Effect of experimental conditions on surface hardness measurements of calcified tissues via LIBS

This paper reports on the effects of LIBS experimental conditions on the measurement of the surface hardness of calcified tissues. The technique mainly depends on a previously demonstrated correlation between the intensity ratio of ionic to atomic spectral lines and the hardness of the target material. Three types of calcified tissues have been examined, namely enamel of human teeth, shells, and eggshells. Laser-induced breakdown spectra were obtained under two different experimental conditions. In the first nano and picoseconds, laser pulses were used in a single-pulse arrangement, while in the second, single- and double-pulse regimes with nanosecond laser excitation were utilized. The results show that the ionic to atomic spectral line intensity ratios are higher in the case of picosecond laser pulse for both Ca and Mg spectral lines. This effect has been justified in view of the repulsive force of the laser-induced shock waves which depends clearly on the target surface hardness and on the laser irradiance. The electron densities ratio (pico/nano) is shown to be strongly depending on the laser irradiance too. In the case of calcium, single-pulse ratios are higher than the double-pulse ratios, while there is no appreciable difference between both in the case of magnesium. The results obtained herein suggest that double-pulse nanosecond arrangement and the choice of a minor element such as Mg furnishes the best experimental conditions for estimating the surface hardness via LIBS spectra. To validate this method, it has been applied on two previously measured groups of teeth enamel, the first is of ancient Egyptians, and the second from Nubians and Ugandans. The results support the usefulness of this method for similar real-life applications.