INTRODUCTION:

Laser Induced Breakdown Spectroscopy has been advancing significantly over the last decade. It is an atomic emission spectroscopy technique can analyze solids, liquids and gases and can return results readily, with very little damage to the sample. Laser Induced Breakdown Spectroscopy (LIBS) is a rapid chemical analysis technology that uses a short and highly energetic laser pulse to create a micro-plasma on the sample surface and provoke optical sample excitation¹. The interaction between focused laser pulses and the sample creates plasma composed of ionized matter². Plasma light emissions can provide “spectral signatures” of chemical composition of many different kinds of materials in solid, liquid, or gas state ³. LIBS can provide an easy, fast, and in situ chemical analysis with a reasonable precision, detection limits, and cost. Additionally, as there is no need for sample preparation, it could be considered as a “put and play” technique suitable for a wide range of applications ¹.

Laser-induced breakdown spectroscopy (LIBS) is an analytical technique in which a high power laser pulse (e.g.; 1-10 MW/cm²) is focused onto a sample, resulting in dielectric breakdown and the formation of a plasma containing the atomized constituents of the sample, which emit light from various electronically excited states. Emissions may result from atomic, ionic, and molecular (typically only diatomic) species in the plasma, and appear at characteristic wavelengths in the 200 – 900 nm spectral region. The wavelength of the emission may be highly diagnostic for a specific element if the spectrometer resolution is sufficient high to determine the peak position. For emission classification or identification purposes (e.g.; ± 0.02 cm^-1) it is recommended that the peak position be known with the following precision as a function of wavelength: 10,000±0.02 Å, 6,000±0.007 Å, 3,000±0.002 Å and 2,000±0.0008 Å⁴. The emission lines are spectrally resolved and recorded, typically about 1 – 5 μs after the laser pulse.

As an analytical technique, LIBS is characterized by minimal sample preparation. Compact instruments have been demonstrated that allow for potential field portability. Reviews of developments in LIBS research address applications of the technique^.5-8 LIBS applications in forensic analysis have been reported for inks⁹ and glass^,10,11 and in manufacturing applications for the analysis of paint¹² and pigments¹³. LIBS has also found use in environmental monitoring of soil¹⁴ and water^15, in the analysis and restoration of archaeological artifacts¹⁶ and works of art¹⁷, and in the analysis of biological specimens¹⁸.

LIBS analyses can be hindered by high background continuum, line-broadening, and self-absorption in strong emission peaks, which manifests itself as a loss of intensity in the center of the peak and an apparent splitting of the emission peak. The precision of LIBS data can suffer as a result of shot-to-shot laser fluctuations (typically 1 – 5%) that can lead to experimental variations in atomic emission intensity that do not follow a normal Gaussian distribution^19,20. Approaches that have been proposed to overcome these problems include “calibration free” LIBS,²¹ semi-quantitative methods,²² and correction for instrumental drift^23. Multivariate data analysis approaches including principal components analysis (PCA) and partial least squares have been examined for the analysis of LIBS data;^{24, 25} however, the accuracy of these analyses is dependent on either normally distributed data or a robust statistical method to overcome the lack of normality. Nonparametric statistical methods that do not rely on normal distribution of the data have also been examined for the analysis of LIBS results^{.26, 27} Both parametric and nonparametric statistical methods were employed to analyze the results in this research and both are addressed in section II.1 (Data analysis methods). The instrument used in this work has previously been reported to give an average of 6.5±1.4 %RSD (relative standard deviation) for a set of 11 emission intensity ratios collected in a single day from averaged LIBS spectra from a NIST SR-610 glass sample. The %RSD increased to 24.5±29.2% for spectra collected over a three day period, which favors limiting the use of this technique to comparisons of spectra collected on the same day and emphasizing the need for statistical testing for discrimination analysis. The precision of LIBS spectra is an important issue for forensic sample discrimination that was addressed in this research.

Advantages:

This analytical technique offers many compelling advantages compared to other elemental analysis techniques. These include:²⁸

· Because such a small amount of material is consumed during the LIBS process the technique is considered essentially non-destructive or minimally-destructive.

· With an average power density of less than one watt radiated onto the specimen there is almost no specimen heating surrounding the ablation site.

· Due to the nature of this technique sample preparation is typically minimised to homogenisation or is often unnecessary where heterogeneity is to be investigated or where a specimen is known to be sufficiently homogeneous,

· This reduces the possibility of contamination during chemical preparation steps.

· One of the major advantages of the LIBS technique is its ability to depth profile a specimen by repeatedly discharging the laser in the same position, effectively going deeper into the specimen with each shot.

· This can also be applied to the removal of surface contamination, where the laser is discharged a number of times prior to the analysing shot. LIBS is also a very rapid technique giving results within seconds, making it particularly useful for high volume analyses or on-line industrial monitoring.

· LIBS is an entirely optical technique, therefore it requires only optical access to the specimen. This is of major significance as fiber optics can be employed for remote analyses. And being an optical technique it is non-invasive, non-contact and can even be used as a stand-off analytical technique when coupled to appropriate telescopic apparatus.

· These attributes have significance for use in areas from hazardous environments to space exploration. Additionally LIBS systems can easily be coupled to an optical microscope for micro-sampling adding a new dimension of analytical flexibility.

· With specialised optics or a mechanically positioned specimen stage the laser can be scanned over the surface of the specimen allowing spatially resolved chemical analysis and the creation of 'elemental maps'. This is very significant as chemical imaging is becoming more important in all branches of science and technology.

· Portable LIBS systems are more sensitive, faster and can detect a wider range of elements (particularly the light elements) than competing techniques such as portable x-ray fluorescence. And LIBS does not use ionizing radiation to excite the sample, which is both penetrating and potentially carcinogenic.

· A sample preparation-free measurement experience

· Extremely fast measurement time, usually a few seconds, for a single spot analysis

· Broad elemental coverage, including lighter elements, such as H, Be, Li, C, N, O, Na, and Mg

· Versatile sampling protocols that include fast raster of the sample surface and depth profiling

· Thin-sample analysis without the worry of the substrate interference

· A typical detection limit of LIBS for heavy metallic elements is in the low-PPM range. LIBS is applicable to a wide range of sample matrices that include metals, semiconductors, glasses, biological tissues, insulators, plastics, soils, plants, soils, thin-paint coating, and electronic materials.

Disadvantages:

Like all other analytical techniques LIBS, is not without limitations.

ü It is subject to variation in the laser spark and resultant plasma which often limits reproducibility.

ü The accuracy of LIBS measurements is typically better than 10% and precision is often better than 5%.

ü The detection limits for LIBS vary from one element to the next depending on the specimen type and the experimental apparatus used. Even so detection limits of 1 to 30 ppm by mass are not uncommon, but can range from >100 ppm to <1 ppm.

PRINCIPLE:

In principle, LIBS can analyse any matter regardless of its physical state, be it solid, liquid or gas. Because, all the elements can emits light of characteristic frequencies, when they excited at sufficient high temperatures^28.

LIBS can detect all elements, limited only by the power of the laser as well as the sensitivity and wavelength range of the spectrograph and detector.

If the constituents of a material to be analyzed are known, LIBS may be used to evaluate the relative abundance of each constituent element, or to monitor the presence of impurities. In practice, detection limits are a function of

a) The plasma excitation temperature,

b) The light collection window, and

c) The line strength of the viewed transition.

LIBS make use of optical emission spectrometry similar to arc/spark emission spectroscopy. LIBS operate by focusing the laser onto a small area at the surface of the specimen; when the laser is discharged it ablates a very small amount of material, in the range of nano-grams to pico-grams, which generates a plasma plume with temperatures in excess of 100,000 K. During data collection, typically after local thermodynamic equilibrium is established, plasma temperatures range from 5,000–20,000 K. At the high temperatures during the early plasma, the ablated material dissociates (breaks down) into excited ionic and atomic species. During this time, the plasma emits a continuum of radiation which does not contain any useful information about the species present, but within a very small timeframe the plasma expands at supersonic velocities and cools. At this point the characteristic atomic emission lines of the elements can be observed. The delay between the emission of continuum radiation and characteristic radiation is in the order of 10 µs, this is why it is necessary to temporally gate the detector.

Design:

Laser Induced Breakdown Spectroscopy is technically very similar to a number of other laser-based analytical techniques, sharing much of the same hardware. These techniques are the vibrational spectroscopic technique of Raman spectroscopy, and the fluorescence spectroscopic technique of laser-induced fluorescence (LIF). In fact devices are now being manufactured which combine these techniques in a single instrument, allowing the atomic, molecular and structural characterisation of a specimen as well as giving a deeper insight into physical properties.

A typical LIBS system consists of following components like:

Laser Configurations for LIBS:

The main device of LIBS is the laser. It generates the energy to induce the plasma and mainly determines the plasma features. The main parameters related to the laser are:

· Pulse time,

· Energy per pulse, the wavelength, and

· Number of pulses per burst ^29.

Obviously, each application works better with a combination of these parameters. Nanosecond-pulsed lasers are the most common for LIBS. Therefore, the most of this section is related to this kind of laser.

Laser Wavelength:

The wavelength influence on LIBS can be explained from two points of view; the laser-material interaction (energy absorption) and the plasma development and properties (plasma-material interaction). When photon energy is higher than bond energy, photon ionization occurs and non-thermal effects are more important. For this reason, the plasma behaviour depends on wavelength in nanosecond LIBS set-ups.

In the same way, the optical penetration is shorter for UV lasers, providing higher laser energy per volume unit of material. In general, shorter the laser wavelength, then higher the ablation rate and lower the elemental fractionation³⁰.

Fig. 2: Typical diagram of LIBS

The plasma ignition and its properties depend of wavelength. The plasma initiation with nanosecond lasers is provoked by two processes; the first one is inverse Bremsstrahlung by which free electrons gain energy from the laser during collisions among atoms and ions. The second one is photoionization of excited species and excitation of ground atoms with high energies. Laser coupling is better with shorter wavelengths, but at the same time the threshold for plasma formation is higher. This is because inverse Bremsstrahlung is more favorable for IR wavelengths ^31. In contrast, for short wavelengths (between 266 and 157 nm) the photoionization mechanism is more important. For this reason, the shorter the wavelength in this range, the lower the fluence necessary (energy per unit area) to initiate ablation ³². In addition, when inverse Bremsstrahlung occurs, part of the nanosecond laser beam reheats the plasma. This increases the plasma lifetime and intensity but also increases the background at the same time. Longer wavelengths increase inverse Bremsstrahlung plasma shielding, but reduce the ablation rate and increase elemental fractionation (elemental fractionation is the redistribution of elements between solid and liquid phases which modifies plasma emission) ³³.

The most common laser used in LIBS is pulsed Nd:YAG³⁴. This kind of laser provides a compact, reliable, and easy way to produce plasmas in LIBS experiments. The fundamental mode of this laser is at 1064 nm and the pulse width is between 6 and 15 ns. This laser can provide harmonics at 532, 355, and 266 nm, which are less powerful and have shorter time pulses (between 4 and 8 nm). The fundamental and the first harmonic are the most common wavelengths used in LIBS. This harmonics can be useful to work with different wavelengths in the same environmental conditions, because a lot of Nd: YAG lasers can produce all of them. Other kinds of lasers can be used in LIBS, such as CO₂ or excimer lasers to work in far IR or UV ranges, respectively. Lasers based on fiber or dye technology can reduce the pulse width if the user is attempting to work with picosecond or femtosecond pulses.

Laser Energy:

The energy parameters related with laser material interaction are fluence (energy per unit area) and irradiance (energy per unit area and time). Ablation processes (melting, sublimation, erosion, explosion, etc.) have different fluence thresholds ³⁰. The effect of changes in the laser energy is related to laser wavelength and pulse time. Hence, it is difficult to analyze the energy effect alone. In general, the ablated mass and the ablation rate increase with laser energy.

These values are for guidance and depend strongly on laser pulse time and environmental conditions, reaching up to around for nitrogen at 760 Torr with a Nd:YAG laser, 1064 nm, 7 ^{ps 2}.

Acquisition Time and Delay:

The first stages of LIBS-induced plasma are dominated by the continuum emission. The time gate of decay of this continuum radiation change with a wide range of experimental parameters, such as laser wavelength and pulse time, ambient pressure or sample features. Besides, these experimental parameters fixes set the time periods of atomic emission, the most interesting stage of LIBS plasmas. Plasma evolution using an IR (Nd:YAG) and a UV (excimer) lasers has been analyzed to discover differences induced by laser wavelength ³¹. The plasma continuum emission stage was around 400 ns for UV laser and several microseconds for IR laser. Laser wavelength can affect the selection of delay time (gate delay) and integration time (gate window) and these parameters are essential to optimize the signal to background ratio. The analysis of plasma evolution for Zn and Cd in sand has been analyzed in other works, with an optimal gate delay of 0.5 μs and a gate window of 1.5 μs ³².

This analysis of optimal gate delay and window can be achieved optimizing the signal to background ratio and the repeatability of this parameter. For a Nd:YAG laser at the fundamental wavelength with power density of approximately 2 GW/cm2, the best compromise between lower relative standard deviation (R.S.D.) and higher signal-to-background ratio was found at a delay of approximately 6 μs. The integration time was fixed at 15 μs ²⁰.

There are different points of view to optimize the gate delay and window, and the big amount of experimental parameters involved in plasma evolution makes it difficult to recommend valid values for these parameters. The selection should be determined case by case in order to achieve a compromise between high-line intensity and low background. Briefly, for Nd:YAG lasers, both times for gate delay and gate window are in the order of microseconds but this values can change if another kind of results are sought.

Sequences of Pulses: Double Pulse LIBS

Proposed twenty-eight years ago to improve the detection limits of some elements ³³, dual pulse LIBS configuration consists on the sequence of two laser pulses temporally spaced in the order of few nanoseconds or microseconds (depending on laser pulse time, the larger the pulse time, and the larger the time delay between pulses). These two pulses can excite the same area and create two temporally spaced plasmas or the second pulse can reheat the plasma induced by the first one.

Using dual pulse LIBS, atomic emission, signal-to-noise ratio, and limit detection can be improved. Conversely, dual pulse LIBS can complicate the LIBS set-up, although the benefits can justify this complication. These improvements provide better detection limits than single pulse configurations due to the induced atomic emission enhancement.

Fig. 3: Dual pulse configurations (a) shows collinear configuration; the first pulse ablates the sample and the second one reheats the plasma ³⁴, (b) is an orthogonal pre-ablative configuration; the first pulse creates a spark on the surrounding media and the second one ablates the sample ^14,
35, (c) shows the same idea as (a), but the plasma is reheated in an orthogonal way ³⁶.

The signal emission enhancement is due to different factors in these dual pulse configurations. This enhancement in collinear and orthogonal reheating is due to energetic issues. In collinear configurations, the second pulse increases the ablated material and, mainly, reheats the plasma, increasing its volume and its emission, but if the time between laser pulses is enough to allow the fading of the plasma, the second pulse interacts with the sample and ablates more material for the reduced atmosphere induced by the first pulse. In the orthogonal configuration, the second pulse does not ablate more material, but reheats the plasma and reexcites the material ablated by the first pulse. Other authors have given reasons for the signal enhancement on preheating orthogonal configurations, such as a reduction in density or pressure of the surrounding media due to the first pulse ^{35, 37–39}.

Spectrometers and Detectors

The spectrometer or spectrograph is a device which diffracts the light emitted by the plasma. There are different designs, such as Littrow, Paschen- Runge, Echelle, and Czerny-Turner ^40,
41. The Czerny-Turner spectrograph is the most common device in LIBS. This spectrograph is composed of an entrance slit, two mirrors, and a diffraction grating. The light comes through the slit and reaches the first mirror which collimates the light, directing it onto the grating. Light is reflected at different angles according to its wavelength. The second mirror focuses the light on the focal plane where the detector is placed.

In recent years, the Echelle spectrograph has been used more extensively ⁴¹. The Echelle spectrograph uses a diffraction grating placed at a high angle, producing a large dispersion in a small wavelength range in each order. As the orders are spatially mixed, a prism is used to separate them. The orders are stacked vertically on the focal plane. For that reason, Echelle devices need a two-dimensional detector. Each vertical portion of the detector contains a part of the spectra and the software composes the whole spectrum.

Different kinds of detectors are used in LIBS, depending of the application. To measure light intensity without spectral decomposition, the photomultiplier tube (PMT) or avalanche photodiode (APD) can be used. On the other hand, for one-dimensional spatial information, the researcher can combine a spectrograph and a photodiode array (PDA) or an intensified photodiode array (IPDA) for time-resolved measurements. If two-dimensional spatial information is required, the most common devices are charge coupled devices (CCD) and intensified CCD (ICCD). A CCD detector provides less background signal, although ICCD improves the signal-to-noise ratio and is better for time-resolved detection using windows of a few nanoseconds ⁴². Another problem related to ICCDs is the price, which is much higher than CCDs.

Collecting and Focusing Light:

There are two main parts related to light acquisition and focusing. Typical laser spots currently must be focused to increase the irradiance to induce plasma formation. Light from plasma must then be collected using devices such as optical fibers, lenses, or combinations of both in order to guide it to the spectrometer.

Focusing Laser Pulses with Lenses:

As the laser beam width from the majority of solid-state lasers is of the order of 6–8 mm, the most common lenses used to focus laser pulses have diameters of 25 or 50 mm and focal lengths between 50 and 150 mm. There are other applications such as stand-off set-ups where a multilens system or different lenses are required. The material of the lens should have maximum transmission at laser wavelength and they must be coated with antireflection layers to minimize transmission losses (the laser beam can lose around 8% of its energy passing through an uncoated lens. Antireflection coatings can reduce this ratio to around 0.5%).

Collecting Light:

A common set-up to acquire plasma light is shown in Figure 4. The first lens collimates the emitted light to improve the focalization of the second one into the fiber probe and to optimize the ratio of acquired light. This set-up can be adapted to different systems using only one lens, a multilens device, and even only the fiber probe positioned in front of the spark. The features of lenses used to focus the plasma light are useful for this purpose, adapting them to different environmental features or set-ups.

Figure 4: Typical light acquisition system.

Optical Fibers:

Sometimes, the sample is far from the detection system, from the lasers, or from both of them, and so a system based on lenses like the one shown in Figure 4 is unpractical⁴³. In these situations or when the environment is too aggressive or the access restriction makes it difficult to induce the plasma or acquire the light, fiber optic cables (FOCs) or bundles are used^44–46. Although the most common optical fibers in LIBS are made of fused silica (with diameters between 50 μm and 1 mm), different kinds of fibers such as photonic-crystal fibers are used too⁴⁷. Approaches using these coiled fibers (including plastic optical fiber (POF) ones) placed around the plasma plume are also used^48,
49. Optical fiber technologies can be used to detect other significant signals from plasma such as shockwaves⁴⁶. Plasma compresses the surrounding media and generates a shockwave. This shockwave can be detected with classical microphones^{50, 51}. However, shockwaves can also be detected using other devices based on optical fiber such as Fiber Bragg Grating⁵². In addition, with this technology based on periodical refractive index changes in the fiber core, temperature, or strain, among other factors, can be detected and measured^{53, 54}.

Algorithms for Quantitative and Qualitative Analysis:

The main goal of LIBS is to achieve a chemical analysis at the atomic level, either qualitatively (i.e., to assess the presence of particular elements) or quantitatively, in which the relative amount of different elements in the sample is quantified from the processing of the acquired spectra. In both cases, a key step is the proper identification of each emission line of a particular element in a neutral or ionized state. If the sample composition is known approximately, the set-up can be adapted to find the optimal spectral range where there are emission lines of the elements under analysis or to discard emission lines of elements outside the sample. At the same time, it is interesting to know the expected relative intensity of each line. If there are doubts about one line that could belong to different elements with very close emission wavelengths, the most probable element can be inferred from the relative intensity and other theoretical and empirical information about emission probabilities ⁵⁵. By the same token, the ionization stage is important too. If two elements in different ionization stages are possible for the same emission line, the element in neutral stage should be more likely. Even more, experimental conditions can determine the emission lines in the spectral signature. For example, lines from Fe(I) and Fe(II) are possible in air, but in vacuum, Fe(III) is possible too because the ionization potential of second ionization stage is lower in this medium.

The latest research in LIBS has tried to improve the accuracy of quantitative chemical analysis. Quantitative analysis has to provide the concentration of species in the sample (parts per million), the absolute mass of species (in a particle), or a surface concentration. This analysis usually finishes in a calibration curve, but this curve is strongly dependent on experimental conditions. For this reason, the conditions should be the same when a new sample is analyzed, and sometimes this is difficult because there are many factors that can change them. An averaging of many acquired spectra can attenuate the variation of spectral emission for the analysis, but the pulse-to-pulse variation can be too high to attempt quantitative analysis.

LIBS Applications

LIBS is useful in a wide range of fields, namely, those which can benefit from a quick chemical analysis at the atomic level, without sample preparation, or even in the field. This paragraph compiles the most important applications at this moment.

LIBS in Archaeology and Cultural Heritage

Samples with archaeological or cultural value are sometimes difficult to analyze. These samples cannot usually be moved or destroyed for analysis, and some chemical techniques to prepare the sample or a controlled environment in a laboratory are needed. In the first place, portable LIBS devices can be used, solving the problem when the sample cannot be moved. In the second place, LIBS does not need contact to analyze the sample, avoiding damage in valuable samples. Although LIBS ablates an amount of the sample, the crater is nearly microscopic and practically invisible to the eye. In addition, this microscopic ablated surface improves the spatial resolution, providing accurate spatial analysis and even a depth profile analysis of the sample. The sample does not need to be prepared; hence the analysis is clean and fast. Besides, LIBS probes based on optical fibers allow the analysis of samples with difficult access. Despite these facts, LIBS is a microdestructive technique and the researcher should pay attention to experimental parameters in order to avoid critical damage in valuable samples. Many cultural heritage artifacts can be analyzed with the right LIBS set-up.

Laser Induced Breakdown Spectroscopy is feasible with virtually all types of materials, for instance ceramics, marble, bones, or metals, usually applying quantitative analysis. The most common analysis attempts to determine the elemental composition of the sample in order to help to date it^56–59, but it works with bones for analysis of paleodiet⁵⁷. LIBS has been used with delicate samples such as Roman coins⁶⁰ or other metallic alloys like bronze⁵⁶, even under water⁶¹. In the field of painting, it can determine the elements that compose the pigments. This analysis of pigments can help to date and authenticate frescos or paintings⁶². Moreover, LIBS can be used, combined with other techniques, in order to sum the potential of them, such as Raman or X-ray fluorescence (XRF)^60-64.

LIBS in Biomedical Applications:

Biomedicine and LIBS are fields that have not been working together for long. For that reason, this field may provide a large number of new developments in a few years. LIBS can analyze chemical compositions of biological samples such as human bones, tissues, and fluids⁶⁵. LIBS can help to detect excess or deficiency of minerals in tissue, teeth, nails, or bones, as well as toxic elements^{65, 66}. In the same way, cancer detection is possible with LIBS and it can provide a surgical device which can detect and destroy tumor cells at the same time⁶⁷. In addition, classification of pathogenic bacteria or virus is possible too^68,
69.

The analysis of samples from plants is difficult, because they need a difficult preparation of the sample based on acid digestion processes in order to obtain accurate analysis of micronutrients. LIBS can provide a fast analysis tool with easy sample preparation, for instance in micronutrient analysis of leaves⁷⁰.

LIBS in Industry

LIBS has been targeting many industrial processes for many years, because it is a fast analytical tool well suited to controlling some manufacturing process. Moreover, LIBS can work at a large range of distances, allowing analysis of samples in hazardous and harsh environments. For example, remote detection of explosives has been assessed with LIBS⁷¹, even at trace levels⁷².

In the nuclear energy industry, the effects of radiation on living beings and devices are widely known. LIBS can work far away from nuclear waste or reactors, using stand-off configuration or with fiber optic probes, avoiding dangerous radiation levels^73,74. In the metallurgical industry, smelters, and final products can reach high temperatures, and LIBS can analyze the alloy compositions in production line or detect impurities in other production sectors, such as the automotive industries^75–77. LIBS can also be useful to detect toxic products like heavy metals in industrial wastes ⁷⁸. These waste products should be recycled or stored, and knowing the elements in them can provide key data to reduce the environmental impact of the process. In the renewable energy field, analysis and detection of impurities in solar cells can be a useful tool to improve the manufacturing processes or to achieve high efficiency solar panels. There are recent research works in this field⁷⁹ although there is a huge amount of work to do.

LIBS and Geological Samples: Towards Extraterrestrial Limits:

Analysis of some kinds of minerals is possible using LIBS, in particular, of soils and geological samples in situ⁸⁰. Sample features can strongly affect the experimental conditions and reduce the accuracy, but quantitative analysis is still possible⁸¹. LIBS analysis can detect traces of toxic material in soils, rocks, or water without sample preparation and in the natural environment of the sample. LIBS can work in a wide range of environmental conditions and with different atmospheres, from air to vacuum. This feature, coupled with the capability to analyze soil samples and the possibility to build a portable set-up, enables the possibility to work in the space. Recently, a spacecraft has been launched to Mars to provide spectral analysis of Mars, geological samples⁸². This spacecraft contains, among other things, a hybrid LIBS-Raman spectrometer.

LIBS Challenges:

Probably, the main challenge that LIBS needs to address is its recognition as a standard in chemical quantitative analysis. Calibration-free algorithms offer a good approximation to this goal, but the results are not perfect yet⁸³. There are different research lines with the goal of a standard quantitative analysis, attempting to improve the calibration-free algorithm or add new capabilities to it. There are recent works based on spectral normalization to improve the final result⁸³ or to detect the elements in the sample automatically⁸⁴. This goal may be the most important and could place LIBS definitively among the most widely used spectrochemical techniques.

In order to widen its use in real applications, new advanced and cost-effective instrumentation is required. Currently, a cumbersome and expensive set-up is needed to achieve accurate analysis, and work is in progress to reduce the size and complexity of LIBS set-ups. New work in micro-LIBS and improvements in laser sources useful to LIBS can enable a compact and accurate set-up which makes it feasible in field work¹⁰¹. A recent (“hyphenated”) approach combines LIBS with other spectrochemical techniques in order to unite the features of them. The Mars Science Laboratory (MSL) is a good example of this because it is a hybrid LIBS-Raman system⁸⁴. Advances in new techniques and approaches for LIBS analysis, such as optical catapulting and molecular LIBS are being explored. Optical catapulting LIBS (OC-LIBS)^85,
86 uses a pulsed laser below the plasma threshold energy on the sample surface to create a solid aerosol which is analyzed with LIBS. Molecular LIBS, on the other hand, analyzes the emission of molecules resulting from sample ablation or from the recombination between target elements and ambient air⁸³. LIBS can improve its performance with this ability and so enable the analysis of organic samples ⁸⁷.

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Received on 16.11.2015 Modified on 05.12.2015

Research J. Pharm. and Tech. 9(1): Jan., 2016; Page 91-100

DOI: 10.5958/0974-360X.2016.00015.9