WATER MONITORING-SYSTEM BASED ON GAS EXTRACTION WITH A SINGLE HOLLOW FIBER MEMBRANE AND GAS-CHROMATOGRAPHIC CRYOTRAPPING

A single, 8.0-cm-long, hollow fiber membrane was used to quantitatively remove 1,1,1-trichloroethane, trichloroethene, and tetrachloroethene from water. The volumetric flow rate of the aqueous sample was below 0.02 mL/min to facilitate exhaustive extraction which results in good precision and accuracy of the determinations. The method involves pumping water containing volatile organic compounds (VOCs) through a single hollow fiber while an inert gas flows countercurrently around the exterior of the fiber. The VOCs diffuse from the liquid phase across the membrane and into the gas phase where they are collected by cryofocusing at the front of the column in a gas chromatograph (GC). Two different instrumental configurations were investigated. In the first configuration a valve was used to introduce the extract and then switch to supply carrier gas for the analysis. Collection of the sample and analysis were determined by cooling and heating the column. The second involved connecting the module directly to the injector of the GC. In this configuration the stripping gas was supplied continuously to the GC and contributed the carrier gas. Quantitative extraction was obtained with the valved configuration which ensured a linear response over several orders of magnitude. The nonporous silicone rubber and microporous polypropylene hollow fiber membranes performed similarly.