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Determination of the δ 15 N and δ 18 O of nitrate in solids; RSIL lab code 2897

Tyler B. Coplen·Haiping Qi·Kinga Revesz·Karen Casciotti·Janet E. Hannon
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Abstract

The purpose of the Reston Stable Isotope Laboratory (RSIL) lab code 2897 is to determine the δ 15 N and δ 18 O of nitrate (NO 3 - ) in solids. The NO 3 - fraction of the nitrogen species is dissolved by water (called leaching) and can be analyzed by the bacterial method covered in RSIL lab code 2900. After leaching, the δ 15 N and δ 18 O of the dissolved NO 3 - is analyzed by conversion of the NO 3 - to nitrous oxide (N 2 O), which serves as the analyte for mass spectrometry. A culture of denitrifying bacteria is used in the enzymatic conversion of NO 3 - to N 2 O, which follows the pathway shown in equation 1: NO 3 - → NO 2 - → NO → 1/2 N 2 O (1) Because the bacteria Pseudomonas aureofaciens lack N 2 O reductive activity, the reaction stops at N 2 O, unlike the typical denitrification reaction that goes to N 2 . After several hours, the conversion is complete, and the N 2 O 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 CO 2 and water removal unit (Costech #021020 carbon dioxide absorbent with Mg(ClO 4 ) 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 N 2 O 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 N 2 O molecule 44, 45, and 46. The ion beams from these m/z values are as follows: m/z = 44 = N 2 O = 14 N 14 N 16 O; m/z = 45 = N 2 O = 14 N 15 N 16 O or 14 N 14 N 17 O; m/z = 46 = N 2 O = 14 N 14 N 18 O. The 17 O contributions to the m/z 44 and m/z 45 ion beams are accounted for before δ 15 N values are reported.

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Tyler B. Coplen, Haiping Qi, Kinga Revesz, Karen Casciotti, Janet E. Hannon. 2007. Determination of the δ 15 N and δ 18 O of nitrate in solids; RSIL lab code 2897. https://doi.org/10.3133/tm10c14

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