I. B. Gornushkin

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.

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.

Effective normalization technique for correction of matrix effects in laser-induced breakdown spectroscopy detection of magnesium in powdered samples

The goal of this research was to investigate the influence of the matrix on the laser-induced spectroscopy of magnesium. Powdered samples were used and were presented to the measurement as thin distributions on adhesive tape. A wide range of NIST certified reference materials were used as samples. With careful sample preparation and correction for sample surface density on the tape (determined by weighing), reasonable consistency in the Mg signal intensity was obtained regardless of sample matrix. Relative error of ∼10% and a precision of 10–20% were obtained for the determination of Mg in several certified samples.

Multivariate classification of pigments and inks using combined Raman spectroscopy and LIBS

AbstractThe authenticity of objects and artifacts is often the focus of forensic analytic chemistry. In document fraud cases, the most important objective is to determine the origin of a particular ink. Here, we introduce a new approach which utilizes the combination of two analytical methods, namely Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS). The methods provide complementary information on both molecular and elemental composition of samples. The potential of this hyphenation of spectroscopic methods is demonstrated for ten blue and black ink samples on white paper. LIBS and Raman spectra from different inks were fused into a single data matrix, and the number of different groups of inks was determined through multivariate analysis, i.e., principal component analysis, soft independent modelling of class analogy, partial least-squares discriminant analysis, and support vector machine. In all cases, the results obtained with the combined LIBS and Raman spectra were found to be superior to those obtained with the individual Raman or LIBS data sets. FigureCombination of Raman spectroscopy and LIBS for improved classification of inks: score plot from PCA and experimental set-up

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.

Identification of particulate materials by correlation analysis using a microscopic laser induced breakdown spectrometer

The goal of this work was the instant identification of particulate geological materials by comparison with spectral libraries stored in a computer. The libraries consisted of representative spectra from different groups of powdered samples. Special attention was paid to identification of particles with very similar chemical composition, such as certain iron ores. Chemical speciation analysis was also carried out. Iron and iron oxides were shown to be reliably identified using statistical correlation methods. Both linear and rank correlations were applied. Both correlation methods yielded probabilities of correct identification close to unity for almost all studied samples. This technique should have applications in the metallurgical, mining, and semiconductor industries, and in medical, environmental and forensic sciences.

Species Differentiation of a Diverse Suite of Bacillus Spores by Mass Spectrometry-Based Protein Profiling

ABSTRACT In this study, we demonstrate the versatility of matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOFMS) protein profiling for the species differentiation of a diverse suite of Bacillus spores. MALDI-TOFMS protein profiles of 11 different strains of Bacillus spores, encompassing nine different species, were evaluated. Bacillus species selected for MALDI-TOFMS analysis represented the spore-forming bacterial diversity of typical class 100K clean room spacecraft assembly facilities. A one-step sample treatment and MALDI-TOFMS preparation were used to minimize the sample preparation time. A library of MALDI-TOFMS spectra was created from these nine Bacillus species, the most diverse protein profiling study of the genus reported to date. Linear correlation analysis was used to successfully differentiate the MALDI-TOFMS protein profiles from all strains evaluated in this study. The MALDI-TOFMS protein profiles were compared with 16S rDNA sequences for their bacterial systematics and molecular phylogenetic affiliations. The MALDI-TOFMS profiles were found to be complementary to the 16S rDNA analysis. Proteomic studies of Bacillus subtilis 168 were pursued to identify proteins represented by the biomarker peaks in the MALDI-TOFMS spectrum. Four small, acid-soluble proteins (A, B, C, and D), one DNA binding protein, hypothetical protein ymf J, and four proteins associated with the spore coat and spore coat formation (coat JB, coat F, coat T, and spoIVA) were identified. The ability to visualize higher-molecular-mass coat proteins (10 to 25 kDa) as well as smaller proteins (<10 kDa) with MALDI-TOFMS profiling is critical for the complete and effective species differentiation of the Bacillus genus.

Novel uses of lasers in atomic spectroscopy. Plenary Lecture

This paper reviews several novel uses of lasers in atomic spectroscopy. A tutorial discussion is given of the basic processes involving the interaction of laser radiation with atoms and the measurement approaches. Laser microprobes, especially laser induced breakdown spectroscopy and laser ablation-inductively coupled plasma-optical emission or mass spectrometry are reviewed thoroughly with respect to principles, instrumentation and applications. Laser excited atomic fluorescence and atomic absorption spectrometry with diode lasers are considered primarily with respect to recent publications. Laser-enhanced ionization, resonance ionization and resonance ionization imaging are also thoroughly reviewed with respect to recent publications. Diagnostical measurements of plasmas and atom reservoirs are discussed. The principles of six laser based atomic absorption methods are given and the methods are compared with respect to detection limits. Finally, future uses of lasers in atomic spectroscopy and a comparison of the characteristics of various atomic methods for trace elements are given.