Evaluation of nitrate determination techniques in fresh vegetables: A comparison of spectrophotometric and chromatography methods

Due to the dynamic nature of nitrate in fresh vegetables and the presence of multiple inaccurate detection techniques, quantifying nitrate concentration remains more challenging than detecting other pollutants. This study evaluated the effectiveness of three common methods, namely high-performance liquid chromatography (HPLC) and two spectrophotometric methods (ISO 6635 and Cadmium-Column Reduction) for measuring nitrate in vegetables. For this purpose, 11 vegetables with varying nitrate accumulation potentials were analyzed. It was observed that the nitrate concentration in onions, as measured by spectrophotometric methods, was lower than the results obtained by HPLC. Consequently, the spectrophotometric methods were modified to improve nitrate measurement in onion samples. The results demonstrated that the average nitrate concentration in leafy vegetables was higher than in tuberous, root, and fruit vegetables. Among the vegetables, spinach, lettuce, and celery, respectively, exhibited the highest nitrate concentrations, while watermelon, tomato, and onion had the lowest levels. A strong correlation was observed across all three methods for determining nitrate content. The HPLC method demonstrated the highest accuracy in samples with high nitrate content, while the spectrophotometric methods were more accurate for samples with low nitrate content. Although the HPLC method incurs higher equipment costs, it is faster, more accurate, and safer than the spectrophotometric methods. Since both Cd-Column reduction and ISO 6635 are based on the Griess reaction, their accuracy depends on factors such as pH, temperature, time, and sample matrix. Therefore, each method can be selected based on laboratory equipment, costs, and specific advantages or limitations.