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The calibration-free methodology described in the literature dealing with the technique of laser induced breakdown spectroscopy has been applied here to several samples under different operating pressures with the aim of classifying the analytical technique as quantitative, semi-quantitative or qualitative (with quantitative estimates only), depending on the relative percentage errors calculated for the major, minor and trace elemental composition of the sample. The results were obtained with a Q-switched Nd:YAG laser (1064 nm), characterized by 5.3 ns duration pulses at a maximum repetition frequency of 10 Hz and with pulse energy up to 300 mJ. The spectra were taken with a grating monochromator coupled with an intensified CCD detector, calibrated from 220 to 700 nm. The samples investigated were aluminum alloy standard disks, standard reference materials of Zn alloy, Cu-Ni alloy, Ti alloy, Ni alloy powder, Cu-Zn alloy and soils. The data are compared with those published in the literature. The results show that the best accuracy (5% relative error) is usually obtained for the matrix element, while in most cases, for the remainder of the components in the sample (major, minor or trace composition), the results can only be considered semi-quantitative (<200% relative error).

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Multi-pulse laser-induced breakdown spectroscopy (LIBS) in the collinear pulse configuration with time-integrating detection was performed on metallic samples in ambient air in an effort to clarify the contributing processes responsible for the signal enhancement observed in comparison with single-pulse excitation. Complementary experiments were also carried out on another LIBS setup using detection by an imaging spectrograph with high time resolution. The effects of laser bursts consisting of up to seven ns-range pulses from Nd-doped solid-state lasers operating at their fundamental wavelength and separated by 8.5-50 micros time gaps was studied. The ablation and emission characteristics of the generated plasmas were investigated using light profilometry, microscopy, plasma imaging, emission distribution mapping, time-resolved line emission monitoring, and plasma temperature calculations. The experimental data suggest that the two contributing processes mainly responsible for the signal enhancement effect are the plume reheating caused by the sequential laser pulses and, more dominantly, the increased material ablation attributed to the lower breakdown threshold for the preheated (molten) sample surface and/or the reduced background gas pressure behind the shockwave of preceding pulses.