Interference of various salts on the determination of selenium by graphite furnace atomic absorption spectrometry: effect of sea water

The effect of Na, Mg, Ca and Sr in their nitrate, chloride and sulfate forms and sea salt, with and without the use of palladium, on the determination of selenium by electrothermal atomic absorption spectrometry was investigated. In the absence of any stabilizing agent, about 50% of selenium was lost as molecular species at low temperatures. The effect of salts on the shape of the atomization signal, the integrated absorbance and the stabilizing effect were highly dependent both on their nature and concentration. For low nitrate concentrations, significant increase of the integrated absorbance was observed. For higher nitrate concentrations, the sequestration of selenium in the corresponding oxides induced an important shift to higher atomization temperatures and, consequently a decrease of the integrated absorbance. Maximum pyrolysis temperatures were in the order Sr > Ca > Mg > Na. For low chloride concentrations, the matrix effect induced was similar to that of nitrate. However, for higher concentrations, the physico-chemical interactions were less important. The shift to higher atomization temperatures and, consequently, the variation of the integrated absorbance depended on the different volatility or hydrolytic properties of the chloride salts. Maximum pyrolysis temperatures were lower than in a nitrate medium. The effect of sulfates were mainly dependent on their concentration and decomposition/vaporization mechanisms. Indeed a strong depressive effect was observed in the presence of Na2SO4 whilst CaSO4 or MgSO4 had a behavior relatively similar to the corresponding nitrates. In the presence of nitrate or chloride salts, palladium had a similar delaying effect, signal shape and integrated absorbance. Pd was less efficient in the presence of sulfate. Maximum pyrolysis temperatures up to 1200 degrees C were obtained, except in the presence of Na2SO4 where the formation of a Pd-Se compound was more difficult due to the sequestration of Se in the Na2S formed in the atomizer. Good recovery of Se was obtained in the presence of Sr, through a modification of the decomposition/vaporization mechanism of sulfate salts; this modification was accompanied by a dramatic decrease of the corresponding background absorbance signal. Therefore, Sr can be recommended as a chemical matrix modifier in the presence of high sulfate concentrations, alone or mixed with Pd. Sea salt induced important modifications of the atomization signal of Se. The signal shape, integrated absorbance and shift to high atomization temperatures depended also on the sea salt mass and the presence of nitric acid. The important role of sea salt magnesium and eventually calcium on the atomization process of selenium is highlighted. In sea water, maximum pyrolysis temperatures up to 1200 degrees C are obtained in the presence of Pd. Consequently, the major part of the sea water matrix could be removed before atomization. However, in the presence of 10% nitric acid, an interference effect and a noisier baseline were observed when the sea salt mass was increased. The detection Limit of Se obtained in sea water by optimizing experimental conditions was 0.8 mu g l(-1) for a 10 mu l sample. (C) 1999 Elsevier Science B.V. All rights reserved.