Validação de testes comerciais para a determinação de nitrato em águas

Abstract

The determination of nitrate in aqueous matrices is of paramount importance for public health and environmental protection due to the risks associated with medical conditions such as methemoglobinemia and the occurrence of eutrophication. In this context, the present study aimed to validate a spectrophotometric analytical method based on the reaction of nitrate with 2,6-dimethylphenol (DMP), according to the DIN 38405-9 standard, using commercial ready-to-use test kits at the Laboratory of Environmental Analysis and Research (LAPAM) of the University of Caxias do Sul. Method validation was conducted following the guidelines of the DOQ-CGCRE-008 guidance document by evaluating selectivity, matrix effect, linearity, accuracy, limits of detection (LOD) and quantification (LOQ), intermediate precision (within-laboratory reproducibility), and interlaboratory comparison. The method proved to be selective in both the absence and presence of raw water, treated water, and wastewater matrices (F-test, α = 0.05). Linearity over the working range of 0.5-25.0 mg L-1 resulted in a coefficient of determination (R2) greater than 0.9945, with no outliers detected by Grubbs' test. Accuracy was confirmed by Student's t-test (tcalculated = 0.543 < ttabulated = 2.1) through comparison with the official phenoldisulfonic acid method (ABNT NBR 12620:1992). The LOD and LOQ were established at 0.1 mg L-1 and 0.5 mg L-1, respectively, demonstrating adequate sensitivity for compliance with the Brazilian regulatory limit of 10 mg L-1. In the interlaboratory comparison, the laboratory achieved a satisfactory z-score of 0.20. The within-laboratory reproducibility study showed a variability of only 0.36%. Furthermore, economic and operational analyses revealed a substantial reduction in analysis time, from 3 h 30 min to only 25 min, while eliminating the need to handle hazardous and corrosive reagents. Overall, the validated method based on the commercial DMP kit proved to be a reliable, rapid, and practical alternative for routine environmental monitoring and water quality control. [resumo fornecido pelo autor]