Chapter 5 The Separation of Cells and Subcellular Particles by Colloidal Silica Density Gradient Centrifugation

Abstract

This chapter describes the practical use of colloidal silica as a gradient medium. It also explains procedures for its use along with its advantages. The use of density gradient centrifugation has played a major role in the recent advances made in cellular and molecular biology. The use of colloidal silica was first reported in 1959 by Mateyko and Kopac. Colloidal silica refers to an aqueous suspension of colloidal particles formed by polymerization of monosilicic acid from SiO 2 , dissolved in water. The colloidal suspension is most stable for storage at pH 8–10. The gradient may be formed by conventional techniques such as the layering of various concentrations, or by using a gradient-forming machine. An attractive feature of colloidal silica as a gradient medium is that it is possible to self-form a gradient in a short time by high-speed centrifugation. The rapid, self-forming features make colloidal silica very convenient. Another advantage of silica is its ability to control the osmotic pressure to match the osmolarity of the particles to be separated. This allows harvesting of cells and particles under iso-osmotic conditions.