Online Measurement Technique on ¹⁵N and ¹⁸O Isotopes of Nitrate in Groundwater

The studying of ¹⁵N and ¹⁸O in soil and water can be used to identify the origin, transportation and transformation of the nitrate, which provides reliable evidence for the remediation of the polluted environment. At present, high-temperature pyrolysis is the most widely used method for assessing ¹⁵N and ¹⁸O. However, NO from the ion source may interfere with the measurement of δ¹⁸O, which is efficiently reduced by the He dilution method. In this article, the selection of four international standards (IAEA-No-3, USGS32, USGS34 and USGS35) and one laboratory standard (CUGL-No-1) are documented to verify the practicability of high-temperature pyrolysis. Over the course of eight months, 238 tests were conducted for 100 groundwater samples by using coupled elemental analyser and MAT 253 stable isotope ratio mass spectrometer (EA-IRMS). Three new conclusions were obtained through the study. Firstly, the low cost target sample of KNO₃ was convenient for the comparison to the international standards. Secondly, it was concluded that online high-temperature pyrolysis was the best method to study ¹⁵N and ¹⁸O in nitrate of standards and samples. In this research, the sample amount of KNO₃ was only 500 μg. Moreover, the values of δ¹⁵N and δ¹⁸O were determined simultaneously in one sample input, which took just 720 s and had the added advantages of being rapid and having high efficiency. The accuracies of δ¹⁵N and δ¹⁸O were 0.25‰ and 0.6‰, respectively, which were close to the corresponding international level. Thirdly, the He dilution method reduced the interference to δ¹⁸O without any improvement of the EA-IRMS system.