Mechanistic Studies on Cationization in MALDI-MS Employing a Split Sample Plate Set-up.

In the analysis of polymers by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), a commonly observed ionization pathway is cation-adduct formation, as polymers often lack easily ionizable (basic/acidic) functional groups. The mechanism of this process has been hypothesized to involve gas-phase cation attachment. In previous experiments, a split sample plate set-up has been introduced, enabling separate deposition of the components on individual MALDI plates. The plates are divided by a small gap of a few micrometers, allowing simultaneous laser irradiation from both plates, while precluding the possibility of any other interactions prior to ablation. Here, we extend on these studies by using different polymer-salt combinations to test the generalizability of a gas-phase ionization process. Clear evidence for in-plume ionization is presented for the model polymers poly (methyl methacrylate) and polystyrene. Furthermore, the contribution of in-plume processes to the overall ion formation by cationization is gauged, providing a first estimate for the importance of this pathway.