Molecular modeling interpretation of IMMS data collected on negative ionization of methyl salicylate using radioactive, corona discharge and photo-ionization

Abstract

Except for electrospray ionization that was introduced later to handle biochemical analyses, the radioactive source has been the benchmark ionization source for ion mobility spectrometry. On the other hand, the licensing requirements accompanying the use of the source has created a desire to replace it with a nonradioactive source with an equivalent ionization capability. Two such sources are the discharge and photoionization sources that are capable of negatively ionizing the internally hydrogen bonded ketoB isomer of methyl salicylate. IMMS data show that if the negative reactant ions are NO 3 − ·HNO 3 (discharge source) or CO 3 − (doped photoionization source), methyl salicylate cannot be ionized. However when the sources are synchronously pulsed with a shutter grid, methyl salicylate can be ionized by the formation of the oxygen anion adduct, O 2 − ·M, similar to radioactive ionization. The ionization capability depends not only upon the partial concentration of oxygen in the immediate ionization region but also on the geometry used to construct the source. Molecular modeling shows that the UV radiation emitted by the sources will photodissociate the reactant ions that reconstitute as the energy of the UV radiation decreases during the pulsing cycle. The reconstitution of CO 3 − is delayed due to a change in geometric symmetry and is the first to leave the ionization region because of its high mobility. This allows methyl salicylate to be ionized by oxygen anions.