Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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 Excitation and Decay: Interplay of electron state and photons – decay from high energy state Absorption – excitation to high energy state Two fundamentally different ways of quantifying the concentration of an element! Emission Emission – flame provides energy to excite atoms Absorption – Gas phase atoms absorb photons from light source Narrow band! (unlike molecular absorption) Instrument Schematic Reference Beam Flame Lamp Chopper V ref V V sample t Light Source: Hollow Cathode Lamp Power Supply + - Cup made of metal of interest window anode light cathode 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 Atomization Atomic absorption and emission requires gasphase 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? 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. Flame Different temperatures required for different elements Air-Acetylene flame Preferred flame for 35 elements Temperature of 2300 C Nitrous Oxide-Acetylene flame Temperature Optimal of 2900 C temperature produces free gas phase atoms but doesn’t ________ ionize them Calibration ________ Beer’s 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 Getting a Concentration Flame Measures something proportional to the ______________ concentration of the sample Analysis in the mg/L range (ppm) Graphite Furnace Measures a _______ mass Analysis in the mg/L range (ppb)