Use of carbon isotopic ratios in nontargeted analysis to screen for anthropogenic compounds in complex environmental matrices

Abstract Analytical data for ultra-high-performance liquid chromatography (UHPLC), nontargeted, high-resolution, mass-spectrometry (HR/MS) molecular features from a wide array of samples are used to calculate 13 C 1 12 C (n-1) / 12 C n isotopologue ratios. These ratios increase with molecular carbon number roughly following a trend defined by atmospheric carbon. When the effective source reservoir 13 C/ 12 C ratio is calculated from the isotopologue ratio (assuming a fractionation factor of unity), features in biotic samples uniformly are tightly grouped, proximate to atmospheric 13 C/ 12 C ratio. In contrast, features in soil natural organic matter (NOM), dust NOM and anthropogenic compounds range from proximate to relatively divergent from atmospheric 13 C/ 12 C. For the NOM, 13 C/ 12 C ratios are consistent with an expected preferential volatilization of 12 C, rendering features in soil NOM 13 C-enriched and some features in dust NOM 13 C-depleted. Anthropogenic compounds tend to diverge most dramatically from atmospheric 13 C/ 12 C, generally toward 13 C-depletion, but pesticides we tested tended toward 13 C-enriched. This pattern is robust and evident in: i) anthropogenic vs natural features in dust; ii) perfluorinated compounds in standards and as soil contaminants; and iii) sunscreen compounds in commercial products and wastewater. Considering the observed wide 13 C/ 12 C range for anthropogenic compounds, we suggest Rayleigh distillation during synthetic processes commonly favors one isotope over the other, rendering a source reservoir that is progressively depleted as synthesis proceeds and, consequently, generates a wide variation in 13 C/ 12 C for man-made products. However, kinetic-isotopic effects and/or synthesis from petroleum/natural gas might contribute to the anthropogenic isotopic signature as well. Regardless of cause, 13 C/ 12 C can be used to cull HR/MS molecular features that are more likely to be of anthropogenic or non-biotic origin.