DETERMINATION OF SILICON IN ORGANIC-SAMPLES BY ATOMIC-EMISSION SPECTROMETRY BY USING A CAPACITIVELY-COUPLED MICROWAVE PLASMA

A cylindrical-shaped plasma has been produced in a helium capacitively -coupled microwave plasma (He-CMP) and applied to the determination of silicon in organic solutions. Samples were introduced by nebulization and thermal vaporization. Spatial profiles of Si emission, microwave power, helium plasma gas flow rat and the addition of molecular gases have been studied The electrodes were fabricated from graphite, titani um and tungsten. By using pneumatic nebulization, the tungsten (W) ele ctrode gave lower background and noise levels, compared to the graphit e electrode. No deterioration of the W electrode was visible after 150 h of operation. A limit of detection (LOD), 3sigma, of 0.3 mug mL-1 w ith a relative standard deviation (RSD) of 2%, was obtained by using n ebulization as the sample introduction technique. Electrode cups, for sample introduction by thermal vaporization, were made from graphite, titanium and tungsten. Graphite cups showed memory effects and a decre ased Si signal, compared to metal cups. The optimum ashing power and a shing time were 135 W and 10 s, respectively. Higher ashing power leve ls or longer ashing times decreased the Si signal, probably due to for mation of silicon carbide. The LOD (3sigma) for thermal vaporization w as 0.03 mug mL-1, while the absolute detection limit is 900 pg with a RSD of 5-18%.