Nitrates and nitrites are inorganic anions which, beyond specific concentration threshold, are classified as water pollutants. Nitrate compounds are commonly used as fertilizers; however, their high concentration in soil and in wastewater, as well as their reduction to nitrites, pose serious environmental and human health risks. Therefore, detecting these ions in water intended for human consumption, zootechnical use, and agricultural applications is essential. This work presents a proof of concept for a spectroscopic prototype setup enabling simple, direct, and simultaneous detection of nitrates and nitrites in water. The device employs solid-state sensor technology and requires no sample pretreatment or chemicals. Ultimately, this apparatus will allow real-time, in-line process analysis. UV absorption bands centered at approximately 302 nm and 355 nm were selected for detecting nitrates and nitrites, respectively. Because nitrite exhibits a slight absorption at 302 nm as well, a straightforward method for simultaneous nitrate and nitrite detection is proposed. The proposed system incorporates a UV deuterium lamp, a 10 cm path length optical cuvette, and a custom home-built silicon carbide detector. This configuration enables testing various concentrations, achieving detection limits of 2.2 mg/L for nitrates and 0.5 mg/L for nitrites. Potential interferences from substances commonly found in drinking and treated agricultural wastewaters, including sodium bicarbonate, sodium sulfate, ammonium chloride, hydrogen peroxide, and sodium hypochlorite, were also investigated. Finally, a compact on-site and online monitoring future device is illustrated.

Integrating cavity spectroscopy is a well-established technique for measuring the optical properties of materials. In this paper, we provide a comprehensive theoretical description of a novel dual-line spectrometer, where the traditional cuvette is replaced by a new type of sample holder, similar to an integrating sphere, and coupled to a silicon photomultiplier for light detection. This setup allows simultaneous measurement of both a reference and a sample solutions contained in two identical innovative spherical holders with highly reflective walls. The effective optical path length increases, with respect to a standard optical cuvette, due to multiple internal reflections occurring within the spherical holders, enhancing absorbance and enabling even trace amounts of compounds in solution to be detected and quantified. Our theoretical model links the absorbance to the geometric and physical properties of the spherical holders, such as their diameter and internal reflectivity. Calculations also reveal a logarithmic relationship between solute concentration and measured absorbance. We also compare the performance of this new spectrometer with that of conventional cuvette-based instruments, particularly with regard to the limit of detection, and explain the origin of the enhanced sensitivity provided by the new design. It should be noted, however, that in the UV range the sensitivity is constrained by the rapid decrease in Teflon reflectivity below 300 nm, as well as by the drop in photon detection efficiency of the employed photosensor in the same spectral region.