An equilibrium model for the radial intensity distribution of analyte lines in the ICP discharge

• Published in: Spectrochimica acta. Part B: Atomic spectroscopy 1983, Vol.38 (1), p.15-27
• Main Authors: Eckert, Hans U., Danielsson, Allan
• Format: Article
• Language: English
• ISSN: 0584-8547; 1873-3565
• DOI: 10.1016/0584-8547(83)80099-8

Simple expressions are derived in terms of exponential functions for the relative intensity distributions of analyte atom and ion lines in the chemical ICP torch for the limiting cases of vanishing and full ionization. The expressions are based upon a concave, parabolic temperature profile, a Gaussian distribution of analyte particle density and upon the existence of thermal equilibrium. The electron density distribution over the radius is assumed to be constant. For given temperature and particle profiles the shape of the line profile is determined by the excitation energy; beyond a critical value it develops off-axis peaks whose separation increases with further increase in excitation energy. The ionization energy of an element in general also affects the distributions of atom and ion line intensities. Its influence vanishes only for atom lines under zero ionization and for ion lines under full ionization. Numerical values of the profile parameters for specific operating conditions are determined by fitting expressions derived from the model to experimental values of peak separation and profile half width for atomic and ionic lines that were obtained through end-on observation and thus represent averages over the torch length. To obtain a theoretical basis for the effect of axial position on the profile shape, the model is made two dimensional by assuming plausible variations of the profile parameters with the axial coordinate. Profiles calculated on this basis show a gradual transition from double peak to single peak with increasing observation height and agree qualitatively with experiments. Finally, it is shown that off-axis profiles of a line as they have been obtained in laser fluorescence experiments can, for a given off-axis distance, be readily calculated from a radial profile of that line.