AUTOMATED IN-SITU GAS-CHROMATOGRAPHIC MASS-SPECTROMETRIC ANALYSIS OF PPT LEVEL VOLATILE ORGANIC TRACE GASES USING MULTISTAGE SOLID-ADSORBENT TRAPPING

A fully automated sampling/injection system for the gas chromatographi c-mass spectrometric (GC-MS) analysis of volatile organic compounds at tropospheric background levels was developed. Organic trace gases fro m air samples up to 100 l were trapped in temperature-controlled solid -adsorbent traps. The instrument utilized a one stage sampling/desorpt ion inlet system designed as a closed device that did not require any replumbing steps between sampling and analysis. For analysis, the adso rbent trap was thermally desorbed and backflushed onto the chromatogra phic column where organic trace gases were directly cryofocused. Adsor bents tested were Carbotrap, Carbotrap C, Carbosieve S III, Tenax TA, Tenax GR and multistage combinations of these adsorbents. Interference s from blanks and adsorbent artifacts were minimized by backflushing t he adsorbent trap, switching valve and transfer line between sampling sequences at temperatures above the levels used during trapping and sa mple transfer. The high sample volumes that could be concentrated and the low background levels allowed structural identification in GC-MS ( electron impact ionization scan mode) at minimum levels of ca. 100 pg per peak (ca. 10-(12) mol) equivalent to atmospheric concentrations at the lower parts per trillion (v/v) level. The volatility range of org anic compounds that could be identified was approximately from pentane to pentadecane. The system was completely computer controlled and cou ld be operated continuously and unattended around the clock for in sit u analysis. The instrument was successfully deployed at the Mauna Loa Observatory, Hawaii, USA in July and August 1992.