Laser-Induced Fluorescence (LIF)
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 “glow.” 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.
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.
Other Resources and Demonstrations
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.