Mass spectrometric elucidation of chlorine location in dichloro fatty acids following 4,4-dimethyloxazoline derivatization, and its application to chlorinated fatty acids in fish

摘要

Fatty acids containing two vicinal chlorine atoms located between C-5 and C-15 were transformed to 4,4-dimethyloxazoline (DMOX) derivatives, which were subjected to electron-impact mass spectrometry (EIMS). The results showed that DMOX derivatization is a useful technique for MS structural analysis of dichloro fatty acids. In contrast to those commonly used derivatives such as alkyl esters which result in mass spectral features indistinguishable among positional isomers, DMOX derivatives of dichloro fatty acids display some distinctive EIMS features which are characteristic of positional isomers. Fragmentation mechanisms responsible for important fragment ions and patterns are inherently related to the position of the vicinal dichloro group on the acyl chain. Among those diagnostic ions and patterns, a homologous series of characteristic chlorodienyl ions, notable by their sizeable intensity and chlorine isotope pattern, are the most valuable structural indicators. The location of the vicinal chlorines in a chlorinated fatty acid can be readily deduced from these ions by equating the mass of the first ion in the series to "144 + 14x", where x represents the location of the first of the vicinal chlorine atoms. In contrast to the "12-mu" rule which, as in the case of monoenoic acids, is only applicable to molecules containing the dichloro group in the central region of the acyl chain, the "144 + 14x" rule is applicable to almost all the positional isomers of vicinal dichloro fatty acids. Using this technique, the 5,6-location of chlorine atoms was confirmed for the previously identified threo-5,6-dichlorotetradecanoic acid in a fish lipid sample.