Haiping Qi Kinga Revesz Karen Casciotti Janet E. Hannon Tyler B. Coplen 2007 The purpose of the Reston Stable Isotope Laboratory (RSIL) lab code 2897 is to determine the &delta;¹⁵N and &delta;¹⁸O of nitrate (NO₃^-) in solids. The NO₃^- 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 &delta;¹⁵N and &delta;¹⁸O 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: <p>NO₃^- &rarr; NO₂^- &rarr; NO &rarr; 1/2 N₂O (1)</p> Because the bacteria <i>Pseudomonas aureofaciens</i> 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 &deg;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 (<i>m/z</i>) of the N₂O molecule 44, 45, and 46. The ion beams from these <i>m/z</i> values are as follows: <i>m/z</i> = 44 = N₂O = ¹⁴N¹⁴N¹⁶O; <i>m/z</i> = 45 = N₂O = ¹⁴N¹⁵N¹⁶O or ¹⁴N¹⁴N¹⁷O; <i>m/z</i> = 46 = N₂O = ¹⁴N¹⁴N¹⁸O. The ¹⁷O contributions to the <i>m/z</i> 44 and <i>m/z</i> 45 ion beams are accounted for before &delta;¹⁵N values are reported. application/pdf 10.3133/tm10C14 en U.S. Geological Survey Determination of the &delta;¹⁵N and &delta;¹⁸O of nitrate in solids; RSIL lab code 2897 reports