Updates to instrumentation and protocols for isotopic analysis of nitrate by the denitrifier method

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Journal Article
Rationale The denitrifier method allows for highly sensitive measurement of the 15N/14N (δ15N value) and 18O/16O (δ18O value) of nitrate dissolved in natural waters and for highly sensitive δ15N measurement of other N forms (e.g., organic N) that can be converted into nitrate. Here, updates to instrumentation and protocols are described, and improvements in data quality are demonstrated. Methods A heart cut of the N2O was implemented in the extraction system to (1) minimize introduction of contaminants into the mass spectrometer, reducing isotopic drift and (2) decrease the fraction of sample lost at the open split to improve sensitivity. Referencing protocols were updated, including a correction scheme for a weak dependence of nitrate δ18O values on nitrate concentration. Analyses of samples from the US GEOTRACES North Atlantic Program and of reference solutions from the same analysis batches were used to characterize performance. Results The drift is typically <0.1‰ for both δ15N and δ18O values. Within-batch and inter-batch replication yields 1 standard deviation (SD) of ≤0.06‰ for δ15N values and ≤0.14‰ for δ18O values down to 5 μM nitrate and ≤0.08‰ and ≤0.23‰ at 2 and 1 μM. The blank is typically 0.06 nmol N, 0.3% of the N in a 20 nmol N sample. Differences between reference materials in seawater are indistinguishable from reported differences for δ15N values, with a contraction for δ18O values of ≤5%. Conclusions The new instrumentation and protocols yield nitrate isotopic data with external precision of ≤0.1‰ for large sample sets such as those derived from oceanographic sections. Further study should investigate the causes of (1) the weak dependence of nitrate δ18O values on nitrate concentration and (2) the inter-batch variation in the δ18O contraction (due mostly to oxygen atom exchange with water). Nevertheless, comprehensive correction schemes are in place for the measurement of both the δ15N and δ18O values of nitrate. © 2016 John Wiley & Sons, Ltd.
Rapid Communications in Mass Spectrometry