Laser-Induced
Fluorescence (LIF)
Description
The Laser-Induced Fluorescence (LIF) petroleum sensor is attached to a probe
that is pushed into the ground with a truck-mounted hydraulic system (a cone penetrometer)
to make real-time
chemical measurements of contaminants,
as part of the Site
Characterization and Analysis Penetrometer System (SCAPS). The chemical
sensor consists of a laser that fires short pulses of light into an optical
fiber that runs through the probe. As the laser pulses pass into the soil
through the probe window, petroleum contaminants in the soil respond by giving
off a characteristic Òglow.Ó This emitted light is carried back to the surface
over a second fiber, and it is measured with a detector. The optical screening
method provides a nearly continuous profile of contaminant distribution as the
probe is pushed into the ground.
This technology is similar to other penetrometer sensors for real-time, in-situ
field screening of metals in soils. The probe can be pushed up to 100 feet into
the ground. Parallel integration of multiple sensors on a single probe more
thoroughly delineates the vertical and horizontal extent of site contamination.
Limitations and Concerns
Field-screening methods do not eliminate the need for laboratory analyses,
but they substantially reduce the number of samples and improve the efficiency
of sample collection. The LIF technology has only been validated as a semi-quantitative
methodology (detect/non-detect); it does not completely eliminate the need for
traditional sampling. As the technology is now applied, random samples are
taken to the laboratory to confirm that the results are accurate and to
recalibrate the LIF sensor.
Detection limits vary among sites. The range of detection levels is 10-100
parts per millions (ppm) in soil and 0.5-10 ppm in groundwater.
Minerals such as calcite, naturally occurring organic matter,
and man-made chemicals also can fluoresce, possibly causing interference
problems.
Extensive experience is required for proper system operation.
The maximum depth for this technology is 150 feet, due to signal attenuation
in the optical fiber cord.
The space above the truck must be free of obstructions such as power lines,
and the subsurface in the immediate vicinity of a cone penetrometer push must
be free of buried objects such as utilities.
The cone penetrometer probes will not penetrate boulders, cobbles, rubble,
well compacted sediment, or sound bedrock.
Cold weather may cause problems by freezing of parts of the cone
penetrometer system.
These systems may be expensive to use on small-scale projects. They have
primarily been used at large sites such as Department of Defense (DoD) and
Department of Energy (DOE) facilities.
Some maintenance of the cone penetrometer tools and the LIF sensors is
required. Downtime due to the breakage of fiber optic cables and push rods, as
well as fogging of the sapphire window, may occur.
Work performed at Dover Air Force Base indicated the potential for smearing
as well as a memory effect on the sensor.
Applicability
The LIF sensor is attached to a cone penetrometer such as the SCAPS, to
detect petroleum hydrocarbons
in the soil. This approach partially replaces the slow, costly traditional
approach, in which subsurface contaminant plumes are
characterized using a drilling rig to install monitoring
wells and collect soil and groundwater
samples that are sent to a laboratory for analysis. The LIF system can also be
to detect petroleum compounds in water. It is best used for screening petroleum
compounds and polycyclic aromatic hydrocarbons and for monitoring cleanup
performance.
Technology Development Status
The laser-induced fluorescence
(LIF) petroleum sensor has been field-tested and is considered commercially
available.
Web Links
http://www.frtr.gov/site/6_1_2.html
http://clu-in.org/char/technologies/lif.cfm
Other Resources and Demonstrations
See also descriptions of SCAPS, and Cone Penetrometer.
See http://www.clu-in.org/download/char/verstate/cone/rost_wrd.pdf
for a technology verification report on the RostTM
system.
See http://www.netl.doe.gov/publications/factsheets/project/Proj369.pdf
for a description of LIF used to detect mercury.