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Abstract
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.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The laser-induced breakdown spectroscopy (LIBS) has been widely recognised as a promising tool for spectrochemical analysis in various areas, since the last few decades, including geochemical, environmental, agriculture, food, metallurgy, and space exploration [1–7]. However, the complex nature of light-matter interaction in laser-induced plasma limits the analytical performance of this technique. One of the major factors that seriously affect the accuracy of LIBS for quantitative analysis is self-absorption. This effect is responsible for reducing the peak intensity of the emission line, which leads to underestimating the elemental concentration and destroys a linear relationship between the spectral line and the elemental concentration. Besides, the effect also introduces distortion on the emission line profile that causes inaccuracy in determining plasma characteristics, such as electron density and plasma temperature, from the emission linewidth. A strong self-absorption effect is observed, particularly for the resonance emission line of an element with high concentration. In this extreme case, a dip, called self-reversal, appears at the centre of the emission line. The existence of self-reversal on the emission line is related to the inhomogeneity of laser-induced plasma along the sight of observation [8].
To date, several efforts have been devoted to addressing the self-absorption problem in LIBS. Researchers have developed theoretical models such as the curve of growth [9], spectral fitting [10–12], self-absorption coefficient [13,14], two-line method [15,16], and internal standard line [17] to estimate and correct the self-absorption effect in LIBS plasma. Besides the theoretical model, some research groups have developed experimental techniques such as doubling mirror [18], atmospheric pressure control [19], laser-stimulated absorption [20–22], microwave-assisted excitation [23], and proper selection of the detection window [24,25] to eliminate the self-absorption effect in LIBS. A comprehensive review of the self-absorption effect in LIBS can be found in a recently published article by Rezeai et al. [26].
Recently, our group also published two articles on strategies to suppress the self-absorption effect in LIBS. In the first paper [27], we report that the self-absorption effect can be reduced when excitation through a helium metastable excited state [28–32] is employed in LIBS. In the second paper [33], we report that the self-absorption effect in LIBS can be overcome by temporarily creating a vacuum-like condition immediately in front of the sample surface. In these techniques, a double-pulse orthogonal laser configuration has been used. However, for practical applications, a system with a simple experimental setup is preferable. Therefore, in this study, we applied the unusual parallel laser irradiation using a single pulse laser to deal with the self-absorption effect in LIBS. It should be noted that a similar experimental configuration has been used in LIBS by De Giacomo et al. [34]. In that study, parallel irradiation was employed to minimize the damage on the sample surface and allow delicate processing of the sample surface. Meanwhile, in our approach, parallel laser irradiation is used to suppress the self-absorption in ordinary single pulse LIBS at atmospheric pressure. In this study, the single pulse laser was used to generate the air breakdown plasma near the sample surface. The strong shockwave produced from this plasma ablates the sample. Atoms ablated from the sample are excited when the atoms enter the hot region of the air breakdown plasma. The presence of air breakdown plasma also creates a temporary rarefied environment near the sample surface, which helps in producing homogenous sample plasma. Under this condition, the spectra of the resonance emission line at high K concentration (K I 766.4 nm and K I 769.9 nm) that are practically free from self-reversal and less affected by the self-absorption were successfully obtained. The quantitative analysis results for elemental K showed that a linear calibration curve over a wide concentration range could be achieved. This proposed method provides an alternative simple way to suppress the self-absorption effect in ordinary single-pulse LIBS.
2. Experimental setup
A schematic diagram of the experimental setup used for parallel laser irradiation is shown in Fig. 1(a). A Q-switch Nd:YAG laser (Quanta Ray, LAB 130–10, USA) operating at a wavelength of 1064 nm with a pulse duration of 8 ns and repetition rate of 10 Hz was employed as the irradiation source. A quartz lens with a focal length of 170 mm was utilised for focusing the laser beam. The beam waist and Rayleigh length of the focused laser beam were calculated to be 45 μm and 5 mm, respectively. Therefore the depth of focus of the laser beam is approximately equal to the sample diameter. Figure 1(b) shows the photograph of the generated air breakdown plasma near the sample. The sample holder was placed on the XYZ manual translation stage. An optical fibre with a numerical aperture of 0.22 was used to collect the emitted light from the plasma. The fibre is positioned at 6 cm on the side of the plasma which provides a visual field with a diameter of 27 mm. This means that with proper alignment, the emitted light from the plasma can be collected efficiently into the fibre. The other end of the optical fibre is connected to the input slit of a Czerny Turner spectrograph (McPherson model 2061, focal length 1000 mm, f/8.6, resolution 0.018 nm at 313.1 nm). The width of the input slit was fixed at 5 µm. A gated intensified charge-coupled device (ICCD; Andor iStar intensified CCD, 1024 × 256 pixels, UK) is mounted at the exit slit of the spectrograph to record the spectra. The ICCD and laser were triggered with a digital delay generator (DDG 535, Stanford Research System, USA). The gate delay and width of the ICCD were fixed at 1 µs and 30 µs, respectively. All the experiments were carried out in the air at atmospheric pressure under a fixed sample position. Each data was accumulated for five shots.
Fig. 1. (a) Schematic diagram of the experimental setup used in this work. (b) Photograph of the air breakdown plasma that is generated close to the sample surface at laser energy of 54 mJ.
The sample used in this study was KCl powder (Wako Chemical, Japan, 4N). For quantitative analysis, the KCl powder were mixed with NaCl powder (Wako Chemical, Japan, 4N) with the K content covering a range of 0.983-52.4 wt.%. The powder sample was finely ground for 6 min and subsequently pressed into pellet with a diameter of 10 mm and a thickness of 2 mm under the pressure of 30 MPa for approximately 90 s.
3. Results and discussion
Initially, we irradiated the sample using a single pulse Nd:YAG laser directed orthogonal to the sample surface. The laser was focused exactly at the sample surface. The spectra of K (K I 764.4 nm and K I 769.9 nm), obtained from two different laser irradiation energies, are presented in Fig. 2. The presence of self-reversal on each spectrum suggests that the temperature gradient or plasma inhomogeneity exists along the direction of observation. Generally, plasma generated in this configuration consists of a bright hot core region near the sample surface surrounded by a cold peripheral region. The hot core region is populated by free electrons, ions, and excited atoms, whereas the cold peripheral region is populated by the ground-state or neutral atoms. When light is emitted from the core region and propagates to the peripheral region, the light may be absorbed by the same species of emitting atoms. As a result, the emission line intensity becomes weak, and in the extreme case, self-reversal appears at the emission line centre. This self-absorption effect is strong for the resonance line of a high-concentration element, as in our case. From Fig. 2, we can see that the self-reversal becomes significant at low laser energy. This can be attributed to the large temperature gradient between the inner and outer regions of the plasma.
Fig. 2. Emission spectra of K I 766.4 nm and K I 769.9 nm from a pure KCl pellet obtained using orthogonal irradiation with laser energies of 54 and 70 mJ.
In the following experiment, to obtain the self-reversal free emission line, especially at low laser energy, we focused the laser beam close to the sample surface, at a distance d = 1 mm, using a parallel configuration [Fig. 1(a)]. In this configuration, the laser beam does not interact with the target but generates the air breakdown plasma near the target surface because d is much larger than the beam waist. The shockwave produced by this plasma propagates to the target and ablates a small amount of the target. When the ablated atoms from the target enter the hot region of the air breakdown plasma, the atoms are excited. The presence of air breakdown plasma also creates a temporary rarefied environment above the target surface. The region of this rarefied environment is relatively large because the air breakdown plasma spreads along the focus cone of the laser beam. When the small amount of ablated atoms from the target expands into this volume, more homogenous plasma with low density can be formed. As a result, the emission lines with negligible self-absorption can be realized. It should be noted that the plasma also satisfied the necessary condition of local thermodynamic equilibrium (LTE) (see Appendix). Figure 3 shows the K I 766.4 nm and K I 769.9 nm emission lines obtained by setting d = 1 mm. As shown in Fig. 3, negligible self-reversal is still observed in the emission line of K I 766.4 nm when the laser energy was set at 70 mJ (laser fluence is 1.23 kJ/cm2 and laser intensity is 154 GW/cm2), because the ablation amount of the target is large at high laser energy. However, when we reduced the laser energy to 54 mJ (laser fluence is 0.951 kJ/cm2 and laser intensity is 119 GW/cm2), the self-reversal disappeared, and sharp and narrow emission lines of K I were observed. The self-reversal-free emission line can also be achieved using a high-energy laser by increasing the focusing distance of the laser beam from the target surface (d = 2 mm), as shown in Fig. 4. In this condition, the shockwave that reaches the target surface is not as strong as in the previous condition, i.e., when d is set to 1 mm. As a result, the ablation amount of the target is not as high as in the previous case.
Fig. 3. Emission spectra of K I 766.4 nm and K I 769.9 nm from a pure KCl pellet obtained using parallel irradiation with d = 1 mm and laser energies of 54 and 70 mJ.
Fig. 4. Emission spectra of K I 766.4 nm and K I 769.9 nm from a pure KCl pellet obtained using parallel irradiation with d = 2 mm and laser energies of 54 and 70 mJ.
In the next experiment, we conducted a series of measurements using several KCl samples with 6 different K concentrations ranging from 0.983 wt.% to 52.4 wt.%, which were prepared from the mixture of pure KCl with NaCl. In this experiment, we set d = 1 mm and laser energy at 54 mJ. We set d = 1 mm because the generated air breakdown plasma has a lateral size of around 2 mm when the laser energy is set at 54 mJ. Therefore the distance (d) between the focused laser beam and the sample surface should be around 1 mm, which is half of the plasma lateral size. Figure 5(a) shows the plot of the measured emission intensities of K I 766.4 nm with respect to the associated K concentration. The self-absorption effect on the spectral line is evaluated from the self-absorption coefficient (SA), which is defined as ratio of the intensity of the measured spectral (I) to the intensity of the ideal spectral without self-absorption (I0), and expressed as [13]
where k is the absorption coefficient and l is the absorption path length. Since $kl$ is proportional to the elemental concentration, the self-absorption coefficient can also be expressed as [19,21,22]. where $\alpha $ is the self-absorption factor and C is the elemental concentration that can be inferred from the exponential fitting of the spectral intensity with respect to the elemental concentration [33,35]. From Eq. (2), we can see that negligible self-absorption is observed when the $\alpha $ value is small or the the SA value is close to one.Fig. 5. (a) Calibration curve for K I 766.4 nm obtained using parallel irradiation with d = 1 mm at atmospheric pressure. (b) The corresponding self-absorption coefficients SAs of K I 766.4 nm.
From the exponential fitting, we found that the value of $\alpha $ is 0.011, indicating negligible self-absorption. The exponential fitting value $R^2$ was 0.9997, indicating the reliability of the $\alpha $ value obtained using the calibration model. It is also observed from Fig. 5(a) that the calibration curve exhibits a linear relationship over a wide measurement range of 0.983-19.7 wt.% and intercept near 0 point. The SA values of K I 766.4 nm for 6 different K concentrations are shown in Fig. 5(b). The SA’s are close to 1 over a wide concentration range of 0.983-19.7 wt.%, which indicates the effectiveness of our proposed technique in suppressing the self-absorption effect at atmospheric pressure using a single pulse laser.
4. Conclusion
In summary, we have demonstrated a simple method for suppressing the self-absorption effect in single-pulse LIBS in atmospheric pressure by employing parallel laser irradiation. By adjusting the laser energy and focusing distance of the laser beam to the sample surface, a free SA resonance emission line of high K concentration can be achieved. We believe that the effectiveness of the proposed method in suppressing the self-absorption effect can help improve the analytical performance of ordinary single-pulse LIBS.
Fig. 6. (a) Emission spectra of Cu I 510.5 nm, Cu I 515.3 nm, and 521.8 nm. (b) Emission spectra of H I 656.2 nm.
Appendix
In this work, we used the McWhirter criterion to verify the LTE condition. We made a pellet sample which was prepared from fthe mixture of 10% CuSO4 powder and 90% KCl powder to estimate the plasma temperature. Figure 6(a) bellow shows the emission spectra of the Cu I 510.5 nm, Cu I 515.3 nm, and Cu I 521.8 nm. The plasma temperature was estimated to be 6800 K from the Boltzmann distribution by calculating the intensity ratio of Cu I 521.8 nm and Cu I 510.5 nm emission lines. The electron density inside the plasma was estimated around 1.2 × 1017 cm-3 by using the linewidth of H I 656.2 nm emission line [Fig. 6(b)] [36]. This value is much larger than the McWhirter criterion (∼5.5 × 1014 cm-3). Therefore, the necessary condition for LTE is satisfied.
Funding
This work is partially supported through a Basic Research Grant in Physics, The Academy of Sciences for the Developing World, Third World Academy of Sciences (TWAS) under contract number (060150 RG/PHYS/AS/UNESCO FR: 3240144882) This project is also partially funded by Badan Riset dan Inovasi Nasional through research grant under contract numbers (1267/LL3/PG/2021, 163/SP2H/LT/DRPM/2021, and 172/LPPM-UPH/IV/2021); and by the Indonesia Ministry of Research, Technology, and Higher Education [Kementerian Riset, Teknologi dan Pendidikan Tinggi through a research grant under contract numbers (225/SP2H/LT/DRPM/2019, 088/LL3/PG/2020, 039/VR.RTT/IV/2019)].
Acknowledgments
The authors express their sincerest thanks to the late Prof. M. O. Tjia for longstanding guidance.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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