Resonant laser-induced breakdown spectroscopy for analysis of lead traces in copper alloys

Resonant laser-induced breakdown spectroscopy (RLIBS) was investigated to improve the limit of detection (LoD) of lead traces in copper alloys. A 5-ns optical parametric oscillator laser pulse was used to ablate the sample (front edge of the pulse) and resonantly excite the lead atoms in the vapor plume (rear edge of the pulse). The laser was tuned to the Pb I 283.31 nm line and the Stokes direct-line fluorescence signal at 405.78 nm was recorded. The experiments were performed in air at atmospheric pressure. The influence of the main experimental parameters, namely the laser wavelength, laser fluence and acquisition delay, on the signal-to-noise ratio (SNR) for the Pb I 405.78 nm line was studied experimentally. We found that the best SNR for the Pb I 405.78 nm line was achieved for a laser fluence of about 1.4 J cm(-2), corresponding to the onset of visible damage on the sample, and an acquisition delay of about 5 ns. When the laser was on resonance a Pb I 405.78 nm signal could however be observed for a fluence as low as 0.25 J cm(-2). For fluences greater than about 5 J cm(-2), similar results were obtained whether the laser wavelength was tuned on resonance or not. Under these optimal conditions, the relative LoD for lead was estimated to be about 8 parts per million, accumulating over 500 laser shots. This amounts to an improvement of about 11-fold when compared to non-resonant LIBS in typical conditions. The corresponding absolute LoD was estimated to be about 0.8 femtograms and was calculated from optical coherence tomography measurements of the crater profiles. The fact that the best performances of RLIBS were obtained near the onset of visible damage on the sample indicates that this approach is particularly suitable for minimally destructive elemental analysis.