Seawater Salinity Measurement Using Laser-Induced Plasma: Acoustic–Spectral Characteristics and Analytical Performances

Seawater salinity is a critical parameter in marine environments; however, the established methods based on conductivity or refractive index detection can hardly obtain the elemental compositions of seawater that affect the salinity. In this work, we proposed a laser-based method that can simultaneously measure the seawater salinity and elemental concentrations. The idea is to generate a laser-induced plasma in water; the acoustic signal from the plasma carries physical information about the sound velocity, while the spectral signal from the plasma provides chemical information on the elemental composition. For this purpose, comprehensive characterizations on the laser-induced breakdown process as well as on the corresponding acoustic and spectral signals were carried out by using shadowgraph imaging, fast imaging, hydrophone measurement, and spectroscopic techniques. Then, an acoustic-spectral data fusion model was built to predict seawater salinity and elemental concentrations. From the calibration curves of salinity, it showed that with the acoustic-spectral combined data, the root mean square error of prediction (RMSEP), average relative error (ARE), and relative standard deviation (RSD) reach the values of 0.5‰, 1.63%, and 1.27%, respectively, which are all lower than with the acoustic data or spectral data only. From the calibration curves of elemental concentrations, the RMSEP values are 519.80 mg/L for Na, 19.04 mg/L for K, and 33.31 mg/L for Ca, while the R² values are above 0.99. This work provides a multimodal laser-based sensing method that could also be extended to other transparent liquid analyses.