Investigation of the Desorption Process in UV Matrix-Assisted Laser Desorption/Ionization with a Liquid 3-Nitrobenzyl Alcohol Matrix by Photoacoustic Analysis, Fast-Flash Imaging, and UV-Laser Postionization^†

Ultraviolet matrix-assisted laser desorption/ionization mass spectrometry (UV-MALDI-MS) is a widely used method for the analysis of a variety of biomolecules. The technique relies on the codesorption of matrix and intact analyze molecules from the condensed to the gaseous phase and Subsequent analyze ionization. In spite of numerous studies, the MALDI processes are not yet fully understood. Here, we used 3-nitrobenzyl alcohol as a liquid matrix and employed three complementary analytical techniques to monitor the phase transition and development of the ejected material plume. Each of these methods provided a time resolution in the nanosecond range. (i) Photoacoustic stress waves, generated by the recoil momentum of the ejected material, were recorded simultaneously with the corresponding MALDI-generated analyte ion signals. (ii) The MALDI plume expansion was monitored by using a second illumination laser for imaging the plume in dark-field and in scattered-light geometries, allowing discrimination between ejected droplets and gaseous plume domains. (iii) Time- and space-resolved UV-laser postionization of desorbed matrix molecules complemented these studies. The photoacoustic data indicate the transition from a primarily molecular ejection according to a quasi thermal model at low laser fluences to a (layer-by-layer) phase explosion above a fluence threshold of about 170 J/m(2). Notably. this threshold coincides well with the onset of sizable ion generation. A few distinct droplets are detected over the whole of the investigated fluence range.