A New Method for Nitrogen Isotopic Analysis of Nitrate in Water by Using Laser Spectrum

Stable isotope techniques are useful to provide insights into the effects of anthropogenic disturbances on aquatic ecosystem functions. ¹⁵N natural abundance (expressed as the δ¹⁵N value) of nitrate NO^-₃ can provide insights into N cycling in both aquatic and terrestrial ecosystems. Isotope ratio mass spectrometry (IRMS) is one of the most popular methods for the measurement of the nitrogen isotopic composition. However, isotope analysis of inorganic nitrogen isotopes in water has been hampered by large sample requirement, and laborious sample preparation and analyses NO^-₃ in solutions being converted to N₂O by chemical conversion or bacterial denitrification methods, and then analyzed by purge-and-trap IRMS. In this study, we established a new method for nitrate-nitrogen isotope ratio analysis in water by using laser spectrum analysis system (LSAS), with an innovative equipment of interface and sample preparation device (ISPD). The LSAS is the type of quantum cascade laser absorption spectroscopy and cavity ring-down spectroscopy (CRDS) instrument using mid-infrared laser absorption spectroscopy to measure N₂O isotope ratios. We examined the overall performance on reference samples on varying initial NO^-₃ concentrations, as well as the water samples from different sources. The results showed that the conversion yields for NO^-₃ to N₂O was over 50% by using the auto ISPD instrument for sample preparation. The limits of quantification (LOQ) obtained from LSAS measurement was in the range of 1.0-2.5 μmol N and the gas amount was applicable for the instrument range. The δ¹⁵N values for nitrate in all reference solutions and water samples were obtained with a good precision (<0.5‰) without isotope fractionation, and agreed with the results obtained by IRMS. The established LSAS method is a promising approach that can batch detection of water samples at nmol level concentration of nitrate and avoids traditional IRMS instruments pre-concentration and cryofocusing steps. It has several advantages for future users to consider, such as more feasible, less extensive lab resources and less expensive.