Determination of nitrite and nitrate by reversed-phase high-performance liquid chromatography using on-line post-column photolysis with ultraviolet absorbance and electrochemical detection

Abstract

Ion-pair (paired-ion) reversed-phase HPLC has proved to be an effective technique for the analysis of inorganic anions. Also referred to as ion-interaction chromatography, it has bee used with conductimetric, ultraviolet (UV) absorbance, refractive index, and electrochemical detection. Nitrite and nitrate are amenable to UV detection at wavelengths of approximately 240 nm and below. In addition, nitrite is readily oxidized at potentials of +0.9 V or higher at a glassy carbon electrode. While nitrate is electrochemically unreactive under these conditions, it will undergo photolysis, with the conversion product (in all likelihood the nitrite anion) generating an oxidative signal. This paper describes the use of a Teflon TM knitted open tubular (KOT) reactor, which when wrapped around a UV source, provides a means of continuous, on-line photolysis. This derivatization step, combined with high-performance liquid chromatography, permits the determination of both nitrite and nitrate using oxidative amperometric detection. We have applied this technique to a number of samples (cured meats, smoked and fresh salmon, smoked cod, spiked water solutions) and have also obtained comparative data for nitrite by UV and direct ( i.e. non-photolytic) oxidative electrochemical detection, as well as by a standard spectrophotometric procedure.