DIRECT DETERMINATION OF ZINC IN SEA-WATER USING ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY WITH ZEEMAN-EFFECT BACKGROUND CORRECTION - EFFECTS OF CHEMICAL AND SPECTRAL INTERFERENCES

The determination of zinc in sea-water using an electrothermal atomic absorption spectrometry system with Zeeman-effect background correctio n is presented. The influence of various chloride and nitrate salts on the atomization signal of zinc was examined. In chloride medium parti cularly, the interference effect induced through losses of zinc chlori de, by the thermohydrolysis of magnesium chloride and simultaneous gen eration of HCI during the pyrolysis step is noted. In nitrate medium, zinc is more stabilized by Mg > Ca > Na > NH4+. The effect of various inorganic and organic acids, used as chemical modifiers, on the atomiz ation of zinc and background absorption signals in sea-water were exam ined. 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 spe ctral Zeeman interference effect induced by the Zeeman splitting of th e absorption bands of NO molecules generated during the decomposition- reduction of nitrate was observed; the induced over-compensation is el iminated by selective pyrolysis at about 850-degrees-C. The chemical i nterference effect (25%) is related to the simultaneous vaporization o f zinc and sodium oxides; the detection limit (3sigma) being about 80 ng l-1 for a 10 mul injected volume of sea-water. In sea-water modifie d with 0.7 mol l-1 oxalic acid, there is no significant interference e ffect and the detection limit in this medium is about 60 ng l-1 for a 10 mul injected volume of sea-water.