Influences of oxidation ability on precision in nitrogen isotope measurements of organic reference materials by EA-IRMS.

Wang, Nan; Liu, Jingyu; Zhang, Yang; Xia, Cuimei; Lin, Deju; Che, Yangli; Wu, Jianzheng; Bao, Rui

RATIONALE The precision of reference materials (RMs) standard measurements is critical to normalize the raw δ15 N values of samples to the international scale. Since the measurement configuration is requested to moderate the properties of both RMs and samples for obtaining an ideal measurement precision, therefore it is of importance to determine the appropriate measurement configurations of RMs to achieve accurate nitrogen isotope compositions of samples. Therefore, it is urgent to systematically examine RMs for optimizing the configurations and further improve the measurement precision. METHODS Gas source isotope ratio mass spectrometry (IRMS) coupled with an online elemental analyzer (EA) equipped with a single quartz reactor was performed to analyze the nitrogen isotope compositions. Some adjustments were executed as follows: (1) as the in-house working standard, urea was used to investigate the influences of the combustion through moderating the different oxygen injections (0~20 mL) and sample delay times (10~12 s), and optimize the combustion conditions in order to enhance the oxidation ability; (2) CO2 from the sample gas stream was removed to prevent interferences between CO2 and N2 ; (3) international RMs including USGS40 (L-glutamic), IAEA600 (caffeine), and soil standard (B2153) with a low organic content, were systematically analyzed under the optimized configurations, and their precisions of the δ15 N values were furthermore examined. RESULTS Our results showed that sufficient oxygen should be injected to improve the sample combustion when analyzing δ15 N in natural samples such as soil or marine sediment with low organic content. In addition, the measurement precision of δ15 N was affected by the tailing of the CO2 peak from the GC column into the subsequent sample measurement if the EA is equipped with a single quartz reactor column. Our adjustments can produce an optimized repeatability and accuracy of the δ15 N value, especially for RMs with low organic content, and the uncertainty of the measurements is improved better than 0.1‰ under optimized configurations. CONCLUSIONS The analytical conditions such as the oxygen flow rate and injection time or sample introduction time into EA, need to be adjusted depending on the combustion conditions of the RMs and samples to obtain a reliable accuracy of measurement. We recommend that when analyzing δ15 N of natural samples such as soil or marine sediment samples with low organic content, more oxygen should be injected to improve the combustion of the samples. In addition, CO2 should be removed from the sample gas stream before introducing into the IRMS when EA is equipped with a single quartz reactor.