Historically a
cesium chloride (CsCl) solution was often used, but more commonly used density gradients are
sucrose or
Percoll. This application requires a solution with high density and yet relatively low viscosity, and CsCl suits it because of its high solubility in water, high density owing to the large mass of Cs, as well as low viscosity and high stability of CsCl solutions. The sample is put on top of the solution, and then the tube is spun at a very high speed for an extended time, at times lasting days. The CsCl molecules become densely packed toward the bottom, so a continuous gradient of layers of different densities (and CsCl concentrations) form. Since the original solution was approximately the same density, they go to a level where their density and the CsCl density are the same, to which they form a sharp, distinctive band.. Following centrifugation at 100 g a density gradient forms in the CsCl solution and the virus particles separate according to their densities. ==Isotope separation==