The effect of plasma composition on chemical and physical events in the direct current plasma

Abstract

Analytical atomic emission spectroscopy has been advanced greatly by the introduction of the plasma as an emission source. The high excitation which it obtains make it an excellent choice for atomic analysis. Still today the excitation processes that occur in the plasma are not clear. In this investigation the chemical and physical effect of plasma com position are studied. The plasma com position was changed by the introduction of other gases into a direct current plasma (DCP). The gases studied here are nitrogen and helium. Various plasma diagnostics were employed to characterize the plasmas. Measurements of atomization efficencies, excitation temperatures, detection limits, and electron densities were made. The atomization efficiency was determined by the degree of dissociation of the refractory metals in the plasma. The excitation temperatures were determined by using a series of Fe lines. The electron densities were determined by the absolute measurement of the continuum intensity in the plasma. Because of errors in the literature, a comprehensive derivation and presentation of the theory involved in continuum measurements is given. Nitrogen was introduced as a nebulization gas into the plasma. The effect of the substitution of the nebulization gas was evaluated. The operation of the plasma under these conditions was favorable with most analyses showing only marginal degradation with the exception of the refractory com pounds. The increase in molecular background was determined to be only marginal since this could be avoided by a prudent choice of analysis wavelength. The use of He in the DCP was applied in three ways: nebulization with He into a Ar plasma, the use o f He/Ar mixtures as the plasma and an all-He plasma. The methods in which He was used with Ar showed little improvement. However, when an all-He plasma was investigated, the added excitation from the He excited states made possible the detection of the halogens by non-resonant emission. Detection limits for all systems were determined.