Picosecond vs. Femtosecond: are all Laser Desorption Ionization created equal?

Three-dimensional (3D) momentum imaging was used to investigate the ionization dynamics of matrix-assisted laser desorption/ionization (MALDI) initiated by both picosecond and femtosecond laser pulses. Over the past three decades a significant effort has been focused on increasing the sensitivity of this method, which remains low for large biomolecules. Traditional MALDI experiments utilize UV pulses to initiate laser desorption of molecules/ions from the surface into the gaseous plume. In the plume, the facilitation of charge transfer is mainly due to collisions between matrix and analyte species. While it is customary to use nanosecond lasers, femtosecond lasers have been implemented in several MALDI studies. The orders of magnitude difference between these pulse durations would suggest that different ionization mechanisms are at play. Here, we have acquired the first 3D momentum images of desorbed ions from 2,5-dihydroxybenzoic (DHB) acid, a common MALDI matrices. The momentum distributions and relative TOF spectra revealed striking difference between the desorption processes initiated by picosecond and femtosecond pulses. The lack of initial ion momentum in all three dimensions from femtosecond pulses activation indicate a suppression of plume formation. This feature can be exploited to enhance the analyte detection sensitivity of MALDI.