元素分析-同位素质谱联用测定微量氮元素同位素方法研究

Determination of δ^(15) N on Microgram Amounts by Modified Element Analysis-Isotope Ratio Mass Spectrometry

在保证仪器正常出峰的情况下,缩小元素分析仪氧化管、还原管的管径,优化色谱柱条件,降低载气流速,进而减小载气对进入质谱仪的N 2稀释比,降低元素分析-同位素质谱(EA-IRMS)测试固体样品中氮同位素的检出限。结果表明,优化前元素分析仪载气流速为80 mL/min,大约20μg氮元素在质谱检测器产生的峰面积约3×10^(-8),δ^(15) N值标准偏差为0.13‰(n=6);优化后载气流速降至20 mL/min,大约4μg氮元素产生的峰面积达到约2×10^(-8),其δ^(15) N值标准偏差为0.18‰(n=6),满足仪器测试氮同位素的标准偏差须小于0.2‰的要求。

Generally,to satisfy the requirement of the standard deviation of 0.2‰(n=6)in testingδ^(15) N by element analysis-isotope ratio mass spectrometry(EA-IRMS),the quality of the absolute content of nitrogen is needed to more than 20μg.However,solid sample packing quality exceeds 30 mg,nitrogen in sample is difficult to fully oxidation by moment in the cobustion tube,it may result in obvious trailing phenomenon of the spectrogram,then lead to low test results of the nitrogen content and produce isotope fractionation.Therefore,it is difficult to obtain an ideal result in testing certain solid samples with low nitrogen content(less than 0.07%)using the current EA-IRMS technique,such as rock powders,marine sediments and poor soil.In this study,to solve the difficulties in testing the nitrogen isotopes of sediments with low nitrogen content,the diameter of combustion tube,reduction tube and scrubber tube and the condition of chromatographic column were optimized on the basis of fully mastering elemental analyzer and the principle of mass spectrometry interface.In addition,the flow rate of the carrier gas was reduced and the peak type of nitrogen gas chromatogram was basically unchanged,eventually reduced the target gas dilution and improved the nitrogen detection limit of EA-IRMS.Before modification,the carrier gas flow rate of the elemental analyzer was 80 mL/min,and the peak area of about 20μg nitrogen in mass spectrometer was about 3×10^(-8),and the standard deviation ofδ^(15) N was 0.13‰(n=6).After modification,the gas flow rate was reduced to 20 mL/min,and the peak area of about 4μg nitrogen reached about 2×10^(-8),the standard deviation ofδ^(15) N was 0.18‰(n=6),which met the requirement that the standard deviation ofδ^(15) N tested by this instrument should be less than 0.2‰.This modification will greatly extend the testing range of EA-IRMS forδ^(15) N in solid samples with low nitrogen content and will have significant application value.