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II
1. Photometers
When a beam of light is transmitted through a
coloured solution, then this beam loses its intensity,
in other words a part of the light is absorbed by the
solution. Depending on the substance in question,
this absorption occurs at a specific wavelength.
The wavelength is selected from the overall spectrum
of white light emitted by a tungsten-halogen lamp
using narrow-band interference filters or other mono-
chromators.
The intensity of the absorption can be characterized
using the transmittance T (or, respectively, T in
percent).
T = I/I
0
I
0
= Initial intensity of the light
I = Intensity of the transmitted light
If the light is not absorbed at all by a solution, then
this solution has a transmittance of 100 %; a com-
plete absorption of the light in the solution means
0 % transmittance.
The measure generally used for the absorption of
light is the absorbance (A), since this correlates
directly with the concentration of the absorbing sub-
stance. The following connection exists between
absorbance and transmittance:
A = – log T
Experiments by BOUGUER (1698 –1758) and
LAMBERT (1728 –1777) showed that the absorbance
is dependent on the thickness of the absorbing layer
of the cell used. The relationship between the absor-
bance and the concentration of the analyte in ques-
tion was discovered by BEER (1825 –1863). The com-
bination of these two natural laws led to the deriva-
tion of
Lambert-Beer’s law, which can be described
in the form of the following equation:
A =
εε
λλ
x
c
x
d
εε
λλ
= Molar absorptivity, in l/mol
x
cm
d = Path length of the cell, in cm
c = Concentration of the analyte, in mol/l
1.1 Photometry
* GLP – Good Laboratory Practice ** AQA – Analytical Quality Assurance
The individual components are optimally coordi-
nated to be fully compatible with each other and
make it possible to conduct state-of-the-art analysis
without any further accessories being required
(according to GLP* and AQA** requirements).
The
Spectroquant
®
Analysis System
consists of the following components:
Incident light Transmitted light
I
0
I
Release 05/04