Field demonstrations of a direct push FO-LIBS metal sensor

A direct push, FO-LIBS sensor probe for real-time, in-situ measurement of metals in soils has been built and its capabilities demonstrated at three sites. The response of the FO-LIBS sensor is affected by the soil matrix conditions of grain size, composition, and water content. For each site, uncontaminated soil was collected and spiked with known quantities of the metal under investigation. These calibration standards were used to generate a site-specific calibration curve, which compensates for variations in soil matrix conditions of grain size, composition, porosity, etc. To compensate for moisture content, extra laser pulses are used to volatilize the water, higher power densities and/or shortened acquisition delay times are used. The LIBS experiment inherently exhibits poor precision since it is a point measurement. When multiple spectra are taken of a single, homogenized soil sample, there is a significant amount of variability in the peak intensities. There is no internal standard available to correct for this variability. In this communication, it is shown that normalizing the peak areas under the spectral curves to one effectively corrects for variation due to differences in the plasma volume. Because the LIBS technique is essentially a point measurement, the FO-LIBS sensor is used as a semiquantitative field screening tool and not as a quantitative analytical method. In the field demonstrations reported in this communication, remarkably good agreement between the in-situ FO-LIBS results and ICP lab results were obtained.