Determination of the δ15N of nitrate in solids; RSIL lab code 2894

Techniques and Methods 10-C12

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The purpose of the Reston Stable Isotope Laboratory (RSIL) lab code 2894 is to determine the δ15N of nitrate (NO3-) in solids. The nitrate fraction of the nitrogen species is dissolved by water (called leaching) and can be analyzed by the bacterial method covered in RSIL lab code 2899. After leaching, the δ15N of the dissolved NO3- is analyzed by conversion of 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 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:
Publication Subtype:
USGS Numbered Series
Determination of the δ15N of nitrate in solids; RSIL lab code 2894
Series title:
Techniques and Methods
Series number:
Version 1.0 - 2007, Version 1.1 - September 2012
Year Published:
U.S. Geological Survey
Publisher location:
Reston, VA
Contributing office(s):
Reston Stable Isotope Laboratory
viii, 35 p.
Larger Work Type:
Larger Work Subtype:
USGS Numbered Series
Larger Work Title:
Chapter 12 of Section C: Stable Isotope-Ratio Methods, Book 10: Methods of the Reston Stable Isotope Laboratory
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