PERFORMANCE EVALUATION OF AN AUTOMATED THREE-STAGE CRYOGENIC PRECONCENTRATOR MODULE FOR THE COMPOUND-SPECIFIC CARBON ISOTOPE ANALYSIS OF BENZENE, ISOPRENE , N-PENTANE, METHACROLEIN, METHYL VINYL CHLORIDE AND TOLUENE

摘要

The instrument specification on precision reported by manufacturers of gas chromatograph-combustion isotope ratio mass spectrometers (GC-C-IRMS) is generally 0.5%o. By comparing the conventional spit and splitless GC injection techniques to on-column injection a decrease in the standard deviation better than 0.3%o for the majority of analytes was reported. In this paper, we present precision values with a standard deviation as little as 0.05‰ measured within the recommended linear range of 1 to 10 nA of the IRMS by using the cooled injector system (CIS 4) (GERSTEL GmbH & Co. KG, Germany) operated in the solvent vent mode. The sample trapping and thermal desorption was achieved using the Thermo Desorption-Large system (TDSG-Large -GERSTEL GmbH & Co. KG, Germany) functioning as part of a fully automated cryogenic preconcentrator/focuser module (CP/F) geared at preconcentrating air volumes as little as 0.1 L and as large as 178 L. The compounds that were tested were benzene, isoprene, methacrolein (MACR), methyl vinyl ketone (MVK), n-pentane and toluene. In addition, the difference between on-line and off-line carbon isotope composition values was determined for all six compounds. The purpose of this work was to test the potential of the new module in terms of reproducibility and accuracy regarding isotope composition for sample volumes up to 178 L after optimizing water trapping temperatures, sample trapping and focusing temperatures, peak resolution between n-pentane and isoprene and the dynamic and static set points and times needed in the sample preparation process using the Gerstel TDSG Large Software (Version 0.5.3.5).