Laser induced breakdown spectroscopy based on single beam splitting and geometric configuration for effective signal enhancement

Landmark Publications in Analytical Atomic Spectrometry: Fundamentals and Instrumentation Development

The almost-two-centuries history of spectrochemical analysis has generated a body of literature so vast that it has become nearly intractable for experts, much less for those wishing to enter the field. Authoritative, focused reviews help to address this problem but become so granular that the overall directions of the field are lost. This broader perspective can be provided partially by general overviews but then the thinking, experimental details, theoretical underpinnings, and instrumental innovations of the original work must be sacrificed. In the present compilation, this dilemma is overcome by assembling the most impactful publications in the area of analytical atomic spectrometry. Each entry was proposed by at least one current expert in the field and supported by a narrative that justifies its inclusion. The entries were then assembled into a coherent sequence and returned to contributors for a round-robin review. A total of 48 scientists participated in this endeavor, contributing a combined list of 1055 individual articles spanning 17 sub-disciplines of spectrochemical analysis into what the current community views as "key" publications. Of these cited articles, 60 received nominations from four or more scientists, establishing them as the most indispensable reading materials. The outcome of this collaborative effort is intended to serve as a valuable resource not only for current practitioners in atomic spectroscopy but also for present and future students who represent coming generations of analytical atomic spectroscopists.

Hydrogen isotope analysis in W-tiles using fs-LIBS

Laser-Induced Breakdown Spectroscopy (LIBS) is a promising technology for in-situ analysis of Plasma-Facing Components in magnetic confinement fusion facilities. It is of major interest to monitor the hydrogen isotope retention i.e. tritium and deuterium over many operation hours to guarantee safety and availability of the future reactor. In our studies we use ultraviolet femtosecond laser pulses to analyze tungsten (W) tiles that were exposed to a deuterium plasma in the linear plasma device PSI-2, which mimics conditions at the first wall. A high-resolution spectrometer is used to detect the Balmer-[Formula: see text] transition of the surface from implanted hydrogen isotopes (H and D). We use Calibration Free CF-LIBS to quantify the amount of deuterium stored in W. This proof-of-principle study shows the applicability of femtosecond lasers for the detection of low deuterium concentration as present in first wall material of prevailing fusion experiments.

Comparative Study of Four Chemometric Methods for the Quantitative Analysis of the Carbon Content in Coal by Laser-Induced Breakdown Spectroscopy Technology

Coal is a heterogeneous mineral substance mainly composed of carbon, along with various amounts of other elements. The carbon content is an important and pertinent parameter for coal quality. To achieve the rapid and accurate online measurement of the carbon content in coal, four different calibration strategies are applied to coal analysis by laser-induced breakdown spectroscopy (LIBS). Four calibration models based on support vector regression (SVR), back-propagation training (BP), random forest (RF), and partial least-squares regression (PLSR) were proposed, and the prediction accuracy, prediction precision, model stability, and training velocity of the four calibration models were compared for the quantitative analysis of the carbon content. A total of 65 coal samples were ablated, and the plasma spectra were used as the input data. Among the four calibration models, the results indicate that SVR and BP are the most promising calibration models for finding a better optimized model with a better prediction accuracy and prediction precision, and PLSR has a better prediction stability and a faster training velocity; however, RF has a prediction performance worse than those of the other three models.

Beam-crossing configuration to control plasma position, improve spatial resolution, and enhance emissions in single-pulse, laser-induced breakdown spectroscopy in gases

We report a relatively simple configuration of laser-induced breakdown spectroscopy (LIBS) that is suitable for gas flow diagnostics with increased spatial resolution, signal intensity, and stability. In this optical configuration, two laser beams are generated by splitting a single laser beam, and then they are focused and crossed orthogonally at the detection volume from two different optical paths. Different from dual-pulse LIBS, this LIBS configuration uses only one laser source, and thus is of relatively low cost. Several advantages were found for this simple beam-crossing LIBS when it was demonstrated in air in the present work, particularly on signal enhancement and stabilization, confining plasma volume, and controlling plasma position. Both of the latter two advantages are relevant to spatial resolution improvement of LIBS in gases, which has rarely been discussed in previous reports. An enhancement factor of 2 was found for atomic hydrogen, nitrogen, and oxygen emissions with respect to conventional LIBS. Another advantage is that the position of breakdown can be precisely controlled through adjustment of the propagation of the two beams, also resulting in smaller plasma volume and stable emission intensity. Furthermore, the technique is moderately tolerant to dust particles neutrally present in the environment, avoiding the spark occurring at a position out of the detection volume. Beyond LIBS, the new configuration has other potential applications, e.g., laser-induced ignition, which is also briefly discussed.

A Perspective to Control Laser-Induced Periodic Surface Structure Formation at Glancing-Incident Femtosecond Laser-Processed Surfaces

The favorable combination of high material removal rate and low influence on the material beneath the ultra-short pulsed laser-processed surface are of particular advantage for sample preparation. This is especially true at the micrometer scale or for the pre-preparation for a subsequent focused ion beam milling process. Specific surface features, the laser-induced periodic surface structures, are generated on femtosecond laser-irradiated surfaces in most cases, which pose an issue for surface-sensitive mechanical testing or microstructural investigations. This work strives for an approach to enhance the surface quality of glancing-incident laser-processed surfaces on the model material copper with two distinctly different grain sizes. A new generalized perspective is presented, in which optimized parameter selection serves to counteract the formation of the laser-induced periodic surface structures, enabling, for example, grain orientation mapping directly on femtosecond laser processed surfaces.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11837-021-04963-w.

Time-resolved characteristics of laser induced breakdown spectroscopy on non-flat samples by single beam splitting

A single-beam-splitting approach was used to enhance the signal intensity of LIBS under the extreme conditions of laser beam grazing of the surface of non-flat samples. Time-resolved spectra show that the laser-ablated plasma presents a stronger spectral intensity and a slower plasma decay in the split beam mode because of the higher laser irradiance. The temporal evolutions of signal enhancement factors indicate that the enhancement effect first rises and then drops with delay time and the maximum enhancement factor of Al plasma comes later than that of Cu plasma under the same laser energy. The mechanisms behind it are discussed. It is also found that the electron density exhibits a faster decay with delay time in the split beam mode, mainly due to the faster plasma expansion. And a slower increase of electron density with laser energy is observed in the split beam mode because of the plasma shielding effect.

Signal enhancement of laser-induced breakdown spectroscopy on non-flat samples by single beam splitting

A single-beam-splitting approach was employed to enhance the signal intensity of LIBS under the extreme condition of laser beam grazing the surface of non-flat samples. Examining the time-integrated emission spectra shows that _{ISplit/ISingle} enhancement factors of 2.5 and 3.5 were achieved at the laser energy of 33 mJ for aluminium alloy and brass, respectively. This factor first increases, reaches its maximum at 33 mJ, and drops gradually with the laser energy further increased. The mechanisms behind the enhanced optical emission and the enhancement factor evolution are discussed by using the proposed laser ablation model and laser-supported detonation (LSD) wave model, respectively. Examining the time-resolved emission spectra show that enhancement effect exists across all the plasma expansion process and the split beam mode allows for a longer plasma lifetime. A remarkable feature is that the trailing phenomenon emerging in the single beam mode vanishes due to the interaction between the plasmas generated by the grazing incident and normally incident laser beams in the split beam mode. The underlying cause is probably that the plasma plume produced by the normally incident laser beam prevents the grazing incident laser beam from further propagating and ablating the sample surface below. These findings not only give an insight into the plasma generation and evolution at grazing incidence of laser beam on sample surface but also provide a more reliable method for outdoor LIBS measurement of irregular samples.

Emission enhancement of laser-induced breakdown spectroscopy for aqueous sample analysis based on Au nanoparticles and solid-phase substrate

In this paper, porous electrospun ultrafine fiber with a nanoparticle coating was proposed as an effective approach to enhance the laser-induced breakdown spectroscopy (LIBS) signal for metal ions in aqueous systems. It is known that the LIBS technique is very limited when used for liquid sample analysis. On the other hand, in practical applications, many LIBS measurements have been accomplished in a liquid environment. A signal enhancement method for aqueous sample LIBS analysis was presented in this work, where Au nanoparticles and a solid-phase support were combined for the first time for aqueous sample analysis with LIBS. The system operation was relatively simple, which only required Au nanoparticles being dropped onto the surface of porous electrospun ultrafine fibers before LIBS analysis. Significant signal enhancement was achieved due to the integration of the merits of the Au nanoparticles and the ultrafine fibers. Nanoparticles possess significant LIBS signal enhancement effects by providing several plasma ignition points and then causing more efficient emissions. In addition, Au nanoparticles could also help to decrease the breakdown threshold of target elements for LIBS analysis. The electrospun ultrafine fibers as solid-phase support can accommodate a larger volume of aqueous sample. Meanwhile, the aqueous solution on the fiber surface was easy to evaporate. The experimental results showed that the limits of detection (LODs) with this method were significantly improved, 0.5 μg/mL for Cr, 0.5 μg/mL for Pb, and 1.1 μg/mL for Cu, respectively, compared with 2.0 μg/mL for Cr and 3.3 μg/mL for Cu in the previous research. In the proposed method, signal enhancement could be achieved without any extra equipment, which makes the LIBS technique feasible for direct measurement of an aqueous sample.

Advances in explosives analysis--part II: photon and neutron methods

The number and capability of explosives detection and analysis methods have increased dramatically since publication of the Analytical and Bioanalytical Chemistry special issue devoted to Explosives Analysis [Moore DS, Goodpaster JV, Anal Bioanal Chem 395:245-246, 2009]. Here we review and critically evaluate the latest (the past five years) important advances in explosives detection, with details of the improvements over previous methods, and suggest possible avenues towards further advances in, e.g., stand-off distance, detection limit, selectivity, and penetration through camouflage or packaging. The review consists of two parts. Part I discussed methods based on animals, chemicals (including colorimetry, molecularly imprinted polymers, electrochemistry, and immunochemistry), ions (both ion-mobility spectrometry and mass spectrometry), and mechanical devices. This part, Part II, will review methods based on photons, from very energetic photons including X-rays and gamma rays down to the terahertz range, and neutrons.