S. Legnaioli

A procedure for correcting self-absorption in calibration free-laser induced breakdown spectroscopy

Abstract A model of the self-absorption effect in laser-induced plasma has been developed, with the aim of providing a tool for its automatic correction in the Calibration-Free algorithm recently developed for standardless analysis of materials by LIBS (Laser Induced Breakdown Spectroscopy). As a test of the model, the algorithm for self-absorption correction is applied to three different certified steel NIST samples and to three ternary alloys (Au, Ag, Cu) of known composition. The experimental results show that the self-absorption corrected Calibration-Free method gives reliable results, improving the precision and the accuracy of the CF-LIBS procedure by approximately one order of magnitude.

Application of laser-induced breakdown spectroscopy technique to hair tissue mineral analysis

The concentration of the main minerals present in human hair is measured on several subjects by Calibration-Free Laser-Induced Breakdown Spectroscopy (CF-LIBS) and compared with the results obtained through a commercial analytical laboratory. The possibility of using CF-LIBS for mineral analysis in hair is discussed, as well as its feasibility for the fast and inexpensive determination of the occurrence of heavy-metal poisoning in hair.

Effect of Laser-Induced Crater Depth in Laser-Induced Breakdown Spectroscopy Emission Features:

The influence of crater depth on plasma properties and laser-induced breakdown spectroscopy (LIBS) emission has been evaluated. Laser-induced plasmas were generated at the surface and at the bottom of different craters in a copper sample. Plasmas produced at the sample surface and at the bottom of the craters were spatially and temporally resolved. LIBS emission, temperature, and electronic number density of the plasmas were evaluated. It is shown that the confinement effect produced by the craters enhances the LIBS signal from the laser-induced plasmas.

Quantitative analysis of aluminium alloys by low-energy, high-repetition rate laser-induced breakdown spectroscopy

With the aim of reducing the dimensions of a laser-induced breakdown spectroscopy (LIBS) apparatus for building a portable instrument, a diode-pumped Nd:YAG mini-laser at high repetition rate was tested as an excitation source for the quantitative analysis of aluminium alloy samples. Moreover, LIBS spectra acquired by using an ICCD-echelle spectrometer detection system were compared with those obtained by a traditional spectrometer coupled to a non-intensified linear array detector. Calibration curves were built and limits of detections were calculated using both detection systems for magnesium, silicon, copper, titanium, manganese, nickel and iron. The results were compared with those obtained by recently proposed LIBS systems based on the use of microchip lasers.

Study of foxing stains on paper by chemical methods, infrared spectroscopy, micro-X-ray fluorescence spectrometry and laser induced breakdown spectroscopy

Abstract Foxing spots appear on the paper as stains of reddish-brown, brown or yellowish color, generally of small dimensions, with sharp or irregular edges, most of which, if excited with UV light, show fluorescence. The formation mechanisms of foxed areas have been studied since 1935, however, despite more recent intensive research there are still no conclusive results. Some authors found evidence of bacterial or fungal growth in some foxed areas sometimes associated with the presence of iron. We decided to focus our attention on the influence of the different iron valence in the formation of stains in the paper. For this reason we artificially induced the formation of foxing by adding to the paper small, known quantities of iron (III) and iron (II) ions. We prepared aqueous solutions of ferric chloride and ferrous sulfate at three different concentrations and we always used the same quantity of each solution (5 μl) to obtain a foxing stain. Part of the paper samples was artificially aged in a climatic chamber at 80 °C, 65% relative humidity for 15 days and part was submitted to aging for the same period at ambient temperatures under UV light at 240 nm. All papers were then analyzed for stain diameter, chromaticity coordinates, fluorescence under UV illumination, water content in the paper and in the spots, carbonyl content and then examined with infrared spectroscopy, X-ray fluorescence spectrometry and laser induced breakdown spectroscopy. Infrared spectra were collected in transmittance from potassium bromide pellets or directly in reflectance under microscope; X-ray fluorescence analysis were carried out using an X-ray microbeam (350 μm beam spot; W X-ray tube) and LIBS analysis with Nd:YAG laser coupled with a Czerny-Turner spectrometer. As a result it is stated that the foxing phenomenon is related to a strong oxidation of the cellulose chain. Concerning the color coordinates there are no great differences between samples treated with iron (III) and iron (II). Carbonyl content, on the contrary, varies for the two sets of samples, especially in relation with the kind of aging. μ-XRF and LIBS measurements show a relationship between iron valence and calcium ion displacement in the foxed areas.

Observation of different mass removal regimes during the laser ablation of an aluminium target in air

The mass removal mechanisms occurring during the ablation of an aluminium target, induced by a Nd:YAG laser at λ = 1064 nm in air, were investigated in the fluence range between 1.5 and 840 J cm−2. The spectroscopic analysis of the plasma emission allowed the calculation of the plasma thermodynamic parameters and an estimation of its atomized mass. Conversely, microscopic analysis of the craters allowed the calculation of the hole volume and the quantification of the material accumulated in the rim around it, as well as a qualitative inspection of the craters profile and appearance. The trends of line intensity, of the atomized plasma mass and of the crater volumes with laser fluence suggest the identification of four laser fluence ranges, where a different role of background gas is played and different mass removal mechanisms seem to occur. A complex picture is drawn where vaporization, melt displacement, melt expulsion and phase explosion take place at different laser fluences.

Temporal and spatial evolution of a laser-induced plasma from a steel target.

The space and time evolution of a laser-induced plasma from a steel target has been studied using optical time-of-flight and shadowgraphic techniques. The results, obtained for two distinct laser energy regimes, allow us to individuate two different regions in the plume, one characterized by air and continuum emissions produced by the shock wave ionization and the other characterized by emissions from ablated material. Moreover, it was shown that a sufficiently high laser fluence and short delay time of acquisition are needed to avoid inhomogeneous effects in the plasma, as required in analytical applications such as laser-induced breakdown spectroscopy.

Effect of laser parameters on plasma shielding in single and double pulse configurations during the ablation of an aluminium target

The process of nanosecond-laser ablation in air is a complex phenomenon, where the drilling process is a plasma mediated mechanism strongly affected by the shielding of the trailing part of the laser pulse. The laser pulse parameters as well as the target and ambient gas properties affect the dynamics of the process, modifying the importance and the initiation times of plasma shielding and laser-supported wave mechanisms. In this paper, the laser?target coupling of a Nd?:?YAG laser pulse in air at 1.06??m impinging on an aluminium target was investigated by spectroscopic analysis of plasma emission and by inspection of craters with video-confocal microscopy. The effects of laser pulse width and energy were investigated both in the single pulse configuration and in the orthogonal pre-pulse double pulse configuration. While in the single pulse case, a strong plasma shielding is observed at irradiances higher than (7?9) ? 108?W?cm?2, which is thought to coincide with the onset of laser-supported wave mechanism, such a change of regime is not observed in the double pulse case. A physical interpretation of the data is proposed.

Recovery of archaeological wall paintings using novel multispectral imaging approaches

New approaches in the application of multispectral imaging to the recovery of archeological wall paintings are presented, based on statistical techniques and on a novel method of image treatment (Chromatic Derivative Imaging – ChromaDI) which offers a way of embedding information coming from four spectral bands into a standard RGB image. The methods are applied to some wall paintings from the Tomb of the Monkey, an Etruscan tomb in the necropolis of Poggio Renzo, near the city of Chiusi (Siena), Italy, dated around 480-470 BC. It is shown that the techniques described are able to highlight and enhance a number of details that cannot be perceived in either any of the original channel images or any single processed output channel.

Micro-Laser-Induced Breakdown Spectroscopy (Micro-LIBS) Study on Ancient Roman Mortars

The laser-induced breakdown spectroscopy (LIBS) technique was used for analyzing the composition of an ancient Roman mortar (5th century A.D.), exploiting an experimental setup which allows the determination of the compositions of binder and aggregate in few minutes, without the need for sample treatment. Four thousand LIBS spectra were acquired from an area of 10 mm2, with a 50 µm lateral resolution. The elements of interest in the mortar sample (H, C, O, Na, Mg, Al, Si, K, Ca, Ti, Mn, Fe) were detected and mapped. The collected data graphically shown as compositional images were interpreted using different statistical approaches for the determination of the chemical composition of the binder and aggregate fraction. The methods of false color imaging, blind separation, and self-organizing maps were applied and their results are discussed in this paper. In particular, the method based on the use of self-organizing maps gives well interpretable results in very short times, without any reduction in the dimensionality of the system.