A Refinement of Nitrogen Isotope Analysis of Coal Using Elemental Analyzer/Isotope Ratio Mass Spectrometry and the Carbon and Nitrogen Isotope Compositions of Coals Imported in China

The ability to accurately analyze nitrogen (N) isotopes of coal is important for evaluating its contribution to NOX emissions to the atmosphere via coal combustion. Although an elemental analyzer/isotope ratio mass spectrometer is highly efficient for analyzing the N isotopes of coal, it requires large amounts of organic matter (>3 mg) to produce enough N-2 to enable N isotope measurements due to the high organic carbon contents of coal with relatively low N contents. Therefore, to completely oxidize the organic matter, more oxidant matter is required to enhance coal combustion in a reactor tube in an elemental analyzer through injecting O-2 during the combustion process. One reference material (RM) (GBW11104) was used to determine the effect of amounts of O-2 on the accuracy of nitrogen isotope values obtained. Our results show that small amounts of O-2 injected into the EA resulted in incomplete coal combustion, which yielded a lower N content and more positive N isotope ratio for RM (GBW11104) than those obtained by complete combustion. Thus, to ensure the complete combustion of coal, large O-2 injection amounts are required. The refined method was applied to provide the N isotope values of coals imported from Southeast Asia, Russia, and Australia. Combined with the carbon isotope values, the imported coals from three coal-producing regions (Southeast Asia, Russia, and Australia) display distinct characteristics.