CaOH Molecular Emissions in Underwater Laser-Induced Breakdown Spectroscopy: Spatial–Temporal Characteristics and Analytical Performances

Abstract

Recently, molecular emissions from the laser-induced plasma in ambient gas have gained increasing interest; however, very little is known about the case in water solutions. In this work, we investigated the spatiotemporal characteristics of molecular emissions, CaOH for instance, in underwater laser-induced breakdown spectroscopy (LIBS) by using time-resolved spectroscopy, spectral-resolved imaging, and shadowgraph techniques. It was shown that clear CaOH molecular bands can be observed in the spectrum at very early times after the laser pulse and presented a much longer lifetime and more homogeneous emission distribution compared with the Ca I and Ca II lines. Such unique characteristics of CaOH molecular emission inspired us to improve the performances of underwater LIBS by using the CaOH molecular bands instead of Ca I and Ca II lines. We demonstrated the excellent quantification results of CaOH with higher stability, less self-absorption, and reduced matrix effect. Meanwhile, the limit of detection (LOD) of Ca with the CaOH molecular band (2.46 ppm) is comparable to that with the atomic line of Ca I (2.07 ppm), and much lower than that with the ionic line of Ca II (13.81 ppm), indicating a good sensitivity of CaOH. This work gives not only some insights into the molecule formation mechanisms in underwater plasmas, but also provides new ideas to improve the analytical performances of underwater LIBS.

Signal Improvement for Underwater Measurement of Metal Samples Using Collinear Long-Short Double-Pulse Laser Induced Breakdown Spectroscopy

“…The ablation of water molecules leads to the significant continuous background in the spectrum. Generally, the emission spectral lines corresponding to the sample elements can still be distinguished from the continuous background in most of the underwater LIBS studies with relatively high laser pulse energy [21,26,27]. However, the spectral lines were completely submerged in the background and noise in the present results of SP-LIBS.…”

“…The lifetime of laser-induced plasma is usually several microseconds in an air atmosphere, but it is only hundreds of nanoseconds in a water environment [20]. The emission of laser-induced plasma is affected by the negative influences of water [21]. Therefore, signal improvement is one of the vital issues for underwater LIBS.…”

Development and Field Tests of a Deep-Sea Laser-Induced Breakdown Spectroscopy (LIBS) System for Solid Sample Analysis in Seawater

“…And the quantification results of CaOH are excellent with higher stability, less self-absorption, and reduced matrix effect. The acquisition of CaOH molecular signal may provide a new approach for improving the performances of underwater LIBS analysis [38]. The results showed that the LIBSea II system was feasible in deep-sea submerged solid detection.…”

Application of Molecular Emissions in Laser-Induced Breakdown Spectroscopy: A Review

“…In addition, CaOH emission can be used to detect the Ca content in underwater conditions. The results showed a LoD = 2.46 ppm with good linearity in a range from 5 to 2000 ppm[52].…”