5.4.1 Raman Spectroscopy
As opposed to absorption of fluorescence, Raman spectroscopy studies the scattering
of photons by molecules. When a sample is irradiated with monochromatic light, a
small proportion of photons (0.0001%) are scattered undergoing a shift in frequency;
this inelastic scattering of photons is known as the Raman effect. The difference in
the frequencies of the input and scattered light corresponds to the quantised energy
levels of the molecule studied. For a more complete treatise on the principles of
Raman and instrumentation used, see Li et al. [27].
Raman spectra are more distinct and less overlapped than UV/Vis/NIR absorption
spectra. Therefore, Raman is complementary to absorption and fluorescence spectroscopy, as it offers a more selective signal. This allows for classification and in
some cases even identification of target substances. It has successfully been applied
to determine organic and inorganic analytes, including various metals, in a water
matrix with examples including polycyclic aromatic hydrocarbons, pesticides, mercury and arsenic [28, 29]. In the case mixtures are analysed, if the components are
known, the relative peak intensities can be used to generate quantitative information
about the mixture’s composition. In the case of water applications, however, the
number of components may be so high that also Raman spectra overlap, and
identification of individual species becomes impossible. Furthermore, the low percentage of photons undergoing inelastic scattering means Raman detection limits are
significantly higher than those achieved with UV/Vis/NIR and are insufficient for
application in online water quality monitoring as discussed here.
Surface-enhanced Raman (SERS) is a technique where Raman scattering is
measured after adsorption of the target molecules on a substrate. This substrate
enhances the sensitivity, in some cases allowing analysis of single molecules or
single cells. As such it has been used to identify bacterial cells. SERS offers potential
for general bacteria classification and pathogen detection [30] and label-free detection of biotoxins [31].
One promising application for Raman spectroscopy is the characterisation of
microplastics. Although sample pretreatment remains a challenge, the characterisation of single particles with Raman, e.g. in combination with a flow cytometer, is a
promising development [32].
5.4.2 Laser-Induced Breakdown Spectroscopy
Whereas all technologies discussed thus far analyse sample composition on a
molecular level, laser-induced breakdown spectroscopy (LIBS) is an elemental
analysis technique. In LIBS, a pulsed laser is used to heat a very small spot of the
sample to extremely high temperatures (in excess of 30,000 K). As a result, a small
amount of the material is transformed into a plasma consisting of free electrons, ions
and excited atoms. As the plasma cools and electrons fall down from high-energy to
lower-energy atomic orbitals, light is emitted at wavelengths characteristic for the
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As opposed to absorption of fluorescence, Raman spectroscopy studies the scattering
of photons by molecules. When a sample is irradiated with monochromatic light, a
small proportion of photons (0.0001%) are scattered undergoing a shift in frequency;
this inelastic scattering of photons is known as the Raman effect. The difference in
the frequencies of the input and scattered light corresponds to the quantised energy
levels of the molecule studied. For a more complete treatise on the principles of
Raman and instrumentation used, see Li et al. [27].
Raman spectra are more distinct and less overlapped than UV/Vis/NIR absorption
spectra. Therefore, Raman is complementary to absorption and fluorescence spectroscopy, as it offers a more selective signal. This allows for classification and in
some cases even identification of target substances. It has successfully been applied
to determine organic and inorganic analytes, including various metals, in a water
matrix with examples including polycyclic aromatic hydrocarbons, pesticides, mercury and arsenic [28, 29]. In the case mixtures are analysed, if the components are
known, the relative peak intensities can be used to generate quantitative information
about the mixture’s composition. In the case of water applications, however, the
number of components may be so high that also Raman spectra overlap, and
identification of individual species becomes impossible. Furthermore, the low percentage of photons undergoing inelastic scattering means Raman detection limits are
significantly higher than those achieved with UV/Vis/NIR and are insufficient for
application in online water quality monitoring as discussed here.
Surface-enhanced Raman (SERS) is a technique where Raman scattering is
measured after adsorption of the target molecules on a substrate. This substrate
enhances the sensitivity, in some cases allowing analysis of single molecules or
single cells. As such it has been used to identify bacterial cells. SERS offers potential
for general bacteria classification and pathogen detection [30] and label-free detection of biotoxins [31].
One promising application for Raman spectroscopy is the characterisation of
microplastics. Although sample pretreatment remains a challenge, the characterisation of single particles with Raman, e.g. in combination with a flow cytometer, is a
promising development [32].
5.4.2 Laser-Induced Breakdown Spectroscopy
Whereas all technologies discussed thus far analyse sample composition on a
molecular level, laser-induced breakdown spectroscopy (LIBS) is an elemental
analysis technique. In LIBS, a pulsed laser is used to heat a very small spot of the
sample to extremely high temperatures (in excess of 30,000 K). As a result, a small
amount of the material is transformed into a plasma consisting of free electrons, ions
and excited atoms. As the plasma cools and electrons fall down from high-energy to
lower-energy atomic orbitals, light is emitted at wavelengths characteristic for the
Spectroscopic Methods for Online Water Quality Monitoring
305