Optimized separation and determination of methyl sulfonemetabolites of polychlorinated biphenyls (PCBs) and p,p′-DDE in biota samples

Abstract

An optimised method is described for the determination of 27 methyl sulfone polychlorobiphenyls (PCBs) and DDE in biota samples. Initially, the samples were extracted by hot Soxhlet and the methyl sulfones were separated by liquid/liquid extraction with concentrated sulfuric acid and back-extracted with hexane. The parameters of the back-extraction were studied and it was found that for a quantitative extraction of the methyl sulfones from the concentrated acid layer, a 50% dilution with cold water should be done. The hexane layer containing the methyl sulfones was further cleaned-up on basic silica (33% KOH) and Florisil. After concentration, the extract was analysed by gas chromatography-mass spectrometry (GC-MS) with electron capture negative ionisation (ECNI) in selected ion monitoring mode (SIM). It was shown that, for methyl sulfones, the ion formation was dependent on the chlorine substitution, position of the MeSO₂-group and the ion source temperature. If the ion source temperature was higher than 200 °C, [M-CH₃]⁻ was the predominant ion for most methyl sulfones. Therefore, for increased sensitivity, quantitation of most congeners was done using [M-CH₃]⁻ ions instead of the molecular ion as used in previously reported methods. The method was validated for the determination of 26 tri- to hepta- 3- and 4-substituted MeSO₂-PCBs and 3-MeSO₂-DDE in animal and human tissues. Good sensitivity and selectivity of the method were obtained. Limits of detection (LODs) ranged from 0.06 to 0.10 ng g⁻¹ lipid weight. Average recoveries of individual congeners from vegetable oil spiked with individual standards (3.33 ng g⁻¹) ranged from 73 to 112% with a mean value of 89%. The coefficients of variation ranged from 5.2 to 12.2%, which is within the acceptable range for environmental analyses.