Smoothing of Doppler spectra line shapes in electrophoretic analysis of biological cell suspensions

The study of cell and particle electrophoresis by laser Doppler techniques requires the application of an electric field to a conducting solution. Steady d.c. electric fields are usually not possible because of electrode surface reactions. Hence, it has been common practice to apply temporally varying electric fields, either pulses of alternating polarity or square-wave fields of constant amplitude. Oscillating particle motion leads to harmonic structure (multiples of the square-wave electric field frequency) in the scattered light spectra. This harmonic structure may be considerably smoothed by a suitable variation of the period of the applied square-wave field. The application of a pseudo-random period oscillating electric field yields line shapes that approach the line shapes to be expected from the application of a continuous d.c. electric field.