Selective sensitivity of highly chlorinated species in negative ion mass spectrometry: Implications for complex mixture analysis

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In attempts to quantify complex chlorinated hydrocarbon mixtures by negative ion mass spectrometry (NI/MS), a selective sensitivity was observed which was not observed using the technique of electron capture detection (ECD) with conventional gas-liquid chromatography. The phenomenon was systematically studied using chlorinated phenols, chlorinated biphenyl isomers and a mixture of chlorinated bicyclic species. Results indicated that for two aromatic series there is an increase in molar response to both ECD and NI/MS with increased degree of chlorination. However, overall an increase in molar response was observed for a series of aliphatic bicyclic compounds only when analyzed by NI/MS; i.e. the ECD molar response did not vary with increasing chlorination of the bicyclic system. A procedure for optimizing mass spectrometer parameters has been developed for quantifying complex chlorinated hydrocarbon mixtures. A simple mixture which mimics the halogenation in a complex mixture is used to determine the response of the instrument to various degrees of chlorination. The recommended calibration procedure optimizes instrumental parameters such as moderating gas pressure, filament current and electron energy so that complex chlorinated mixtures such as those found in toxaphene residues can be reliably quantified.