Consideration of norm temperature and local thermodynamic equilibrium models for emission and ionization in inductively coupled plasmas

Abstract

Norm temperature and local thermodynamic equilibrium (LTE) models of emission intensity and ionization in ICPs are discussed. Based on each of the two models, detailed predictions of emission intensities and total atom, singly charged ion and doubly charged ion fractions are presented. The predicted behavior is compared to previously published experimental data. The norm temperature model results appear to be consistent with the experimentally observed behavior of the peak in emission intensity as a function of height in the plasma. However, assumptions made in the norm temperature model are inconsistent with LTE. LTE model predictions do not match experimentally observed data as a function of temperature. The LTE model results do not show a peak in emission intensity or ion to atom intensity ratio as a function of temperature from 3000 to 10000 K. The LTE model predicts that analytes in He ICPs should be more highly ionized than in LTE Ar plasmas. However, experimentally, He plasmas should be further from LTE. Possible causes of the disagreement between model predictions and experimental data include radiative deexcitation, non-LTE electron temperatures, mass transport effects, slow vaporization or ionization kinetics and non steady-state conditions.