The effect of thin conductive layers on glass on the performance of radiofrequency glow discharge optical emission spectrometry

The effect of thin conductive gold layers, on glass samples of different thickness, is investigated in detail in the performance of a radiofrequency (rf) glow discharge (GD) coupled to optical emission spectrometry (OES) in terms of sample sputtering rates, analytical emission intensities and emission yields using the rf-GD in the common operational mode constant pressure-constant forward power''. Guillot et al. have pointed out recently that the deposition of a conductive top layer on an insulator gives rise to an increase of the ratio plasma voltage/input voltage'' and to the enhancement of the analytical emission signals for the non-conducting substrate. Therefore, the present paper is meant to study such an effect on parameters of analytical interest. Gold layers 10 and 50 nm thick were investigated, while the selected glass thicknesses were in the interval 1.8-4.8 mm. In most experiments a forward power of 20 W was used, while in some studies the interval 20-30 W was investigated. The pressure was kept at 450 Pa. As expected, a decrease in the sputtering rates is obtained as the glass investigated gets thicker. However, this effect turned out to be more serious for homogeneous glasses than for those coated with gold. Studies on the effect of GD forward power on sputtering rates, investigated for homogeneous glasses, gold coated glasses and two conducting materials, showed that coated glasses exhibit an intermediate behavior between the insulating glasses and the conducting materials. Higher analytical signals were obtained for coated glasses than for the homogeneous samples. The differences in the signals were more noticeable the greater the thickness of the glass. Emission yields for Si and Ca were also investigated. Interestingly, they were found to be virtually independent of the insulator thickness, the presence of conductive layers, or the forward powers in the interval used ( from 20 to 30 W).