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Abstract
A rapid, accurate, and sensitive method has been developed for the simultaneous determination of nitrite and nitrate. A low capacity strong anion exchange PRP-X100 Hamilton, 150 × 4.1 mm, 10 μm, with exchange capacity 0.19 ± 0.02 meq/g, analytical column was used, with a mixture of 0.1 M NaCl-CH3OH, at a volume ratio 45:55. The flow rate of 2 mL/min, lead to a pressure of 150 kg/cm2. Detection was performed with a variable wavelength UV-visible detector, at 230 nm, resulting in detection limits of 0.1 ng and 0.2 ng for nitrite and nitrate, respectively, per 20 μL injection. For the quantitative determination, iodide was used as internal standard, at a concentration of 7.0 ng/μL. A rectilinear relationship was observed up to 12 ng/μL for nitrite and up to 25 ng/μL for nitrate. Analysis time was less than 6 min. The statistical evaluation of the method was examined performing intra–day (n = 8) and inter-day calibration (n = 8) and was found to be satisfactory, with high accuracy and precision results.
The method was applied to the direct determination of nitrite and nitrate in drinking water (tap, table, and ground water) and in biological fluids (human serum). Solid phase extraction was tested for sample clean-up and analyte retention using SAX Lichrolut Merck cartridges. Low recovery of nitrite ca. 70% and high recovery of nitrate, approximately 130%, indicated the partial oxidation of nitrite during sample preparation. Solid phase extraction of solutions containing solely nitrite proved that a 40% oxidation is taking place, while the solid phase extraction of spiked serum samples yielded a 30% oxidation of nitrite anions. Therefore, a sample pre-treatment assay avoiding the solid phase extraction technique was applied, leading to approximately 80% recovery of nitrite and 120% recovery of nitrate.
The pre-treatment involves protein precipitation, using organic solvent and centrifugation of the sample. Endogenous compounds of human serum did not interfere in both cases, after SPE or without SPE. Total nitrogen recovery appeared to be quite the same, 98.6% and 98.0% respectively. Ascorbic acid used as anti-oxidant did not prevent the oxidation of nitrite.