Non-linear intensity response in glow discharge emission spectroscopy: Excitation, radiative transfer and self-absorption

Abstract

A comprehensive treatment is presented of signal response in glow discharge emission spectroscopy as function of the analyte concentration and the sputter rate of the sample matrix. Deviations from linear response are expressed by 1/Γ factors. Atomization, excitation, radiative transfer and self-absorption in the Grimm-type discharge is discussed. Experimental data on non-linear intensity response, line profiles of selected lines and some matrix effects are presented in the spectra of copper and zinc in the analysis of CuZnAl alloys and in the spectra of Fe, Ni, Cr. Plausible explanations of the findings are offered, based on the fundamentals of the discharge. Besides self-absorption, non-linear intensity response can also be caused by selective excitation processes due to heavy particle collisions, and even an apparent enhancement of some lines with a rising analyte concentration, a trend opposite to that caused by self-absorption. Charge transfer from argon ions to the atoms of some analyte elements is involved and other heavy particle reactions affecting the spectra, that may be occurring on the cathode surface or in its close vicinity. Similarities and differences with other analytical plasmas are discussed, including laser-induced breakdown spectroscopy.