R. Fantoni

A comparison of single and double pulse laser-induced breakdown spectroscopy of aluminum samples

Abstract Single and double pulse laser-induced breakdown spectroscopy (LIBS) was carried out on aluminum samples in air. In the case of double pulse excitation, experiments were conducted by using the same laser source operated at the same wavelength (1064 nm in most cases here presented). A lowering of the second pulse plasma threshold was observed, together with an overall enhancement in line emission for the investigated time delay between the two pulses (40–60 μs). The laser-induced plasma originated by a single and double pulse was investigated near ignition threshold with the aim to study possible dynamical mechanisms in different regimes. Currently available spectroscopic diagnostics of plasma, such as the line broadening and shift due Stark effects, have been used in the characterization in order to retrieve electron densities, while standard temperature measurements were based on Boltzmann plot. Plasma relevant parameters, such as temperature and electron density, have been measured in the plasma decay on a long time scale, and compared with crater shape (diameter and inferred volume). The comparison of double with single pulse laser excitation was carried out while keeping constant the energy per pulse; the influence of laser energy was investigated as well. Results here obtained suggest that use of the double pulse technique could significantly improve the analytical capabilities of LIBS technique in routine laboratory experiments.

Self-absorption model in quantitative laser induced breakdown spectroscopy measurements on soils and sediments ☆

Abstract Application of laser induced breakdown spectroscopy (LIBS) in the quantitative analysis of elemental composition of soils with different origins and Antarctic marine sediments has been considered. The analytical method followed includes the usual plasma modeling at local thermal equilibrium (LTE) based on average temperature and electron density values, as well as spectra normalization, introduced in order to reduce the effects related both to the substrate optical and thermal properties and to the influence of laser parameters on quantitative data. The computational algorithm takes into account only atomic species and their first ionization states, which is sufficient at the plasma temperature measured in the experiments. Calibration curves are finally generated for each element of interest measured on certified samples with different provenience and matrix composition. In this paper a model is developed which takes into account the effects responsible for non-linearities in the relationship between line intensity and elemental concentration. The model properly includes line re-absorption and contributions from space regions with different plasma densities. Its application permits us to obtain the correlation coefficients between the LIBS measured and certified concentration of each element analyzed. These coefficients, specific for a given experimental layout and atomic lines data base, are successively applied in analytical LIBS measurements allowing for the direct determination of a single element concentration in any sample, regardless of its unknown matrix composition. The LIBS method presented here was tested on a priori unknown samples, and gave uncertainties in concentration varying from 15 to 40% over a large concentration range covering several orders of magnitude. The measuring error depends on element type, on the concentration value and also on the number of certified samples used for the initial calibration. The present results are already significant for some field application, such as on-board marine sediment analysis where a significant matrix variation with layer depth is common.

Determination of heavy metals in soils by Laser Induced Breakdown Spectroscopy

Laser Induced Breakdown Spectroscopy (LIBS) is a recent analytical technique that is based upon the measurement of emission lines generated by atomic species close to the surface of the sample, thus allowing their chemical identification. In this work, the LIBS technique has been applied to the determination of total contents of heavy metals in a number of reference soil samples. In order to validate the technique, LIBS data were compared with data obtained on the same soil samples by application of conventional Inductively Coupled Plasma (ICP) spectroscopy. The partial agreement obtained between the two sets of data suggested the potential applicability of the LIBS technique to the measurement of heavy metals in soils.

Laser-induced breakdown spectroscopy for semi-quantitative and quantitative analyses of artworks—application on multi-layered ceramics and copper based alloys ☆

Abstract In the present work, we report on the analyses of different types of artworks, such as medieval glazed Umbrian pottery and copper based alloys from Roman and modern periods, performed by means of Laser Induced Breakdown Spectroscopy (LIBS). The semi-quantitative analyses on the multi-layered ceramic findings regard glaze, luster and pigment decorations present on the surface. The composition for each decorative layer was determined by estimating the contribution of the ceramic layer beneath the examined one to the whole plasma emission. Two types of ancient luster have been considered: red and gold, while the pigments examined include painted decorations of different blue tonalities. The measured elemental composition of the decorative layers resulted partially correlated with the color of the painted surface, measured by a standard UV-VIS spectrometer. In LIBS analyses of bronze samples, a procedure was developed, which improves data repeatability and extends quantitative measurements to minor elemental constituents. Results of the quantitative analyses gave indications about the manufacturing process of the artwork, its actual degree of conservation and the presence of residual surface decorations.

Quantitative laser induced breakdown spectroscopy analysis of ancient marbles and corrections for the variability of plasma parameters and of ablation rate

White marble samples from ancient quarries have been analyzed by Laser Induced Breakdown Spectroscopy (LIBS) both on the bulk material and surface encrustations. With the aim to achieve quantitative results by LIBS, until now not reported on marble materials, calibration standards with CaCO3 matrices doped with certified soils were realized. Very different emission intensities and plasma parameters were observed on the standards and natural marbles. In order to compare so different spectra, a method for data analysis was developed, which takes into account variability of the ablation rate, plasma temperature and electron density. It was experimentally demonstrated that ablated volume is well correlated to the emission intensity of plasma continuum for a wide range of laser energies. LIBS signal normalization on the adjacent continuum level, together with introduction of correction factors dependent on plasma parameters, allowed the measuring of concentrations both for major and trace elements in marbles. The analytical procedure was validated by comparative SEM-EDX and ICP-OES measurements. Quantitative LIBS analyses were also performed during encrustation removal and could be applied to control laser-cleaning processes. The quantification of metal contents in the encrustations supported the occurrence of sulfates in the outer layers exposed to environmental agents via a catalytic process.

Self-calibrated quantitative elemental analysis by laser-induced plasma spectroscopy: application to pigment analysis

Abstract A new laser-based spectroscopic technique, called calibration-free LIPS (laser-induced plasma spectroscopy), is proposed for fast and precise elemental analysis in the field of cultural heritage conservation and study. Quantitative stratigraphic results, obtained by CF-LIPS on ancient Roman frescoe samples, are shown. The application of this calibration-free procedure frees the LIPS technique from the need of reference samples or an internal standard. This characteristic, along with the intrinsic speed (the whole process of data collection and analysis takes a few minutes) and precision (of the order of a few parts percent) make the CF-LIPS a viable technique for in situ quantitative analysis of artworks.

On board LIBS analysis of marine sediments collected during the XVI Italian campaign in Antarctica

Abstract The Laser-induced Breakdown Spectroscopy technique was applied on board the R/V Italica during the XVI Antarctic campaign (2000–2001) to carry out elemental chemical analysis of marine sediments collected using different sampling systems. To this end, a compact system has been built, which was suitable to operate also in the presence of mechanical vibrations, induced by the ship motion. Qualitative and quantitative analyses were performed on dried samples, without any further pre-treatment. Qualitative analyses have shown similar elemental composition among different collected sediments, except for significant differences in the case of rock fragments and manganese nodule. The latter also contains some heavy metals that in sediment layers were detected only in traces. The methodology to retrieve relative or absolute elemental concentration in heterogenous samples has been optimized and is scarcely sensitive to variations of sediment physical properties with depth, and to experimental parameters such as laser defocusing because of surface irregularities, and laser energy fluctuations. The relative distribution of the major elemental constituents, both from a bio-organic and mineral origin, was measured as a function of sediment depth. Measurements, once limited to specific spectral sections, and data analyses are fast and very reproducible. Most of the elements show a gradually varying distribution along the sampled core, except for silicon and barium, whose steep decrease with depth is strongly related to their biogenic origin. Quantitative LIBS analyses were performed on a limited number of samples and the results reported here, are comparable to the certified element contents in a reference sample of Antarctic sediments.

LIBS as a diagnostic tool during the laser cleaning of copper based alloys: experimental results

In spite of difficulties in quantitative LIBS analysis on copper based alloys, the very low invasiveness of the technique strongly sustains attempts to use it with cultural heritage materials, including ancient bronzes. Analytical results obtained with calibration curves and a calibration free model are compared here on a set of ancient roman coins. An attempt is presented to monitor the laser ablation process on bronze coins and artificially aged standards during the cleaning. The double pulse technique showed that LIBS analytical results could benefit from synchronization between the UV laser sources used, respectively, for cleaning (266 nm) and for LIBS analysis (335 nm).

Enhancement of LIBS signal by spatially confining the laser-induced plasma

A small chamber in brass has been developed in order to enhance the sensitivity of single-pulse Laser-Induced Breakdown Spectroscopy (LIBS) for element determination in air. Its use results in an increase of a factor of 3 and 10 for As and Fe signals detected in the UV range (230–240 nm).

Laser synthesis and crystallographic characterization of ultrafine SiC powders

High purity, ultrafine SiC powders have been produced from gas phase reactants (SiH{sub 4}, C{sub 2}H{sub 2}) in a CO{sub 2} laser induced process. The flow reactor designed to operate with a medium power (10--50 W) continuous wave CO{sub 2} laser source is described. The mechanism of gas phase reactions involved has been investigated by means of on-line optical diagnostics. Powders produced have been characterized by means of conventional chemical and spectroscopic methods. The x-ray results point out a growth mechanism by coalescence; i.e., whole islands move in the flame to take part in binary collisions, analogously to that observed for particles produced by inert gas evaporation.