Indirect determination of nitrate by electrothermal atomic absorption spectrometry using an on-line cadmium microcolumn

Abstract

The proposed indirect automatic method is based on the reduction of nitrate to nitrite using a cadmium reductant microcolumn where the flow of the sample through the microcolumn oxidizes the metallic cadmium to CdII, which is measured by electrothermal atomic absorption spectrometry. The automatic system is achieved simply by replacing the sample tip of the autosampler arm with the reductant microcolumn. This system results in a powerful integrated system which permits fully automated operation, avoiding time-consuming manual work and enhancing the sensitivity. The manifold was evaluated with respect to accuracy, precision and sensitivity for nitrate determination in water, the detection limit achieved being 0.05 µg ml^–1 (3s) with a sampling rate of about 35 h^–1. A 2.3% relative standard deviation was obtained in a repeatability study (n = 11) at the 0.2 µg ml^–1 level of nitrate and 2.6% at the 8.0 µg ml^–1 level. Some ions which commonly occur with nitrate in water samples, such as Na⁺, Ca²⁺ and Mg²⁺, interfered to a certain extent, although these interferences were minimized by passing of the sample through an Amberlite cation-exchange resin. The method was applied to samples of bottled and spring waters with good results. The matrix effects were minimized by using the standard addition method.