Laser-induced molecular desorption and particle ejection from organic films

We present a comparative study on UV matrix-assisted laser desorption (MALD ), laser-induced thermal desorption (LITD), IR laser ablation, and desorpti on via electrical substrate heating. In order to determine to what extent t he material is desorbed as individual molecules or ejected as particles, re spectively, the ablated material was intercepted by a trapping plate and th e particle size distribution was analyzed by atomic force microscopy (AFM). The ratio of ejected particles to free molecules is strongly dependent on the laser volatilization technique used. High energy densities in the sampl e, for example in UV MALD, favor molecular desorption leading to very smoot h films on the trapping plate. In contrast, IR polymer ablation and LITD of micrometer thick organic films is dominated by ejection of clusters and pa rticulates. Volatization by electrically heated flash filaments is not sign ificantly different from substrate mediated thermal laser desorption (LD). It may allow to miniaturize and simplify mass spectrometric instrumentation for applications requiring desorption-ionization techniques. (C) 1999 Else vier Science B.V. All rights reserved.