A new approach to determine method detection limits for compound-specific isotope analysis of volatile organic compounds.
Compd.-specific isotope anal. (CSIA) was established as a useful tool for environmental science, particularly for assessing polluted sites. What limits the use of gas chromatog./isotope ratio mass spectrometry (GC/IRMS) is the low sensitivity of the method vs. with GC/MS anal.; however, development of suitable extn. and enrichment techniques for important groundwater pollutants will extend application fields for GC/IRMS. Thus far, purge and trap (P&T) is the most effective, known pre-concn. technique for online CSIA, with lowest reported method detection limits (MDL) in the low mg/L range. With a goal of improving sensitivity of a fully automated GC/IRMS anal. method, a com. available P&T system was modified. The method was evaluated for 10 monoarom. compds. (benzene, toluene, p-xylene, ethylbenzene, propylbenzene, isopropylbenzene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, fluorobenzene) and 10 halogenated volatile org. compds. (VOC: dichloromethane, cis-1,2-dichloroethene, trans-1,2-dichloroethene, CCl4, chloroform, 1,2-dichloroethane, trichloroethene, perchloroethene, 1,2-dibromoethane, bromoform). The effect of method parameters, including purge gas flow rates and purge times, on target compd. d13C values was evaluated. The P&T method demonstrated good reproducibility, high linearity, and small isotopic fractionation. MDL were detd. by consecutively calcg. d13C mean values. The last concn. for which the d13C value was within this iterative interval and for which the std. deviation was <+-0.5.permill. for triplicate measurements was defined as the MDL. MDL for monoarom. compds. from 0.07 to 0.35 mg/L were the lowest values reported for continuous-flow isotope ratio measurements using an automated system. MDL for halogenated hydrocarbons were 0.76-27 mg/L. The environmental applicability of the P&T-GC/IRMS method in the low mg/L range was demonstrated in a case study of groundwater from a former military air field polluted by VOC.
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