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 e lemental emission signals were generated from even strongly emitting s pecies at high concentrations (e.g. Al(I) from commercially pure alumi num). The spectrum in these cases was dominated by spectrally broad em issions (continuum radiation) from the plume (T < 3000 K). The use of a repetitive spark pair (RSP), however, produced a plasma (T similar t o 8,880 K) on the second pulse useful to monitor the elemental composi tion of the metals. In the RSP, a pair of pulses was focused on the me tals at 10 Hz with the separation between pulses adjusted from 30 to 1 80 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 ove r two orders of magnitude were obtained compared to the RSS. Using the RSP, detection limits of 367, 520, 1200, and 1190 ppm were obtained f or Cr, Cu, Mn, and Si in steel. Using time-resolved imaging of the las er plumes/plasmas plasmas formed on the metal samples under water, som e characteristics of the excitation process were evaluated. It was det ermined 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 i s then interrogated by the second pulse. The second pulse then forms a plasma resulting in excitation similar to that produced by a single l aser spark formed on metal in air. (C) 1997 Elsevier Science B.V.