10
Section 3 - Theory and Practice of Spectroscopy Measurement
3.1
Theory of Spectroscopy Measurement
UV-visible spectroscopy is the measurement of the absorbance of light at a specific wavelength in a sample. This is
used to identify the presence and concentration of molecular entities within the sample. The Beer-Lambert law is
used to relate the absorption of light to the properties of the sample through which the light is travelling through.
The Beer-Lambert law states that:
A = Ɛ l c
A
is the absorbance
Ɛ
is the molar absorption coefficient (l mol
-1
cm
-1
)
l
is the path length (cm)
c
is the concentration (mol I
-1)
This law shows that absorbance is linear to concentration but this is only true for low concentrations. For
absorbance levels above 3 the concentration starts to move away from the linear relationship.
Transmittance is the proportion of the light which passes through the sample:
Where:
l
o
is the incident light
l
t
is the transmitted light
l
is the path length
Therefore:
Absorbance is inversely related to transmittance:
I
o
I
I
t
I
o
I
t
T =
T
Log 1
A =
I
o
I
I
t
I
o
I
t
T =
T
Log 1
A =
I
o
I
I
t
I
o
I
t
T =
T
Log 1
A =
