Matrix-Assisted Laser Desorption/Ionization of Size- and Composition-Selected Aerosol Particles

Matrix-assisted laser desorption/ionization (MALDI) was performed on individual, size-selected aerosol particles in the 2-8 μm diameter range. Monodisperse aerosol droplets containing matrix, analyte, and solvent were generated and entrained in a dry stream of air. The dried particles were drawn from atmospheric pressure directly into the source region of a reflecting field time-of-flight mass spectrometer. Individual particles were then detected by light scattering and analyzed on-the-fly by MALDI. The particle size and composition were systematically varied, and both liquid and solid matrices were studied. As the analyte-to-matrix mole ratio increased, the analyte signal intensity first increased and men leveled off. Quenching effects were observed when two different peptides were present in the same particle. These dependences were interpreted on the basis of analyte surface activity and adsorption isotherms. With a liquid matrix, the analyte is thought to partition between the particle surface and bulk. The signal intensity increases with analyte surface coverage until a monolayer is formed. With a solid matrix, the analyte is thought to adsorb on the surface of the original droplet containing matrix, analyte, and solvent When the solvent evaporates, the analyte deposits on the dry particle surface. Again, the signal intensity increases with analyte surface coverage until a monolayer is formed. With either a solid or liquid matrix, signal quenching is observed when multiple analytes compete for surface adsorption.