Gregory A. Theriault

A real‐time fiber‐optic LIBS probe for the in situ delineation of metals in soils

Detection-limit determination and field-deployment results for the SCAPS fiber-optic laser-induced breakdown spectroscopy (LIBS) cone penetrometer probe are presented. Probe design and issues concerning sample presentation and in situ matrix effects are also addressed. It was observed that a design in which measurements are made by a scanning optical system through a sapphire window in the probe produces detection limits that are at or below the EPAs site screening levels (SSLs) for Pb, Cd, and Cr in sand.

Optimization of the optical characteristics of a fiber-optic-guided laser fluorescence technique for the in-situ evaluation of fuels in soils

We have developed a system for the in situ determination of petroleum hydrocarbons in soils. This system uses a pulsed N2 laser coupled with a photodiode array detectorto make fluorescent measurements via optical fibers. The measurement is made through a sapphire window on a probe that is pushed into the ground with a truck-mounted cone penetrometer. This is the first reported direct optical detector for contaminants in soils. Remote in situ fluorometric measurements over long lengths of optical fibers give rise to several complications not encountered with conventional laboratory fluorescence measurements. The effects of these issues on the calibration and response of the optical detector are discussed. While we specifically discuss calibration of measurements of diesel fuel marine (DFM) by UV fluorescence, we believe that the calibration techniques and optical issues we are addressing will be germane to most if not all in situ optical measurements of contaminants in soils. In orderto improve the in situfluorescent quantitation of petroleum hydrocarbons, we have calibrated the fluorescent response of fuels as a function of soil type and conditions. The fluorescent response of DFM vanes by an order of magnitude or more as a function of soil type. Experiments to determine the causes of this variability have shown that the controlling variable is surface area of the substrate, although there are secondary effects as a function of grain size, mineralogy, and degree of soil aggregation. We have found that normalizing contaminant concentration to available soil surface area allows for a much more predictable response factor. It should be noted, however, that the variation in fluorescent response of OEM in three of four EPA soils tested is relatively small, with only one showing a large divergence. Soils with mixtures of grain types and sizes fall in a relatively tight response range, while soils such as very pure sands or clays diverge significantly. Preliminary studies on the effects of moisture content on the fluorescent response of OEM in soils suggestthat the addition of waterto the matrix begins to exclude DFM from the grain surfaces, forcing the fluorophore into the grain interstices, and greatly decreasing the difference in fluorescent response between soil types. The moisture effect is the smallest in mixtures of sand and clay. We are currently developing specific fluorescence calibration algorithms as a function of soil type, and correlating actual soil types to soil classifications derived from cone penetrometer strain gauge data.

Remote in-situ detection of heavy metal contamination in soils using a fiber optic laser-induced breakdown spectroscopy (FOLIBS) system

The current status of a fiber optic laser induced breakdown spectroscopy probe for subsurface deployment via a cone penetrometer truck is discussed. In this work the parameters involved in delivering both the LIBS excitation and emission over optical fibers are discussed and laboratory detection limit results for Pb contaminated sand and soil are presented from a prototype FOLIBS probe.

Subsurface screening of petroleum hydrocarbons in soils via laser-induced fluorometry over optical fibers with a cone penetrometer system

A novel field screening method is described that couples a fiber optic based chemical sensor system to a truck mounted cone penetrometer. The system provides the capability for real- time, in situ measurement of petroleum hydrocarbon contamination and soil type to depths of 50 m. The technique uses a hydraulic ram in a truck with a 20 ton reaction mass to push an instrumented probe into the ground. Fluorescence is excited through a sapphire window in the probe by 337 nm light from a pulsed nitrogen laser. The excitation pulse is transmitted down the probe over a 100 m silica clad silica optical fiber. The resulting fluorescence from aromatic hydrocarbons in the soil is returned to the surface over a second fiber, dispersed with a spectrograph, and quantified with an intensified linear photodiode array. Field test data is presented that demonstrates how the system can be used for rapid three-dimensional delineation of a POL (Petroleum-Oil-Lubricant) contaminant plume at a hazardous waste site. Fluorescent fingerprints from 14 samples of 9 fuel types are used to show how spectral differences can be used for identifying contaminant sources. The effects of volatilization of different fuel types on the measured fluorescent signal are discussed.

Fiber optic fluorometer based on a dual-wavelength laser excitation source

A remote fiber optic fluorometer system which incorporates a dual wavelength UV laser excitation source is described. The system provides increased specificity for detection of multiple fluorophores without sacrificing real time sensing capability. Limitations imposed by UV transmission in fused silica fibers are discussed.

Field deployment of a LIBS probe for rapid delineation of metals in soils

A fiber optic probe for use in the in situ delineation of subsurface metal contamination is described. The probe is based on the technique of laser-induced breakdown spectroscopy and is designed for deployment via a standard cone penetrometer truck. Initial measurements of the detection limits of the probe on Pb and Cr contaminated sands are in the low ppm range.

Application of LIBS to in-situ assessment of metal-contaminated soils

In this communication, we discuss the development of a fiber optic probe for use in field screening of waste sites that are contaminated with heavy metals. The prove, which is deployed via cone penetrometer truck, uses the principle of laser induced breakdown spectroscopy to gather qualitative and quantitative about metal contamination in-situ.