Laser doppler spectrometer for study of electrokinetic phenomena

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Abstract
The laser Doppler spectrometer consists of a laser source, a scattering cell, a photomultiplier, a spectrum analyzer, and electrodes for applying an electric field. In a suitably applied field, bioparticles or macromolecules in the scattering cell will be forced to move along the electric field lines with a velocity given by the product of the electrophoretic mobility and the field amplitude. This induced motion causes a Doppler shift of the scattered laser light which is detected and measured by the spectrometer. Both the electrophoretic mobility and the diffusion constant can be measured simultaneously by measuring the scattered light spectrum in this way. Several procedures are used to obtain high resolution Doppler spectra. Convective instabilities due to Joule heating are minimized by use of rectangular electrodes with a very narrow gap (200 μ to 1 mm). Gas bubbling is minimized by a platinization of the Pt electrodes and by use of a square wave electric field. Examples of high resolution Doppler spectra from suspensions of polystyrene spheres are given and the various resolution limiting factors are described.