Quantitation improvement of underwater laser induced breakdown spectroscopy by using self-absorption correction based on plasma images

Simplified LIBS-based intensity-ratio approach for concentration estimation (SLICE): an approach for elemental analysis using laser induced breakdown spectroscopy

We propose what we believe to be a new approach for elemental analysis using laser induced breakdown spectroscopy (LIBS). This method offers enhanced convenience and simplicity for elemental analysis as it eliminates the necessity of Boltzmann/ Saha-Boltzmann plot. It is an intensity-ratio based approach that provides several notable advantages. One of the key benefits is its ability to perform comprehensive elemental analysis using only a few spectral lines; specifically, only n + 1 emission lines are sufficient for a sample containing n elemental species. This offers a great flexibility in the choice of emission lines which do not suffer from self-absorption. Further, high accuracy can be obtained as many repeated estimations from a single measurement are possible. We demonstrate the theory and working procedure of this technique by experimentally recording the data of two samples (binary and ternary copper alloys). A nanosecond Nd:YAG pulsed laser of ∼7 ns pulse duration and 532 nm incident wavelength is used. The results are in good agreement with CF-LIBS and Energy-dispersive X-ray spectroscopy (EDS).

Modulate the laser phase to improve the ns-LIBS spectrum signal based on orbital angular momentum

Aiming to enhance the ns-LIBS signal, in this work, we introduced orbital angular momentum to modulate the laser phase of the Gaussian beam into the vortex beam. Under similar incident laser energy, the vortex beam promoted more uniform ablation and more ablation mass compared to the Gaussian beam, leading to elevated temperature and electron density in the laser-induced plasma. Consequently, the intensity of the ns-LIBS signal was improved. The enhancement effects based on the laser phase modulation were investigated on both metallic and non-metallic samples. The results showed that laser phase modulation resulted in a maximum 1.26-times increase in the peak intensities and a maximum 1.25-times increase in the signal-to-background ratio (SBR) of the Cu spectral lines of pure copper for a laser energy of 10 mJ. The peak intensities of Si atomic spectral lines were enhanced by 1.58-1.94 times using the vortex beam. Throughout the plasma evolution process, the plasma induced by the vortex beam exhibited prolonged duration and a longer continuous background, accompanied by a noticeable reduction in the relative standard deviation (RSD). The experimental results demonstrated that modulation the laser phase based on orbital angular momentum is a promising approach to enhancing the ns-LIBS signal.

Self-absorption correction method based on intensity self-calibration of doublet lines

The self-absorption effect in an optically thick plasma seriously affects the spectral line intensity and the measurement accuracy of laser-induced breakdown spectroscopy (LIBS). In this work, a self-absorption correction method based on intensity self-calibration of doublet lines belonging to the same multiplet is proposed. The K/Δλ0 parameter and self-absorption coefficient (SA) of the doublet lines of the analytical element can be calculated based on the measured actual lines intensity ratio and the K parameters ratio. Compared with the generally applied self-absorption correction methods, this method can effectively reduce the influence of laser energy and plasma plume fluctuations and the non-uniformity distribution of the element in the plasma, and is independent of the availability of Stark broadening coefficients. So it has obvious advantages of high computation efficiency, high analysis accuracy and good applicability. Univariate quantitative analysis results of aluminum (Al) show that the correlation coefficient of calibration curves and the measurement accuracy of elemental content have been significantly improved with the self-absorption correction.

A numerical procedure for understanding the self-absorption effects in laser induced breakdown spectroscopy

Optical emission spectroscopic techniques, such as laser-induced breakdown spectroscopy (LIBS), require an optimal state of plasma for accurate quantitative elemental analysis. Three fundamental assumptions must be satisfied in order for analytical results to be accurate: local thermodynamic equilibrium (LTE), optically thin plasma, and stoichiometric ablation. But real-life plasma often fails to satisfy these conditions, especially the optical thinness of plasma, resulting in the reabsorption of emitted radiation called self-absorption. To study the self-absorption effect, we simulated optically thick emission spectrum at typical laser-produced plasma conditions. The simulation of the spectrum involves four stages, including the estimation of the ratio of the number density of various ionisation states in the plasma using the Saha-Boltzmann equation, the peak intensity of a spectral line using the Boltzmann equation, the full-width half maxima of each spectral line using the Stark broadening method, and the generation of full spectra by providing a Lorentzian profile for each spectral line. Then self-absorption is applied to the simulated spectrum. We investigated the dependence of the self-absorption coefficient on the plasma temperature, optical path length, and element concentration in the sample. Self-absorption decreases with increased plasma temperature, whereas it increases with increasing optical path length and analyte concentration. We also investigated the role of self-absorption in quantitative analysis by calibration-free LIBS with and without resonance lines of the binary alloy (Mg 50% & Ca 50%). We observed a drastic reduction in error from 27% to 2% in the composition estimation when excluding resonance lines.

Long-term repeatability improvement using beam intensity distribution for laser-induced breakdown spectroscopy

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.