Chemical Sciences Circular Dichroism Spectroscopy Lab Experiments

To Study the operational details of a CD spectrometer.

As we are already familiar with the concepts of circular dichroism, ellipticity etc. from the previous experiments, here we are going to provide only a brief review of the CD.
CD instruments (known as spectropolarimeters) measure the difference in absorbance between the L and R circularly polarized components (ΔA = A L - A R), and generally report this in terms of the ellipticity (θ) in degrees. It should be noted that θ = tan-1 (b/a) where b and a are the minor and major axes of the resulting ellipse . There is a simple numerical relationship between ΔA and ellipticity (in degrees), namely θ = 32.98 ΔA. The CD spectrum is obtained when the dichroism is measured as a function of wavelength. In a CD instrument, plane polarised light is split into the L and R components by passage through a modulator subjected to an alternating electric field (50 kHz is the frequency most commonly employed). The modulator normally used consists of a piezoelectric quartz crystal and a thin plate of isotropic material (e.g., fused silica) tightly coupled to the crystal. The alternating electric field induces structural changes in the quartz crystal; these make the plate transmit circularly polarised light at the extremes of the field. As the transmitted radiation is switched between L and R components, these are detected in turn by the photomultiplier tube.