Enantioselective determination of two persistent chlorobornane congeners in sediment from a toxaphene-treated Yukon Lake

The composition of toxaphene residues and the enantiomeric ratios of the ma jor congeners were studied in the top 10 slices (1935-1992) of a sediment c ore collected from Hanson Lake in the Yukon Territories (Canada). This lake was treated with toxaphene as a picsicide in July 1963. Electron-capture n egative ion mass spectrometry and electron-capture detection were applied i n combination with enantioselective gas chromatography using modified cyclo dextrin columns. Of the 10 toxaphene peaks identified in each of the sedime nt slices, two congeners, 2-exo, 3-endo,6-exo,8,9, 10-hexachlorobornane (B6 -923 or Hx-Sed) and 2-endo,3-exo,5-endo,6-exo,8,9,10-heptachlorobornane (B7 -1001 or Hp-Sed), were the most abundant. Each of the three chiral stationa ry phases used in this study was successful at resolving the enantiomers in at least of one of the two congeners. Among them was permethylated beta-cy clodextrin (Chirasil-Dex), which, up until now, has never been used for ena ntiomer separation of compounds of technical toxaphene. B6-923 and another less abundant, as of yet unknown, hexachlorobornane were racemic in all sed iment core slices. For B7-1001, the latter eluting enantiomer was always mo re Predominant. Furthermore, the enantiomeric ratio was found to consistent ly decrease with time from approximately 0.8 in the early slices (similar t o 1950) to 0.7 in the slice representing 1992, implying that the first elut ed B7-1001 enantiomer is less persistent than the second. B6-923 and B7-100 1 are most likely metabolites of the more highly chlorinated congeners. A s tructural evaluation provided evidence that reductive dechlorination at car bons with geminal chlorine atoms is the primary microbial degradation pathw ay in sediments from Hanson Lake.