van Holde - Weischet Tutorial

Experimental Design

Careful experimental design is an important factor determining the amount and resolution of the information obtained from an experiment. The following considerations influence the quality of the results:

• Range of scans: Always collect data for sedimentation spanning the entire cell. It is easy to discard unusable data later on, but impossible to generate missing data later on. Always start collecting immediately as soon as rotor speed is reached. Scans that didn't clear the meniscus can be discarded for the van Holde - Weischet analysis, but may be useful for fitting in the finite element analysis.

• Number of scans: Get as many scans as possible. The more scans are available, the higher the confidence in the linear extrapolation, and the more easily problems are identified. Always minimize the time between scans to maximize the number of scans collected for each cell.

• Rotor speed: run as fast as possible since the faster the speed, the higher the resolution of the data (this is a tradeoff with the number of scans the machine can physically measure in the shorter time period resulting from the higher speed).

• Keep in mind that later scans generally show an improved resolution of multi-component systems. Analyze the system with different cropping of scans (the later scans) to see if resolution improves.

• For unknown samples, always measure 3 different concentrations spanning at least a 10-fold concentration range, better more. This can often be accomplished by changing the wavelength, or by changing to interference optics, if available.

• It is important that the temperature stays constant throuhout the run. Precooling the rotor and equilibrating the rotor at run temperature for at least 30-45 minuted before the run starts is recommended.