The purpose of a double beam instrument is to help

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The purpose of a double beam instrument is to help compensate for drift of the output of the hollow cathode lamp or PMT. If the HCL output drifts slowly the subtraction process described immediately above will correct for this because both beams will drift equally on the time scale of the analysis. Likewise if the PMT response changes the double beam arrangement take this into account. Ignition, Flame conditions, and Shut Down The process of lighting the AAS flame involves first putting the optical cell in place and connecting the hydride gas transfer line. Next the fuel and the oxidant are turned on and then the flame is lit with the instrument's auto ignition system (a small flame or red-hot glow plug). After only a few minutes the flame is stable. Deionized water or a dilute acid solution can be aspirated between samples (but experimentation is required to ascertain what produces the best reproducibility). An aqueous solution with the correct amount of acid and no analyte is often used as the blank. To stabilize the HGAAS system the acidic blank is often flowed through the sample inlet tube for 5 or 10 minutes; al- though the longer this goes on, the more acidic waste is produced. Careful control of the fuel/air mixture is important because each element's response depends on successful decomposition of the volatile hydride in the heated optical cell. Remember that the flame's heat must break down the hydride and reproducibly create the elemental form of the analyte atom. Optimiza- tion is accomplished by aspirating a solution containing the element (with analyte content about that of the middle of the linear response range) and then adjusting the fuel/oxidant mix until the maxi- mum light absorbance is achieved. Also the position of the burned head, optical cell, and sample uptake rate are similarly "tuned." Most computer controlled systems can save variable settings so that methods for different elements can be easily saved and reloaded. Shut down involves aspirating deionized water through all three inlet tubes (borohydride, acid, and sample inlets) for a short period and then closing the fuel off first. Most modern instruments control the ignition and shutdown procedures automatically. The plastic tubing that is stretched around the peristaltic pump head is loosened to length its lifetime. Finally the purge gas is turned off. These notes were written by Dr. Thomas G. Chasteen ; Department of Chemistry, Sam Houston State University, Huntsville, Texas 77341. Copyright 2000. Unk Blank Std 3 Std 2 Std 1 50% HCl 0.35% NaBH 4 Volatile hydride Detector Hollow cathode lamp (HCL) Monochromator
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  • Fall '06
  • CHASTEEN
  • Atom, Light, hydride generation, Optical Cell

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