Spectrophotometric method to determine nitrates in fresh vegetables using sodium salicylate reagent

The aim of the study is to validate the spectrophotometric method for the determination of nitrate ions in vegetables using sodium salicylate as a nitrating reagent using the method of solid-phase extraction on the SAX ion exchange cartridge. Research objectives: selection of conditions for solid-phase extraction; evaluation of the linearity and stability of the calibration dependence; comparison of the obtained results with the control HPLC method. The paper presents the results of the development of an accessible and relatively simple spectrophotometric method for the determination of nitrates in fresh vegetables using a reagent based on sodium salicylate. The studies were based on the colorimetric reaction of electrophilic substitution (nitration) between the nitronium ion NO2+ and salicylate in the presence of sulfuric acid, with the formation of nitrosalicylic acid, which has a stable yellow color at pH > 7. In order to purify the sample from the influence of soluble dyes and carbohydrates and concentrate, solid-phase extraction on a SAX ion-exchange sorbent was used. The detection limit for nitrates was 18 μg with a linear range of up to 1 mg using a sample volume of 5-10 cm3. The nitrate content (median) according to this method ranged from 8 to 94 mg/kg in various types of vegetables. The results for each group of measurements had a high degree of correlation in comparison with the data obtained by the HPLC method for the same samples. The degree of finding the additive averaged 108 % of the calculated one. The parameters of the linear model were estimated by the least squares method. The statistical significance of the equation was tested using the coefficient of determination R2 and Fisher's test. Thus, the high precision of the results was confirmed, which can be successfully used to measure the content of nitrates in vegetables in a wide range, while giving values comparable with the method of high performance liquid chromatography.