**2. Laser-induced breakdown spectroscopy (LIBS)**

The spectrochemical analysis exploits the electronic quantized transitions, which are characteristic of each individual element. An energy source (i.e. flame, plasma, laser, arc) can excite atomic species that emit specific wavelengths or frequencies upon returning to their fundamental state. The emitted light is spectrally resolved and detected to determine the elemental composition of the sample. The use of laser for the ablation process in the spectrochemical and elemental analysis was first proposed by Brech and Cross in 1962 [6]. At that time, the luminous plume produced by the ruby laser on the surface of metallic and non-metallic materials was able to remove a small mass of the target in the form of atoms and small particles, but too weak to provide usable spectra. Thus, an auxiliary electrical spark for excitation of vaporized and atomized material was added [7]. This study marks the conception of laser-induced breakdown spectroscopy (LIBS). Although the instrumentation proposed by Brech and Cross was soon commercially available, the interest in using LIBS rise in the 1980s. Advances in technology, more specifically the development of powerful and robust lasers, high-resolution optics, high sensitive detectors, and fast electronics for data acquisition have all contributed to its acceptance in routine analysis and increasing adoption as a cost-effective alternative analytical technique. Besides the ablation of the sample surface, using powerful modern lasers a microplasma is formed that excites the ablated atoms, ions, and molecular fragments. The plasma continues this excitation, and can also vaporize small ablated particles, atomize and excite atoms, ions, and molecules obtaining a rich emission spectrum used for qualitative and quantitative analysis [8]. LIBS is a highly versatile and adaptable spectroscopy technique. It has increasingly become a powerful tool for fast multi-elemental analysis in several fields of applications such as industrial, agriculture, environmental, food, geological, and biomedical. The growing interest on LIBS is probably due to the simplicity of the technique and instrumentation as sampling and subsequent excitation of atoms, ions and molecules rapidly occurs in one step and in the same system, i.e. there is no transportation of ablated material to another instrument as in LA-ICP-MS. It is a universal technique because any type of sample can yield a LIBS spectrum [8, 9]. The most notorious advantages of LIBS are the direct solid analysis capability, the quasi nondestructive analysis, and the portable instruments for analysis in the field. On the other hand, the technique presents limited detectability and it is not useful for trace element analysis unless physical and chemical enhancement strategies are applied to overcome this drawback.

## **2.1 Fundamentals and principals of operation**

In practice, LIBS is a very simple spectroscopic technique to implement. A highpowered density pulsed laser beam is focused through lenses in (liquid or gaseous) or on (solid) the sample to produce dielectric breakdown leading to plasma formation. Plasma is a partially ionized gas containing atoms, ions and free electrons, and electrically neutral. Once initiated, the plasma induces the ablation of a finite quantity of the sample surface to a condition that may then be excited by the energy

**139**

*Laser Chemical Elemental Analysis: From Total to Images*

supplied by the same pulse or by a subsequent pulse of the laser beam (double pulse strategy) [9, 10]. Excitation is followed by emission of electromagnetic radiation which is collected, often through a fiber optic cable, and directed into a spectrom-

The interaction between a laser pulse and mater to create LIBS plasmas involves a process dependent on characteristics of both the laser, *e.g.* wavelength, irradiance and pulse duration, and the sample, *e.g.* gases, liquids, and solids, which can be conductive and non-conductive samples. When the laser pulse in the nanosecond time regime reaches the sample, the dominant mechanism is the thermal ionization process [9, 10]. A process of scattering transfers the laser energy to the lattice of the spot targeted on the sample causing the melting and generation of larger particles, thus enlarging the size distribution of the aerosol particles [11]. Plasma life stages include plasma ignition, plasma expansion and cooling and particle ejection and condensation. The physics of the plasma generation, evolution, and termination processes are complex. In the case of the analysis of solid samples, *i.e.* the majority of LIBS applications, plasma is initiated when the power density of the laser exceeds

W cm−2 is

eter where it is spectrally resolved and further instrumentally detected.

the breaking limit of a solid surface. In general, irradiance above 108

solid sample exfoliation, not interesting for LIBS analysis [10].

After 10−9 s and 10−8 s of the ignition, plasma induced by laser in the nanosecond regime becomes opaque due to laser pulse absorption by electrons in the plasma. It reduces the ablation rate because only a fraction of radiation reaches the sample surface. This phenomenon is called shielding and induces a crater with melted and deposited material around it [10]. Meanwhile, plasma is reheated, and the lifetime and size of plasma are higher. On the other hand, ultra-short pulses (femtoseconds) lead minimum plasma shielding and crater with highly defined edges without melted or deposited materials [9, 10]. This regime of pulse is too short to induce thermal effects in the breakdown process. Electronic excitation and ionization, and Coulomb explosion, are the main bond breaking and plasma ignition mechanisms, making the ablation mechanism mostly based on photochemical reactions resulting in vaporization presenting sharper size-distribution solid aerosol than those

needed. The breakdown is promoted by the intense electric field gradient of the laser, and atoms, ions, and electrons result from the deposition of energy into the target. Afterward, a high-pressure vapor is produced while the plume compresses the surrounding gas. A shockwave at supersonic speed is generated from the surface towards the surrounding atmosphere during vapor expansion. The absorption of the incident laser energy and transfer to the plasma occurs through the inverse Bremsstrahlung absorption involving interactions between free electrons, atoms, and ions. Free electrons in the hot vapor absorb photons from the incident laser increasing their kinetic energy to ionize additional atoms by collisions. The new electrons absorb more photons from the remaining pulse so that a cascade of ionization is generated [8–10]. Plasma is created having distinctive characteristics, high temperatures (10.000 K), and high electron densities (>1017 cm−3). The removed material in the ablation process of the solid samples contributes to plasma formation and expansion. In fact, the plasma is only sustained due to the presence of the ablated material [9]. The plasma expands in a similar way the initial breakdown occurs, at supersonic velocities producing a shockwave, also through the process of inverse Bremsstrahlung absorption. Meanwhile, electron number density and temperature of the plasma changes. The remaining laser energy from the laser duration of the pulse is continuously absorbed by the plasma. Collisions with the surrounding gas reduce plasma velocity of propagation, and then plasma cools down by self-absorption and recombination between electrons and ions, generating neutral species and clusters after plasma extinction [8–10]. Part of ablated mass are particles and these particles create condensed vapor, liquid sample ejection, and

*DOI: http://dx.doi.org/10.5772/intechopen.94385*

#### *Laser Chemical Elemental Analysis: From Total to Images DOI: http://dx.doi.org/10.5772/intechopen.94385*

*Practical Applications of Laser Ablation*

**2. Laser-induced breakdown spectroscopy (LIBS)**

the discussion regarding the instrumentation, some processes involved in the image acquisition and formation, and also on the analytical results and figures of merit for a diversity of methods involving different areas as geochemical, environmental, biological, medical, and forensic. Some trends aspects and perspectives in the application of laser in chemical analysis are also the focus of this Chapter.

The spectrochemical analysis exploits the electronic quantized transitions, which are characteristic of each individual element. An energy source (i.e. flame, plasma, laser, arc) can excite atomic species that emit specific wavelengths or frequencies upon returning to their fundamental state. The emitted light is spectrally resolved and detected to determine the elemental composition of the sample. The use of laser for the ablation process in the spectrochemical and elemental analysis was first proposed by Brech and Cross in 1962 [6]. At that time, the luminous plume produced by the ruby laser on the surface of metallic and non-metallic materials was able to remove a small mass of the target in the form of atoms and small particles, but too weak to provide usable spectra. Thus, an auxiliary electrical spark for excitation of vaporized and atomized material was added [7]. This study marks the conception of laser-induced breakdown spectroscopy (LIBS). Although the instrumentation proposed by Brech and Cross was soon commercially available, the interest in using LIBS rise in the 1980s. Advances in technology, more specifically the development of powerful and robust lasers, high-resolution optics, high sensitive detectors, and fast electronics for data acquisition have all contributed to its acceptance in routine analysis and increasing adoption as a cost-effective alternative analytical technique. Besides the ablation of the sample surface, using powerful modern lasers a microplasma is formed that excites the ablated atoms, ions, and molecular fragments. The plasma continues this excitation, and can also vaporize small ablated particles, atomize and excite atoms, ions, and molecules obtaining a rich emission spectrum used for qualitative and quantitative analysis [8]. LIBS is a highly versatile and adaptable spectroscopy technique. It has increasingly become a powerful tool for fast multi-elemental analysis in several fields of applications such as industrial, agriculture, environmental, food, geological, and biomedical. The growing interest on LIBS is probably due to the simplicity of the technique and instrumentation as sampling and subsequent excitation of atoms, ions and molecules rapidly occurs in one step and in the same system, i.e. there is no transportation of ablated material to another instrument as in LA-ICP-MS. It is a universal technique because any type of sample can yield a LIBS spectrum [8, 9]. The most notorious advantages of LIBS are the direct solid analysis capability, the quasi nondestructive analysis, and the portable instruments for analysis in the field. On the other hand, the technique presents limited detectability and it is not useful for trace element analysis unless physical and chemical enhancement strategies are applied

**138**

to overcome this drawback.

**2.1 Fundamentals and principals of operation**

In practice, LIBS is a very simple spectroscopic technique to implement. A highpowered density pulsed laser beam is focused through lenses in (liquid or gaseous) or on (solid) the sample to produce dielectric breakdown leading to plasma formation. Plasma is a partially ionized gas containing atoms, ions and free electrons, and electrically neutral. Once initiated, the plasma induces the ablation of a finite quantity of the sample surface to a condition that may then be excited by the energy supplied by the same pulse or by a subsequent pulse of the laser beam (double pulse strategy) [9, 10]. Excitation is followed by emission of electromagnetic radiation which is collected, often through a fiber optic cable, and directed into a spectrometer where it is spectrally resolved and further instrumentally detected.

The interaction between a laser pulse and mater to create LIBS plasmas involves a process dependent on characteristics of both the laser, *e.g.* wavelength, irradiance and pulse duration, and the sample, *e.g.* gases, liquids, and solids, which can be conductive and non-conductive samples. When the laser pulse in the nanosecond time regime reaches the sample, the dominant mechanism is the thermal ionization process [9, 10]. A process of scattering transfers the laser energy to the lattice of the spot targeted on the sample causing the melting and generation of larger particles, thus enlarging the size distribution of the aerosol particles [11]. Plasma life stages include plasma ignition, plasma expansion and cooling and particle ejection and condensation. The physics of the plasma generation, evolution, and termination processes are complex. In the case of the analysis of solid samples, *i.e.* the majority of LIBS applications, plasma is initiated when the power density of the laser exceeds the breaking limit of a solid surface. In general, irradiance above 108 W cm−2 is needed. The breakdown is promoted by the intense electric field gradient of the laser, and atoms, ions, and electrons result from the deposition of energy into the target. Afterward, a high-pressure vapor is produced while the plume compresses the surrounding gas. A shockwave at supersonic speed is generated from the surface towards the surrounding atmosphere during vapor expansion. The absorption of the incident laser energy and transfer to the plasma occurs through the inverse Bremsstrahlung absorption involving interactions between free electrons, atoms, and ions. Free electrons in the hot vapor absorb photons from the incident laser increasing their kinetic energy to ionize additional atoms by collisions. The new electrons absorb more photons from the remaining pulse so that a cascade of ionization is generated [8–10]. Plasma is created having distinctive characteristics, high temperatures (10.000 K), and high electron densities (>1017 cm−3). The removed material in the ablation process of the solid samples contributes to plasma formation and expansion. In fact, the plasma is only sustained due to the presence of the ablated material [9]. The plasma expands in a similar way the initial breakdown occurs, at supersonic velocities producing a shockwave, also through the process of inverse Bremsstrahlung absorption. Meanwhile, electron number density and temperature of the plasma changes. The remaining laser energy from the laser duration of the pulse is continuously absorbed by the plasma. Collisions with the surrounding gas reduce plasma velocity of propagation, and then plasma cools down by self-absorption and recombination between electrons and ions, generating neutral species and clusters after plasma extinction [8–10]. Part of ablated mass are particles and these particles create condensed vapor, liquid sample ejection, and solid sample exfoliation, not interesting for LIBS analysis [10].

After 10−9 s and 10−8 s of the ignition, plasma induced by laser in the nanosecond regime becomes opaque due to laser pulse absorption by electrons in the plasma. It reduces the ablation rate because only a fraction of radiation reaches the sample surface. This phenomenon is called shielding and induces a crater with melted and deposited material around it [10]. Meanwhile, plasma is reheated, and the lifetime and size of plasma are higher. On the other hand, ultra-short pulses (femtoseconds) lead minimum plasma shielding and crater with highly defined edges without melted or deposited materials [9, 10]. This regime of pulse is too short to induce thermal effects in the breakdown process. Electronic excitation and ionization, and Coulomb explosion, are the main bond breaking and plasma ignition mechanisms, making the ablation mechanism mostly based on photochemical reactions resulting in vaporization presenting sharper size-distribution solid aerosol than those

generated by nanosecond lasers [11]. In the case of picosecond laser pulses, both thermal and nonthermal processes can occur depending on the laser irradiance.

Initially, the emission spectrum is dominated by a background continuum, while ionic and atomic emission increases with time. The continuum is the main source of background signal (BG) in LIBS, which predominates at the first instants of the plasma life [12]. It is a result of the bremsstrahlung emission (radiant loss of energy due to electron deceleration) and banding (e.g. OH, N2 + , NH, and NO). A delay time is required to start analytical measurement to avoid high continuous emission. The decrease in the emission intensity of the continuum occurs at a higher rate than the excited atoms or ions in the plasma. Thus, temporal separation is feasible and detectability is improved. In LIBS analysis, the delay time must be experimentally evaluated in order to obtain maximum signal-to-background ratio [12].

Undesirable matrix effects may lead to inaccurate determinations and lower sensitivity. These are usually a consequence of i) physical properties of the sample which change the ablation parameters altering the amount of ablated mass, ii) the presence of an element alters the emission features of another one, and *iii)* the plasma–particle interaction processes, which are time and space-dependent due to the transient nature of the plasma and its spatial inhomogeneity [13]. Elemental fractionation is defined as a non-stoichiometric effect which also depreciates the quality of the results [10, 13]. It occurs when the ablated material failure to represent the real composition of a sample due to preferential evaporation of volatile elements, selective segregation, surface temperature distribution inhomogeneity, among others. Elemental fractionation and matrix effects in laser sampling-based spectrometry methods have been discussed in detail by Zhang et al. [14]. Adequate laser wavelength, energy density, pulse width, and proper calibration using standard reference materials with known composition and matrix-matching strategy can overcome these drawbacks. An advantage of LIBS compared to LA-ICP-MS is fractionation can occur only during ablation, as there is no transportation of ablated material from ablation chamber to excitation/ionization source, or from ICP to mass spectrometer.

Plasma optical thickness is an important parameter in laser-induced plasmas. The emitted radiation is successively reabsorbed by atoms/ions located in the coldest plasma region leading to self-absorption and pronounced non-linear effects [15]. The plasma is called optically thick and usually occurs for the most intense emission lines of elements and for less intense emission lines at higher elemental concentrations.

#### **2.2 Instrumentation**

The main components of a generalized LIBS apparatus include (i) laser source; (ii) focusing lenses; (iii) sample support; (iv) optical fiber; (v) spectrometer (vi) detector and (vii) computer for precise control of temporal events, such as: pulse trigger laser and spectrum recording. In the case of a particular application, the specification of each component may be considered and changed. Elements to be monitored, expected concentration, type of analysis (quali or quantitative), sample characteristic (physical state, homogeneity and matrix composition) are common factors to consider when selecting the instrumental specifications.

The laser source generates the laser beam, which main properties are the wavelength and pulse width, both dependent on the laser source and its technological developments. The initial works employing laser in the chemical analysis used visible and infrared (IR) laser sources, such as 693 nm ruby [7, 16] and 1064 nm Nd:YAG (neodymium-doped yttrium aluminum garnet) [17] with a pulse width of μs and ns, respectively. However, further researches demonstrated that the ablation

**141**

high continuous emission [17].

probe [10].

*Laser Chemical Elemental Analysis: From Total to Images*

efficiency of transparent samples using visible or IR wavelengths was harmed due

The evolution of optical technologies led the development of Nd:YAG with wavelengths of 266 and 213 nm by quadrupling and quintupling, respectively, the natural frequency of the Nd:YAG emitted laser. On the other hand, the use of gas excimer as a laser source also enhanced the possibilities of shorter laser wavelengths, such as 193 and 157 nm ArF lasers, which works in the deep-UV region [8–10]. As previously mentioned, the pulse duration is also an important parameter of the laser beam and impacts on the mechanism of interaction between the laser beam and the solid target, the amount of the ablated material, and the crater shape [8–10]. Nowadays, the typical pulsed laser used in LIBS works with a pulse width of nanoseconds or femtoseconds. The best type of laser used for LIBS depends on the

Laser pulses can be focused on the sample using lenses or mirrors. Focal length, diameter, and material are important parameters to achieve minimum spot size (highest power density on target), maximum transmission, and minimum back reflections. For systems requiring an adjustable focus, i.e. lens-to-sample distance may change, a multi-lens system may be required [17]. The collection of the emitted radiation and direct it into the spectrometer is possible by employing lenses and fiber optic or only lenses. Fiber optic transmits the light using total internal reflection, and it is especially useful when the detection system cannot be positioned close to the sample target [17]. A combination of lens and fiber optic is typical as the lens collimates the emitted light improving the focalization into the fiber

Once emitted light by the plasma reaches the spectrometer, it is diffracted in order to obtain spectra in terms of signal intensity as a function of wavelength. Czerny Turner is a sequential dispersive system that combines two collimating mirrors and one grating, while Paschen-Runge optics use a concave grating to separate wavelengths allowing simultaneous multielement analysis. Although both designs have been employed in LIBS for many years, Czerny Turner optical mounting is limited by the monoelemental characteristic in which sequential multielemental analysis is impossible in the case of inhomogeneous samples as the elemental composition varies shot to shot, and Paschen-Runge presents relatively low resolution [9]. High-resolution and high spectral coverage devices are essential in LIBS due to the complexity of the emission spectra. Echelle optics combine a low-density grating with a prism and is called an order-sorting device, i.e. light is diffracted in two dimensions. It offers typical spectral bandwidth of 5 pm and high spectral coverage emission, approximately from 200 to 900 nm, which can be recorded with a single laser pulse. In recent years, the Echelle spectrograph has been more extensively

Detectors devices convert the diffracted optical signal into an electric signal. Photodiodes multiply the current produced by incident light striking photocathode by multiple dynode stages. This is the simplest and inexpensive device but useful only for one-dimensional spatial information from the spectrometer (e.g. Czerny Turner and Paschen-Runge). On the other hand, charge-coupled device (CCD) is constituted by a bi-dimensional configuration of several sensors, each made up of three electrodes over a common substrate of p-type silicon, which allow the acquisition of two-dimensional spatial information obtained by the Echelle system. More recently proposed, intensified CCD (ICCD) is a CCD coupled to microchannel plates to provide time-gated detection of the laser plasma [8–10]. Thus, timeresolved detection down to a few nanoseconds is possible, which is essential to avoid

to the poor absorbance of the laser energy by these types of samples.

application and the desired laser wavelength and pulse duration.

used, but require a two-dimensional detector [10, 17].

*DOI: http://dx.doi.org/10.5772/intechopen.94385*

#### *Laser Chemical Elemental Analysis: From Total to Images DOI: http://dx.doi.org/10.5772/intechopen.94385*

*Practical Applications of Laser Ablation*

ICP to mass spectrometer.

**2.2 Instrumentation**

due to electron deceleration) and banding (e.g. OH, N2

generated by nanosecond lasers [11]. In the case of picosecond laser pulses, both thermal and nonthermal processes can occur depending on the laser irradiance.

Initially, the emission spectrum is dominated by a background continuum, while ionic and atomic emission increases with time. The continuum is the main source of background signal (BG) in LIBS, which predominates at the first instants of the plasma life [12]. It is a result of the bremsstrahlung emission (radiant loss of energy

time is required to start analytical measurement to avoid high continuous emission. The decrease in the emission intensity of the continuum occurs at a higher rate than the excited atoms or ions in the plasma. Thus, temporal separation is feasible and detectability is improved. In LIBS analysis, the delay time must be experimentally

Undesirable matrix effects may lead to inaccurate determinations and lower sensitivity. These are usually a consequence of i) physical properties of the sample which change the ablation parameters altering the amount of ablated mass, ii) the presence of an element alters the emission features of another one, and *iii)* the plasma–particle interaction processes, which are time and space-dependent due to the transient nature of the plasma and its spatial inhomogeneity [13]. Elemental fractionation is defined as a non-stoichiometric effect which also depreciates the quality of the results [10, 13]. It occurs when the ablated material failure to represent the real composition of a sample due to preferential evaporation of volatile elements, selective segregation, surface temperature distribution inhomogeneity, among others. Elemental fractionation and matrix effects in laser sampling-based spectrometry methods have been discussed in detail by Zhang et al. [14]. Adequate laser wavelength, energy density, pulse width, and proper calibration using standard reference materials with known composition and matrix-matching strategy can overcome these drawbacks. An advantage of LIBS compared to LA-ICP-MS is fractionation can occur only during ablation, as there is no transportation of ablated material from ablation chamber to excitation/ionization source, or from

Plasma optical thickness is an important parameter in laser-induced plasmas. The emitted radiation is successively reabsorbed by atoms/ions located in the coldest plasma region leading to self-absorption and pronounced non-linear effects [15]. The plasma is called optically thick and usually occurs for the most intense emission lines of elements and for less intense emission lines at higher elemental concentrations.

The main components of a generalized LIBS apparatus include (i) laser source;

The laser source generates the laser beam, which main properties are the wavelength and pulse width, both dependent on the laser source and its technological developments. The initial works employing laser in the chemical analysis used visible and infrared (IR) laser sources, such as 693 nm ruby [7, 16] and 1064 nm Nd:YAG (neodymium-doped yttrium aluminum garnet) [17] with a pulse width of μs and ns, respectively. However, further researches demonstrated that the ablation

(ii) focusing lenses; (iii) sample support; (iv) optical fiber; (v) spectrometer (vi) detector and (vii) computer for precise control of temporal events, such as: pulse trigger laser and spectrum recording. In the case of a particular application, the specification of each component may be considered and changed. Elements to be monitored, expected concentration, type of analysis (quali or quantitative), sample characteristic (physical state, homogeneity and matrix composition) are common

factors to consider when selecting the instrumental specifications.

evaluated in order to obtain maximum signal-to-background ratio [12].

+

, NH, and NO). A delay

**140**

efficiency of transparent samples using visible or IR wavelengths was harmed due to the poor absorbance of the laser energy by these types of samples.

The evolution of optical technologies led the development of Nd:YAG with wavelengths of 266 and 213 nm by quadrupling and quintupling, respectively, the natural frequency of the Nd:YAG emitted laser. On the other hand, the use of gas excimer as a laser source also enhanced the possibilities of shorter laser wavelengths, such as 193 and 157 nm ArF lasers, which works in the deep-UV region [8–10]. As previously mentioned, the pulse duration is also an important parameter of the laser beam and impacts on the mechanism of interaction between the laser beam and the solid target, the amount of the ablated material, and the crater shape [8–10]. Nowadays, the typical pulsed laser used in LIBS works with a pulse width of nanoseconds or femtoseconds. The best type of laser used for LIBS depends on the application and the desired laser wavelength and pulse duration.

Laser pulses can be focused on the sample using lenses or mirrors. Focal length, diameter, and material are important parameters to achieve minimum spot size (highest power density on target), maximum transmission, and minimum back reflections. For systems requiring an adjustable focus, i.e. lens-to-sample distance may change, a multi-lens system may be required [17]. The collection of the emitted radiation and direct it into the spectrometer is possible by employing lenses and fiber optic or only lenses. Fiber optic transmits the light using total internal reflection, and it is especially useful when the detection system cannot be positioned close to the sample target [17]. A combination of lens and fiber optic is typical as the lens collimates the emitted light improving the focalization into the fiber probe [10].

Once emitted light by the plasma reaches the spectrometer, it is diffracted in order to obtain spectra in terms of signal intensity as a function of wavelength. Czerny Turner is a sequential dispersive system that combines two collimating mirrors and one grating, while Paschen-Runge optics use a concave grating to separate wavelengths allowing simultaneous multielement analysis. Although both designs have been employed in LIBS for many years, Czerny Turner optical mounting is limited by the monoelemental characteristic in which sequential multielemental analysis is impossible in the case of inhomogeneous samples as the elemental composition varies shot to shot, and Paschen-Runge presents relatively low resolution [9]. High-resolution and high spectral coverage devices are essential in LIBS due to the complexity of the emission spectra. Echelle optics combine a low-density grating with a prism and is called an order-sorting device, i.e. light is diffracted in two dimensions. It offers typical spectral bandwidth of 5 pm and high spectral coverage emission, approximately from 200 to 900 nm, which can be recorded with a single laser pulse. In recent years, the Echelle spectrograph has been more extensively used, but require a two-dimensional detector [10, 17].

Detectors devices convert the diffracted optical signal into an electric signal. Photodiodes multiply the current produced by incident light striking photocathode by multiple dynode stages. This is the simplest and inexpensive device but useful only for one-dimensional spatial information from the spectrometer (e.g. Czerny Turner and Paschen-Runge). On the other hand, charge-coupled device (CCD) is constituted by a bi-dimensional configuration of several sensors, each made up of three electrodes over a common substrate of p-type silicon, which allow the acquisition of two-dimensional spatial information obtained by the Echelle system. More recently proposed, intensified CCD (ICCD) is a CCD coupled to microchannel plates to provide time-gated detection of the laser plasma [8–10]. Thus, timeresolved detection down to a few nanoseconds is possible, which is essential to avoid high continuous emission [17].

#### **2.3 Analytical features and strategies to improve sensitivity**

The majority of LIBS measurements involve the analysis of solids. Some applications are limited by the relatively low sensitivity of the technique. Limits of detection of LIBS usually range from 1 to 100 parts per million (mg kg−1). Therefore, most applications are focus on major elements, as the technique cannot meet the demands for the detection of trace elemental analysis (parts per billion, μg kg−1). Physical and chemical strategies have been demonstrated to enhance the LIBS detection limits and sensitivity [18]. Increased plasma temperature and electron density are achieved by double-pulse laser method using two laser sources, the use of spatial and magnetic constraint devices, and controlling the atmosphere in which the sample is placed with inert gas (e.g., N2, Ar, and He) [19]. Nanoparticles (NPs) deposited on surfaces of the solid samples favors ablation processes which mechanisms differ for conductors and insulators samples. For liquid sample analysis, liquid–liquid extraction, liquid–solid conversion, and surface-enhanced LIBS (liquid sample is dried onto the surface of a selected solid substrate before the analysis) have been exploited to overcome problems due to laser-liquid sample interaction, laser energy dissipation, low plasma temperature and sample splashing which depreciate repeatability and reproducibility. The reader is referred to a recent review paper by Fu et al. [18].

The possibility of measuring the molecular emission in LIBS allows the determination of some non-metallic elements from emission bands of diatomic molecules, e.g. fluorine and chlorine have been detected by the emission of CaF, BaF, MgF, CaCl, SrCl, or MgCl [20, 21]. Isotopic analysis based on the discrimination between emission bands of molecules formed by two different isotopes has been reported. The different masses of the isotopes affect the vibrational and rotational energy levels results in molecular isotopic shifts which are exploited in the isotopic determinations [21].

Different data acquisition modes are possible in LIBS analysis and it is selected depending on the goal of the experiment. Using just one laser pulse or using repetitive laser pulses, localized microanalysis with lateral and depth profiling information is easily obtained. For image-based analysis, a generation of a series of plasmas at different positions on the sample following a scan sequence is necessary. Most of LIBS imaging instruments rely on an XY stage that moves the sample instead of moving the laser beam because of a greater collection efficiency from a fixed plasma plume. LIBS imaging analysis is later discussed in this text, and additional information can be found in the review by Jolivet et al. [22].

An important innovation in LIBS is the handheld instruments commercially available for analysis in the field, especially useful when the sample cannot be moved. Some instruments present capability of chemometric analysis by means of proprietary software, video targeting, and an argon purge of atmosphere neighboring the target in order to improve sensitivity. Applications in agriculture, environment, industry, and cultural heritage can provide information to solve important economic and historical issues. An impressive breakthrough of this technique is the use in planetary geology on a Mars mission for remote sensing.
