The automated calibration and analysis of very low mixing ratios of the reactive volatile organic halocarbons CH3I, CHCl3, C2H5I, 2-C3H7I, CH2Br2, CH2ClI, CHBr2Cl, 1-C3H7I, CH2BrI, CHBr3 and CH2I2 for long term atmospheric field measurements are described. Analytes were pre-concentrated from 3 l of air onto an adsorbent trap cooled to 210 degreesC using Peltier plates, and rapidly transferred to a gas chromatograph ( GC) by resistive heating. A two stage Carboxen 1016/Carbotrap C adsorbent trap allowed good analyte recovery and rapid desorption without the need for post-desorption cryofocussing. Halocarbons were detected using a mass spectrometer ( MS) in selective ion mode. Detection limits were between 0.02 and 0.12 pptv ( parts per trillion by volume) for approximately hourly samples of CHCl3, CH3I, C2H5I, 1-C3H7I, 2-C3H7I, CH2ClI, CH2Br2, CHBr2Cl, CH2BrI, CHBr3 and CH2I2 with a precision of 3 - 8%. A novel calibration system was constructed which utilised fixed volume ( 50 mul) injections of the output of thermostatted permeation tubes into a stream of nitrogen gas in order to dilute parts per million by volume (ppmv) mixing ratios into pptv. The calibration was completely automated, allowing multi-point calibrations during routine operation. The overall accuracy of the measurements is estimated to be +/- 15%. The instrument was used continuously for automated atmospheric measurements during a 4-month research cruise from Germany to Antarctica, and a 6 week field campaign at Mace Head, Ireland. The results for CHCl3 during the latter campaign were within 13% of measurements made by a GC-MS operating continuously at the site within the long term Advanced Global Atmospherics Gases Experiment.
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