Introduction and Brief Summary of the LIBS Development

The reader can find the concept of laser-induced breakdown spectroscopy (LIBS) described in almost any LIBS paper. Everyone knows that 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 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. 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, according to the Scopus database, more than 14?000 papers have been published in the field of LIBS, covering fundamentals, instrumentation, and applications. Figure 1 reveals 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. Moreover, the field is still growing.

Figure 1 LIBS papers evolution according to Scopus database using specific key words.

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, pulse duration, focusing condition, wavelength, and beam quality), on the characteristics of the sample (thermal conduction, melting and vaporization temperature, and so on), and on the ambient atmosphere (pressure, composition, and thermal conduction) 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 (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 as technique 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.

During the second period, from 1990 to 2000, 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 LIBS reached a milestone with the first conference devoted to LIBS 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. In addition, 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. We have seen some growth as well in R&D reflected by several regional symposium that has been organized in Asia (ASLIBS) and Latin America (LASLIBS).

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 multielement analysis now achieves a level that is equal to, or even better than in some cases, that of classical methods. LIBS is currently considered one of the most active research areas in the field of analytical spectroscopy.

After the brief history and the introduction, this the first part of this book provides a brief explanation of the physics involved in plasma generation and the features of this plasma in LIBS (Chapter 1), then followed by a description of the basic components (Chapter 2), which compose a LIBS instrumentation. These devices are described associating their features with the properties of the laser-induced plasma. Finally, some key LIBS applications is described in Chapter 3.

This part will introduce the reader to the basic of LIBS, its fundamentals, instrumentation, and applications. It is not intended to be exhaustive survey of LIBS literature nor the state of art of the technique. It will bring generally to the reader a brief overview for the necessary ingredients needed to use the LIBS technique as analytical method for a given application and help understanding how to correlate spectra to composition and the factors affecting that correlation. It will provide a brief explanation of the physics involved in plasma generation and the features of this plasma in LIBS, followed by a description of the basic devices, which compose a LIBS instrumentation. These components will be described associating their features with the...