Circular dichroism (CD) is the difference in light absorbance between left- (L-CPL) and right-circularly polarised light (R-CPL) and circular dichroism spectrometers (spectrophotometers) are highly specialised variations of the absorbance spectrophotometer.
A circular dichroism spectrophotometer is also commonly termed a circular dichroism spectropolarimeter or a circular dichrograph. Most modern circular dichroism instruments operate on the same principles, which is demonstrated in the slide show at the bottom of the page. There is a source of monochromatic linearly polarised light which can be turned into either left- or right-circularly polarised light by passing it through a quarter-wave plate whose unique axis is at 45 degrees to the linear polarisation plane as described in the section about polarised light.
Instead of a static quarter-wave plate, a circular dichroism spectrophotometer has a specialised optical element called a photo-elastic modulator (PEM). This is a piezoelectric element cemented to a block of fused silica. At rest, when the piezoelectric element is not oscillating, the silica block is not birefringent; when driven, the piezoelectric element oscillates at its resonance frequency (typically around 50 kHz), and induces stress in the silica in such a way that it becomes birefringent. The alternating stress turns the fused silica element into a dynamic quarter-wave plate, retarding first vertical with respect to horizontal components of the incident linearly polarised light by a quarter-wave and then vice versa, producing left- and then right- circularly polarised light at the drive frequency. The amplitude of the oscillation is tuned so that the retardation is appropriate for the wavelength of light passing through the silica block.
On the other side of the sample position there is a light detector. When there is no circularly dichroic sample in the light path, the light hitting the detector is constant. If there is a circularly dichroic sample in the light path, the recorded light intensity will be different for right- and left-CPL. Using a lock-in amplifier tuned to the frequency of the PEM, it is possible to measure the difference in intensity between the two circular polarisations (vAC). The average total light intensity across many PEM oscillations (vDC) can be used to scale the size of the lock-in amplifier signal to take into account variations in total light level. Both signals can be recorded and from them the circular dichroism signal can be calculated easily by dividing the vAC component by the vDC signal.
G is a calibration-scaling factor to provide either ellipticity or differential absorbance. The section about CD Units and their inter-conversion explains how ellipticity and differential absorbance are related.
Principle of operation of a Circular Dichroism Spectrophotometer (click to progress through slideshow)