Wang Nan, Liu Jingyu, Zhang Yang, Xia Cuimei, Lin Deju, Che Yangli, Wu Jianzheng, Bao Rui
Frontiers Science Center for Deep Ocean Multispheres and Earth System, Key Lab of Submarine Geosciences and Prospecting Techniques, MOE and College of Marine Geosciences, Ocean University of China, Qingdao, 266100, China.
Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao, 266100, China.
Rapid Commun Mass Spectrom. 2021 Jul 31;35(14):e9122. doi: 10.1002/rcm.9122.
The precision of the standard measurements of reference materials (RMs) is critical to normalize the raw δ N values of samples to the international scale. Because the measurement configuration is required to moderate the properties of both RMs and samples for obtaining an ideal measurement precision, it is important to determine the appropriate measurement configurations of RMs to achieve accurate nitrogen isotope compositions of samples. Therefore, it is critical to systematically examine RMs for optimizing the configurations and further improve the measurement precision.
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 made as follows: (a) as the in-house working standard, urea was used to investigate the influences of combustion through moderating the different oxygen injections (0-20 mL) and sample delay times (10-12 s) and optimize the combustion conditions to enhance oxidation ability; (b) CO from the sample gas stream was removed to prevent interferences between CO and N ; (c) 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 the precisions of the δ N values were further examined.
Our results showed that sufficient oxygen should be injected to improve the sample combustion when analyzing δ N in natural samples such as soil or marine sediment with low organic content. In addition, the measurement precision of δ N was affected by the tailing of the CO peak from the gas chromatography 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 δ N value, especially for RMs with low organic content, and the uncertainty of the measurements is improved to be better than 0.1‰ under optimized configurations.
The analytical conditions such as the oxygen flow rate and injection time or sample introduction time into the 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 δ 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, CO should be removed from the sample gas stream before being introduced into the isotope ratio mass spectrometer when the EA is equipped with a single-quartz reactor.
参考物质（RM）标准测量的精度对于将样品的原始δN值归一化至国际标准至关重要。由于需要测量配置来调节RM和样品的性质以获得理想的测量精度，因此确定RM的合适测量配置以实现样品准确的氮同位素组成很重要。因此，系统地检查RM以优化配置并进一步提高测量精度至关重要。
采用配备单石英反应器的在线元素分析仪（EA）与气体源同位素比率质谱仪（IRMS）联用分析氮同位素组成。进行了如下一些调整：（a）以尿素作为内部工作标准，通过调节不同的氧气注入量（0 - 20 mL）和样品延迟时间（10 - 12 s）来研究燃烧的影响，并优化燃烧条件以增强氧化能力；（b）去除样品气流中的CO以防止CO和N之间的干扰；（c）在优化配置下系统地分析了国际RM，包括USGS40（L - 谷氨酸）、IAEA600（咖啡因）和低有机含量的土壤标准物质（B2153），并进一步检查了δN值的精度。
我们的结果表明，在分析低有机含量的天然样品（如土壤或海洋沉积物）中的δN时，应注入足够的氧气以改善样品燃烧。此外，如果EA配备单石英反应器柱，δN的测量精度会受到气相色谱柱中CO峰拖尾进入后续样品测量的影响。我们的调整可以产生优化的δN值重复性和准确性，特别是对于低有机含量的RM，并且在优化配置下测量不确定度提高到优于0.1‰。
需要根据RM和样品的燃烧条件调整诸如氧气流速、注入时间或进入EA的样品引入时间等分析条件，以获得可靠的测量精度。我们建议，在分析低有机含量的天然样品（如土壤或海洋沉积物样品）中的δN时，应注入更多氧气以改善样品燃烧。此外，当EA配备单石英反应器时，应在将样品气流引入同位素比率质谱仪之前去除其中的CO。
