This paper describes an on-line instrument, capable of measuring the size and chemical composition of single, aerosol particles. Possible applications include monitoring aerosol reactors and studying atmospheric chemistry. The main conclusion is that a working prototype has been built and tested. It uses a three stage vacuum system to generate an aerosol beam with a low divergence angle and a high transmittance. The pressure is reduced sufficiently to allow the application of a time-of-flight mass analyzer. The aerosol beam is probed in the analysis section by the focused beam of a low-power helium-neon laser. Every particle crossing the laser beam scatters light, which is detected by two photomultiplier tubes, mounted at angles of 45 and 90°. The signal is stored when both detectors produce a pulse simultaneously, and this event triggers the chemical analysis cycle. A pulsed Nd: YAG laser vaporizes the particle and generates ions, which are next analyzed by a time-of-flight mass spectrometer. In this way combined information on the size and the composition of the particle is obtained.

The development of an on-line aerosol analysis instrument, making use of laser pyrolysis and T.O.F. mass spectroscopy, requires a particle beam generator to introduce the particles in the ionization chamber. The results of previous investigators and theoretical models have been used to improve the design of the beam system. The apparatus consists of a 0.5 mm nozzle and three vacuum chambers in series (0.8 mbar, 10−4 mbar and 4×10−6 mbar resp.), separated by skimmers. To minimize the solid angle and to limit the influence of particle size a capillary nozzle was selected. The distances between nozzle and skimmer and between both skimmers were optimized taking the formation of shock waves into account. The beam diameter and particle transmission efficiency were measured by examining depositions of 2.82 μm PSL spheres with an optical microscope. A slight contraction of the beam just behind the nozzle was observed, while about 4 % of the particles behind the nozzle reached the third vacuum chamber. To reduce the solid angle the use of sheath air is recommended.