van Holde - Weischet Tutorial

Diagnostics: Aggregation and Loss of Material

The van Holde - Weischet analysis is based on a solution of the Lamm equation which assumes an infinitely long cell. In an infinitely long cell, material never pellets. In addition, in an infinitely long cell there will never be any back-diffusion from the bottom of the cell. Both pelleting and back-diffusion occurs because of the boundary conditions of the sector shaped cell. Since this boundary condition is not considered in the solution of the Lamm equation employed by the van Holde- Weischet analysis, all data scans that contain absorption profiles resulting from boundary effects will distort the solution. These boundary effects incidently also include the meniscus - scans that haven't cleared the meniscus are not appropriate for inclusion in the van Holde - Weischet analysis, since the boundary cannot be divided into the corresponding boundary fractions.

There are several conditions that result in data containing back-diffusion or data pelleting:

• Speed too high/experiment too long (samples will pellet, causing loss of material)
• Speed too low/experiment too long (this approaches equilibrium conditions and causes back-diffusion over time)
• Aggregation (samples will pellet, causing loss of material)
• Improper selection of scan files for the van Holde - Weischet analysis (see section on data editing for details)
• Range of S-values is too large (samples will pellet, causing loss of material while slower components haven't cleared the meniscus)