Quantitative site-specific (2)H 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 (2)H nuclear magnetic resonance (NMR) spectroscopy for seven commercially available MTBE products. The delta(2)H values of the methoxy and tertiary butyl groups ranged from -103 per thousand to -171 per thousand, and from -76 per thousand to -104 per thousand, reflecting their production from methanol and isobutene, respectively. Several MTBE products whose whole-compound delta(13)C and delta(2)H MTBE values were within error of each other, as measured by isotope ratio mass spectrometry (IRMS), had demonstrably different delta(2)H values for their methoxy and tertiary butyl groups measured by (2)H NMR. Site-specific isotopic variations were large enough to provide proof of principle that quantitative site-specific (2)H 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 delta(2)H values for MTBE, derived as weighted averages of (2)H 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 (2)H/(1)H within a molecule offers considerable promise as a new environmental tool to track the source and fate of environmental contaminants.