Calculation Method for Accurate Determination of High Abundance 5N₂ by Mass Spectrometry

Stable isotope ratio mass spectrometers (IRMS) have been more and more widely used in the agricultural, ecological, environmental studies and geological resources, among them, gas IRMS is mainly used to measure the natural or low enriched ¹⁵N abundance samples in the process of nitrogen research. However, in terms of determination of high abundance of ¹⁵N₂, it was found that the given results of the automatic calculation of mass spectrometer calculator had a great deviation in precision, especially in accuracy. In order to explore the causes of above problem, 10, 25, 50, 70 and 99.14 atom% series of high abundance gas samples of ¹⁵N₂ were prepared precisely according to the method that ammonium sulfate and sodium bromate solution could produce N₂ by reaction. Then, m/z 28, 29 and 30 of the ¹⁵N₂ samples were determinated by MAT-253 IRMS. In this study, three different formulas for calculating the abundance of ¹⁵N₂ samples were deduced, respectively as ¹⁵N atom%=1/1+2(I₂₈/I₂₉)×100, ¹⁵N atom%=(I₂₉+2I₃₀)/2(I₂₈+I₂₉+I₃₀)×100 and ¹⁵N atom%=2/(I₂₉/I₃₀+2)×100. Finally, the datas of m/z 28, 29 and 30 of ¹⁵N₂ samples were put into the three formulas for calculating the abundance of the samples. The result showed that, formulas ¹⁵N atom%=1/1+2(I₂₈/I₂₉)×100 and ¹⁵N atom%=(I₂₉+2I₃₀)/2(I₂₈+I₂₉+I₃₀)×100 were suitable for calculation of the ¹⁵N₂ abundance about 10 atom%, and formula ¹⁵N atom%=2/(I₂₉/I₃₀+2)×100 was applicable to calculate the abundance higher than 10 atom% of ¹⁵N₂ samples. For all of three formulas, adopting the peak areas of m/z 28, 29 and 30 to calculate the abundance were more accurate than using the peak heights, because the amplification of receivers of m/z 28, 29 and 30 were different, so magnifications were to be converted to the same level just at the artificial calculation. The receiving cups in the actual measurement process were different from the theory, the larger the differences, the greater the deviation from the results. However, the peak areas were high resistance to the same multiple cases, so there was no error amplification factor. In addition, the abundance of N₂ was larger, the combination probability of ¹⁵N was higher, and the ion current intensity of m/z 30 was stronger, so the calculation of high abundance ¹⁵N₂ samples were mainly determined by the intensity of m/z 30. Therefore, the results of the automatic calculation of mass spectrometer calculator without m/z 30 were wrong. As a result, using the m/z 28, 29 and 30 of ¹⁵N₂ and the three abundance calculation formulas deduced in this paper could obtain the accurate ¹⁵N₂ abundance, which provides an accurate and reliable calculation method for the mass spectrometric determination of high abundance ¹⁵N₂ in the future.