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The production of doubly charged sample ions by “charge transfer and ionization” (CTI) in analytical GD-MS

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Abstract

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.

Graphical abstract: The production of doubly charged sample ions by “charge transfer and ionization” (CTI) in analytical GD-MS

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Publication details

The article was received on 17 Nov 2016, accepted on 08 Feb 2017 and first published on 23 Feb 2017


Article type: Paper
DOI: 10.1039/C6JA00415F
Citation: J. Anal. At. Spectrom., 2017, Advance Article
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    The production of doubly charged sample ions by “charge transfer and ionization” (CTI) in analytical GD-MS

    S. Mushtaq, E. B. M. Steers, D. Barnhart, G. Churchill, M. Kasik, S. Richter, J. Pfeifer and K. Putyera, J. Anal. At. Spectrom., 2017, Advance Article , DOI: 10.1039/C6JA00415F

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