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Abstract
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.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The recent development of laser-induced breakdown spectroscopy (LIBS) as a new and versatile analytical tool has attracted many researchers worldwide, as demonstrated by the increasing number of publications in that field. Research on its basic mechanism of excitation, including the study of plasma temperature, electron density, local thermodynamic equilibrium (LTE) and several models for explaining plasma formation, has been pursued with great enthusiasm and has been well documented. Its initial uses in qualitative and quantitative analysis of solids, liquid, gaseous and aerosol samples have diversed into a new field of biological applications, including reagentless analysis of molecularly complex biological materials or clinical specimens [1–3]. These developments have been further followed by additional applications of LIBS research on environmental pollution, including river, lake and sea contamination; archeological studies; isotopic analysis; chemical warfare agents and explosive material analysis; and mineral and rock analysis, including a recent expedition to Mars [4–12]. The rapid development of LIBS is mainly due to its rapidity (no sample preparation), sensitivity (down to ppb level detection limit), low cost investment for equipment, and applicability for performing in-situ/field analysis. Another great advantage of LIBS over other established analytical techniques is its nondestructive analysis, which means that this technique can be applied to high value samples, such as antique or ancient picture paint analysis and jewelry analysis.
However, it is well known that the effect of self-absorption occurs in LIBS, especially for emission lines originating from direct resonant transition involving the ground state and at high concentrations of the analyte atoms [2]. The appearance of this effect is related to the failure to obtain linear calibration lines, which is crucial for reliable quantitative analysis. Many attempts have been made to minimize the self-absorption effect, for example, by using the columnar density Saha-Boltzmann plot method proposed by Cristoforetti et al., [13] and the C-sigma model proposed by Aragon et al., [14]. Both techniques employed self-absorption emission lines in LIBS. However, their mathematical complexity limited the application of both models. Recently the two methods have been merged as an extended C-sigma approach, which simplifies mathematics [15]. Another experiment performed by Hai et al., [16] concluded that an argon atmosphere is preferable for decreasing self-absorption and that appropriate selection of the time window for the detection system effectively prevents self-absorption. Another interesting work by Xiong et al., [17] proposed the use of fiber laser ablation LIBS and found a significant reduction in self-absorption for Mg and Ca lines. Another impressive work, performed by Ming et al., [18] in decreasing self-absorption in K, Mn, and Al, employed LIBS assisted by laser-stimulated absorption (LSA-LIBS). They also showed a reduction in the full width at half maximum (FWHM) of those self-reversed emission lines.
The self-absorption effect, which disturbs the linearity of the calibration curve of Mn in steel, was also studied by Tang et al., [19]. They studied the temporal evolution of the self-reversed emission line of Mn and found that the detection window of 0.2–0.4 µs yielded the best result for the suppression of self-absorption of Mn in steel. The same authors, Tang et al., [20] also proposed microwave-assisted excitation in LIBS (MAE-LIBS) to reduce the effect of self-absorption in a wide spectral range of 200–900 nm. They also achieved a small reduction in the FWHM of the self-reversed emission lines. Due to the high impact of the self-absorption effect on the linearity of the calibration line in LIBS, many studies have also been proposed that cannot be pointed out one by one in this manuscript. However, they can be considered excellent references for the study of self-absorption [21–37].
In a series of our previous publications, which were mainly dedicated to the analysis of H and D in a zircaloy tube used in a nuclear power plan [38–41], we found that the H and D emission lines, which are separated by only 0.18 nm, can be excellently resolved by employing excitation through a Penning-like energy transfer process from He metastable excited state atoms to the analyte atoms. In this case, a special double-pulse technique is developed in which the first laser is used to create a strong He gas plasma, and the second laser is employed to ablate the target. The He gas plasma is generated first, and after a certain delay, the ablation laser is fired. The most important point of this technique is that the delay between the two-laser systems should be optimally searched; namely, the ablated target atoms should be in the ground state before entering the previously generated, relatively cool He gas plasma. In such cases, the excitation of the ablated target atoms is due solely to the Penning-like energy transfer process through collision between the ablated atoms in their ground state with the metastable excited He atoms, which have a long enough life time of approximately 50 µs. We have coined the acronym LIPS-He* (laser-induced plasma spectroscopy utilizing the He metastable excited state) for this technique [42].
Encouraged by the above unusual result, we naturally sought further elucidation and substantiation of the previous experimental results to obtain further evidence that this technique can completely suppress self-absorption since the excitation of the ablated target is not thermal excitation. This experiment is conducted by using pellet samples of pure KCl and NaCl as well as standard samples containing high concentrations of Al. In addition, it is also important to show the linearity of the calibration curve of high-concentration Al alloy. To that end, two sets of samples are prepared. The first set is a mixture of KCl and SrCl2 pellet samples containing different K concentrations, and the second set of samples are an aluminum alloys containing 99.8%, 93.0% and 87.0% Al. Finally, the FWHM values of Na, K and Al were also measured in this study.
2. Experimental setup
Figure 1 shows the experimental setup used in this work, which is almost the same as that reported in our previous works [42]. It consists of a two-laser system as an irradiation source, a sample holder with no chamber and a high-resolution spectrograph to analyze the plasma emission from the target. The two-laser system is exactly the same as the one reported before [42], but it is described again in Fig. 1 for easy reference. The first laser is a YAG laser, which operates at a wavelength of 1,064 nm (Quanta Ray, LAB 130-10, USA, 10 Hz). The laser energy is fixed at 122 mJ throughout the entire experiments. This laser is focused from above the sample holder with a quartz lens of 150 mm focal length to generate a He gas plasma approximately 6 mm from the sample surface. The helium gas used in this experiment is supplied by Air Liquid with 5N purity and delivered to the gas breakdown area through a flexible pipe at a constant rate of 5 l/min. In this stage, we confirmed the production of strong He gas plasma by the occurrence of a strong orange color associated with the triplet He I 587.5 nm emission and singlet He I 667.8 nm emission, as shown in the inset of Fig. 1. When the gas flow is reduced below 3 l/min, air breakdown plasma accompanies the He gas plasma, and this condition should be avoided. The second YAG laser with a 355 nm wavelength is used for target ablation (Quanta Ray, INDI 10, USA, 10 Hz and operated at 15 mJ during all experiments). This laser is horizontally focused on the sample surface using a quartz lens of 100 nm focal length to create the target plasma, as shown in the inset of Fig. 1. Based on repeated measurements, it was found that a 3 µs delay of the second laser pulse produces the optimal quality needed to suppress the self-absorption of the emission spectrum.
With this special arrangement, both the He gas plasma and target plasma will have significantly cooled down upon the arrival of the ablated atoms in the He gas plasma, as depicted in the inset of Fig. 1. The emission spectra around the He gas plasma are collected by an optical fiber with one of its ends positioned 3 cm alongside from the He gas plasma so that the fiber will collect emissions only from the region of the He gas plasma without interference from the target plasma, as shown in Fig. 1. Another end of the fiber is connected to the input slit of a spectrograph (Andor model 2061, focal length 1000 mm, f/8.6 Czerny Turner configuration) with its exit slit coupled to a gated intensified charge-coupled device (ICCD; Andor iStar’ intensified CCD 1,024 × 256 pixels, UK). A digital delay generator (DDG 535, Stanford Research System, USA) is used to trigger the ICCD and to synchronize the two lasers. The gate delay and gate width of the ICCD are fixed at 200 ns and 30 µs, respectively, after the initiation of the second laser during all of the experiments.
The sample used in this experiment were KCl and NaCl powder (Wako Chemical, Japan, 4N) and standard Al sample containing 87.0%, 93.0% and 99.8% Al. The powder samples were ground until the grain size was approximately 50 µm and then pressed into pellet at 30 MPa pressure for approximately 90 s. The resulting pellet had a diameter of 10 mm and a thickness of 2 mm. All experiments were conducted under the fixed sample position with 5 data accumulations.
3. Results and discussion
Prior to data acquisition, it is important to know whether the target will be ablated if only the He gas plasma is generated. This is closely related to the energy of the first laser and the distance between the He gas plasma and the target surface. However, we need strong He gas plasma to excite the ablated atoms of the target. Therefore, for this purpose, we fixed the first laser energy at approximately 122 mJ, which generates strong orange He gas plasma. Under this condition, we varied the distance between the He gas plasma and the target surface, and we concluded that at 6 mm separation, there is no target ablation when only the first laser is used. Therefore, this 6 mm distance separation was used throughout the ensuing experiments.
Figure 2 (dashed line) shows the emission spectra of the KCl pellet when only the second laser is fired in an ambient air at asmospheric pressure. A strong self-absorption was observed for the K I 766.4 nm and K I 769.9 nm emission lines, with an extremely large FWHM of approximately 4.8 nm for the K I 766.4 nm emission line. When both lasers fire with the first laser fired at 3 µs before the second laser in ambient asmosphere of He gas, the appearance of the spectrum is completely changed, as shown in Fig. 2 (solid line). The FWHM of K I 766.4 nm decreases significantly from 4.8 nm to 0.8 nm. In addition, the emission intensity of the K emission lines is approximately the same as in the case of single-laser operation. This means that the total number of K atoms involved in the LIPS-He* is not smaller than the number involved in the thermal excitation, as in case of single laser. Inset in Fig. 2 shows the emission spectrum of the KCl pellet in ambient air obtained by employing double-pulse LIBS configuration. The appearance of dip in the emission spectrum clearly indicates the present of the self-absorption effect. Therefore, this result supports the role of LIPS-He* in supressing the self-absorption effect.
Fig. 2. The emission spectra of the KCl pellet showing the emission lines of K I 766.4 nm and K I 769.9 nm when only the second laser is used in surrounding atmospheric air (dashed line), and when the LIPS-He* configuration is used in surrounding atmospheric He (solid line). The first laser is fired 3 µs prior to the second laser. The gate delay and gate width of the ICCD are fixed at 200 ns and 30 µs after the initiation of the second laser.
The same experiments were also carried out for the NaCl pellet, and Fig. 3 (dashed line) presents the emission spectrum of the NaCl pellet when only the second laser is fired, while Fig. 3 (solid line) shows the spectrum when both lasers are fired with the first laser fired at 3 µs before the second laser in an ambient atmosphere of He gas. Similar effects are observed as in the case shown in Fig. 2, and the FWHM of Na I 588.9 nm is 1.4 nm in the case of a single-pulse laser and only 0.15 nm when a LIPS-He* configuration is used.
Fig. 3. The emission spectra of the NaCl pellet showing the emission lines of Na I 588.9 nm and Na I 589.5 nm. Dashed line and solid line are the same condition as in Fig. 2.
We also used an aluminum (Al) alloy samples with Al concentrations of 87.0%, 93.0% and 99.8%. The emission spectra for both single-laser irradiation and the LIPS-He* technique are shown in Fig. 4 for the 99.8% Al sample. Self-absorption clearly occurs in Al I 394.4 nm and Al I 396.1 nm emission lines when only a single laser is used, as shown in Fig. 4 (dashed line). Moreover, the FWHM of the emission line Al I 396.1 nm is 0.44 nm for the case of a single-laser and 0.12 nm when LIPS-He* is employed, as shown in Fig. 4. By using the spectral line of Al I 396.1 nm in Fig. 4, we can estimate the electron density in the plasma to be 0.35×1016/cm3. Similar results to the experiments shown in Fig. 4 were also obtained when the 99.8% Al sample was replaced with an Al alloy sample containing 93.0% Al, and the result is presented in Fig. 5. The figure shows the same pattern of Al emission lines as in the case of the 99.8% Al sample. The FWHM in this case is also narrower for LIPS-He*. Figure 5 shows the proportionality of the emission intensity of Al I 396.1 nm compared with Fig. 4. Encouraged by this result, a calibration line was obtained for single-laser irradiation and LIPS-He* (Fig. 6). Actually, an emission line with self-absorption should not be used for making a calibration line. However, for comparison, a calibration line obtained from single-laser irradiation is also shown. In the case of LIPS-He*, the calibration line of Al I 396.1 nm is exactly a straight line with the intercept at zero.
Fig. 4. The emission spectra of Al alloy containing 99.8% Al showing the emission lines of Al I 394.4 nm and Al I 396.1 nm. Dashed line and solid line are the same condition as in Fig. 2.
Fig. 5. The emission spectra of Al alloy containing 93.0% Al showing the emission lines of Al I 394.4 nm and Al I 396.1 nm. Dashed line and solid line are the same condition as in Fig. 2.
Fig. 6. Calibration line of Al I 396.1 nm taken from samples containing 99.8%, 93.0%, and 87.0% Al based on the same condition as in Fig. 2.
However, we have only 3 Al alloy samples with different concentrations (87.0%, 93.0% and 99.8% Al sample). The most interesting part of this result is the fact that even at such high concentrations of Al, there was no saturation effect in the calibration curve. This means that all of the Al atoms entering the He gas plasma are totally excited by the He metastable excited state atoms. This result is completely different when thermal excitation is in effect, which is another advantage of excitation through He* (LIPS-He*) in addition to narrow FWHM, high S/N and the capability of exciting elements with high electronic energy [42].
Finally another calibration line was also prepared by using pellet samples. We combined KCl and SrCl2 samples to make pellets in which the final concentration of K was 21%, 28.3%, 33.4% and 40%. Using these four kinds of samples, we likewise obtained a linear calibration line with zero intercept, as shown in Fig. 7. This again means that all K atoms entering the He gas plasma are excited through He*, as in the case of Al alloy. Last, it should be mentioned that in this work, we used very high-concentration samples to demonstrate the total suppression of self-absorption in LIPS-He*. For example, in the Al case, we used high concentrations of Al ranging from 87.0% - 99.8% and for the K case, we used high concentrations of K ranging from 21% - 40%.
Fig. 7. Calibration line of K I 766.4 nm taken from pellet sample containing 21%, 28.3%, 33.4% and 40% K in a mixture of KCl and SrCl2. The data were obtained following the procedure in Fig. 2 for LIPS-He*.
4. Conclusion
It has been shown in this work that LIPS-He* completely suppress the self-absorption that occurs in ordinary LIBS, which disturbs the linearity of the calibration line. This is possible because the excitation of the analyte atoms occurs through a Penning-like energy transfer process in which the ablated atoms are excited through collision with the He metastable excited atoms in the He gas plasma. We believe that He gas plasma can be used for future excitation sources in atomic emission spectroscopy.
Funding
Third World Academy of Sciences (060150 RG/PHYS/AS/UNESCO FR:3240144882); Kementerian Riset Teknologi Dan Pendidikan Tinggi Republik Indonesia (039/VR.RTT/V/2019, 12/AKM/PNT/2019, 7/E/KPT/2019).
Acknowledgments
The authors intended to express their sincerest thanks to the late Prof. M.O. Tjia for his guidance during experiments and manuscript preparation.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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