Viral vectors that are of potential use in gene therapy would clearly benefit from isolation methods which are both effective and cause little or no damage to the viral particles. Density gradient centrifugation has always played an important part in the concentration and purification of virus particles but the main gradient media that have been used (sucrose and CsCl), pose a number of problems.

Both are highly hyperosmotic at the densities used to band viruses (sucrose solutions are also very viscous). Both have to be removed either by pelleting the virus or by dialysis, prior to further processing or analysis. CsCl also leads to poor recoveries and low infectivity of parvovirus and rAAV isolates. 

Because of the very low water activity of CsCl solutions, viruses tend to have significantly higher density in this medium compared to media such as sucrose or any of the iodinated density gradient media, although the magnitude of this difference varies from virus to virus. 

Many viruses in CsCl have a density of approx 1.34 g/ml, in iodixanol the density range is generally 1.16-1.22 g/ml, although some viruses may be as low as 1.14 g/ml or as high as 1.24 g/ml.