Improving laser-induced breakdown spectroscopy (LIBS) performance for iron and lead determination in aqueous solutions with laser-induced fluorescence (LIF)

Abstract

The combination of laser-induced breakdown spectroscopy (LIBS) and laser-induced fluorescence (LIF) was investigated to improve the limit of detection (LoD) of trace elements in liquid water, while preserving the distinctive on-line monitoring capabilities of LIBS analysis. The influence of the main experimental parameters, namely the ablation fluence, the excitation fluence, and the inter-pulse delay was studied to maximize the fluorescence signal. The plasma was produced by a 266 nm frequency-quadrupled Q-switched Nd:YAG laser and the trace elements under investigation were then re-excited by a nanosecond optical parametric oscillator (OPO) laser, delivering pulses in the sub-mJ energy range, and tuned to strong absorption lines of the trace elements. The reproducibility of the measurements was improved using a home-made flow-cell, and relative standard deviations as low as 6.7% for a series of 100 shots were attained with a repetition rate of 0.7 Hz. Using the LIBS-LIF technique, we demonstrated LoDs of 39 ppb and 65 ppb for Pb and Fe, respectively, accumulating over 100 laser shots only, which correspond to an improvement of about 500 times with respect to LIBS.