Mechanistic study on the ionization of trace gases by an electrospray plume

The fact that electrosprays of pure solvents can efficiently ionize gas-phase analytes has been known for decades, although this method has not been widely exploited. With the advent of ambient mass spectrometry, this approach is becoming increasingly popular. However, the mechanism by which vapors become ionized in the encounter with electrospray plumes remains largely unknown. This has been our motivation in this study in which we have exposed a set of amine vapors towards electrosprays of water, water/methanol (1/1) and methanol. The ionized vapors were characterized via ion mobility-mass spectrometry. We further tested a recently developed model to predict ionization probabilities for the encounter and charge transfer of vapors with ions or charged droplets emerging from an electrospray source. We found: (i) the highest sensitivity with water sprays and the poorest with methanol; (ii) an approximate correlation of sensitivity with vapor mass, being the heaviest species insoluble in water; (iii) different electrical mobility spectra for the same compounds ionized from the liquid phase and from the gas phase (i.e. one main feature for gas-phase and 3 features for liquid-phase); (iv) a closer agreement with the model for ion-molecule reactions than for droplet-vapor charge exchange. We conclude that the analytes could not possibly be dissolved in the droplets to be reemitted as if they were originally present in the electrospray solution. Our observations suggest that the vapors are ionized via ion-molecule reactions. (C) 2011 Elsevier B.V. All rights reserved.