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

RATIONALE: The denitrifier method allows for highly sensitive measurement of the N-15/N-14 (delta(15)Nvalue) and O-18/O-16 (delta O-18 value) of nitrate dissolved in natural waters and for highly sensitive delta N-15 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 delta O-18 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 d15N and d18O values. Within-batch and inter-batch replication yields 1 standard deviation (SD) of <= 0.06% for delta N-15 values and <= 0.14% for delta O-18 values down to 5 mu M nitrate and <= 0.08% and <= 0.23% at 2 and 1 mu M. 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 delta N-15 values, with a contraction for delta O-18 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 delta O-18 values on nitrate concentration and (2) the inter-batch variation in the delta O-18 contraction (due mostly to oxygen atom exchange with water). Nevertheless, comprehensive correction schemes are in place for the measurement of both the delta N-15 and delta O-18 values of nitrate. Copyright (C) 2016 John Wiley & Sons, Ltd.