Laser-induced breakdown spectroscopy; LIBS; liquid; plasma; laser ablation;AQUEOUS-SOLUTIONS; VEGETABLE-OILS; WASTE-WATER; LIBS; METALS; ELEMENTS; SODIUM; PLASMA; PULSE; ACIDS

We studied changes in laser-induced breakdown spectroscopy (LIBS) signal intensity with the thickness of a liquid layer placed on a solid substrate, where an easily evaporating methanol sample was used. For a certain optimal liquid film thickness we obtained a manifold increase of the LIBS signal from methanol. Progressive liquid film thinning leads to a reduction and a successive disappearance of laser-induced splashes; the latter condition drastically reduces the sample consumption and allows measurements to be repeated many times on a single liquid droplet. In following, we developed two methods for actively controlled deformation, i.e., thinning of a liquid droplet (volume similar to 10 ml) prior to its sampling by LIBS. Control of the droplet's height was achieved on a Si-SiO2 wafer substrate by electro-wetting in the case of water solutions or by target rotation in the case of viscous liquids. The chosen substrate also has the advantages of low cost, easy manipulation, and very high purity, thus minimizing interference with analytes. Through the droplet deformation, in a single-pulse excitation at moderate laser energy (70 mJ), we clearly detected Fe and Mn in peanut oil, which represent trace elements in edible oils (similar to 1 part per billion), according to results published in the literature.

QUANTITATIVE-ANALYSIS; WASTE-WATER; SURFACE; ABLATION; DROPLET; LIBS; SUBSTRATE; SODIUM; DIESEL; OIL

Droplets of organic liquids on aluminum substrate were probed by an Nd:YAG laser, both in a steady state and during rotation at speeds 18-150 rpm. Rotation transforms the droplet into film, which estimated thickness at high speeds was below 3 mu m and 20 mu m for diesel and peanut oil, respectively. Line intensities from the liquid (C I) and the support (Al I) material were tracked as a function of the film thickness and the laser energy. By film thinning, the line intensities from liquid sample were enhanced up to a factor 100x; simultaneously, the LIBS signal fluctuations were reduced 5-10 times with respect to the steady droplet. In certain experimental conditions, the line intensities from the support material become very weak with respect to the C I line, indicating an efficient screening of the substrate by highly excited plasma from the liquid layer. At a fixed rotation speed, there is a laser energy threshold, dependent on the liquid thickness, above which the LIBS signal becomes stable. Here, we discuss the relative processes and optimization of the experimental conditions for the LIBS measurements frome one laser shot to another.