LIBS: Fundamentals, Benefits, and Advice to New Users

Laser-induced breakdown spectroscopy (LIBS) has developed significantly in recent years, and adoption of the technique has increased at the same time. Yet many spectroscopists are still quite unfamiliar with the technique. In this interview, Mohamad Sabsabi of the National Research Council of Canada explains the fundamentals of the technique, its recent evolution, and its benefits and limitations, and offers advice for new users.

Can you describe the concept of laser-induced breakdown spectroscopy (LIBS), for those who are new to the technique?

The reader can find the concept of LIBS described in almost any LIBS paper. Basically, everyone knows when we focus a laser beam on a sample, the irradiation in the focal volume leads to local heating of the material. When the irradiance of the laser pulse exceeds the threshold of material ablation (> MW/cm²), there is vaporization and a hot ionized gas (called a plasma) is formed. In this plasma, atoms and ions are in excited states that emit light by radiative decay. Quantitative and qualitative analyses can be carried out by collecting and spectrally analyzing the plasma light and monitoring the spectral line emission positions and intensities. The technique based on that approach is called laser-induced breakdown spectroscopy (LIBS).

The LIBS technique is a form of atomic emission spectroscopy of plasma generated by a laser focused on the material to be analyzed. It is similar to other optical emission spectroscopy techniques based on plasmas, such as spark ablation, glow discharge, inductively coupled plasma, or arc plasma techniques. However, these techniques use an adjacent physical device (electrodes or a coil) to produce the plasma, whereas LIBS uses the laser-generated plasma as the hot vaporization, atomization, and excitation source. This gives LIBS the advantage that it can interrogate samples at a distance and analyze the material without contact, independent of the nature of the sample, thus making it suitable for in-the-field and real-time analysis of any type of material, whether in the solid, liquid, slurry, or gas phase. The capabilities of LIBS to effectively carry out fast, in situ, real-time, and remote spectrochemical analysis with minimal sample preparation, and its potential applications to detect traces of a wide variety of materials, make it an extremely versatile analytical technique.

Can you tell about how LIBS has evolved?

These attributes of LIBS attracted the interest of spectroscopists, analytical chemists, and physicists since the invention of the laser in the 1960s; indeed, the first work on LIBS appeared in 1962. Since then, more than 13,700 papers have been published in the field of LIBS, covering both fundamentals and applications. Any literature will reveal the significant increase in the annual number of LIBS papers in recent decades, from a few in the 1960s to an annual rate of more than 900 today. And the field is still growing.

LIBS has been the subject of several books in recent years (1-7) and numerous review papers, and some of these document the history of the technique. In particular, I refer the reader to a chart provided in the introduction of the Cremers and Radziemski book (1) that illustrates the evolution of LIBS.

When we look at the development of the technique, we need to consider that the LIBS plasma is quite simple and yet complicated at the same time. You need a laser as a source of energy to generate the plasma. The plasma formed depends on the characteristics of the laser (energy, duration, focussing condition, wavelength, beam quality), on the characteristics of the sample (thermal conduction, melting and vaporization temperature, and so on), and on the ambient atmosphere where it is created. To extract the information from the light emitted, you need a spectrometer to diffract the light, and a detector to convert photons to an electrical signal you can work with. It involves several fields of science, such as laser–matter interaction, plasma physics, atomic physics, plasma chemistry, spectroscopy, electro-optics, and signal processing. The LIBS plasma is transient (that is, it is space- and time-dependent), unlike an inductively coupled plasma, arc plasma, or glow discharge plasma, which are all stationary. This characteristic dictates some restrictions on the ability to transfer tools used with other emission spectroscopy techniques to LIBS. Therefore, the development of LIBS over the years has been closely tied to the development of enabling tools (such as pulsed lasers, detectors, and spectrometers) and ongoing improvements in their performance.

We can distinguish four periods in the development and use of LIBS over the last five decades. During the first period, prior to the 1990s, the plasma was generated by inadequate lasers, and the emission of the plasma was observed mostly time- and space-integrated, with the limited use of single channel photomultipliers (PMT) as detectors for time-resolved spectroscopy, so only limited analytical quantification was achievable.

Then, in the 1990s, the arrival of the intensified charge-coupled device (ICCD) detector after the Cold War made it possible to observe time-resolved emission for several lines simultaneously in a given spectral window, rather than only one line as allowed by the single channel photomultiplier tube (PMT). This ability attracted some research groups to develop the understanding of the LIBS plasma and how it can be used for spectrochemistry. This development provided new capabilities for LIBS at the end of the 1990s and beginning of the 2000s, which allowed LIBS to address new emerging applications.

In addition, the echelle spectrometer coupled with an intensified charge-coupled device (ICCD) camera allowed time-resolved broadband spectra, and opened new ways to extract more information from the LIBS plasma. This capability was strengthened by the arrival of the Sony linear CCD array chip, which enabled the use of a low cost gated CCD camera. The combination of a gated CCD with low cost compact Czerny Turner spectrometers enabled a growth in the number of laboratories working on LIBS along with newcomers, and an increase of new applications that became feasible with the new capabilities. More importantly, it encouraged some LIBS spin off companies to enter the market.

In the third period, from 2000 to 2010, the first conference devoted to LIBS was held. It was organized in Pisa in 2000 by Vincenzo Palleschi's group. Since then, the series of LIBS International conferences has been organized every two years, alternating with the Euro-Mediterranean symposium conference (EMSLIBS), which was started in Cairo by Mohamed Abdel Harith's group in 2001. A similar LIBS symposium began in North America in 2007 and was organized by Jagdish Singh and Andrzej Miziolek. During that period, LIBS found its way across a variety of applications and disciplines in geology, metallurgy, planetary science, defense, food, environment, industry, mining, biology, automotive, materials science, aerospace, forensics, pharmaceuticals, security, and more. Also, more companies entered the market to commercialize LIBS systems.

In the last 10 years, the miniaturization of LIBS equipment has opened new opportunities to perform real time measurements and respond to emerging needs under conditions in which other spectroscopic techniques cannot be applied. In addition, the progress of laser technologies, such as the diode pumped laser and the fiber laser, with the improvement of the beam quality, led to better conditions for plasma generation and better analytical performance. Furthermore, the high repetition rate and the low cost of ownership of these devices have met the requirements of acceptance for several industrial applications in terms of speed of analysis and cost. Big players entered the market, and now offer handheld LIBS systems. Nowadays, as an example, the operating lifetime of a fiber laser is around 100,000 hours, or 11 years, of 24/7 use without any consumables, which is better than the TV in our houses. I remember, 26 years ago, in the first LIBS work we carried out in our laboratory at the National Research Council (NRC) of Canada, we were using an excimer laser to generate the plasma. During operation, you started with a certain energy per pulse in the morning and ended with half that energy at noon. At the same time, the solid YAG laser was bulky and very fragile, even for laboratory use.

To summarize, during the last three decades, extensive research has been carried out on the influence of the parameters affecting the analytical signal, to improve LIBS performance. Meanwhile, dynamic technological development in the field of solid state lasers, electro-optical detectors, and signal processing was successfully harnessed for LIBS. The analytical performance of LIBS for a multi-element analysis now achieves a level that is equal to, or even better than, that of classical methods. LIBS is currently considered one of the most active research areas in the field of analytical spectroscopy.

Furthermore, in the last decade, we have been witnessing more newcomers in the LIBS field from different regions of the globe, in particular from China. An analysis of the literature during this period shows clearly that China has surpassed the United States as the largest contributor to the LIBS field in terms of the number of papers.

What benefits does LIBS offer over alternative spectroscopic techniques?

To answer that fairly, the right question would be: What are the attributes of LIBS that can offer an advantage over other spectroscopic techniques for a given application? In my opinion, all techniques are useful for what they are able to do and for their ability to achieve the expected analytical performances for their appropriate use. For instance, if you have a broken leg, you go to the hospital to see an orthopedist, not an eye doctor. Both types of doctors are needed, but for different types of problems. Following the same logic, the benefits of LIBS over alternative spectroscopic techniques stem from the fields of use. The analytical community should understand that logic. In my opinion, the time has come to accept LIBS as a technique among other spectroscopic techniques.

What are the features of LIBS that make it beneficial?

I already mentioned the key features of LIBS in a previous answer. In summary, LIBS can be applied on any type of material (conductive or not), independently of the nature of the sample or its phase (solid, liquid, or gas). It has the ability to interrogate a sample in situ and remotely. It needs a minimal to no sample preparation. It can analyze any element in the periodic table, regardless of its Z number (low or high). It ablates a minute mass of materials (in the ng to µg range). It is suitable for fast, online analysis.

The literature shows clearly that LIBS has emerged as a new member of the family of analytical methods, with strengths and weaknesses like other techniques. To make this clearer for the reader, I will highlight some general features by citing a few examples (not an exhaustive list), comparing the LIBS to conventional techniques.

Solid analysis: When you analyze nonconducting samples such as mineral ore, soil, drug, ash, plastic, or wood, LIBS has the advantage of analyzing light elements (Z < 20) that cannot be analyzed by X-ray fluorescence (XRF), whether with a portable system or in the laboratory. Furthermore, the sensitivity of LIBS is better than XRF using a handheld system, not only for light elements, but also for other elements such gold and precious elements.

Analysis of metals: For the analysis of most solid metals, arc/spark spectroscopy is mostly used as the standard technique. LIBS and arc/spark spectroscopy have similar detection limits. However, LIBS is more suitable for fast analysis, whether for online or at line measurement, compared to arc/spark spectroscopy. In addition, LIBS can analyze a sample without preparation like cutting or polishing, and the laser can clean oxide or dust from the surface prior to analysis. LIBS can be used for depth profilometry with very high spatial depth resolution in the nanometer range, and also for microanalysis.

Analysis of liquids: Atomic absorption (AA) and ICP are standard techniques for the analysis of liquids. Both techniques have better relative detection limit than LIBS. However, if the need is for at line or online fast analysis, or for analysis in a harsh environment, LIBS has a clear advantage. Again, it has the advantage of eliminating the sampling step and avoids contamination.

Microanalysis: LIBS has a clear advantage for microanalysis because it operates in air at atmospheric conditions, so there is no need for a vacuum. In addition, it provides fast analysis, thanks to the high repetition rate of the laser (which can be in the range of MHz for a fiber laser).

The analysis of molten metals: ICP and arc/spark techniques cannot be applied to the analysis of molten metals because of their offline character and their need for physical contact with the sample. LIBS offers unique capabilities for that purpose, and has come to be considered a standard technique for this application. Our laboratory developed and patented an approach for LIBS analysis of molten metals (8) that is being implemented worldwide.

For the analysis of particles or inclusions, LIBS has the advantages in terms of sensitivity and speed of analysis. ICP, XRF and glow discharge–optical emission spectroscopy are not suitable for particle detection, because of difficulties related to the handling and more importantly the small mass of the particles, which is well below their minimal absolute detection limits. LIBS is a very appropriate tool for this application because of its very low absolute limit of detection (LOD) (which is element-dependent but in the range of attograms to femtograms), and also the minute mass needed for analysis.

What are the limitations of LIBS?

For some analytical applications, LIBS has some limitations and inconveniences. Here are a few examples, not to be considered an exhaustive list.

Laboratory analysis of liquids for environmental applications: Because of the small bulk of the LIBS plasma (a few mm³) and its transient nature, LIBS suffers from a relatively higher LOD compared to ICP or AA (which have stationary plasmas, with few cm³ of volume). The LOD of LIBS is element dependent, but mostly in the ppm range, or even higher in the case of a portable system. This makes it ill suited to the analysis of liquids when a lower LOD is required, such as in environmental applications. Inductively coupled plasma optical emission spectroscopy (ICP-OES) and atomic absorption spectroscopy enjoy LODs that are three orders of magnitude better than that of LIBS for the analysis of liquids.

Laboratory analysis of solid samples: The probed bulk from a sample to be analyzed by XRF is much larger than the volume ablated by LIBS during the same measurement time, which gives better sampling in XRF than in LIBS for some applications. This means that XRF is less dependent on sample homogeneity than LIBS. This statement should be taken with precaution, however. Someone would argue that this issue can be solved to some extent by using a laser with a high repetition rate, or by using a better sampling strategy. This is true for some cases, but not always feasible.

Analysis of mineral ore samples: For the analysis of precious metals in mineral ore, the analysis that can be done by LIBS is of the surface, which is not always representative of the bulk material. AA, in turn, requires samples to be digested; although digestion is a lengthy and time-consuming process, it helps ensure representative sampling. The surface analysis issue is not limited to LIBS, however. Other analytical techniques, such as XRF, infrared and Raman spectroscopy, also analyze only the surface of a sample rather than the bulk material.

Laser safety for standoff analysis: Given the ability of LIBS to interrogate a sample remotely, there are also concerns related to laser safety in an open path beam. The pulsed laser used for LIBS is considered a Class IV device, and special care should be taken in some applications to avoid exposure of the eyes to the laser beam or even its reflection or diffracted light. Sometimes, this can be done by restricting access to the area, by wearing goggles that do not transmit the laser beam, or by containing the laser beam, thus shifting the device classification from IV to I.

What are the most common application areas for this technique?

Clearly, there has been much progress in the application of LIBS. Based on the literature, we can confirm that LIBS is gaining acceptance in many industrial applications, and continues to receive significant research emphasis around the world. And we increasingly see LIBS units in corporate laboratories, and even controlling industrial processes.

LIBS has been used in many areas such as geology, ecology, geochemistry, forensics, pharmaceuticals, semiconductors, consumer electronics, metallurgy, mining, photovoltaics, metallurgy, planetary science, defense, food, the environment, industry, mining, biology, automotive, materials science, aerospace, forensics, pharmaceuticals, security, and battery applications, where consistent payback has been found using this technology. In addition, there are new areas for standoff measurements and real-time continuous process monitoring in industry, such as in raw-material or product screening for impurities and contamination, where LIBS has been implemented.

Do you have any advice for analysts using LIBS for the first time?

There is still room for newcomers to improve the analytical performance of LIBS in resolving new challenges and to explore new applications. In particular, the advent of new enabling tools (laser, detector and spectrometer) will promise bright future for reaching new areas not studied yet. Here are some recommendations for newcomers to LIBS:

1) Attend tutorial courses given at LIBS conferences to know the state of the art.

2) Remember that it is very easy to generate the plasma, but it is very complicated to make it useful for spectrochemistry.

3) For the analytical chemist and the use of chemometrics: the LIBS spectrum is very rich, and there is a lot of useful information hidden in the spectrum. Prior to any chemometrics work, check the accuracy of your experimental data, and understand their experimental conditions. Remember that we correlate the spectrum taken from the laser-generated plasmas to the sample by assuming the plasma composition is representative of the sample. This composition varies with the conditions of creating the plasmas. That condition is valid only if you control the plasma generation in a reproducible way from shot to shot. This assumes the same ablation process of creating reproducible plasma and excitation. If we put garbage data in, then the model will provide garbage data out.

4) Newcomers should be careful in using the phrase "It has been done for the first time." They should look not only at recent publications but also at the old literature. There is no excuse for newcomers, and also not for editors, for insufficient literature searches. In the patent domain, a complete search is a normal practice used by a patent agent or examiner to determine for patentability. Why can it not be done properly for papers?

5) LIBS companies should be working on solutions tailored toward the needs of users, and make it easy for them to implement the technology and use it in their day-to-day work. Not all users can afford to hire a PhD to develop a method and run a system. Users need a fast, practical solution and not a system that is so difficult to use that it ends up being shelved, thus hindering further adoption of the technique.

8) LIBS R&D institutions should focus on programs responding to both the short and long term questions where the technology can have a real impact, and avoid reinventing the wheel.

9) As LIBS continues to mature, manufacturers will go through similar learning curves to what we saw with XRF and OES technologies. Mobile OES and handheld XRF analyzers have been around for decades, and the core components have vastly improved in performance and accuracy.

What is your outlook for LIBS?

The principle of operation of LIBS is quite simple, although the physical processes involved in the laser–matter interaction are complex and still not completely understood. LIBS remains an evolving technology, as optics and photonics researchers seek new ways to take advantage of its strength and to overcome some of its challenges. In my opinion, the LIBS field is becoming crowded but not saturated yet.

In particular, as the need for quick and on-the-spot analysis is increasing, the adoption of portable and handheld instruments is gaining momentum. That importance of such instruments is due to their ability to support online analysis of samples where it is difficult to carry benchtop instruments. Their key application areas include drug identification, food inspection, environmental applications, metallurgy, and the defense sector. Portable instruments are gaining more importance especially in the food and healthcare industries. Higher growth is expected in many regions of the world, where the need for safety in drugs, food, and environmental health is increasing. Portable instruments do not require the use of reagents, do not produce analytical waste, are fast and allow on-the-spot analysis, and have increasing features and functionality. Thus, portable instruments are good candidates to respond to the growing needs for in-situ analysis and they also contribute to keeping the environment green. The portability aspect of the LIBS devices constitutes a major asset of this evolving technology. However, the level of portability needed for some applications imposes some restrictions on the choice of many of the core components used in a low cost LIBS handheld sensor unit.

References

(1) D.A. Cremers and L.J. Radziemski, Handbook of Laser-induced Breakdown Spectroscopy (Wiley. Chichester, West Sussex, UK, 2006).

(2)A.W. Miziolek, V. Palleschi, and I. Schechter, Eds., Laser Induced Breakdown Spectroscopy (LIBS): Fundamentals and Applications (Cambridge University Press, Cambridge, UK, 2006).

(3) J.P. Singh and S.N. Thakur, Eds., Laser-Induced Breakdown Spectroscopy (Elsevier, Oxford, UK, 2007).

(4) Y.-I. Lee and J. Sneddon, Laser Induced Breakdown Spectroscopy: A Practical and Tutorial Approach (Taylor & Francis, Abingdon, Oxfordshire, UK, 2016).

(5) R. Noll, Laser Induced Breakdown Spectroscopy: Fundamentals and Applications (Springer, Heidelberg, Germany, 2012).

(6) M. Baudelet, Laser-Induced Breakdown Spectroscopy: A Fundamental Approach for Quantitative Analysis (Momentum Press, Buchanan, NY, 2014).

(7) S. Musazzi, U. Perini, Eds., Laser-Induced Breakdown Spectroscopy: Theory and Applications (Springer, Heidelberg, Germany, 2012).

(8) J.M. Lucas, M. Sabsabi, and R. Heon, Method and apparatus for molten material analysis by laser induced breakdown spectroscopy. Filed in 2002, US patent 6909505B2, 2005.

Mohamad Sabsabi

Mohamad Sabsabi is a principal research officer at the National Research Council (NRC) of Canada, which he joined in 1992. Sabsabi and his team have pioneered and implemented LIBS technology for many applications. He initiated and led, for four years, the NRC High Efficiency Mining (HEM) program to improve the mining value chain by developing advanced sensors, process technologies, and advanced materials. He holds 18 patents and has more than 500 publications (papers and conference presentations) covering fundamental aspects and industrial applications of laser-induced plasmas.