Direct determination of zinc in sea-water using electrothermal atomic absorption spectrometry with Zeeman-effect background correction: effects of chemical and spectral interferences

Abstract

The determination of zinc in sea-water using an electrothermal atomic absorption spectrometry system with Zeeman-effect background correction is presented. The influence of various chloride and nitrate salts on the atomization signal of zinc was examined. In chloride medium particularly, the interference effect induced through losses of zinc chloride, by the thermohydrolysis of magnesium chloride and simultaneous generation of HCl during the pyrolysis step is noted. In nitrate medium, zinc is more stabilized by Mg > Ca > Na > NH⁺₄. The effect of various inorganic and organic acids, used as chemical modifiers, on the atomization of zinc and background absorption signals in sea-water were examined. In unmodified sea-water, a Zeeman interference effect related to the vaporization of the chloride matrix leading to a systematic under-compensation and consequently to erroneous zinc concentration values was observed. In sea-water, modified with 1 mol l^–1 nitric acid, a spectral Zeeman interference effect induced by the Zeeman splitting of the absorption bands of NO molecules generated during the decomposition–reduction of nitrate was observed; the induced over-compensation is eliminated by selective pyrolysis at about 850 °C. The chemical interference effect (25%) is related to the simultaneous vaporization of zinc and sodium oxides; the detection limit (3σ) being about 80 ng l^–1 for a 10 µl injected volume of sea-water. In sea-water modified with 0.7 mol l^–1 oxalic acid, there is no significant interference effect and the detection limit in this medium is about 60 ng l^–1 for a 10 µl injected volume of sea-water.