ABSTRACT
This chapter confines itself to a discussion of the general requirements of gradient media, a description of their physicochemical properties, how suitable gradient solutions are produced, how the density of their solutions is measured and how the media may affect subsequent analytical techniques. There are now five main classes of density gradient medium: polyhydric alcohols, polysaccharides, inorganic salts, iodinated compounds based on metrizoic acid and colloidal silica. Further development of the X-ray contrast media resulted in the production of a series of nonionic, iodinated media, which were both clinically more acceptable and more useful as density-gradient media. The importance of the water activity of a solution used as a density-gradient medium is seen in the effect it has upon the degree of hydration of biological particles exposed to it. No gradient medium should provide any problems regarding effective and rapid removal.
