A solid-state dedicated circular dichroism spectrophotometer: Development and application

A solid-state dedicated circular dichroism (CD) spectrophotometer (J-800KCM) was designed and constructed. As a CD spectrophotometer is a polarization–modulation instrument, CD spectra are necessarily accompanied by artifacts due to macroscopic anisotropies such as linear birefringence (LB) and linear dichroism (LD) which are unique to the solid state. A photomultiplier with the least polarization characteristics and a photoelastic modulator (PEM) with the least residual static birefringence were selected for the new instrument, which was based on the electrical and optical systems of a commercially available spectrophotometer. A phased-locked loop circuit was introduced to a PEM driver, and a sample rotation holder, a stage controller, and an analyzer were installed. We have designed and built a special solid-state sample holder to enable the cancellation of artifact CD, and a lens unit for smaller samples. A set of procedures for obtaining true CD has been devised based on the Mueller matrix method, and a program was written to facilitate the otherwise cumbersome measurements. Using the J-800KCM instrument and this approach, we could obtain nearly zero CD for achiral polyvinyl alcohol (PVA) film dyed with achiral Congo Red by cancelling the substantial apparent CD arising from large macroscopic anisotropies. Experiments on chiral single crystals of α-Ni(H2O)6⋅SO4, one with larger and one with smaller LB, have established that, by our method, we could also eliminate parasitic artifacts to obtain the true CD.