Especiação de compostos nitrogenados empregando a espectrometria de absorção molecular de alta resolução com fonte contínua (HR-CS MAS)

Abstract

The present work presents a new technique for the speciation of nitrogenous compounds in environmental and food samples, using the high-resolution continuum-source molecular absorption technique (HR-CS MAS). The nitrite ion was determined by the generation of vapor (CVG) of NO coupled to HR-CS MAS in the presence of hydrochloric acid and ascorbic acid. Factors such as the temperature of the quartz cell, carrier gas flow, CVG system flow and acid concentration were optimized in this step. Nitrite and nitrate speciation methods were studied, using the photoreduction of nitrate to nitrite - irradiation of ultraviolet light (UV) - in the samples containing nitrate in buffered medium with ammonium chloride at pH 7,97, in the presence of EDTA. In this way, the nitrate concentration, using the reduction process, is added to the concentration of the previous nitrite, being speciated by subtraction of the final concentration of the initial one. P-nitrophenol (PNP) was determined by breaking the C-N bond in the structure, employing UV radiation in the presence of hydrogen peroxide at pH 11.8, generating nitrite, which was determined by the reduction to NO vapor. The optimized conditions of the proposed method for the determinations (speciation) through NO formation were: (I) nitrite determination: absorption line in 214,803 nm, quartz cell temperature of 600 oC, carrier gas flow of 6 L.H-1, CVG system flow of 8.0 mL.min-1, [HCl] 3.0 mol.L-1 and [Ascorbic acid] 3.5% (m.v-1); (II) Determination of nitrate: [NH4Cl] 2.0 mol.L-1 at pH 7.97, [EDTA] 1.0 mmol.L-1 and 20 seconds of UV irradiation (254 nm and 15 W). (III) Determination of PNP: [H2O2] 1.0 mmol.L-1 at pH 11.8 and 20 seconds in UV irradiation (254 nm and 15 W). Calibration curves were constructed in order to evaluate the behavior of the species in each condition of selective determination, and, later, the method was applied in the samples. Both the conserve water and the solid food, oven dried at 70 oC up to constant mass, were analysed and then crushed in ultra-pure deionized water at 70 oC for 40 minutes. The samples were centrifuged and then filtered. Addition and recovery tests were performed in the samples to evaluate the efficiency of the method and possible sources of interference in the system. The values of R2, LOD (mg.L-1) and LOQ (mg.L-1) were, respectively, 0.9903; 0.047 and 0.16 for nitrite; 0.9904; 0.090 and 0.30 for nitrate and 0.9957; 0.034 and 0.11 for PNP.