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Moderate cylindrical cavity was used to regularize the laser-induced plasma for signal strength enhancement and precision improvement in laser-induced breakdown spectroscopy (LIBS). A polytetrafluoroethylene (PTFE) plate of 1.5 mm thickness with diameter of 3 mm was fabricated. It was placed closely on a sample surface and a laser pulse was shot through the center of the hole to the sample. Using coal as samples, it was verified that the configuration both enhanced the spectral line intensity and reduced shot-to-shot fluctuation, showing its great potential in improving the precision of LIBS analysis.
©2012 Optical Society of America
1. Introduction
Laser-induced breakdown spectroscopy (LIBS) is a spectrometry technology based on measuring the atomic emission of a plasma induced by a high power laser pulse. LIBS have numerous advantages. It can study samples of any physical form (powder, liquid, solid, gas, etc.). Sample preparation is simple and the analysis is fast. Therefore, LIBS find its application in many real-time and online chemical assays or monitoring [1, 2].
Due to fluctuations of various factors such as the laser energy and laser-sample interaction, the precision of LIBS analysis is always of concern. Efforts have been spent on system improvement and spectral processing algorithms to solve the problem. Researchers have proposed various methods to enhance the signal stability like the multiple pulse excitation technique [3–6], introduction of inert gas [7], and usage of spectra normalization method [8–11].
Currently, confinement has been proved to be an effective approach for signal enhancement [12–22]. A plasma confined by a cavity wall is physically different from a free expanding plasma. Firstly, the expansion of the plasma is guided and bounded by the cavity wall so that the plasma temperature (T) and electron density (ne) may vary from those in free expansion. Secondly, the shockwave reflected from the cavity wall may reheat and maintain the plasma at a higher temperature. Different cavity configurations were studied for their confinement effects. Hedwig et al. created a cavity (1 mm in diameter and 1 mm in depth) by shooting laser pulses repeatedly on a quartz sample. Results showed that the cavity enhanced the spectra line intensity [12]. Zeng et al. fabricated truncated cone cavities in the sample surface and the cavity was of 0.5 mm in depth with different diameters (80 μm, 165 μm, and 490 μm). Results showed that the cavity enhanced T and ne as well as the spectral line intensity [19]. Guo et al. put a hemispherical cavity of 11 mm diameter onto the steel target and measured a 12-fold enhancement in manganese atomic line signal. The hemispherical cavity was fabricated from aluminum, with a polished internal surface and with a 2 mm hole at the top for laser incoming and plasma emission outcoming [20]. Popov et al. used a cylinder cavity of dimension 4 mm by 4 mm to confine plasmas generated from soil, steel and aluminum samples. The limit of detection (LOD) was improved by two to five times for different elements [15, 21]. Yeates et al. diagnosed the evolution of a plasma confined by a rectangular cavity via ICCD imaging and Langmuir probes [23]. Shen et al. used the cylindrical pipes of various diameters as cavities and showed that the signal enhancement could be explained by the change of plasma temperature [22]. However, the researches on the confinement effect reported so far mainly focused on the mechanism and the intensity enhancement, but rarely on the signal repeatability for LIBS measurement as described above. Only Popov et al. studied the signal precision but with a even higher relative standard deviation (RSD) when the cavity was introduced [21].
The main objective of the present work is to seek the possibility of improving the stability of the LIBS signal using spatial confining the plasma while keeping the advantage of signal enhancement. Ideally, to obtain a high repeatable LIBS signal, the laser induced plasma should have the same morphology. Yet, due to the variation of surface condition as well as laser surface interaction, the propagation of the laser induced plasma would be affected in different way, resulting in different plasma morphology and generating relatively higher measurement uncertainty than other technologies such as inductive coupled plasma /optical emission spectroscopy (ICP-OES). Spatial confinement is possible applicable to modulate the laser induced plasma to obtain more repeatable spectra.
2. Basic logics
Figure 1 explains the basic ideas of using a confinement for plasma morphology modulate. Without the cavity wall (Fig. 1(a)), the propagation of the plasma from a same sample is affected by the surface condition (roughness), laser-sample interaction, and ambient environment and the resulted plasma morphology can differ from pulse to pulse. Furthermore, this uncontrollable behavior can change the plasma temperature, electron density, and specie density which result in additional uncertainty of LIBS signal.
Theoretically, plasma expanded axisymmetrically. With the presence of an axisymmetrical cavity (Fig. 1(b)), the propagation of a plasma is confined inside the hollow cavity. Although variation in laser-sample interaction, the roughness of the sample surface and other factors still affect the expansion of the plasma, due to the compression of the reflection of shockwave, the size and shape of the plasma could be more stable between different laser pulses. That is, there is possible that a more stable and homogeneous plasma with lower measurement uncertainty can be obtained. However, as mentioned above, there was no research reporting the improvement with the application of confinement. It was hypothesized that the undesirable effect was resulted from either too small confinement [12–14, 19], in which the plasma touches the confinement material, or too large confinement [15, 16, 20, 21], in which the reflected shockwave is not able to regularize the plasma due to energy dissipation, or the utilization of asymmetric confinement [4, 23]. With a suitable cavity diameter and shape, it was applicable to improve signal precision using confinement. Besides, the confinement by the cavity wall may re-shape the plasma such that the elongated plume becomes optically thinner with respect to the light collection optics, which is beneficial to reduce the self-absorption effects.
Furthermore, the reflected shockwave from the cavity wall would reheat the plasma, resulting in an increase in the plasma temperature, electron density, as well as signal strength, which would also be helpful to improve the measurement precision as explained below:
Under the local thermodynamic equilibrium (LTE) condition, for an atomic emission induced by an electron transition from an upper energy level i to a lower energy level j, the emitted line intensity, I, can be expressed by [24]:
where R can be calculated by the Saha equation [25]:In the equations, the superscripts I and II refer to atomic and ionic states, while the subscripts i refers to the upper energy level and j refers to the lower energy level of the transition. nI and nII, are the number density of the neutral atom and the ion at the first ionization stage, respectively. The rest of the parameters are: F – gain factor of the instrument; A – transition probability; g – degeneracy; U(T) – partition function; Ei – upper level energy of excitation; Eion – ionization energy of ground state atoms; – the ionization potential lowering factor with a typical value on the order of 0.1 eV; ns – specie number density; h – Planck constant; k – Boltzmann constant.In this case, the carbon line C(I) 193.09 nm is chosen for illustration. Using Eq. (1) and (2), the intensity of this carbon line at different plasma conditions can be calculated (Fig. 2 ). Within the simulation range of T from 7000 to 25000 K and ne from 1.5 × 1017 to 3 × 1018 cm−3, it can be observed that the intensity peaks at around T = 17000K and ne = 3 × 1018 cm−3, i.e., the red region or the region near the ridge. Far away from the ridge, the intensities can be extremely sensitive to a slightly change of T or ne. In the blue region, however, the intensity is too low. In practice, if a plasma can be engineered such that its condition maintains at the red region, then the emission signal would be maximized and become more stable. In other words, the LIBS measurement will have a higher precision.
Fig. 2 Theoretical intensity distribution of C(I)193.09nm at different plasma conditions. The intensities are normalized by their maximum value.
It should be noted that Fig. 2 is derived from theory. In reality, the optimal emission state can shift due to many factors such as the sample matrix or the ambience which alter the emission characteristic. Moreover, with our current LIBS apparatus, the spectrum was recorded for one millisecond and during the recorded time, the plasma temperature and electron density shifted greatly, weaken the relationship shown above to some extent. The emphasis of this simulation is only to show the non-linearity characteristic of the emission stability with the plasma temperature and electron number density.
3. Experimental setup
The LIBS system used was Spectrolaser 4000 (XRF, Australia) which was already described in detail in a previous paper [26]. Briefly, the laser source was a Q-switched Nd:YAG laser of 532 nm with a pulse width of 5 ns (Fig. 3 ). The detector consists of four Czerny–Turner spectrometers which covered a spectral range from 190 to 940 nm with a nominal resolution of 0.09 nm. The gate delay and integration time of the detectors were fixed at 1μs and 1 ms, respectively. The sample was bituminous coal powder. Its concentration was certified by the China Coal Research Institute (CCRI). The major composition were C (78.98%wt), H (4.95%wt), N (1.38%wt), S (1.70%wt) and Al (1.2%). For easier sampling, the powder was pressed into pellets of 30 mm diameter with a hydraulic press.
Fig. 3 Schematic of the experimental setup (not to scale) and the PTFE plate with cylindrical cavities.
The plasma generated using the current device is about 1-2 mm, and therefore to avoid direct contact of plasma and cavity. The diameter of the cylindrical cavity was chosen as 3 mm. The cylindrical cavity was drilled in a 1.5 mm thick polytetrafluoroethylene (PTFE) plate. Eleven holes were drilled on the plate. The plate was fixed closely on the pellet surface so that they would move simultaneously on a motorized sample stage in the sample chamber. The laser beam could be aimed to the center of the cavity with the help of the imaging camera of the LIBS system. The ambience condition was air at normal pressure. In the study, a spectrum was sampled from each of the 10 different locations for one experimental setting. Therefore, each setting could be completed with one pellet. This could minimize undesired signal variations due to different samples and the ambient condition. The recorded spectrum was background subtracted before processing. For each setting, the experiment was repeated three times, that is to say, three pellets.
4. Results and discussion
In this study, the laser energy was the only variable. It affected not only the plasma temperature and electron density, but also the ablated mass, and the size, lifetime, and propagation speed of the plasma. These characteristics could be directly or indirectly modified by the cavity wall. Two laser energy settings were chosen: 130 mJ and 80 mJ. The former was similar to the energies adopted in our other coal studies [27, 28], while the latter served as comparison purpose as the confinement effect was expected to be weaker at a lower ablation energy. Ten spectra were collected from each pellet and three pellets were studied for each setting. Thus, unless otherwise stated, the results presented in this section were an average of 30 data points.
4.1 Spectral intensity, plasma temperature, and electron number density
Carbon is the common element of interest in coal analysis. Two carbon lines, C(I) 193.09 nm and C(I) 247.856 nm, were studied. Figure 4 shows the enhancement of the emission by the cavity at the two energy settings. For both settings, the signal enhancement of the carbon lines can be readily observed. Table 1 summarizes the average intensities and their average standard deviations under different conditions, where the intensity means the peak area of a characteristic line and the average standard deviation was the average of three standard deviations from three repeated experiments. As Fig. 4(b) suggests that the C(I) 247.856 nm line could be saturated, only the intensities of C(I) 193.09 nm line were listed. The enhancement was more than two times in the cavity configuration. In addition, the profile of line C(I) 247.856 nm showed some saturation, therefore line C(I)193.09 nm was utilized for analyses in the present work.
Fig. 4 Spectra showing the C(I) 193.09nm and C(I) 247.856nm lines. Laser energy was (a) 80 mJ and (b) 130 mJ
Table 1. Average Line Intensity of 193 nm (a.u.) under Different Conditions
The enhancement could also be seen from the changes of plasma temperature, T, and electron density, ne, two common parameters to explain LIBS’s phenomena. Here, the temperature was obtained from the Boltzmann plots of six aluminum lines (237.312nm, 257.509nm, 308.215nm, 309.271nm, 394.401nm, 396.152nm), while the electron number density was calculated from the full width at half maximum (FWHM) of Hα line,with [25, 29]:
C(T,ne) is a weak function of T and ne, and can be obtained from Ref [25]. Table 2 lists the average plasma parameters and their average standard deviation for different settings. For both energy settings, the plasma temperature was higher when there was a cavity. This confirmed that the increment of temperature caused by the cavity could be a reason for intensity improvement.Table 2. Plasma Parameters under Different Conditions
The electron density, however, remained unchanged with the laser energy in the case of without cavity. This confirmed the free expansion characteristic of the plasma when no cavity was introduced. When the cavity was added, the cavity wall and the reflected shockwave helped to cage the plasma and made it smaller, so the electron densities were much higher.
According to the McWhirter criterion [29], a higher electron number density can help the plasma reach the LTE condition. Since LTE condition is a basic assumption for quantitative analysis, it is necessary to investigate whether the cavity can help the plasma to reach LTE condition. According to Ref [30], the LTE condition can be considered to be satisfied when the relaxation time is much shorter than the expansion time of the plasma and the diffusion length during the relaxation time is much shorter than plasma diameter. Table 3 shows the relaxation time trel and the diffusion length λ. The trel and λ were calculated using the average value in Table 2 according to the method in Ref [30].
Table 3. Relaxation Time and Diffusion Length under Different Conditions
For a typical laser induced plasma, the expansion time is about 10−6~10−5s and the plasma diameter is about several millimeter [30]. Table 3 clearly shows that the cavity can shorten the relaxation time and diffusion length, which can help to ensure the LTE condition. This may help to improve the accuracy of quantitative analysis of LIBS. It need to be pointed out that due to the compression effect of the reflected shock wave, the plasma with a cavity was expected to be slimmer, which will reducing the significance of the great reduction in diffusion length to some extent. In our future work, the plasma size will be investigated to further evaluate the significance of the diffusion length reduction.
4.2 Relative standard deviation of the line intensity
Repeatability of measurements is a key issue in LIBS applications and is the main focus of this study. The RSD is an indicator to assess the signal stability. Table 4 shows the RSD of the C(I) 193 nm line of each experiment and the standard deviation of the RSD, where the standard deviation was calculated from the three RSD of three repeated experiments.
Table 4. RSD (%) of the C(I) 193 nm of Each Experiment Setting
At the laser pulse energy of 80 mJ, the RSD of the signal were decreased from 5.2 to 4.1 when a confinement was added. When the laser energy was increased to 130 mJ, the RSD of the signal were decreased from 12.2 to 7.8. The improvement of RSD may be attributed to the following: 1) the temperature and the electron density was pushed to a certain range such that the intensity was less sensitive to T and ne, such as the “ridge” in Fig. 2; 2) the shockwave reflected from the cavity wall regulates the expansion process of the plasma to makes the plasma more stable and homogeneous; 3) the confinement effect may reduce the signal sensitivity to morphologic effect.
In addition, the standard deviation of the RSD was also much reduced when a cavity was added. This further indicated that the cavity not only improve the pulse-to-pulse signal repeatability, but also improve the sample-to-sample repeatability.
The results proved the applicability of the utilization of cavity to enhance signal strength as well as to improve signal stability. It was also noticed that laser energy, delay time, and other parameters such as the cavity diameter/thickness, the confinement shape, and the material of the cavity change final effect of signal repeatability improvement. Systematic studies of the energy-dependent, spatial and temporal variation of LIBS plasma are planned to further investigate the mechanism and effects of the moderate confinement.
5. Conclusions
Moderate cylindrical cavity was used to confine and regularize the laser induced plasma. Compared to the cases without cavity, the line intensity was enhanced due the increase in plasma temperature and electron density. More important, the uncertainty of the signal can be reduced by the confinement as shown in our experiments. That is, the confinement effect shows great potentials to improve precision of LIBS analysis. Yet further studies were required to understand the role of the cavity and the reactions between the shockwave and the plasma. Other parameters, such as the delay and integration time of the spectrometer or the dimension of the cavity, were also critical, and should be investigated in detail as well in future studies.
Acknowledgment
The authors are grateful for the financial support from National Natural Science Foundation of China (No. 51276100).
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