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Abstract
In this paper, the spatially-resolved characteristics, such as emission intensity, plasma temperature, and electron density, of femtosecond filament induced soil plasma were experimentally studied, and the spatial evolution of the limit of detection (LOD) was obtained along the filament channel propagation. The experiment results show that the spectrum intensity and LOD of Pb I 405.78 nm trended opposingly along the filament channel propagation, the maximum spectrum intensity and the minimum LOD are obtained at a certain distance along the filament channel, and the minimum LOD value for Pb element is 1.31 ± 0.04 ppm.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Heavy metal pollution in soil by the large quantity of industrial waste slag, chemical fertilizer, wastewater irrigation, and continuously produces heat into it, which affecting the crop production, ecological environment and human’s health, so the detection of heavy metal pollution in soil has become a hotspot in recent years. In the past, the detection manner of soil species was largely used the chemical analysis method, such as inductively coupled plasma-atomic emission spectroscopy (ICP-AES) which was expensive, labor intensive and time-consuming [1,2]. Popov et al. [3] have measured the Ag, Cu, Mo and Pb multi-elements simultaneous analysis in soil by ns-LIBS method, which indicates that LIBS is feasible for detecting heavy metal in soil and other mixed material. laser induced breakdown spectroscopy (LIBS) was considered a promising spectrum detection method due to its unique features [4], such as in situ, multi-element detection, no sample preparation, rapid and a wide range of materials can apply in many fields. However, there are also some drawbacks such as plasma shield effect, self-absorption effect and matrix effect for the conventional ns-LIBS [5,6], and these drawbacks will strongly affect the detection sensitivity and accuracy of ns-LIBS.
Recent years, comparing to ns laser for LIBS studies, fs-LIBS has been extensively investigated due to its characteristics such as high ablation efficiency, precision ablation threshold and ideally minimal laser-plasma plume interaction [7–11]. Therefore, fs-LIBS can improve the stability and detection sensitivity of LIBS. The filamentation of ultrafast lasers, resulting from a dynamic balance between optical-Kerr self-focusing and plasma defocusing effect, is capable of intensity in the range of 5×1013 W/cm2 for the “intensity clamping” effect, and it is sufficient to ablate and excite solid samples at large stand-off distances, and the filaments can propagate over very large distances without suffering from diffraction effects. So the filament induced breakdown spectroscopy (FIBS) is a very promising method for stand-off application, and FIBS is widely used to many detection fields such as long-distance detection [12], polluted soil [13], chemical biological pollutants [14]and explosive residue [15],and shown higher detection sensitivity and accuracy [16].
Previous reported works have been done mainly how to generate long, stable filament and use it to carry out the proof-of-principle experiments [17,18]. Harilal et al. [19] studied the plasma temperature clamping by filament ablation brass target, and obtained the spatial evolutions of emission intensity and the electron density along the filament channel propagation. Ghebregziabher et al. [20] reported the propagation distance-dependent characteristics of filament-induced copper plasma, and the trends of spectral intensity, plasma temperature, electron density and SBR in the different position of filament propagation were studied. Yang et al. [21] measured the spectral intensity of produced Cu plasma along the filament channel and obtained the distribution of Cu(I) intensity versus the distance between sample and focused lens. Yao et al. [22] reported that the femtosecond pulsed laser energy can affect plasma characteristics of filament ablation of soil because of the assisted ablation effect of the laser energy in the “energy reservoir”. FIBS can be used in implicated fields such as polluted soil[13], discrimination of bimetallic alloy targets [23], isotopic analysis [24] and chemical biological pollutants [14] for its unique ablated properties of filament comparing to that of tight focused ablation of femtotecond laser [25,26]. However, the investigation of spatial evolutions of plasma parameters of filament-assisted ablation of soil sample and the LOD of FIBS along the filament channel propagation have not been reported.
In this paper, by using the plasma optical emission spectroscopy, the spatial evolutions of plasma parameters of FIBS of soil along with the filament channel propagation, such as emission intensity, plasma temperature and density were studied, and presented the LOD and the linear correlation coefficient R2 of calibration curve of FIBS at various locations along the filament channel. The correlation between the plasma emission spectral intensity and LOD of FIBS along with the filament channel propagation was analyzed. The quantitative analysis of Pb in soil was performed to evaluate the improvement in reliability and stability of FIBS, and overcome the detection ability of LIBS depend on the distance between the laser beam lens and sample surface.
2. Experiment setup
2.1 Experimental setup
A schematic diagram of the experimental setup is shown in Fig. 1. We use an ultrafast femtosecond laser (Libra-Usp-He, USA, Coherent) with center wavelength of 800 nm, repetition rate of 1kHz, maximum pulsed energy of 4 mJ, energy stability of 0.5% and pulse width of 50 fs. The laser energy is attenuated to about 3 mJ using energy attenuation system (a half-wave plate and thin-film polarized devices) during the whole experiment.
The femtosecond laser beam is focused by a fused lens L1 with a focal length of 500 mm and formed afilament with a length of 40 mm. A soil sample with diameter of 30 mm and thick of 4 mm is placed in filament channel and ablated to induce breakdown spectroscopy. The lens L1 is attached to a one-dimensional (1D) translation stage in steps of 2 mm and changed the position in filament channel for soil target, and the soil is mounted on a X-Y-Z translation stage for ensuring each laser spot ablating the fresh surface.
The FIBS emission spectrum of soil is 1:1 imaged by a fused quartz lens L2 (with 75 mm focal length) and then couple into a grating spectrometer (, Spectra Pro500i, Princeton Instruments, USA) equipped with an intensified charge coupled device(ICCD) detector (PI MAXII,1024×256 pixel) with a slit width of 30 $\mu m$, spectral resolution of 0.05 nm @ 1200 lines/mm at 500 nm blaze wavelength. The gate width of ICCD detector is fixed at 200 ns during the whole experiment. The ICCD detector trigged by SDG (Signal Delay Generator) signal of femtosecond amplifier is operated in the gate mode, and the gate delay and width are adjusted to capture the time resolved spectroscopy. For ensure the signal stability and minimize error over each experiment, FIBS spectra are averaged of 100 shots.
2.2 Sample preparation
In the experiment, the soil samples are made up of the different quality of Lead Nitrate (Pb(NO3)2, purity of 99%) adding to the 5 g of soil standard reference materials (serial number GBW07458 (ASA-7)) via the process of the dissolve, dry, grind and compress, then a soil sample with a diameter of 30 mm and a thickness of 4 mm is formed, the Pb concentration in weight of soil are 0.097%, 0.181%, 0.256% , 0.323%, 0.367%, 0.445%, 0.501%, 0.551%, 0.592% and 0.656%, respectively.
3. Results and discussion
3.1 Spectral emission intensity
The spatial evolutions of FIBS intensity of the two spectral lines of Fe I 405.78 nm and Pb I 404.58 nm at different delay times (100 ns, 200 ns and 300 ns) along the filament channel propagation are shown in Fig. 2. It can be observed that the spectral intensities of Fe I 404.58 nm and Pb I 405.78 nm all shown the two peaks at the position of 480 mm and 498 mm (near the geometrical focus of the lens) in the filament channel propagation, and the measurable plasma emission is evident throughout the channel length (∼40 mm). In Fig. 2, the spectral intensities of Pb I 405.78 nm and Fe I 404.58 nm in the filament channel propagation all decrease with the increasing of delay time is due to the rapid expansion and the cooling effect of the surrounding air [22], and the decay of spectral intensity at the location of 480 mm is faster than that of the two ends of filament. During the filament ablated the soil, the laser energy in the energy reservoir surrounding the filament may also participate for assisted ablation. Therefore, the ablation efficiency of filament depends both on clamped filament intensity and the rest of the laser energy in the energy reservoir of filament, which means that there is non-uniform distribution for the plasma emission spectral intensity along the filament channel propagation, and the location of highest emission intensity along the filament channel propagation is at 480 mm for the combined ablated effects of filament and energy reservoir. The weak peak appears in the tails of the filament (496 mm) near to the geometrical focal point of Lens1(focal length of 500 mm) can be attributed to the combined effects of the spectral broadening contributing to the filament ionization efficiency and geometrical focal effect of the rest of the laser energy in the energy reservoir of filament. These two effects result in higher plasma density as the filament propagate, which enhances the emission intensity.
Fig. 2. Spatial evolutions of spectral intensity of Fe I 404.58 nm and Pb I 405.78 nm (delay time of 100 ns, 200 ns and 300 ns)
3.2 Electron temperature and density
The plasma electron temperature and density, which depend on the spectra intensity and spectrum broadening width, are two important parameters for laser induced plasma, hence the plasma temperature and density can be obtained at various locations along filament channel using optical emission spectroscopy (OES). The plasma electron temperature and density were calculated by using the Boltzmann plot method and the Stark broadening method, respectively.
According to the Local Thermodynamic Equilibrium (LTE), with optically thin plasma, the plasma electron temperature T can be given by the Boltzmann plot method [27].
where, $I$ is the spectral intensity, $\lambda$ is the emission wavelength, g is the degeneracy of upper level, A is the transition probability, $E$ is upper energy level, ${k_B}$ is Boltzmann constant, h is Plank constant, c is speed of light, $L$ is the characteristic length of plasma, n is the number density of atoms and P is partition function. The plasma temperature T is obtained by the slope of the type $- 1/{k_B}T$ in Eq. (1). In this way, the calculated plasma temperatures at various locations along the filament channel propagation are given in Fig. 3, and the spectral parameters are given as Table 1.Fig. 3. Spatial evolutions of the plasma temperature (left) and electron density (right) at delay time of 100 ns, 200 ns and 300 ns
Table 1. Spectral parameters of spectral lines of FeI
The electron density is related to the spectral broadening width of emission line, and the main spectrum broadening is from Stark effect [28], then the electron density can be calculated as following:
where $\Delta {\lambda _{1/2}}$ is the spectral line width, ${n_e}$ is the electron density, $\omega$ is the electron impact parameter. The spectral line of Pb I 405.78 nm was used to calculate the electron density and the electron impact parameter value $\omega$ was 0.176 nm [29]. The electron densities at various locations along the filament channel propagation are given in Fig. 3.The spatial evolutions of plasma temperature and electron of soil plasma along the filament channel propagation are shown in Fig. 3. From the Fig. 3, the plasma electron temperature and density at delay time of 100 ns, 200 ns and 300 ns all show two peaks, the main peak at 480 mm and a weak peak at 498 mm, along the filament channel propagation, which correspond to the evolution of spectral intensity as shown in Fig. 2. But for the electron temperature, the difference between the highest and the lowest values are smaller, which means that the plasma temperature along the filament channel is clamped due to the laser intensity clamping effect [19], and for the electron density, the difference between the highest and the lowest are larger, which is similar to the case of spectral intensity. These results indicate that the significant changes in plasma spectral emission intensity as well as electron density occur although the temperature along the filament is clamped, which means that the femtosecond pulsed energy keeping in the filament reservoir will ablate the soil, increase the ablation efficiency, spectral intensity and electron density. Due to the effects of the plasma expansion and surround air cooling, the electron temperature and density at various locations of filament channel are all decreased with the increasing of the delay time from 100 ns to 300 ns.
It is needed to point out that the calculated critical electron density is $7.51 \times {10^{15}}c{m^{ - 3}}$ by the McWhiter criterion [30], which is lower than the experimental minimum value of $7.88 \times {10^{15}}c{m^{ - 3}}$, therefore, the condition of LTE is validity.
3.3 Limits of detection of FIBS
The limit of detection (LOD) is an important parameter for quantitative analysis of material, and it can reflect the detection sensitivity of LIBS. The relative standard deviation (RSD) is an important parameter reflecting the experimental repeatability of LIBS. The linear correlation coefficient R2 value of calibration curve is an important parameter reflecting the reliability of LIBS measurement. The LOD (${C_{\lim }}$) can be expressed as Eq. (3) [3].
where, ${S_{bac}}$ is the standard deviation of background signal, M is the slope of the calibration curve. We select the uninterrupted continuum emission at 402.5 nm near to the spectral line Pb I 405.78 nm as the background signal, which is selected according to definition of the International Union of Pure and Applied Chemistry (IUPAC). Under same experimental conditions, we acquire 50 times of FIBS, then to calculate the standard deviation of background signal, and the ${S_{bac}}$ of the FIBS along the filament channel propagation is shown in Table 2. The spectral line of Pb I 405.78 nm is selected for the quantity analysis of Pb element in soil, the calibration curve at the location of 480 mm along the filament channel is shown in Fig. 4, the LODs of the FIBS along the filament channel propagation is shown in Fig. 5.Table 2. Standard deviation of background signal of different positions
From the Fig. 5, it is known that the LODs of FIBS at various location along the filament channel propagation change from 1.31 ± 0.04 ppm to 1.51 ± 0.04 ppm, and the minimum LOD value is 1.31 ± 0.04 ppm at the location of 480 mm in filament channel. For the linear correlation coefficient R2 value of calibration curve at various locations along the filament channel propagation are all larger than 0.98 and can even up to 0.995, which means that the experiment data have good measurement reliability of LIBS. Compare the Fig. 2 and Fig. 5, The spectrum intensity and LOD are shown the opposite variation trend along with the filament channel propagation, and the location of the maximum spectrum intensity and the minimum LOD are found at 480 mm along the filament channel propagation.
4. Conclusion
In summary, we studied the spatially-resolved characteristics such as emission intensity, plasma temperature and electron density of filament assisted ablation soil plasma along filament channel propagation. The significant changing in plasma emission signal intensity is correlated to the changes in electron density although the temperature is clamped for the laser intensity clamping inner filament. The minimum LOD value is 1.31 ± 0.04 ppm at the location of 480 mm in filament channel. For the linear correlation coefficient R2 value of calibration curve at various location along the filament channel propagation are all larger than 0.98.The varieties of spectrum intensity and LOD were shown the opposite trend along with the filament channel propagation, and the location of the maximum spectrum intensity and the minimum LOD all can be found at 480 mm along the filament channel propagation.
Funding
National Natural Science Foundation of China (NSFC) (61575030); Natural Science Foundation of Jilin Province (20180101283JC).
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