SOME IMPROVEMENTS ON THE PARTICLE BEAM GENERATOR

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 x 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-mu-m 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.