Determination of Nitrite in Milk- and Soy-Based Nutritional Ingredients by Derivatization with 2,3-Diaminonaphthalene and Fluorescence Spectrometry

Nitrite (NO2-) is an inorganic anion that can be found in various powdered milk- and soy-based nutritional ingredients as an incidental contaminant. Reliable determination of NO2- in nutritional ingredients is of paramount importance to ensure the safety of finished products. The derivatization reaction of NO2- with 2,3-diaminonaphthalene with the formation of fluorescent 2,3-naphtotriazole has been adapted to milk- and soy-based nutritional ingredients. The sample preparation consisted of protein precipitation with Carrez solution, simple pass through cleanup of extracts utilizing a carbon black-based cartridge and derivatization, followed by batch fluorometry. The method was validated in six representative ingredient matrixes-i.e., whole-milk powder, nonfat dry milk, milk protein concentrate, whey protein concentrate, sodium caseinate, and soy protein isolate. Recovery values were 82-109%, whereas within-day and intermediate precision were 0.6-5.2 and 3.6-11% (RSDs), respectively. The method LOQ was 0.1 or 0.2 mu g/g sodium nitrite (NaNO2), depending on the ingredient matrix. Surveyed NO2- concentration levels in 25 lots of 10 types of nutritional ingredients ranged from between less than 0.1 to 29 mu g/g NaNO2. This method is proposed as a more sensitive and rugged alternative to the widely used ion chromatographic and colorimetric approaches.