Laser Induced Breakdown Spectroscopy

by Andrzej W. Miziolek, Vincenzo Palleschi, Israel Schechter

Overview

Laser Induced Breakdown Spectroscopy (LIBS) is an emerging technique for determining elemental composition. With the ability to analyse solids, liquids and gases with little or no sample preparation, it is more versatile than conventional methods and is ideal for on-site analysis. This is a comprehensive reference explaining the fundamentals of the LIBS phenomenon, its history and its fascinating applications across eighteen chapters written by recognized leaders in the field. Over 300 illustrations aid understanding. This book will be of significant interest to researchers in chemical and materials analysis within academia and industry.

Product Details

ISBN-13:	9780521852746
Publisher:	Cambridge University Press
Publication date:	09/07/2006
Pages:	640
Product dimensions:	7.01(w) x 10.04(h) x 1.42(d)

About the Author

ANDRZEJ MIZIOLEK is a Senior Research Physicist at the US Army Research Laboratory. His work is currently concentrated on nanomaterials research and on the development of the LIBS sensor technology.

VINCENZO PALLESCHI is a researcher in the Institute for Chemical-Physical Processes at the Italian National Research Council and, in particular, the Applied Laser Spectroscopy Laboratory.

ISRAEL SCHECHTER is Professor of Chemistry in the Dept of Chemistry at the Technion - Israel Institute of Technology. His main scientific interest is in new methods for fast analysis of particulate materials.

Read an Excerpt

Cambridge University Press
978-0-521-85274-6 - Laser-Induced Breakdown Spectroscopy (LIBS) - Fundamentals and Applications - Edited by Andrzej W. Miziolek, Vincenzo Palleschi and Israel Schechter
Excerpt

1

History and fundamentals of LIBS

David A. Cremers
Chemistry Division, Los Alamos National Laboratory

Leon J. Radziemski
Physics Department, Washington State University

1.1 Introduction

Laser-induced breakdown spectroscopy (LIBS) is a method of atomic emission spectroscopy (AES) that uses a laser-generated plasma as the hot vaporization, atomization, and excitation source. Because the plasma is formed by focused optical radiation, the method has many advantages over conventional AES techniques that use an adjacent physical device (e.g. electrodes, coils) to form the vaporization/excitation source. Foremost of these is the ability to interrogate samples in situ and remotely without any preparation. In its basic form, a LIBS measurement is carried out by forming a laser plasma on or in the sample and then collecting and spectrally analyzing the plasma light. Qualitative and quantitative analyses are carried out by monitoring emission line positions and intensities. Although the LIBS method has been in existence for 40 years, prior to 1980, interest in it centered mainly on the basic physics of plasma formation. Since then the analyticalcapabilities have become more evident. A few instruments based on LIBS have been developed but have not found widespread use. Recently, however, there has been renewed interest in the method for a wide range of applications. This has mainly been the result of significant technological developments in the components (lasers, spectrographs, detectors) used in LIBS instruments as well as emerging needs to perform measurements under conditions not feasible with conventional analytical techniques. A review of LIBS literature shows that the method has a detection sensitivity for many elements that is comparable to or exceeds that characteristic of other field-deployable methods.

1.2 Basic principles

1.2.1 Introduction

Setting . . . the surface of the Moon

The laser played across the cliff face in double waves. First a gentle scan lit up every millimeter of the sheer sedimentary surface, while widely spaced recording devices read its reflections, noting every

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microscopic contour and color variation. Then, when that first scan was finished, the machine sent forth a much more powerful second beam, which seared away a thin layer wherever it touched. The monitors now recorded glowing spectra from these vapors, taking down elemental compositions in minute detail.

The above excerpt is from the science fiction book Murasaki [1] and very obviously describes the use of LIBS for the analysis of lunar terrain preceded by what is apparently reflection spectroscopy. Although at present fiction, the use of LIBS for space exploration is actively being pursued for use in the near future [2]. This potential application, along with other implemented and proposed applications of LIBS, highlights the versatility of the LIBS method. In fact a review of these applications and the LIBS literature in general leads to the conclusion that LIBS is perhaps the most versatile method yet developed for elemental analysis. In this chapter we present a brief history of LIBS, its development since the invention of the laser, and some early applications. In addition, some of the fundamental characteristics of the method are described to lay a foundation for subsequent chapters in this book. There are several reviews relating to LIBS that will serve as good background material to understanding the method [3–8].

LIBS, an analytical method born along with the invention of the laser, has had a checkered past. First, the ablation produced by the action of the laser pulse on the sample surface was exploited as a sampling method for use with the electrode-generated spark because all materials could be ablated and the finely focused laser pulse provided micro-sampling capabilities [9]. Subsequently, it was realized that the laser plasma generated during ablation could be used as an excitation source itself. However, with the development of high-performance laboratory-based elemental analysis methods (i.e. the inductively coupled plasma or ICP), the LIBS method was (temporarily) relegated to merely a scientific curiosity, with published literature devoted more to studying fundamental characteristics of the laser plasma than to its analytical capabilities. In recent years, however, there has been strong, renewed interest in LIBS as revealed by the number of published papers in refereed journals over the past several years. This is shown in Figure 1.1. Although there has always been a steady flow of publications dealing with LIBS, beginning in about 1995 the number per year has increased dramatically. The data in Figure 1.1 were compiled from a single database using LIBS as the keyword. Many more publications than listed here dealt with the phenomenology of the

Figure 1.1. Number of LIBS-related publications during the past 11 years. At the time of this writing, not all publications for 2002 had yet been entered into the database.

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laser plasma, but LIBS, which is more closely associated with analytical applications of the plasma, was the interest here.

   The renewed interest in LIBS can be related to several factors. First is the need for a new method of analyzing materials under conditions not possible using current analytical methods. This is driven in part by new regulations mandating that materials and operations be monitored to ensure the health and safety of workers and the public, as well as by the need for improved industrial monitoring capabilities to increase the efficiency and reduce costs of production. Second, over the past five years there have been substantial developments in reducing the size and weight while increasing the capabilities of lasers, spectrographs, and array detectors. This makes feasible the development of compact and rugged instrumentation for use in applications outside the laboratory. The number of patents issued for LIBS-based devices also shows increased interest in the method from a technology viewpoint. Currently there are over 65 LIBS-related patents world-wide.

1.2.2 Atomic emission spectroscopy

LIBS is one method of atomic emission spectroscopy (AES). The purpose of AES is to determine the elemental composition of a sample (solid, liquid, or gas). The analysis can range from a simple identification of the atomic constituents of the sample to a more detailed determination of relative concentrations or absolute masses. Basic steps in AES are:

• atomization/vaporization of the sample to produce free atomic species (neutrals and ions),
• excitation of the atoms,
• detection of the emitted light,
• calibration of the intensity to concentration or mass relationship,
• determination of concentrations, masses, or other information.

Examination of the emitted light provides the analysis because each element has a unique emission spectrum useful to “fingerprint” the species. Extensive compilations of emission lines exist [10–12]. The position of the emission line(s) identifies the element(s) and, when properly calibrated, the intensity of the line(s) permits quantification. The specific procedures and instrumentation used in each step of AES are determined by the characteristics of the sample and by the type of analysis (i.e. identification vs. quantification). It should be noted that because the first step in AES is atomization/vaporization, AES methods are generally not suitable to determine the nature of compounds in a sample. In specific cases, however, information can be obtained about molecular origins.

The beginnings of AES can be traced back to the experiments of Bunsen and Kirchhoff (c. 1860) in which atomization and excitation were provided by a simple flame [13, 14]. Following this, more robust and controllable methods of excitation were developed by using electrical current to interrogate the sample. Some of the more well-known methods of vaporization and excitation include electrode arcs and sparks, the ICP, the direct coupled plasma (DCP), the microwave-induced plasma (MIP), and hollow cathode lamps [15]. These traditional sources typically require significant laboratory support facilities and some form of sample preparation prior to performing the actual analysis. In special cases, novel sampling methods have been developed for some of these sources for specific applications. Examples are an air-operated ICP providing direct analysis of particles contained in air and the introduction of particles collected on a filter into the hollow electrode of a conventional spark discharge. For various reasons, these methods saw very limited use. LIBS is an extension of the vaporization/excitation scheme to optical frequencies [16].

1.2.3 The discovery of LIBS

The production of dielectric breakdown by optical radiation, the process generating the laser plasma used by LIBS, had to wait until the development of the laser in 1960 [17]. Prior to 1960, however, the ability to produce dielectric breakdown in gases had been known for at least 100 years. These discharges can be produced fairly easily in low-pressure gas tubes with or without electrodes, at frequencies in the range of hundreds of kilohertz to a few tens of megahertz. Examination of the spectra from these sources reveals atomic emissions characteristic of the gas composition. In subsequent years, the breakdown of gases induced by frequencies on the order of gigahertz was demonstrated at reduced pressures using microwave range electromagnetic fields. Experiments were carried out at reduced pressures because the breakdown threshold shows a minimum in rarefied gases. At atmospheric pressure, the electric field required for breakdown by static and microwave fields is on the order of tens of kilovolts per centimeter. At optical frequencies the situation requires much stronger fields on the order of 10 MV/cm. Such strong fields are not attainable using conventional optical sources, thereby requiring the development of a new light source.

   In 1960, laser operation was first reported in a ruby crystal. Following this in 1963 came the development of a “giant pulse” or Q-switched laser. This laser had the capability of producing high focused power densities from a single pulse of short duration sufficient to initiate breakdown and to produce an analytically useful laser plasma (also called the laser spark). This was the “birth” of the LIBS technique and in subsequent years significant milestones were made in the development of the method. Here is a list of some of the more important milestones.

• 1960 – First laser demonstrated.
• 1962 – Brech and Cross [18] demonstrate the first useful laser-induced plasma on a surface.
• 1963 – The first analytical use, involving surfaces, hence the birth of laser-induced breakdown spectroscopy.
• 1963 – First report of a laser plasma in a gas.
• 1963 – Laser micro-spectral analysis demonstrated, primarily with cross-excitation.
• 1964 – Time-resolved laser plasma spectroscopy performed.
• 1966 – Characteristics of laser-induced air sparks studied.
• 1966 – Molten metal directly analyzed with the laser spark.
• 1970 – Continuous optical discharge reported.
• 1970 – Q-switched and non-Q-switched lasers used and results compared.
• 1972 – Steel analysis carried out with a Q-switched laser.
• 1980 – LIBS developed for analysis of hazardous aerosols.
• 1980 – LIBS used for diagnostics in the nuclear power industry.
• 1984 – Analysis of liquid samples demonstrated.
• 1989 – Metals detected in soils using the laser plasma method.
• 1992 – Portable LIBS unit for monitoring surface contaminants developed.
• 1992 – Stand-off LIBS for space applications demonstrated.
• 1993 – Underwater solid analysis via dual-pulse LIBS.
• 1995 – Demonstration of LIBS using fiber optic delivery of laser pulses.
• 1997 – Use of LIBS for pigment identification in painted artworks.
• 1998 – Subsurface soil analysis by LIBS-based cone penetrometers.
• 2000 – Demonstration of LIBS on a NASA Mars rover.

1.3 Characteristics of LIBS

1.3.1 The LIBS method in brief

In LIBS, the vaporizing and exciting plasma is produced by a high-power focused laser pulse. A typical LIBS set-up is shown in Figure 1.2. Pulses from a laser are focused on the sample using a lens and the plasma light is collected using a second lens or, as shown in Figure 1.2, by a fiber optic cable. The light collected by either component is transported to a frequency dispersive or selective device and then detected. Each firing of the laser produces a single LIBS measurement. Typically, however, the signals from many laser plasmas are added or averaged to increase accuracy and precision and to average out non-uniformities in sample composition. Depending on the application, time-resolution of the spark may

Figure 1.2. Diagram of a typical laboratory LIBS apparatus. Here: L = laser; M = mirror; LP = laser pulse; CL = lens; P = plasma; T = target; FOC = fiber optic cable; S = spectrograph; AD = array detector; GE = gating electronics; C = computer.

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Figure 1.3. Left: the laser plasma formed on soil by a spherical lens is about 4–5 mm in height. Right: the long spark formed on a filter by a cylindrical lens is 7–8 mm in length.

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improve the signal-to-noise ratio or discriminate against interference from continuum, line, or molecular band spectra.

   Photos of laser plasmas formed on soil (by a spherical lens) and on a filter (by a cylindrical lens) are shown in Figure 1.3. To the eye, the plasma appears as a bright flash of white light emanating from the focal volume. Often the plasma formed by a spherical lens appears triangular shaped owing to formation of the initial breakdown at the focal point followed (during the laser pulse) by growth of the plasma back towards the focusing lens. Accompanying the light is a loud snapping sound owing to the shock wave generated during optical breakdown.

   Because the laser plasma is a pulsed source, the resulting spectrum evolves rapidly in time. The temporal history of a laser-induced plasma is illustrated schematically in Figure 1.4a. At the earliest time, the plasma light is dominated by a “white light” continuum that has little intensity variation as a function of wavelength. This light is caused by

Figure 1.4. (a) The important time periods after plasma formation during which emissions from different species predominate. The box represents the time during which the plasma light is monitored using a gatable detector. Here t_d is the delay time and t_b the gate pulse width. The timing here corresponds to an RSS experiment. (b) Important timing periods for a double-pulse RSP measurement. Here Δt is the time between the closely spaced double pulses.

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bremsstrahlung and recombination radiation from the plasma as free electrons and ions recombine in the cooling plasma. If the plasma light is integrated over the entire emission time of the plasma, this continuum light can seriously interfere with the detection of weaker emissions from minor and trace elements in the plasma. For this reason, LIBS measurements are usually carried out using time-resolved detection. In this way the strong white light at early times can be removed from the measurements by turning the detector on after this

Figure 1.5. The LIBS spectrum evolves as the plasma cools. The spectra in the column on the right are expanded regions of the corresponding spectra on the left. Here, t_b  =  0.5 μs (top eight spectra); t_b  =  2 μs (bottom two spectra).

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white light has significantly subsided in intensity but atomic emissions are still present. The important parameters for time-resolved detection are t_d, the time between plasma formation and the start of the observation of the plasma light, and t_b, the time period over which the light is recorded (Figure 1.4a).

   The majority of LIBS measurements are conducted by using the RSS (repetitive single spark) in which a series of individual laser sparks are formed on the sample at the laser repetition rate (e.g. 10 Hz). In some cases, to enhance detection capabilities, the RSP (repetitive spark pair) is used. The RSP is a series of two closely spaced sparks (e.g. typically 1–10 μs separation) used to interrogate the target at the laser repetition rate. The timing arrangement in this case is shown in Figure 1.4b. Note that t_d is measured from the second laser pulse in this case. The spark pair may be formed by two separate lasers or by a single laser.

   Figure 1.5 shows the evolution of the LIBS spectrum from a soil sample. Several important features should be noted. First, note the significant decrease in line widths as the delay time changes from 0 to 7 μs. This is particularly evident in the two strongest lines (once-ionized Ca) on the left side of the figure. Second, as the line widths decrease, it becomes evident at t_d = 0.5 μs that two additional lines (neutral Al between the Ca lines) appear that were masked by the strong Ca lines. Third, comparison of the relative intensities of the Ca and Al lines shows that these change as the plasma cools with the once-ionized Ca lines decreasing more in comparison with the neutral Al lines with increased delay time. These same features are evident in the expanded portions of the spectrum displayed on the right side of the figure showing Fe and Sr lines.

1.3.2 The physics and chemistry of the laser plasma

A life cycle schematic for a laser-induced plasma on a surface is shown in Figure 1.6. The physics of the breakdown phase was well reviewed by Weyl in 1989 [19]. Briefly, there are two steps leading to breakdown due to optical excitation [20]. The first involves having or generating a few free electrons that serve as initial receptors of energy through three-body collisions with photons and neutrals. The second is avalanche ionization in the focal region. Classically, free electrons are accelerated by the electric fields associated with the optical pulse in the period between collisions, which act to thermalize the electron energy distribution. As the electron energies grow, collisions produce ionization, other electrons, more energy absorption, and an avalanche occurs. In the photon picture, absorption occurs because of inverse bremsstrahlung. The breakdown threshold is usually specified as the minimum irradiance needed to generate a visible plasma.

Following breakdown, the plasma expands outward in all directions from the focal volume. However, the rate of expansion is greatest towards the focusing lens, because the optical energy enters the plasma from that direction. A pear- or cigar-shaped appearance results from this nonisotropic expansion. The initial rate of plasma expansion is on the order of 10⁵ m/s. The loud sound that one hears is caused by the shock wave coming from the focal volume.

Figure 1.6. Life cycle diagram showing main events in the LIBS process.

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   Between its initiation and decay, the plasma evolves through several transient phases, as it grows and interacts with the surroundings. These are well described, for different irradiance regimes, by Root [21]. The three models for propagation and expansion are the laser-supported combustion (LSC), laser-supported detonation (LSD), and laser-supported radiation (LSR) waves. They differ in their predictions of the opacity and energy transfer properties of the plasma to the ambient atmosphere. At the low irradiances used in LIBS experiments, the models that most closely match experiment are LSC and LSD. In these, the plasma is at relatively low temperature and density. The plasma and the boundary with the ambient atmosphere are transmissive enough to allow the incoming laser radiation to penetrate, at least for laser wavelengths shorter than that of the CO₂ laser (10.6 μm).

   Throughout the expansion phase the plasma emits useful emission signals. It cools and decays as its constituents give up their energies in a variety of ways. The ions and electrons

Figure 1.7. Air plasma temperature as a function of time after plasma formation. Data abstracted from reference [3] using Saha and Boltzmann data from carbon and beryllium lines.

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recombine to form neutrals, and some of those recombine to form molecules. Energy escapes through radiation and conduction.

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© Cambridge University Press

Table of Contents

Preface R. Russo and A. W. Miziolek; 1. History and fundamentals of LIBS D. A. Cremers and L. J. Radziemski; 2. Plasma morphology I. Schechter and V. Bulatov; 3. From sample to signal in laser induced breakdown spectroscopy: a complex route to quantitative analysis E. Tognoni, V. Palleschi, M. Corsi, G. Cristoforetti, N. Omenetto, I. Gornushkin, B. W. Smith and J. D. Winefordner; 4. Laser induced breakdown in gases: experiments and simulation C. G. Parigger; 5. Analysis of aerosols by LIBS U. Panne and D. Hahn; 6. Chemical imaging of surfaces using LIBS J. M. Vadillo and J. J. Laserna; 7. Biomedical applications of LIBS H. H. Telle and O. Samek; 8. LIBS for the analysis of pharmaceutical materials. S. Béchard and Y. Mouget; 9. Cultural heritage applications of LIBS D. Anglos and J. C. Miller; 10. Civilian and military environmental contamination studies using LIBS J. P. Singh, F. Y. Yueh, V. N. Rai, R. Harmon, S. Beaton, P. French, F. C. DeLucia, Jr., B. Peterson, K. L. McNesby and A. W. Miziolek; 11. Industrial applications of LIBS R. Noll, V. Sturm, M. Stepputat, A. Whitehouse, J. Young and P. Evans; 12. Resonance-enhanced LIBS N. H. Cheung; 13. Short-pulse LIBS: fundamentals and applications R. E. Russo; 14. High-speed, high resolution LIBS using diode-pumped solid state lasers H. Bette and R. Noll; 15. LIBS using sequential laser pulses J. Pender, B. Pearman, J. Scaffidi, S. R. Goode and S. M. Angel; 16. Micro LIBS technique P. Fichet, J-L, Lacour, D. Menut, P. Mauchien, A. Rivoallan, C. Fabre, J. Dubessy and M-C. Boiron; 17. New spectral detectors for LIBS M. Sabsabi and V. Detalle; 18. Spark-induced breakdown spectroscopy: a description of an electrically-generated LIBS-like process for elemental analysis of airborne particulates and solid samples A. J. R. Hunter and L. G. Piper.