Ionization interferences under various operating conditions in a 9, 27 and 50 MHz ICP, and a study of shifts in level populations of calcium through simultaneous absorption-emission measurements in a 9 MHz ICP
Radially resolved absorption and emission measurements were employed for a better understanding of the excitation mechanism of nebulized species operating under conditions favourable for the occurrence of ionization interferences in an atmospheric pressure 9 MHz ICP. Three monitored spectral lines of calcium were used to observe changes in ground and excited level populations of atoms and ions, in ion excitation temperatures using the two-line method. Observations were made at a fixed height, namely 25 mm above the rf coil and varying carrier gasflows from 2 to 51 min-1 and were correlated with the position of the "initial radiation zone" (IRZ) in the plasma. Ionization interferences occurring only inside the IRZ indicate an excitation mechanism depleting ion ground level population and populating excited atom and ion levels. No changes in atom absorbances or excitation temperatures were observed ruling out ionization suppression as dominating mechanism. Indications are that increased collisional excitation for Ca ions and ambipolar diffusion may be the dominant excitation mechanism operating in the analyte channel. Recombination reactions (three body or radiative) or charge transfer reactions may be responsible for an increase of excited atom level populations. It is obvious that non-thermal processes are operating under conditions favourable for ionization interferences occurring in the ICP. © 1985.