Rapid Determination of Nitrate Nitrogen Isotope in Water Using Fourier Transform Infrared Attenuated Total Reflectance Spectroscopy (FTIR-ATR) Coupled with Deconvolution Algorithm

Nitrate is a prominent pollutant in water bodies around the world. The isotopes in nitrate provide an effective approach to trace the sources and transformations of nitrate in water bodies. However, determination of isotopic composition by conventional analytical techniques is time-consuming, laborious, and expensive, and alternative methods are urgently needed. In this study, the rapid determination of (NO3-)-N-15 in water bodies using Fourier transform infrared attenuated total reflectance spectroscopy (FTIR-ATR) coupled with a deconvolution algorithm and a partial least squares regression (PLSR) model was explored. The results indicated that the characteristic peaks of (NO3-)-N-14/(NO3-)-N-15 mixtures with varied N-14/N-15 ratios were observed, and the proportion of (NO3-)-N-15 was negatively correlated with the wavenumber of absorption peaks. The PLSR models for nitrate prediction of (NO3-)-N-14/(NO3-)-N-15 mixtures with different proportions were established based on deconvoluted spectra, which exhibited good performance with the ratio of prediction to deviation (RPD) values of more than 2.0 and the correlation coefficients (R-2) of more than 0.84. Overall, the spectra pretreatment by the deconvolution algorithm dramatically improved the prediction models. Therefore, FTIR-ATR combined with deconvolution and PLSR provided a rapid, simple, and affordable method for determination of (NO3-)-N-15 content in water bodies, which would facilitate and enhance the study of nitrate sources and water environment quality management.

Automated summary

This study explored a rapid method for determining (NO3-)-N-15 content in water bodies using Fourier transform infrared attenuated total reflectance spectroscopy (FTIR-ATR) coupled with a deconvolution algorithm and partial least squares regression (PLSR) model. The characteristic peaks of (NO3-)-N-14/(NO3-)-N-15 mixtures with different ratios were observed, and the (NO3-)-N-15 proportion was negatively correlated with the wavenumber of absorption peaks. PLSR models based on deconvoluted spectra showed good performance with ratio of prediction to deviation (RPD) values above 2.0 and correlation coefficients (R-2) above 0.84. The use of FTIR-ATR combined with deconvolution and PLSR provided a rapid, simple, and affordable method for determining (NO3-)-N-15 content in water bodies, facilitating the study of nitrate sources and water environment quality management.