The bacterial denitrification method for isotopic analysis of nitrate using N2O generated from Pseudomonas aureofaciens may overestimate δ15N values by as much as 1–2‰ for samples containing atmospheric nitrate because of mass-independent 17O variations in such samples. By analyzing such samples for δ15N and δ18O using the denitrifier Pseudomonas chlororaphis, one obtains nearly correct δ15N values because oxygen in N2O generated by P. chlororaphis is primarily derived from H2O. The difference between the apparent δ15N value determined with P. aureofaciens and that determined with P. chlororaphis, assuming mass-dependent oxygen isotopic fractionation, reflects the amount of mass-independent 17O in a nitrate sample. By interspersing nitrate isotopic reference materials having substantially different δ18O values with samples, one can normalize oxygen isotope ratios and determine the fractions of oxygen in N2O derived from the nitrate and from water with each denitrifier. This information can be used to improve δ15N values of nitrates having excess 17O. The same analyses also yield estimates of the magnitude of 17O excess in the nitrate (expressed as Δ17O) that may be useful in some environmental studies. The 1-σ uncertainties of δ15N, δ18O and Δ17O measurements are ±0.2, ±0.3 and ±5‰, respectively.
|Publication Subtype||Journal Article|
|Title||Using dual-bacterial denitrification to improve δ15N determinations of nitrates containing mass-independent 17O|
|Series title||Rapid Communications in Mass Spectrometry|
|Contributing office(s)||Toxic Substances Hydrology Program|
|Online Only (Y/N)||N|
|Additional Online Files (Y/N)||N|
|Google Analytic Metrics||Metrics page|