Mutual interference between 3,6-dichlorinated carbazole and p,p′-dichlorodiphenyltrichloroethane in gas chromatography mass spectrometry analysis

The widespread contamination of environment by polyhalogenated carbazoles (PHCZs) has been increasingly observed during the past decade. Among numerous PHCZ congeners, 3,6-dichlorocarbazole (36-CCZ) is often among the most frequently detected with higher concentrations. Although the environmental level of legacy pesticide p,p ′-dichlorodiphenyltrichloroethane ( p,p ′-DDT) has been declining, it continues to be ubiquitously detected. These two compounds were found to interfere with each other during analyses using gas chromatographic (GC) couples with single- or triple-quadrupole low resolution mass spectrometry (MS or MS/MS). The base peak in mass spectra was that of m/z 235 for both compounds. In MS/MS with multiple reaction monitoring (MRM), the same transitions (235®200 and 235®165) were often used. Under the same GC operating conditions, one of the two GC columns used in this work did not resolve the two compounds at baseline. Pre-treatment using cleanup column chromatography can fractionate the sample extract, with the two compounds separated in different fractions before instrumental analyses. Reversed phase HPLC columns also work for resolving 36-CCZ and p,p′ -DDT. But possible overlaps in GC retention and similarity in MS spectra might have caused data inaccuracy for 36-CCZ as well as DDT in some studies published to date, and steps to avoid the interference should be taken into quality control protocols in future research.