Use of liquid chromatography/tandem mass spectrometry to detect distinctive indicators of in situ RDX transformation in contaminated groundwater

An important element of monitored natural attenuation is the detection in groundwater of distinctive products of pollutant degradation or transformation. In this study, three distinctive products of the explosive RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) were detected in contaminated groundwater from the Iowa Army Ammunition Plant; the products were MNX (hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine), DNX (hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine), and TNX (hexahydro-1,3,5-trinitroso-1,3,5-triazine). These compounds are powerful indicators of RDX transformation for several reasons: (a) they have unique chemical features that reveal their origin as RDX daughter products, (b) they have no known commercial, industrial, or natural sources, and (c) they are well documented as anaerobic RDX metabolites in laboratory studies. The products were analyzed by LC/MS/MS (liquid chromatography/ mass spectrometry/mass spectrometry) with selected reaction monitoring and internal standard quantification using [ring- U-15N]RDX. Validation tests showed the novel LC/ MS/MS method to be of favorable sensitivity (detection limits ca. 0.1 μg/L), accuracy, and precision. The products, which were detected in all groundwater samples with RDX concentrations of > ca. 1 μg/L (25 out of 55 samples analyzed), were present at concentrations ranging from near the detection limit to 430 μg/L. MNX was the typically the most abundant of the three nitroso-substituted products; concentrations of the products seldom exceeded 4 mol % of the RDX concentration, although they ranged as high as 26 mol % (TNX). Geographic and temporal distributions of RDX, MNX, DNX, and TNX were assessed. A degradation product resulting from RDX ring cleavage, methylenedinitramine, was not detected by LC/MS/MS in any sample (detection limit ca. 0.6-4 μg/L). This extensive field characterization of MNX, DNX, and TNX distributions in groundwater by a highly selective analytical method (LC/ MS/MS) is significant because very little is known about the occurrence of intrinsic RDX transformation in contaminated aquifers.