Quantitative analysis of metallic traces in water-based liquids by UV-IR double-pulse laser-induced breakdown spectroscopy

Double-pulse laser-induced breakdown spectroscopy (DP-LIBS) was performed for quantitative analysis of three metallic trace elements: Fe, Pb and Au in aqueous solutions. The plasma was generated using a UV (266 nm) frequency-quadrupled Q-switched Nd:YAG laser (7 ns) and then reheated by a 1064 nm Q-switched Nd:YAG laser (7 ns) in a quasi-collinear geometrical configuration. In order to improve the reproducibility of LIBS measurements, a circulation cell was used, providing a reproducibility of about 4% with a laser repetition rate of 0.3 Hz. The influence of the inter-pulse delay and the fluence of the second laser pulse on the signal-to-noise ratio (SNR) for the analytical lines was investigated and optimized. Analytical figures of merit of DP-LIBS and conventional single-pulse LIBS (SP-LIBS) were evaluated by establishing the calibration curves for the Fe I 358.12 nm, Pb I 405.78 nm and Au I 267.60 nm lines. The signal was greatly enhanced in DP-LIBS while the noise level did not vary as much. An improvement of the relative limit of detection of about 10 was achieved using DP-LIBS when compared to UV SP-LIBS in all cases. Measurement of the electron density as a function of time indicates that the plasma plume lifetime is longer in DP-LIBS. Similar trends in the excitation temperature were not observed for reasons that we attribute to larger uncertainties related to the Boltzmann plot method.