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Spatially selective excitation was proposed to improve excitation efficiency in laser-induced breakdown spectroscopy combined with laser-induced fluorescence (LIBS-LIF). Taking chromium (Cr) and nickel (Ni) elements in steels as examples, it was discovered that the optimal excitation locations were the center of the plasmas for the matrix of the iron (Fe) element but the periphery for Cr and Ni elements. By focusing an excitation laser at the optimal locations, not only excitation efficiency but also the analytical accuracy and sensitivity of quantitative LIBS-LIF were better than those with excitation at the plasma center in conventional LIBS-LIF. This study provides an effective way to improve LIBS-LIF analytical performance.
© 2017 Optical Society of America
1. Introduction
Laser-induced breakdown spectroscopy (LIBS) is a laser-ablation-based method of spectrochemical analysis. Elemental information is deduced by analyzing spectra emitting from laser-induced plasma [1, 2]. Due to its attractive advantages, including fast response, remote sensing, real-time monitoring, in situ detection, noncontact, no or simple sample preparation, and simultaneous multielement determination, LIBS has shown great potential in environmental protection [3], solar cells [4], nuclear reaction monitoring [5], space exploration [6], mining [7], archeological verification [8], and coal [9], etc. Although LIBS has been developed quickly in recent years, unsatisfactory analytical sensitivity and accuracy greatly hinder its continued development.
LIBS combined with laser-induced fluorescence (LIBS-LIF) is a promising enhancement method for improving sensitivity and accuracy in LIBS [10]. In LIBS-LIF, a wavelength-tunable laser is focused into plasma to resonantly excite specific atoms. When the laser wavelength is equal to an excited line of a target element, the atoms of the target element absorb laser photons and then emit strong fluorescence. Detection of trace cobalt [10, 11], iron [12], lead [12–16], cadmium [17], phosphorus [18], boron [19], thallium [17], and ytterbium [20] using LIBS-LIF has been reported, which demonstrates that LIBS-LIF is effective in improving analytical sensitivity and accuracy. In these works, no spectral lines from matrices were close to the excited lines from the target elements; and the excitation laser energy was absorbed only by the atoms of target elements. However, when spectral lines from matrix and target elements are very close, a large number of matrix atoms absorb much laser energy; and then the laser energy absorbed by the atoms of target elements lessens, resulting in less improvement of analytical sensitivity. Moreover, the matrix interference deteriorates because the atoms of the matrix and target elements are simultaneously excited.
To solve this problem, a new approach using spatially selective excitation in plasma was proposed, in which the excitation locations inside the plasma were optimized to increase excitation efficiency. Chromium (Cr) and nickel (Ni) in steels were selected as typical examples to demonstrate the feasibility of spatially selective excitation in LIBS-LIF. The varied spectral intensity, with excitation laser spot locations inside the plasma, was investigated; and the accuracy and sensitivity of quantitative analyses was discussed.
2. Experimental
A schematic diagram of the LIBS-LIF setup used in this work is shown in Fig. 1. A Q-switched Nd:YAG laser (Beamtech Optronics, Nimma series, pulse duration of 6 ns, flattened Gaussian beam) operating at 532 nm and 10 Hz was used. The laser beam was reflected by a dichroic mirror and then focused onto sample surfaces. To mitigate the laser damage to the samples, the laser energy and focal length for ablation were set at 2 mJ and 25 mm, respectively. The focal spot on the sample is 100 μm. The laser ablation fluence was 25.5 J/cm−2. The diameter and depth of the crater is estimated to be about 100 um and 0.01 um. An optical parametric oscillator (OPO) wavelength-tunable laser (OPOTEK, Inc., Vibrant HE 355 LD, wavelength range of 225-2400 nm, pulse duration of 10 ns, linewidth of 9 cm−1 at UV range) operating at 10 Hz and 2 mJ was used as the excitation source. The OPO laser beam was focused at the plasma by a UV-grade quartz lens (f = 150 mm). In order to avoid being blocked by the sample edges, the OPO beam passed the lens above the optical axis of the lens. The OPO beam after the lens was slightly slanted down. The estimated angle between OPO beam (after the lens) and the optical axis of the lens was about 4°.The spot and irradiance of the OPO laser focused at the plasma were about 1 mm and 25.5 MW/cm2, respectively. By adjusting the two micrometers on the lens mount, the OPO spot at the plasma could be adjusted two dimensionally. The fluorescent light emission from the plasma was collected by a light collector (Ocean Optics, 84-UV-25, wavelength range: 200-2000 nm) and coupled into a Czerny-Turner spectrometer (Andor Technology, Shamrock 500, grating of 1800 lines per mm, 100 μm slit, resolution about 0.02 nm) through a multicore fiber. An intensified charge-coupled device (ICCD) (Andor Technology, iStar 320T) was equipped to record the spectra. The two laser devices and the ICCD were sequentially triggered by a digital delay generator (Stanford Research Systems, DG535). The interpulse delay of the two laser was optimized to be 3 μs. The OPO laser and the ICCD gate were simultaneously switched. The ICCD gate was 10 ns, equal to the OPO laser pulse duration. The signals generated by 100 pulses of LIBS laser and 100 pulses of OPO laser are accumulated on the detector to generate one spectrum. Each sample was measured repeatedly ten times.
Twenty-two certified steel samples (seven from Central Iron & Steel Research Institute of China, eight from the National Institute of Standards and Technology, and seven from Ansteel Research Institute), were used in this work. The certified concentrations of Cr and Ni are listed in Table 1.
Table 1. Certified concentrations of Cr and Ni elements in the steel samples (wt.%).
3. Results and discussion
3.1 Matrix interference
Due to thousands of iron lines, steels are troublesome matrices with strong interference. Under the same experimental conditions, the limits of detection (LoDs) of trace elements in steels are worse than in most other matrices [11]. Figure 2 shows normalized LIBS spectra near excited lines of Cr and Ni acquired from pure Cr, Ni, and Fe samples. Clearly, Fe I 358.11 nm and Fe I 234.55 nm were close to the LIBS-LIF excited lines of Cr I 357.87 nm and Ni I 234.56 nm. Though Fe excitation efficiency was not high due to the small gaps between the iron lines and excited lines, the amount of Fe atoms was much larger than trace Cr and Ni atoms. Therefore, the excitation laser energy absorbed by Fe atoms was nonnegligible.
With the OPO laser excitation, the LIBS-LIF spectra acquired from steel sample No. 16 are shown in Fig. 3. It was demonstrated that the Cr and Fe lines were simultaneously enhanced by the 357.87 nm laser (the Cr excited line), and the Ni and Fe lines were simultaneously enhanced by the 234.56 nm laser (the Ni excited line). The obvious enhancement of Fe lines implied that much laser energy was absorbed by the Fe atoms, and the excitation efficiency of Cr and Ni in the steels decreased correspondingly.
3.2 Spatial scanning of excitation laser spot
For discussion purposes, the XY coordinate system in plasma was defined as shown in Fig. 4. Two micrometers along the X and Y axes were equipped on the adjustable lens mount. By adjusting the two micrometers (10 μm graduations, ± 3 μm uncertainty), the OPO focal spot at the plasma could scan two dimensionally in steps of 0.25 mm. The LIBS-LIF spectral intensities, with the location information of each point, were recorded and combined into scanning maps.
Figure 5 shows the scanning intensity maps for the target and matrix elemental lines in the XY coordinate system. It is illustrated that the optimal locations for LIBS-LIF intensity between Cr and Fe is distinctly different in Fig. 5(a). The intensity map of the Cr I 425.43 nm line had a dip on the plasma center where a high intensity Fe I 427.17 nm line was located, which proved that Fe atoms located at the plasma center absorbed too much OPO laser energy and lowered the Cr excitation efficiency. From the center to the periphery of the plasma, the Fe line intensity decreased and the Cr line intensity increased. This occurred because different particles have different distributions in plasmas [21, 22]. The optimal excitation locations were X = 1.25 mm and Y = 0.25 mm for Cr, while X = 0 mm and Y = 0 mm for Fe.
Fig. 5 The scanning intensity maps of Cr I 425.43 nm/Fe I 427.17 nm (a) and Ni I 305.08 nm/Fe I 306.72 nm (b).
Figure 5(b) shows intensity maps of Ni I 305.08 nm and Fe I 306.72 nm excited by an OPO laser with a wavelength of 234.56 nm. As with the Cr line in Fig. 5(a), the Ni atomic line also had a lower intensity in the plasma center, where a high intensity Fe atomic line was located and the Ni excitation efficiency decreased. The optimal locations were X = 0.75 mm and Y = 0 mm for Ni, while X = 0 mm and Y = 0.25 mm for Fe.
To simplify the description, the LIBS-LIF with resonant excitation at the optimal locations of the target elements (such as Cr and Ni in this work) was defined as LIBS-LIF-T; and the LIBS-LIF with resonant excitation on an area of matrix elements (such as Fe in this work) was defined as LIBS-LIF-M.
3.3 Elemental determination
With the above optimal excitation locations, the acquired LIBS and LIBS-LIF spectra are shown in Fig. 6. As expected, LIBS-LIF spectra were much stronger than LIBS spectra. Comparing the two LIBS-LIFs in Fig. 6(a), LIBS-LIF-T had a 2.0 times stronger intensity of Cr I 425.43 nm than LIBS-LIF-M, while LIBS-LIF-M had a 9.6 times stronger intensity of Fe I 428.24 nm than LIBS-LIF-T. In Fig. 6(b), LIBS-LIF-T had a 2.1 times stronger intensity of Ni I 305.08 nm than LIBS-LIF-M, while LIBS-LIF-M had a 3.2 times stronger intensity of Fe I 306.72 nm than LIBS-LIF-T. These results show that the enhancements of matrix and target elements at different excitation locations are quite different.
To evaluate the advantages of spatially selective excitation, calibration curves of Cr and Ni were established by measuring the 22 steel samples using LIBS and LIBS-LIF (Fig. 7). Generally, analytical accuracy is evaluated by the determination coefficient R2 of the calibration curves [23]; and analytical sensitivity is indicated by slopes and LoDs [24] in calibration curves. The LoDs were calculated according to the 3σ criterion. The noise level measured by the standard deviation of the continuum background. The backgrounds of Cr and Ni detection range were chosen in 431.8~432 nm and 308.6~308.8 nm, respectively. All of the criteria were calculated and listed in Table 2.
Fig. 7 The calibration curves of LIBS-LIF-T (blue scatters and lines), LIBS-LIF-M (red scatters and lines), and LIBS (green scatters and lines).
Table 2. Quantitative comparison of LIBS-LIF-T, LIBS-LIF-M, and LIBS.
In Fig. 7 and Table 2, LIBS-LIF-T had R2 slightly higher than LIBS-LIF-M, which represent similar accuracy. Compared with LIBS under the same experimental conditions, LIBS-LIF had obviously greater R2 and better accuracy. In Cr determination, the slope of LIBS-LIF-T was 2.05 and 20.8 times higher than that of LIBS-LIF-M and LIBS under the same experimental conditions, respectively. Per unit elemental content represents a larger number of detector counts in LIBS-LIF-T than in LIBS-LIF-M and LIBS. Furthermore, under the same experimental conditions, the LoDs were 2.33 μg/g in LIBS-LIF-T, 23.6 μg/g in LIBS-LIF-M, and 24.4 μg/g in LIBS. They can be further improved by increasing ablation laser energy and lengthening time in measuring. The LoD superiority in the LIBS-LIF-T was much greater than the slope superiority because the noise in LIBS-LIF-M was much higher. The possible reason for such high noise in LIBS-LIF-M is that optimal excitation of the matrix atoms intensified the atomic collision in the plasma. These results demonstrate that LIBS-LIF-T is significantly more sensitive in Cr determination than LIBS-LIF-M and LIBS under the same experimental conditions. Similarly, LIBS-LIF-T in Ni determination came to the same conclusion.
4. Conclusions
In summary, spatially selective excitation in LIBS-LIF was proposed to improve excitation efficiency of target elements. Upon analyzing the LIBS-LIF spectra excited at different locations of the plasma, it was discovered that the excitation efficiency of the matrix and target elements was quite different between the center and periphery of the plasma. By selecting the optimal locations for excitation at the plasma, both the analytical accuracy and sensitivity were greatly improved under the same experimental condition. The results demonstrated that spatially selective excitation is an effective approach to improving LIBS-LIF analytical performance.
Funding
National Instrumentation Program of China (No. 2011YQ160017); National Natural Science Foundation of China (61575073).
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