<title>Field deployment of a LIBS probe for rapid delineation of metals in soils</title>

Abstract

Abstract A fiber optic probe for use in the in situ delineation of subsurfacemetal contamination is described. The probe is based on the technique oflaser-induced breakdown spectroscopy and is designed for deploymentvia a standard cone penetrometer truck. Initial measurements of thedetection limits of the probe on Pb and Cr contaminated sands are in thelow ppm range.Keywords: FOLIBS, fiber optic, laser induced breakdown spectroscopy,LIBS, SCAPS, CPT, cone penetrometer, in situ metal detection 1. Introduction Laser-induced breakdown spectroscopy (LIBS) involves theanalysis of the spectral emission from a laser-induced spark. In a typicalLIBS measurement, the high power emission from a pulsed laser isfocused to a small spot on a sample material. The resulting powerdensity in the small volume about the focal point is usually in excess ofseveral GW/cm2. Under these conditions, rapid heating, vaporizationand ionization of a small amount of the sample material occurs and thesubsequent laser-induced plasma emission is spectroscopically analyzedto yield information about the elemental species present in the sample.The LIBS technique has been used in the identification andquantification of elemental species in solids, liquids and gases.' Themethod is promising as an in situ detection method for heavy metalcontamination in soil because it is highly sensitive while requiring nosample preparation and maintaining a high degree of specificity to aparticular metal.2 The technique usually involves delivery of the laserexcitation energy through the air due to the high power densitiesrequired to initiate the spark. The feasibility of generating a laser-induced spark on soil using fiber optic excitation to make LIBSmeasurements with high sensitivity has recently been demonstrated.34As a result, fiber optic LIBS (FOLIBS) can be applied to situations inwhich the sample is located at a considerable distance from the exciting