Isotopic analysis of N and O in NO3- by selective bacterial reduction to N2O for groundwater pollution.

We describe a method to determine the nitrogen and oxygen isotopic composition of nitrate in groundwater samples ((15)N/(14)N and (18)O/(16)O, respectively), which is based on the analysis of nitrous oxide gas (N2O) that is produced quantitatively from nitrate by denitrifying bacteria. This method which is simple, inexpensive and effective in the removal of nitrite is greatly selective for NO2(-) and was used for mixed samples containing both NO2(-) and NO3(-) with little or no measurable cross-contamination. The precision of δ(15)N and δ(18)O are 0.3 and 0.17‰ respectively, compared to that of 0.1 and 0.5‰ abroad (Brand et al. in Org Geochem 21:585-594, 1994; Begley and Scrimgeour in Anal Chem 69(8): 1530-1535, 1997; Kornexl et al. in Rapid Commun Mass Spectrom 13(16):1685-1693, 1999; Böhlke et al. in Rapid Commun Mass Spectrom 17:1835-1846, 2003; Gehre and Strauch in Rapid Commun Mass Spectrom 17(13):1497-1503, 2003; Werner in Isot Environ Health Stud 39:85-104, 2003).