Spark discharge has been proved to be an effective way to enhance the LIBS signal while moderate cylindrical confinement is able to increase the signal repeatability with limited signal enhancement effects. In the present work, these two methods were combined together not only to improve the pulse-to-pulse signal repeatability but also to simultaneously and significantly enhance the signal as well as SNR. Plasma images showed that the confinement stabilized the morphology of the plasma, especially for the discharge assisted process, which explained the improvement of the signal repeatability.
© 2014 Optical Society of America
Relatively low sensitivity and pulse-to-pulse repeatability has always been regarded as the obstacle for wide commercialization of laser induced breakdown spectroscopy (LIBS). For applications such as trace element analysis, a very low limit of detection (LOD) is desired, while for on-line measurement and inhomogeneous sample analysis, high repeatability is necessary. Therefore, a strong and highly repeatable signal is of great importance for LIBS applications. Researchers have proposed various methods to enhance the signal such as dual-pulse excitation technique [1–4], spark discharge [5–9], and spatial confinement [10–17]. The commonly applied methods to improve the signal repeatability is normalization  and other data processing methods [19, 20].
Spark discharge has been proved to be an effective way to enhance the signal of LIBS [5–9], but its effect on pulse-to-pulse signal repeatability has barely been discussed yet. As shown in our previous work [16, 17], a cylindrical cavity confinement can both increase the signal repeatability and achieve signal enhancement.
In the present work, the effects of combining spatial confinement and spark discharge were investigated comparing with spark discharge or spatial confinement alone. It was demonstrated that the spark discharge was more effective in signal enhancement while moderate cylindrical cavity confinement was more effective in improving pulse-to-pulse signal repeatability. Combining spark discharge and moderate cylindrical cavity confinement, the signal was enhanced significantly and the pulse-to-pulse signal repeatability was also improved. That is, the combination of these two methods is able to provide a better signal than either method alone and conventional LIBS. The fluctuation of the plasma images was quantitatively analyzed and clearly demonstrated the repeatability improvement of plasma morphology using cavity confinement.
2. Experimental setup
The schematic of the experimental setup was shown in Fig. 1. A Spectrolaser 4000 LIBS system (XRF, Australia) was utilized for the experiments as in our previous work [16, 17]. A Q-switched Nd:YAG laser with a wavelength of 532 nm was applied and the focal spot size was estimated to be 200 μm in diameter. Spatial and time integrated spectra were collected from an angle of about 45 degree with the laser beam. The plasma emitted light was focused into the fiber through a lens with diameter 0.5 inch and focal length 2 cm. Four Czerny–Turner spectrometers and CCD detectors covered the spectral range from 190 to 940 nm with a nominal resolution of about 0.1 nm. The gate width was 1 millisecond and delay time was 1 μs. The plasma images were captured by an ICCD camera (Andor, iStar 334T) from the direction at an angle of 60 degree with the laser beam. The delay time and gate width of ICCD were the same as that of the spectrometer.
The sample used in this work was powdery bituminous coal with grain diameter less than 0.2mm. The main components of the sample are C (78.98%wt), H (4.95%wt), N (1.38%wt), Si (1.7%wt), S (1.70%wt), which were certified by the China Coal Research Institute (CCRI). About 3 gram of coal powder was pressed with pressure of 20 tons to form a pellet with 30mm in diameter. In addition, the coal pellet sample surface was made very smooth by using smooth pressing mould.
The cylindrical cavity used in this work was 1.5mm high and 3mm diametric, which was drilled in a 1.5mm thick polytetrafluoroethylene (PTFE) plate. For cases with cavity, the PTFE plate was placed closely on the sample surface, and the laser was shot through the center of the cylindrical hole. For cases without cavity, nothing was placed on the sample surface. Two cylindrical electrodes (3mm in diameter) with hemispherical endpoint were paced 15 degree with the horizontal and the lowest point of the electrodes was 2.5mm above the sample surface. An electric capacitor with 20nF and a high-voltage direct current power supply were parallel connected with the electrodes. In this work, the high-voltage was set to be 7.5kV. The discharge process was triggered passively, which means the laser induced plasma increases the conductivity of air near the electrode so as to automatically trigger the discharge process.
For each sample, 40 spectra and 40 plasma images were collected from 40 different locations on the sample surface, and these 40 spectra were used to calculate the average parameters and RSD of the signal. Each spectrum and image was from a single laser shot. After each pulse, an air flow was used to clear the laser ablated aerosols above the sample. The spectra were background subtracted before data processing. Carbon is a major element in coal and is of most interest in coal analysis so the carbon line C(I) 193.09 nm was selected for analyses in this work. Some other spectral lines such as C(I)247.856 nm, Si(I)288.158 nm were also analyzed and the results were found to be similar with that of C(I) 193.09nm, so they had not been displayed.
3. Results and discussion
As described above, the main objective of the present work is combining moderate cylindrical cavity confinement and spark discharge to improve signal repeatability and enhance the signal. In this section, the plasma character and optical signals of four configurations were compared, namely the conventional LIBS (without confinement and spark discharge), cylindrical cavity confined LIBS, spark discharge assisted LIBS, and the combination of cylindrical cavity confinement and spark discharge LIBS.
3.1 Plasma character
Plasma temperature and electron number density are two key parameters to characterize the plasma. Five Si I spectral lines (243.515nm, 250.69nm, 251.432nm, 251.61nm, 288.158nm) were used to build Boltzmann plot for temperature calculation. The full width at half maximum (FWHM) of Hα (656.28nm) line was used to calculate the electron number density according to [21, 22].
Figure 2 shows that the spatial confinement increased the plasma temperature and electron number density distinctly for all laser energies, mainly because the reflected shockwave added extra energy to the plasma [16, 17]. While for spark discharge, the plasma temperature and electron density were lower than conventional LIBS. Our results is mainly agreed with the results obtained by other researchers , where the spark discharge has no obvious effect on plasma temperature and electron density. Though the spark discharge adds additional electric energy into the laser induced plasma, it can increase the plasma volume (seen in Fig. 3). Therefore, the plasma temperature and electron density may be higher or lower than that of conventional LIBS for different samples, laser energy, delay time and gate width. For example, Zhou et al.  indicated that the spark discharge can obviously increase the plasma temperature and electron density for soil samples, with the voltage of 8kV and capacitor of 4nF. The difference between Zhou et al. and the present work may come from the samples’ matrix and electric circuit property such as the voltage and capacitor. More detailed researches are needed to investigate the effect of spark discharge on plasma temperature and electron density.
3.2 Plasma morphology stabilization
For each configuration, 40 plasma images from 40 laser pulses were captured. 10 typical plasma images for each configuration were shown in Fig. 3. As Fig. 3(c) shows, when only using spark discharge assistance, the plasma morphology can be more unstable than that of conventional LIBS, which may increase signal uncertainty. The probable reason is that even a slight plasma morphology difference at the initial expansion stage may be enlarged by the discharge process. As shown, the cavity can stabilize the expansion and morphology of the plasma. Especially for Figs. 3(c) and 3(d), the discharge process and the plasma were much more stable when the cavity confinement was used. The probable reason is the cavity confinement reduced the pulse-to-pulse fluctuation of the air conductivity between the plasma and the electrodes so to stabilize the discharge process.
The fluctuation of plasma morphology was investigated more quantitatively below. The intensities at different pixels on ICCD, which indicates the spatial distribution of the plasma, were analyzed statistically. The relative standard deviation (RSD) of 40 data for each pixel were calculated and plotted in Fig. 4 for each configuration. Figure 4(c) shows that the instability of the discharge process caused a large fluctuation to the plasma. The larger region with lower fluctuation in Figs. 4(b) and 4(d) clearly demonstrated the repeatability improvement of the plasma morphology using cavity confinement. The average values of RSD for all pixels were 20.8%, 7.8%, 30.3% and 12.6% for Figs. 4(a)–4(d), respectively. It was noticed that in Figs. 4(a) and 4(c) there were also some small regions with very low fluctuation of the plasma morphology, which might also lead to an acceptable signal repeatability by carefully making the optical alignment. The mechanism of the cylindrical cavity confinement for plasma morphology stabilization had been discussed in our previous work [16, 17], mainly due to the regulation of the reflected shockwave.
3.3 Signal enhancement, SNR improvement and uncertainty reduction
The line intensity (peak area) and signal-to-noise ratio (SNR) of the C(I) 193.09 nm for different configurations under different laser energy were shown in Fig. 5. The noise was defined as the standard deviation of the continuum emission intensity near the characteristic line. Both the cavity confinement and spark discharge are able to increase the line intensity, so the combination of cylindrical cavity confinement and spark discharge generated the strongest line intensity in four configurations. For larger laser energy, such as 80mJ, the cavity confinement enhanced the line intensity more than spark discharge, probably because the higher laser energy generates a stronger shockwave.
Comparing with the signal intensity, SNR was more important for the sensitivity and limit of detection (LOD) of LIBS analysis. Figure 5 shows that when combining cylindrical cavity confinement and spark discharge, the SNR was the highest in four configurations, which was helpful to improve the LOD and sensitivity of LIBS analysis.
For the signal repeatability, we find that the spark discharge increase the pulse-to-pulse RSD of carbon line, as shown in Fig. 6. When combining cylindrical cavity confinement and spark discharge, the pulse-to-pulse RSD can be greatly reduced, similar with that of cylindrical cavity confinement alone. This result was accordance with the fluctuation of the plasma morphology in section 3.2. As shown in Fig. 6, the effect of cavity confinement on uncertainty reduction was more significant for spark discharge assisted LIBS. As discussed in section 3.2, when using discharge alone, the plasma morphology was more unstable than that of conventional LIBS. Therefore, the plasma stabilization effect of cavity confinement will become more prominent and necessary for spark discharge assisted LIBS. The signal repeatability can be influenced by voltage, capacitance, electrode distance, cavity shape, etc., more detailed experiments and optimization are necessary in the future research.
Using coal sample and C(I)193.09 nm for analysis, the spark discharge had been found to be an effective way to enhance the LIBS signal, while it may decrease the pulse-to-pulse signal repeatability due to the instability of the discharge process. Spatial confinement can regulate the expansion the plasma so as to stabilize the discharge process and finally stabilize the plasma morphology. Combining the moderate cylindrical cavity confinement and spark discharge, the pulse-to-pulse signal repeatability, signal intensity, and SNR were all improved comparing with only using confinement or discharge. The fluctuation of the plasma images was quantitatively analyzed to demonstrate the effect of cavity confinement on plasma morphology stabilization.
The authors are grateful for the financial support from National Natural Science Foundation of China (NO. 51276100) and National Basic Research Program (973 Program) of China (NO. 2013CB228501).
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