质谱准确测定高丰度^15N2的计算方法

Calculation Method for Accurate Determination of High Abundance ^(15)N_2 by Mass Spectrometry

近年来,稳定同位素质谱(IRMS)在农业、生态、环境以及地质资源中的应用越来越广泛。在氮素研究过程中,稳定同位素质谱仪主要测定的是^(15)N自然丰度或^(15)N富集较低的气体样品,在测定高丰度的^(15)N_2时,质谱仪自动计算出的结果在精确度和准确度上,都有很大的偏差。为了探究造成这种现象的原因,利用(NH4)2SO4与NaBrO溶液反应产生N_2,精确地配制了10、25、50、70、99.14atom%的^(15)N_2系列高丰度气体样品,利用MAT-253稳定同位素质谱仪测定气体样品的^(15)N_2丰度,并采用推导出的3种不同的公式:^(15)N atom%=1/1+2I28/I29×100、^(15)N atom%=I29+2I30/2(I28+I29+I30)×100和^(15)N atom%=2/I29/I30+2×100计算^(15)N_2系列样品的丰度,并与仪器给出的结果进行对比。结果表明,在测定^(15)N atom%≥10atom%的N_2样品丰度时,质谱仪计算机软件计算出的^(15)N丰度结果并不准确,必须采用公式进行人工计算。公式^(15)N atom%=1/1+2I28/I29×100和^(15)N atom%=I29+2I30/2(I28+I29+I30)×100只适用于计算丰度约为10atom%的N_2样品的^(15)N丰度,并要求以m/z28、29和30的峰面积代入公式进行计算;而当N_2样品^(15)N atom%≥10atom%时,均可采用公式^(15)N atom%=2/I29/I30+2×100计算^(15)N_2的丰度。该方法可为质谱测定高丰度^(15)N_2提供准确可靠的计算方法。

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＋2I28/I29×100,^15N atom%=I20＋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 ^15 N2 samples were put into the three formulas for calculating the abundance of the samples. The result showed that, formulas ^15N atom% =1/1＋2I28/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 the peak areas of m/z using the peak heights 10 atom% of ^15N2 samples. 28, 29 and 30 to calculate the For all of three abundance were because the amplification of receivers of m/z formulas, adopting more accurate than 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 ~SN2 abundance, which provides an accurate and reliable calculation method for the mass spectrometric determination of high abundance ^15N2 in the future.