Particles and monoliths are used for providing an ionized analyte for mass analysis by photon desorption having a size in the range of 0.5-100 .mu.m, wherein the particles or monoliths are modified with a chemical compound capable of absorbing photons having a wave-length of at least 300 nm. The particles and monoliths allow the use of MALDI-MS for the high throughput screening of molecules having a molecular weight lower than 700 .mu. without interfering signals or with only a limited number of background signals.