Optimizations of the EA-IRMS system for δ15N analysis of trace nitrogen

The elemental analyzer-isotope ratio mass spectrometer (EA-IRMS) technique is widely utilized to measure the nitrogen isotope ratio (delta N-15) in various environmental samples. However, this method has limitations when analyzing samples with low nitrogen content (e.g., soils, rocks and atmospheric deposition) due to significant sample dilution (>99%) before entering the ion source. To reduce sample size requirement and increase sensitivity, modifications can be made to the elemental analyzer and analytical conditions to transfer more sample gas to the ion source. In this study, we presented four modifications to the EA-IRMS instrument, including the reaction tube, water trap, gas chromatography (GC) column, and inlet splitter connected to ConFlo, along with the analytical conditions of the GC column, carrier gas flow rate, and split ratio for improving sensitivity. The results show that reducing the carrier gas flow rate from 80 to 30 mL/min increased the sensitivity of the EA-IRMS system by twofold. By increasing the split ratio from 3:2 to 3:1, the sensitivity increased from 2.32 Vs/mu g N to 3.93 Vs/mu g N. The optimized EA-IRMS has a sample size requirement of at least 12 mu g N and a standard deviation of <0.2 parts per thousand for delta N-15 analysis. We found that the modifications of the GC column and water trap had a negligible influence on sample peak and measurement and thus were deemed unnecessary. The effective modifications in the study were reducing the carrier gas flow rate and increasing split ratio, which increased the concentration of the analyte gas per unit volume and more efficient sample transfer to the ion source. This upgraded method can provide crucial technical support for environmental samples with low nitrogen content.

Automated summary

The elemental analyzer-isotope ratio mass spectrometer (EA-IRMS) technique is commonly used for measuring nitrogen isotope ratio in various environmental samples. However, analyzing samples with low nitrogen content has limitations due to significant sample dilution. In this study, modifications were made to the instrument and analytical conditions to improve sensitivity and reduce sample size requirement. The results showed that reducing the carrier gas flow rate and increasing the split ratio significantly increased sensitivity.