ABSTRACT
Self-generated gradients offer simplicity of preparation and very high reproducibility, as their formation and the gradient density profile that is produced depends solely on the centrifugation time, relative centrifugal field, temperature and starting concentration of the gradient solute. The only important requirement is that no air bubbles are introduced which may disturb the lower density layers above; for this reason a syringe with a metal filling cannula is the best tool for this procedure. Thus although vertical and near-vertical rotors may be the rotors of choice for self-generated gradients, fixed-angle rotors, particularly those with a low angle and small tube volume, maybe satisfactory. Swinging-bucket rotors, which tend to have the longest sedimentation path-lengths, are almost never used as such for self-generated gradients; their density profiles never change from S-shaped. The mode of harvesting depends very much on the type of tube used for the gradient, the distribution of particles in the gradient and the aim of the fractionation.
