Spectral characteristics of underwater laser-induced breakdown spectroscopy under high-pressure conditions

Laser-induced breakdown spectroscopy (LIBS) has been proven to be an attractive technique for in situ oceanic applications. However, when applying LIBS into deep-sea, the pressure effect caused by different ocean depths is inescapable and could have great influence on the LIBS signals. In this work, spectral characteristics of underwater LIBS were investigated as a function of pressure in the range of 0.1-45 MPa. A high-pressure chamber built in the laboratory was used to simulate the high-pressure deep-sea environment. Optimal laser energy and detection delay were first determined under different pressure conditions and were shown to be independent of the external pressure. The increase in pressure has a significant impact both on the peak intensity and line broadening of the observed spectra. The peak intensity of Na, Li and K lines increases with the increasing pressure until a maximum intensity is reached at 12.5 MPa. Above this value, the peak intensity decreases gradually up to 45 MPa. For Ca line, the maximum intensity was observed at 30 MPa. The line broadening keeps constant at low pressures from 0.1-10 MPa, while it increases linearly at higher pressures, indicating a higher electron density caused by the compression effect of the high external pressure. We also compared the spectral data obtained from the high-pressure chamber and from the field sea trials, and the good consistency between the laboratory data and sea-trial data suggested the key role of pressure effect on underwater LIBS signals for practical deep-sea applications.