Does asymmetric charge transfer play an important role as an ionization mode in low power–low pressure glow discharge mass spectrometry?

We report results of comprehensive studies using the Nu Instruments Astrum high-resolution glow discharge mass spectrometer (GD-MS) and optical emission spectrometry (OES) to investigate the relative importance of discharge mechanisms, such as Penning ionization (PI) and asymmetric charge transfer (ACT), at low-power/low-pressure discharge conditions. Comparison of the ratios of the ion signals of each constituent element to that of the plasma gas shows that for oxygen, the ratio in krypton is more than ten times higher than in argon (oxygen ground state ions are produced by Kr–ACT). For many elements, the ratios are very similar but that for tungsten is higher with krypton, while for iron, the reverse holds. These effects are linked to the arrangement of ionic energy levels of the elements concerned and the resulting relative importance of ACT and PI. The GD-MS and GD-OES results have shown that the ACT process can play an important role as the ionization mode in low-power/low-pressure discharges. However, OES results have shown that the magnitude of change in spectral intensities of elements studied are dependent on the discharge conditions. •GD-MS and GD-OES used to study the role of ACT in low power GD as in the Astrum GD-MS.•Ar and Kr plasmas used for Ti sample;10-fold greater O relative signal using Kr.•Increase due to Kr–ACT of ground state O ion, though ACT often thought irrelevant.•Higher W sensitivity with Kr due to ACT production of excited ions and decay to a6D.•ACT, known to be important in normal GD-OES, still important if power reduced.