Observation of inconsistent carbon isotope compositions of chlorine-isotopologue pairs of individual organochlorines on gas chromatography-high resolution mass spectrometry

Abstract

ABSTRACT Relative abundances of chlorine and carbon isotopologues of individual organochlorines (e.g. polychlorinated organic pollutants) are generally recognised to comply with stochastic distribution, and thus can be calculated by the binomial theorem. As a result, carbon isotope ratios derived from different chlorine-isotopologue pairs, e.g. 12C2 35Cl4 vs. 12C13C35Cl4, should be consistent. Yet this recognition may not exactly reflect the reality. This study investigated the consistency/inconsistency of carbon isotope compositions of chlorine-isotopologue pairs of individual organochlorines including two chloroethylenes, three polychlorinated biphenyls, methyl-triclosan and hexachlorobenzene. The raw carbon isotope ratios were measured by gas chromatography-high resolution mass spectrometry. Data simulations in terms of background subtraction, background addition, dual 13C-atoms substitution, deuterium substitution and hydrogen-transfer, along with measurements at different injection concentrations were conducted to confirm the validity of measured carbon isotope ratios and their differences. Inconsistent carbon isotope ratios derived from chlorine-isotopologue pairs of individual organochlorines were observed, and the isotopologues of each organochlorine were thus inferred to be non-randomly distributed. Mechanistic interpretation for these findings was tentatively proposed according to a basic principle in clumped-isotope geochemistry, reaction thermodynamics and kinetics, along with isotope effects occurring in electron ionisation mass spectrometry. This study sheds light on the actual carbon isotope compositions of chlorine-isotopologue pairs of organochlorines, and yields new insights into the real distributions of carbon and chlorine isotopologues. The inconsistent carbon isotope compositions of chlorine-isotopologue pairs are anticipated to benefit the exploration of formation conditions and source identification of organochlorine pollutants.