Abstract: Light (632.8 nm) from a linearly polarized 2-mW He - Ne laser was focused by a 2.5-cm-focal length lens to traverse an Ortho model 300-051100 sheath-flow cuvette in which the sample stream was injected along the axis of a sheath stream under laminar-flow conditions, and was detected by a 1-mm2 Si photodiode connected through a current-to-voltage converter to a digital voltmeter. The cuvette was located ~1.5 cm past the beam waist (waist spot size ~15 µm) and near the centre of the beam axis. A set of diffraction fringes was formed at the extremities of the field by interaction of the beam with the body of the cuvette, the intensity at the centre of the fringe system being markedly perturbed by the presence of the sample stream. The detection limit for a 20 µm-radius sample stream corresponded to a refractive index difference of 3 x 10^-6, i.e., ~15 pg for glycerol. Application to flow injection analysis and gradient-elution HPLC is envisaged.

Abstract: The cuvette (illustrated) consists of a square-bore flow chamber (250 µm) with thick (1.5 mm) silica windows. The sample stream (e.g., an aqueous suspension of polystyrene latex spheres) is introduced into the centre of a flowing sheath stream and illuminated with a polarized He - Ne laser beam of 10 µm spot size; the scattered light is collected with a microscope. The detection volume of the system can be reduced to 5 fl provided a tightly focused laser beam is used. The system can be applied in liquid chromatography and flow injection analysis.