Lott Michael J, Howa John D, Chesson Lesley A, Ehleringer James R
IsoForensics, Inc., 421 Wakara Way, Suite 100, Salt Lake City, UT, 84108, USA.
Rapid Commun Mass Spectrom. 2015 Aug 15;29(15):1381-8. doi: 10.1002/rcm.7229.
Elemental analyzer systems generate N(2) and CO(2) for elemental composition and isotope ratio measurements. As quantitative conversion of nitrogen in some materials (i.e., nitrate salts and nitro-organic compounds) is difficult, this study tests a recently published method - thermal decomposition without the addition of O(2) - for the analysis of these materials.
Elemental analyzer/isotope ratio mass spectrometry (EA/IRMS) was used to compare the traditional combustion method (CM) and the thermal decomposition method (TDM), where additional O(2) is eliminated from the reaction. The comparisons used organic and inorganic materials with oxidized and/or reduced nitrogen and included ureas, nitrate salts, ammonium sulfate, nitro esters, and nitramines. Previous TDM applications were limited to nitrate salts and ammonium sulfate. The measurement precision and accuracy were compared to determine the effectiveness of converting materials containing different fractions of oxidized nitrogen into N(2).
The δ(13) C(VPDB) values were not meaningfully different when measured via CM or TDM, allowing for the analysis of multiple elements in one sample. For materials containing oxidized nitrogen, (15) N measurements made using thermal decomposition were more precise than those made using combustion. The precision was similar between the methods for materials containing reduced nitrogen. The %N values were closer to theoretical when measured by TDM than by CM. The δ(15) N(AIR) values of purchased nitrate salts and ureas were nearer to the known values when analyzed using thermal decomposition than using combustion.
The thermal decomposition method addresses insufficient recovery of nitrogen during elemental analysis in a variety of organic and inorganic materials. Its implementation requires relatively few changes to the elemental analyzer. Using TDM, it is possible to directly calibrate certain organic materials to international nitrate isotope reference materials without off-line preparation.
元素分析仪系统会生成用于元素组成和同位素比率测量的氮气和二氧化碳。由于某些材料（即硝酸盐和硝基有机化合物）中的氮难以进行定量转化，本研究测试了一种最近发表的方法——不添加氧气的热分解法——用于分析这些材料。
使用元素分析仪/同位素比率质谱仪（EA/IRMS）比较传统燃烧法（CM）和热分解法（TDM），后者在反应中不添加额外的氧气。比较使用了含有氧化态和/或还原态氮的有机和无机材料，包括尿素、硝酸盐、硫酸铵、硝酸酯和硝胺。之前热分解法的应用仅限于硝酸盐和硫酸铵。比较测量精度和准确性，以确定将含有不同比例氧化态氮的材料转化为氮气的有效性。
通过CM或TDM测量时，δ(13)C(VPDB)值没有显著差异，从而可以对一个样品中的多种元素进行分析。对于含有氧化态氮的材料，使用热分解法进行的(15)N测量比燃烧法更精确。对于含有还原态氮的材料，两种方法的精度相似。通过TDM测量的%N值比CM更接近理论值。购买得到的硝酸盐和尿素的δ(15)N(AIR)值在使用热分解法分析时比燃烧法更接近已知值。
热分解法解决了各种有机和无机材料在元素分析过程中氮回收率不足的问题。其实施对元素分析仪的改动相对较少。使用TDM，无需离线制备就可以直接将某些有机材料校准到国际硝酸盐同位素参考材料。
