= Density-gradient centrifugation
<Density-gradient centrifugation> uses a liquid whose <density> varies along the separation direction. A band does not by itself establish which mechanism produced it: <rate-zonal centrifugation> separates by migration rate before equilibrium, whereas <isopycnic centrifugation> uses equilibrium positions set by <buoyant density>.
The distinction is between a moving clock and a restoring balance. In <rate-zonal centrifugation>, two species may occupy different positions because their <sedimentation coefficients> differ, even when their <buoyant densities> agree. In <isopycnic centrifugation>, two differently sized species can converge to the same band because their <buoyant densities> agree. Conversely, equal <mass> does not ensure equal migration <velocity>: shape affects <drag force>, and medium <density> affects <buoyancy>.<An Introduction to Ultracentrifugation by Beckman Coulter Life Sciences>[The Physics Behind the Technology; Rate Zonal Centrifugation; Isopycnic Centrifugation: migration properties versus matching density]{ref}
The worked <finite-time rate-zonal separation> and <linear-gradient isopycnic focusing> examples isolate these mechanisms in chosen <mathematical models>. The actual <DNA> experiment described under <isopycnic centrifugation> illustrates the second mechanism; its initial collection of <biological cells> into <centrifugation pellets> is a different operation from the later molecular banding. Collecting liquid fractions from a tube can recover material at different positions, but the position alone does not identify its <molecules> or establish purity.
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