NEW DESIGN FOR FOURIER-TRANSFORM INFRARED VIBRATIONAL CIRCULAR-DICHROISM SPECTROMETERS

Fourier transform infrared (FT-IR) spectrometers are commonly designed with small-area detectors and tight focusing mirrors. Vibrational cir cular dichroism (VCD) measurements made with such FT-IR instruments co ntain polarization artifacts, and VCD measurements on both enantiomers (or one enantiomer and racemic mixture) are required in order to redu ce these artifacts. This restriction limits the VCD measurements to on ly those samples for which both enantiomers (or one enantiomer and rac emic mixture) are available. Recently a modified design was reported i n the literature where the mirrors between sample and detector were re placed with a BaF2 lens, and a larger-area detector was substituted fo r the smaller-area counterpart. These modifications successfully allev iated some of the artifact problems. This design, however, is not suit able for polarizing interferometers, where polarizations exiting the i nterferometer are to be preserved to a high degree of purity. In addit ion, it is not clear whether the throughput enhancement advantage real ized with a larger-area detector completely offsets the disadvantage f rom increased noise with detector area. Furthermore, BaF, lenses reduc e the broad range routinely available on an FT-IR instrument. Here we report a new design that replaces all the mirrors at the exit port of the interferometer with two KBr lenses and retains the full spectral r ange (4000-400 cm(-1)) of mid-infrared FT-IR spectrometers. VCD measur ements obtained with small-(1 x 1 mm) and large-(4 x 4 mm) area detect ors are found to have similar signal quality.