Geoscience Reference
In-Depth Information
Solar radiation
Solution of quinine
Excitation filter <400 nm
(blue-glass from church window)
Emission filter (yellow-glass
of wine transmits >400 nm
Figure 5.1. Sir George Gabriel Stokes (1852) and his filter fluorimeter.
For example, how does the instrument change in time, how does it compare with
other instruments, and how do measurements compare from laboratory to laboratory?
Furthermore, what is the meaning of fluorescence units? These questions and others are
examined in this chapter with the aim of providing a simple and clear understanding of
what is inside the “black box” and an appreciation of its relevance to achieving fluores-
cence measurements that are truly representative of the sample.
5.2 Methods of Optical Spectroscopy
Optical spectroscopy is the science of the interaction of optical radiation with matter. In
many cases this involves specific transitions between the energy levels (states) of a sample,
which are monitored experimentally by the absorption or emission of electromagnetic radi-
ation. In these types of interactions the radiation is considered to be composed of packets
of energy called photons, and they have a dual character in that they have both particle- and
wave-like properties. As discussed in
Chapter 1
, the photon energy is related to its wave-
length and frequency by:
Eh
hc
=
ν
λ
(5.1)
where
E
is the photon energy,
h
is Planck's constant,
ν
is the frequency,
c
is the speed of
light, and
λ
is the wavelength.
There are many types of radiation-matter interactions, such as diffraction, refraction,
reflection, and scattering that do not involve transitions between energy levels. However,
these interactions may cause changes in the measured optical radiation due to direction or
polarization and are often the result of bulk material properties rather than specific chem-
ical species.
In the ultraviolet to infrared spectral regions the instrumental needs are similar in terms
of the optical materials required for focusing, steering, and dispersing light. “Spectrometry”
