Investigation on self-absorption at reduced air pressure in quantitative analysis using laser-induced breakdown spectroscopy

Study on the radiation and self-absorption characteristics of plasma under various background gases

The self-absorption effect is a primary factor responsible for the decline in the precision of quantitative analysis techniques using plasma emission spectroscopy, such as laser-induced breakdown spectroscopy (LIBS). In this study, based on the thermal ablation and hydrodynamics models, the radiation characteristics and self-absorption of laser-induced plasmas under different background gases were theoretically simulated and experimentally verified to investigate ways of weakening the self-absorption effect in plasma. The results reveal that the plasma temperature and density increase with higher molecular weight and pressure of the background gas, leading to stronger species emission line intensity. To reduce the self-absorption effect in the later stages of plasma evolution, we can decrease the gas pressure or substitute the background gas with a lower molecular weight. As the excitation energy of the species increases, the impact of the background gas type on the spectral line intensity becomes more pronounced. Moreover, we accurately calculated the optically thin moments under various conditions using theoretical models, which are consistent with the experimental results. From the temporal evolution of the doublet intensity ratio of species, it is deduced that the optically thin moment appears later with higher molecular weight and pressure of the background gas and lower upper energy of the species. This theoretical research is essential in selecting the appropriate background gas type and pressure and doublets in self-absorption-free LIBS (SAF-LIBS) experiments to weaken the self-absorption effect.

Rapid determination of lead (Pb) in the soil-plant system by laser-induced breakdown spectroscopy (LIBS): case study of Pb-pollution from perovskite solar cells

With the widespread usage of lead (Pb)-containing perovskite solar cells (PSCs), it is critical to monitor Pb pollution from PSCs in the environment. Among different analytical techniques, laser-induced breakdown spectroscopy (LIBS) has demonstrated good performance in the fast quantification of many elements in solid samples, without using toxic and expensive chemical reagents. Therefore, LIBS offers significant potential for detecting and quantifying Pb in the environment. In this study, a Pb migration model in the PSCs-soil-Houttuynia plants system was assessed based on the LIBS data. The Pb transfer rates and the Pb uptake coefficients were calculated to evaluate Pb migration from PSCs to plants. The results showed that the R² of quantitative results were all greater than 0.98, with the root-mean-square error of cross-validation (RMSECV) being less than 1.53 wt.%. Furthermore, above 49% Pb from PSCs was swiftly diffused into soil under watering conditions, while Houttuynia plants absorbed over 10% Pb from polluted soil. This study revealed that Pb leakage from PSCs should not be underestimated and that LIBS is a viable and fast analytical method for monitoring Pb in the environment.

Simple defocus laser irradiation to suppress self-absorption in laser-induced breakdown spectroscopy (LIBS)

This study introduces a novel and simple way to suppress the self-absorption effect in laser-induced breakdown spectroscopy (LIBS) by utilizing a defocusing laser irradiation technique. For this purpose, a Nd:YAG laser with a wavelength of 1,064 nm and repetition rate of 10 Hz with energy in the range of 10 mJ–50 mJ was used. The laser irradiation was focused by using a 150-mm-focal-length plano-convex lens onto the sample surface under defocusing of approximately –6 mm. Potassium chloride (KCl) and sodium chloride (NaCl) pellet samples were used to demonstrate this achievement. When the defocus position is adjusted to –6 mm for KCl and NaCl samples, the self-reversal in the emission lines of K I 766.4 nm, K I 769.9 nm, Na I 588.9 nm, and Na I 589.5 nm vanish. Meanwhile, the FWHM values of K I 766.4 and K I 769.9 nm are 0.29 nm and 0.23 nm, respectively, during –6 mm defocus laser irradiation, as opposed to 1.24 nm and 0.86 nm under tight focus laser irradiation. Additionally, this work demonstrates that, when the laser energy is changed between 10 and 50 mJ, no self-reversal occurs in the emission lines when –6 mm defocus laser irradiation is applied. Finally, a linear calibration curve was generated using KCl at a high concentration ranging between K concentrations from 16.6% to 29%. It should be noted that, even at such high K concentrations, the calibration curve is still linear. This means that self-absorption is almost negligible. This simple change in defocus laser irradiation will undoubtedly contribute to the suppression of the self-absorption phenomenon, which disrupts LIBS analytical results.

Spatial confinement effects of laser-induced breakdown spectroscopy at reduced air pressures

Spatial confinement is a simple and cost-effective method for enhancing signal intensity and improving the detection sensitivity of laser-induced breakdown spectroscopy (LIBS). However, the spatial confinement effects of LIBS under different pressures remains a question to be studied, because the pressure of the ambient gas has a significant influence on the temporal and spatial evolution of plasma. In this study, spatial confinement effects of LIBS under a series of reduced air pressures were investigated experimentally, and the plasma characteristics under different air pressures were studied. The results show that the reduced air pressure can lead to both earlier onset and weakening of the enhancement effect of the spatial confinement on the LIBS line intensity. When the air pressure drops to 0.1 kPa, the enhancement effect of the emission intensity no longer comes from the compression of the reflected shock wave on the plasma, but from the cavity's restriction of the plasma expansion space. In conclusion, the enhancement effect of spatial confinement technology on the LIBS is still effective when the pressure is reduced, which further expands the research and application field of spatial confinement technology.

Accuracy improvement of single-sample calibration laser-induced breakdown spectroscopy with self-absorption correction

The single sample calibration laser-induced breakdown spectroscopy (SSC-LIBS) is quite suitable for the fields where the standard sample is hard to obtain, including space exploration, geology, archaeology, and jewelry identification. But in practice, the self-absorption effect of plasma destroys the linear relationship of spectral intensity and element concentration based on the Lomakin-Scherbe formula which is the guarantee of the high accuracy of the SSC-LIBS. Thus, the self-absorption effect limits the quantitative accuracy of SSC-LIBS greatly. In this work, an improved SSC-LIBS with self-absorption correction (SSC-LIBS with SAC) is proposed for the promotion of quantitative accuracy of SSC-LIBS. The SSC-LIBS with SAC can correct the intensity ratio of spectral lines in the calculation of SSC-LIBS through relative self-absorption coefficient K without complicated preparatory information. The alloy samples and pressed ore samples were used to verify the effect of the SSC-LIBS with SAC. Compared with SSC-LIBS, for alloy samples, the average RMSEP and average ARE of SSC-LIBS with SAC decreased from 0.83 wt.% and 13.75% to 0.40 wt.% and 4.06%, respectively. For the pressed ore samples, the average RMSEP and average ARE of SSC-LIBS with SAC decreased from 4.77 wt.% and 90.48% to 2.34 wt.% and 14.60%. The experimental result indicates that SSC-LIBS with SAC has a great improvement of quantitative accuracy and better universality compared with traditional SSC-LIBS, which is a mighty promotion of the wide application of SSC-LIBS.

Unusual parallel laser irradiation for suppressing self-absorption in single pulse laser-induced breakdown spectroscopy

This study demonstrates a new approach for suppressing the self-absorption effect in single-pulse laser-induced breakdown spectroscopy (LIBS) using unusual parallel laser irradiation. A nanosecond Nd:YAG laser with a wavelength of 1064 nm was fired parallel to and focused at a very close distance of 1 mm to the sample surface. The experiment was carried out in air at atmospheric pressure. In this configuration, the sample was ablated by a shockwave generated from the air breakdown plasma formed near the sample surface. Under this condition, we successfully obtained spectra of the resonance emission line for high concentration K (K I 766.4 nm and K I 769.9 nm) that are free from self-reversal and weakly affected by the self-absorption. Furthermore, the quantitative analysis results for the element K showed that a linear calibration curve over a wide concentration range could be achieved, which indicates the effectiveness of this technique in reducing the self-absorption effect and improving the analytical performance of ordinary single-pulse LIBS.

Suppression of self-absorption in laser-induced breakdown spectroscopy using a double pulse orthogonal configuration to create vacuum-like conditions in atmospheric air pressure

Self-absorption, which is known to severely disturb identification of the emission peak intensity in emission-based spectroscopy, was first studied using ordinary single pulse laser-induced breakdown spectroscopy (LIBS). It was found that severe self-absorption, with an evident self-reversal, occurs in the resonance emission lines of high concentration Na, K, and Al, and thus it is impossible to obtain the linear calibration curve required for quantitative analysis. To overcome this problem, we introduce a double pulse orthogonal technique in which the first laser is fired in a parallel orientation at a varied distance of 2-6 mm from the sample surface. It is well known that the strong shock wave generated by this laser irradiation temporarily creates a vacuum-like condition immediately in front of the sample surface. This action is followed by a second laser irradiation oriented perpendicular to the sample surface. The sample ablated by the second laser irradiation expands following the shockwave excitation process in the vacuum-like air atmosphere created by the first laser. The obtained spectra of the resonance emission lines of high concentration Na, K, and Al are free from the self-reversal and weakly affected by the self-absorption effect. A linear calibration curve that intercepts near zero point for K element over a wide concentration range is also demonstrated in this study. This simple modification is considered notably helpful in overcoming the self-absorption that occurs in ordinary single pulse atmospheric pressure LIBS.

Suppression of self-absorption effect in laser-induced breakdown spectroscopy by employing a Penning-like energy transfer process in helium ambient gas

A unique approach for achieving total suppression of the self-absorption effect in laser-induced breakdown spectroscopy (LIBS) has been demonstrated employing a previously published technique of laser-induced plasma spectroscopy utilizing a helium (He) metastable excited state (LIPS-He*).This achievement was attained by the use of the He metastable excited state (He*) and a Penning-like energy transfer mechanism for the delayed excitation of the ablated analyte atoms. KCl and NaCl samples showed the disappearance of the self-absorption emission lines of K I 766.4 nm, K I 769.9 nm, Na I 588.9 nm, and Na I 589.5 nm, and the FWHM values of K I 766.4 and Na I 588.9 nm were found to be 0.8 nm and 0.15 nm, respectively, by LIPS-He* as compared to 4.8 nm and 1.4 nm, respectively, by single-laser operation. A standard Al sample also showed the total disappearance of the self-absorption emission lines Al I 394.4 nm and Al I 396.1 nm. The FWHM of Al I 396.1 nm was 0.12 nm when LIPS-He* was employed compared to 0.44 nm when a single laser was used. A remarkable linear calibration line with zero intercepts was also obtained for high-concentration Al samples (87.0%, 93.0% and 99.8%). Thus, it is established that the self-absorption effect can be completely neglected when excitation through He* is employed in LIBS.

Improving the spectral qualities of major elements in soil by controlling the ambient pressure in time-resolved laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is a powerful tool in the soil monitoring field, but the poor spectral quality limits its application. To improve the spectral quality of major elements in soil samples, a method based on controlling the ambient pressure and time sequence was introduced. Spectral qualities that include signal-to-background ratio (SBR), spectral stability, and spectral profile were all studied in different ambient pressures and delay times. The results show that the SBRs of Na and K elements increased from 20 to about 300, when the air pressure and delay time were controlled. Meanwhile, the relative standard deviations were improved from more than 30% to less than 5% due to the release of the self-absorption effect. This work proved that the spectral qualities of LIBS can be improved a lot by controlling the ambient pressure in the field of detecting major elements in soil.

Quantitative Analysis of Carbon with Laser-Induced Breakdown Spectroscopy (LIBS) Using Genetic Algorithm and Back Propagation Neural Network Models

Carbon content detection is an essential component of the metal-smelting and classification processes. An obstacle in carbon content detection by laser-induced breakdown spectroscopy (LIBS) of steel is the interference of carbon lines by the adjacent Fe lines. The emission line of C(I) 247.86 nm generally has higher response and transmission efficiency than the emission line of C(I) 193.09 nm, but it blends with the Fe(II) 247.86 nm line. Therefore, this study proposes a method of back propagation (BP) neural network modeling, which incorporates a genetic algorithm (GA), evaluates the method of parameter modeling and prediction based on GA to optimize the BP neural network (GA-BP), and realizes a quantitative analysis of the C(I) 247.86 nm line. The achieved root mean square error for the GA-BP model is 0.0114. The obtained linear correlation coefficient shows a significant improvement after correction, indicating that the proposed method is effective. The method is concise, easy to implement, and can be applied in the carbon content detection of steels and iron-based alloys.

Investigation of the self-absorption effect using time-resolved laser-induced breakdown spectroscopy

Self-absorption seriously affects the accuracy and stability of quantitative analysis in laser-induced breakdown spectroscopy (LIBS). To reduce the effect of self-absorption, we investigated the temporal evolution of the self-absorption effect by establishing exponential calibration curves. Meanwhile, the temporal evolution mechanism of the self-absorption effect was also investigated. The results indicated that self-absorption was weak at the early stage of plasma expansion. For determination of manganese (Mn) in steel, as an example, the concentration of upper bound of linearity (C_int) was 2.000 wt. % at the early stage of plasma expansion (in a time window of 0.2-0.4 μs)-much higher than 0.363 wt. % at a traditional optimization time window (2-3 μs). The accuracy and stability of quantitative analysis at the time window of 0.2-0.4 μs was also much better than at the time window of 2-3 μs. This work provides a simple method for improving quantitative analysis performance and avoiding the self-absorption effect in LIBS.

Comparative study on self-absorption of laser-induced tungsten plasma in air and in argon

The onset of self-absorption of laser-induced plasma poses a problem for converting emission line intensities to concentrations, which is one of the main bottlenecks in quantitative laser-induced breakdown spectroscopy (LIBS) measurements. In this paper, the effects of atmosphere and laser fluence on self-absorption reduction of the plasma induced on tungsten-copper alloy target were investigated with nanosecond infrared (1064 nm) laser pulse over a range of 2.9 to 18.2 J/cm². The time-resolved features of self-absorption, and temperature and electron density of the plasma were characterized in atmospheric air and argon, respectively. The experimental results show the effect of self-absorption can be significantly reduced by increasing the laser pulse energy. The argon atmosphere is more helpful for self-absorption reduction. The time-resolved diagnostics of emission spectra in the early stage of the plasma formation are very effective to prevent self-absorption to improve the LIBS analytical performance.

Accuracy improvement of quantitative analysis in spatially resolved fiber-optic laser-induced breakdown spectroscopy

Fiber-optic laser-induced breakdown spectroscopy (FO-LIBS) has been employed in many applications because of the flexibility of optical fiber cable. However, the inhomogeneous elemental distribution of plasmas can cause a self-absorption effect and, hence, significantly hinder the determination of FO-LIBS. Here, to solve this flaw, we took iron (Fe), magnesium (Mg), and zinc (Zn) elements in aluminum alloy as examples to investigate the self-absorption reduction and accuracy improvement using spatially resolved FO-LIBS. Spatially resolved FO-LIBS means the spectra were collected at different positions along the direction parallel to the surface of the sample rather than at the center of the plasma. With this method, the self-absorption effect could be improved by selecting different acquisition positions along the X-axis. The root mean square error of cross-validations (RMSECV) for Fe, Mg, and Zn were reduced from 0.388, 0.348, and 0.097 wt. % to 0.172, 0.224, and 0.024 wt. %, respectively. Generally, spatial resolution is an effective method of self-absorption reduction and accuracy improvement in FO-LIBS.

One-point and multi-line calibration method in laser-induced breakdown spectroscopy

The calibration-free laser-induced breakdown spectroscopy (CF-LIBS) and its variations are low cost, short time consumption, and high adaptability. However, seeking a more flexible and simple quantitative analysis method remains a challenge. A one-point and multi-line calibration (OP-MLC) was presented as a simple quantitative analysis method of LIBS. The results showed that OP-MLC-LIBS method can achieve quantitative analysis using only one standard sample, and the average relative errors (AREs) are 9, 22, 21 and 36% for Mn, Cr, Ni and Ti elements in six tested low-alloy steel samples, respectively. The method requires neither a large number of standard samples nor complicated calculations, which provides a flexible and low-cost quantitative analysis approach for development and application of LIBS.

Reutilization of nanosecond pulse laser energy and its performance in single particle triggered LIBS

A method that can reutilize the energy of a nanosecond pulse laser beam in LIBS was studied. When the pulse energy is not sufficient to generate the plasma, the overlapped point in this method can reach the threshold.

Spatially selective excitation in laser-induced breakdown spectroscopy combined with laser-induced fluorescence

Spatially selective excitation was proposed to improve excitation efficiency in laser-induced breakdown spectroscopy combined with laser-induced fluorescence (LIBS-LIF). Taking chromium (Cr) and nickel (Ni) elements in steels as examples, it was discovered that the optimal excitation locations were the center of the plasmas for the matrix of the iron (Fe) element but the periphery for Cr and Ni elements. By focusing an excitation laser at the optimal locations, not only excitation efficiency but also the analytical accuracy and sensitivity of quantitative LIBS-LIF were better than those with excitation at the plasma center in conventional LIBS-LIF. This study provides an effective way to improve LIBS-LIF analytical performance.

Quantitative analysis of steel samples using laser-induced breakdown spectroscopy with an artificial neural network incorporating a genetic algorithm

In this work, a genetic algorithm (GA) was employed to select the intensity ratios of the spectral lines belonging to the target and domain matrix elements, then these selected line-intensity ratios were taken as inputs to construct an analysis model based on an artificial neural network (ANN) to analyze the elements copper (Cu) and vanadium (V) in steel samples. The results revealed that the root mean square errors of prediction (RMSEPs) for the elements Cu and V can reach 0.0040 wt. % and 0.0039 wt. %, respectively. Compared to 0.0190 wt. % and 0.0201 wt. % of the conventional internal calibration approach, the reduction rates of the RMSEP values reached 78.9% and 80.6%, respectively. These results indicate that the GA combining ANN can excellently execute the quantitative analysis in laser-induced breakdown spectroscopy for steel samples and further improve analytical accuracy.