Attenuated Total Reflectance Crystal of Silicon for Rapid Nitrate Sensing Combining Mid-Infrared Spectroscopy.

Rapid detection of NO -N is critical to address the challenges of food security, environmental degradation, and climate change. Conventional methods for sensing NO -N in water demand pretreatments and chemical reagents, which are time- and cost-consuming. Consequently, Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy has been well applied for the determination of NO -N. However, the conventional ATR crystals, i.e., zinc selenide (ZnSe) and diamond, showed a weakness in duration or cost since the ZnSe material was relatively soft and diamond was relatively expensive. In this study, comparing with ZnSe-ATR and diamond-ATR, a silicon-based ATR (Si-ATR) accessory was developed and used to explore the applicability and stability for sensing NO -N combining mathematic algorithms. It was found that partial least-squares regression (PLSR) showed a good performance comparing with the algorithms of principal component analysis (PCA) and linear regression (LR), and it was recommended for quantifying NO -N. For ZnSe-ATR, the residual prediction deviation () was more than 1.80, the determination coefficient ( ) was more than 0.7725, and the root-mean-square error () was less than 2.73 mg L. For diamond-ATR, the was more than 1.76, the was more than 0.7309, and the was less than 8.22 mg L. For Si-ATR, the was more than 1.42, the was 0.5198, and the was less than 11.02 mg L. It was confirmed that all three types of ATR could be applied in the quantification of NO -N in water for high nitrate concentrations. However, for the quantification of low nitrate concentrations (0-1 mg L NO -N), ZnSe-ATR and diamond-ATR acquired the same accuracy, while Si-ATR had a lower accuracy. The pretreatment of Si-ATR-based spectra using the deconvolution algorithm could improve the prediction accuracy compared to water deduction for predicting low NO -N. Furthermore, a Si-ATR accessory was developed using Si-ATR, which was reliable for NO -N concentration quantification in water with the advantage of its low cost and long durability. Totally, samples with high nitrate concentrations implied a more reliable prediction for all crystals, and comprehensively, ZnSe-ATR was recommended for sensing low nitrate concentrations; diamond-ATR was recommended for samples with strong acid or base corrosion; and for sensing relatively high nitrate concentrations, such as in natural water bodies, Si-ATR was more economical because of its low cost and relatively long use life.