Photon Correlation Spectroscopy

In photon correlation spectroscopy (PCS) or quasi-elastic light scattering (QELS) the Brownian motion (movement in random direction) of sub-micron particles is measured as a function of time. A laser beam is diffracted by particles in suspension. The diffusion of particles causes rapid fluctuations in scattering intensity around a mean value at a certain angle (varying from 10 to 90°). These intensity fluctuations depend on particle size. The calculated correlation function results in a diffusion coefficient for a given temperature and viscosity which can be converted to particle size.

The technique is used for determination of average particle size in a range between 3 and 3000 nm. The measurements are performed on a Coulter N4 or an ALV 5000. The report consists of a table with average and mode effective hydrodynamic diameter and polydispersity index, which is a measure for the width of the distribution.