Influences of oxidation ability on precision in nitrogen isotope measurements of organic reference materials using...

The precision of the standard measurements of reference materials (RMs) is critical to normalize the raw δ¹⁵ 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) CO₂ from the sample gas stream was removed to prevent interferences between CO₂ and N₂ ; (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 δ¹⁵ N values were further examined.

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 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 δ¹⁵ 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 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 being introduced into the isotope ratio mass spectrometer when the EA is equipped with a single-quartz reactor.