反硝化法硝酸盐同位素分析的仪器和协议更新

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

作者信息

Weigand M Alexandra, Foriel Julien, Barnett Bruce, Oleynik Sergey, Sigman Daniel M

机构信息

Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ, 08544, USA.

Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1-1E-1 Ookayayama Meguro-ku, Tokyo, 152-8550, Japan.

出版信息

Rapid Commun Mass Spectrom. 2016 Jun 30;30(12):1365-83. doi: 10.1002/rcm.7570.

DOI:10.1002/rcm.7570

PMID:27197029

Abstract

RATIONALE

The denitrifier method allows for highly sensitive measurement of the (15) N/(14) N (δ(15) N value) and (18) O/(16) O (δ(18) O value) of nitrate dissolved in natural waters and for highly sensitive δ(15) N 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 N2 O 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 δ(18) O 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 δ(15) N and δ(18) O values. Within-batch and inter-batch replication yields 1 standard deviation (SD) of ≤0.06‰ for δ(15) N values and ≤0.14‰ for δ(18) O 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 δ(15) N values, with a contraction for δ(18) O 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 δ(18) O values on nitrate concentration and (2) the inter-batch variation in the δ(18) O contraction (due mostly to oxygen atom exchange with water). Nevertheless, comprehensive correction schemes are in place for the measurement of both the δ(15) N and δ(18) O values of nitrate. Copyright © 2016 John Wiley & Sons, Ltd.

摘要

原理

反硝化法能够高度灵敏地测量天然水体中溶解硝酸盐的（15）N/（14）N（δ（15）N值）和（18）O/（16）O（δ（18）O值），以及能够转化为硝酸盐的其他氮形态（如有机氮）的δ（15）N值。本文描述了仪器和实验方案的更新情况，并展示了数据质量的提升。

方法

在萃取系统中对N2O进行“中心切割”，以（1）尽量减少污染物进入质谱仪，减少同位素漂移，（2）降低在开放分流处损失的样品比例，提高灵敏度。更新了参比方案，包括针对硝酸盐δ（18）O值对硝酸盐浓度的微弱依赖性的校正方案。利用美国全球海洋通量研究（GEOTRACES）北大西洋项目的样品以及同一分析批次的参比溶液的分析结果来表征性能。

结果

对于δ（15）N和δ（18）O值，漂移通常<0.1‰。对于δ（15）N值，批内和批间重复测量的1标准偏差（SD）在硝酸盐浓度低至5μM时≤0.06‰，在2μM和1μM时分别≤0.08‰；对于δ（18）O值，相应的标准偏差在硝酸盐浓度低至5μM时≤0.14‰，在2μM和1μM时分别≤0.23‰。空白值通常为0.06 nmol N，是20 nmol N样品中氮含量的0.3%。海水中参考物质之间的差异与报道的δ（15）N值差异无法区分，δ（18）O值的收缩幅度≤5%。