Lianbo Guo

Enhancement of optical emission from laser-induced plasmas by combined spatial and magnetic confinement

To enhance optical emission in laser-induced breakdown spectroscopy, both a pair of permanent magnets and an aluminum hemispherical cavity (diameter: 11.1 mm) were used simultaneously to magnetically and spatially confine plasmas produced by a KrF excimer laser in air from pure metal and alloyed samples. High enhancement factors of about 22 and 24 in the emission intensity of Co and Cr lines were acquired at a laser fluence of 6.2 J/cm2 using the combined confinement, while enhancement factors of only about 11 and 12 were obtained just with a cavity. The mechanism of enhanced optical emission by combined confinement, including shock wave in the presence of a magnetic field, is discussed. The Si plasmas, however, were not influenced by the presence of magnets as Si is hard to ablate and ionize and hence has less free electrons and positive ions. Images of the laser-induced Cr and Si plasmas show the difference between pure metallic and semiconductor materials in the presence of both a cavity and magnets.

Plasma confinement by hemispherical cavity in laser-induced breakdown spectroscopy

An aluminum hemispherical cavity (diameter: 11.1 mm) was used to confine plasmas produced by a KrF excimer laser in air from a steel target with a low concentration manganese in laser-induced breakdown spectroscopy. A significant enhancement (factor >12) in the emission intensity of Mn lines was observed at a laser fluence of 7.8 J/cm2 when the plasma was confined by the hemispherical cavity, leading to an increase in plasma temperature about 3600 K. The maximum emission enhancement increased with increasing laser fluence. The spatial confinement mechanism was discussed using shock wave theory.

Accuracy improvement of quantitative analysis by spatial confinement in laser-induced breakdown spectroscopy

To improve the accuracy of quantitative analysis in laser-induced breakdown spectroscopy, the plasma produced by a Nd:YAG laser from steel targets was confined by a cavity. A number of elements with low concentrations, such as vanadium (V), chromium (Cr), and manganese (Mn), in the steel samples were investigated. After the optimization of the cavity dimension and laser fluence, significant enhancement factors of 4.2, 3.1, and 2.87 in the emission intensity of V, Cr, and Mn lines, respectively, were achieved at a laser fluence of 42.9 J/cm(2) using a hemispherical cavity (diameter: 5 mm). More importantly, the correlation coefficient of the V I 440.85/Fe I 438.35 nm was increased from 0.946 (without the cavity) to 0.981 (with the cavity); and similar results for Cr I 425.43/Fe I 425.08 nm and Mn I 476.64/Fe I 492.05 nm were also obtained. Therefore, it was demonstrated that the accuracy of quantitative analysis with low concentration elements in steel samples was improved, because the plasma became uniform with spatial confinement. The results of this study provide a new pathway for improving the accuracy of quantitative analysis of LIBS.

Optimally enhanced optical emission in laser-induced breakdown spectroscopy by combining spatial confinement and dual-pulse irradiation

In laser-induced breakdown spectroscopy (LIBS), a pair of aluminum-plate walls were used to spatially confine the plasmas produced in air by a first laser pulse (KrF excimer laser) from chromium (Cr) targets with a second laser pulse (Nd:YAG laser at 532 nm, 360 mJ/pulse) introduced parallel to the sample surface to re-excite the plasmas. Optical emission enhancement was achieved by combing the spatial confinement and dual-pulse LIBS (DP-LIBS), and then optimized by adjusting the distance between the two walls and the interpulse delay time between both laser pulses. A significant enhancement factor of 168.6 for the emission intensity of the Cr lines was obtained at an excimer laser fluence of 5.6 J/cm(2) using the combined spatial confinement and DP-LIBS, as compared with an enhancement factor of 106.1 was obtained with DP-LIBS only. The enhancement mechanisms based on shock wave theory and reheating in DP-LIBS are discussed.

Sensitivity improvement in the detection of V and Mn elements in steel using laser-induced breakdown spectroscopy with ring-magnet confinement

To improve the detection sensitivity of vanadium (V) and manganese (Mn) elements in steel using laser-induced breakdown spectroscopy (LIBS), a ring magnet was employed to spatially and magnetically confine plasmas produced from steel samples using an Nd:YAG laser. The results showed that the optical emission and signal-to-noise ratios (SNRs) for both V I 437.92 nm and Mn I 403.08 nm lines were enhanced by the ring-magnet confinement. The enhancements were found to be due to an increase in the plasma temperature and electron density as a result of both spatial and magnetic confinement. The calibration curves of V I 437.92 nm and Mn I 403.08 nm with/without confinement were established. The 3σ-limits of detection (LoDs) for V and Mn in steels were 11 and 30 ppm with the ring magnet, lower than the 18 and 41 ppm with a degaussed magnet and the 41 and 56 ppm in open air, respectively.

Analytical-performance improvement of laser-induced breakdown spectroscopy for steel using multi-spectral-line calibration with an artificial neural network

A multi-spectral-line calibration (MSLC) approach based on an artificial neural network (ANN) was developed to improve the accuracy and precision of steel analysis using laser-induced breakdown spectroscopy (LIBS). The intensity ratios of multiple spectral lines of target and matrix elements were used to train an ANN. The resulting model was able to relate the spectra to the concentrations of target elements more accurately than the conventional internal calibration approach, which led to improvements in the accuracy and precision of the LIBS analysis. This approach was applied to LIBS analysis of steel samples to predict the Cr and Ni concentrations. Compared with a conventional internal calibration approach, the root-mean-square errors of cross-validation for Cr and Ni decreased from 0.018 and 0.067 wt% to 0.010 and 0.023 wt%, respectively, using the proposed MSLC, and the average values of the relative standard deviation for Cr and Ni decreased from 11.3 and 19.5% to 6.4 and 12.9%, respectively.

Determination of cobalt in low-alloy steels using laser-induced breakdown spectroscopy combined with laser-induced fluorescence

Cobalt element plays an important role for the properties of magnetism and thermology in steels. In this work, laser-induced breakdown spectroscopy combined with laser-induced fluorescence (LIBS-LIF) was studied to selectively enhance the intensities of Co lines. Two states of Co atoms were resonantly excited by a wavelength-tunable laser. LIBS-LIF with ground-state atom excitation (LIBS-LIFG) and LIBS-LIF with excited-state atom excitation (LIBS-LIFE) were compared. The results show that LIBS-LIFG has analytical performance with LoD of 0.82μg/g, R(2) of 0.982, RMSECV of 86μg/g, and RE of 9.27%, which are much better than conventional LIBS and LIBS-LIFE. This work provided LIBS-LIFG as a capable approach for determining trace Co element in the steel industry.

Accuracy improvement on polymer identification using laser-induced breakdown spectroscopy with adjusting spectral weightings

A new approach to polymer identification by laser-induced breakdown spectroscopy (LIBS) with adjusting spectral weightings (ASW) was developed in this work aiming at improving the identification accuracy. This approach has been achieved through increasing the intensities of specific characteristic spectral lines which are important to polymer identification but difficult to be excited. Using the ASW method, the identification accuracies of all 11 polymers were increased to nearly 100%, while the accuracies of PE, PU, PP and PC were only 98%, 74%, 90% and 98%, respectively, without using the ASW method.

Accuracy improvement of quantitative analysis in laser-induced breakdown spectroscopy using modified wavelet transform

A modified algorithm of background removal based on wavelet transform was developed for spectrum correction in laser-induced breakdown spectroscopy (LIBS). The optimal type of wavelet function, decomposition level and scaling factor γ were determined by the root-mean-square error of calibration (RMSEC) of the univariate regression model of the analysis element, which is considered as the optimization criteria. After background removal by this modified algorithm with RMSEC, the root-mean-square error of cross-validation (RMSECV) and the average relative error (ARE) criteria, the accuracy of quantitative analysis on chromium (Cr), vanadium (V), cuprum (Cu), and manganese (Mn) in the low alloy steel was all improved significantly. The results demonstrated that the algorithm developed is an effective pretreatment method in LIBS to significantly improve the accuracy in the quantitative analysis.

Acidity measurement of iron ore powders using laser-induced breakdown spectroscopy with partial least squares regression

Laser-induced breakdown spectroscopy (LIBS) with partial least squares regression (PLSR) has been applied to measuring the acidity of iron ore, which can be defined by the concentrations of oxides: CaO, MgO, Al₂O₃, and SiO₂. With the conventional internal standard calibration, it is difficult to establish the calibration curves of CaO, MgO, Al₂O₃, and SiO₂ in iron ore due to the serious matrix effects. PLSR is effective to address this problem due to its excellent performance in compensating the matrix effects. In this work, fifty samples were used to construct the PLSR calibration models for the above-mentioned oxides. These calibration models were validated by the 10-fold cross-validation method with the minimum root-mean-square errors (RMSE). Another ten samples were used as a test set. The acidities were calculated according to the estimated concentrations of CaO, MgO, Al₂O₃, and SiO₂ using the PLSR models. The average relative error (ARE) and RMSE of the acidity achieved 3.65% and 0.0048, respectively, for the test samples.