Analytical Ultracentrifugation - sedimentation velocity experiment

The sedimentation velocity experiment is one of the most popular Analytical Ultracentrifugation techniques. The data obtained can be evaluated in various ways yielding a large scope of results. Without any assumptions, the sedimentation coeffifient distribution (s-distribution in short) is obtained, to give information on the following:

  • How many components are contained?
  • Are these components poly- or monodisperse?
  • Do the components sediment rather quickly or slowly?

Thus, measurements of sedimentation velocity are suitable if you want to determine the number of species composing your system and if their sedimentation velocity is sufficient for characterisation. Often, however, this experiment is rather a starting point for further evaluations.

Introducing assumptions and models, further results are derived from the s distribution, such as

  • particle size distributions
  • particle density distributions
  • molar mass distributions
  • diffusion coefficients

The s distribution is independent on the nature of the particles. It simply gives a measure for how rapidly a particle will sediment in a given field of gravity. No assumptions are carried into evaluation. The s distribution is the primary result of measurement.

sedimentation velocity experiment

No specific expertise is required for this rather simple experiment, making it the cheapest among all Analytical Ultracentrifugation experiments. In the neighboring figure, consecutive scans of a sedimentation velocity experiment are shown. They show the sedimentation profiles of a sedimenting species at constant time intervals.

The sedimentation constant of a particle with given molar mass, density and shape is also dependent on temperature, pressure, solvent density and solvent viscosity. Thus, the experiment is either conducted under standard conditions or the constant is corrected for these conditions.

By means of further calculation, the s distribution can be converted into particle size and density distributions. Molecular mass distributions and diffusion constants are also available from this primary result. Specifically:

  • Particle size distributions can be obtained by direct calculation with known particle density.
  • With unknown particle density, a density distribution is obtained by comparing s distributions of two sedimentation velocity experiments performed in solvents of different density. This procedure is called density variation.
  • Molar mass distributions can be obtained from the density variation technique as well.
  • Diffusion coefficients are calculated from the time-dependent broadening of the sedimentation boundary.