Time-resolved imaging of the MALDI linear-TOF ion cloud : Direct visualisation and exploitation of ion optical phenomena using a position and time sensitive detector

In this study, we describe the implementation of a position- and time-sensitive detection system (Timepix detector) to directly visualize the spatial distributions of the matrix-assisted laser desorption ionization ion cloud in a linear-time-of-flight (MALDI linear-ToF) as it is projected onto the detector surface. These time-resolved images allow direct visualization of m/z-dependent ion focusing effects that occur within the ion source of the instrument. The influence of key parameters, namely extraction voltage (E (V) ), pulsed-ion extraction (PIE) delay, and even the matrix-dependent initial ion velocity was investigated and were found to alter the focusing properties of the ion-optical system. Under certain conditions where the spatial focal plane coincides with the detector plane, so-called x-y space focusing could be observed (i.e., the focusing of the ion cloud to a small, well-defined spot on the detector). Such conditions allow for the stigmatic ion imaging of intact proteins for the first time on a commercial linear ToF-MS system. In combination with the ion-optical magnification of the system (similar to 100x), a spatial resolving power of 11-16 mu m with a pixel size of 550 nm was recorded within a laser spot diameter of similar to 125 mu m. This study demonstrates both the diagnostic and analytical advantages offered by the Timepix detector in ToF-MS.