Photoionization pathways and free electrons in UV-MALDI.

The recently developed model for primary and secondary UV-MALDI ion formation (Knochenmuss, R. J. Mass Spectrom. 2002, 37, 867-877. Knochenmuss, R. Anal. Chem. 2003, 75, 2199.) is applied to questions regarding photoionization pathways and electron versus negative ion production. Two-photon ionization of the matrix in direct contact with analyte is possible under some circumstances (Kinsel, G.; Knochenmuss, R.; Setz, P.; Land, C. M.; Goh, S.-K.; Archibong, E. F.; Hardesty, J. H.; Marynik, D. J. Mass Spectrom. 2002, 37, 1131-1140.), and is added to the model. When analyte is present in large mole ratios (such as when matrix suppression is desired), this effect contributes modestly to the ion yield. Generally, matrix exciton pooling remains dominant. The interfacial layer of thin samples on a metal substrate may also be ionizable in a 2-photon process. A mechanism is proposed, and the correspondingly modified model gives excellent agreement with electron emission versus laser intensity data. Capture in, or escape of low-energy electrons from a thick sample (or on a nonmetallic substrate) is also examined. Because the mean free path for MALDI electrons in a solid matrix is on the order of 10 nm, below such depths, any electrons generated are captured to form negative ions. Only a surface layer can emit free electrons. This surface emission effect is also well reproduced by the model, up to a laser intensity limit caused by surface charging. This charging phenomenon is investigated and illustrated by molecular dynamics calculations.