Investigation of laser-induced breakdown spectroscopy and multivariate analysis for differentiating inorganic and organic C in a variety of soils

Laser-induced breakdown spectroscopy (LIBS) along with multivariate analysis was used to differentiate between the total carbon (C), inorganic C, and organic C in a set of 58 different soils from 5 soil orders. A 532 nm laser with 45 mJ of laser power was used to excite the 58 samples of soil and the emission of all the elements present in the soil samples was recorded in a single spectrum with a wide wavelength range of 200–800 nm. The results were compared to the laboratory standard technique, e.g., combustion on a LECO-CN analyzer, to determine the true values for total C, inorganic C, and organic C concentrations. Our objectives were: 1) to determine the characteristic spectra of soils containing different amounts of organic and inorganic C, and 2) to examine the viability of this technique for differentiating between soils that contain predominantly organic and/or inorganic C content for a range of diverse soils. Previous work has shown that LIBS is an accurate and reliable approach to measuring total carbon content of soils, but it remains uncertain whether inorganic and organic forms of carbon can be separated using this approach. Total C and inorganic C exhibited correlation with rock-forming elements such as Al, Si, Fe, Ti, Ca, and Sr, while organic C exhibited minor correlation with these elements and a major correlation with Mg. We calculated a figure of merit (Mg/Ca) based on our results to enable differentiation between inorganic versus organic C. We obtained the LIBS validation prediction for total, inorganic, and organic C to have a coefficient of regression, r² = 0.91, 0.87, and 0.91 respectively. These examples demonstrate an advance in LIBS-based techniques to distinguish between organic and inorganic C using the full wavelength spectra.     

Femtosecond and nanosecond laser induced breakdown spectroscopic studies of NTO,HMX, and RDX

We present our results from the laser induced breakdown spectroscopic studies of 5-Nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO), Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX) investigated using nanosecond and femtosecond pulses. The presence of C, CN peaks in the spectra, signatures of high energy materials, was confirmed and persistence of emissions has been measured. Some of the Nitrogen peaks in fs LIBS spectra were found to be lower in magnitude (after normalization with N 868.60 nm peak) compared to the ns LIBS spectra. The presence of an additional CN peak in the fs spectra was identified for all samples. The ratio of CN peaks (388.28 nm, 387.08 nm, 386.16 nm) to C peak (247.82 nm), recorded with similar fluences, was discovered to be stronger in the fs case. Some of the possible mechanisms ensuing from our studies towards discrimination of such materials are outlined.     

Onboard calibration igneous targets for the Mars Science Laboratory Curiosity rover and the Chemistry Camera laser induced breakdown spectroscopy instrument

Accurate characterization of the Chemistry Camera (ChemCam) laser-induced breakdown spectroscopy (LIBS) on-board composition targets is of prime importance for the ChemCam instrument. The Mars Science Laboratory (MSL) science and operations teams expect ChemCam to provide the first compositional results at remote distances (1.5–7 m) during the in situ analyses of the Martian surface starting in 2012. Thus, establishing LIBS reference spectra from appropriate calibration standards must be undertaken diligently. Considering the global mineralogy of the Martian surface, and the possible landing sites, three specific compositions of igneous targets have been determined. Picritic, noritic, and shergottic glasses have been produced, along with a Macusanite natural glass. A sample of each target will fly on the MSL Curiosity rover deck, 1.56 m from the ChemCam instrument, and duplicates are available on the ground. Duplicates are considered to be identical, as the relative standard deviation (RSD) of the composition dispersion is around 8%. Electronic microprobe and laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) analyses give evidence that the chemical composition of the four silicate targets is very homogeneous at microscopic scales larger than the instrument spot size, with RSD < 5% for concentration variations > 0.1 wt.% using electronic microprobe, and < 10% for concentration variations > 0.01 wt.% using LA ICP-MS. The LIBS campaign on the igneous targets performed under flight-like Mars conditions establishes reference spectra for the entire mission. The LIBS spectra between 240 and 900 nm are extremely rich, hundreds of lines with high signal-to-noise, and a dynamical range sufficient to identify unambiguously major, minor and trace elements. For instance, a first LIBS calibration curve has been established for strontium from [Sr] = 284 ppm to [Sr] = 1480 ppm, showing the potential for the future calibrations for other major or minor elements.     

Role of laser pre-pulse wavelength and inter-pulse delay on signal enhancement in collinear double-pulse laser-induced breakdown spectroscopy

Dual-pulse (DP) laser-induced breakdown spectroscopy (LIBS) provides significant improvement in signal intensity as compared to conventional single-pulse LIBS. We investigated collinear DPLIBS experimental performance using various laser wavelength combinations employing 1064 nm, 532 nm, and 266 nm Nd:YAG lasers. In particular, the role of the pre-pulse laser wavelength, inter-pulse delay times, and energies of the reheating pulses on LIBS sensitivity improvements is studied. Wavelengths of 1064 nm, 532 nm, and 266 nm pulses were used for generating pre-pulse plasma while 1064 nm pulse was used for reheating the pre-formed plasma generated by the pre-pulse. Significant emission intensity enhancement is noticed for all reheated plasma regardless of the pre-pulse excitation beam wavelength compared to single pulse LIBS. A dual peak in signal enhancement was observed for different inter-pulse delays, especially for 1064:1064 nm combinations, which is explained based on temperature measurement and shockwave expansion phenomenon. Our results also show that 266 nm:1064 nm combination provided maximum absolute signal intensity as compared to 1064 nm:1064 nm or 532 nm:1064 nm.     

Laser-induced breakdown spectroscopy analysis of minerals: Carbonates and silicates

Laser-induced breakdown spectroscopy (LIBS) provides an alternative chemical analytical technique that obviates the issues of sample preparation and sample destruction common to most laboratory-based analytical methods. This contribution explores the capability of LIBS analysis to identify carbonate and silicate minerals rapidly and accurately. Fifty-two mineral samples (18 carbonates, 9 pyroxenes and pyroxenoids, 6 amphiboles, 8 phyllosilicates, and 11 feldspars) were analyzed by LIBS. Two composite broadband spectra (averages of 10 shots each) were calculated for each sample to produce two databases each containing the composite LIBS spectra for the same 52 mineral samples. By using correlation coefficients resulting from the regression of the intensities of pairs of LIBS spectra, all 52 minerals were correctly identified in the database. If the LIBS spectra of each sample were compared to a database containing the other 51 minerals, 65% were identified as a mineral of similar composition from the same mineral family. The remaining minerals were misidentified for two reasons: 1) the mineral had high concentrations of an element not present in the database; and 2) the mineral was identified as a mineral with similar elemental composition from a different family. For instance, the Ca–Mg carbonate dolomite was misidentified as the Ca–Mg silicate diopside. This pilot study suggests that LIBS has promise in mineral identification and in situ analysis of minerals that record geological processes.     

Production of aerosols by optical catapulting: Imaging,performance parameters and laser-induced plasma sampling rate

Optical catapulting (OC) is a sampling and manipulation method that has been extensively studied in applications ranging from single cells in heterogeneous tissue samples to analysis of explosive residues in human fingerprints. Specifically, analysis of the catapulted material by means of laser-induced breakdown spectroscopy (LIBS) offers a promising approach for the inspection of solid particulate matter. In this work, we focus our attention in the experimental parameters to be optimized for a proper aerosol generation while increasing the particle density in the focal region sampled by LIBS. For this purpose we use shadowgraphy visualization as a diagnostic tool. Shadowgraphic images were acquired for studying the evolution and dynamics of solid aerosols produced by OC. Aluminum silicate particles (0.2–8 μm) were ejected from the substrate using a Q-switched Nd:YAG laser at 1064 nm, while time-resolved images recorded the propagation of the generated aerosol. For LIBS analysis and shadowgraphy visualization, a Q-switched Nd:YAG laser at 1064 nm and 532 nm was employed, respectively. Several parameters such as the time delay between pulses and the effect of laser fluence on the aerosol production have been also investigated. After optimization, the particle density in the sampling focal volume increases while improving the aerosol sampling rate till ca. 90%.     

Different level of fluorescence in Raman spectra of montmorillonites

The paper presents the study of selected montmorillonite standards by Raman spectroscopy and microscopy supported by elemental analysis, X-ray powder diffraction analysis and thermal analysis. Dispersive Raman spectroscopy with excitation lasers of 532 nm and 780 nm, dispersive Raman microscopy with excitation laser of 532 nm and 100× magnifying lens, and Fourier Transform-Raman spectroscopy with excitation laser of 1064 nm were used for the analysis of four montmorillonites (Kunipia-F, SWy-2, STx-1b and SAz-2). These mineral standards differed mainly in the type of interlayer cation and substitution of octahedral aluminium by magnesium or iron. A comparison of measured Raman spectra of montmorillonite with regard to their level of fluorescence and the presence of characteristic spectral bands was carried out. Almost all measured spectra of montmorillonites were significantly affected by fluorescence and only one sample was influenced by fluorescence slightly or not at all. In the spectra of tested montmorillonites, several characteristic Raman bands were found. The most intensive band at 96 cm⁻¹ belongs to deformation vibrations of interlayer cations. The band at 200 cm⁻¹ corresponds to deformation vibrations of the AlO₆ octahedron and at 710 cm⁻¹ can be assigned to deformation vibrations of the SiO₄ tetrahedron. The band at 3620 cm⁻¹ corresponds to the stretching vibration of structural OH groups in montmorillonites.     

Influence of sample temperature on the expansion dynamics and the optical emission of laser-induced plasma

We investigate the influence of sample temperature on the dynamics and optical emission of laser induced plasma for various solid materials. Bulk aluminum alloy, silicon wafer, and metallurgical slag samples are heated to temperature T_S ≤ 500 °C and ablated in air by Nd:YAG laser pulses (wavelength 1064 nm, pulse duration approx. 7 ns). The plasma dynamics is investigated by fast time-resolved photography. For laser-induced breakdown spectroscopy (LIBS) the optical emission of plasma is measured by Echelle spectrometers in combination with intensified CCD cameras. For all sample materials the temporal evolution of plume size and broadband plasma emission vary systematically with T_S. The size and brightness of expanding plumes increase at higher T_S while the mean intensity remains independent of temperature. The intensity of emission lines increases with temperature for all samples. Plasma temperature and electron number density do not vary with T_S. We apply the calibration-free LIBS method to determine the concentration of major oxides in slag and find good agreement to reference data up to T_S = 450 °C. The LIBS analysis of multi-component materials at high temperature is of interest for technical applications, e.g. in industrial production processes.     

Towards a two-dimensional laser induced breakdown spectroscopy mapping of liquefied petroleum gas and electrolytic oxy-hydrogen flames

Two-dimensional mapping of the laser-induced breakdown spectroscopy (LIBS) signal of chemical species information in liquefied petroleum gas (LPG) and electrolytic oxy-hydrogen (EOH) flames was performed with in situ flame diagnostics. Base LIBS signals averaged from measurements at wavelengths of 320 nm to 350 nm describe the density information of a flame. The CN LIBS signal provides the concentration of fuel, while the H/O signal represents the fuel/air equivalence ratio. Here, we demonstrate the meaningful use of two-dimensional LIBS mappings to provide key combustion information, such as density, fuel concentration, and fuel/air equivalence ratio.     

Feasibility study for diagnosis of stomach adenoma and cancer using IR spectroscopy

The feasibility of infrared (IR) spectroscopy as a biomedical analysis tool for the diagnosis of stomach malignancy including adenoma and cancer has been studied using unstained biopsy samples. Biopsy samples were acquired from 11 subjects. IR spectra were collected for these samples using a microscope (aperture: 25 μm × 25 μm). The samples were stained again and the spots where the IR spectra were collected were re-examined by a pathologist to ensure the spectra represented the correct diagnostic information. The spectral features were compared among the averaged spectra of normal and malignant tissues. The spectral contrasts could be correlated to the differences in the molecular structure of the membrane lipids of the two tissue types as well as the variation in their glycogen contents. However, the spectral features between the adenoma and cancer tissues could not be distinguished. Initially we used principal component analysis (PCA) to examine the degree of separation between tissue types. Soft independent modeling of class analogies (SIMCA) was employed to evaluate the prediction accuracy of IR spectroscopy for the diagnosis of stomach adenoma and cancer. The prediction accuracies for normal, adenoma and cancer tissues were 77%, 30% and 87%, respectively, using SIMCA. IR microscopy successfully differentiated normal and malignant tissues. However, a more sophisticated algorithm will be required in order to effectively extract relevant information for the differentiation between stomach adenoma and cancer.     

Determination of silicon in plant materials by laser-induced breakdown spectroscopy

In spite of the importance of Si for improving the productivity of many important crops, such as those from the Poaceae family (e.g. sugar cane, maize, wheat, rice), its quantitative determination in plants is seldom carried out and restricted to few laboratories in the world. There is a survey of methods in the literature, but most of them are either laborious or difficult to validate in view of the low availability of reference materials with a certified Si mass fraction. The aim of this study is to propose a method for the direct determination of Si in pellets of plant materials by laser-induced breakdown spectroscopy (LIBS). The experimental setup was designed by using a Q-switched Nd:YAG laser at 1064 nm (5 ns, 10 Hz) and the emission signals were collected by lenses into an optical fiber coupled to an Echelle spectrometer equipped with an intensified charge-coupled device. Experiments were carried out with leaves from 24 sugar cane varieties, with mass fractions varying from ca. 2 to 10 g kg^− 1 Si. Pellets prepared from cryogenically ground leaves were used as test samples for both method development and validation of the calibration model. Best results were obtained when the test samples were interrogated with laser fluence of 50 J cm^− 2 (750 μm spot size) and measurements carried out at Si I 212.412 nm emission line. The results obtained by LIBS were compared with those from inductively coupled plasma optical emission spectrometry after oven-induced alkaline digestion, and no significant differences were observed after applying the Student's t-test at 95% confidence level. The trueness of the proposed LIBS method was also confirmed from the analysis of CRM GBW 07603 (Bush branches and leaves).     

Laser-induced breakdown spectroscopic characterization of impurity deposition on the first wall of a magnetic confined fusion device: Experimental Advanced Superconducting Tokamak

Our recent investigations have indicated the potential of laser-induced breakdown spectroscopy (LIBS) for analysis of the co-deposited layers on the first wall in the Experimental Advanced Superconducting Tokamak (EAST) in the vacuum environment. Detailed information of compositions at the superficial and in-depth positions of the first wall of divertor tiles can be obtained by analyzing the spectra from 200 to 980 nm. The decrease in concentrations of the depositional elements (such as D, Li) was clearly observed in the depth from 0 to 100 μm, but the concentrations of the substrate elements were found to be relatively uniform in the depth after dozens of laser pulses. The linear correlation approach has been applied for improving the impurity depth profile accuracy and identifying the interface boundary between the deposition layer and the substrate for the first time. This would help us to develop LIBS technique to monitor the fuel retention and impurity deposition on the first wall of EAST.     

Near-infrared spectroscopy: A powerful tool in studies of acid-treated clay minerals

The benefit of near-infrared (NIR) spectroscopy in studies of acid-treated clay minerals is demonstrated. The effects of mineral type, composition and content of non-swelling interlayers on the dissolution rate are investigated. Detailed analysis of the NIR region is performed by comparing the first overtone (2ν_OH) and combination (ν + δ)_OH bands with the fundamental stretching (ν) and bending (ν) vibrations. Spectra of acid-treated samples show a gradual decrease in the intensities of the structural OH overtone (near 7100 cm^?1) and combination (4600–4300 cm^?1) bands reflecting a fewer number of octahedral atoms. The appearance of the 2ν_SiOH vibration for terminal (isolated) SiOH groups near 7315 cm^?1 indicates the formation of a protonated silica phase. The band near 7130 cm^?1 remaining in the spectra of acid-treated samples is assigned to 2ν_HOSiOH of geminal silanol groups. Thus the creation of geminal silanols, previously detected by ²⁹Si MAS-NMR spectroscopy in acid-treated hectorite, is confirmed also by NIR spectroscopy. The assignment of the 4555 cm^?1 band to the (ν + δ)_SiOH combination enabled calculation of the wavenumber for the SiO–H bending vibration (～810 cm^?1) that is not observable in the mid-IR region due to overlapping with the Si–O band of amorphous silica (～800 cm^?1). The NIR spectra confirm that trioctahedral hectorite is much more susceptible to dissolution in HCl than dioctahedral nontronite. The dissolution rate of kaolinite present in the Badin clay as an admixture is lower than that of the main mineral nontronite. The accessibility of the interlayers for protons significantly influences the stability of clay minerals in HCl. Mixed-layered mineral illite/smectite with only 30% of swelling interlayers dissolves more slowly than smectite of similar chemical composition containing mainly swelling interlayers.     

A multimodal spectrometer for Raman scattering and near-infrared absorption measurement

Raman and infrared (IR) spectroscopy are complementary spectroscopic techniques. However, measurement of Raman and IR spectra are commonly carried out on separate instruments. A dispersive system that enables both Raman spectroscopy and NIR spectroscopy was designed, built, and tested. The prototype system measures spectral ranges of 2600–300 cm⁻¹ and 752–987 nm for Raman and NIR channels, respectively. A wavelength accuracy better than 0.6 nm and spectral resolution better than 1 nm (14.4 cm⁻¹ for Raman channel) could be achieved with our configuration. The linearity of spectral response was better than 99.8%. The intensity stability of the instrument was found to be 0.7% and 0.4% for Raman and NIR channels, respectively. The performance of the instrument was evaluated using binary aqueous solutions of ethanol and ovalbumin. It was found that ethanol concentrations (2–10%) could be predicted with a root mean squared error of prediction (RMSEP) of 0.45% using Raman peak height at 882.2 cm⁻¹. Quantification of ovalbumin concentration (8–16 g/L) in aqueous solutions and in denatured states yielded RMSEP values of 1.05 g/L and 0.74 g/L, respectively. Using concentration as external perturbation in two-dimensional correlation spectroscopy (2DCOS), heterospectral correlation analysis revealed the relationship between NIR and Raman spectra.     

FTIR investigation of spray-dried milk protein concentrate powders

Fourier transform infrared (FTIR) spectroscopy was used to examine the conformation of proteins in spray-dried milk protein concentrate (MPC) powders and to determine if the spectral changes could be related to nitrogen solubility of these powders. MPC samples (83–92% protein, dry basis) were prepared using a range of processing conditions and stored for 4 weeks at 21 °C. FTIR spectra were collected in the mid infrared (MIR) region between 4000 and 600 cm⁻¹. FTIR data was pre-processed to remove physical effects causing discrimination between samples using firstly second derivatives and normalization and secondly the extended multiplicative scatter correction (EMSC) technique. The FTIR spectral changes were subsequently assessed using second derivative spectroscopy and principal components analysis (PCA) in the amide I and II regions (1700–1400 cm⁻¹) and the fingerprint region (1800–700 cm⁻¹). PCA analysis showed that the different powder preparations could be separated on scores plots but the separation was not related to nitrogen solubility per se. However, changes in nitrogen solubility of individual MPC powders during storage could be correlated to changes in FTIR spectra. PCA analysis of FTIR spectra could generally discriminate between MPC powders that had lost significant nitrogen solubility (9–20%) and those in which nitrogen solubility was preserved on storage. There were changes in intensity and/or position of bands at 1630 cm⁻¹ when the solubility of a stored sample decreased substantially. The results of this work also show that EMSC data pre-processing for these samples gives comparable results when compared with more complicated data pre-processing for the removal of physical effects.     

Quantitative analysis of samples at high temperature with remote laser-induced breakdown spectrometry using a room-temperature calibration plot

A method for measurement of composition in samples at high temperature using a calibration curve performed at low temperature is presented. The method uses a transfer function which avoids the need for building a calibration plot at high temperature. The method has shown suitability for high-temperature measurements up to 850 °C in steel slags exhibiting coefficients of correlation of 0.991 and 0.989 for Si / Ca and Mg / Ca ratios, respectively. The accuracy of the method has been assessed by comparing the results with remote LIBS at high temperature to those by XRF at room temperature. As an example of its versatility, the method has been applied to the determination of the basicity index of steel slags at high temperature.     

A combined laser-induced breakdown and Raman spectroscopy Echelle system for elemental and molecular microanalysis

Raman and laser-induced breakdown spectroscopy is integrated into a single system for molecular and elemental microanalyses. Both analyses are performed on the same ~ 0.002 mm² sample spot allowing the assessment of sample heterogeneity on a micrometric scale through mapping and scanning. The core of the spectrometer system is a novel high resolution dual arm Echelle spectrograph utilized for both techniques. In contrast to scanning Raman spectroscopy systems, the Echelle–Raman spectrograph provides a high resolution spectrum in a broad spectral range of 200–6000 cm^? 1 without moving the dispersive element. The system displays comparable or better sensitivity and spectral resolution in comparison to a state-of-the-art scanning Raman microscope and allows short analysis times for both Raman and laser induced breakdown spectroscopy. The laser-induced breakdown spectroscopy performance of the system is characterized by ppm detection limits, high spectral resolving power (15,000), and broad spectral range (290–945 nm). The capability of the system is demonstrated with the mapping of heterogeneous mineral samples and layer by layer analysis of pigments revealing the advantages of combining the techniques in a single unified set-up.     

Measurement of concentration of chlorine attached to a stainless-steel canister material using laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) in the single-pulse or orthogonal double-pulse configuration was performed for the measurement of the concentration of chlorine, which induces the occurrence of stress corrosion cracking (SCC), attached to stainless-steel (UNS S30403). The chlorine spectra were measured for samples sprayed with synthetic seawater with chlorine concentrations from 0.1 to 1.0 g/m². The chlorine emission intensity decreased between chlorine concentrations of 0.4 and 1.0 g/m² as determined in the single-pulse measurement. The chlorine concentration dependence of the chlorine emission intensity in the single-pulse configuration was unchanged even when the laser energy was set between 30 and 100 mJ. On the other hand, the chlorine emission intensity increased linearly versus chlorine concentration from 0.1 to 1.0 g/m² with the orthogonal double-pulse configuration. The results suggest that LIBS is promising for the inspection of the environmental condition for SCC initiation, which can occur when the chlorine concentration is greater than or equal to 0.8 g/m².     

Nanocrystalline hydroxyapatite powders by a chitosan–polymer complex solution route: Synthesis and characterization

Nanocrystalline hydroxyapatite (HAp) powders were successfully synthesized by a simple method using chitosan–polymer complex solution. To obtain HAp nanopowders, the prepared precursor was calcined in air at 400–800 °C for 2 h. The phase composition of the calcined samples was studied by X-ray diffraction (XRD) technique. The XRD results confirmed the formation of HAp phase with a small trace of monotite phase. With increasing calcination temperature, the crystallinity of the HAp increased, showing the hexagonal structure of HAp with the lattice parameter a in a range of 0.94030–0.94308 nm and c of 0.68817–0.68948 nm. The particle sizes of the powder were found to be 55.02–73.36 nm as evaluated by the XRD line broadening method. The chemical composition of the calcined powders was characterized by FTIR spectroscopy. The peaks of the phosphate carbonate and hydroxyl vibration modes were observed in the FTIR spectra for all the calcined powders. TEM investigation revealed that the prepared HAP samples consisted of rod-like nanoparticles having the particle size in the range of 100–300 nm. The corresponding selected-area electron diffraction (SAED) analysis further confirmed the formation of hexagonal structure of HAp.     

Detection of zinc and lead in water using evaporative preconcentration and single-particle laser-induced breakdown spectroscopy

A novel laser-induced breakdown spectroscopy (LIBS)-based measurement method for metals in water is demonstrated. In the presented technology a small amount of sodium chloride is dissolved in the sample solution before spraying the sample into a tubular oven. After water removal monodisperse dry NaCl aerosol particles are formed where trace metals are present as additives. A single-particle LIBS analysis is then triggered with a scattering based particle detection system. Benefits are the highly increased metal concentration in the LIBS focal volume and the static NaCl-matrix which can be exploited in the signal processing procedure. Emitted light from the emerged plasma plume is collected with wide angle optics and dispersed with a grating spectrometer. In an aqueous solution, the respective limits of detection for zinc and lead were 0.3 ppm and 0.1 ppm using a relatively low 14 mJ laser pulse energy. Zn/Na peak intensity ratio calibration curve for zinc concentration was also determined and LIBS signal dependence on laser pulse energy was investigated.