Changes in analyte emission intensities occur when either easily or non-easily ionizable elements are present as concomitant species at a concentration of 0.05 M. The direction (enhancement or depression of emission signals) and magnitude of the matrix effect are strongly dependent on radial and vertical location in the plasma. At some heights in the ICP, matrix-induced depressions of the emission intensity in the center are equal to enhancements off-center. As a result, no change in the line-of-sight emission intensity is observed. Initial fluorescence measurements suggest that the number of analyte ions in the normal analytical zone decreases in the presence of each of the concomitant species studied. However, it appears that the presence of concomitant species enhances the fraction of ions that are excited and that therefore emit light. The presence of Na and K resulted in larger enhancements in the fraction of ions excited than did the presence of Fe, Ni, or Ba.
John W. Olesik and Eric J. Williamsen, "Easily and Noneasily Ionizable Element Matrix Effects in Inductively Coupled Plasma Optical Spectrometry," Appl. Spectrosc. 43, 1223-1232 (1989)