Environmental Engineering Reference
In-Depth Information
spectroscopic techniques implemented in these tools for
in situ
investigation include
laser-induced fluorescence (LIF) and Raman spectroscopy. These types of sensors
offer many advantages over non-optical technologies. The optical sensor systems
are generally very light and miniaturized, making them very convenient for field
applications. Moreover, optical techniques are inherently immune to
electromagnetic interference and since they are usually made of glass are highly
resistant to degradation in aggressive environments.
12.3.2.4. Cone penetrometer with LIF
Laser and fluorescence are related to the ability of a material to produce light
while it is subjected to radiant energy such as ultraviolet, X, etc. In addition, the
laser is a device containing a crystal, gas or another suitable substance in which
atoms, once stimulated by focused light waves, amplify them, concentrating and
emitting them in a narrow and very intense beam.
[SIE 99] indicate that LIF probes operate on the fundamental principle of
fluorescence, which is one of two possible forms of luminescence (the other being
the phosphorescence). When the laser is focused on materials, they tend to
fluoresce; a phenomenon used to determine the molecular structure of materials. If a
material is bombarded with a laser beam (photons) of a certain frequency or
wavelength, these molecules tend to absorb some of the photons. This changes the
molecules to an excited and unstable state that cannot be maintained and the
molecules return to a stable state by emitting a photon. This emission is called
fluorescence. The particular molecular type is encoded in the incoming photon
fluorescence; the concentration of molecules is encoded in the intensity of
fluorescence.
A new class of environmental sensors based on the use of laser and fluorescence
was fitted to the cone penetrometer [LIE 91]. At first glance the combination of CPT
and LIF represents a powerful geoenvironmental tool. In this system, two fiber optic
cables are added to the electrical cables of the CPT. A fiber optic cable transmits the
laser beam through a prism − a sapphire window mounted on the surface along the
side of the penetrometer approximately 12mm from the tip of the cone [REU 98].
Fluorescence is redirected to the surface through another fiber optic cable linked
with the detection system in the CPT vehicle. The detection system accurately
measures the intensity of fluorescence, which is related to the concentration of the
contaminant in question and soil type.
LIF probes are particularly amenable to the detection of common contaminants
such as fuel oil, jet fuels, gasoline and diesel that generally show strong
fluorescence signatures [GIL 92]. However, the drawback of the CPT-LIF system is
that some organic contaminants at sites, such as trichlorethylene and
perchloroethylene, do not fluoresce and therefore cannot be detected. Penetrometer
