PREPARATION OF NEAR HOMOGENEOUS SAMPLES FOR THE ANALYSIS OF MATRIX-ASSISTED LASER-DESORPTION IONIZATION PROCESSES

Highly uniform thin films of samples for matrix-assisted laser desorpt ion/ionisation (MALDI) have been fabricated by depositing a saturated solution of ferulic acid onto a soda lime glass disc and crushing with polished aluminium, the films covering large areas of the substrate a nd having a thickness between 45-60 mu m. The effects that different s ubstrates and crushing materials as well as sample concentration and s ample recrystallisation have on these films has been examined by scann ing electron microscopy. Such films have been shown to have a lower th reshold fluence for matrix ion detection than standard dried-droplet s amples, the reduction being approximately 15% for three of the five ma trices analysed, An explanation for this is proposed in terms of crush ed samples possessing a greater average energy per unit volume coupled to them by the laser due to their improved surface uniformity. Furthe rmore, samples that are dried at refrigerated temperatures (similar to 2.25 degrees C) are shown to have a much improved macroscopic uniform ity over samples dried at room temperature. Refrigerated and crushed M ALDI samples yield analyte ions with good spot-to-spot and pulse-to-pu lse reproducibility and both preparation steps appear to improve the r esolution of spectra obtained with a time-of-flight mass spectrometer.