Dichroic samples, preparation

CD samples must be isotropic, at least in the direction perpendicular to the light path, since linear dichroism (LD, birefringence) causes large distortion of CD spectra. Therefore, samples of solid dispersions, membranes, films, gels, and liquid crystals should be prepared with great care to minimize anisotropy. Furthermore, the measurement of nonsolution samples should be repeated several times after successive rotation of the cell around the light beam axis to check the independency from linear dichroic interference. Some reliable studies show LD data with CD curve in order to examine artifacts. The linear dichroic interference may be too large for solid-state samples, and hence their measurement may require specialized instruments. Chiral liquid crystal (nematic or smectic C phase) exhibits a characteristic circular dichroic phenomenon irrelevant to... 

Another problem often encountered in absorbance subtraction is the preferential orientation of one of the samples relative to the other. Unfortunately, there is no general method or algorithm to eliminate this orientation problem because each band has its own characteristic dichroic behavior. The only solution is to prepare the samples with reproducible or random orientation. Unless this is accomplished, three-dimensional tilted sample methods must be used. The three-dimensional tilted sample method removes the orientation of the sample and generates a pure structure-factor spectrum prior to absorbance subtraction. 

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