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Atomic-Absorption (AA) Spectroscopy

Atomic-Absorption (AA) Spectroscopy. Overview. Excitation and Decay: Interplay of electron state and photons Emission Absorption Instrument Components Double beam configuration Light source Production of free atoms Graphite furnace Flame Detector.

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Atomic-Absorption (AA) Spectroscopy

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  1. Atomic-Absorption (AA) Spectroscopy

  2. Overview • Excitation and Decay: Interplay of electron state and photons • Emission • Absorption • Instrument Components • Double beam configuration • Light source • Production of free atoms • Graphite furnace • Flame • Detector

  3. Excitation and Decay: Interplay of electron state and photons • Emission – decay from high energy state • Absorption – excitation to high energy state • Two fundamentally different ways of quantifying the concentration of an element! • Emission – flame provides energy to excite atoms • Absorption – Gas phase atoms absorb photons from light source • Narrow band! (unlike molecular absorption)

  4. Flame V Chopper t Instrument Schematic Reference Beam Beam Recombiner Monochromator Electronics Detector Lamp V ref V sample

  5. Light Source: Hollow Cathode Lamp Cup made of metal of interest • The electric potential ionizes rare gas atoms and accelerates them into the cathode where they sputter metal atoms into the gas phase • Collisions with gas atoms or electrons excite the metal atoms • On decay the metal atoms emit light Power Supply window anode + light - cathode

  6. Atomization • Atomic absorption and emission requires gas-phase atoms • Liquid and solid samples must be vaporized • Flame • Graphite furnace • The gas-phase atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. • What about the inevitable subsequent emission of photons on decay?

  7. Detector • Photo multiplier tube • Extremely sensitive • Can detect single photons • Produce a small current that is proportional to the number of photons • Op-amps produce a voltage that is then digitized for analysis.

  8. Flame • Different temperatures required for different elements • Air-Acetylene flame • Preferred flame for 35 elements • Temperature of 2300 C • Nitrous Oxide-Acetylene flame • Temperature of 2900 C • Optimal temperature produces free gas phase atoms but doesn’t ________ them ionize

  9. Calibration • ________ Law • Non linearity (according to Perkin Elmer) • Stray light • Non homogeneities of temperature and space in the absorbing cell • Line broadening • Absorption at nearby lines • What else could be causing non linearity? Beer’s Correct for this by subtracting Vdark What is the physics? emission

  10. Getting a Concentration • Flame • Measures absorbance proportional to the ______________ of the sample • Analysis in the mg/L range (ppm) • Graphite Furnace • Measures an absorbance peak with an area proportional to _______ of the sample • Analysis in the mg/L range (ppb) concentration mass

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