Buoyant density is the surrounding liquid density at which a species has no net centrifugal drift under the stated conditions. It depends on interactions with the medium, including associated solvent, rather than necessarily equaling the dry substance's density. Equal buoyant densities can give overlapping bands even for differently sized proteins or DNA fragments.[ref]
https://www.beckman.com/resources/technologies/centrifugation/ultracentrifugation/. An Introduction to Ultracentrifugation, Isopycnic Centrifugation: buoyant density depends on the surrounding medium
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