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.

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⁻², 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⁻², 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⁻² and an excitation fluence of about 1.1 J cm⁻². 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.

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.

F ₂ -Fluorination of Fe/N/C catalysts poisons FeN ₄ , but not CNx sites. Main causes of instability in PEMFCs are either FeN ₄ demetalation for Fe/N/C or H ₂ O ₂ when FeN ₄ sites are poisoned by fluorination or absent as in MOF_CNx_Ar + NH ₃ .

We measure the surface ablation threshold fluence of porcine corneal stroma for 100 fs laser pulses, with wavelengths between 800 nm and 1450 nm, generated by a Ti:sapphire-pumped optical parametric amplifier. The ablation threshold was found to vary only slightly within this wavelength range, between 1.5 and 2.2 J/cm². The data suggest a rapid increase of the ablation threshold for wavelengths up to about 1000 nm, followed by a plateau for longer wavelengths. This behavior is partly confirmed by a simple theoretical model of the ablation process. The influence of the wavelength on the physics of surface ablation is discussed on the basis of the model.