| Abstract
| - Compound-specific carbonisotope analysis (CSIA) hasbecome an important tool in biological, archeological, andgeological 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 applicationssuch as, for example, in situremediation studies to assess contaminated environments, identificationof pollutant degradation pathwaysand kinetics, distinction between degradation/formationmechanisms, or, verification of contaminant sources. Withthis goal in mind, we have developed methods to determine ∂13C values of commonly reported groundwatercontaminants in low-microgram per liter concentrations.Several injection and preconcentration techniques wereevaluated for this purpose, i.e., on-column injection, split/splitlessinjection, solid-phase microextraction (SPME),and purge and trap (P&T) in combination with gaschromatography−isotope ratio mass spectrometry. The∂13C values of the target compounds were determinedbyliquid injections of the analytes dissolved in diethyl etheror, in the case of P&T and SPME, by extraction from waterspiked with the analytes. P&T extraction was the mostefficient preconcentration technique reaching methoddetection limits (MDLs) from 0.25 to 5.0 μg/L. These arethe lowest MDLs reported so far for continuous-flowisotope ratio determinations, using a commercially available and fullyautomated system. Isotopic fractionationresulting from preconcentration and injection was investigated andquantified for the priority groundwater pollutants methyl tert-butyl ether (MTBE), chloroform, tetrachloromethane,chlorinated ethylenes, benzene, andtoluene. The isotopic fractionations caused by the extraction techniqueswere small but highly reproducible andcould therefore be corrected for. P&T was characterizedby a higher reproducibility and smaller isotopic fractionations thanSPME. Among the liquid injection techniques,cold on-column injection resulted in slightly better precision comparedto split/splitless injection. However, theMDLs determined for liquid injections were 4−6 ordersof magnitude higher (i.e., 9.5−2800 mg/L) than for P&Tand SPME. Since both of the latter methods are solventless, a betterchromatographic resolution was obtainedthan for the liquid injection techniques. The P&T andSPME methods described here are also applicable forCSIA of D/H ratios, which require 10−20 times higheranalyte concentrations than 13C/12C analysis.Finally, theapplicability of the described methods is demonstratedfor pollutant concentrations of only 5−60 μg/L in environmentalsamples.
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