Determinations of N-Nitrosodimethylamine at Part-per-Trillion Concentrations in Contaminated Groundwaters and Drinking Waters Featuring Carbon-Based Membrane Extraction Disks

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Abstract
A new solid-phase extraction procedure extracts N-nitrosodimethylamine (NDMA) at part-per-trillion (ng/L) concentrations from aqueous samples using a C18 (reversed-phase) membrane extraction disk layered over a recently introduced carbon-based extraction disk. The reversed-phase disk removes nonpolar water-insoluble neutrals and is set aside; the carbon-based disk is extracted with a small volume of dichloromethane. NDMA is quantified in the organic extract using a gas chromatograph equipped with both a short-path thermal desorber and a chemiluminescent nitrogen detector. The detection limit for the procedure, calculated using two statistically unbiased protocols, is 3 ng of NDMA/L; the analyte recovery is ∼57%. A related procedure substitutes a standard automatic sampler for the short-path thermal desorber and is suitable for determining NDMA in heavily contaminated (>300 ng of NDMA/L) aqueous samples. The detection limit for the procedure, calculated in the same manner as above, is 300 ng of NDMA/L, with an analyte recovery of ∼64%. The detection limits and measured recovery values are comparable to those observed in earlier work, in which a conventional continuous overnight extraction with dichloromethane was used to remove NDMA from the aqueous samples. The newer procedures described herein offer a 50-fold savings in extraction time and a 100-fold reduction in dichloromethane consumed per sample while maintaining the wide range (3−4 orders of magnitude concentrations of NDMA) observed for the original procedures used in tandem. Authentic contaminated groundwaters are extracted using both the conventional and disk-based extraction procedures and analyzed; the observed NDMA concentrations are virtually identical over a target range spanning 100−10000 ng of NDMA/L.