Editing Circular dichroism (section)

=== Analysis of solid samples ===
It is possible to analyze solid-state samples by CD spectroscopy. There are a few options to accomplish this.

Samples are often prepared in the form of discs, either by compression with an inert dispersant such as potassium bromide or potassium chloride or by deposition of the sample as a thin film on top of the disc. This technique is well established in IR absorbance spectroscopy, and it has also been used in UV/visible spectroscopy. The sample is measured in transmission mode, with the light passing through the sample to a detector mounted in the spectrometer transmission port. To eliminate the effect of [[anisotropy]], the sample is usually mounted in a wheel and can be manually rotated to allow measurements to be made at several angular positions. The spectra of all positions can then be averaged.

If the solid samples exist in the form of a powder, they can be measured via diffuse reflectance by using a device known as an [[integrating sphere]].

Integrating spheres were developed early in the twentieth century and were originally used to measure the total output of a light source without reference to the original direction of the light. They are now commonly used when measuring the UV/visible or IR absorption spectra of solid samples, and their use can be extended to the UV/visible CD spectroscopy of solids.

Light enters the sphere through an inlet port, and the internal surfaces of the sphere are usually white, reflective, and diffuse, so that the light becomes equally distributed within the sphere through multiple scattering reflections. A detector is placed at an outlet port and the intensity of the light is measured. For CD spectroscopy, the sample can be placed either at the inlet port, so that the light passes through it before entering the sphere, or at a point on the sphere opposite the inlet port, so that light is diffusely reflected from the sample after entering the sphere. These two operating modes are called transmission and diffuse reflectance, respectively. In both cases the sample is prepared in powder form. For transmission mode, it must then be dispersed in either potassium bromide or potassium chloride and compressed into a disc. For diffuse reflectance mode, it remains as a free powder, if necessary dispersed in a diluent such as barium sulphate or polytetrafluoroethylene (PTFE).