A new method for on-line chemical analysis is under development. Particles entering the source region of a mass spectromter scatter light form a helium-neon laser beam. The scattered light triggers an excimer laser cousing one-step desorption and ionization of the particle. Low laser irradiances preferentially vaporize the particle surface; higher irradiances vaporize a larger fraction of the particle. A complete mass spectrum of the ionized material is subsequently recoreded with a time-of-flight mass analyzer. The method will be used to stduy surface-active organic and inorganic coatings on micron-size particles that are relevant to atmospheric aerosols. The ability of laser desorption to distinguish molecular species on the surface of n airosol particle from those in the total volume will be tested as a means of distringuishing surface and total concentrations of ammonium salt in layered particles. Laser desorption will be used to detect fractional crystallization on the surface of a particle during drying. The ultimate goal of this research is the development of a method for rapid on-line chemical analysis of atmospheric aerosol particles.
