With the hope of applying LIBS to solid target detection in deep-sea, the influences of laser focus to sample distance (LFTSD) on the plasma characteristics were investigated using spectra-image approach with the laser energies at sub- and super- threshold irradiance of solution. The experimental results show that LFTSD is a critical parameter which can directly influence the plasma shapes, by changing the laser fluence on sample surface. The plasma is divided into two parts under pre-focus condition, while the plasma only forms at the surface of Cu target under de-focus condition. Moreover, the “seed electron” generated from Cu sample can reduce the breakdown threshold of the solution. By comparing the laser energy, it seems to be inefficient by using super-threshold energy due to the plasma shielding effect of the liquid. High quality spectra can be observed by using lower laser energy and longer gate delay (25 mJ and 1000 ns, in this work).
Laser Induced Breakdown Spectroscopy (LIBS) has been developing as a promising technique with many
advantages, including on-line, real time, stand off and multi-element detection <i>etc</i>. At present, LIBS application
has involved many areas; more and more researchers have studied LIBS as an <i>in-situ</i> measurement for oceanic
sensing. In this paper, a novel approach for LIBS enhancement has been introduced and used to improve the
sensitivity of underwater detection. In this approach, the replacement reaction assisted with the electric field was
utilized to strengthen the reaction and increase local cations concentration for depositing on the underlay surface.
With the aid of replacement reaction and electric field, the LIBS sensitivity for cations detection underwater was
found to be enhanced significantly, hence the detection limit. The detection limit of 16 ppb was achieved for
copper cations (Cu<sup>2+</sup>) detection in the water solution of CuSO<sub>4</sub>. The obtained results suggested that this novel
approach has great potential to be developed as an effective method for cations detection underwater.