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Determination of the δ15N and δ18O of nitrate in water; RSIL lab code 2900

Techniques and Methods 10-C17

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Abstract

The purpose of the Reston Stable Isotope Laboratory (RSIL) lab code 2900 is to determine the δ15N and δ18O of nitrate (NO3-) in water. The δ15N and δ18O of the dissolved NO3- are analyzed by converting the NO3- to nitrous oxide (N2O), which serves as the analyte for mass spectrometry. A culture of denitrifying bacteria is used in the enzymatic conversion of the NO3- to N2O, which follows the pathway shown in equation 1:

NO3- → NO2- → NO → 1/2 N2O (1)

Because the bacteria Pseudomonas aureofaciens lack N2O reductive activity, the reaction stops at N2O, unlike the typical denitrification reaction that goes to N2. After several hours, the conversion is complete, and the N2O is extracted from the vial, separated from volatile organic vapor and water vapor by an automated -65 °C isopropanol-slush trap, a Nafion drier, a CO2 and water removal unit (Costech #021020 carbon dioxide absorbent with Mg(ClO4)2), and trapped in a small-volume trap immersed in liquid nitrogen with a modified Finnigan MAT (now Thermo Scientific) GasBench 2 introduction system. After the N2O is released, it is further purified by gas chromatography before introduction to the isotope-ratio mass spectrometer (IRMS). The IRMS is a Thermo Scientific Delta V Plus continuous flow IRMS (CF-IRMS). It has a universal triple collector, consisting of two wide cups with a narrow cup in the middle; it is capable of simultaneously measuring mass/charge (m/z) of the N2O molecule 44, 45, and 46. The ion beams from these m/z values are as follows: m/z = 44 = N2O = 14N14N16O; m/z = 45 = N2O = 14N15N16O or 14N14N17O; m/z = 46 = N2O = 14N14N18O. The 17O contributions to the m/z 44 and m/z 45 ion beams are accounted for before δ15N values are reported.

Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Determination of the δ15N and δ18O of nitrate in water; RSIL lab code 2900
Series title:
Techniques and Methods
Series number:
10-C17
Edition:
Version 1.0 - 2007, Version 1.1 - September 2012
Year Published:
2007
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Reston, VA
Contributing office(s):
Reston Stable Isotope Laboratory
Description:
viii, 35 p.
Larger Work Type:
Report
Larger Work Subtype:
USGS Numbered Series
Larger Work Title:
Chapter 17 of Section C: Stable Isotope-Ratio Methods, Book 10: Methods of the Reston Stable Isotope Laboratory
Number of Pages:
45
Online Only (Y/N):
Y