**3. Laser ablation inductively coupled plasma mass spectrometry: LA-ICP-MS**

With the development of laser technologies and the increasing demand for specific applications in the direct analysis of solids, in 1985 Alan Gray [16] demonstrated the coupling between a ruby laser ablation system with an inductively

**143**

sub-section.

*Laser Chemical Elemental Analysis: From Total to Images*

6 years (source: www.sciencedirect.com, September 10th, 2020).

by conventional elemental analysis and sample pretreatment.

coupled plasma mass spectrometer (LA-ICP-MS) for monitoring Si, Al, Fe, Ca, Mg, Na, K, Mn, and Ti in pelletized standard powdered rock samples. Since its application, the area has been growing and one indicator of that is the number of scientific works published: more than 15,000 research papers related to LA-ICP-MS in the last

The interest in the LA-ICP-MS technique is related to its impressive characteristics, such as the possibility to acquire elemental and isotopic information by direct sampling of nano, and even pico or femtograms of solid materials in the micrometric scale, and limits of detection around low parts per million (in some specific cases hundreds of parts per billion). Additionally, the image-based analysis to study sample heterogeneity and elemental or isotopic distribution in a surface contributed to its acceptance [23, 24]. These almost unique characteristics permit the use of LA-ICP-MS in many fields of science, such as geochemistry, forensic, environmental, materials, medical, and biological, in specific applications that were not possible

The LA-ICP-MS technique works based on the coupling of a solid sampling system, the LA system, with a powerful elemental/isotopic analytical technique, the ICP-MS. A high-power laser beam, focused on the sample surface through an optical system promotes a huge and instant increase of the temperature of the target sample and transfers a discrete sample volume to the vapor phase. A solid aerosol is generated in an ablation chamber because of this interaction between the laser

The solid aerosol generated carries the information of the elemental and isotopic composition of the sample ablated (analytes and matrix components). To acquire this information in terms of analytical data, the aerosol is transferred to the ICP-MS by a gas stream through a connection tube to the ICP. In the ICP, a high-temperature argon plasma induced by a radiofrequency, the aerosol is digested, and its solid

The ions generated into the ICP are extracted, through the interface region, to the mass spectrometer (MS), the instrumental component responsible to separate and detect these ions. After the extraction by the interface region, the ion optics conduces the ion beam up to the mass analyzer device, a high vacuum region in which separation of the ions based on its mass-to-charge ratio (m/z) occurs. The ions are further detected by a specific detector that generates an electric impulse due to the interaction

The analytical signal profile observed in a typical LA-ICP-MS analysis is a transient signal characterized, in general, by the counts per second (intensity) of the monitored ion as a function of the ablation time. The specific characteristics of the LA-ICP-MS instruments available for chemical analysis are dependent on each instrumental component type and its principles of operation discussed in the next

As mentioned before, the LA-ICP-MS permits different instrumental setups that can impact on mechanisms of the interaction between the laser beam and the solid sample, the separation of the ions in the mass analyzer, and the analytical response of the instrument during analysis. The main components of LA-ICP-MS instrument are the laser source, ablation chamber equipped with a CCD camera, and transport tubing connected to the ICP-MS. The ICP-MS, in turn, can be composed of different

beam and the sample, and this process is named laser ablation [2, 25].

constituents are vaporized, atomized, and ionized [2, 25].

**3.2 Instrumentation and its fundamental characteristics**

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

**3.1 Principles of operation**

of the ions on its surface [2, 25].

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

coupled plasma mass spectrometer (LA-ICP-MS) for monitoring Si, Al, Fe, Ca, Mg, Na, K, Mn, and Ti in pelletized standard powdered rock samples. Since its application, the area has been growing and one indicator of that is the number of scientific works published: more than 15,000 research papers related to LA-ICP-MS in the last 6 years (source: www.sciencedirect.com, September 10th, 2020).

The interest in the LA-ICP-MS technique is related to its impressive characteristics, such as the possibility to acquire elemental and isotopic information by direct sampling of nano, and even pico or femtograms of solid materials in the micrometric scale, and limits of detection around low parts per million (in some specific cases hundreds of parts per billion). Additionally, the image-based analysis to study sample heterogeneity and elemental or isotopic distribution in a surface contributed to its acceptance [23, 24]. These almost unique characteristics permit the use of LA-ICP-MS in many fields of science, such as geochemistry, forensic, environmental, materials, medical, and biological, in specific applications that were not possible by conventional elemental analysis and sample pretreatment.

## **3.1 Principles of operation**

*Practical Applications of Laser Ablation*

review paper by Fu et al. [18].

determinations [21].

tion can be found in the review by Jolivet et al. [22].

use in planetary geology on a Mars mission for remote sensing.

**3. Laser ablation inductively coupled plasma mass spectrometry:** 

With the development of laser technologies and the increasing demand for specific applications in the direct analysis of solids, in 1985 Alan Gray [16] demonstrated the coupling between a ruby laser ablation system with an inductively

**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

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

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 informa-

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

**142**

**LA-ICP-MS**

The LA-ICP-MS technique works based on the coupling of a solid sampling system, the LA system, with a powerful elemental/isotopic analytical technique, the ICP-MS. A high-power laser beam, focused on the sample surface through an optical system promotes a huge and instant increase of the temperature of the target sample and transfers a discrete sample volume to the vapor phase. A solid aerosol is generated in an ablation chamber because of this interaction between the laser beam and the sample, and this process is named laser ablation [2, 25].

The solid aerosol generated carries the information of the elemental and isotopic composition of the sample ablated (analytes and matrix components). To acquire this information in terms of analytical data, the aerosol is transferred to the ICP-MS by a gas stream through a connection tube to the ICP. In the ICP, a high-temperature argon plasma induced by a radiofrequency, the aerosol is digested, and its solid constituents are vaporized, atomized, and ionized [2, 25].

The ions generated into the ICP are extracted, through the interface region, to the mass spectrometer (MS), the instrumental component responsible to separate and detect these ions. After the extraction by the interface region, the ion optics conduces the ion beam up to the mass analyzer device, a high vacuum region in which separation of the ions based on its mass-to-charge ratio (m/z) occurs. The ions are further detected by a specific detector that generates an electric impulse due to the interaction of the ions on its surface [2, 25].

The analytical signal profile observed in a typical LA-ICP-MS analysis is a transient signal characterized, in general, by the counts per second (intensity) of the monitored ion as a function of the ablation time. The specific characteristics of the LA-ICP-MS instruments available for chemical analysis are dependent on each instrumental component type and its principles of operation discussed in the next sub-section.

#### **3.2 Instrumentation and its fundamental characteristics**

As mentioned before, the LA-ICP-MS permits different instrumental setups that can impact on mechanisms of the interaction between the laser beam and the solid sample, the separation of the ions in the mass analyzer, and the analytical response of the instrument during analysis. The main components of LA-ICP-MS instrument are the laser source, ablation chamber equipped with a CCD camera, and transport tubing connected to the ICP-MS. The ICP-MS, in turn, can be composed of different mass analyzer types, which presents specific analytical features, and the detector. The main characteristics of each LA-ICP-MS component are discussed below, focusing on principles of operation, commenting on the potentialities and limitations, as well as its technological developments and improvements.

## *3.2.1 Laser ablation system*

The laser source is the heart of a laser ablation system because it confers the main properties of the laser beam and thus, the main characteristic of the ablated material. It was previously mentioned, in the LIBS section, the coupling between laser radiation and the solid sample surface promotes the ablation depending mainly on the laser wavelength and pulse width. Readers are referred to as sections 2.2 and 2.3. Besides the efficient coupling, these parameters impact the size-distribution of the aerosol particles and the crater shape. In general, the ablation rate increases by increasing the laser energy (shorter wavelengths), and smaller particles of the solid aerosol are obtained using femtosecond lasers, which are required to guarantee the efficiency of the transport and posterior digestion of these particles into the ICP. The most common pulsed laser used in LA-ICP-MS present nanoseconds or femtoseconds pulse width and short wavelengths (e.g. 213 nm, 5th harmonic solid-state Nd:YAG laser).

Different from LIBS which sample holder can be open support, in LA coupled systems, such as LA-ICP-MS, a closed gas-tight compartment denominated ablation chamber is required. The ablation process occurs in the ablation chamber, which presents a transparent window in the wavelength of the laser beam, and the gas streaming transports the aerosol to the ICP. The ablation chamber is coupled to a CCD camera to improve sample visualization and to define the specific position of the sampling target on the sample surface using the LA software. The types of ablation cell designs are vast, including commercial and customized, but some requirements are needed for the efficiency of the role of ablation chambers.

In general, ablation chambers must provide a gas environment that permits an expansion of the aerosol generated to ensure a small-size particle-distribution, and an adequate gas-flow for an efficient transport of the solid aerosol to the ICP avoiding memory and fractionation effects. Thus, the ablation volume, the flow dynamics of the gas stream, and the diameter and length of the transport tube are determinant for the features of an efficient ablation chamber [26]. The carrier gas also plays an important role in chemical analysis. Lighter carrier gases, such as helium, improves the efficiency of the ablation process and particle transportation enhancing sensitivity of the analytical method and reducing drawbacks related to fractionation in the transport process [27]. Recent developments in the design of low dispersion systems, rapid response ablation cells and their impact on bioimaging applications was recently discussed by Van Malderen et al. [28].

## *3.2.2 ICP and mass analyzer devices*

LA system generates a laser beam that is responsible for removing an amount of sample and transferring its constituents to the vapor phase. From this point, is the role of the ICP-MS to analyze this material and provide analytical information regarding its elemental and isotopic composition. Because the analytical entities monitored by ICP-MS are mainly mono-charged positive ions, ICP-MS instruments must first be able to generate ions from the aerosol constituents of the sample. The ICP's are ion sources in which plasmas are produced by the energy transferred from a radiofrequency generator to a gas flow into a concentric quartz tube (torch) via a magnetic field through an induction coil [29]. The plasma has the ability to ionize

**145**

*Laser Chemical Elemental Analysis: From Total to Images*

most atoms of the elements presented in the periodic table due to its high-temperature (around 10,000 K) and high electron density. The inductively coupled plasma formation mechanism will not be discussed in this chapter, but we encourage the

As the ions of the sample composition are formed in the plasma, including the analytes and concomitants, they are conducted towards the interface region to enter the mass spectrometer. This interface region is composed of metallic cones, normally two (sampler and skimmer cones) which extracts these ions. Then, ions go to the mass spectrometer via the ion optics, which are a set of electrostatic lenses that conduce the positive ions to the mass analyzer, and deviates the neutral and nega-

The heart of the mass spectrometer is the mass analyzer. This component has the function to, based on the mass-to-charge ratio, separate the analyte ions from concomitants, permitting the detector count and acquire the abundancy of the specific element under analysis. For this, there are different arranges of mass analyzers with their principles of operation to separate the ions. The most employed mass analyzer types in LA-ICP-MS are quadrupole systems, magnetic sector, and

The quadrupole system is composed of two pairs of metallic bars, positioned oppositely. It operates as a mass analyzer by applying specific direct and alternate current (AC/DC) in the pair of rods that in consequence, for each specific AC/DC applied, only the desirable m/z of the ions presents a stable trajectory in the quadrupole, reaching the detector. The trajectory of the other ions will be unstable and will exit the quadrupole before reaching the detector. This mass selection occurs sequentially, and each m/z determined has an optimum AC/DC voltage applied on the quadrupole. This characteristic, as well as the design of the quadrupole, confers a resolution ranging from 0.7–1.0 *a.m.u*. It is possible to optimize the parameters of quadrupole to increase the mass resolution, but a compromise with the sensitivity must be attempted. This relatively low resolution impacts some applications due to the interferences that can be present in the analysis. Some isobaric and polyatomic species formed in the plasma can present the same m/z of the analyte and then depreciate the accuracy of the analysis. For this, the new quadrupole-based ICP-MS's presents a collision/reaction cell or interface that uses a reactive or nonreactive gas to break polyatomic interference species, reduce their kinetic energy or react with the analyte generating a new analyte ion free of interference, then allowing accurate results. Although the limitations of single quadrupole devices in terms of resolution, the low price, and suitability for most quantitative applications make it the most widely used mass analyzer in ICP-MS analysis. The newest inductively coupled plasma tandem mass spectrometry, popularly called triple-quadrupole ICP-MS presents an additional quadrupole located before the collision/reaction cell. It confers the possibility to filter non-analyte ions in the first quadrupole, preventing the production of unwanted species that could immediately interfere on the m/z of interest and the second mass analyzer to deal with interferences using mass-shift or on-mass strategies. It has shown great results of accuracy in some specific dif-

Another mass analyzer type is the double-focusing magnetic sector technology. This mass analyzer device operates with two analyzers, a magnetic and an electrostatic. This design of mass analyzer presents greater resolution and interference overcoming compared to the quadrupole, and is applied in studies that need this resolution, such as isotopic ratio monitoring and analyte quantification in complex samples. The principles of m/z separation in such mass analyzer are based on the dependence of the deflection angle of the ion beam by a magnetic field to the mass of the ions, and the alignment of the ions with the same m/z in the electrostatic

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

readers to look for it in Thomas, 2013 [30].

tive species, as well as the photons [30].

ficult applications for single quadrupole ICP-MS [30].

time-of-flight [30].

*Practical Applications of Laser Ablation*

*3.2.1 Laser ablation system*

harmonic solid-state Nd:YAG laser).

*3.2.2 ICP and mass analyzer devices*

mass analyzer types, which presents specific analytical features, and the detector. The main characteristics of each LA-ICP-MS component are discussed below, focusing on principles of operation, commenting on the potentialities and limitations, as

The laser source is the heart of a laser ablation system because it confers the main properties of the laser beam and thus, the main characteristic of the ablated material. It was previously mentioned, in the LIBS section, the coupling between laser radiation and the solid sample surface promotes the ablation depending mainly on the laser wavelength and pulse width. Readers are referred to as sections 2.2 and 2.3. Besides the efficient coupling, these parameters impact the size-distribution of the aerosol particles and the crater shape. In general, the ablation rate increases by increasing the laser energy (shorter wavelengths), and smaller particles of the solid aerosol are obtained using femtosecond lasers, which are required

to guarantee the efficiency of the transport and posterior digestion of these particles into the ICP. The most common pulsed laser used in LA-ICP-MS present nanoseconds or femtoseconds pulse width and short wavelengths (e.g. 213 nm, 5th

ments are needed for the efficiency of the role of ablation chambers.

ing applications was recently discussed by Van Malderen et al. [28].

Different from LIBS which sample holder can be open support, in LA coupled systems, such as LA-ICP-MS, a closed gas-tight compartment denominated ablation chamber is required. The ablation process occurs in the ablation chamber, which presents a transparent window in the wavelength of the laser beam, and the gas streaming transports the aerosol to the ICP. The ablation chamber is coupled to a CCD camera to improve sample visualization and to define the specific position of the sampling target on the sample surface using the LA software. The types of ablation cell designs are vast, including commercial and customized, but some require-

In general, ablation chambers must provide a gas environment that permits an expansion of the aerosol generated to ensure a small-size particle-distribution, and an adequate gas-flow for an efficient transport of the solid aerosol to the ICP avoiding memory and fractionation effects. Thus, the ablation volume, the flow dynamics of the gas stream, and the diameter and length of the transport tube are determinant for the features of an efficient ablation chamber [26]. The carrier gas also plays an important role in chemical analysis. Lighter carrier gases, such as helium, improves the efficiency of the ablation process and particle transportation enhancing sensitivity of the analytical method and reducing drawbacks related to fractionation in the transport process [27]. Recent developments in the design of low dispersion systems, rapid response ablation cells and their impact on bioimag-

LA system generates a laser beam that is responsible for removing an amount of sample and transferring its constituents to the vapor phase. From this point, is the role of the ICP-MS to analyze this material and provide analytical information regarding its elemental and isotopic composition. Because the analytical entities monitored by ICP-MS are mainly mono-charged positive ions, ICP-MS instruments must first be able to generate ions from the aerosol constituents of the sample. The ICP's are ion sources in which plasmas are produced by the energy transferred from a radiofrequency generator to a gas flow into a concentric quartz tube (torch) via a magnetic field through an induction coil [29]. The plasma has the ability to ionize

well as its technological developments and improvements.

**144**

most atoms of the elements presented in the periodic table due to its high-temperature (around 10,000 K) and high electron density. The inductively coupled plasma formation mechanism will not be discussed in this chapter, but we encourage the readers to look for it in Thomas, 2013 [30].

As the ions of the sample composition are formed in the plasma, including the analytes and concomitants, they are conducted towards the interface region to enter the mass spectrometer. This interface region is composed of metallic cones, normally two (sampler and skimmer cones) which extracts these ions. Then, ions go to the mass spectrometer via the ion optics, which are a set of electrostatic lenses that conduce the positive ions to the mass analyzer, and deviates the neutral and negative species, as well as the photons [30].

The heart of the mass spectrometer is the mass analyzer. This component has the function to, based on the mass-to-charge ratio, separate the analyte ions from concomitants, permitting the detector count and acquire the abundancy of the specific element under analysis. For this, there are different arranges of mass analyzers with their principles of operation to separate the ions. The most employed mass analyzer types in LA-ICP-MS are quadrupole systems, magnetic sector, and time-of-flight [30].

The quadrupole system is composed of two pairs of metallic bars, positioned oppositely. It operates as a mass analyzer by applying specific direct and alternate current (AC/DC) in the pair of rods that in consequence, for each specific AC/DC applied, only the desirable m/z of the ions presents a stable trajectory in the quadrupole, reaching the detector. The trajectory of the other ions will be unstable and will exit the quadrupole before reaching the detector. This mass selection occurs sequentially, and each m/z determined has an optimum AC/DC voltage applied on the quadrupole. This characteristic, as well as the design of the quadrupole, confers a resolution ranging from 0.7–1.0 *a.m.u*. It is possible to optimize the parameters of quadrupole to increase the mass resolution, but a compromise with the sensitivity must be attempted. This relatively low resolution impacts some applications due to the interferences that can be present in the analysis. Some isobaric and polyatomic species formed in the plasma can present the same m/z of the analyte and then depreciate the accuracy of the analysis. For this, the new quadrupole-based ICP-MS's presents a collision/reaction cell or interface that uses a reactive or nonreactive gas to break polyatomic interference species, reduce their kinetic energy or react with the analyte generating a new analyte ion free of interference, then allowing accurate results. Although the limitations of single quadrupole devices in terms of resolution, the low price, and suitability for most quantitative applications make it the most widely used mass analyzer in ICP-MS analysis. The newest inductively coupled plasma tandem mass spectrometry, popularly called triple-quadrupole ICP-MS presents an additional quadrupole located before the collision/reaction cell. It confers the possibility to filter non-analyte ions in the first quadrupole, preventing the production of unwanted species that could immediately interfere on the m/z of interest and the second mass analyzer to deal with interferences using mass-shift or on-mass strategies. It has shown great results of accuracy in some specific difficult applications for single quadrupole ICP-MS [30].

Another mass analyzer type is the double-focusing magnetic sector technology. This mass analyzer device operates with two analyzers, a magnetic and an electrostatic. This design of mass analyzer presents greater resolution and interference overcoming compared to the quadrupole, and is applied in studies that need this resolution, such as isotopic ratio monitoring and analyte quantification in complex samples. The principles of m/z separation in such mass analyzer are based on the dependence of the deflection angle of the ion beam by a magnetic field to the mass of the ions, and the alignment of the ions with the same m/z in the electrostatic

field, due to its kinetic energy. The combination of these two mechanisms of separation improves the resolution and the precision of the measurements. With a single detector, the mass separation is sequential, by varying the magnetic field in the deflection separation, allowing a specific m/z at a time to reach the outer slit and the detector. But, if this double-focused magnetic sector technology is arranged with an electrostatic sector field followed by a magnetic sector field and a detector plane, known as multi-collector, the detection of the ions is simultaneous and the precision of the ion measurement is improved. This technology is the state-ofthe-art instrumentation for high precise and accurate isotopic ratio studies. The disadvantages of the double-focusing magnetic sector technology are related to the price of these instruments and, in the case of multi-collector, the loss of resolution, impacting on the analysis of complex matrices [30].

The last discussed mass analyzer technology is the time-of-flight (ToF) mass analyzer. Its principles of operation are based on the velocity dependence of the ions to the m/z, for ions generated at the same time and with the same kinetic energy. For this, packages of ions are simultaneously introduced in the mass analyzer by a specific ion optics, and, in a flight tube, the package of ions is accelerated due to a constant voltage, through a known distance. As the kinetic energy of the ions is the same, they will reach the detector in different times-of-flight due to their differences in m/z. In general, lighter isotopes reach the detector first, followed by medium m/z ions and, finally, heavier ions. These principles of operation can produce 20,000 mass spectra per second and, in contrast with the quadrupole technology, there is no dependence on the resolution and the sensitivity for the ToF analyzers. So, it came in the scientific community as an instrument with higher resolution compared with the quadrupole system, faster and is applicable not only for isotope ratio studies but also for fast qualitative screening and quantitative analysis of complex matrices [30].

As can be seen, there are many possibilities of LA-ICP-MS setup, whether the LA system, with the properties of the laser source and ablation chamber, or the ICP-MS specificities, such as the ICP conditions, mass analyzer devices, and their particularities, not mentioning the detector types, that are not less important, but were not discussed in this chapter due to de variety of designs in the market. In this way, LA-ICP-MS hardware's designs will provide specific analytical features for each instrument setup, that will be commented in general in the next sub-section, to a better understanding of LA-ICP-MS potentialities and limitations.

An innovative approach that exploits the combination of the two techniques, LIBS and LA-ICP-MS, is known as Tandem LA/LIBS and commercially available (**Figure 1**). It provides simultaneous and complementary information for total and spatially resolved mapping of major and trace elements. Emission spectra are simultaneously monitored from the micro plasma created by the laser during ablation/ sampling.

## **3.3 Analytical features, challenges and limitations**

The capabilities of LA-ICP-MS as an analytical technique starts with the characteristics of the LA system. It has the ability to direct sampling with micrometric spatial resolution, varying from 4 to 200 μm in most instruments. As the ablation chamber can move on the axis x and y, the ablation can be performed in spot analysis and also by making ablation lines over the sample area of interest, and this is the sampling principle for image-based LA-ICP-MS applications later discussed [2, 24, 25].

For quantitative applications, and especially using quadrupole-based ICP-MS, the LA-ICP-MS can achieve a precision of 2–5% for the determination

**147**

*Laser Chemical Elemental Analysis: From Total to Images*

of homogeneous elements in solid samples. For isotopic ratio studies and using double-focusing magnetic sector ICP-MS with multi-collector, the LA-ICP-MS allows a precision, in some cases, of 0.001–0.005% in the isotope ratio measurement. For heterogeneous samples, the precision is not the goal of the analysis, but the evaluation of the elemental distribution and its concentration in different areas

In terms of limits of detection, it varies depending on the sample matrix and instrumental setup employed, usually ranging from hundreds of ng g−1 to a few μg g−1. Due to its limit of detection and the low mass ablated, LA-ICP-MS can cover a wide range of concentration, allowing the determination of trace, micro and major

Although the potentiality of LA-ICP-MS, there are some drawbacks that the scientific community have devoted effort to overcome, and that the users must give special attention. The critical limitations of LA-ICP-MS are derived from the nature of the interaction between the laser beam and the solid sample and the transport process of the solid aerosol generated through the ICP. Since the mass removed by the laser beam is dependent on the matrix of the sample and the characteristics of LA system and parameters (laser fluence, spot size diameter, and repetition rate), thus, the analytical signal obtained in the ICP-MS is also matrix-dependent. This fact impacts the accuracy of the analytical method, which will be achieved in the condition that the same mass is ablated from the sample and the calibration standards, and the analyte undergoes the same transportation effects and processes in

However, it is not simple to achieve a suitable solid calibration standard for the vast sample matrices that are analyzed by LA-ICP-MS. The ideal condition is the use of certified reference materials (CRMs), but a limited number of matrix types is available as CRM. It is especially difficult for the analysis of heterogeneous and non-powdered samples. To overcome this drawback, the scientific community has been studying different approaches to calibration. The main strategies are the use of matrix-matched materials, such as lab-made standards, by spiking the analytes in the powdered material that has approximately the same composition of the sample, drying and pressing it into a pellet. In the case of non-matrix-matched standards, solution-based strategies are employed, for example, the use of solution nebulization of liquid standard and mixing it with the aerosol of the sample, to calibrate the method. In all these cases, the homogeneity of the standard must be monitored to guarantee the analytical performance of the external calibration procedure. Additionally, internal standardization is usually needed to correct signal

elements directly in the solid sample analyzed [2, 24, 25].

*Schematic diagram of the for simultaneous LIBS and LA-ICP-MS.*

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

of the sample [2, 24, 25].

**Figure 1.**

the plasma until be ionized [2, 25].

*Practical Applications of Laser Ablation*

analysis of complex matrices [30].

impacting on the analysis of complex matrices [30].

field, due to its kinetic energy. The combination of these two mechanisms of separation improves the resolution and the precision of the measurements. With a single detector, the mass separation is sequential, by varying the magnetic field in the deflection separation, allowing a specific m/z at a time to reach the outer slit and the detector. But, if this double-focused magnetic sector technology is arranged with an electrostatic sector field followed by a magnetic sector field and a detector plane, known as multi-collector, the detection of the ions is simultaneous and the precision of the ion measurement is improved. This technology is the state-ofthe-art instrumentation for high precise and accurate isotopic ratio studies. The disadvantages of the double-focusing magnetic sector technology are related to the price of these instruments and, in the case of multi-collector, the loss of resolution,

The last discussed mass analyzer technology is the time-of-flight (ToF) mass analyzer. Its principles of operation are based on the velocity dependence of the ions to the m/z, for ions generated at the same time and with the same kinetic energy. For this, packages of ions are simultaneously introduced in the mass analyzer by a specific ion optics, and, in a flight tube, the package of ions is accelerated due to a constant voltage, through a known distance. As the kinetic energy of the ions is the same, they will reach the detector in different times-of-flight due to their differences in m/z. In general, lighter isotopes reach the detector first, followed by medium m/z ions and, finally, heavier ions. These principles of operation can produce 20,000 mass spectra per second and, in contrast with the quadrupole technology, there is no dependence on the resolution and the sensitivity for the ToF analyzers. So, it came in the scientific community as an instrument with higher resolution compared with the quadrupole system, faster and is applicable not only for isotope ratio studies but also for fast qualitative screening and quantitative

As can be seen, there are many possibilities of LA-ICP-MS setup, whether the LA system, with the properties of the laser source and ablation chamber, or the ICP-MS specificities, such as the ICP conditions, mass analyzer devices, and their particularities, not mentioning the detector types, that are not less important, but were not discussed in this chapter due to de variety of designs in the market. In this way, LA-ICP-MS hardware's designs will provide specific analytical features for each instrument setup, that will be commented in general in the next sub-section, to a

An innovative approach that exploits the combination of the two techniques, LIBS and LA-ICP-MS, is known as Tandem LA/LIBS and commercially available (**Figure 1**). It provides simultaneous and complementary information for total and spatially resolved mapping of major and trace elements. Emission spectra are simultaneously monitored from the micro plasma created by the laser during ablation/

The capabilities of LA-ICP-MS as an analytical technique starts with the characteristics of the LA system. It has the ability to direct sampling with micrometric spatial resolution, varying from 4 to 200 μm in most instruments. As the ablation chamber can move on the axis x and y, the ablation can be performed in spot analysis and also by making ablation lines over the sample area of interest, and this is the sampling principle for image-based LA-ICP-MS applications later

For quantitative applications, and especially using quadrupole-based ICP-MS, the LA-ICP-MS can achieve a precision of 2–5% for the determination

better understanding of LA-ICP-MS potentialities and limitations.

**3.3 Analytical features, challenges and limitations**

**146**

sampling.

discussed [2, 24, 25].

**Figure 1.** *Schematic diagram of the for simultaneous LIBS and LA-ICP-MS.*

of homogeneous elements in solid samples. For isotopic ratio studies and using double-focusing magnetic sector ICP-MS with multi-collector, the LA-ICP-MS allows a precision, in some cases, of 0.001–0.005% in the isotope ratio measurement. For heterogeneous samples, the precision is not the goal of the analysis, but the evaluation of the elemental distribution and its concentration in different areas of the sample [2, 24, 25].

In terms of limits of detection, it varies depending on the sample matrix and instrumental setup employed, usually ranging from hundreds of ng g−1 to a few μg g−1. Due to its limit of detection and the low mass ablated, LA-ICP-MS can cover a wide range of concentration, allowing the determination of trace, micro and major elements directly in the solid sample analyzed [2, 24, 25].

Although the potentiality of LA-ICP-MS, there are some drawbacks that the scientific community have devoted effort to overcome, and that the users must give special attention. The critical limitations of LA-ICP-MS are derived from the nature of the interaction between the laser beam and the solid sample and the transport process of the solid aerosol generated through the ICP. Since the mass removed by the laser beam is dependent on the matrix of the sample and the characteristics of LA system and parameters (laser fluence, spot size diameter, and repetition rate), thus, the analytical signal obtained in the ICP-MS is also matrix-dependent. This fact impacts the accuracy of the analytical method, which will be achieved in the condition that the same mass is ablated from the sample and the calibration standards, and the analyte undergoes the same transportation effects and processes in the plasma until be ionized [2, 25].

However, it is not simple to achieve a suitable solid calibration standard for the vast sample matrices that are analyzed by LA-ICP-MS. The ideal condition is the use of certified reference materials (CRMs), but a limited number of matrix types is available as CRM. It is especially difficult for the analysis of heterogeneous and non-powdered samples. To overcome this drawback, the scientific community has been studying different approaches to calibration. The main strategies are the use of matrix-matched materials, such as lab-made standards, by spiking the analytes in the powdered material that has approximately the same composition of the sample, drying and pressing it into a pellet. In the case of non-matrix-matched standards, solution-based strategies are employed, for example, the use of solution nebulization of liquid standard and mixing it with the aerosol of the sample, to calibrate the method. In all these cases, the homogeneity of the standard must be monitored to guarantee the analytical performance of the external calibration procedure. Additionally, internal standardization is usually needed to correct signal fluctuations during the analysis. A detailed review of calibration strategies for LA-ICP-MS quantification method was written by Miliszkiewicz et al. [31], and can be checked for more information about calibration strategies [2, 25].

Another drawback in LA-ICP-MS analysis is the fractionation that can occur in an ablation procedure or transport process. As previously mentioned in the LIBS section of this chapter, femtosecond UV lasers could suppress this effect, due to the minimization of the thermal process during the ablation. Aligned with the femtosecond UV laser, a well-designed ablation chamber and transport tubing will permit the small size distribution of the aerosol particles, avoiding fractionation during the ablation and the transportation through the ICP. Another point, and not less important, the optimization of the LA-ICP-MS instrument parameters are recommended to guarantee its suitability to the sample matrix analyzed. Such parameters are the laser frequency, spot size diameter, repetition rate, carrier gas flow rate, auxiliary gas flow rate, and radiofrequency power [2, 25].

In spite of these limitations, a careful dealing with LA-ICP-MS analysis, followed by an adequate optimization of the instrumental parameters, monitoring the possible sources of analyte fractionation, and a suitable calibration strategy allow the LA-ICP-MS users to achieve impressive information of solid samples that could not be accessed using traditional solution-based ICP-MS analysis, enlarging the possibilities of application.

## **3.4 LA-ICP-MS applications**

The fields of science that LA-ICP-MS is applicable are vast, including forensic, environmental, materials, biological, geological, etc. Although the image-based analysis using LA-ICP-MS composes the state-of-the-art of this technique, the use of LA-ICP-MS in studies involving total elemental analysis are also useful and confers interesting information about plenty kinds of samples that are pointed out above.

The use of LA-ICP-MS in forensic science can provide information of crime evidence without destructing the sample (usually obtained in a small amount). For example, the elemental composition of tape packaging samples could be used to classify them according to their origin rolls [32], and the elemental composition of glass evidence also allows for forensic crime elucidation [33]. The analyses of difficult to prepare samples are feasible using LA-ICP-MS, which is the case of hair samples. Ash and He [34] demonstrated that LA-ICP-MS was used to understand the poisoning dynamics of thallium in a criminal case, including understanding the increasing of doses of poison and the time interval. Levels of Cu, Zn, and Hg monitored in hair samples of different grizzly bears showed adequate correlation with the duration of salmon consumption and the amount of it [35], which is an important environmental monitoring method of mammal's wildlife.

LA-ICP-MS is applied also in conjunction with separation techniques, such as thin-layer chromatography (TLC), to improve the quality of the data. The use of TLC-LA-ICP-MS could provide quantitative information of gold nanoparticles (AuNPs) by separating them from gold ions, as well as AuNP size information by using a specific mobile phase in the TLC separation [36]. TLC was also employed in fractionation studies of S, Ni, and V in petroleum using LA-sector field ICP-MS [37].

The reader is referred to critical reviews of recent LA-ICP-MS applications, such as by Lobo et al. [38], where isotopic analysis in biological studies are discussed; or Limbeck et al. [39], where the challenges and advances in the quantitative analysis are detailed. Another interesting work by Pozebon et al. [40] demonstrates the use of LA-ICP-MS analysis of biological samples bringing information of novel

**149**

**Figure 2.**

*Laser Chemical Elemental Analysis: From Total to Images*

modern demands, such as single-cell analysis and NP uptake.

imaging process *via* LA-ICP-MS or LIBS can be seen in **Figure 2**.

There are different software for data processing and generation of chemical images, such as Microsoft Excel, LA-iMageS [47], and MATLAB [48]. In the case of LIBS imaging, the use of chemometrics tools has been a great ally in the data treatment due to the complexity of the emission spectra data. Principal component analysis (PCA) and partial least squares (PLS) are commonly used, and also can

*Steps of the imaging process via LA-ICP-MS and LIBS. Final 3D and 2D nanoparticles images by Gimenez* 

*et al. [46], reproduced with permission of Springer Nature.*

developments in instrumentation, methods of calibration, and applications in

**4. Elemental distribution imaging via LA-ICP-MS and LIBS techniques**

Images are present in daily life of the population in different contexts, such as through the image created by the eyes, enabling observation of the environmental around us; through pictures and photographs that record different personal or historical moments; and also, via magnetic resonance imaging (MRI) or X-ray computed tomography (CT) which contribute to medical diagnostics. In the chemistry context, imaging is a process that transforms the spectral information of atoms and molecules present in the solid sample surface, in a high resolution image through the application of powerful spectral techniques as LA-ICP-MS, LIBS, Raman [41], X-Ray Spectroscopy [42] and Secondary Ion Mass Spectrometry (SIMS) [43]. For the LA-ICP-MS and LIBS techniques, the imaging process occurs through the applications of laser pulses directed at specific regions of the sample surface (*x,y* coordinate), via point by point or continuous lines performing the chemical measurements and obtaining spectral information of the region of interest [22, 44]. In this case, a document (.txt or .log format, for example) is generated for each point or a line containing the signal intensity data of all the elements measured [45]. Due to the complexity and a large number of data obtained, appropriate software for data processing is required to separate the information and generate a data matrix, thus allowing the generation of a final image for each element. A two (2D) or a three (3D) dimensions image can be created for each element in the sample. The 2D image is equivalent just to the surface image and the signal intensity or concentration of the analyte, obtained by the conventional method describe previously. The 3D image can be obtained in two ways, i) analyzing and combining each layer of the sample (volume reconstruction of several 2D images), ii) repeated pulses of laser in the same region, allowing in-depth elemental imaging [46]. The scheme of the

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

*Practical Applications of Laser Ablation*

gas flow rate, and radiofrequency power [2, 25].

possibilities of application.

above.

**3.4 LA-ICP-MS applications**

fluctuations during the analysis. A detailed review of calibration strategies for LA-ICP-MS quantification method was written by Miliszkiewicz et al. [31], and can

Another drawback in LA-ICP-MS analysis is the fractionation that can occur in an ablation procedure or transport process. As previously mentioned in the LIBS section of this chapter, femtosecond UV lasers could suppress this effect, due to the minimization of the thermal process during the ablation. Aligned with the femtosecond UV laser, a well-designed ablation chamber and transport tubing will permit the small size distribution of the aerosol particles, avoiding fractionation during the ablation and the transportation through the ICP. Another point, and not less important, the optimization of the LA-ICP-MS instrument parameters are recommended to guarantee its suitability to the sample matrix analyzed. Such parameters are the laser frequency, spot size diameter, repetition rate, carrier gas flow rate, auxiliary

In spite of these limitations, a careful dealing with LA-ICP-MS analysis, followed by an adequate optimization of the instrumental parameters, monitoring the possible sources of analyte fractionation, and a suitable calibration strategy allow the LA-ICP-MS users to achieve impressive information of solid samples that could not be accessed using traditional solution-based ICP-MS analysis, enlarging the

The fields of science that LA-ICP-MS is applicable are vast, including forensic, environmental, materials, biological, geological, etc. Although the image-based analysis using LA-ICP-MS composes the state-of-the-art of this technique, the use of LA-ICP-MS in studies involving total elemental analysis are also useful and confers interesting information about plenty kinds of samples that are pointed out

The use of LA-ICP-MS in forensic science can provide information of crime evidence without destructing the sample (usually obtained in a small amount). For example, the elemental composition of tape packaging samples could be used to classify them according to their origin rolls [32], and the elemental composition of glass evidence also allows for forensic crime elucidation [33]. The analyses of difficult to prepare samples are feasible using LA-ICP-MS, which is the case of hair samples. Ash and He [34] demonstrated that LA-ICP-MS was used to understand the poisoning dynamics of thallium in a criminal case, including understanding the increasing of doses of poison and the time interval. Levels of Cu, Zn, and Hg monitored in hair samples of different grizzly bears showed adequate correlation with the duration of salmon consumption and the amount of it [35], which is an

LA-ICP-MS is applied also in conjunction with separation techniques, such as thin-layer chromatography (TLC), to improve the quality of the data. The use of TLC-LA-ICP-MS could provide quantitative information of gold nanoparticles (AuNPs) by separating them from gold ions, as well as AuNP size information by using a specific mobile phase in the TLC separation [36]. TLC was also employed in fractionation studies of S, Ni, and V in petroleum using LA-sector field

The reader is referred to critical reviews of recent LA-ICP-MS applications, such as by Lobo et al. [38], where isotopic analysis in biological studies are discussed; or Limbeck et al. [39], where the challenges and advances in the quantitative analysis are detailed. Another interesting work by Pozebon et al. [40] demonstrates the use of LA-ICP-MS analysis of biological samples bringing information of novel

important environmental monitoring method of mammal's wildlife.

be checked for more information about calibration strategies [2, 25].

**148**

ICP-MS [37].

developments in instrumentation, methods of calibration, and applications in modern demands, such as single-cell analysis and NP uptake.
