Application of Dynamic Dilution on δ¹³C and δ¹⁵N Measurements Using EA-IRMS System

In natural environment, there are wide varieties in C and N element contents and carbon-nitrogen ratio (C/N) for different samples. Element analysis-stable isotope ratio mass spectrometry (EA-IRMS) is selected to dilute these sample gas through strict weight controls to keep the intensities of sample gas consistent with the reference gas. However, this kind of fixed dilution method is commonly time consuming and labor intensive. In this study, a fast analysis method of EA-IRMS equipped with a dynamic dilution component was established for the measurements of the bulk δ¹³C and δ¹⁵N, which was expected to improve the measurement efficiency. For comparison, different dilution ratios of 0 (no dilution), 57% (fixed dilution) and 0.96% (dynamic dilution) were set. To detect the repeatability of detection for δ¹³C and δ¹⁵N, nine kinds of samples (honey, pork fat, pork, chicken, hair, spinach, apple, soil and caffeine) were measured by the dynamic dilution method. As for reproducibility assessment at different times, the above samples were measured on 4th March and 19th April 2016, respectively. The results indicated that EA-IRMS had an identical measurement result for δ¹³C and δ¹⁵N through both the dynamic and fixed dilution methods (p>0.05). For the 9 kinds of samples with different mass and C/N values, the EA-IRMS gave good repetitions for δ¹³C and δ¹⁵N measurements. Based on the dynamic dilution method, the measurement precision ranged from 0.07‰ to 0.32‰ for δ¹³C, and 0.05‰ to 0.27‰ for δ¹⁵N, respectively. There was no significant time drifts for the new EA-IRMS equipped with the dynamic dilution component during the 45 days of study periods. During these two measurement periods, the differences were less than 0.2‰ for δ¹³C, and 0.3‰ for δ¹⁵N, respectively. Because it was unnecessary to accurately measure the mass of different kinds of samples, the new EA-IRMS equipped with a dynamic dilution component would have the advantages of time and labor saving. It should have a broad application prospect for the rapid measurements of δ¹³C and δ¹⁵N in future research.