Elemental analysis of metals under water using laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been evaluated for the detection of metals located under water. Repetitive laser sparks were formed on the metals by focused pulses from a Q-switched Nd:YAG laser (1064 nm). Using repetitive single sparks (RSS) at 10 Hz, only weak elemental emission signals were generated from even strongly emitting species at high concentrations (e.g. Al(I) from commercially pure aluminum). The spectrum in these cases was dominated by spectrally broad emissions (continuum radiation) from the plume (T < 3000 K). The use of a repetitive spark pair (RSP), however, produced a plasma (T similar to 8,880 K) on the second pulse useful to monitor the elemental composition of the metals. In the RSP, a pair of pulses was focused on the metals at 10 Hz with the separation between pulses adjusted from 30 to 180 mu s. The pulse pair was generated from a single laser. Only a weak dependence of the emission intensities on the separation between the pulses of the spark pair was observed although signal enhancements over two orders of magnitude were obtained compared to the RSS. Using the RSP, detection limits of 367, 520, 1200, and 1190 ppm were obtained for Cr, Cu, Mn, and Si in steel. Using time-resolved imaging of the laser plumes/plasmas plasmas formed on the metal samples under water, some characteristics of the excitation process were evaluated. It was determined that the first pulse of the RSP forms a large bubble or vapor cavity (maximum diameter similar to 8 mm) on the metal surface which is then interrogated by the second pulse. The second pulse then forms a plasma resulting in excitation similar to that produced by a single laser spark formed on metal in air. (C) 1997 Elsevier Science B.V.