Compound-Specific Carbon Isotope Analysis of Volatile Organic Compounds in the Low-Microgram per Liter Range

Abstract

Compound-specific carbon isotope analysis (CSIA) has become an important tool in biological, archeological, and geological studies as well as in forensics, food sciences, and organic chemistry. If sensitivity could be enhanced, CSIA would further have an improved potential for environmental applications such as, for example, in situ remediation studies to assess contaminated environments, identification of pollutant degradation pathways and kinetics, distinction between degradation/formation mechanisms, or, verification of contaminant sources. With this goal in mind, we have developed methods to determine delta13C values of commonly reported groundwater contaminants in low-microgram per liter concentrations. Several injection and preconcentration techniques were evaluated for this purpose, i.e., on-column injection, split/ splitless injection, solid-phase microextraction (SPME), and purge and trap (P&T) in combination with gas chromatography-isotope ratio mass spectrometry. The delta13C values of the target compounds were determined by liquid injections of the analytes dissolved in diethyl ether or, in the case of P&T and SPME, by extraction from water spiked with the analytes. P&T extraction was the most efficient preconcentration technique reaching method detection limits (MDLs) from 0.25 to 5.0 microg/L. These are the lowest MDLs reported so far for continuous-flow isotope ratio determinations, using a commercially available and fully automated system. Isotopic fractionation resulting from preconcentration and injection was investigated and quantified for the priority groundwater pollutants methyl tert-butyl ether (MTBE), chloroform, tetrachloromethane, chlorinated ethylenes, benzene, and toluene. The isotopic fractionations caused by the extraction techniques were small but highly reproducible and could therefore be corrected for. P&T was characterized by a higher reproducibility and smaller isotopic fractionations than SPME. Among the liquid injection techniques, cold on-column injection resulted in slightly better precision compared to split/splitless injection. However, the MDLs determined for liquid injections were 4-6 orders of magnitude higher (i.e., 9.5-2800 mg/L) than for P&T and SPME. Since both of the latter methods are solventless, a better chromatographic resolution was obtained than for the liquid injection techniques. The P&T and SPME methods described here are also applicable for CSIA of D/H ratios, which require 10-20 times higher analyte concentrations than 13C/12C analysis. Finally, the applicability of the described methods is demonstrated for pollutant concentrations of only 5-60 microg/L in environmental samples.

Isotope‐ratio detection for gas chromatography

“…enrichment of 13 C relative to bulk (offline) analyses. Zwank et al [42] have recently provided the most comprehensive comparison of on-column, splitless, split (50 : 1), SPME, and purge-and-trap injection methods for volatile hydrocarbons and halocarbons. They conclude that while "all methods are suitable for environmental applications," on-column injection provided the best isotopic precision while purge-and-trap provided the lowest detection limits.…”

Application of compound-specific stable carbon isotope analysis for the biotransformation and trophic dynamics of PBDEs in a feeding study with fish

“…This type of analysis is based on the principle that the same chemical substance, from different manufacturers or synthesis processes, may have different isotope signatures and that degradation generally results in a 13 C-enrichment in the remaining compounds and a 13 C-depletion in its degradation products (Schmidt et al, 2004). To date, this technique has been utilized for volatile and several semi-volatile organic compounds, such as BTEX (Dempster et al, 1997), chlorinated ethylenes (Zwank et al, 2003), methyl tert-butyl ether (Elsner et al, 2007;Kuder et al, 2005), polycyclic aromatic hydrocarbons (PAHs) (Bergmann et al, 2011;Ou et al, 2011), and polychlorinated biphenyls (PCBs) (Yanik et al, 2003;Drenzek et al, 2001). Few studies have examined the stable carbon isotope ratios of individual PBDE congeners in the environment.…”

Demonstrating a Natural Origin of Chloroform in Groundwater Using Stable Carbon Isotopes

“…The δ 13 C of industrial chloroform was comprised between −43.2‰ and −63.6‰ (n = 1−6; 1σ = 0.08−0.39‰) consistent with its production from methane. 29 The similarities in δ 13 C values between some chloroform suppliers suggest that the samples may come from the same carbon feedstock and similar production processes. It is also possible that chloroform from different suppliers is produced by the same manufacturer.…”

“…The most sensitive commercial method usually used in groundwater stable isotope studies of VOCs is purge and trap coupled to gas chromatography isotope-ratio mass spectrometry with an estimated detection limit of 2.3 μg/L for chloroform. 29 However, such a detection limit is not sufficient to evaluate if low levels chloroform represent a plume fringe or correspond to a natural background. Therefore, a method using a large scale online purge and trap is validated by evaluating the linearity and reproducibility of the method.…”