SPATIALLY RESOLVED LASER-INDUCED FLUORESCENCE STUDIES ON A 3-ELECTRODE DIRECT-CURRENT PLASMA

Spectrochemical emission from analytes in a three-electrode direct current plasma has been found to be sensitive to the presence of an easily ionisable element (EIE) as a concomitant in the sample matrix. In order to gain some insight into this effect, a fluorescence spectrometer was used as a spatial and spectral probe to obtain relative analyte population distribution profiles in the direct current plasma. Three-dimensional fluorescence profiles were collected at a variety of vertical spatial positions. These profiles show that the spatial distribution of barium ion ground states are dictated primarily by flow dynamics around the argon plasma jet. The area in the plasma where emission is observed was found to be relatively small compared with the over-all spatial distribution of the analyte, which suggests that the amount of analyte contributing to the emission is a small fraction of the total analyte population present. Three-dimensional fluorescence profiles of barium ions in the plasma were determined with and without the presence of sodium as the EIE. The presence of the EIE in the sample did not alter the spatial distribution of the Ba + population significantly, but it did appear to cause an ionisation suppression. Emission profiles of the Ba I 553.548-nm and Ba II 493.409-nm lines were also obtained with and without an EIE present in the sample matrix. For both atomic and ionic species, the presence of the EIE caused emission enhancements, but the effect was greater for the Ba I than the Ba II line.