Circular dichroism

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.

 
 amcrasto@gmail.com

http://newdrugapprovals.org/

DRUG APPROVALS BY DR ANTHONY MELVIN CRASTO

 
 amcrasto@gmail.com

http://newdrugapprovals.org/

DRUG APPROVALS BY DR ANTHONY MELVIN CRASTO

NORWAY\

























Norway: Oslo to Bergen by train










ALESUND




Stave church Heddal, Norway






Norway Bergen Tramway







At the top of Preikestolen with view over the Lysefjord, Norway 





Norway - Bergen Cityscape by AgiVega

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