UV/Vis, (N)IR and fluorescence spectroscopy are all based on absorption, in the
case of fluorescence followed by emission (Table 1). Photons can be absorbed if the
atom/molecule has energy states that differ by the same amount of energy
corresponding to the photon energy. For ultraviolet and visible radiation, absorption
of a photon results in one of the molecule’s valence electrons being excited to a
higher energy state, while infrared absorption changes the vibrational energy of a
molecular bond. The possible energy transitions that a molecule can undergo are
discrete and determined by its molecular structure and by its environment in
solution. As a result, absorption measurements as a function of the wavelength
(reciprocal of the photon energy) result in spectra that are fingerprints for atoms/
molecules. Figure 2 shows a simplified view of a photon’s absorption and subsequent emission.
Although the possible energy transitions for one molecule are very precisely
defined, in practice the signals observed in absorption spectroscopy and fluorescence
are not as well defined. Whereas spectral lines for individual transitions may be
visible in a vacuum, in a solution effects of temperature, inhomogeneities, solutesolvent interactions and in particular hydrogen bonding mean that each molecule
may have slightly different vibrational levels. Especially in complex molecules, the
possible energy transitions can be so close together that they cannot be distinguished
and are observed as one overlapping signal (Fig. 3). The result is a number of closely
Fig. 1 The electromagnetic spectrum
Table 1 The fundamental light-matter interactions, which form the basis for most spectroscopic
methods
Absorption
Matter absorbs light and undergoes a change in energy
Emission
Matter releases radiative energy, e.g. after excitation by light
Elastic scattering and
reflection
Interaction between matter and light that changes the direction but not
the energy of the photons
Inelastic scattering
Interaction between matter and light that changes the direction as well
as the energy of the photons
Spectroscopic Methods for Online Water Quality Monitoring
287
case of fluorescence followed by emission (Table 1). Photons can be absorbed if the
atom/molecule has energy states that differ by the same amount of energy
corresponding to the photon energy. For ultraviolet and visible radiation, absorption
of a photon results in one of the molecule’s valence electrons being excited to a
higher energy state, while infrared absorption changes the vibrational energy of a
molecular bond. The possible energy transitions that a molecule can undergo are
discrete and determined by its molecular structure and by its environment in
solution. As a result, absorption measurements as a function of the wavelength
(reciprocal of the photon energy) result in spectra that are fingerprints for atoms/
molecules. Figure 2 shows a simplified view of a photon’s absorption and subsequent emission.
Although the possible energy transitions for one molecule are very precisely
defined, in practice the signals observed in absorption spectroscopy and fluorescence
are not as well defined. Whereas spectral lines for individual transitions may be
visible in a vacuum, in a solution effects of temperature, inhomogeneities, solutesolvent interactions and in particular hydrogen bonding mean that each molecule
may have slightly different vibrational levels. Especially in complex molecules, the
possible energy transitions can be so close together that they cannot be distinguished
and are observed as one overlapping signal (Fig. 3). The result is a number of closely
Fig. 1 The electromagnetic spectrum
Table 1 The fundamental light-matter interactions, which form the basis for most spectroscopic
methods
Absorption
Matter absorbs light and undergoes a change in energy
Emission
Matter releases radiative energy, e.g. after excitation by light
Elastic scattering and
reflection
Interaction between matter and light that changes the direction but not
the energy of the photons
Inelastic scattering
Interaction between matter and light that changes the direction as well
as the energy of the photons
Spectroscopic Methods for Online Water Quality Monitoring
287