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Optical emission of laser ablation plasma on a shale target surface provides sensitive laser-induced breakdown spectrometry (LIBS) detection of major, minor or trace elements. An exploratory study for the characterization of the plasma induced on shale materials was carried out with the aim to trigger a crucial step towards the quantitative LIBS measurement. In this work, the experimental strategies that optimize the plasma generation on a pressed shale pellet surface are presented. The temporal evolution properties of the plasma induced by ns Nd:YAG laser pulse at the fundamental wavelength in air were investigated using time-resolved space-integrated optical emission spectroscopy. The electron density as well as the temperatures of the plasma were diagnosed as functions of the decay time for the bulk plasma analysis. In particular, the values of time-resolved atomic and ionic temperatures of shale elements, such as Fe, Mg, Ca, and Ti, were extracted from the well-known Boltzmann or Saha–Boltzmann plot method. Further comparison of these temperatures validated the local thermodynamic equilibrium (LTE) within specific interval of the delay time. In addition, the temporal behaviors of the signal-to-noise ratio of shale elements, including Si, Al, Fe, Ca, Mg, Ba, Li, Ti, K, Na, Sr, V, Cr, and Ni, revealed the coincidence of their maximum values with LIBS LTE condition in the time frame, providing practical implications for an optimized LIBS detection of shale elements. Analytical performance of LIBS was further evaluated with the linear calibration procedure for the most concerned trace elements of Sr, V, Cr, and Ni present in different shales. Their limits of detection obtained are elementally dependent and can be lower than tens of parts per million with the present LIBS experimental configurations. However, the occurrence of saturation effect for the calibration curve is still observable with the increasing trace element content, indicating that, due to the complex composition of shale materials, the omnipresent “matrix effect” is still a great challenging for the performance of quantitative LIBS measurement even in the framework of the LTE approach.

Rapid and accurate identification of multiple types of rocks using spectroscopic techniques has a wide market application prospect and is always challenging due to similar chemical composition and complex matrix effects. In recent years, laser induced breakdown spectroscopy (LIBS) coupled with supervised machine learning and chemometrics methods (e.g. k-nearest neighbor (kNN), support vector machine (SVM), partial least squares (PLS), artificial neural network (ANN)) and combined with feature engineering techniques (e.g. principal component analysis (PCA)), has demonstrated great capabilities for efficient identification of materials with similar chemical composition. To further increase the classification accuracy, LIBS coupled with a convolutional neural network with two-dimensional input (2D CNN) is here investigated for the identification of rock samples, including dolomites, granites, limestones, mudstones and shales. A regularized network structure was first designed, according to the performance of validation dataset, to enable the most reliable discrimination of the rock specimens. The accuracy of test dataset was then evaluated by the determined model. Results indicated that validation and test set of the 2D CNN was able to reach an accuracy of 0.9877 and 1, respectively. Finally, the performance was compared with other identification methods, including: one-dimensional convolutional neural network (1D CNN), kNN, PCA-kNN, SVM, PCA-SVM, PLS-DA, and Human-Assisted ANN (HA-ANN). The proposed approach has demonstrated that CNN has a great potential for the lithological identification and could be a feasible and useful tool for LIBS spectral data processing.

Gas shale is one of the important unconventional hydrocarbon source rocks, whose composition, such as mineral components and redox sensitive trace elements, has been proved as important geochemical proxies playing essential roles in indicating the gas potential and gas productivity in recent geological researches. Fast and accurate measurements for the shale composition, especially those with spatial resolution, will reveal rich information for the understanding and evaluation of gas shale reservoirs. In this paper, we demonstrated the potentiality as well as feasibility of laser-induced breakdown spectroscopy as an effective technique to perform spectrochemical analysis for shale samples. In case of the bulk analysis of pressed shale pellet, spectral analysis of the plasma emission revealed high sensitivity of LIBS for major, minor and even trace elements. More than 356 lines emitted by 19 different elements can be found. Among these species, redox sensitive trace elements such as V, Cr, and Ni were detected with high signal-to-ratios. Two-dimensional surface micro-analysis for the concerned major or minor elements with strong emissions was then applied to the smoothed shale slab. Local thermodynamic equilibrium for the plasma was first verified with a line profile point-by-point on the sample surface, the matrix effect was then assessed as negligible by the extracted electron density and temperature of the plasmas induced at each position on the same profile. Concentration mappings for the major elements of Si, Al, Fe, Ca, Mg, Na and K were finally constructed with their measured relative variations of line emission intensities. The distribution and correlations of these elements in concentration may reflect changes of shale mineral components with respected to the variations of the depositional environments and provide an important clue in identifying sedimentary processes when combined with other geological or geochemical evidences. These results well demonstrated the potential of LIBS technique for shale studies.

A novel analysis system based on a low power atmospheric pressure pulsed direct current (Pdc) microplasma is described for the determination of ultra-trace mercury in natural water by cold vapor generation atomic emission spectrometry (CV-AES). The plasma was generated with a miniaturized home-built high-voltage Pdc power supply which decreased the volume and weight of the whole experiment setup. The CV-Pdc-AES system is based on the preconcentration of mercury vapor on a gold filament trapping micro-column prior to detection that provides fast, reproducible absorption and desorption of mercury. The micro-column is produced by winding 30 μm diameter 100 m long gold filament to a small ball and then insert it into a quartz tube of 6 mm i.d, 8 mm o.d. Under the optimized experimental conditions, the new system provides high sensitivity (detection limit: 0.08 pg mL − 1 ) and good reproducibility (RSD 3.0%, [Hg] = 20 pg mL − 1 , n = 11). The calibration curve is linear at levels near the detection limit up to at least 200 pg mL − 1 and the accuracy is on the order of 1–4%. The proposed method was applied to 5 real water samples for mercury ultra-trace analysis. The advantages and features of the newly developed system include high sensitivity, simple structure, low cost, and compact volume with field portable potential.

The correct identification of rock types is critical for understanding the origins and history of any particular rock body. Laser-induced breakdown spectroscopy (LIBS) has developed into an excellent analytical tool for geological materials research because of its numerous technical advantages compared with traditional methods. The coupling of LIBS with advanced multivariate analysis has received increasing attention because it facilitates the rapid processing of spectral information to differentiate and classify samples. In this study, we collected LIBS datasets for 16 sedimentary rocks from Triassic strata in Sichuan Basin. We compared the performance of two types of spectrometers (Czerny–Turner and Echelle) for classification of rocks using two advanced multivariate statistical techniques, i.e., partial least squares discriminant analysis (PLS-DA) and support vector machines (SVMs). Comparable levels of performance were achievable when using the two systems in the best signal reception conditions. Our results also suggest that SVM outperformed PLS-DA in classification performance. Then, we compared the results obtained when using pre-selected wavelength variables and broadband LIBS spectra as variable inputs. They provided approximately equivalent levels of performance. In addition, the rock slab samples were also analyzed directly after being polished. This minimized the analysis time greatly and showed improvement of classification performance compared with the pressed pellets.

Plasma-cavity ringdown spectroscopy is a powerful absorption technique for analytical measurement. It combines the inherent advantages of high sensitivity, absolute measurement, and relative insensitivity to light source intensity fluctuations of the cavity ringdown technique with use of plasma as an atomization/ionization source. In this review, we briefly describe the background and principles of plasma-cavity ringdown spectroscopy(CRDS) technology, the instrumental components, and various applications. The significant developments of the plasma sources, lasers, and cavity optics are illustrated. Analytical applications of plasma-CRDS for elemental detection and isotopic measurement in atomic spectrometry are outlined in this review. Plasma-CRDS is shown to have a promising future for various analytical applications, while some further efforts are still needed in fields such as cavity design, plasma source design, instrumental improvement and integration, as well as potential applications in radical and molecular measurements.

A low power micro-plasma device was designed, constructed and evaluated for nonmetal detection, particularly for the direct measurement of phosphorus and sulfur. The device employed an open discharge chamber to generate an argon plasma discharge at atmospheric pressure of about 2 watts power and 150 nL volume for elemental excitation and molecular fragmentation. The emission light from the miroplasma source was collected and coupled to a palm-sized spectrometer through an optical fiber for spectral measurement. The characteristics of the low power micro-nano-plasma discharge, such as relationship of voltage–current, were studied to optimize discharge parameters. Two spectral lines of 253.6 nm and 469.4 nm were used for detection of phosphorus and sulfur, respectively. The stability of the low power micro-plasma device was quantitatively evaluated, and the analytical figures of merit for nonmetal detection were reported. The detection limits are 68 ppbv for phosphorus, and 1.0 ppmv for sulfur, respectively. The sensitivities of the device for nonmetal detection are comparable with those of conventional methods. However, our device provides significant advantages of small size, low power, low cost, and fieldable detection.

Microwave plasma (MWP) sources have found extensive use in spectrochemical analysis during the past decades. As the MWP offers some attractive features, it has been widely used either as an excitation source for atomic emission spectrometry (MWP-AES) or as an ionization source for mass spectrometry (MWP-MS). The use of MWPs as an atomization source for atomic absorption spectrometry (MWP-AAS) and atomic fluorescence spectrometry (MWP-AFS) has also appeared. The historical development and recent improvements in these MWP techniques are presented in this review with emphasis on the analytical performance, characteristics, interferences and applications. Research on the fundamental properties of MWPs is also given. Both the advantages and limitations of MWPs in atomic spectrometry are discussed.

A chemical vapor-generation (CVG) method is compared with solution nebulization for determination of nonmetals by microwave plasma-torch mass spectrometry (MPT-MS). In contrast to solution nebulization, the CVG approach produces volatile species that are more efficiently transported to the plasma and influence the discharge to a lesser extent; furthermore, it enables analyte species to be separated from the sample matrix and solvent, so the likelihood of spectral interferences is lessened. The optimal operating conditions for generating sulfur, chlorine, bromine and iodine vapors have been determined, and several analytical characteristics (including linear dynamic range, detection limits and matrix effects) have been evaluated. The detection limits for several nonmetal elements are in the range from low ng ml −1 to high pg ml −1 , between one and three orders of magnitude better than those for the same elements obtained with an inductively coupled plasma on the same mass spectrometer. The linear dynamic range is between four and five orders of magnitude.

A new on-line vapor-generation nebulization (VGN) method is explored for non-metal detection by microwave plasma torch-mass spectrometry (MPT-MS). In the new method, a three-channel “Y” is used to introduce the sample and reactant solutions simultaneously into a nebulizer. The VGN method offers higher sampling efficiency than does simple nebulization and improves sensitivity roughly two-fold for the determination of non-metallic elements. The device is suggested to be of potential benefit also in the determination of species that do not readily form volatile products.

This paper describes an improvement of detection capability for microwave-induced plasma atomic absorption spectrometry (MIP-AAS) by using a modified desolvation system, which includes heated tubing, a water-cooled condenser and a concentrated sulfuric acid desiccator. The influence of heating temperature, carrier-gas flow rate and microwave power on the analytical performance of MIP-AAS has been examined and the relationship between these experimental parameters discussed. The desolvation efficiencies of the different steps, such as in the water-cooling system and sulfuric acid desiccator, have been quantitatively measured using silica gel traps. Under the optimized experimental conditions, an overall desolvation efficiency of as high as 99% is achieved by using this recommended desolvation system. The analytical figures of merit for MIP-AAS are compared and evaluated under different desolvation conditions.

An investigation was performed to evaluate the coupling of a 150 W atmospheric pressure helium microwave plasma torch (MPT) with mass spectrometric detection. The interface of an existing inductively coupled plasma mass spectrometer was modified to sample the helium plasma. Optimization studies illustrate the dependence of analytical signals on various instrumental parameters, including sampling depth, gas and sample solution flow rates, and ion optic settings. With the use of a pneumatic nebulization-desolvation sample introduction system, detection limits for seven non-metallic elements range from 12 ng ml to 1.0 μg ml , comparable to those determined with an Ar ICP and the same mass spectrometer. Interference effects of concomitant cations and isotope ratios for Cl and Br are also presented.