Abstract

A lot of data have been accumulated to date on the sources of formation of polychlorinated dibenzodiox� ins (PCDD) and dibenzofurans (PCDF), their envi� ronmental levels, and effects on living organisms, but there relatively few data about the properties of these compounds (1). Indeed, the octachlorodibenzop� dioxin (OCDD) phenomenon, in particular, the fact that it is detected in very high concentrations in soils that have not been affected by any human interven� tion, while the concentrations of other PCDD in these soils exponentially decrease and the PCDF level is negligibly low, has not been explained as yet. Often, this abnormal profile is detected in hot climate areas such as Australia, Thailand (2, 8), and Vietnam (4). Moreover, this compound is not characteristic of any industrial process. The source of OCDD in these areas is unknown but probably detection of some precursors or metabolites would help to estimate the theory of natural formation of OCDD from pentachlorophenol (PeCP), which was indirectly supported by our earlier study (5). Here, we present the results of detailed analysis of a soil sample with abnormally high OCDD concentra� tion collected at the Sa Pa mountain range in the North of Vietnam. Due to the low absolute concentra� tions of PCDD/PCDF, these compounds are deter� mined most efficiently by gas chromatography/high� resolution mass spectrometry (GC/HRMS) with recording of selected characteristic ions. Since the compounds to be determined were unknown before� hand, we applied GC/HRMS in the full scan mode, i.e., recorded the full highresolution mass spectrum. EXPERIMENTAL The soil and bottom sediment samples were taken in 2010 by expeditions organized by the Russian- Vietnam Tropical Center. The samples were analyzed at the Laboratory for Analytical Ecotoxicology of the Institute of Ecology. In all samples, PCDD, PCDF, and PeCP were determined by gas chromatogra� phy/highresolution mass spectrometry (GC/HRMS) with isotope dilution (4). The possible OCDD metab� olites were determined by recording GC/MS patterns in the full scan mode and the selected ion detection (SID) mode. The samples were extracted with a toluene-ace� tone mixture and with an aqueous solution of sodium hydroxide; this was followed by acidification of the aqueous phase and reextraction into dichloromethane. The extracts were analyzed without purification and after passing through a silica gel layer impregnated with sulfuric acid.

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