Measurement of Hydrofluorocarbons, Perfluorocarbons and Chlorofluorocarbons in Ambient Air by Gas Chromatography and Mass Spectrometry

摘要

Since measurements at concentrations near detection levels are subject to some uncertainties, measurement conditions for ambient levels target halogenates were enhanced as much as possible to achieve signals well above their detection limits to provide a good assessment of their concentrations. These actions included: 1. Trapping with multi-layer hydrophobic adsorbents at -48 °C to effectively snare tetrafluoromethane and others, and allow oxygen, methane and carbon dioxide to pass through to vent, 2. Cryofocusing at -195 °C to sharpen injection of tetrafluoromethane and others into the analytical column, 3. Inserting a Nafion dryer to remove sample water prior to injection to avoid trap plugging at the required subzero trap settings and the resulting loss of proper loading of a sample aliquot, 4. Activating Extended Dynamic Range to automatically enhance detection of peaks with low responses as well as monitoring ones with major signals, 5. Using of Selected Ions Monitoring to both confirm peak identities and calculate concentrations and to dramatically lower measurement noise by eliminating nonessential mass ions, 6. Choosing several quantifier ions combined to enhance peak sizes for low concentration analytes, as long as added signal noise does not degrade peak area assignments, 7. And installing an in-line sample pressure regulator to properly load standards from pressurized vessels for direct comparison with samples at different pressures, such as real-time sampling of ambient air at atmospheric pressure.