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In ESI-MS, we ionise the molecules using a high voltage. Why wouldn't this high voltage cause the ejection of an electron from the analyte itself, generating a M+ ion. Why do we have to assume the the positive charge comes from protonation of solvent molecules?

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  • $\begingroup$ The ions have to get into the mass spectrometer, and you need to have a spray, i.e. you need the Raleigh instability. Take the case of quaternary ammonium ions like tetramethylammonium. If you ran "electrospray" with no voltage on the ion source, you would see a vastly, vastly attenuated signal for tetramethylammonium, even though it is permanently charged and doesn't need protonation by the solvent for detection. Sure, electrochemical reactions at the capillary walls $\endgroup$
    – Curt F.
    Commented Jul 20, 2023 at 0:55

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You might be asking two questions here.

(1) Can the high electric field strength induce electron detachment in the vapor? No. Normal electrospray voltages are not high enough to do so. Air, for instance, has a breakdown voltage of around 30 kV/cm, and liquids and aerosol particles are ionized only at even higher electric field strengths. Electrospray voltages are around one-tenth of that.

(2) Can the positive potential at the capillary walls induce oxidation of $M$ to $M^+$? In principle, yes, but only if there are no other species present with higher oxidation potentials than $M$ -- such as the metal walls of the capillary.

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