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

TL;DR

This study reveals that carbon isotope ratios of chlorine-isotopologue pairs in organochlorines are inconsistent, challenging assumptions of random distribution and providing new insights into their formation and source identification.

Contribution

It demonstrates the inconsistency of carbon isotope compositions in chlorine-isotopologue pairs and offers mechanistic explanations based on isotope effects and reaction thermodynamics.

Findings

Inconsistent isotope ratios observed in individual organochlorines.

Isotopologue distributions are non-random and influenced by reaction mechanisms.

Provides new insights into formation conditions and source tracing of organochlorines.

Abstract

This study investigated the consistency/inconsistency of carbon isotope compositions of chlorine-isotopologue pairs, e.g., 12C235Cl4 vs. 12C13C35Cl4, 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 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 on electron ionization 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.