Analysis of nitrate ion in nettle (Urtica dioica L.) by ion-pair chromatographic method on a C30 stationary phase

Nitrate ion is a frequent pollutant not only in soil and natural water resources but in vegetables and foods as well. In our study we focused on nettle due to its increased ability to accumulate nitrate ions. A new, simple method for the separation and determination of nitrate ion based on reversed-phase ion-pair chromatography has been elaborated. A new four-step sample pretreatment method enables the precipitation of proteins and oxidative degradation of compounds that may disturb the identification of the nitrate ion: (1) extraction of the total nitrate content, (2) precipitation of proteins with acetonitrile, (3) oxidative degradation of the organic contaminants with H2O2, (4) evaporation of the solvent and taking up of the residue in water. The chromatographic separations were carried out on a high-density C30 stationary phase under isocratic conditions. The optimal mobile-phase composition was 10% (v/v) acetonitrile and 90% (v/v) 20 mmol L(-1) phosphate buffer, containing 2 mmol of tetrabutylammonium hydroxide at pH 6.0. The method could also be used for the separation of IO3(-), SeO3(2-), BrO3(-), NO2(-), Br-, SeO4(2-), and I- ions. The validated method is sensitive (the detection limit is 0.18 ng of nitrate ion). The method is linear in a high concentration range (0.031-30.66 microg mL(-1)). Recoveries varied between 98% and 103%. Reproducibility of the elaborated sample pretreatment method showed 1.54%. The method can be used for the determination of nitrate ion from different plants.