Quantitative Analysis of the Detection Limits for Heavy-Metal-Contaminated Soils by Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is a rapid remote measurement method for detection of metals in the environment. A major factor in the quantitative use of this technique involves the minimum detection limits under laboratory and field operations. Nd:YAG laser pulses (125 mJ, lambda = 1.06 micrometers) are focused on sample surfaces to produce laser sparks (plasmas). Plasma emissions are recorded after interference from broadband emissions is reduced. Results are reported on the lower detection limits of As, Cd, Cr, Hg, Pb and Zn in sand, silt, clay and kaolin. Detection limits are significantly lower in sand than silt and clay. An excimer laser (125 mJ, lambda = 248 nm) was used for comparative LIBS analysis of lead (Pb) in silt. No significant difference was found in the lower detection limits using excimer and Nd:YAG lasers. The LIBS method must provide accurate data when used in a cone penetrometer. Conditions encountered by a cone penetrometer-based LIBS system have been simulated by compressing soil samples and then allowing them to relax for specific intervals before LIBS analysis. Results are presented of the dependence of LIBS measurements on the relaxation time after soil sample compression. This data is important for a firm understanding of detection limits for field-deployable LIBS systems.