Irradiation effects in MALDI and surface modifications - Part I: Sinapinic acid monocrystals

The origin of the irradiation effect on the ion production in MALDI processes, using large sinapinic acid crystal, was studied in conjunction with the surface modifications. Characterization of the crystal irradiated surfaces by a commercial N-2 laser, from which ion production was quenched at the normal laser threshold energy, with scanning electron microscopy and light microscopy show very dramatic modifications of the surface structure. Three main regions with different morphologies were observed. A region with small craters in the center of the spot, a strong perturbed contiguous area with hill-valley patterns and a bead periodical structure extending up to the rim of the spot. Periodical bead structure mainly is responsible for the disappearance of ion production. Theses structures are eliminated after one or two shots at high energy. The analysis of the uppermost layers of the irradiated and non-irradiated surfaces, performed by chemical ionization with quadrupole mass analyzer, indicates that the matrix, at surface, yields mainly to decarboxylated sinapinic acid. The physical properties of the modified matrix can be responsible for the structural changes on the irradiated target and the vanishing of ion production.