Single Particle Optical Sizing (SPOS). I. Design of an improved SPOS instrument and application to stable dispersions.

We describe an instrument for single particle optical sizing, based upon small angle light scattering by individual particles. A colloidal dispersion is hydrodynamically focused into a narrow stream of 5 m width, which is illuminated over a length of 30 m by a perpendicular laser beam with a constant intensity across this detection volume, using a specially designed elliptical optical focus. As the particles pass one-by-one through the detection volume, the light pulses are detected under a scattering angle of 5°. The signals are stored in a multichannel analyzer and electronically processed. This set-up enables one to measure accurately the particle size and particle size distribution of both stable and aggregating colloidal dispersions. In this paper, we give some illustrative results for stable systems