Applications of laser induced breakdown spectroscopy.
Applications of laser induced breakdown spectroscopy (LIBS) in solid and liquid phases have been presented. Use of single pulse and double pulse LIBS to enhance the detection limit of LIBS has been desctribed. A correlation between plasma temperature and the LIBS signal in liquid phase has been presented. Potentials of LIBS in biomedia for example hair and nail have been presented. A difference in LIBS spectrum for normal and malignant tissue has been shown.
LIBS is a technique that is able to analyze any matter in solid (Marquardt B.J et al., 1996), liquid (Kumar et al., 2003) or gaseous (Martin M. et al., 2000) form using an intense pulse of laser beam (Yueh, F.Y et al., 2000). In LIBS, a high power pulsed laser (typically in the range 10-100 mJ) is used to create a high temperature plasma spark (typically >10,000 Kelvin) which in turn vaporizes a small amount of the target sample material (typically micro-gram to pico-gram). This technique is also known as LIPS or Laser Induced Plasma Spectroscopy. The target sample material emits light that is characteristic of the elemental composition (duration less than ~10 ms). The excited plasma subsequently cools down, and the light emitted from the laser-created plasma is helpful in diagnosing the samples. The spectrum is then captured using a spectrometer in the 200 to 900 nm wavelength range. The emission from the plasma is fed to the spectrometer and the CCD detector to record the atomic signature of different elements present in the material.
Understanding the plasma dynamics is becoming more and more important because of the large number of technological applications in different areas of science and technology. Some of the research fields that include the plasma physics are material science, fluid dynamics, chemical physics, biology, space propulsions, and astrophysics (Margetic Vet al., 2003). The basic goal in different areas of research is to discover the different plasma parameters and harness plasma for useful purposes. Using lasers is one of the easiest ways to create plasma from any material.
Currently, laser produced plasma of solid and liquid material is of utmost interest, especially in the fields of laser diagnostics, laser fusion, thin film growth and chemical analysis. Laser Induced Breakdown Spectroscopy (LIBS) is an analytical technique in which laser light is used to create the plasma and the emission of the plasma is used to recognize the composition of the material. (Rusak, D.A., 1997; Singh et al., 1997). The shot-to-shot variation of this technique can be high, but spectral samples can easily be captured at rates of hundreds of hertz, producing good statistical results. Some of the major advantages to this technique lie in its simplicity. Little or no sample preparation is required, reducing the possibility of contamination. It is minimally invasive since a very small amount of sample can give good results. Since data are easily interpretable, skilled analysts are not required. The instruments themselves can be made rugged and portable. This technique is quite similar to discharge/arc emission spectroscopy in which elements are excited to a higher energy state by the electric discharge. Using a laser beam, it is possible to create the discharge like situation. A highly focused laser beam is directed onto the surface of the sample. The intense electric field of the laser light creates the breakdown and the plasma in the sample. The basic principle of laser induced ablation is shown in Figure 1.
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MATERIALS AND METHODS
It is possible to generate the LIBS signal using a high energy pulsed Nd: YAG laser or an excimer laser. Typical experimental arrangement for recording the LIBS spectra of liquid samples is shown in Figure 2. Laser light from a Q-switched frequency doubled Nd: YAG laser (Continuum Surelite III) that delivers energy up to 400 mJ in 8 ns time duration at 532 nm was focused on the center of the liquid mist stream of the nebulizer (Meinhard TR-30-C6) using a 20 cm focusing lens. The laser was operated at 10 Hz frequency, and its beam diameter was about 9 mm. In order to measure laser power, a OPHIR-OPTICS model no. 30A-P-CAL power meter can be used. In the given arrangement for liquid samples, a Meinhard nebulizer was originally designed to work in an optimized condition of 30 psi gas pressure, a gas flow rate of about 1000 ml/min, and a liquid flow rate of 5.7 ml/min for ICP measurements. However, in the case of LIBS, we observed that a liquid flow rate of 3.5 ml/min and a gas flow rate of 300 ml/min was the optimum for a good signal. In this experimental arrangement, a capillary of an internal diameter of ~ 0.22 - 0.32 mm carrying the liquid has been used. The same nebulizer can be used to create a fine jet if one stops purging the gas and increases the liquid flow rate. In the present experimental condition, we also recorded the LIBS signal for magnesium in the jet mode. The flow rate of liquid in the jet mode was 17.5 ml/min. The mist produced by the nebulizer is an aerosol that exits the tip at a very high speed, drops vertically approximately 30 mm (depending on gas and liquid flow rate), and finally disperses. The laser was focused on the center of the mist around three mm below the tip of the nebulizer to achieve a stable and reproducible signal. The closed loop system has been used for circulating the solution. Emission collected in the backward direction was coupled to an optical fiber bundle via two UV-grade quartz lenses of focal length 50 cm and 10 cm. The light entrance slit width of the spectrometer was 100 [micro]m. The fiber bundle that collects the emission from the plasma plume was a collection of 80 single fibers of 0.1 mm core diameter. The other end of the fiber was connected to the entrance slit of a remote spectrograph (Model HR 460, Instrument SA, Inc., Edison, NJ). Finally, emitted light was dispersed by a 2400-1/mm diffraction grating of dimension 75 mm x 75 mm and converted into an electrical signal by an Intensified CCD camera (Princeton Instruments Corporation, Princeton, NJ) connected to the exit end of the spectrograph. The typical gate delay used for recording the calibration curve was chosen from the experimental condition that gave the best signal-to-noise and signal-to-background ratio. Noise here is defined as the standard deviation of the background signal. The typical spectra were recorded between 1 to 10 [micro]s at a constant gate width of 10 [micro]s. The signal was processed and stored in a computer. One hundred laser shots were accumulated to obtain one spectrum, and thirty spectra were recorded under the same experimental conditions to increase the sensitivity and reduce the standard deviation.
[FIGURE 2 OMITTED]
Determination of the Elements Present in Solid and Liquid Phases:
Before starting to monitor the concentration of trace elements in the real samples, the LIBS system must be calibrated. The calibration curve for the elements can be obtained by plotting the signal intensity versus the concentration of the elements. The slope of the calibration curve along with the standard deviation in the signal decides the Limit of Detection (LOD) of the apparatus. LOD is defined as the minimum concentration of the element when the signal is three times more intense than the background noise fluctuation. The fluctuation in the background signal and the original characteristic signal lying on top of the background fluctuates in almost the same proportion. Mathematically, the LOD is defined as LOD = 3[sigma]/R, where [sigma] is the standard deviation in the background and R is the slope of the signal intensity versus concentration curve. The calibration curve of Cr signal is shown in Figure 3.
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RESULTS AND DISCUSSIONS
Application of Applications of Laser Induced Breakdown Spectroscopy in Liquid Medium:
Applications of laser induced breakdown spectroscopy of liquids presents more problems than do solids because of higher breakdown threshold, splashing of liquid and turbulences causing loss of laser energy. Different techniques such as LIBS on liquid jet both thick and thin modes, application of magnetic field around the jet (Rai et al., 2003) and use of two sequential laser pulses have been adopted to improve the technique for liquid application.
Using double pulse technique, a significant enhancement in the LIBS signal (approximately 7 times) and a consequent increase in sensitivity (i.e. decrease in LOD) to trace metals in water have been observed (Nakamura et al 1996).
Single pulse and double pulse laser induced breakdown spectroscopy of trace elements in water has been done successfully by focusing a laser beam on a thin stream of liquid in jet form. Commercially available jets, for example Meinhard Nebulizer in mist mode as well as thin jet mode, have been successfully used. (Kumar et al 2003) It has also been possible to measure different plasma parameters such as electron density and electron temperature using the Boltzmann's local thermal equilibrium (LTE) approximation and Stark broadening of the hydrogen alpha line. (Rai et al 2003)
In double pulse LIBS, the first laser ablates the liquid and the second laser finds the ablated plasma after a delay of a few microseconds. The second laser re-ionizes the rapidly cooling plasma created by the first laser, hence bringing a larger number of atomic species into an excited state. The effect of the time delay between the two laser pulses on the LIBS signal as well as on electron density has been studied. (Rai et al 2003)
An example of LIBS of Mg solution using Nd: YAG laser is presented here. Two laser pulses with a few microsecond inter-pulse delays have been used to produce and re-excite the plasma. The correlation between the electron density in the laser produced plasma at various time delays between the two lasers and the corresponding LIBS signal has been studied. The electron density is determined by using the Stark broadening of [H.sub.[alpha]] line. In this study, a Meinhard Nebulizer has been used to create the thin liquid jet that introduces the liquid sample to the laser plasma. The Meinhard Nebulizer used for LIBS was originally developed for ICP (Inductively Coupled Plasma) applications, which can work both in very fine thin jet and mist modes depending on the requirement. The experimental setup to record the plasma emission of Mg in 2% acidic solution has been shown earlier in Figure 2.
In brief, a Q-switched, frequency doubled Nd: YAG laser that gives 532 nm radiation of 8 ns duration has been used to ablate the laser jet. To enhance the LIBS signal, another Q-switched, frequency doubled Nd: YAG laser is used to re-ablate and excite the laser produced plasma. The emission from the plasma was focused using a 20-cm focusing lens on the liquid jet/mist stream of a Nebulizer (Meinhard TR-30-C6) with a nozzle diameter of about 0.3 mm. The Meinhard Nebulizer was originally made for ICP measurements. It creates a mist and is optimized to work in conditions of 30 psi gas pressure with an argon gas flow rate of about 1000 ml/min and a liquid flow rate of 5.7 ml/min. But in the case of LIBS, we observed that the liquid sample solution flow rate of 3.5 ml/min and gas (argon) flow rate of 300 ml/min gave the best signal. Also, the same nebulizer has been used to produce a very fine and stable liquid jet at the liquid flow rate of 17.5 ml/min. A closed loop system has been used for circulating the solution. Emissions have been collected in the backward direction by coupling UV grade quartz lenses of focal length 50 cm and 10 cm lenses on the optical fiber bundle. The fiber bundle is a collection of 80 single fibers of 0.1 mm core diameter. The other end of the fiber is connected to a remote spectrograph equipped with a 2400-1/mm diffraction grating. The emission signal was recorded with an Intensified CCD Camera connected to the exit end of the spectrograph. The signal is finally processed and stored in a computer. One hundred shots of spectra are accumulated for one spectrum. Thirty such spectra were recorded under the same conditions for a good statistical average.
Comparison of Double Pulse Applications of Laser Induced Breakdown Spectroscopy Signal in Mist and Thin Jet Modes:
It has been widely reported that two pulse LIBS is preferable to single pulse. Normally, after plasma is created by one pulse, it expands and the various species in their ionized state return to their neutral state by the electron-ion recombination process. The normal life-time of the plasma is of the order of 20-30 [micro]s. Light emission from the plasma is the sum of the two distinct processes. The first process is recombination of free electrons and the free ions that contribute to the background emission. The second process is the emission from the atoms and ions due to the transition from one bound state to the other. The second process causes the line emission. After creation of the laser induced plasma, both kinds of emissions decay with time but at different decay rates. Background emissions, which are stronger in the initial few microseconds, decay faster than the emissions from the atoms and ions. The bound states emissions are used to characterize the elements present in the plasma/material. The purpose of the second laser pulse is to re-excite the plume created by the first laser pulse and, after a certain time delay, re-ionize the expanding plume. The delay timing between the lasers is selected in such a manner that the ionization of the concerned species (heavy metal atoms that expand little more slowly than the lighter atomic components of water and surrounding gas) is more than the unwanted species. As a result, the enhanced LIBS signal of the element of interest (Mg in the present case) is observed.
Variations of LIBS signal intensity of 20 ppm Mg in water in the jet mode and the mist mode using the Meinhard Nebulizer is shown in Figure 4. In Figure 4, the energy of two lasers is 150 mJ and 130 mJ respectively. The gate delay and gate width are 3[micro]s and 10 [micro]s respectively which are selected after initial optimization. It is evident from Figure 4 that as we increase the time delay between the two lasers, the LIBS signal increases in the case of jet mode and decreases in the case of mist mode. It is also clear from Figure 4 that on increasing the time delay between the lasers, the LIBS signal of Mg increases. The LIBS signal maximizes around a time delay of 3 [micro]s and then starts decreasing. In mist mode, the behavior is different than in jet mode. In mist mode the LIBS signal goes on decreasing as the delay between the lasers is increased. In fact, the situation in mist is very different than in case of jet. In mist, the liquid particles are coming out in the form of fine droplets immersed in the gas (argon in the present case). When the laser beam is focused on the mist, each droplet behaves as a tiny lens that can focus the laser beam at random locations. Therefore, the second laser, which is supposed to re-excite the plasma plume created by the first pulse, may not find the right spot.
Comparison of Plasma Density in Double Pulse LIBS in Mist and Thin Jet modes:
The variation of electron density with the time delay between the lasers has been compared in Figure 5. To estimate the electron density, the [H.sub.[alpha]] line at 656.2 nm is recorded in the same experimental conditions as were used to record the LIBS spectra of magnesium lines. Stark broadening of [H.sub.[alpha]] line was used to estimate the electron temperature.
It is clear from the Figure 5, that initially when the time delay between the two lasers is zero, they behave as a single laser. The electron density in mist mode is higher than that in the jet mode due to the low ionization potential of the purging gas used in the mist mode. The laser plasma was generated in the argon atmosphere in the mist mode while it is generated in the water surrounded by the ambient gas in the jet mode. Also, for the jet mode on increasing the time delay, the electron density initially decreases then and then finally starts increasing. After the plasma generated by the first laser expands to a certain size, the second laser can effectively re-excite the plasma and increase the plasma density. Initially, when the time delay between the two lasers is zero, they work as a single laser with higher laser intensity. As the laser delay time equals 200 ns, LIBS signal intensity increases while the electron density goes down.
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The plasma density goes down because the plasma density generated by the first laser is lower than when the two laser pulses were together. It seems that initially the size of the plasma plume is so small and electron density is so high that the second laser is not able to reach the material and create its own ablation and plasma. Rather, it re-excites the plasma created by the first laser, causing an increase of the LIBS signal. With time, the size of the plasma plume becomes bigger and bigger, and the area of the plume of the plasma between the influence of the second laser becomes smaller and smaller. As a result, the density of the plasma is reduced. Beyond a critical time (delay = 1.5 [micro]s), the plasma created by the first laser becomes diluted so that the second laser is able to penetrate the plume, creating the plasma from the material. As a result, the plasma density starts increasing and reaches a maximum. Further, on increasing the time delay, the influence of the first laser becomes weaker and weaker, and thus only the characteristic of the second laser plasma becomes prominent. By comparing double pulse LIBS signals in the jet mode and mist mode, it is very obvious that jet mode is beneficial for the enhancement of the signal.
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Applications of Laser Induced Breakdown Spectroscopy in Solid Media:
Applications of laser induced breakdown spectroscopy technique have successfully been used in monitoring the trace elements in different metals, glasses and both organic and inorganic media including bio-media. An example of detection of malignant cells using the LIBS spectroscopy is presented in this paper. In common practice, a tedious time consuming procedure is used in which the sample must be sent to a sophisticated laboratory and results are awaited for days. Laser induced breakdown spectroscopy (LIBS) is an equally good technique for the diagnosis of a solid medium or a mixed medium where solids and liquids coexist together. It has the same capability to provide real-time, multi-element detection of the trace element in the human body as in liquids and solids separately. In order to record the LIBS spectrum, an Echelle spectrometer has been used that can identify the different atomic and ionic lines in the spectral region 200 to 800 nm.
The correlation of human health to trace element analysis is widely reported in literature (Reinhold, J. G. et al., 1966). It is reported that the imbalance of trace elements in the human body can lead to several acute problems such as nervousness, cancer, tumors, etc (Pihl et al., 1980). Efforts have been made to use LIBS to identify different trace elements (for example iron, sodium and magnesium) present in the human blood, nails, hair and cancer cells (Kwiatek, W.M. et al., 1996). Concentration of the vital elements is directly related to the health of the human body. Using this technique, the imbalance of trace elements in the body can be immediately detected and preventive measures can be adopted. Normally, patients must go to laboratories for pathological tests and wait to obtain the test results. By using LIBS technique, doctors and medical professionals can know immediately the levels of the trace elements in the body by just shining a spark of laser light on the nail or hair.
[FIGURE 6 OMITTED]
It will be possible to explore these variations in the human body. During laser surgery, it will be possible to monitor the malignant portion of the body (Kumar et al 2004). Applications of laser induced breakdown spectroscopy spectrum of malignant and normal cells from a dog liver are depicted in Figure 6. The difference in the two spectra is very minute. However, when the ratio of intensity of different elemental lines with calcium is plotted (Figure 7) for malignant and normal cells, the distinction between the two is very evident.
It has been demonstrated that laser light can be used to extract blood from the human body. When the laser light is focused beneath the skin, it ablates the skin and provides a path for blood to exit. In this technique, no danger or infection is expected as opposed to the usage of conventional needles for blood collection. The whole procedure requires less than a minute, with no pain or physical damage to the patient. An attempt has been made to monitor trace elements in hair and nails. It has been reported that the nails and hair have similar structures and similar contents of trace elements. The normal growth rate of nails varies from 4 mm to 8 mm per month. For persons who have a deficiency of minerals, sampling different portions of the nail can monitor the effect of medication.
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Typical LIBS spectra of hair and nails obtained in the laboratory are shown respectively in Figures 8 and 9. It is very apparent from these figures that the intensity lines of magnesium, calcium and carbon are very strong whereas sodium and potassium are very weak in nails and hair spectra. It has been observed that lines of carbon and magnesium are stronger in nails as compared to hair as shown in Figures 8 and 9. The spectra shown in Figures 8 and 9 are recorded under similar experimental conditions at laser energy of 20 mJ, gate delay of 1 [micro]s and gate width of 10 [micro]s. From the spectra of hair and nails, one can observe the similarity of their structure. This similarity has been described elsewhere (Stevens B.J, 1983). Performing the LIBS experiment with nails is easier in comparison to hair because of the nails' large flat area. There is no need of special alignment of the laser focal point to the nail surface as in the case of hair. The nail is also firmer and more solid in comparison to hair. The LIBS is more pronounced for nails (approximately three times) in comparison to hair. In LIBS literature, this phenomenon is described as the matrix effect. Mass of the material ablated in the laser induced breakdown spectroscopy depends on the dielectric properties of the material. The trace elements detected by LIBS technique give different signal intensities in different solid matrices. This is commonly known as matrix effect.
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Future applications of laser induced breakdown spectroscopy:
Laser induced breakdowns are a relatively new diagnostic tool, and it has been steadily gaining a lot of importance in different areas of technology and pure fundamental research. However, issues such as reproducibility of experimental results and production of a clean and controlled plasma using laser light still need to be worked on before LIBS becomes a standard industry tool. New additions such as use of femto-second laser (Margetic et al., 2003) and dual-pulse LIBS have taken LIBS into its next generation and have helped to resolve a number of issues being faced with this technique. There are huge potentials for LIBS in industry and the military. In the future, the development of more advanced detection systems and the cheap femto-second laser will bring this technology close to everybody's life in the coming years.
In summary, LIBS are a useful and very fast real-time technique to monitor trace elements in almost all phases of media. It has been observed that double pulse LIBS is beneficial than single pulse as this gives a lower limit of detection (LOD) of trace elements. Also it has been established by this work that there is a correlation between the LIBS signal and the plasma density. It has been shown that LIBS technique can differentiate between normal and malignant tissue. It has also been shown that LIBS is equally effective in detecting trace elements in nails and hair samples.
The authors thank Provost Dr. Luther S. Williams and Dr. L. L. Burge, Jr., for their support. Thanks are also due to Dr. H. Jain, Chair Professor, Department of Physics, Lehigh University for his interest. We thankfully acknowledge Dr. Eugene Zakar, Program Manager, Army Research Laboratory, Adelphi, MD, for providing research funds to complete this work.
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Akshaya Kumar and Prakash C. Sharma
Department of Physics, College of Engineering, Architecture and Physical Sciences
Tuskegee University, Tuskegee, AL 36088
Correspondence: Sharma, Prakash (PCSHARMA@tuskegee.edu)
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|Author:||Kumar, Akshaya; Sharma, Prakash C.|
|Publication:||Journal of the Alabama Academy of Science|
|Date:||Jul 1, 2006|
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