Spectral Interference Elimination in Soil Analysis Using Laser-Induced

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Spectral interference elimination in soil analysis using laser-induced breakdown spectroscopy assisted by laser-induced fluorescence Rongxing Yi, Jiaming Li, Xinyan Yang, Ran Zhou, Huiwu Yu, Zhongqi Hao, Lianbo Guo, Xiangyou Li, Xiaoyan Zeng, and Yongfeng Lu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03969 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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Analytical Chemistry

Spectral interference elimination in soil analysis using laserinduced breakdown spectroscopy assisted by laser-induced fluorescence Rongxing Yi, Jiaming Li, Xinyan Yang, Ran Zhou, Huiwu Yu, Zhongqi Hao, Lianbo Guo, Xiangyou Li *, Xiaoyan Zeng and Yongfeng Lu Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology (HUST), Wuhan, Hubei 430074, P. R. China ABSTRACT: The complex and serious spectral interference makes it difficult to detect trace elements in soil using laser-induced breakdown spectroscopy (LIBS). To address it, LIBS assisted by laser-induced fluorescence (LIBS-LIF) was applied to selectively enhance the spectral intensities of the interfered lines. Utilizing this selective enhancement effect, all the interference lines could be eliminated. As an example, the Pb I 405.78 nm line was enhanced selectively. The results showed that the determination coefficient (R2) of calibration curve (Pb concentration range: 14-94 ppm), the relative standard deviation (RSD) of spectral intensities, and the limit of detection (LoD) for Pb element were improved from 0.6235 to 0.9802, 10.18% to 4.77%, and 24 ppm to 0.6 ppm using LIBS-LIF, respectively. These demonstrate that LIBS-LIF can eliminate spectral interference effectively and improve the ability of LIBS to detect trace heavy metals in soil.

LIBS is a spectrochemical analysis method based on analyzing spectra of plasmas that generated by pulsed lasers. With its advantages of fast, real-time, and stand-off analyses,1-3 LIBS has been widely used in many areas, such as steels,4-6plastics,7 ceramics and soil.8-12 For ideal matrixes of LIBS, the peak lines of their spectra are not overlapped with each other and there are linear relationships between the elemental concentrations and their spectral intensities. However, for the case of soil, spectral interference cannot be neglected due to the complex matrix, which will influence the accuracy of quantitative analysis in LIBS seriously.13 To reduce the influence of spectral interference, signal processing methods, such as curve fitting,14,15 wavelet transform (WT),16,17 and artificial neural networks (ANNs) etc.,18 have been studied. Although these methods can reduce spectral interference in some extent, there are some disadvantages. For example, the curve fitting method is difficult to obtain the number of individual peaks and the accurate peak positions.14 For the WT method, different original parameters such as decomposition levels and wavelet function will lead to different results, and the selection of these parameters is based on the types of samples.19 For the ANNs method, there are no uniform rules to determine the training samples and testing samples, and the models need to be rebuilt for different samples.19 All in all, the above mathematical methods can only reduce the spectral interference, but not eliminate it thoroughly. Therefore, it is still a challenge to find a method to eliminate all the overlapped spectral peaks. By irradiating the plasma with a wavelength tunable laser in LIBS-LIF, the ground state atoms of the target elements can be selectively stimulated to an excited state, resulting in a great increase in atom density in the corresponding excited state,20, 21 and then transition lines

from the excited state can be greatly enhanced. This might be a chance to eliminate the spectral interference. In this study, LIBS-LIF was used to eliminate spectral interference for Pb element in soil plasma and quantitative analysis of trace Pb element in soil was also carried out.

Experimental setup and method

Experimental setup. The LIBS experimental setup used in this study is schematically shown in Figure 1. A Qswitched Nd: YAG laser (Beamtech, Nimma 400, 532 nm; 10 Hz) was focused below 2 mm under the surface of the soil sample using a 150 mm focal length lens to produce plasma. A wavelength-tunable optical parametric oscillator (OPO) laser (OPOTEK, VIBRANT, 10 Hz) was used for selective enhancement. The plasma was stimulated by the OPO laser horizontally with the optical axes at about 1 mm above the sample surface. The diameter of the OPO laser beam was about 4 mm, which could fully cover the plasma generated by the 532 nm laser. Plasma emission was coupled into an optical fiber by a light collector and then collected by a Czerny-Turner spectrometer (Andor Tech., Shamrock 500i, grating: 1800 l/mm, slit width: 50 µm). A digital delay generator (SRS, DG535) was adopted to trigger the 532 nm laser, the OPO laser, and the intensified charge-coupled device (ICCD) camera (Andor Tech., iStar 320T: DH320T-18F-E3-26mm). To avoid over ablation, the sample was mounted on a stepper motor stage, and the laser beam scanned the sample in a straight line.

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Sample preparation. Certified soil samples (GWE070008

and GW07407) that approved by the State Administration of Quality Supervision, Inspection, and Quarantine of China, were used to study the influence of overlapped peaks of Pb element. Sample GWE070008 was used to optimize the experimental parameters due to its high concentration (675 ppm) of Pb. Sample GW07407 was used to prepare samples with different Pb concentrations due to its low Pb concentration (14 ppm). By adding appropriate amounts of nitrate (Pb(NO3)2) solution in the soil sample GW07407, 12 new samples were prepared (Pb concentration range: 14414 ppm). The soil powder samples were then pressed into pellets with a diameter of 4 cm under a pressure of 20 MPa. The concentrations of Pb, Mn, Ti, Fe elements in the GWE070008 and GW07407 are listed in Table 1. Table 1. Elemental concentrations of prepared samples. Sample No. Pb concentration (ppm) Mn concentration (ppm) Ti concentration (ppm) Fe2O3 concentration (%)

GWE070008

GW07407

675

14

2100

1780

4120

20200

6.95

18.76

3

Fig. 1. LIBS experimental setup.

soil. Fig. 2 also shows that, the spectral intensities of soil plasmas decreased rapidly as the delay times increased, and almost all the spectra disappeared when the delay time reached to 14 µs. The results indicate that, if the Pb I 405.78 line was selectively enhanced at a longer delay time, the spectral interference could be eliminated. 4 Delay 4 µs Fe 406.36 nm Delay 6 µs Delay 10 µs Delay 14 µs Ti 406.02 nm Pb: 64 ppm 2 Pb 405.78 nm

Intensity (10 Counts)

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Mn 405.55 nm

Mn 405.89 nm

0 405.0

405.5 406.0 Wavelength (nm)

406.5

Fig. 2. Spectra of sample GW07407 in the spectral range 405.0406.5 nm at different delay times.

Principle of LIBS-LIF. To determine trace Pb element in soil, LIBS-LIF was applied to eliminate the spectral interference. Taking Pb as an example, the principle of LIBSLIF is shown in Figure 3. It is shown that, the photon energy of the OPO laser (283.31 nm) is equal to the energy gap between E0 and E2 of Pb atom. When the atoms of Pb element at the ground state were irradiated by the OPO laser (283.31 nm), they were stimulated to the excited state (E2) selectively. Afterwards, the spontaneous transition appeared in the exited plasma, the excited lead atoms transited from E2 to E1 and the 405.78 nm fluorescence emitted from the excited lead atoms, simultaneously. Finally, the atoms went back to ground state (E0) by radiationless transition.

RESULTS AND DISCUSSION

Spectral interference. The time-integrated spectra of laser-induced soil plasma were obtained in the spectral range of 405.0-406.5 nm at different delay times, as shown in Figure 2. The plasmas were generated by 532 nm Nd: YAG laser with pulse energy of 10 mJ, and were collected with a gate width of 6 µs and delay times of 4, 6, 10, and 14 µs, respectively. Each spectrum was accumulated 30 pulses, and 10 spectra were taken for each sample and averaged. As shown in Fig. 2, the solid line represents the spectrum obtained at the delay time of 4 µs, the intensity of Pb I 405.78 nm line is not strong enough due to its low concentration and it is interfered by Mn I 405.55 nm, Mn I 405.89 nm, Ti I 406.02 nm and Fe I 406.36 nm lines. As shown in Table 1, sample GBW07407 contains Mn: 1780 ppm, Ti: 20200 ppm, and Fe2O3: 18.76 %, the high concentrations of these elements lead to serious spectral interference. Because Mn, Ti, and Fe are very common elements in soil, and the Pb I 405.78 nm line is easily interfered by these elements, it is very difficult to analyze trace Pb element in

Fig. 3. Principle of LIBS-LIF.

Optimization of experimental parameters. OPO laser energy and inter-pulse delay are two main parameters in LIBS-LIF, which influence the signal to noise ratios (SNRs) of the LIF signals significantly. Figure 4 shows the SNRs for Pb I 405.78 nm as a function of the OPO laser energies EOPO. The ablation energy was 10 mJ, the gate width was 20 ns (to obtain the whole LIF signal), and the inter-pulse delay time was 14 µs. For lower OPO laser energies (less than 0.2 mJ), the LIF signals increased linearly with the


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OPO laser energies. For higher OPO laser energies (more than 0.2 mJ), the LIF signals tend to saturate and keep nearly constant.

Pb I: 405.78 nm 4 SNR

Elimination of spectral interference. Under the optimized parameters, the spectra of Pb element obtained by the 283.31 nm OPO laser and LIBS only are shown in Figure 6. The ablation laser energy was 10 mJ, the OPO laser energy was 0.5 mJ, the inter-pulse delay time was 14 µs, and the gate width was 20 ns. In the spectral range of 405.0-406.5 nm, there is no signal in the LIBS spectrum (dot line). However, the LIBS-LIF spectrum (solid line) contains the Pb I 405.78 nm line, the long inter-pulse delay time and narrow gate width of the ICCD made all the other lines disappeared. Obviously, the interference was eliminated using LIBS-LIF. 1.2

LIBS-LIF LIBS 0.9 Pb: 64 ppm Intensity (a.u.)

As the OPO laser energy and the photon numbers are closely related, when the OPO laser energy is less than 0.2 mJ, the photon numbers are not large enough to stimulate all the ground state Pb atoms, and when the OPO laser energy is higher than 0.2 mJ, all the ground state Pb atoms could be stimulated, which causes the saturation of SNRs. The Pb concentration of the sample used in this optimization experiment is 675 ppm, which is much higher than the Pb concentrations of the samples used for quantitative analysis. It means that, the 0.2 mJ OPO laser energy is high enough to stimulate all the Pb atoms, and the influences by OPO laser energy fluctuation on the SNRs could be reduced effectively due to the saturation of SNRs.

2

0.6 0.3 0.0 405.0

0 0.0

0.2

0.4 0.6 0.8 OPO Energy (mJ)

1.0

Fig. 4. SNRs for Pb LIF signals at 405.78 nm as a function of the OPO laser energies EOPO.

Influence of the inter-pulse delays between two lasers on the SNRs for Pb I 405.78 nm line is shown in Figure 5. The ablation energy was 10 mJ, the OPO energy was 0.5 mJ, and the gate width was 20 ns. When the inter-pulse delay reached to 14 µs (much later than conventional LIBS), the SNRs for Pb LIF signal at 405.78 nm reached to a maximum, and then the SNRs decreased gradually. It shows that, the best SNR could be obtained at the inter-pulse delay time of 14 µs.

9

SNR

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Pb I: 405.78 nm

6

0

20 40 Inter-pulse delay (µs)

406.5

Fig. 6. Spectra of sample GW07407obtained by LIBS-LIF and conventional LIBS.

Quantitative analysis. As shown in Figures. 7(a) and 7(b), the spectral intensities of Pb 405.78 nm line was used to build the calibration curves of LIBS-LIF and conventional LIBS, and the error bars indicated the uncertainties of spectral intensities. Fig. 7(a) shows the calibration curves of Pb elements in the range of 19-414 ppm. It was found that, the R2 factor of LIBS-LIF was improved from 0.9428 to 0.9837 of conventional LIBS, as well as the LoD of Pb element calculated by the 3σ criterion22 was improved from 24 ppm to 0.6 ppm, which showed that both the accuracy and the sensitivity became better. Furthermore, the calibration curves in the range of 19-94 ppm were also obtained as shown in Fig. 7(b), it was interesting that the R2 factor was improved from 0.6235 of conventional LIBS to 0.9837 of LIBS-LIF dramatically. That is to say, Pb lines in low concentrations were very easy to be interfered by other lines under conventional LIBS, which led to the worse R2. However, the Pb line was not interfered by other lines under LIBS-LIF.

3

0

405.5 406.0 Wavelength (nm)

60

Fig. 5. Influence of the inter-pulse delays between two lasers on the SNR for Pb I 405.78 nm.



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4.77%, and 0.6 ppm using LIBS-LIF, which were much better than the results of conventional LIBS. The results have shown that LIBS-LIF can be used to eliminate spectral interference, and it has obvious advantages over conventional LIBS in improving the ability of quantitative analysis in detecting trace heavy metals in soil.

AUTHOR INFORMATION

Corresponding Author * Phone: 86-27-87541423. Fax: +86-27-87541423. E-mail: [email protected]

Notes The authors declare no competing financial interest.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS

This research was financially supported by the Major Scientific Instruments and Equipment Development Special Funds of China (No. 2011YQ160017), the National Natural Science Foundation of China (No. 51429501, 61575073, and 61378031), and the Fundamental Research Funds for the Central Universities of China (HUST: 2015TS075).

Fig. 7. Calibration curves of Pb element obtained by LIBS and LIBS-LIF in the range of 19-414 ppm (a), and 19-94 ppm (b).

To verify the advantages of LIBS-LIF more clearly, the R2, RSD and LoD values for LIBS and LIBS-LIF in the range of 19- 94 ppm were compared in Table 2. All the R2, RSD and LoD values were improved significantly using LIBS-LIF due to the elimination of spectral interference in the spectra of soil. Table 2. The R2, RSD and LoD values for LIBS and LIBSLIF.

LIBS

R2 0.6235

RSD 10.18%

LoD (ppm) 24

LIBS-LIF.

0.9802

4.77%

0.6

CONCLUSIONS

To eliminate spectral interference, LIBS-LIF was investigated in soil analysis and the influences of the main experimental parameters on the LIF signals were studied. With the stimulations of 283.31 nm OPO lasers, the line intensity of Pb 405.78 nm line was selectively enhanced at optimized parameters with an ablation laser energy of 10 mJ, an OPO laser energy of 0.5 mJ, an inter-pulse delay time of 14 µs, and a gate width of 20 ns. With such a long inter-pulse delay time and a short gate width, all the spectral interferences were eliminated. Furthermore, successful quantitative analysis of trace Pb element in soil by LIBSLIF was carried out. In the range of 19-94 ppm, the R2 factor, RSD and LoD values of Pb element were 0.9802,

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Spectral Interference Elimination in Soil Analysis Using Laser-Induced

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