Performance improvement of underwater LIBS qualitative and quantitative analysis by irradiating with long nanosecond pulses.

Long nanosecond pulses have been proven to be efficient at enhancing underwater LIBS emission. However, the quantitative analytical capability of underwater long-pulse LIBS has yet to be further revealed. In this work, we investigated the spectral characteristics by irradiating with a laser pulse of 120 ns duration. The alkali and alkaline earth metals Li, K and Ca and the transition element Mn were selected for analysis. It is shown that obvious self-reversal structures were observed in the spectra at high concentrations, making the calibration curves saturated. Correction was performed using the approximate Voigt function fitting method, which significantly improves the linearity of the calibration curves. In addition to the target metal elements, atomic lines of the matrix elements H and O in water were also observed, which can serve as promising internal standards for quantitative analysis. A comparison of the quantification performance with and without the internal standards demonstrates that the use of the internal standards is conducive to improving the robustness of the calibration approaches with higher determination coefficients. More importantly, the underwater LIBS signal stability is improved by more than 3 times, and the prediction error for validation samples is reduced by 2-4 times. The present results suggest that long ns pulses are favorable to significantly improving the qualitative and quantitative performance of underwater single-pulse LIBS, enabling long-pulse LIBS to have great potential to be applied to underwater in situ chemical analysis.

Investigation of laser-induced plasma characteristics in bulk water under different focusing arrangements.

Laser-induced breakdown spectroscopy (LIBS) has drawn more attention as a new technique for in situ detection of seawater, especially for hydrothermal areas. In order to evaluate the focusing geometry effect on laser-induced plasma in bulk water, four focusing arrangements were tried out with a single lens as well as with a double-lens combination. We demonstrated that, for the same transmission distance in water, the double-lens combination with shorter effective focal length generated more condensed plasma, as shown by the spectroscopic and fast imaging results. Accordingly, the moving breakdown phenomenon significantly decreased with well-improved LIBS intensity, signal-to-noise ratio, and stability. The plasma emissions evidently attenuated along the laser transmission due to the strong absorption effect of the water medium. Based on the acquired results, the performance of a practical detection window was evaluated by combining the regular lens with a customized lens-shaped optical window, and a positive outcome was also reached. The obtained results suggested that improved LIBS detection could be easily achieved via settling another lens window to LIBS system, which is considered helpful for better in situ submarine application.

Development of a compact underwater laser-induced breakdown spectroscopy (LIBS) system and preliminary results in sea trials.

The exploitation and research of deep-sea hydrothermal vent has been an issue of great interest in ocean research in recent years. Laser-induced breakdown spectroscopy (LIBS) has great potential for ocean application due to the capabilities of stand-off, multiphase, and multielement analysis. In this work, a newly developed compact 4000 m rated LIBS system (LIBSea) is introduced with preliminary results of sea trials. The underwater system consists of an Nd:YAG single-pulsed laser operating at 1064 nm, an optical fiber spectrometer, an optics module, and an electronic controller module. The whole system is housed in an L800 mm×ϕ258 mm pressure housing with an optical window on the end cap. It was deployed on the remote operated vehicle Faxian on the research vessel Kexue, and in June 2015 was successfully applied for hydrothermal field measurements at the Manus area. The obtained results are shown that the LIBS system is capable of detecting elements Li, Na, K, Ca, and Mg in the hydrothermal area. Profiles of LIBS signals of elements K and Ca have also been obtained during the sea trial. The results show that the K emission line is gradually broadened with depth from sea surface to sea floor (1800 m or so); the K intensity shows a hump shape with maximum value at about 1050 m. The Ca emission line is rapidly broadened below 400 m and slowly narrowed to the sea floor; the Ca intensity shows no obvious change below 400 m and increases continuously to sea floor. A very interesting finding is that the small fluctuations of intensity profile curve of Ca show a degree of correlation with seawater temperature change. The sea trial results prove the performance of LIBSea. After further optimization, it is hoped to apply the LIBS system to the in situ mineral deposits and hydrothermal vent fluid detection in deep sea.

[Study on the Spatial Distribution of Laser Induced Plasma Emission Underwater with Different Laser Energies].

Laser-induced breakdown spectroscopy (LIBS), as a promising in-situ underwater detection technology, has received extensive attention in the field of ocean exploration. Improving the remote sensing ability of LIBS is crucial in bringing this technology into practical applications, hence higher laser energies, i.e. over threshold, are required. To characterize the plasma induced with super threshold energies and have a better understanding of resulted moving breakdown process, some extensive investigations into KCl water solution have been carried out with spectra-image jointed analysis. The spatial span and the brightest spot position of plasma radiation, with different laser energies from 1 to 20 mJ, were determined from the recorded plasma images. It was found that the plasma stretched from 0.49 to1.83 mm, as the laser pulse energy increased from 1 to 20 mJ, with a center position shift of 0.79 mm towards the incident laser beam. The obvious power dependence has also been observed from the obtained spatial resolved atomic spectra. Although the axial distributions at different energies were similar, both the position and the intensity of potassium atomic emission maximum varied. The optimal laser energy was determined to be 5 mJ with the emission intensity maximum higher than that at any other investigated energies. The obtained results suggested that the power dependence of atomic emission should be taken into account on increasing laser energy to meet the needs of stand-off LIBS applications. The FWHM and signal to background ratio of K Ⅰ 769.90 nm under different laser energies have also been investigated.

[Comparative investigation of underwater-LIBS using 532 and 1 064 nm lasers].

With the hope of applying laser induced breakdown spectroscopy (LIBS) to the ocean applications, the laser energy at 532 and 1 064 nm wavelength with 3 and 40 mj respectively was used, which was near their breakdown threshold. Extensive experimental investigations of LIBS from CaCl2 water solution were carried out in this paper using different laser wavelengths of 532 and 1 064 nm. The obtained results show that compared with the 532 nm laser, the 1 064 nm laser can induce the plasma in water with higher emission intensity and longer lifetime, while the reproducibility of LIBS signal under 1 064 nm laser is poorer. On the other hand, due to the different attenuation ratios of 532 and 1 064 nm laser energies in water, the LIBS signal of 1 064 nm laser decreases a lot within the transmission distance range 2-5 cm, while LIBS signal of 532 nm remains the same, because that the wavelength of 532 nm lies in the "transmission window" of the water solution. This study will provide valuable design considerations for the development of LIBS-sea system in near future.

Identification of microplastics using a convolutional neural network based on micro-Raman spectroscopy.

Microplastics (MPs) pose a threat to human and environmental health, and have emerged as a global environmental issue. Because MPs are small and complex, methods of quickly and reliably classifying and identifying them are either lacking or in the early stages of development. In this study, micro-Raman spectroscopy and a convolutional neural network (CNN) were combined to establish identification models for 10 MP references and three environmental samples. In addition, an interaction network was established based on pair-wise correlations of Raman bands to determine the influence of environmental stress on MPs. The CNN model achieved average classification accuracies of 96.43% and 95.6% for the 10 MP references and the three environmental samples, respectively. For MPs exposed to environmental stressors, an interaction network can provide highly sensitive, information-dense, and universally applicable signatures for characterizing the environmental processes affecting MP spectra. The results of this study can help establish efficient and automatic analysis for accurate identification of MPs as well as an intuitive exhibition of spectral changes on environmental exposure.