The procedure for isolating (purifying) retroviruses is well known:

Layers of sucrose solution (bands) of known density are set up in a centrifuge tube.

A sample from a culture is gently placed on the top of the solution and the tube spun at high speed for many hours whereupon retroviral particles will descend though the sucrose solution until their progress is arrested by a layer of the same density.

By definition retroviruses are arrested, that is, they band in a layer where the density of sucrose is 1.16 gm/ml.