Determination of δ15N on Microgram Amounts by Modified Element Analysis-Isotope Ratio Mass Spectrometry

Generally, to satisfy the requirement of the standard deviation of 0.2‰ (n=6) in testing δ¹⁵N by element analysis-isotope ratio mass spectrometry (EA-IRMS), the quality of the absolute content of nitrogen is needed to more than 20 μg. 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 20 μg nitrogen in mass spectrometer was about 3×10^-8, and the standard deviation of δ¹⁵N was 0.13‰ (n=6). After modification, the gas flow rate was reduced to 20 mL/min, and the peak area of about 4 μg nitrogen reached about 2×10^-8, the standard deviation of δ¹⁵N was 0.18‰ (n=6), which met the requirement that the standard deviation of δ¹⁵N tested by this instrument should be less than 0.2‰. This modification will greatly extend the testing range of EA-IRMS for δ¹⁵N in solid samples with low nitrogen content and will have significant application value.