¹³C/¹²C Analysis of Ultra-Trace Amounts of Volatile Organic Contaminants in Groundwater by Vacuum Extraction

Abstract

We developed a method for the vacuum extraction (VacEx) of volatile organic compounds (VOCs) from water samples for ultratrace determinations of carbon isotopic signatures. Our method permits compound-specific stable carbon isotope analysis (CSIA) at VOC concentrations as low as 0.03-1.34 microg/L. VacEx was developed to extract and preconcentrate VOCs for subsequent carbon-CSIA by the standard technique purge-and-trap (P&T) coupled to an isotope-ratio mass spectrometer (IRMS). Even without complete extraction, the delta(13)C signatures of VOCs determined by VacEx-P&T-IRMS were in good agreement (deviation <1 per thousand) with signatures determined by P&T-IRMS. This indicates that VacEx does not cause isotopic discrimination. Limits of quantification (LOQs) for delta(13)C analysis were: 0.03-0.06 microg/L for benzene, toluene, o-xylene, m-p-xylene and ethylbenzene, 0.09 microg/L for methyl tert-butyl ether (MTBE), and 0.18-0.27 microg/L for trans-DCE, cis-DCE, TCE and PCE. These are the lowest LOQs reported to date for continuous-flow isotope-ratio determinations using a commercially available and automated system. To our knowledge, analytical protocols adopted from noble gas analysis in water were applied for the first time to determine the isotope composition of organic contaminants. We applied VacEx in a field study to illustrate how the determination of VOC isotopic signatures at very low concentrations opens new avenues in the in situ assessment of these priority groundwater pollutants.

Multi-element isotope fractionation concepts to characterize the biodegradation of hydrocarbons — from enzymes to the environment

“…In case ME-CSIA data cannot solely prove in situ biodegradation, additional methods should be employed as geochemical approaches, metabolite analyses, in situ microcosms or expression analyses of functional genes [3,48]. For environmental samples containing low concentrated, almost water insoluble hydrocarbons, new techniques have been recently described for extraction and carbon and/or hydrogen isotope analysis, allowing, for example, measuring PAHs in the sub-ng L À1 range [49][50][51]. Similar approaches have been done for the isotopic analysis of micropollutants [12].…”

Current challenges in compound-specific stable isotope analysis of environmental organic contaminants

“…Before measurement, the compounds are released by heating and are carried in a He stream into the injection port of a gas chromatograph. Reliable isotope values in the low ppb concentration range have been obtained for numerous volatile groundwater contaminants [133][134][135][136][137]. Larger amounts of water samples (up to 100 mL) can be extracted in an automated process, and interfering non-volatile matrix components are left behind leading to "clean" chromatograms.…”

Enantioselective stable isotope analysis (ESIA) — A new concept to evaluate the environmental fate of chiral organic contaminants

“…Thus, an efficient preparative procedure (clean-up) that will reduce the matrix effects is needed for field samples (wastewater, soil, biota, etc.) (Amaral et al, 2010;Berg et al, 2007;Meyer et al, 2008;Penning and Elsner, 2007). III.…”