Filtern
Dokumenttyp
- Zeitschriftenartikel (12) (entfernen)
Sprache
- Englisch (12)
Referierte Publikation
- ja (12)
Schlagworte
- LIBS (12) (entfernen)
The Radon transform is tested as a method for reconstruction of the emissivity distribution of asymmetric laser induced plasmas. Two types of experiments were carried out. First, the plasma asymmetry is introduced via focusing the laser by a cylindrical lens to create plasma plumes elongated along the symmetry axis of the lens. Second, an asymmetric power distribution across the laser beam is created by reflecting the latter from a damaged mirror. Various effects on the quality of the plasma emissivity reconstructed by the Radon tomography method are investigated. The understanding of these effects appears to be essential to design a proper experimental setup to study LIBS plasmas by the Radon tomography method. It is demonstrated that the Radon tomography can successfully be used for experimental studies of asymmetric LIBS plasmas.
The Radon transform tomography is used for reconstruction of the emissivity distribution in single- (SP) and double-pulse (DP) laser induced plasmas in orthogonal geometry. The orthogonal DP plasma is intrinsically asymmetric and thus suitable for the Radon reconstruction. The DP plasma consists of two plasmas separated by a short time interval of ~ 1 µs. The first plasma is created in air near the surface of a Si wafer and is followed (pre-ablation mode) or preceded (post-ablation mode) by the second plasma induced on this surface. A spectrometer moves in a semi-circular path around the plasma keeping the plasma in the rotation center. The optical detection is arranged so that a thin plasma layer parallel to the target surface is monitored. The axial symmetry of the SP plasma is investigated by comparing data from the Abel inversion taken at different angles and Radon reconstruction. The multi-angle measurements are used to estimate errors of the Abel reconstruction due to asymmetries of the plasma. Time-resolved Radon reconstruction in white light is performed for the DP plasma in both pre- and post-ablation modes. In the former case, the effect of ablated aerosol on the formation of the air plasma is monitored. In the latter case, a formation of an asymmetric compression shock created by the target plasma inside the air plasma is visualized. This observation is supported by computer simulations. An interaction of the two plasmas is studied by spectrally resolved Radon reconstruction revealing a complex distribution of target and ambient species inside the plasma at all studied delay times. Overall, it is demonstrated that Radon-based tomography is an informative tool to study transient asymmetric laser induced plasmas.
Gated detection with intensified detectors, e.g., ICCDs, is today the accepted approach for detection of plasma emission in laser-induced breakdown spectroscopy (LIBS). However, these systems are more cost-intensive and less robust than nonintensified CCDs. The objective of this paper is to compare, both theoretically and experimentally, the performance of an intensified (ICCD) and nonintensified (CCD) detectors for detection of plasma emission in LIBS. The CCD is used in combination with a mechanical chopper, which blocks the early continuum radiation from the plasma. The detectors are attached sequentially to an echelle spectrometer under the same experimental conditions. The laser plasma is induced on a series of steel samples under atmospheric conditions. Our results indicate that there is no substantial difference in the performance of the CCD and ICCD. Signal-to-noise ratios and limits of detection achieved with the CCD for Si, Ni, Cr, Mo, Cu, and V in steel are comparable or even better than those obtained with the ICCD. This result is further confirmed by simulation of the plasma emission signal and the corresponding response of the detectors in the limit of quantum (photon) noise.
Calibration-Free Laser-Induced Breakdown Spectroscopy (CF-LIBS) has been proposed several years ago as an approach for quantitative analysis of Laser-Induced Breakdown Spectroscopy spectra. Recently developed refinement of the spectral processing method is described in the present work. Accurate quantitative results have been demonstrated for several metallic alloys. However, the degree of accuracy that can be achieved with Calibration-Free Laser-Induced Breakdown Spectroscopy analysis of generic samples still needs to be thoroughly investigated. The authors have undertaken a systematic study of errors and biasing factors affecting the calculation in the Calibration-Free Laser-Induced Breakdown Spectroscopy spectra processing. These factors may be classified in three main groups: 1) experimental aberrations (intensity fluctuations and inaccuracy in the correction for spectral efficiency of a detection system), 2) inaccuracy in theoretical parameters used for calculations (Stark broadening coefficients and partition functions) and 3) plasma non-ideality (departure from thermal equilibrium, spatial and temporal inhomogeneities, optical thickness, etc.). In this study, the effects of experimental aberrations and accuracy of spectral data were investigated, assuming that the analytical plasma is ideal. Departure of the plasma conditions from ideality will be the object of future work. The current study was based on numerical simulation. Two kinds of metallic alloys, iron-based and aluminum-based, were studied. The relative weight of the error contributions was found to depend on the sample composition. For the here-investigated samples, the experimental aberrations contribute to the overall uncertainty on the quantitative results more than theoretical parameters. The described simulation method can be applied to the Calibration-Free Laser-Induced Breakdown Spectroscopy analysis of any other kind of sample.
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.
The purpose of this work is to improve the performance of a linear correlation method used for material identification in laser induced breakdown spectroscopy. The improved correlation procedure is proposed based on the selection and use of only essential spectral information and ignoring empty spectral fragments. The method is tested on glass samples of forensic interest. The 100% identification capability of the new method is demonstrated in contrast to the traditional approach where the identification rate falls below 100% for many samples.
The 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.
High repetition rate laser-induced breakdown spectroscopy using acousto-optically gated detection
(2014)
This contribution introduces a new type of setup for fast sample analysis using laser-induced breakdown spectroscopy (LIBS). The novel design combines a high repetition rate laser (up to 50 kHz) as excitation source and an acousto-optical modulator (AOM) as a fast switch for temporally gating the detection of the emitted light. The plasma radiation is led through the active medium of the AOM where it is diffracted on the transient ultrasonic Bragg grid. The diffracted radiation is detected by a compact Czerny-Turner spectrometer equipped with a CCD line detector. Utilizing the new combination of high repetition rate lasers and AOM gated detection, rapid measurements with total integration times of only 10 ms resulted in a limit of detection (LOD) of 0.13 wt.% for magnesium in aluminum alloys. This short integration time corresponds to 100 analyses/s. Temporal gating of LIP radiation results in improved LODs and consecutively higher sensitivity of the LIBS setup. Therefore, an AOM could be beneficially utilized to temporally detect plasmas induced by high repetition rate lasers. The AOM in combination with miniaturized Czerny-Turner spectrometers equipped with CCD line detectors and small footprint diode pumped solid state lasers results in temporally gateable compact LIBS setups.
A mobile double-pulse laser-induced breakdown spectroscopy system for industrial environments is presented.
Its capabilities as a process analytical technique for the recovery of metals from molten inorganic wastes are investigated. Using low-melting glass doped with different amounts of additives as a model system for recycling slags, the optimum number of shots, laser inter-pulse and acquisition delay times are optimized for solid and liquid (1200 °C) glass. Limits of detection from 7 ppm (Mn) to 194 ppm (Zn) are achieved working at a distance of 75 cm from the sample. To simplify the quantification of molten samples in an industrial furnace, the possibility is examined of using solid standards for analysis of molten material.
A spatial heterodyne spectrometer (SHS) is tested for the first time in combination with laser-induced breakdown spectroscopy (LIBS). The spectrometer is a modified version of the Michelson interferometer in which mirrors are replaced by diffraction gratings. The SHS contains no moving parts and the gratings are fixed at equal distances from the beam splitter. The main advantage is high throughput, about 200 times higher than that of dispersive spectrometers used in LIBS. This makes LIBS-SHS a promising technique for low-light standoff applications. The output signal of the SHS is an interferogram that is Fourier-transformed to retrieve the original plasma spectrum. In this proof-of-principle study, we investigate the potential of LIBS-SHS for material classification and quantitative analysis. Brass standards with broadly varying concentrations of Cu and Zn were tested. Classification via principal component analysis (PCA) shows distinct groupings of materials according to their origin. The quantification via partial least squares regression (PLS) shows good precision (relative standard deviation , 10%) and accuracy (within 6 5% of nominal concentrations). It is possible that LIBS-SHS can be developed into a portable, inexpensive, rugged instrument for field applications.