Direct detection of particles formed by laser ablation of matrices during matrix-assisted laser desorption/ionization

We report the detection of nanoparticles formed by irradiating matrix-assisted laser desorption/ionization (MALDI) matrix samples. This is direct evidence for the ejection of large size aggregates in the MALDI process. Nanometer-size particles were generated via a tunable solid-state UV laser, irradiating a sample placed in a nitrogen atmosphere. Size distribution measurements were performed using a differential mobility analyzer and a condensation particle counter. Particles in the 10-1000 nm size range were detected. The dependence of the particle size distribution on the laser fluence, wavelength and matrix was investigated. The observed effects are discussed and related to the MALDI ablation dynamics and gas-phase processes.