Laser-induced volatilization and ionization of microparticles

Abstract

We have developed a method for the laser-induced volatilization and ionization of individual microscopic particles on a continuous, real-time basis. A beam of particles is produced by the expansion of an aerosol into a vacuum through a capillary nozzle and skimmer system. The particle in the beam is then hit by a high-energy Nd-YAG laser pulse for its volatilization and ionization. Mass spectral measurements using a quadrupole mass spectrometer on the ions thus generated from potassium biphthalate particles (1.96 μm in diameter) have been made. The combined ionization efficiency for K+ ion is found to be ∼10−6. The advantages of laser-induced volatilization and ionization along with the real-time capability of the present method make it very useful for the chemical analysis of aerosol particles by mass spectrometry. For the synchronization of the particle with the YAG laser pulse, the velocity of the particles in the beam has also been determined. This was accomplished by measuring the time-of-flight of monodisperse aerosol particles between two He–Ne laser beams. The velocities of different size particles are found to decrease with their increasing diameters and the velocity distribution for a given size is sharply peaked. The characteristic velocity may be used for the aerodynamic size measurement of aerosol particles and for relating the aerodynamic size to the optical size. Volatilization and ionization of microparticles in the beam eliminates any possible substrate-sample matrix interferences in the analysis. The method may also find applications in other spectroscopic and chemical kinetic studies.