...
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This note was uploaded on 02/13/2012 for the course UML 84.314 taught by Professor Ryan during the Fall '11 term at UMass Lowell.
- Fall '11