An automated gas chromatographic (autoGC) system was used to collect and analyze both nonpolar and polar volatile organic compounds (VOCs) in ambient air. This system combines the use of dual multiadsorbent traps to provide continuous air sampling for 57 min of each hour; a dry helium purge to remove extraneous gases, including some residual water vapor retained in the sorbent packing; thermal desorption of analytes onto a VOC-focusing trap cooled by a small Stirling-cycle refrigerator; and GC/mass spectrometric detection using ion trap technology. Cleanliness, linearity, method detection limits (MDLs), precision, and accuracy of the autoGC were determined for 41 VOCs. For most of the compounds tested, MDLs were less than 0.10 ppbv, response was linear over the 1-40 ppbv range, accuracy was (20%, and trap-to-trap precision was (20%. Linear response for a set of polar VOCs was also determined over the 5-50 ppbv range. The autoGC was successfully operated in a mobile laboratory at a field site in Axis, AL, for 10 days, during which time the system was in operation 24 h/day with minimal interruptions. This autoGC is designed for monitoring subsets of the 97 VOCs among the 189 hazardous organic compounds that are listed in Title III of the Clean Air Act Amendments of 1990.

A carefully designed study was conducted during the summer of 1998 to collect samples of ambient air by canisters and compare the analysis results to direct sorbent preconcentration results taken at the time of sample collection. Thirty-two 1 h sample sets were taken, each composed of a "near-real-time" sample analyzed by an autoGC-MS XonTech 930/Varian Saturn 2000 system, and Summa and Silco canisters. Hourly total non-methane organic carbon (TNMOC), ozone, and meteorological measurements were also made. Each canister was analyzed on the autoGC-MS system for a target list of 108 volatile organic compounds (VOCs) and on a manual cryosampling GC-FID system. Comparisons were made between the collection and analysis methods. Because of the low sample loading (150-250 ppbC TNMOC), these comparisons were a stringent test of sample collection and analysis capabilities. The following specific conclusions may be drawn from this study. Reasonable precision (within 15% mean difference of duplicate analyses from the same canister) can be obtained for analyses of target VOCs at low-ppbC concentrations. Relative accuracy between the GC-MS and GC-FID analysis methods is excellent, as demonstrated by comparisons of analyses of the same canisters, if measurements are sufficiently above the detection limits. This is especially significant as the GC-MS and GC-FID were independently calibrated. While statistically significant differences may be observed between the results from canister and near-real-time samples, the differences were generally small and there were clear correlations between the canister results and the near-real-time results. Canister cleanliness limits detection below the EPA Method TO-14 acceptance standard of 0.2 ppbv (0.2-2 ppbC for target analytes).

No abstract

Abstract. Ambient gaseous organic compounds were monitored on an hourly basis with an automated gas chromatograph from June 15 to 27, 1995, at the New Hendersonville site near Nashville, Tennessee, as part of the Southern Oxidants Study (SOS). These compounds and their estimated average contributions to the total were as follows: isoprene (2.1 ppbC), methyl vinyl ketone (1.1 ppbC), methacrolein (0.6 ppbC), c•-pinene plus/3-pinene (1.3 ppbC), and those nonmethane hydrocarbons, minus isoprene, monitored routinely in the Photochemical Assessment Monitoring Stations (44.6 ppbC). The sum of n-aldehydes from butanal through nonanal averaged 12.6 ppbC, where a conservative estimate of artifact n-aldehydes has been subtracted. Strong diurnal variations in the concentrations of n-aldehydes and the apparent influence of wind shifts on concentration support the existence of n-aldehydes at significant levels in the ambient air. The empirical fit to data as developed by Jobson et al. Because of the sample conditioning required to prepare air samples for gas chromatographic separation, the specific design of an autoGC is critical to preserving sample integrity of many VOCs. The sample conditioning involves several features, including (1) transport of VOCs through a manifold, (2) filtering of particulate matter, (3) ozone scrubbing to prevent reactions of ozone with cocollected VOCs, or system components leading to artifact formation, (4) water management, and (5) the method of concentrating VOCs. The transport manifold is usually heated to prevent water vapor condensation and to reduce adsorption of target gases, and various types of inlet filters are used to exclude particulate matter from entering the main air manifold. Different approaches to the use and type of ozone scrubbers is a topic area that has recently been reviewed [Helmig, 1997], although one of the conclusions is that exper-22,509