Geoscience Reference
In-Depth Information
Detector
Monochromator
100 w
Tungsten
Halogen
Lamp
I
o
I
t
c
,
α
l
Figure 5.3. Single-channel absorption spectrophotometer.
For the measurement of absorption of a sample a typical arrangement of a single-channel
absorption spectrophotometer in relation to the sample is shown in
Figure 5.3
.
5.2.2 Optical Emission Spectroscopy
Atoms of molecules can be brought to a so-called excited state via the interaction with
a radiation source (electromagnetic radiation) as in photoluminescence or a nonradia-
tional source such as a flame, DC current, high-voltage spark, or pulsed laser heating.
Photoluminescence relies on a radiative emission from the sample after excitation by
excited atoms or molecules.
Figure 5.4
demonstrates a typical instrument arrangement for
measuring optical emission spectroscopy.
Ideally, sodium atoms emit a characteristic radiation frequency when excited in a flame
by a collisional process. When in a thermal equilibrium (an ideal scenario), it generates a
statistical distribution of excited states. Once generated, the number of atoms, n, at a par-
ticular energy level,
i
, is given by the Boltzmann distribution:
−
E
nge
i
kT
ti
n
=
(5.3)
i
−
E
∞
∑
0
ge
i
kT
i
i
=
