Indirect ultra-trace determination of nitrate and nitrite in food samples by in-syringe liquid microextraction and electrothermal atomic absorption spectrometry

A simple and rapid method for simultaneous separation and preconcentration of ultra-trace amounts of nitrate and nitrite using in-syringe liquid microextraction is developed. The extraction procedure is based on the selective ion-pairing complex formation of nitrate and nitrite with Cu(I)-neocuproine in 100 mu L of 1-butyl-3-methylimidazolium hexafluorophosphate as an extracting phase within a syringe. The atomic-absorption signal of copper in the ionic liquid layer is proportional to the nitrate and nitrite concentrations. When barbituric acid is used, nitrite is converted into violuric acid and thus it is removed from the environment. In this case, the selectivity of the modified-ligand is limited to nitrate. Under optimum experimental conditions, the calibration graph was in the range from 0.1 to 25 mu g L-1 for nitrate and 0.05 to 10 mu g L-1 for nitrite, respectively. The detection limits for these two anions were around 0.013 mu g L-1. The resultant repeatability expressed as relative standard deviations (RSDs, n = 6) was < 5.1%. The accuracy of the proposed method was confirmed using addition/recovery test and analyzing with the reference method. At two different addition levels for each analyte, recoveries were determined to be within the range from 94.2 to 103.3% for nitrate and 93.4 to 98.2% for nitrite, respectively. There was also no significant difference between the results when t-test is used with 95% confidence (n = 6). The applicability of the method was extended to the extraction and preconcentration of nitrate and nitrite from tuna fish, tomato paste and infant food sample.