An instrument has been developed to analyze the size and chemical composition of individual, air-borne particles. The particles are introduced into the vacuum chamber of a time-off-light mass spectrometer where they are detected and sized using an aerodynamic principle. After sizing, the particles are vaporized by an excimer laser and the resulting ions are analyzed. Experiments have shown that polystyrene particles 1 μm in diameter are not fully vaporized by the laser pulse. The instrument has been characterized and some examples of measurements on ambient and artificial aerosols are presented.

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 Particle Technology Group at Delft University of Technology is developing an instrument for on-line, real-time measurement of size and chemical composition of individual aerosol particles. Particles are sampled from an aerosol by using aerosol beam techniques. Collimated by the beam generator system the particles enter the ionization chamber of a time-of-flight mass spectrometer. There they are detected by the use of a photomultiplier when travelling through the beam of a small He-Ne laser and thereby scattering light. The photomultiplier signal is also used to trigger a Q-switched YAG laser which is vaporizing and ionizing the same particle. Analysis of the generated ions provides information on the chemical composition of the particles and the photomultiplier signal contains the particle size information.

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.

The aim of this work is the development of an instrument to analyze the size and chemical composition of individual aerosol particles "in-situ," on a continuous and reel-time basis. It is anticipated that its applications include aerosol research, monitoring on behalf of environmental end health effects, clean-room technology and monitoring of production processes. [...]