The volumetric Kα emission rate emitted from the Electron Cyclotron Resonance (ECR) heated plasma of an ECR ion source (ECRIS) can be used to estimate the inner shell ionization rate of a gas species. Additionally, the thick target radiation originating from the walls of the plasma chamber can be used to estimate the energetic electron losses from the magnetic confinement system of the ECRIS. With a previous investigation performed on the JYFL 14 GHz ECRIS and the 14.5 GHz Grenoble Test Source (GTS) at iThemba LABS, Kα diagnostics was used to determine the influence of the source tune parameters on the inner shell ionization rate. With this investigation it was established that the neutral gas pressure and the absorbed microwave power had the most pronounced effect on the volumetric emission rate. Specifically, it was observed that the volumetric emission rate increases with increasing neutral pressure. As an extension of this investigation, the current study probes the influence of the volumetric Kα emission rate of the working gas as a function of various mixing gases. By adding a lighter mixing gas to the working gas, the intensity of high charge states of the heavier element is known to increase. This effect is not completely understood but common explanations for its effectiveness include ion cooling, the dilution effect, increasing the electron density and increasing the plasma stability. With the current investigation the relative importance of all of these effects were assessed and the results will be discussed.