False colour scanning electron micrograph of Sephadex beads. These beads are used in the separation of molecules according to size (molecular weight). They are made of an inert polymer, in this case dextran, and each bead contains pores of a specific size. A glass column is packed with beads in water and the mixture of proteins, or protein and salt, to be separated is poured in. Small molecules can penetrate the pores in the matrix and their progress through the column is impeded, while larger molecules proceed downwards at the rate of flow of the solvent. This technique is known as column chromotography or gel filtration. Magnification: x200 at 6x4.5cm size.