Enhancement of LIBS signal in lithium solution at the interface between liquid and an aluminum substrate

Detecting lithium (Li) in seawater in situ is of great significance for searching deep-sea hydrothermal activities and prospecting oceanic lithium reservoirs. Underwater LIBS (Laser-induced breakdown spectroscopy) has been proven to be an attractive technique for in situ element analysis, but its detecting sensitivity is still hard to satisfy the needs of less rich element analysis. Here, we propose an effective approach to enhance the LIBS signal of Li in bulk water with an immersed metal Al substrate. In this work, a laser beam focused on the substrate's surface with focal offsets between −2.5 mm to 1.5 mm from the surface. A 24 times enhancement factor of the Li signal was achieved with a typical pulse energy of 22 mJ at the optimal focal offset of −0.5 mm. The required minimum pulse energy for 100% breakdown was reduced by 5 times, and the plasma lifetime was 3 times longer, compared with the case without substrate assistance. The quantitative results show that both the LOD and analytical prediction capability of Li can be improved by twice compared to the direct measurement. Moreover, using the background as an internal reference, the average RSD at different concentrations can be further reduced from 45% to 18%. This study not only demonstrates the existence of a spectral enhancement effect at the liquid-solid interface of underwater LIBS, but also reveals an easy-to-implement signal enhancement method for upgrading the development of underwater in situ LIBS sensors.