Relative sensitivity factors in glow discharge mass spectrometry: The role of charge transfer ionization

A model is developed to explain the experimental relative sensitivity factors (RSF) in GDMS, based on transport and ionization of the sputtered atoms. The densities of argon metastable atoms and argon ions and the flux energy distribution of electrons throughout the discharge, which are needed to calculate the ionization factor, are obtained from explicit modelling work of the dc-GD, instead of using fitting parameters for these quantities, as was done in previous models. Since the literature contains values for cross-sections of asymmetric charge transfer ionization for only a few elements, the work was carried out in reverse order. This process was neglected in the first instance. It was found that the model based on transport and Penning ionization only (electron impact ionization is of minor importance), is not able to explain the variations in experimental RSFs for all elements. The discrepancy is clearly correlated with the (non)availability of suitable energy levels for charge transfer of the element ions. This correlation strongly suggests that ionization by asymmetric charge transfer between argon ions and the spattered atoms is able to explain the variations in the RSF values.