Stability evaluation of water droplets levitated by a TinyLev acoustic levitator for laser induced breakdown spectroscopy

Stability of acoustic levitated water droplets, for laser induced breakdown spectroscopy (LIBS), is required in order to obtain shot-to-shot reproducibility and low limits of detection. Although stabilization is possible by increasing the TinyLev levitator's operational voltage, at high voltages liquid droplets shatter when inserted at the nodes of the acoustic levitator. An alternative for stabilization, it is the use of a four beam laser arrangement, similar to an optical trap. In this case, we have found adequate not to use focusing lenses, since the goal is just to exert certain lateral pressure to counteract the lateral pressure instabilities generated by the acoustic levitator and simplify the optical arrangement. For an operational voltage of 8.5 V, a reduction on the droplet's horizontal deviation of 42% is attained at 650 mW laser power. At these experimental conditions, LIBS measurements were conducted on water samples doped with different metals. It is shown that, at this droplet's deviation, it is possible to analyze liquid samples despite a complete liquid evaporation is not reached. For Al samples a limit of detection (LoD) of 0.22 ppm is calculated from a calibration curve, and LoD of 1.18, 0.15, 0.94 and 0.7 ppm for Cu, Mn, Pb and Ni respectively, are estimated by signal to noise ratios values. These results show that even though, the final size of the liquid droplets is large, there is not a reduction on the lowest detectable concentration obtained, compared to other LIBS works on liquid samples. Leading up to conclude that pre-concentration effect takes more relevance than residual liquid's detrimental effect.