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 13C112C(n-1)/12Cn isotopologue ratios. These ratios increase with molecular carbon number roughly following a trend defined by atmospheric carbon. When the effective source reservoir 13C/12C ratio is calculated from the isotopologue ratio (assuming a fractionation factor of unity), features in biotic samples uniformly are tightly grouped, proximate to atmospheric 13C/12C ratio. In contrast, features in soil natural organic matter (NOM), dust NOM and anthropogenic compounds range from proximate to relatively divergent from atmospheric 13C/12C. For the NOM, 13C/12C ratios are consistent with an expected preferential volatilization of 12C, rendering features in soil NOM 13C-enriched and some features in dust NOM 13C-depleted. Anthropogenic compounds tend to diverge most dramatically from atmospheric 13C/12C, generally toward 13C-depletion, but pesticides we tested tended toward 13C-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 13C/12C 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 13C/12C 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, 13C/12C can be used to cull HR/MS molecular features that are more likely to be of anthropogenic or non-biotic origin.