Pneumatically assisted desorption/ionization: 1. Some thoughts on the possible ionization mechanism(s)

We have observed that spraying solvent droplets on a zopiclone tablet produced MH(+) ions also in the absence of any electrical field and without the addition of organic acids to the sprayed solvent. The choice of a drug tablet as test bench has been done for the signal stability, higher than that observed when the drug is directly placed on a stainless steel surface. This behavior indicates that the formation of MH(+) ions is mainly due to pneumatical effects and the results are discussed with respect to those obtained by other research groups. Different mechanisms contributing to MH(+) production under these conditions are proposed and discussed. The local heating of the solvent thin layer present on the surface has been calculated and the small temperature increase (and the consequent small decrease of pK(a) value) suggests that this effect can play only a minor role. However, different solvents have been employed for studying this aspect and, quite surprisingly, the best results have been obtained with acetonitrile (ACN). Experiments performed by spraying CD(3)CN showed again the formation of MH(+) and not MD(+), and this excludes the role of ACN as protonating medium. A further thought was stimulated by the behavior observed by varying the sheath gas (N(2)) flow, showing that the MH(+) ion intensity increases by increasing the flow. Side effects related to the highest kinetic energy of the spraying droplets can be considered, but an active role of N(2) in the MH(+) formation could be taken into account, by considering the possible ionization of N(2) by collisional phenomena. The N(2)(+*) ions could undergo a charge-exchange reaction with analyte molecules leading to a short-lived odd electron ion which behaves as protonating media for neutral molecules. The above-described mechanism does not require either the presence of electrical fields nor the addition of organic acid to the sprayer solvent and can give a rationale for what was observed when only pneumatical conditions are employed.