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auction in the excitation temperature and a concomitant decrease in the electron concentration of the plasma, but increased both ion and neutral atom excitation. Such reductions are inconsistent with the enhancements observed. In a similar investigation, Boumans et al. (12) reported both en hancement and suppression effects due to the presence of easily ionized elements such as cesium and calcium. In these instances (12, 34), the effects are due to a combination of phenome na. Both CMP systems have design deficiencies in that the sample aerosol is injected into the plasma along its peripheral boundaries (see Figure 1), while the light emitted in the central portion of the plasma is selected for measurement. The problem is conse quently that of entry of sample into the central plasma which is overcome with the ICP by use of the toroidal configuration (1, 2). Moreover, the de gree of lateral diffusion of aerosol into the central region must change with solution composition as shown by West et al. (66, 67). The importance of this is particularly apparent for the CMP used by Murayama (34). Bou mans et al. (12) have also noted that as the amount of Cs present in solu tion increases, the geometry and zonal structure of the plasma undergo rath er striking changes. As a result, the re gion of maximum excitation shifts ac cordingly. To overcome or alleviate these problems, it would seem advis able to arrange for sample injection through the central core .of the plas ma. Enhancement effects on calcium and barium due to the presence of eas ily ionized elements were also noted for a MIP by Kawaguchi and asso ciates (44). This appears to have been due to typical ionization repression ef fects because the only observed en hancements involved elements for which ion line intensities were mea sured. A subsequent investigation (26) examined the effect of sodium on emission and reported no effect at so dium levels 1000-fold above the con centration of the analyte. Later stud ies have demonstrated that avoidance of such interference effects depends on the choice of the nebulizer and aerosol transport facilities (43). The primary requirement is the use of a nebulizer which produces droplets in the 1-2 μ mean size range. Solute va porization effects such as the phos phate repression of calcium emission have been reported for CMP (44) and MIP (26, 44) systems. Based on this overview, plasmas produced with low-power microwave supplies show considerable promise as spectral excitation sources. The rela tively simple spectra emitted are com prised primarily of lines originating
CIRCLE 88 ON READER SERVICE CARD 618 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
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