Citation:

Washington, J., C. Rosal, E. Ulrich, AND T. Jenkins. Use of carbon isotopic ratios in nontargeted analysis to screen for anthropogenic compounds in complex environmental matrices. JOURNAL OF CHROMATOGRAPHY A. Elsevier Science Ltd, New York, NY, 1583:73-79, (2019). https://doi.org/10.1016/j.chroma.2018.11.013

Impact/Purpose:

The last decade has seen rapid advances in nontargeted mass-spectral analyses of complex environmental samples. Commonly generating ~10,000 molecular features per sample comprised largely of compounds naturally present in the environmental sample, the vast size of these datasets can challenge common EPA objectives of identifying anthropogenic compounds for deeper scrutiny. While anthropogenic compounds containing large numbers of halogens often can be identified using mass-defect filtering, screening for anthropogenic compounds not bearing halogens, e.g., organophosphates, carbamates and pyrethroids, remains a challenge. Here we describe the use of stable carbon isotopes in nontargeted high-resolution mass spectrometry to screen for anthropogenic compounds in complex environmental matrices.

Description:

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.

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