N. Omenetto

Laser-Induced Breakdown Spectroscopy (LIBS), Part II: Review of Instrumental and Methodological Approaches to Material Analysis and Applications to Different Fields

The first part of this two-part review focused on the fundamental and diagnostics aspects of laser-induced plasmas, only touching briefly upon concepts such as sensitivity and detection limits and largely omitting any discussion of the vast panorama of the practical applications of the technique. Clearly a true LIBS community has emerged, which promises to quicken the pace of LIBS developments, applications, and implementations. With this second part, a more applied flavor is taken, and its intended goal is summarizing the current state-of-the-art of analytical LIBS, providing a contemporary snapshot of LIBS applications, and highlighting new directions in laser-induced breakdown spectroscopy, such as novel approaches, instrumental developments, and advanced use of chemometric tools. More specifically, we discuss instrumental and analytical approaches (e.g., double- and multi-pulse LIBS to improve the sensitivity), calibration-free approaches, hyphenated approaches in which techniques such as Raman and fluorescence are coupled with LIBS to increase sensitivity and information power, resonantly enhanced LIBS approaches, signal processing and optimization (e.g., signal-to-noise analysis), and finally applications. An attempt is made to provide an updated view of the role played by LIBS in the various fields, with emphasis on applications considered to be unique. We finally try to assess where LIBS is going as an analytical field, where in our opinion it should go, and what should still be done for consolidating the technique as a mature method of chemical analysis.

Comparing several atomic spectrometric methods to the super stars: special emphasis on laser induced breakdown spectrometry, LIBS, a future super star

The “super stars” of analytical atomic spectrometry are electrothermal atomization-atomic absorption spectrometry (ETA-AAS), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS). Many other atomic spectrometric methods have been used to determine levels of elements present in solid, liquid and gaseous samples, but in most cases these other methods are inferior to the big three super star methods. The other atomic methods include glow discharge emission, absorption and mass spectrometric methods, laser excited fluorescence emission and ionization methods, and flame and microwave plasma emission and mass spectrometric methods. These “lesser” methods will be compared to the “super star” methods based on a number of figures of merit, including detection power, selectivity, multi-element capability, cost, applications, and “age” of the methods. The “age” of the method will be determined by a modification of the well-known Laitinen “Seven Ages of an Analytical Method” (H.A. Laitinen, Anal. Chem., 1973, 45, 2305). Calculations will show that certain methods are capable of single atom detection, including several atomic absorption methods, as well as laser atomic ionization and fluorescence methods. The comparison of methods will indicate why the “super stars” of atomic spectrometric methods will continue to retain their status and what must be done for the lesser atomic methods to approach “super star” status. Certainly most of the lesser atomic spectrometric methods will have a limited place in the analytical arena. Because of the wide current interest and research activity, special emphasis will be placed on the technique of laser induced breakdown spectrometry (LIBS). Its current status and future developments will therefore be reviewed.

Curve of growth methodology applied to laser-induced plasma emission spectroscopy

Abstract The curve-of-growth (COG) method was applied to a laser-induced plasma. The plasma was produced by a Nd:YAG laser on the surface of steel samples containing 0.007–1.3% of Cr. The emission was collected from the top of the plasma by means of a 45° pierced mirror and aligned onto an intensified charge-coupled device (ICCD) with a gate width of 1 μs and a variable delay time. The resonance 425.4 nm Cr line was used for construction of the COG. The temperature of the plasma (∼8000 K at 5-μs delay) was determined from a Boltzmann plot. The damping constant a, proportional to the ratio of the Lorentzian to the Doppler line widths, was found from the best fit of a series of calculated COG to the experimental data points and was 0.20±0.05. The number density of neutral Cr atoms which corresponded to the transition between low and high optical densities, was ≈6.5·1012 cm−3. The cross-section for broadening collisions of Cr atoms with atmospheric species (presumably N2) was calculated to be (66±16) A. The shape of the 425.4-nm Cr line was additionally checked by scanning an ultra-narrow cw Ti:Sapphire laser across the atomic transition and found to be in agreement with preliminary estimates. The potential of the COG method for laser breakdown spectroscopy is discussed.

Review of Multielement Atomic Spectroscopic Methods

A comparison of multielement atomic spectroscopic methods is given. The atomic spectroscopic methods, including atomic absorption flame and nonflame spectrometry, atomic fluorescence flame and nonflame spectrometry, and atomic emission spectrometry (including sources consisting of flames, inductively coupled plasmas, and microwave plasmas) are reviewed and compared especially with respect to their inherent advantages and disadvantages for multielement analysis. The types of optical detection systems, including temporal devices, spatial devices, and multiplex devices whether the spectral information is collected sequentially, simultaneously in parallel mode, or simultaneously in multiplexed mode are also compared with respect to their advantages and limitations for measuring atomic spectra and for multielement analysis. In this review, a comparison of atomic spectroscopic methods and optical detection devices is given. The comparison is given with respect to signal-to-noise ratio, experimental limits of detection, and other practical analytical figures of merit. It is hoped that this review will aid the analyst in selecting a multielement atomic method for trace analysis and especially make the analyst aware of the unavoidable tradeoffs in such a selection.

Laser induced breakdown spectroscopy as a tool for discrimination of glass for forensic applications

Materials analysis and characterization can provide important information as evidence in legal proceedings. The potential of laser induced breakdown spectroscopy (LIBS) for the discrimination of glass fragments for forensic applications is presented here. The proposed method is based on the fact that glass materials can be characterized by their unique spectral fingerprint. Taking advantage of the multielement detection capability and minimal to no sample preparation of LIBS, we compared glass spectra from car windows using linear and rank correlation methods. Linear correlation combined with the use of a spectral mask, which eliminates some high-intensity emission lines from the major elements present in glass, provides effective identification and discrimination at a 95% confidence level.

On-line and in-situ detection of lead aerosols by plasma-spectroscopy and laser-excited atomic fluorescence spectroscopy

Abstract A set-up for on-line and size-segregated detection of lead in ultrafine aerosols was developed. Lead nitrate aerosols with particle diameters between 10 and 300 nm were generated by ultrasonic nebulization of aqueous Pb(NO3)2-solutions. A differential mobility particle sizer (DMPS) was used for size-resolved mass calibration. Either a miniaturized acetylene-air-flame or a laser-induced plasma (LIP) was employed for atomization. Lead was detected with a spectrograph and a gateable, intensified CCD-camera by atomic emission spectroscopy (AES) and laser excited atomic fluorescence (LEAF). Due to the lower sensitivity, for LIP-AES no size-resolved calibration was possible and for calibration with polydisperse aerosols a detection limit of 155 μg m−3 was found for lead. With LEAF and flame atomization, a linear calibration curve was obtained with on-line detection limits of 47 ng m−3 for lead. No dependence of the detection limit on the particle diameter was observed. For LEAF with a laser-induced plasma as atom source, a correlation between the detection limit and the particle diameter was found. The detection limit increased from 55 ng m−3 for a particle diameter of 48 nm to 130 ng m−3 for a particle diameter of 300 nm. The increasing detection limit with increasing particle diameter was probably due to the incomplete atomization of larger particles in the colder periphery of the plasma.

Modeling an inhomogeneous optically thick laser induced plasma: a simplified theoretical approach ☆

Abstract A simplified theoretical approach is developed for an optically thick inhomogeneous laser induced plasma. The model describes the time evolution of the plasma continuum and specific atomic emission after the laser pulse has terminated and interaction with a target material has ended. Local thermodynamic equilibrium is assumed allowing the application of the collision-dominated plasma model and standard statistical distributions. Calculations are performed for a two-component Si/N system. The model input parameters are the number of plasma species (or plasma pressure) and the ratio of atomic constituents. Functions are introduced which describe the evolution of temperature and size of the plasma. All model inputs are experimentally measurable. The model outputs are spatial and temporal distributions of atom, ion and electron number densities, evolution of an atomic line profile and optical thickness and the resulting absolute intensity of plasma emission in the vicinity of a strong non-resonance atomic transition. Practical applications of the model include prediction of temperature, electron density and the dominating broadening mechanism. The model can also be used to choose the optimal line for quantitative analysis.

Quantitative analysis of low-alloy steel by microchip laser induced breakdown spectroscopy

The development of a compact laser induced breakdown spectroscopy (LIBS) system increases the possibilities of applying the technique in industrial arenas, field applications and process monitoring. Significant progress has been achieved in miniaturization of optical detectors and lasers, allowing portable, low-cost LIBS equipment to be devised. Conventional lasers for LIBS, like actively Q-switched Nd:YAG lasers are limited by their bulkiness, the need for a cooling system and high power consumption. The use of a miniature solid state microchip laser overcomes these drawbacks and offers further advantages of good beam quality, high pulse repetition frequency and less damage to target. In this work we studied the quantification of elemental composition of low alloy steel samples using a higher power microchip (“powerchip”) laser. The possibility of real time, in situ quantification of such materials by powerchip LIBS enhances the applicability of the technique to process monitoring in the steelmaking industry. The performance of the LIBS technique based on a powerchip laser and a portable non-intensified, non-gated detector for elemental quantification is evaluated and compared to that obtained using an intensified detector. Calibrations were achieved for Cr, Mo, Ni, Mn and Si with linear regression coefficients between 0.98–0.99 and limits of detection below 100 ppm in most cases.

Characterisation of hydroxide complexes of uranium(VI) by time-resolved fluorescence spectroscopy

Deconvolution of composite fluorescence spectra and related decay curves for UVI in the pH ranges 0–5 and 9–11 at different total U concentrations, yielded decay times and individual fluorescence spectra for various hydroxo-complexes (UO2)m(OH)n(2m–n), where (m, n)=(1, 1); (1, 3); (2, 2) and (3, 5). Depending on the emission wavelength, the fluorescence efficiencies of (2, 2) and (3, 5) were found to be, respectively, 7 to 85, and 3 to 4 times higher than that of the uranyl ion, whereas the latter is ca. 3 times more fluorescent than the (1, 1) complex. Thus, relatively small concentrations of polynuclear species contribute appreciably to the overall fluorescence of aqueous UVI solutions. The fairly large difference between decay times and the high sensitivity of the equipment used made it possible to detect even small amounts of the hydroxo complexes. In the alkaline pH range 10–12, the (1, 3) hydroxo-complex was found to predominate. In the presence of 0.05 mol dm–3 phosphate and in the same pH range, there was a pronounced change in the fluorescence spectrum indicating that the chemical speciation in the system had changed. These observations make it necessary to revise the current equilibrium data for uranium (VI) phosphate complexes at high pH.

Microchip laser-induced breakdown spectroscopy: a preliminary feasibility investigation.

A commercial, 7 μJ/pulse, 550 ps microchip laser is used to induce plasma on Pb, Si, Cu, Fe, Ni, Ti, Zn, Ta, and Mo foils and a Si wafer. The measured plasma lifetime is comparable with the duration of the laser pulse (a few ns). The plasma continuum radiation is low, while some of the strong resonance lines (e.g., Zn 213.86 nm) show self-reversal. Quantitative analysis is possible using non-gated detectors but analytical lines should be chosen with care to avoid reduction in the linear dynamic range. The mass removed (0.5–20 ng/pulse) is sufficient to yield spectra that are detectable with portable grating spectrometers equipped with non-gated, non-intensified detector arrays. The spectrum of Cd is detected with a broadband portable spectrometer (200–950 nm). The combination of the broadband spectrometer and the microchip laser is very promising for material identification, especially in field applications.