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BACKGROUND: Variations in plasma properties among spectra and samples lead to significant signal uncertainty and matrix effects in laser-induced breakdown spectroscopy (LIBS). To address this issue, direct compensation for plasma property variations is considered highly desirable. However, reliably compensating for the total number density variation is challenging due to inaccurate spectroscopic parameters. For reliable compensation, a total number density compensation (TNDC) method was presented in our recent work, but its applicability is limited to simple samples because of its strict assumptions. In this study, we propose a new pre-processing method, namely extended TNDC (ETNDC), to reduce signal uncertainty and matrix effects in the more complex analytical task of uranium determination. RESULTS: ETNDC reflects the total number density variation with a weighted combination of spectral lines from all major elements and incorporates temperature and electron density compensation into the weighting coefficients. The method is evaluated on yellow cake samples and combined with regression models for uranium determination. Using the typical validation set and line combination, the mean relative standard deviation (RSD) of U II 417.159 nm in validation samples decreases from 4.92% to 2.27%, and the root mean square error of prediction (RMSEP) and the mean RSD of prediction results decrease from 4.81% to 1.93% and from 1.92% to 1.56%, respectively. Furthermore, the results of 10 validation sets and 216 line combinations show that ETNDC outperforms baseline methods in terms of average performance and robustness. SIGNIFICANCE: For the first time, ETNDC explicitly addresses the temperature and electron density variations while compensating for the total number density variation, where the inaccurate spectroscopic parameters are avoided by fitting related quantities using concentration information. The method demonstrates effective and robust improvement in signal repeatability and analytical performance in uranium determination, facilitating accurate quantification of the LIBS technique.

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The relatively low measurement repeatability has long been considered as a major obstacle to the widespread use and commercialization of laser-induced breakdown spectroscopy (LIBS). Although many efforts have been made to improve the signal repeatability in the short term, how to improve the long-term signal repeatability is critical in practical applications and has rarely been studied. Moreover, the mechanisms behind the degradation of long-term repeatability are not fully revealed. This study proposes a new method to improve the long-term repeatability of LIBS measurement, which modifies the spectral intensity based on laser beam intensity distribution. It first pre-processes the beam intensity distribution profiles and spectral intensity. Then the relationship between the relative deviations of beam and spectral intensities is modelled using Partial Least Squares Regression (PLSR). The proposed method was tested on copper and silicon samples, and the spectra and laser beam intensity distribution were recorded for more than thirty days. Day-to-day variations in beam intensity distribution were observed. Such variations can lead to changes in spectral intensity, resulting in degraded signal repeatability. By modifying the spectral intensity, the long-term signal repeatability was improved. Specifically, in terms of day-mean spectral intensity, the valid correction rates were above 70% for both of copper silicon sample in most cases. Long-term RSD decreased from ∼13.5% to ∼4% for copper and decreased from ∼10.7% to 6.5% for silicon sample. These results indicate that the proposed method provides a viable method for improving the long-term repeatability of LIBS measurement.

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Real-time quantitative detection of uranium in ores is one of the major challenges for uranium exploration. Laser-induced breakdown spectroscopy (LIBS) has been regarded as a most promising technique for this application. However, due to the matrix complexity as well as low uranium concentration of ore, the detection sensitivity of LIBS for uranium in ores is still unsatisfactory. This work explored the potential of a beam-shaping plasma modulation method to improve the limit of detection of uranium in ores. By shaping the profile of laser beam from normally Gaussian distribution to flat-top, the plasma was modulated to be more excited with reduced peak electron density at the laser-plasma interaction point for plasma shielding reduction especially at high laser energy as well as to be more morphologically stable for LIBS signal repeatability improvement. It was further found that this method enhanced LIBS signal intensity mainly by increasing the plasma temperature, while the electron density was almost unchanged, which was very attractive for uranium detection in ore since one of the major problem for uranium detection was that it is hard to find clear uranium spectral lines for analysis due to the high dense emission lines of ore samples in real cases and lower electron density, indicated less line broadening and less line overlap or interferences. A clear uranium emission line has been found in crowded ore spectra, and the intensity of U II 409.013 nm based on flat-top beam was about 5 times higher than that of Gaussian beam, and the relative standard deviation (RSD) of the signal was reduced by about 50%. Moreover, the LOD of uranium in ores was estimated to be 21.2 ppm with flat-top beam, indicating that beam shaping is a promising method for rapid and accurate detection of uranium in ores.

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High signal uncertainty has been regarded as a critical obstacle for the quantitative analysis of laser-induced breakdown spectroscopy (LIBS). One of the most effective ways for uncertainty reduction is to directly compensate for the variation of plasma properties, especially total number density. However, reliable compensation for the variation of total number density is hard to implement. In this work, we propose a data pre-processing method, called total number density compensation (TNDC), to reduce signal uncertainty. It is established on an assumption extended from the internal standard method and utilizes a weighted sum of emission lines from all major elements to reflect the variation of total number density. The TNDC method is tested on 29 brass samples and outperforms common normalization methods based on the spectral area in terms of signal repeatability and analytical performance. For Cu, the mean pulse-to-pulse relative standard deviation (RSD) of signals is greatly decreased from 5.10% to 1.03%, which is almost the best signal repeatability that LIBS can achieve and is comparable to that of ICP-OES. The root mean square error of prediction (RMSEP) and the mean RSD of prediction are decreased from 6.56% to 0.60% and from 12.00% to 1.03%, respectively. While for Zn, the mean RSD of signals improves from 6.43% to 4.12%, and the RMSEP is reduced from 1.57% to 0.59% with the RSD of prediction from 5.41% to 4.18%. Results demonstrate that TNDC can be an effective method for LIBS analysis especially for repeatability improvement.

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Laser-induced breakdown spectroscopy (LIBS) is a promising multi-elemental analysis technique and has the advantages of rapidness and minimal sample preparation. In traditional LIBS measurement, sample spectra are generally collected based on a single set of fixed experimental parameters, such as laser energy and delay time. When samples have the same main components and similar component concentrations, the difference in their spectral intensities becomes less obvious. This can lower the sensitivity of LIBS measurement and pose a threat to the accuracy and robustness of LIBS qualitative analysis. In this work, we propose a new method to increase the spectral difference between similar samples, namely multiple-setting spectra. For each sample, it adopts different sets of experimental parameters and obtains a group of spectra to increase the fingerprint spectral information. The effectiveness of the proposed method is theoretically verified and then tested on 11 similar coal samples. Specifically, the sample spectra were collected with different laser energy and delay time, and processed by principal component analysis (PCA) and Davies-Bouldin index (DBI). The results show that the use of multiple-settings spectra can significantly improve the sample discrimination accuracy from 81.8% to 96.4%. In addition, the proposed method can maintain the efficiency and cost of LIBS measurement.

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Developments in femtosecond laser induced breakdown spectroscopy (fs-LIBS) applications during the last two decades have further centered on innovative métier tie-in to the advantageous properties of femtosecond laser ablation (fs-LA) introduced into LIBS. Yet, for industrially-oriented application like coal analysis, no research has exposed to view the analytical capabilities of fs-LA in enhancing the physical processes of coal ablation and the impact into quantitative correlation of spectra and data modeling. In a huge coal market, fast and accurate analysis of coal property is eminently important for coal pricing, combustion optimization, and pollution reduction. Moreover, there is a thirst need of precision standardization for coal analyzers in use. In this letter, the analytical performance of a one-box femtosecond laser system is evaluated relative to an industrially applied coal analyzer based on five objectives/measures: spectral correlation, relative sensitivity factors, craters topology, plasma parameters, and repeatability. Despite high-threshold operation parameters of the fs system, competitive results are achieved compared to the optimized analytical conditions of the ns-coal analyzer. Studies targeting the in-field optimization of fs-LIBS systems for coal analysis can potentially provide insights into fs-plasma hydrodynamics under harsh conditions, instrumental customization, and pave the way for a competitive next-generation of coal analyzers.

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Repeatability is of utmost importance as it is directly linked to measurement accuracy and precision of a technique and affects its cost, utility, and commercialization. The present paper contributes to explain enhanced repeatability of the femtosecond laser-induced breakdown spectroscopy (fs-LIBS) technique, remarkably significant for its industrial applications and instrumental size reduction. A fs-laser with 7 mJ pulse energy was focused to create a transient titanium plasma, and a high-resolution spectrometer was used to study time-resolved spectra and single-shot drilling sampling repeatability. Time-resolved spectroscopy study at a delay time interval of 0-1600 ns showed 200-400 ns as the optimum delay time zone for data acquisition with 2-4% line intensity RSDs. Plasma temperature RSDs were <1.8% for the investigated delay interval and reached 0.5% at 200 ns where the temperature recorded a maximum value of 22,000 K. Electron density reached 5.7 × 1017 cm-3 at 200 ns, and RSDs were <3% with the least fluctuation of 0.7%. Shot-to-shot RSDs were 3.5-5% at 15-30 drilling shot intervals for line intensities, <2% for plasma temperature, and <6.5% for electron density. Using an uncertainty propagation formula, total number density RSDs were calculated to be 1.9-5.3% for 50 single-shot drilling scenarios. Considering physics behind results, fs-plasmas are "stable ablation sources" due to their electrostatic formation mechanisms and confined hydrodynamic evolution. The fs-laser opens up new directions for LIBS applications where accuracy is significantly enhanced.

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The self-absorption effect due to optically thick property greatly influences the measured line intensities as well as the performance of quantification for laser-induced breakdown spectroscopy (LIBS) especially for calibration-free LIBS which requires proper correction. In this paper, a new self-absorption correction method for Calibration-Free LIBS (CF-LIBS), called blackbody radiation referenced self-absorption correction (BRR-SAC) is proposed. An iterative algorithm was designed to calculate the plasma temperature and normally hard-to-obtain collection efficiency of the optical collection system by directly comparing the measured spectrum with the corresponding theoretical blackbody radiation for self-absorption correction. Compared with generally applied self-absorption correction methods based on the principle of curve of growth, the proposed method has obvious advantages of simpler programming, higher computation efficiency, and its independency of the availability or accuracy of line broadening coefficients. Experiments were conducted on titanium alloy samples. The experimental results showed that the self-absorption was corrected with increased linearity of the Boltzmann plots and the measurement accuracy of the elemental concentration was significantly improved through BRR-SAC. Compared with the traditional CF-LIBS with self-absorption correction, the proposed method also showed better performance. In addition, BRR-SAC provides a simple way to obtain the collection efficiency of the experimental setup, which benefits the plasma diagnostics and quantitative analysis.

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Collecting strong enough and repeatable signals from laser-induced plasmas is the primary goal of laser-induced breakdown spectroscopy optical detection systems. Typically, the light emitted from the plasma is refracted by the lens, collected by the fiber, and measured by the spectrometer. In the present work, we established a three-dimensional model to systematically evaluate the overall emission collected from different positions of the plasma for a typical optical collection system composed of a focus lens and a collection fiber, and sensitivity analyses were further performed. In addition, experiments were conducted and partially validated the model. Results showed that for the collection system with an optical fiber located on the focal point of the collection lens, the collection efficiency distribution is almost constant within a large cylindrical-shaped area, while for that located off the focal point, there is a rhombus-shaped area with higher collection efficiency than other areas. This much higher collection efficiency area is small in size but has a large impact on the detected spectral intensity. The spatially distributed collection efficiency on the lens parameters, such as size and position, was further discussed to clarify the impacts of the collection system. Furthermore, sensitivity analyses were performed to evaluate the impact of the collection system on the signal repeatability. Based on these calculations, recommendations for the design of the collection for optimized spectral intensity and stability were proposed.

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A calibration method based on homogeneous material for correcting laser-induced breakdown spectroscopy (LIBS) measurement-error bias in the case of dust pollution under laboratory conditions is proposed. The measured plasma spectra of the sample can be corrected by measuring the spectral integral of the homogeneous material. Thus, we can effectively minimize the dust pollution effect on LIBS and guarantee its precision. Results show that the mean absolute errors of CaO, MgO, Fe2O3, Al2O3, and SiO2 in cement samples are decreased notably from 1.02%, 0.06%, 0.15%, 0.57%, and 0.80% to 0.41%, 0.02%, 0.04%, 0.35%, and 0.39%, respectively. Combination of this calibration method with the traditional optical dustproof methods will significantly extend the LIBS equipment maintenance cycle and make preliminary preparations for the next practical industrial application.

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Using cavity confinement to enhance the plasma emission has been proved to be an effective way in LIBS technique while no direct visual evidence has been made to illustrate the physical mechanism of this enhancing effect. In this work, both laser-induced plasma plume images and shockwave images were obtained and synchronized for both flat surface case and rectangular cavity case. Phenomena of shockwave reflection, plasma compression by the reflected shockwave and merge of the reflected shockwave into plasma were observed. Plasma emission intensities recorded by ICCD in both cases were compared and the enhancement effect in the cavity case was identified in the comparison. The enhancement effect could be explained as reflected shockwave "compressing" effect, that is, the reflected shockwave would compress the plasma and result in a more condensed plasma core area with higher plasma temperature. Reflected shockwave also possibly contributed to plasma core position stabilization, which indicated the potential of better plasma signal reproducibility for the cavity case. Both plasma emission enhancement and plasma core position stabilization only exist within a certain temporal window, which indicates that the delay time of spectra acquisition is essential while using cavity confinement as a way to improve LIBS performance.

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Spark discharge has been proved to be an effective way to enhance the LIBS signal while moderate cylindrical confinement is able to increase the signal repeatability with limited signal enhancement effects. In the present work, these two methods were combined together not only to improve the pulse-to-pulse signal repeatability but also to simultaneously and significantly enhance the signal as well as SNR. Plasma images showed that the confinement stabilized the morphology of the plasma, especially for the discharge assisted process, which explained the improvement of the signal repeatability.

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In our previous work, we found that there was great potential to improve the pulse-to-pulse signal repeatability using a moderate cylindrical cavity confinement. However, the improvement was achieved only with certain experimental parameters; while under other conditions, there was no improvement or even worse repeatability. In the present work, the experimental configuration was redesigned and unexpected uncertainty from the variation of the laser and cavity alignment and the laser ablated aerosols were avoided. With these two improvements, we demonstrated that the cavity can always increase the signal repeatability. In addition, image taken by ICCD verified that the confinement improved the stability of the plasma morphology as expected.

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Moderate cylindrical cavity was used to regularize the laser-induced plasma for signal strength enhancement and precision improvement in laser-induced breakdown spectroscopy (LIBS). A polytetrafluoroethylene (PTFE) plate of 1.5 mm thickness with diameter of 3 mm was fabricated. It was placed closely on a sample surface and a laser pulse was shot through the center of the hole to the sample. Using coal as samples, it was verified that the configuration both enhanced the spectral line intensity and reduced shot-to-shot fluctuation, showing its great potential in improving the precision of LIBS analysis.

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Quantitative carbon measurement in anthracites remains difficult with laser-induced breakdown spectroscopy (LIBS) due to its relatively high measurement uncertainty. To improve the measurement repeatability, binders to bind the anthracite powder together were utilized for LIBS measurement. Results showed that the optimized binder Na(2)SiO(3)·9H(2)O, with Si from the binder as the internal calibration element, can yield the overall best measurement precision and accuracy. Using 15 anthracites for calibration and 7 anthracites for validation and with optimized percentage of Na(2)SiO(3)·9H(2)O as binder, the average value of the measurement's relative standard deviation (RSD), R(2), and root mean square error of prediction (RMSEP) were 12.1%, 0.76, and 6.25%, respectively, proving the applicability of binder for carbon measurement in anthracites.

RESUMEN

Moderate cylindrical cavity was used to regularize the laser-induced plasma for signal strength enhancement and precision improvement in laser-induced breakdown spectroscopy (LIBS). A polytetrafluoroethylene (PTFE) plate of 1.5 mm thickness with diameter of 3 mm was fabricated. It was placed closely on a sample surface and a laser pulse was shot through the center of the hole to the sample. Using coal as samples, it was verified that the configuration both enhanced the spectral line intensity and reduced shot-to-shot fluctuation, showing its great potential in improving the precision of LIBS analysis.