Linear Dichroism - Alignment Methods

Alignment Methods

Couette flow

The Couette flow orientation system is the most widely used method of sample orientation for UV LD. It has a number of characteristics which make it highly suitable as a method of sample alignment. Couette flow is currently the only established means of orientating molecules in the solution phase. This method also requires only very small amounts of analysis sample ( 20 - 40 µl) in order to generate an LD spectrum. The constant recirculation of sample is another useful property of the system, allowing many repeat measurements to be taken of each sample, decreasing the effect of background noise on the final recorded spectrum.

Its mode of operation is very simple, with the sample sandwiched between a spinning tube and a stationary rod. As the sample is spun inside the cell, the light beam is shone through the sample, the parallel absorbance calculated from horizontally polarised light, the perpendicular absorbance from the vertically polarised light. Couette flow UV LD is currently the only commercially available means of LD orientation.

Stretched film

Stretched film linear dichroism is a method of orientation based on incorporating the sample molecules into a polyethylene film. The polyethylene film is then stretched, causing the randomly oriented molecules on the film to ‘follow’ the movement of the film. The stretching of the film results in the sample molecules being oriented in the direction of the stretch.