Quasi-elastic light scattering from large anisotropic particles: application to the red blood cells

The usefulness of dynamic light scattering measurements for the determination of particle size and shape in suspensions of biological objects is well established. However, when the particle sizes are larger than the wavelength of the incident light, so that structural information on the scattering particles can be made available, the analysis of the correlation function of the scattered light requires a careful examination, owing to the problem involved in its theoretical description. In this note, dynamic light scattering techniques were employed for the determination of the size of human normal erythrocytes in physiological saline solution (isotonic solution) with the aim to discuss in detail the different effects that contribute to the scattered light intensity distribution and to evidentiate how correct information can be made available only when the anisotropic contribution to the translational diffusion coefficient of the erythrocyte cells is properly taken into account. In the case of erythrocytes, this effect produces deviation of the order of 8-10% in the evaluation of the cell size (the cell diameter and cell thickness) that can be accounted for by an appropriate dependence upon time of the autocorrelation function. Once the electric field autocorrelation function, where the intraparticle correlation function contains the correction for the anisotropic contribution, is considered, reasonable agreement was obtained between the data deduced from light scattering methods and those from scanning electron microscope pictures.