Detection limits for lines widely differing in “hardness” and intensity ratios of ionic to atomic lines measured in 50 and 100 MHz inductively coupled plasmas

Using the same torch and nebulizer, a 50 and a 100 MHz inductively coupled plasma (ICP) with argon as working gas were rationally optimized to achieve maximum detection power. The detection limits (actually source SBRs) of 55 elements were measured in both ICPs under the respective compromise conditions using 84 spectral lines, comprising 30 atomic and 54 ionic lines. These detection limits did not differ essentially and therefore it appeared likely that the excitation conditions did not differ substantially either. It was considered whether this feature was reflected in the changes of the ratios of the intensities of ionic to atomic lines derived from the same set of measurements. A rational interpretation of the results in terms of Saha-Boltzmann relationships led to the conclusion that the changes in the ionic to atomic line intensity ratios could reflect closely similar excitation conditions only if a rigorous LTE coupling between the (electron) temperature (T) and the electron concentration (n_e) was precluded, in other words, the results pointed to the likelihood of a model in which T and n_e vary independently, n_e in fact being assumed constant within the range of conditions under consideration (T = 6300 ± 300 K). On the whole, the conclusions should be treated with caution, since no Abel inversion has been applied. However, the approach as such may be of interest as a basis for more rigorous experiments.