Development of A Spectrophotometric Method with Enhanced Sensitivity for the Determination of Nitrate Contamination in Vegetables

Sensitive and environmental friendly sequential method for the determination of nitrate in vegetables was proposed. The method was based on the use of a new combination of Griess reagents (sulfanilic acid and N(1-napthyl) ethylenediamine dihydrochloride) in solid form added directly to nitrate solution in acid medium. The method was optimized for the amounts of charcoal, zinc, diazo-coupling reagents and pH. The validated method had detection and quantitation limits of 0.022 and 0.066 ppm respectively. The dynamic linear range extended between 0.088 and 1.2 ppm (R2 = 0.9999). The method recovery in aqueous solutions was between 96.25% ∓4.40% and 99.70% ∓ 4.20% for seven measurements with an average of 98.52% ∓4.11% indicating high method accuracy and optimum conversion of nitrate to nitrite. The method was successfully applied for the determination of nitrate in two leafy crops (mint and coriander) and two root vegetables (white radish and carrot) collected from five different local markets in the capital of Yemen, Sana’a. The average recovery values of nitrate in the four vegetables matrices ranged between 95.63% and 107.50%. The assessment of nitrate in the vegetables samples revealed that carrots contained the least amount of nitrate (77.90 mg/kg) among the tested vegetables flowed by white radish (641.84 mg/kg), mint (786.86 mg/kg) and coriander (1740.00 mg/kg). The data of the present study confirmed that the proposed method has the required sensitivity to determine nitrate bellow the regulated level in various kinds of leafy and root vegetables.