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Abstract
Boron is an essential element for industry, but it is hard to accurately and rapidly determine high boron content with conventional laser-induced breakdown spectroscopy (LIBS), due to the matrix and self-absorption effect. Using molecular emission is an alternative method for boron content analysis, but its weak spectra are major challenges. Here, boron monoxide (BO) radicals were used to establish calibration assisted by LIBS and laser-induced radical fluorescence (LIBS-LIRF). Two types of BO radical excitations, vibrational ground state excitation (LIRFG) and vibrational excited state excitation (LIRFE), were compared. The results showed that LIRFG achieved better sensitivity with a limit of detection of 0.0993 wt.%, while the LIRFE was more accurate with a root mean square error of cross validation of 0.2514 wt.%. In conclusion, this work provided a potential approach for molecular emission analysis with LIBS-LIRF.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Boron has been widely used in agriculture [1], pharmaceutical [2], metallurgical [3], and nuclear industries [4, 5]. The applications’ properties, such as high hardness, melting resistance, electrical conductivity, and chemical inertness, usually depend on its boron content. The standard boron analytical methods include the colorimetric curcumin method [6], inductively coupled plasma optical emission spectrometry (ICP-OES) [7], and atomic absorption spectrometry (AAS) [8], etc. Generally, specifically trained operators and complex, time-consuming operations in these methods are both required, which hinders the applications of these methods. Therefore, it is urgent to search for a fast and accurate analytical method for boron content determination.
Laser-induced breakdown spectroscopy (LIBS) is an ideal spectrometry technique for the material components analysis. Having advantages of rapid, in situ, real-time, remote analysis and simple sample preparation, LIBS has been widely used in the fields of space exploration [9], solar energy [10], environmental protection [11], food monitoring [12], and other fields [13–16]. Atomic emission, generally used in conventional LIBS, still has several drawbacks [17], such as the matrix [18] and saturation effects [19]. To overcome these drawbacks, researchers have been concentrating on molecular emission in recent years. For example, Bhatt et al. [20] performed a comparative study of the quantitative and qualitative analysis of atomic and molecular emission from LIBS spectra to improve the accuracy of strontium analysis. Pisonero et al. [21] investigated molecular emission to determine the fluorite (CaF2) mass content of powdered ore samples. Yao et al. [22] adopted molecular CN to rapidly measure unburned carbon in fly ash using LIBS. All of these works proved the feasibility of element determination using molecular emission, but one of the major challenges of molecular emission is its weak spectra. Thus, to enhance the intensity of spectra in molecular emission, laser-induced breakdown spectroscopy, assisted by laser-induced radical fluorescence (LIBS-LIRF), was adopted. The plasma created by LIBS was excited with a wavelength-tunable laser to emit fluorescence. The effectiveness of spectra enhancement of atomic emission in LIBS has been reported in various elements, such as thallium in soil [23], uranium in glass [24], iron in aqueous solution [25], phosphorus in steel [26], and lead in brass [27]. Considering these applications of LIBS assisted by laser-induced fluorescence (LIF), it is probable that molecular spectra can be enhanced with the assistance of laser-induced radical fluorescence (LIRF). However, few studies have been reported about the enhancement of molecular emission with LIBS-LIRF [28].
In this study, a new approach for accurate, fast determination of boron content with molecular emission using LIBS combined with LIRF was studied. Two BO excitations, vibrational ground state excitation (LIRFG) and vibrational excited state excitation (LIRFE), were investigated to enhance the spectra of molecular emission based on the LIBS-LIRF principle.
2. LIBS-LIRF Principle
The LIBS-LIRF used in this work was nonresonance fluorescence. The intensity of fluorescence emitted by the measured radicals Ifυ is given by [29]:
where Iαυ represents intensity of the excitation laser which is absorbed by measured radicals at frequency υ; α represents the attenuation coefficient of fluorescence, which is mainly caused by the self-absorption effect; Φ represents the fluorescence efficiency, which is the ratio of the emitted fluorescence power to absorbed excitation laser power; and ΔΩ represents the solid angle of fluorescence radiation.Considering the attenuation in the excitation laser propagation, the Beer-Lambert law is applied for Iαυ [30]:
where the Ioυ represents the intensity of the excitation laser emitted by an optical parametric oscillator (OPO); Kυ represents the absorption coefficient; and L represents the absorption path propagated by excitation laser.Combined with the Taylor expansion of Eq. (2), the intensity of fluorescence in Eq. (1) can be rewritten as:
Integrating If over υ, we obtain:As shown in Eq. (4), the fluorescence intensity mainly depends on the absorption coefficient. Considering the linear light source of the excitation laser, the absorption coefficient is obtained by formula [29]:
where C refers to the constant coefficient, and ΔυD refers to Doppler broadening of the lower state of the excitation line. Combining Eqs. (4) and (5), the intensity of the fluorescence emitted by the measured radicals depends mainly on two factors: particle number of the lower state Nm and the oscillator strength fnm.Therefore, two representative excitation lines were investigated for fluorescence excitation of the BO B2Σ+ (υ = 0) →X2Σ+ (υ = 2) transition in this work. One has the maximum particle number of lower state Nm (vibrational ground state excitation, LIRFG), while the other has the maximum oscillator strength fnm (vibrational excited state excitation, LIRFE) [31], as shown in Fig. 1.
3. Experiment Details
3.1. Experiment Setup
The experiment setup for the LIBS-LIRF used in this study is shown in Fig. 2. A Q-switched Nd:YAG pulsed laser (Quantel Brilliant; wavelength: 1064 nm; pulse duration: 6 ns; beam diameter: 5 mm; repetition rates: 10 Hz) was used for sample ablation. The laser beam was reflected by a mirror and focused by an ultraviolet-grade quartz lens (f = 100 mm) with a focal point of 2 mm below the sample surface. An OPO wavelength-tunable laser (Opotek, VIBRANT HE 355 LD; wavelength range: 225-2400 nm; pulse duration: 10 ns; FWHM: 0.035-0.052 nm@240 nm) was used for fluorescence excitation. The sample was placed on an X-Y-Z translation platform so that the laser pulse would ablate with a fresh sample each time. A Czerny-Turner spectrometer (Princeton Instruments, IsoPlane® SCT 320; 3600 lines/mm) equipped with an intensified charge-coupled device (ICCD) camera (Princeton Instruments, PI-MAX®3; 1024 × 256 pixels) was used to record spectra. The 1064 nm laser, the OPO laser, and the ICCD camera were all triggered by a digital delay generator (Stanford Research Systems, DG535) in experiments. The slit width used in experiments was 100 µm, and the corresponding spectral resolution was about 0.08 nm.
3.2. Sample Preparation
Experimental Samples were prepared by using mixtures of H3BO3 (99.7%) and C6H12O6•H2O (99.7%), which were both purchased from Sinopharm Chemical Reagent Co., Ltd. The mixtures were pressed with 20 MPa pressure into four-centimeter diameter pellets. The numerical details were given in Table 1.
Table 1. Concentrations of H3BO3, C6H12O6•H2O, and B in the samples.
3.3. Analytical Criteria
To evaluate the performance of boron content determination, the coefficient of determination (R2), root mean square error of cross validation (RMSECV), and limit of detection (LoD) were calculated. The R2 is a coefficient to evaluate linearity of determination, which is given as:
where xi and yi represent the value of the concentration and signal intensity of the sample i, respectively; and represent the average value of xi and yi over n samples, respectively. The RMSECV was applied to evaluate the predicted results using the leave-one-out cross-validation (LOOCV) method. It is mathematically expressed as:where yj and represent the certificated and predicted concentration of sample i, and n represents the number of samples. The LoD is an analytical criteria to evaluate sensitivity. It is mathematically expressed as [32]:where N represents the spectral noise, and S represents the slope of the calibration curve.4. Results and Discussions
4.1. LIBS Analysis
The molecular spectra were measured at optimal parameters according to spectral intensities: the laser energy was 60 mJ/pulse, the ICCD gate delay was 5 μs, and the gate width was 50 μs. To reduce the intensity deviation, each molecular spectrum was collected with 50 laser shots, repeated for 10 times at different places. Figure 3(a) showed several representative spectra of different samples. The spectra of BO B2Σ+(υ = 0)→X2Σ+(υ = 2)transition band head (255.14 nm) were weak, especially for the low boron content samples. Figure 3(b) showed the calibration curve of the band head 255.14 nm. The R2 was 0.9723; and the LoD was 1.2397 wt.%, respectively. The results meant that the determination of BO molecular emission still had room for improvement in LIBS.
4.2. LIBS-LIRF Analysis
To enhance the spectra of molecular emission, we introduced laser-induced radical fluorescence to assist LIBS. The optimal parameters for recording of LIBS-LIRF spectra were a little different from conventional LIBS. The ICCD gate delay and gate width were 675 ns and 10 ns, due to the synchronization of the fluorescence and the OPO laser excitation [33]. The interpulse delays between the two lasers were optimized according to fluorescence intensities of two BO molecular emission excitations, as shown in Fig. 4. The highest points of fluorescence intensity were collected at an interpulse delay of 5 μs in LIRFG and 6 μs in LIRFE, respectively. The summary of parameters used in LIBS, LIBS-LIRFG, and LIBS-LIRFE was listed in Table 2.
Table 2. Summary of parameters used in LIBS, LIBS-LIRFG, and LIBS-LIRFE.
Figure 5 showed the enhancement of the BO B2Σ+ (υ = 0) →X2Σ+ (υ = 2) transition in LIRFG and LIRFE collected from Sample 13. The enhancement factors of these two excitations were 29.35 and 22.61, respectively. These results proved that the BO radicals were successfully irradiated by the two types of excitations, and their spectra were enhanced effectively.
The determinations of boron molecular emission were shown in Fig. 6. The intensities of BO were represented by B2Σ+ (υ = 0) → X2Σ+ (υ = 2) 255.14 nm, which both exhibited great linear relationships with the content of boron. Obviously, the LIBS-LIRF was capable of enhancing the spectra of BO molecular emission and improving the quantitative performance of boron determination. The R2, RMSECV, and LoD of LIBS, LIRFG, and LIRFE were calculated and listed in Table 3. RMSECVs were 0.5158 and 0.2514 wt.% for LIRFG and LIRFE, respectively. LoDs were 0.0993 and 0.2239 wt.% for LIRFG and LIRFE, respectively. The results showed that LIRFG performed better in terms of sensitivity while LIRFE performed better in terms of accuracy. The reasons were the different fluorescence intensity of the two excitations, which was influenced by the particle number of lower state Nm and oscillator strength fnm, as mentioned above.
Table 3. Summary of analytical performances in LIBS, LIRFG, and LIRFE.
5. Conclusion
In summary, we proposed an approach to determine boron molecular emission using laser-induced BO radical fluorescence in LIBS. Two BO radical excitations, LIRFG and LIRFE, were investigated to enhance the molecular spectra. The results showed that these two radical excitations were both capable of enhancing the spectra of BO molecular emission and improving the quantitative performance of boron determination. In addition, through the comparison of the two excitations, it was determined that the LIRFG had better sensitivity, with an LoD of 0.0993 wt.%, while the LIRFE was more accurate, with a RMSECV of 0.2514 wt.%. This work resulted in a new way to enhance the spectra and improve the determination of molecular emission with LIBS-LIRF.
Funding
This research was financially supported by National Natural Science Foundation of China (No.51429501 and 61575073) and China Postdoctoral Science Foundation (No.2017M622415).
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