Underwater Laser-induced Breakdown Spectroscopy Research Articles

Solid substrate assisted enhanced laser induced breakdown spectroscopy for metal element analysis in aqueous solution.

Due to plasma quenching caused by the dense water medium, laser-induced breakdown spectroscopy (LIBS) faces challenges such as strong continuous background radiation and weak and broadened characteristic spectral lines when directly detecting metal elements in liquids. In this work, we introduced a simple approach to improve underwater LIBS signals with a solid substrate-assisted method, which requires no sample pre-treatment and simple operation and thus has potential for in situ marine applications. In this method, four submerged solid substrates (Zn, Cu, Ni, and Si) were employed to investigate the breakdown characteristics of underwater LIBS and the mechanism of spectral enhancement by using a CaCl2 solution. The results demonstrated a significant improvement in the detection sensitivity of Ca with these substrates even at a short laser pulse with a relatively low laser energy (10 mJ). Among them, the semiconductor Si substrate exhibited the best enhancement effect, with an enhancement factor of over 75 for the Ca ionic lines at 393.4 nm and 396.8 nm and an enhancement factor of 29 for the Ca atomic line at 422.7 nm, respectively. This is mainly because the presence of substrate decreases the breakdown threshold of the liquid sample, and a higher plasma excitation temperature and electron density are obtained, which, in turn, leads to higher signal intensity. Furthermore, significant plasma emission enhancements for a wide range of elements are also achieved from seawater. These findings can contribute to the development of compact underwater in situ LIBS sensors with low power consumption, while ensuring high detection sensitivity.

Enhancement of LIBS signal in lithium solution at the interface between liquid and an aluminum substrate

Detecting lithium (Li) in seawater in situ is of great significance for searching deep-sea hydrothermal activities and prospecting oceanic lithium reservoirs. Underwater LIBS (Laser-induced breakdown spectroscopy) has been proven to be an attractive technique for in situ element analysis, but its detecting sensitivity is still hard to satisfy the needs of less rich element analysis. Here, we propose an effective approach to enhance the LIBS signal of Li in bulk water with an immersed metal Al substrate. In this work, a laser beam focused on the substrate's surface with focal offsets between −2.5 mm to 1.5 mm from the surface. A 24 times enhancement factor of the Li signal was achieved with a typical pulse energy of 22 mJ at the optimal focal offset of −0.5 mm. The required minimum pulse energy for 100% breakdown was reduced by 5 times, and the plasma lifetime was 3 times longer, compared with the case without substrate assistance. The quantitative results show that both the LOD and analytical prediction capability of Li can be improved by twice compared to the direct measurement. Moreover, using the background as an internal reference, the average RSD at different concentrations can be further reduced from 45% to 18%. This study not only demonstrates the existence of a spectral enhancement effect at the liquid-solid interface of underwater LIBS, but also reveals an easy-to-implement signal enhancement method for upgrading the development of underwater in situ LIBS sensors.

Elemental Characterization of Indonesian Coral Skeleton Using Underwater Laser-Induced Breakdown Spectroscopy (LIBS), X-Ray Fluorescence Spectroscopy (XRF), and Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES)

The identification and determination of elements from the Indonesian coral skeleton are required to characterize anthropogenic activities. To this end, various spectroscopy methods were used for elemental analysis including underwater laser-induced breakdown spectroscopy (LIBS), x-ray fluorescence spectroscopy (XRF), and inductively coupled plasma – optical emission spectroscopy (ICP-OES). The results show that the LIBS was able identify the major elements Ca, Mg, and Sr. Using ICP-OES, all elements including major, minor, and trace elements were quantitatively determined. The results show that coral skeletons from Karimunjawa contain the highest concentrations of trace elements, which may be due to expanded human activities.

Early dynamics of laser-induced plasma and cavitation bubble in water

The knowledge of early laser-induced plasma and cavitation bubble dynamics are of great importance for a better understanding of underwater laser-induced breakdown spectroscopy. In this work, we investigated the temporal evolution of the plasma and bubble at an early stage before 2 μs, by using fast imaging and shadowgraph techniques. It showed that the initial plasma forms at the laser focal point and then grows up toward the incident laser. As time evolves, the plasma expands simultaneously forward and backward to its peripheral region and a sub-plasma structure can be observed at low laser energies. The plasma has a good pulse-to-pulse repeatability during the laser pulse, while soon after the laser pulse (from 20 to 50 ns), it undergoes a severe pulse-to-pulse fluctuation with an inhomogeneous emission distribution. At longer times, the plasma becomes much stable again. To clarify the interrelationship between the bubble dynamics and the plasma characteristics, shadowgraph images of the bubble were shown and compared with the plasma images. It showed that the morphology of the bubble and its evolution have a good consistency with the morphology of plasma. The bubble radius as a function of time can be well expressed by the equation R=at0.4, and a is a constant related to the laser energy. By inspecting the evolution curve of the bubble wall and the bubble pressure calculated based on the Gilmore model, a transition phase from 20 to 50 ns can be identified between the moving breakdown phase and the thermal expansion phase. In this transition phase, both the bubble expansion speed and the bubble pressure drop drastically that may account for the severe plasma fluctuations as observed in this period. These results of early bubble dynamics provide insights into the critical role of bubble-plasma interaction on the characterization of laser-induced plasma in water.

Effect of repeated irradiation on laser-induced breakdown spectroscopy of copper immersed in a sodium chloride aqueous solution and normalization with bubble collapse time

Strong correlations were found between underwater LIBS signals and bubble collapse time. Signal fluctuation caused by the repeated irradiation at a fixed position was successfully reduced by the normalization with bubble collapse time.

Underwater determination of calcium and strontium ions in oilfield produced water by laser-induced breakdown spectroscopy (LIBS).

Laser-induced breakdown spectroscopy (LIBS) was applied to the determination of scaling ions in oilfield-produced water employing underwater measurements. Initially, the stability of plasma was verified using four different optical setups and expansion of the laser beam, and a combination of an achromatic lens with a meniscus lens were necessary to stabilize the plasma. Preliminary experiments demonstrated that only the determinations of Ca(II) and Sr(II) ions were feasible while the signal for the Mg(II) ion was absent and the sensitivity for Ba(II) was very low. The laser pulse repetition rate was evaluated and rates of 10 and 20 Hz provided a more stable breakdown in water compared to repetition rates of 2 to 7 Hz, besides imparting higher intense signals. The increase in salinity showed a small matrix effect, decreasing the sensitivities of the calibration curves by 8-13% when standard solutions with a salinity of 30‰ were used instead of water. Under optimized conditions with a laser pulse energy of 31 mJ, gate delay of 300 ns, gate width of 5.0 μs, repetition rate of 10 Hz, and accumulation of 500 laser shots, a linear range from 25 to 150 mg L-1 was obtained, with limits of detection of 0.58 and 0.85 mg L-1 for Ca(II) and Sr(II), respectively. The underwater determination of scaling ions in produced water by LIBS provided results that do not significantly differ from those obtained by inductively coupled plasma atomic emission spectroscopy (ICP OES) at a confidence level of 95%, with relative errors of up to 5.2%. These results demonstrate the potential of underwater LIBS measurements as an analytical tool for the determination of alkaline-earth metal ions in produced water, which can help the oil industry to overcome the problems related to scale formation.

Quantitation improvement of underwater laser induced breakdown spectroscopy by using self-absorption correction based on plasma images

Laser-induced breakdown spectroscopy (LIBS) is a practical technique for in-situ detection, but self-absorption effect has been a big issue for quantitative applications of this technique. In presented work, a method was developed to correct self-absorption to improve the quantitation of underwater LIBS. We proposed “relative self-absorption coefficient” as the critical parameter to evaluate self-absorption, and the plasma image was employed as the reference to determine the coefficient value. Based on that, the LIBS detection was successfully corrected by the coefficient to realize quantitative analysis, and the “Dominant Factor-PLS” was used as assistance. The results indicated that our method greatly improved LIBS quantitation in practice. More importantly, the calibration curve was able to be established with high linearity (R2 = 0.9999) to cover a large concentration range (0–103 ppm). It is hoped that our method could be a contribution to developing LIBS as an analytical tool for field measurements.

Analysis of pulse-to-pulse fluctuation in underwater Laser-Induced Breakdown Spectroscopy on the basis of error propagation calculation

We study the pulse-to-pulse variation of the emission spectral intensity of underwater laser-induced breakdown spectroscopy. Emission spectral intensity and its dispersion were measured as a function of the fluence of the laser pulse at a metal target surface immersed in water. The coefficient of variation, which is an index of the dispersion, showed a minimum at a certain fluence. The dispersion at the low fluences was attributed to the variations of the population density and the atomic excitation temperature, according to the error propagation analysis of the theoretical spectral line intensity based on the Boltzmann distribution. The population density and the temperature showed a negative correlation, which is consistently explained by the unstable division of pulse energy into two parts, i.e., the early part of a pulse is attributed to materials ablation and hence to the population density, while the other part to plasma heating and hence to the temperature.

Normalization of underwater laser-induced breakdown spectroscopy using acoustic signals measured by a hydrophone.

Laser-induced breakdown spectroscopy (LIBS) signals in water always suffer strong pulse-to-pulse fluctuations that result in poor stability of the spectrum. In this work, a spectrum normalization method based on acoustic signals measured by a hydrophone immersed in water was developed and compared with laser energy normalization. The characteristics of the acoustic signals were studied first, and the correlations between the acoustic signals and LIBS spectra were analyzed. It showed that the spectral line intensity has a better linear relationship with the acoustic energy than with the laser energy. Consequently, the acoustic normalization exhibited better performance on the reduction of LIBS spectral fluctuation versus laser energy normalization. Calibration curves of Mn, Sr, and Li were then built to assess the analytical performance of the proposed acoustic normalization method. Compared with the original spectral data, the average RSD_C values of all analyte elements were significantly reduced from 5.00% to 3.18%, and the average RSD_P values were reduced from 5.09% to 3.28%, by using the acoustic normalization method. These results suggest that the stability of underwater LIBS can be clearly improved by using acoustic signals for normalization, and acoustic normalization works more efficiently than laser energy normalization. This work provides a simple and cost-effective external acoustic normalization method for underwater LIBS applications.

Pressure effects on underwater laser-induced breakdown spectroscopy: an interpretation with self-absorption

Laser-induced breakdown spectroscopy (LIBS) has been successfully applied for in situ detection in the sea, and the water pressure is found to be the main influence on the LIBS signal compared to temperature or salinity.

Signal enhancement in underwater long-pulse laser-induced breakdown spectroscopy for the analysis of bulk water

Significant signal enhancement in underwater laser-induced breakdown spectroscopy for the analysis of bulk water by using a long ns pulse.

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

In recent years, the investigation and exploitation of hydrothermal region and polymetallic mineral areas has become a hot topic. The emergence of underwater vehicle platforms has made it possible for new chemical sensors to be applied in marine in-situ detection. Laser-induced breakdown spectroscopy (LIBS), with its advantages of rapid real-time analysis, sampling without pretreatment, simultaneous multi-element detection and stand-off detection, has great potential in marine applications. In this paper, a newly more compact and lighter underwater LIBS system based on the LIBSea system named LIBSea II was developed and tested both in the laboratory and sea trials. The system consists of a Nd:YAG single-pulse laser at 1064 nm, a fiber spectrometer, optical layout, a power supply module and an internal environment sensor. The system is encapsulated in a pressure vessel (Φ 190 mm × L 588 mm) with an optical window on the end cap. Experimental parameters of the system including laser energy and delay time were firstly optimized in the laboratory. Then, field test of the system in nearshore was performed with various samples, including pure metal and alloy samples as well as a manganese nodule sample from deep sea, to verify the detection performance of the LIBSea II system. In 2019, the system was deployed on a remotely operated vehicle (ROV) of Haima for deep sea trial, and atomic lines of K, Na, Ca and strong molecular bands of CaOH from a carbonate rock sample were obtained for the first time at depths of 1400 m. These results show that the LIBSea II system has great potential to be used in deep-sea geological exploration.

A Review of Underwater Laser-induced Breakdown Spectroscopy of Submerged Solids.

Fundamentals to applications of underwater laser-induced breakdown spectroscopy (LIBS) of submerged solid targets are reviewed. Since the deformation of spectral lines caused by plasma confinement has been the most serious problem in underwater LIBS, the methods developed to overcome this effect are overviewed. Deep-sea LIBS, which is the most successful application of underwater LIBS, is described in some detail as well as the hydrostatic pressure effects. Since the downsizing of underwater LIBS instrument is an important task for the applications to on-site measurements, studies on non-gated LIBS are covered. In the subsequent section, the methods for the quantitative analysis of the underwater LIBS signal are reviewed. The basics of the plasma parameters used in quantitative analysis are explained, and various methods of quantitative analysis applicable to underwater LIBS are described.

Laser-induced plasma in water at high pressures up to 40 MPa: A time-resolved study.

The knowledge on the laser-induced plasma emission in water at high pressures is essential for the application of laser-induced breakdown spectroscopy (LIBS) in the deep-sea. In this work, we investigate the spectral features of ionic, atomic and molecular emissions for the plasma in water at different pressures from 1 to 40 MPa. By comparing between the time-resolved spectra and shadowgraph images, we demonstrate that the dynamics of the cavitation bubble at high pressures plays a key role on the characterization of plasma emission. The initial plasma emission depends weakly on the external pressure. As time evolves, the cavitation bubble is more compressed by the higher external pressure, leading to a positive confinement effect to maintain the plasma emission. However, at very high pressures, the bubble collapses extremely fast and even earlier than the cooling of the plasma. The plasma will gain energy from the bubble collapse phase, but quench immediately after the collapse, leading to a sharp reduction in the plasma persistence. These effects caused by bubble dynamics explain well the observed spectral features and are further proved by the temporal evolutions of the plasma temperature and electron density. This work gives not only some insights into the laser-induced plasma and bubble dynamics in high pressure liquids but also better understanding for the application of underwater LIBS in the deep-sea.

Depth Profiling Investigation of Seawater Using Combined Multi-Optical Spectrometry.

Depth profiling investigation plays an important role in studying the dynamic processes of the ocean. In this paper, a newly developed hyphenated underwater system based on multi-optical spectrometry is introduced and used to measure seawater spectra at different depths with the aid of a remotely operated vehicle (ROV). The hyphenated system consists of two independent compact deep-sea spectral instruments, a deep ocean compact autonomous Raman spectrometer and a compact underwater laser-induced breakdown spectroscopy system for sea applications (LIBSea). The former was used to take both Raman scattering and fluorescence of seawater, and the LIBS signal could be recorded with the LIBSea. The first sea trial of the developed system was taken place in the Bismarck Sea, Papua New Guinea, in June 2015. Over 4000 multi-optical spectra had been captured up to the diving depth about 1800 m at maximum. The depth profiles of some ocean parameters were extracted from the captured joint Raman-fluorescence and LIBS spectra with a depth resolution of 1 m. The concentrations of and the water temperatures were measured using Raman spectra. The fluorescence intensities from both colored dissolved organic matter (CDOM) and chlorophyll were found to be varied in the euphotic zone. With LIBS spectra, the depth profiles of metallic elements were also obtained. The normalized intensity of atomic line Ca(I) extracted from LIBS spectra raised around the depth of 1600 m, similar to the depth profile of CDOM. This phenomenon might be caused by the nonbuoyant hydrothermal plumes. It is worth mentioning that this is the first time Raman and LIBS spectroscopy have been applied simultaneously to the deep-sea in situ investigations.

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.

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

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.

Mineral carbonate dissolution with increasing CO2 pressure measured by underwater laser induced breakdown spectroscopy and its application in carbon sequestration

In this study, the ability of laser-induced breakdown spectroscopy (LIBS) to measure the in situ aqueous dissolution of various mineral carbonates with increasing CO2 pressure was examined. Dissolution experiments included four geologically common mineral carbonates (CaCO3, MgCO3, MnCO3, SrCO3) and the CO2 pressure ranged from ambient to 250 bar. The ensuing plasma emission was spectrally analyzed, and the intensities of Ca, Mg, Mn, and Sr emission lines were used to monitor the respective metal cations released to the aqueous solution. The strong emission lines of Ca (Ca II 393.36, Ca II 396.84, Ca I 422.67 nm), Mg (unresolved magnesium doublet: Mg I 383.230, Mg I 383.829 nm), Mn (unresolved manganese triplet: Mn I 403.076, Mn I 403.307, Mn I 403.449 nm), and Sr (Sr II 407.77, Sr II 421.55, Sr I 460.73 nm) were identified in the spectra. The amounts of metals released from their respective carbonates were estimated at different time intervals following the CO2 injection (5 m, 1, 2, 3, 4, 24 h) and at different pressures (50, 100, 150, 200, 250 bar) using calibration models developed at corresponding pressure settings. The results demonstrated that the pressure-induced dissolution of all carbonates was consistent with their expected and selective pH-dependent solubility. The dissolution rate of CaCO3, MgCO3, and SrCO3 was found to be higher than that of MnCO3. The dissolution of constituents in a Mt. Simon sandstone associated with a deep saline reservoir at elevated CO2 pressure was also studied and Ca release was quantified. The results demonstrated that real-time monitoring of carbonate dissolution by LIBS may provide a useful indirect detection system indicative of CO2 leakage from geologic carbon storage sites.

Effects of Ambient Temperature on Laser-Induced Plasma in Bulk Water.

Laser-induced breakdown spectroscopy (LIBS) has been successfully applied to ocean exploration, but the changes in marine environmental factors could have an important impact on the LIBS signals. The aim of the research is to investigate the ambient water temperature effects on laser-induced plasma in bulk water. Both the spectroscopic and fast imaging techniques are used to observe the plasma emission with the temperatures in the range of 5-60 ℃. It is shown that as the ambient temperature increases, an obviously increasing trend of emission intensity is observed, both for the atomic and ionic lines of Ca. Higher plasma temperature and electron density can be obtained at higher ambient temperature. The image results demonstrate that hotter and larger plasmas can be produced in water with the increase of ambient temperature. In addition, it is found that the changes of plasma emission and morphologies could be related to the changes of physical property parameters of water such as thermal expansivity and viscosity with ambient temperature. The results suggest that the ambient temperature has great influences on laser-induced plasma, which needs to be taken into account in underwater LIBS measurement, especially on-site marine applications.

Salinity effects on elemental analysis in bulk water by laser-induced breakdown spectroscopy.

The effects of salinity on underwater laser-induced breakdown spectroscopy (LIBS) were investigated with salinities ranging from 2‰ to 50‰. Both spectroscopic and fast imaging techniques were used to observe plasma emission. It was shown that as the salinity increased, emission intensities of the atomic lines increased, while intensities of the ionic lines were suppressed. The signal-to-background ratios of the spectral lines decreased as a function of salinity, but the signal-to-noise ratios changed irregularly with salinity. Image results demonstrated that brighter and longer plasma could be produced at higher salinity with higher plasma temperature and electron density. The calibration curves at different salinities indicated that the high salinity environment did not limit the detection capability of LIBS. The obtained results reveal the significant influences of salinity on underwater LIBS detection, which plays an important role in promoting applications of LIBS in the ocean.