Light Induced Fluorescence – Technical Information
The UVOST® (UltraViolet Optical Screening Tool) instrument works on the principle of LIF – Light (Laser or LED) Induced Fluorescence. The UVOST® is an in-situ fluorescence spectrometer that delivers laser energy to the subsurface and records the fluorescence spectral output. This information can be used to delineate oils and other free phase petroleum products in the subsurface.
In simple terms fluorescence is the absorption and subsequent release of radiation by fluorophores – molecules that fluoresce. When fluorophores are excited by exposure to photons (energy) of a particular wavelength their valence electrons undergo a change in their electron state, rising from the ground state to an excited state. As the electrons return to the ground state they release energy as fluorescence. This process can be visualized in a Jablonski diagram.
Phosphorescence is a related phenomenon, the main difference being the time involved. Fluorescence occurs almost instantaneously while phosphorescence is delayed by further chemical action. Both processes are governed by quantum mechanics. This is important from a site characterization context as it means fluorophores behave in a specific, predictable manner when exposed to photons. This characteristic makes fluorescence useful for environmental analytical chemistry.
Polycyclic Aromatic Hydrocarbons
Polycyclic Aromatic Hydrocarbons (PAH)s have several relevant characteristics that allow in-situ LIF the ability to see free phase NAPL in the subsurface. Most importantly PAHs exhibit fluorescence, and so the instrument is able to detect them.
PAHs also have a relatively high octonal–water partition coefficient (Kow). Larger PAHs also have a low vapour pressure. This means that when petroleum products are spilled into the environment the majority of the PAHs tend to stay dissolved in the free phase organic solvent.
These characteristics mean that the PAHs will remain in the free phase product at orders of magnitude higher concentrations than in the surrounding environment. And by virtue of their fluorescence the in-situ LIF instruments will be able to detect their presence. This confluence of phenomena allows for the in-situ detection of free phase NAPL using LIF.
The UVOST® capitalizes on the PAHs found in NAPL and their associated fluorescence response. The tool consists of a laser, oscilloscope, down-hole probe and the associated fiber optics, processer and software. A typical UVOST® reading proceeds as follows in this simplified explanation: as the probe is advanced through the subsurface (at ~ 2cm/s) the Xenon Chloride in the Excimer Laser is stimulated by high voltage and sends out a UV pulse (308 nm photon). This pulse is amplified and directed by mirrors to the subsurface through the fiber optic cables.
Once the UV pulse travels to the down-hole probe it is directed out through a sapphire window and strikes the subsurface. Any fluorescence generated is collected by the same mirrors and directed back up the fiber optic cables. These photons travel back to the surface and the spectrum passes through a cutoff block to filter out any reflected laser light and other noise. This signal is then split through separate, time delay channels in order to break the signals spectrum into four channels. This produces the readout as %RE (percent of reference emitter – an amalgamation of the channels) and as the waveforms of each channel.
The semi-quantitative data comes from the %RE. As each reading is calibrated to the same reference product subsequent changes in the %RE (increases/decreases) give an indication of the relative concentration of the NAPL in-situ. The waveforms produced by the four channels give insight into the relative concentrations of PAHs in the NAPL. The compositions of PAHs in NAPL vary based of fuel type. As such the channels and their various waveforms allow the UVOST® tool to discern different fuel types in-situ.
Depth is constantly logged as the probe advances allowing the practitioner to delineate the NAPL in-situ. Experienced interpretation of the results in conjunction with other site data allows for an unprecedented understanding of the NAPL dynamics on site. Application of this data has tremendous implications for informed decision making concerning the management of contaminated sites.
Example of typical UVOST® reading displaying both qualitative and semi-quantitative data with depth
The UVOST® instrument is able to detect NAPL at relatively low concentrations in the range of 10-1000 ppm in lab experiments. As the PAHs represent a small fraction of the NAPL their detection limit is much lower. The UVOST® machine cannot see dissolved phase PAHs as their concentration in water is orders of magnitude lower than in the NAPL.
UVOST® cannot detect BTEX range and other MAHs (Monocyclic Aromatic Hydrocarbons). The wavelength of their fluorescence is absorbed by the fiber optical cables.
Detection of heavy oils and crudes can be limited by “quenching”. Higher molecular weight PAHs tend to absorb the fluorescence of surrounding fluorophores and reemit that energy. In some cases heavy NAPL products contain such high concentrations of large PAHs that “quenching” can lead to no signal or a limited signal.
Being a direct push technology the UVOST® may be limited by the geology on site.
SCG recommends contacting us for discussion before applying LIF on site. SCG can help define the benefits and limitation on a site-specific basis to ensure the best results.
Please contact SCG to learn more about LIF and how it can add value to characterization and remediation of hydrocarbon impacted sites.