Thermal Proton Transfer Reactions in Ultraviolet Matrix-Assisted Laser Desorption/Ionization

One of the reasons that thermally induced reactions are not considered a crucial mechanism in ultraviolet matrix-assisted laser desorption ionization (UV-MALDI) is the low ion-to-neutral ratios. Large ion-to-neutral ratios (10^–4) have been used to justify the unimportance of thermally induced reactions in UV-MALDI. Recent experimental measurements have shown that the upper limit of the total ion-to-neutral ratio is approximately 10^–7 at a high laser fluence and less than 10^–7 at a low laser fluence. Therefore, reexamining the possible contributions of thermally induced reactions in MALDI may be worthwhile. In this study, the concept of polar fluid was employed to explain the generation of primary ions in MALDI. A simple model, namely thermal proton transfer, was used to estimate the ion-to-neutral ratios in MALDI. We demonstrated that the theoretical calculations of ion-to-neutral ratios exhibit the same trend and similar orders of magnitude compared with those of experimental measurements. Although thermal proton transfer may not generate all of the ions observed in MALDI, the calculations demonstrated that thermally induced reactions play a crucial role in UV-MALDI.