PARTS-PER-TRILLION DETERMINATION OF TRIHALOMETHANES IN WATER BY PURGE-AND-TRAP GAS-CHROMATOGRAPHY WITH ELECTRON-CAPTURE DETECTION

A gas chromatographic method using a purge-and-trap concentrator and electron capture detection to monitor trihalomethanes (THMs) at the low parts-per-billion (ppb) and the parts-per-trillion (ppt) levels is described in this paper. The method shows good linearity over 4 orders of magnitude with a detection limit between 0.01 and 0.05 ppt for each THM normally found after chlorination: chloroform, bromodichloromethane, dibromochloromethane, and bromoform. Special procedures, for sample handling and the preparation of blank water and analytical standards, required at such a low concentration, are also discussed. The analytical method has been successfully applied in the analysis of samples taken at different points of the water treatment plant of Hydro-Quebec's Gentilly 2 nuclear power station. Analysis of samples taken after chlorination and demineralization revealed the formation of all four THMs and other unidentified halogenated volatile compounds. Chloroform and bromodichloromethane were found at the low ppb level, while dibromochloromethane and bromoform were present at the ppt level.