Coupling of a Headspace Autosampler with a Programmed Temperature Vaporizer for Stable Carbon and Hydrogen Isotope Analysis of Volatile Organic Compounds at Microgram per Liter Concentrations

One major challenge for the environmental application of compound-specific stable isotope analysis (CSIA) is the necessity of efficient sample treatment methods, allowing isolation of a sufficient mass of organic contaminants needed for accurate measurement of the isotope ratios. Here, we present a novel preconcentration techniquethe coupling of a headspace (HS) autosampler with a programmed temperature vaporizer (PTV)for carbon (δ¹³C) and hydrogen (δ²H) isotope analysis of volatile organic compounds in water at concentrations of tens of micrograms per liter. The technique permits large-volume injection of headspace samples, maintaining the principle of simple static HS extraction. We developed the method for multielement isotope analysis (δ¹³C and δ²H) of methyl tert-butyl ether (MTBE), benzene, toluene, ethylbenzene, and o-xylene (BTEX), and analysis of δ¹³C for chlorinated benzenes and ethenes. Extraction and injection conditions were optimized for maximum sensitivity and minimum isotope effects. Injection of up to 5 mL of headspace sample from a 20 mL vial containing 13 mL of aqueous solution and 5 g of NaCl (10 min of incubation at 90 °C) resulted in accurate δ¹³C and δ²H values. The method detection limits (MDLs) for δ¹³C were from 2 to 60 μg/L (MTBE, BTEX, chlorinated ethenes, and benzenes) and 60–97 μg/L for δ²H (MTBE and BTEX). Overall, the HS–PTV technique is faster, simpler, isotope effect-free, and requires fewer treatment steps and less sample volume than other extraction techniques used for CSIA. The environmental applicability was proved by the analysis of groundwater samples containing BTEX and chlorinated contaminants at microgram per liter concentrations.