Analytical Ultracentrifugation: density gradient
This technique is used for the determination of particle densities and density distributions. In contrast to the density variation method, this an equilibrium technique. You may need this experiment, when
- you want to resolve species with slightly different densities,
- you want to measure densities, excluding transport processes.
The principle: A dense solvent is added to the system. Exposed to a strong gravity field, the mixture will partially separate, the more dense solvent accumulating towards the cell bottom. A solvent and thus, density gradient is established. The dispersed particles will accumulate at the radial position where their own density is matched. The particles' density and, to some extent, molecular masses can be calculated. The resolution of this method reaches the fourth digit in density.
The Figure 1 shows the theoretical density gradient inside the sample cell and (dashed) the measured concentration profile. The dominating
component has a density of 1 g/mL, another component can be observed near the cell bottom.
Increasing angular velocity allows us to zoom into the density range of interest. Such, a high resolution is achieved. The only drawback of this experiment is that it is rather time consuming because an equilibrium has to be attained. This fact is responsible for the higher costs of this experiment.
Theoretical calculations help select a solvent mixture that is suitable for a density gradient enclosing the assumed densities of the particles. It is not always possible to provide a feasible mixture. If you describe your system, we will check whether a gradient experiment can be performed.
Further theoretical background can be found on our scientific website under www.kolloidanalytik.de (in German).