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Correcting for background nitrate contamination in KCl‐extracted samples during isotopic analysis of oxygen and nitrogen by the denitrifier method
Author(s) -
Bell Michael D.,
Sickman James O.
Publication year - 2014
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.6824
Subject(s) - chemistry , molar concentration , nitrate , nitrogen , analytical chemistry (journal) , contamination , environmental chemistry , reagent , ecology , organic chemistry , biology
RATIONALE Previous research has shown that the denitrifying bacteria Pseudomonas chlororaphis ssp. aureofaciens ( P. aureofaciens ) can be used to measure the δ 15 N and δ 18 O values of extracted soil nitrate (NO 3 – ) by isotope ratio mass spectrometry. We discovered that N 2 O production from reference blanks made in 1 M KCl increased relative to blanks made of deionized water (DIW). Further investigation showed that isotopic standards made in KCl yielded δ 15 N and δ 18 O values different from the standards prepared in DIW. METHODS Three grades of crystalline KCl were dissolved in DIW to create solutions of increasing molarity (0.1 M to 2 M), which were added to P. aureofaciens broth and measured as blanks. Reference standards USGS‐32, USGS‐34, and USGS‐35 were then dissolved in a range of KCl concentrations to measure isotopic responses to changing KCl molarity. Reference blanks and standards created in DIW were analyzed as controls to measure the impact of KCl on the δ 15 N and δ 18 O values. RESULTS The amount of N 2 O in the KCl blanks increased linearly with increasing molarity, but at different rates for each KCl grade. The isotopic values of the reference standards measured in KCl were systematically different from those measured in DIW, suggesting contamination by background NO 3 – in the KCl reagents. However, we also noted reduced conversion of NO 3 – into N 2 O as the KCl molarity increased, suggesting there is a physiological response of P. aureofaciens to KCl. CONCLUSIONS There is a small amount of NO 3 – present in crystalline KCl, which can bias isotopic measurement of NO 3 – at low sample concentrations. This can be minimized by making standards and blanks in the same KCl as is used in samples, diluting all samples and standards to the appropriate NO 3 – concentration using matched KCl solutions, and adding samples and standards to the broth at a constant volume to standardize the KCl molarity in the reaction vial. Copyright © 2014 John Wiley & Sons, Ltd.