5.2.4 Oil in Water
Contamination with mineral oils is a recurring issue in surface- and groundwater.
Fluorescence sensors can be used to detect such contaminations. Monitoring the
removal of mineral oils in industrial wastewater treatment, before discharge to sewer
systems or surface waters, is another application. Mineral oils can be monitored with
fluorescence as they typically consist of a mixture of aromatic and aliphatic hydrocarbons. It is the aromatic fraction which is detectable. Using fluorescence sensors
employing fixed excitation-emission wavelength pairs in the UV range, either
monocyclic aromatics (BTX) or polycyclic aromatic hydrocarbons (PAH) are measured. The presence of substances from these groups is used as an indicator for
contamination for different types of mineral oil products, e.g. BTX as indicator for
refined oil products (such as gasoline, diesel and kerosene), which are rich in these
components.
More detailed analysis of the oil type can be done using fluorescence spectroscopy, where a high-resolution spectrometer is used as detector instead of the typical
filter-covered photomultiplier or photodiode. Such instruments, often using laserinduced fluorescence to get sufficient signal to noise levels, can distinguish between
different oil types. These laser-induced fluorescence (LIF) systems are primarily
used in the offshore industry, refineries and applications where contamination with
lubricating or cooling oils is common, e.g. industrial or bilge water in ships. The oils
in these applications are low in aromatic contents and therefore difficult to detect
using LED-induced fluorescence. A further issue with oil and hydrocarbon products
is their poor miscibility with water, meaning a submersed sensor may not detect a
contamination as is poorly mixed or floats on the surface of the water. To detect
floating layers, remote sensing can be used; using a laser mounted above the water,
the oil layer is illuminated, and the induced fluorescence is recorded. This allows
detection of oil films down to 1 μm.
5.3 NIR
Infrared (IR) spectroscopy is similar to the previously described UV/Vis spectroscopy but uses a lightsource with a higher wavelength, i.e. lower-energy photons. The
infrared spectrum is divided into near infrared (NIR) (750–2,500 nm), mid infrared
(MIR) (2.5–16 μm) and far infrared (16–1,000 μm). For water quality analysis, the
NIR spectrum is most widely used.
Because photon energies in IR are lower than in UV/Vis, it probes a different
property of the molecule: instead of exciting electrons to a higher electronic energy
level, it changes the vibrational state. IR radiation changes the vibrations of atomic
bonds, with the behaviour of specific bonds depending on the atoms in the bond and
their environment (both within the larger molecular environment as well as the
physical environment, e.g. temperature, solution state). Similar to the electronic
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Contamination with mineral oils is a recurring issue in surface- and groundwater.
Fluorescence sensors can be used to detect such contaminations. Monitoring the
removal of mineral oils in industrial wastewater treatment, before discharge to sewer
systems or surface waters, is another application. Mineral oils can be monitored with
fluorescence as they typically consist of a mixture of aromatic and aliphatic hydrocarbons. It is the aromatic fraction which is detectable. Using fluorescence sensors
employing fixed excitation-emission wavelength pairs in the UV range, either
monocyclic aromatics (BTX) or polycyclic aromatic hydrocarbons (PAH) are measured. The presence of substances from these groups is used as an indicator for
contamination for different types of mineral oil products, e.g. BTX as indicator for
refined oil products (such as gasoline, diesel and kerosene), which are rich in these
components.
More detailed analysis of the oil type can be done using fluorescence spectroscopy, where a high-resolution spectrometer is used as detector instead of the typical
filter-covered photomultiplier or photodiode. Such instruments, often using laserinduced fluorescence to get sufficient signal to noise levels, can distinguish between
different oil types. These laser-induced fluorescence (LIF) systems are primarily
used in the offshore industry, refineries and applications where contamination with
lubricating or cooling oils is common, e.g. industrial or bilge water in ships. The oils
in these applications are low in aromatic contents and therefore difficult to detect
using LED-induced fluorescence. A further issue with oil and hydrocarbon products
is their poor miscibility with water, meaning a submersed sensor may not detect a
contamination as is poorly mixed or floats on the surface of the water. To detect
floating layers, remote sensing can be used; using a laser mounted above the water,
the oil layer is illuminated, and the induced fluorescence is recorded. This allows
detection of oil films down to 1 μm.
5.3 NIR
Infrared (IR) spectroscopy is similar to the previously described UV/Vis spectroscopy but uses a lightsource with a higher wavelength, i.e. lower-energy photons. The
infrared spectrum is divided into near infrared (NIR) (750–2,500 nm), mid infrared
(MIR) (2.5–16 μm) and far infrared (16–1,000 μm). For water quality analysis, the
NIR spectrum is most widely used.
Because photon energies in IR are lower than in UV/Vis, it probes a different
property of the molecule: instead of exciting electrons to a higher electronic energy
level, it changes the vibrational state. IR radiation changes the vibrations of atomic
bonds, with the behaviour of specific bonds depending on the atoms in the bond and
their environment (both within the larger molecular environment as well as the
physical environment, e.g. temperature, solution state). Similar to the electronic
Spectroscopic Methods for Online Water Quality Monitoring
303