Yankuang ceshi (Mar 2013)
Online Measurement Technique on 15N and 18O Isotopes of Nitrate in Groundwater
Abstract
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.
