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Abstract
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.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser-induced breakdown spectroscopy (LIBS) is a multielement analytical technology. In LIBS, a focused laser beam ablates a sample to generate plasmas; and then the optical emission emitted by the plasma is collected for elemental analysis [1–3]. Because of the advantages of real-time, in situ analysis and no sample preparation, LIBS has great potential in many areas, such as steel detection [4,5], cultural heritage [6–8], space exploration [9], archeological verification [10,11], the coal industry, and other fields [12,13]. Although conventional LIBS was developed quickly in recent years, the complex optical system structure limits its industrial applications in complex and versatile environments [14–17].
Fiber-optic laser-induced breakdown spectroscopy (FO-LIBS), which delivers laser pulses by optical fiber cable [2,15], is a promising method for portable LIBS equipment. For instance, Davies et al. [18] utilized an FO-LIBS system to achieve remote detection in hostile environments. Rai et al. [14,19,20] demonstrated an FO-LIBS system to detect molten Al alloy in a laboratory furnace. Dumitrescu et al. [16] reported the first gas-phase LIBS measurements using a fiber-optically delivered spark. Saggese et al. [21] presented a comparison of the performance of a LIBS system with free space delivery of the laser beam versus the performance using an optical fiber probe. Guirado et al. [22–24]conducted a series of experiments to evaluate a remote LIBS instrument based on a fiber optic cable to recognize and identify archeological materials submerged in seawater at depths of up to 30 m.
Nevertheless, because of the complicated laser-matter interactions, the plasma induced by the fiber-optic laser beam is often inhomogeneous [25]. In ideal LIBS conditions, the plasma is optically thin and the light emitted from the center of the plasma will not be re-absorbed [26]. When the plasma is optically thick and inhomogeneous, there are not only excited atoms but also cold atoms. The emission light emitted from the excited atoms will be reabsorbed by the same kind of cold atoms [27,28]. Re-absorption cannot be neglected especially when the plasma inhomogeneity is strong. Thus, this plasma inhomogeneity may cause the self-absorption effect and, hence, significantly hinder the determination in FO-LIBS. Generally, two approaches have been proposed to overcome the self-absorption effect in LIBS by researchers. In the first approach, some researchers were focusing on modifying the experimental setup to solve the self-absorption problem [29–31]. In the second approach, some other researchers were interested in employing data processing methods to sample spectra [32,33]. These methods have improved the determination successfully in conventional LIBS, while it seems too complicated for FO-LIBS. Thus, it is desirable to find a simpler method to improve the determination in FO-LIBS.
Spatially resolved technology is a simple method to obtain higher quality spectra by avoiding self-absorption parts in plasma. Many researchers have studied the spatially resolved spectra in LIBS [34–39]. However, up to now, no studies of using the spatially resolved method to get a better determination in FO-LIBS have been reported.
In this work, we utilized spatially-resolved acquisition to solve the self-absorption effect and accuracy deterioration caused by plasma inhomogeneity in FO-LIBS. Iron (Fe), magnesium (Mg), and zinc (Zn) elements in aluminum alloy were selected as examples to verify the feasibility of spatially resolved FO-LIBS.
2. Experiments and methods
2.1 Experimental setup
The schematic diagram of the FO-LIBS setup used in this study is shown in Fig. 1(a). A compact Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) pulsed laser (wavelength: 532 nm; pulse duration: 7 ns; and repetition rate: 10 Hz) was used as the ablation source. Transmitted through a 3 m long optical fiber (core diameter: 1 mm) and reflected by a dichroic mirror, the laser beam was focused onto the sample surface by an ultraviolet (UV)-grade quartz lens with a focal length of 100 mm. The sample was placed on an X-Y-Z translation platform. The plasma spectrum was transmitted through two lenses and then collected by an echelle spectrometer (Andor Tech., Mechelle 5000, wavelength range: 240–880 nm; spectral resolution: λ/∆λ = 5000). The spectrometer is equipped with an intensified charge-coupled device (ICCD) (Andor Tech., iStar 334T). The collecting direction was perpendicular to the laser beam. The laser pulse energy measured before the sample was about 24 mJ, which was limited by the damage threshold of laser-delivering fiber. The estimated laser fluence was about 4.78 J/cm2. To reduce the intensity deviation, each spectrum was collected using 50 laser shots, repeated 10 times at different places.
Fig. 1 (a) Schematic diagram of the experimental setup and (b) self-defined coordinate system for the plasma and detection direction.
The coordinate system for FO-LIBS of plasmas was defined as shown in Fig. 1(b), and the collecting position was adjusted by two micrometers along the X-axis. The detection direction of the plasma emission was along the Y-axis.
2.2 Samples
The six certified aluminum alloy samples used in this work were purchased from Southwest Aluminum (Group) Co., Ltd. The matrix elements in all of these samples were aluminum with a content of over 90%. The concentrations of the Fe, Mg, and Zn elements are listed in Table 1.
Table 1. Certified concentrations (wt. %) of Zn, Mg, and Fe in the aluminum alloy samples.
2.3 Quantitative criteria
The R2 is the determination coefficient, which provides a measure of how well-observed outcomes are replicated by the calibration curves. In this study, R2 is given as:
where and represent the value of the concentration and signal intensity ratio of the sample i, respectively; and and represent the average value of and over n samples, respectively.The root mean square error of cross-validation (RMSECV) was applied to evaluate the predicted results using the leave-one-out cross-validation (LOOCV) method. It is mathematically expressed as:
where and represent the certificated and predicted concentration of sample i, and n represents the number of samples.2.4 Self-absorption factor
The self-absorption factor α is defined as a coefficient to evaluate the self-absorption effect in this work [40,41], which can be obtained according to the exponential fitting parameters of the calibration curve. For non-optically thin plasma, the self-absorption coefficient (SA) can be expressed as [42]:
where k(λ0) is the absorption coefficient, l is the absorption path length. k(λ0)l represents the optical depth, which is proportional to the concentration of element C [34]. Assuming that k(λ0)l = αC, Eq. (3) can be transformed as:Thus, the higher the α value is, the smaller the SA value is, and the more serious the self-absorption effect is.
3. Results and discussion
3.1 Quantitative analysis with conventional FO-LIBS
The characteristic emission spectra of Fe, Mg, and Zn species obtained by conventional FO-LIBS are shown in Fig. 2(a). The spectra were acquired from Sample No.E312. To get a sufficiently large line intensity for analytical elements to perform quantitative analysis, we chose Fe I 438.41 nm, Zn I 481.13 nm, and Mg I 516.83 nm as the analysis spectral lines. Temporal evolutions of signal-to-noise ratios (SNRs) of three species are depicted in Fig. 2(b). We can get the best SNRs for three species when the gate delay time is 0.5 µs and gate width is 1 µs. Thus, we have selected 0.5 µs and 1 µs as the gate delay and gate width values for quantitative analysis.
Fig. 2 (a)The characteristic emission spectra of Fe, Zn, and Mg obtained by FO-LIBS and (b) the evolution of SNR as a function of gate delay time.
To analyze the content of each element, internal standardization was adopted as the quantitative analysis method, which used intensity ratios (analysis line/background near the analysis line) instead of peak heights to construct the calibration curve. It is capable of minimizing shot-to-shot variations in the LIBS emission signals and improving measurement stability.
Figure 3 shows the calibration curves for (a) Fe, (b) Mg, and (c) Zn. Notably, these species were fitted with both linear functions and exponential functions. The determination coefficients R2 of linear fitting for these three species were too low to perform quantitative analysis. In contrast, the exponential functions showed much higher determination coefficients R2 (>98%), proving the existence of the self-absorption effect for these lines. Thus, we calculated the self-absorption factors (α) of the three lines, and all of these species showed great self-absorption.
To illustrate the quantitative results clearly, Table 2 shows the R2 and RMSECVs of linear curve fitting and self-absorption factors obtained from exponential functions for the three species. As listed in the table, the quantitative results of conventional FO-LIBS were not good because of the existence of the self-absorption effect. Thus, we tended to investigate the characteristics of plasma plumes generated by FO-LIBS systems to reduce the self-absorption effect and improve the accuracy of quantitative analysis.
Table 2. Quantitative results of Fe, Mg, and Zn with linear fitting in conventional FO-LIBS.
3.2 Spectral intensity distribution
Considering that the self-absorption effect is caused by plasma inhomogeneity, we observed the temporal evolution of the plasma plumes of Sample No.E312a. Figure 4 shows the fast images of five different delay times (marked in the pictures). These images were acquired using an attached Andor ICCD (iStar DH334T) with a Nikon lens (105 mm, f/2.8 G). Each image was obtained by accumulating 50 pulses. As shown in Fig. 4, the spectral intensity had a big drop beyond the delay time of 1.0 μs and became too weak to measure beyond the delay time of 1.5 μs. In other words, the lifetime of the plasma induced by FO-LIBS was approximately 1.5 μs.
However, fast imaging was not spectrally resolved. To obtain the separated intensities of different species in the plasma, we also took the Sample No.E312a as an example and observed the spectral intensities of analytical lines with different acquisition positions along the X-axis at Z 0.3 mm. (As shown in Fig. 1(b), the Z-axis is perpendicular to the sample surface; and the X-axis is parallel to the sample surface.) We selected 11 positions along the X-axis in the range of −1.5 mm to 1.5 mm, and the interval of each position was 0.3 mm. The intensities displayed in the picture were normalized. As shown in Fig. 5, the spectral intensities of different elements were distributed approximately. Intensities at the central part (X = 0 mm approximately) were lower than the peripheral part (X = ± 0.9 mm approximately). The positions where X was greater than 1.2 mm showed very low intensities, suggesting the absence of laser-induced plasma. In conclusion, the intensity near X = 0 mm was the lowest, the intensity near X = 0.9 mm was the highest, and there were few plasma emissions at X = 1.2 mm.
Fig. 5 Normalized intensity of Fe, Mg, and Zn with different acquisition positions along the X-axis.
Generally, fast imaging and spectrally resolved intensities of the sample are consistent. The coordinate value X = 0 mm had lower spectral intensities, which was consistent with the plasma center area of fast imaging. The coordinate value X = 0.9 mm had the highest intensities, which corresponded to the position of strongest optical emission in the plasma. The coordinate value X = 1.2 mm had much lower spectral intensities, which corresponded to the edge of the plasma with weak optical emission spectra. Along the X-axis, the plasma distribution was inhomogeneous. The emission of the plasma increased slowly from X = 0 mm to X = 0.9 mm and then decreased rapidly from X = 0.9 mm to X = 1.5 mm.
3.3 Quantitative analysis with spatially resolved FO-LIBS
In this work, we selected some typical positions along the X-axis to analyze the quantitative accuracy. As shown in Section 3.2, the plasma was almost symmetrical, and the emission at X = 1.5 mm was too low. Thus, we selected x = 0 mm, 0.3 mm, 0.6 mm, 0.9 mm and 1.2 mm as the typical acquisition positions, and the Z-axis coordinate was 0.3 mm. The coordinate was depicted in Fig. 6, and the emission from a certain position would not be affected by other parts since the core diameter of collecting fiber was small enough for the whole plasma. The experimental parameters were the same as the experiment in Section 3.1 except the acquisition positions.
Calibration curves of the linear fitting for (a) Fe, (b) Mg, and (c) Zn with five different acquisition positions are shown in Fig. 7. For these species, the best determination coefficient R2 and RMSECV occurred at X = 1.2 mm, X = 1.2 mm, and X = 0.9 mm, respectively. The determination coefficients R2 for these species were all above 0.96, much higher than the quantitative results mentioned in Section 3.1, which means the higher reliability of the calibration curving fitting.
Fig. 7 Calibration curves of linear fitting for (a) Fe, (b) Mg, and (c) Zn with spatially resolved FO-LIBS.
Figure 8 shows the acquisition position dependence of α and plasma temperature. The self-absorption factor α of each position was obtained from the exponential fitting function, and the plasma temperature was estimated using iron atomic lines of Sample No.E312a by the Boltzmann plot method. The Boltzmann plot method rests on the essential assumption that the plasma is in local thermal equilibrium (LTE), which can be considered to be satisfied when the electron density is high enough to ensure a high collision rate. The corresponding lower limit of electron density is given by the McWhirter criterion [43], which was in the range of 1.4 1016 for all five positions, while the electron densities were in the range of 3.0 1016 calculated by Stark broadening method. Therefore, the LTE conditions have been fully satisfied.
As shown in Fig. 8, the spectra collected at X = 0 mm have the highest α value, while those collected at X = 1.2 mm have the lowest α value. The plasma temperatures and self-absorption factors for these species of different acquisition positions had opposite trends. The position which had the lowest self-absorption factors (X = 1.2 mm) had the highest temperature.
The self-absorption effect is one of the main flaws of determination and always caused by reabsorbing the photon emitted by the same species. Considering that high-density atoms usually have characteristically strong temperatures and electron density gradients, the cold atoms and excited atoms distribute differently in the plasma. In our experiment, the lowest temperature appeared at X = 1.2 mm, which means this area should be populated by cold atoms, residing mostly in the ground state. While the highest temperature appeared at X = 1.2 mm, which means this area should contain a higher density of excited atoms. The emitted photons corresponding to resonance transitions will have a high probability of being absorbed by the colder atoms in the low-temperature layer at X = 0 mm, thereby reducing the observed intensity of the emission line. This is why the position at X = 0 mm showed the strongest self-absorption effect. Thus, selecting a position with a high temperature can reduce the self-absorption effect and get a better determination.
To compare the quantitative analysis abilities of conventional LIBS and spatially resolved FO-LIBS, we calculated some parameters to measure the quantitative results of these two methods, as listed in Table 3. For spatially resolved FO-LIBS, we selected the position with the highest determination coefficient (X = 1.2 mm) as the representative. The spatially resolved FO-LIBS greatly improved the accuracy of quantitative analysis, measured by the determination coefficient R2 and RMSECV. The determination coefficients R2 were prompted from 0.918 to 0.986 for the Fe species, from 0.951 to 0.965 for the Mg species, and from 0.786 to 0.983 for the Zn species; and the RMSECVs were reduced from 0.388 to 0.172 wt. % for the Fe species, from 0.348 to 0.224 wt. % for the Mg species, and from 0.097 to 0.024 wt. % for the Zn species. Simultaneously, the self-absorption factor of α was greatly reduced from 1.364 to 0.199 for the Fe species, from 0.995 to 0 for the Mg species, and from 10.487 to 3.025 for the Zn species.
Table 3. Comparison of quantitative results of conventional FO-LIBS and spatially resolved FO-LIBS.
4. Conclusions
In this work, we proposed spatially resolved FO-LIBS to improve the quantitative accuracy with the aluminum alloy samples because of the inhomogeneous distribution of elements and plasma temperatures in plasma. The results showed that the self-absorption effect can be reduced by selecting different acquisition positions along the X-axis. The RMSECVs for Fe, Mg, and Zn improved from 0.388, 0.348, and 0.097 wt. % to 0.172, 0.224, and 0.024 wt. %, respectively. The R2 of linear fitting for Fe, Mg, and Zn species improved from 0.918, 0.951, and 0.786 to 0.986, 0.965, and 0.983, respectively. This work demonstrated that spatially resolved FO-LIBS is an effective approach to reducing the self-absorption effect and getting a better analytical performance.
Funding
National Natural Science Foundation of China (No. 61705064); Project of the Hubei Provincial Department of Education (No. B2016183); Natural Science Foundation of the Hubei Province (2018CFB773).
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