1. Overview and Specifications
1.1 Overview
1.1.1 Principles of CD operation
When linearly-polarized light passes through an optically-active substance, its two circularly
polarized components (i.e. the right and left circularly polarized beams of light) travel at
different speeds, and are absorbed in differing degrees by the substance. Thus, the light that
passes through the substance is elliptically polarized, and the substance is said to have
“Circular Dichroism (CD)”. The magnitude of circular dichroism is usually expressed in terms
of the molar ellipticity [], which is determined according to the following formula:
= −
4500
10( )log
L R e
(1-1)
where,
L
and
R
are the molecular extinction coefficients for the right and left circularly
polarized beams of light. The difference () between
L
and
R
is determined using the
following formula:
=−
L
R
RL
I
I
LC
10
log
1
(1-2)
Substituting this expression into equation (1) gives:
=
L
R
e
I
I
LC
10
log10log
4500
(1-3)
In equations (2) and (3), L represents the thickness (in cm) of the absorbing layer, C represents
the molar concentration, and I
R
and I
L
represent the intensities of the right and left circularly
polarized beams of light, respectively, after passing through the substance. Theoretically, the
molar ellipticity can be derived using equation (3). However, in practice, determining [] with a
high degree of accuracy is very difficult using equation (3), because the value of I
R
/I
L
is nearly
1. To avoid this difficulty, we substitute the following quantities.
(1-5)
Since S/2
A
is less than 1, equation (3) can be re-expressed as follows, by substituting the
expressions from equations (4) and (5):
−
+
=
e
I
S
LC
I
S
I
S
LC
A
e
A
A
e
10
10
log10log
4500
2
1
2
1
log10log
4500
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