  A recent article reported a comparison study concerning compound-specific
chlorine isotope analysis (CSIA-Cl) of organochlorines using gas
chromatography- isotope ratio mass spectrometry (GC-IRMS) and gas
chromatography-quadrupole mass spectrometry (GC-qMS). Comparable precisions
between the two instruments were achieved and trueness of the analysis results
was confirmed. The CSIA-Cl method using GC-qMS was originally developed by
Sakaguchi-S\"oder et al.and further improved by several relevant studies,and
has been used to evaluate organic contaminant attenuation in the environment.
The essential principle of the method is to calculate the chlorine isotope
ratio (37Cl/35Cl) by using the first pair of neighboring chlorine isotopologues
(two-mass apart) of either the molecular ion or a fragmental ion of a target
analyte, or using all the first isotopologue pairs of all molecular and
fragmental ions. These isotope-ratio calculation schemes using chlorine
isotopologue pair(s) were based on the binomial theorem and the prerequisite
hypothesis that the measured abundances of chlorine isotopologues of individual
analytes were binomially distributed. However, it is not true with the
hypothesis.
