Generation and Propagation of MALDI Ion Packets Probed by Sheet-Like Nanosecond UV Laser Light

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A sheet-like ultraviolet (UV) probe laser is used to investigate the ejection and propagation of ion packets of matrix CHCA, which are produced by matrix-assisted laser desorption and ionization (MALDI). Laser irradiation of the expanding MALDI plume induced photodissociation of the CHCA-related ions, which existed in a sheet-like volume, leading to their absence in their MALDI signal profiles. The MALDI spectra were measured under varying conditions: the temporal delay of the lasers and the distance of the sheet-like probe laser from the MALDI sample surface. It was found that the center of the (CHCA)H⁺ packets were ejected at 46±11 ns after MALDI laser irradiation, while the (CHCA)₂H⁺ packets were ejected at 64±12 ns, regardless of the magnitude of acceleration static high-voltage in 3.5–5.5 kV. This suggests that (CHCA)₂H⁺ is formed by a proton transfer reaction from (CHCA)H⁺ to (CHCA)₂ in the heated condensed phase and/or near the surface. This study represents the first experimental determination of ion ejection time in the MALDI process, which is also applicable to other species in the MALDI plume.