An analytical ultracentrifuge consists of an optical detection system integrated into an ultracentrifuge, allowing for the real-time detection of the evolution of the concentration distribution of particles subjected to centrifugation. Two major experimental methods are employed in analytical ultracentrifugation, which differ in the applied centrifugal force: sedimentation velocity (SV) and sedimentation equilibrium (SE).
An SV experiment is basically the observation of the free fall of particles in solution under the influence of a strong gravitational field: in the reference frame of the spinning solution column, the centrifugal force is equivalent to a gravitational force.
SE experiments are conducted in the same instrument used for SV experiments. These two methods are complementary and share many common practical considerations.
Obvious key differences are:
- equilibrium conditions can be theoretically derived from equilibrium thermodynamics, without reference to the dynamics through which equilibrium is attained and therefore allow modeling the sedimentation behavior of solutions that may be kinetically intractable;
- since there is no net transport at equilibrium, kinetic considerations and hydrodynamic friction are irrelevant for an equilibrium analysis;
- in SE, lower rotor speeds are used such that the back-diffusion region imposed by the impermeable bottom reaches throughout the solution column;
- the equilibrium experiment usually requires much longer times
(Source: Schuck, P., Zhao H., Brautigam C-A., Ghirlando R., (2016) Basic Principles of Analytical Ultracentrifugation. CRC Press)