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Normally, in analytical GD-MS, the doubly charged metallic ion signals from the sample are several orders of magnitude less than the corresponding singly charged signals. However, we have observed that using a neon plasma, the M++ signals of some elements, which have double ionization energies close to the first ionization energy of neon, are of the same order as the M+ signal. Doubly charged ions may be produced directly in the discharge cell by electron ionization (EI), and also by two electron Penning ionization (TEP), but these processes cannot explain the above effect. In this paper, we suggest that an additional process named as ‘Charge Transfer and Ionization’ (CTI) produces such ions either in their ionic ground state or in an excited state. To confirm that this process is typical of the discharges used in GD-MS and not an artefact of any particular form of cell and ion extraction system, we have carried out comprehensive experimental measurements using three different GD-MS instruments, viz., Nu Astrum, VG9000 and ELEMENT GD and our results provide clear evidence for CTI. This is the first time the process has been identified as an ionization process in analytical GD-MS. CTI must be differentiated from Asymmetric Charge Transfer (ACT), which is a “selective” process and requires a close energy match (e.g.ΔE ＜ 0.5 eV for a strong effect). On the other hand, CTI is “non-selective” in the sense that a close energy match is not required (e.g. a strong effect is observed with ΔE∼ 2 eV), although the process only occurs for a limited number of elements, depending on the plasma gas used and the total energy required to doubly ionize the metallic atom. [ABSTRACT FROM AUTHOR]

Extensive investigations have been carried out in recent years on the effects of the presence of small amounts of molecular gases in analytical glow discharges (GDs) using argon as the plasma gas. Complementary studies using neon have shown that, when hydrogen is added to a neon discharge with a copper sample, the intensities of lines from most of the upper energy levels in the Cu II 3d9(2D)4p sub-configuration decrease. However, the 224.700 nm spectral line and other lines from the same upper level (3Po2) increase in intensity and this effect cannot be produced by the excitation processes normally considered. We show that asymmetric charge transfer (ACT) excitation by hydrogen molecules (H2-ACT) is the most likely explanation, the first time this process has been reported for any element in analytical GDs. We also explain why a similar effect is not observed with added nitrogen, although the ionization energies of hydrogen and nitrogen molecules are very similar. [ABSTRACT FROM AUTHOR]

In low pressure glow discharges such as the Grimm-type source, the relative intensities of lines in the analyte spectrum depend primarily on the plasma gas. The probability of selective excitation of certain analyte spectral lines by asymmetric charge transfer (ACT) involving carrier gas ions is increased if excited levels are close to resonance. Traces of light gaseous elements, present either as constituents in the sample such as oxide, hydrides, nitrides or occluded within the sample, can be involved in selective excitation or affect selective excitation by the plasma gas. Asymmetric charge transfer involving oxygen ions (O-ACT) has been observed in spectra recorded with the Imperial College high resolution vacuum UV Fourier transform spectrometer (FTS). In addition, analyte lines excited by asymmetric charge transfer with argon ions (Ar-ACT) are relatively weaker, probably due to quenching of argon ions and corresponding reduction of Ar-ACT. Recent glow discharge mass spectrometry results have shown that significant quenching of the argon ion population occurs with the progressive addition of oxygen to glow discharges. ACT by oxygen ionic metastable states is reported and discussed. [ABSTRACT FROM AUTHOR]