AUTOMATED HEADSPACE ANALYSIS OF FUMIGANTS 1,3-DICHLOROPROPENE AND METHYL ISOTHIOCYANATE ON CHARCOAL SAMPLING TUBES

Charcoal tubes are widely used for collecting organic vapor in the atmosphere, and the measurement is usually completed by analyzing an aliquot of the solvent phase following solvent extraction, typically with carbon disulfide. However, the sensitivity of this method is limited and sometimes too low for monitoring contaminants at trace levels in the environmental atmosphere. In this study, the potential of static headspace analysis techniques was explored on two common fumigants, 1,3-dichloropropene (1,3-DCP) and methyl isothiocyanate (MITC), on both coconut- and petroleum-based charcoal sampling tubes, using an automated and programmable headspace sampler. Three important parameters in the headspace analysis, equilibrating temperature and time, and amount of extracting solvent, were optimized individually for each compound-charcoal tube combination to achieve maximum sensitivity of GC analysis. Higher stability was observed for both isomers of 1,3-DCP and MITC on petroleum-based charcoal, and 180 and 190 degrees C, and 5 min were selected as the equilibrating temperatures and time, respectively. On coconut-based charcoal tubes, however, all the compounds were more sensitive to the temperature, and 160 and 140 degrees C, and 5.0 and 3.0 min were therefore determined as the equilibrating temperatures and times for the 1,3-DCP isomers and MITC, respectively. Reducing solvent volume from 3 to 1 ml in 9-ml headspace vials improved the sensitivity and 1.0 ml benzyl alcohol was therefore selected for all the compound-charcoal tube combinations. Compared to the conventional extraction method with CS2, the optimized headspace methods were 10-35 times more sensitive, and equivalently reproducible except for MITC on coconut-based ORBO-32 tubes. Better sensitivity and precision of measurements were consistently obtained on petroleum-based charcoal tubes, and the minimum detection limits were estimated as 0.2 and 0.5 ng per tube for the (Z)- and (E)-isomers of 1,3-DCP, respectively, and 2.0 ng per tube for MITC. With the automated headspace method, sample preparation was simplified and sample throughput was greatly enhanced, and up to 200 samples could be analyzed on a 24-h basis under the optimum conditions.