Quantitative Site-Specific 2H NMR Investigation of MTBE: Potential for Assessing Contaminant Sources and Fate

Site-specific isotopic values of methyl tertiary butyl ether (MTBE) were measured using quantitative site-specific 2H nuclear magnetic resonance (NMR) spectroscopy for seven commercially available MTBE products. The δ2H values of the methoxy and tertiary butyl groups ranged from −103‰ to −171‰, and from −76‰ to −104‰, reflecting their production from methanol and isobutene, respectively. Several MTBE products whose whole-compound δ13C and δ2H MTBE values were within error of each other, as measured by isotope ratio mass spectrometry (IRMS), had demonstrably different δ2H values for their methoxy and tertiary butyl groups measured by 2H NMR. Site-specific isotopic variations were large enough to provide proof of principle that quantitative site-specific 2H NMR may provide an additional parameter for contaminant sourcing at field sites. Isotopic variations were small enough to not bias the comparability of degradation-associated isotopic enrichment factors determined using different MTBE products. Calculated δ2H values for MTBE, derived as weighted averages of 2H NMR measurements of the two functional groups, showed good agreement with IRMS measurements. The ability to gain accurate information about the site-specific isotopic ratios of 2H/1H within a molecule offers considerable promise as a new environmental tool to track the source and fate of environmental contaminants.