Purification and analysis of Group VI (ss)RNA-RT viruses: Retroviridae:
Lentivirus: Human immunodeficiency virus – 1 (HIV-1) and lentivirus
A simple iodixanol density cushion can be used to concentrate the virus. Depending on the density of the cushion, such a technique may also partially purify the virus. A small volume of very dense cushion [1,2] was used for banding viral particles from a very large volume of virus-containing fluid.
This stage can be carried out at 50,000 g for a short time (1.5-3 h). Should it be necessary to remove the iodixanol after recovery of the banded material, the virus can be efficiently pelleted from a small volume by gentle centrifugation for a long period . Low-speed centrifugation of the original large volume on the other hand would lead to very poor recovery.
These procedures lead to improved
infectivity, compared to simple high-speed pelleting. Low-density cushions of 8.4%  and 6% 
(w/v) iodixanol have been used to pellet HIV-1 and HIV-1 cores respectively and to separate them
from more slowly-sedimenting contaminating particles and soluble proteins.