Effect of Humidity on Ionisation and Ion Chemistry in Active-Capillary Dielectric Barrier Discharge Ionisation

Dielectric barrier discharge ionisation (DBDI) in an active-capillary configuration (including the commercial SICRIT source) is widely used as a soft, ambient-pressure ionisation method for vapours and gases, yet the role of water vapour in controlling reagent ion chemistry and analytical behaviour has not been mechanistically described. Thus, it is necessary to understand how carrier gas humidity governs reagent ion populations and product ion formation in a DBDI source.

A SICRIT ion source was coupled to a triple-quadrupole mass spectrometer and operated under constant discharge conditions while switching the carrier gas between humidified and nominally dry air. Ketones and short-chain carboxylic acids were introduced at trace vapour concentrations using permeation or diffusion tubes. Reagent ion and analyte mass spectra were acquired in full-scan mode. Selected analyte ions were subjected to collision-induced dissociation (CID) to compare in-source fragments with those formed from protonated molecules.

Under humid conditions, the reagent ion population was dominated by hydrated hydronium ions. Analytes yielded mainly protonated molecules, MH⁺ and their hydrates, with minimal fragmentation. Switching to a dry carrier gas reduced the total ion current by more than an order of magnitude and led to extensive, structure-dependent fragmentation. Comparison with CID spectra of MH⁺ indicates that the fragments observed under dry conditions arise from the fragmentation of protonated molecules rather than from the direct dissociation of radical cations. This is explained by highly exothermic proton transfer from a dominant m/z 29 reagent ion (most plausibly N₂H⁺).

These results highlight humidity as a key operational active-capillary DBDI parameter; varying it enables both low-fragmentation ionisation in humid mode and more energetic fragmenting ionisation in dry mode. The humid mode facilitates reproducible quantitation, whereas the dry mode allows structural discrimination, exemplified by differentiation of hexanone isomers.