Materials Used to Generate a Gradient

The most common materials used to generate density gradients are sucrose, Ficoll, and Cs salts. Sucrose solutions can be as concentrated as 65% w/w, with a maximum density of 1.32 g/cm3 at 4 °C however, concentrated solutions of sucrose have high osmotic strength and viscosity. Ficoll is a brand name for a synthetic polysaccharide with an average MW of 400,000 and a maximum density in aqueous solution of 1.23. It is very useful to separate osmotically sensitive particles, like mammalian cells. 

Colloidal particles of silica have also been used to generate density gradients. These particles are approximately 20 nm in diameter, and are often coated with an 

Dense iodinated organic compounds (e.g., iodixanol) derived from X-ray contrast materials can be used in applications that require higher densities than Percoll (as high as 1.5 g/cm3) as well as low ionic strength and low osmolarity. 

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