Resonance-enhanced laser-induced breakdown spectroscopy (RELIBS) was investigated with the aim to improve the limit of detection of trace elements in the context of elemental analysis of aluminium alloys. A Q-switched Nd:YAG laser pulse (7 ns, 1064 nm) was used for ablation of the samples and was followed, after a suitable delay, by an Optical Parametric Oscillator (OPO) laser pulse (7 ns), tuned at 396.15 nm, to resonantly excite the aluminium host atoms. In particular, the Mg I 285.21 nm and Si I 288.16 nm lines were observed in the acquisition spectral window. We investigated the influence of the main experimental parameters, namely, the excitation wavelength, the interpulse delay and the ablation and excitation fluences, on the signal-to-noise ratio for the Mg I 285.21 nm line. We found that, at low ablation fluences, typically less than a few J cm À2 , the Mg signal at 285.21 nm achieved using RELIBS was significantly enhanced when compared to LIBS using the same ablation fluence. At fluences higher than 8 J cm À2 , the effect of the excitation pulse became unnoticeable and similar results were observed for both approaches. The optimum conditions were achieved for an interpulse delay of about 30 ns, an ablation fluence of about 3.8 J cm À2 and an excitation fluence of about 1.1 J cm À2 . The corresponding absolute LoDs were 0.7 and 50 fg, for Mg and Si, respectively, using RELIBS. When using LIBS, they were 4 and 128 fg, instead. Finally, the applicability of RELIBS in the context of a minimally destructive elemental analysis is discussed.

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Leakage of injected carbon dioxide (CO2) or resident fluids, such as brine, is a major concern associated with the injection of large volumes of CO2 into deep saline formations. Migration of brine could contaminate drinking water resources by increasing their salinity or endanger vegetation and animal life as well as human health. The main objective of this study was to investigate the effect of sodium chloride (NaCl) concentration on the detection of calcium and potassium in brine samples using laser-induced breakdown spectroscopy (LIBS). The ultimate goals were to determine the suitability of the LIBS technique for in situ measurements of metal ion concentrations in NaCl-rich solution and to develop a chemical sensor that can provide the early detection of brine intrusion into formations used for domestic or agricultural water production. Several brine samples of NaCl-CaCl2 and NaCl-KCl were prepared at NaCl concentrations between 0.0 and 3.0 M. The effect of NaCl concentration on the signal-to-background ratio (SBR) and signal-to-noise ratio (SNR) for calcium (422.67 nm) and potassium (769.49 nm) emission lines was evaluated. Results show that, for a delay time of 300 ns and a gate width of 3 μs, the presence of and changes in NaCl concentration significantly affect the SBR and SNR for both emission lines. An increase in NaCl concentration from 0.0 to 3.0 M produced an increase in the SNR, whereas the SBR dropped continuously. The detection limits obtained for both elements were in the milligrams per liter range, suggesting that a NaCl-rich solution does not severely limit the ability of LIBS to detect trace amount of metal ions.