L.M. Cabalín

In situ analytical assessment and chemical imaging of historical buildings using a man-portable laser system.

In this work, the capability of laser-induced breakdown spectrometry for the in situ analytical assessment and chemical mapping of the façade of the cathedral of Malaga (Spain) has been demonstrated. The task required the use of a portable laser analyzer that allowed real-time spectral acquisitions in the field. A man-portable laser, based on a Q-switched Nd:YAG laser operating at its fundamental wavelength, has been utilized to generate a LIBS plasma of the sample surface. A chemical characterization of the different materials employed in the construction of this building has been performed. The purpose of this study was to use LIBS spectrochemical analysis to qualitatively discriminate between sandstone, limestone, marble, and cement mortar, which are the main components used in this class of historical monument. The field analysis was performed in two zones: the northern façade and the “girola”; the total areas of analysis of the two regions were 250 m2 and 650 m2, respectively. Chemical images of Si/Ca and Ca/Mg ratios from both parts of the building were generated. During the measurement campaign, a protocol of analysis was chosen so as to achieve an accurate description of the building materials with respectable spatial resolutions.

Line-focused laser ablation for depth-profiling analysis of coated and layered materials

The performance features of line-focused laser ablation for the characterization of interfaces in layered materials by laser-induced plasma spectrometry (LIPS) have been compared with the point-focusing method in terms of signal precision, signal-to-noise ratio, ablation rates, and surface sensitivity. In both optical configurations a pulsed Nd:YAG laser beam operating at 532 nm, with a homogeneous energy distribution (flattop laser), is used to generate point and microline plasmas on the sample surface. Subsequent light from the plasma is spectrally resolved and detected with an imaging spectrograph and an intensified charge-coupled-device detector that is binned along the slit-height direction. Line-focusing LIPS permits much higher laser power input while maintaining relatively low laser fluence, thus yielding better surface sensitivity and improved detection power. Values of the signal-to-noise ratio are improved by a factor of 6. In addition the ablation rate is 9 nm/pulse with the microline approach compared with 23 nm/pulse obtained with the point-focusing method. The results demonstrate that the microline-focusing approach is suitable for the depth analysis of coated and layered materials.

Surface interaction and chemical imaging in plasma spectrometry induced with a line-focused laser beam

a ´´ Acerinox S.A, E-11379 Los Barrios (Cadiz), Spain b ´ Abstract An experimental line imaging arrangement for surface characterization of solid samples in air at atmospheric pressure using laser-induced plasma spectrometry (LIPS) is presented. A microline plasma was formed by focusing a Nd:YAG laser beam operating at 532 nm with a cylindrical lens on the sample surface. The spatial resolution was investigated as a function of beam focal parameters including cylindrical lens focal length, beam diameter and pulse energy. While focal length affected lateral resolution only slightly, a strong dependence of crater width on beam diameter and laser pulse energy was observed. For routine sample imaging the crater width obtained was close to 15 mm while the spatial resolution along the spectrometer slit was 17.4 mm. Compositional distribution maps of inclusion constituents (Mg, Ca, Si, Al and Ti) in stainless steel are shown. The results demonstrate the potential of microline imaging for fast analysis of large area samples and for screening purposes. 2002 Elsevier Science B.V. All rights reserved.

Spectrochemical study for the in situ detection of oil spill residues using laser-induced breakdown spectroscopy.

Laser-induced breakdown spectroscopy (LIBS) has been used to identify the differences or similarities between crude oil and fuel residues. Firstly, a man portable LIBS analyzer was used for the on-site environmental control and analysis of the oil spill from The Prestige. An exhaustive analysis of crude oil and oil spill residues (collected during the field campaign in the Galician Coast) was performed in the laboratory. Characteristics elements in petroleum such as C, H, N, O, Mg, Na, Fe and V were detected. In addition, contributions from Ca, Si and Al in the composition of residues have been found. The use of intensity ratios of line and band emissions in the original fuel (crude oil) and in the aged residues allowed a better characterization of the samples than the simple use of peak intensities. The chemical composition between the crude oil and the fuel residues was found completely different. As well, a statistical method was employed in order to discriminate residues. Although significant differences were observed, no conclusions in terms of age and provenance could be reached due to the unknowledgment in the origin of the samples.

Automated Line-Focused Laser Ablation for Mapping of Inclusions in Stainless Steel

An automated line imaging arrangement for two-dimensional (2D) and three-dimensional (3D) generation of chemical maps of inclusions in stainless steel by laser-induced plasma spectrometry (LIPS) is presented. The plasma was generated in air at atmospheric pressure by focusing a flat-top Nd:YAG laser beam operating at 532 nm to a microline on the sample surface. The emitted light from the microline plasma was projected through an imaging spectrograph onto a charge-coupled device (CCD) detector to generate a spatially and spectrally resolved data set. Compositional distribution maps of inclusion constituents (Mn, Mg, Ca, Al, and Ti) in stainless steel of different grades have been generated. Comparative studies with the point-to-point LIPS mapping method have been performed, resulting in a 51-fold reduction in the number of pulses and analysis time when the microline imaging approach is employed. The results illustrate the capability of microline imaging LIPS for fast-automated acquisition of tomographic maps with spatial resolution of 50 μm between adjacent craters and 4.8 μm along the microline.

Chemical imaging using microline laser ablation: Performance comparison of Gaussian and flat top lasers

In previous studies, the potential of a microline imaging arrangement for surface characterization of solid samples in air at atmospheric pressure using laser-induced plasma spectrometry (LIPS) has been demonstrated. An improvement of this approach to obtain a uniform ablation along the microline focus that leads to a representative spatial distribution of elements on the sample surface is presented. For this purpose, the influence of beam energy distribution of two types of Nd: YAG pulsed lasers (a Gaussian laser and a flat top laser) on lateral resolution, intensity profiles of elements, chemical distribution maps, and plasma electron temperature has been investigated. As models of homogeneous and compositionally patterned samples, a stainless steel sample and a photovoltaic cell were chosen for this study. The results demonstrated that the Gaussian laser provides the best lateral resolution, but more redeposition at both sides of the microline crater, while the chemical maps obtained with the flat top laser matched with the distribution of the constituents of the sample surface.

Distinction strategies based on discriminant function analysis for particular steel grades at elevated temperature using stand-off LIBS

This work presents experimental discrimination strategies based on advanced chemometric tools for the differentiation of steel grades using stand-off laser-induced breakdown spectroscopy. The experiments are aimed at simulating the conditions existing in the continuous casting line of a steel factory. For this purpose, measurements of the as-cast samples provided by steelmakers have been carried out at a distance of 4.5 m, with samples in motion heated at temperatures up to 900 °C. Coaxial double-pulse laser excitation was used. The main challenge of the present study derives from the formation of an oxide layer on the surface when the steel sample is heated in contact with air. The chemical composition of such a layer deviates significantly from that of the bulk material, and not in a linear way. Moreover, complete removal of the scale layer from the surface with a single laser shot is not feasible. Therefore, quantitative analysis or calibration-based methods have to be discarded and specific chemometric strategies for identification purposes were developed. Discriminant Function Analysis (DFA) has been chosen as the semi-quantitative statistical method for the differentiation of steel grades from the corresponding scale layers. The statistical parameters derived from DFA and discriminant functions are presented with good correlation between real and predicted steel grades.

Effect of Pulse Duration in Multi-Pulse Excitation of Silicon in Laser-Induced Breakdown Spectroscopy (LIBS)

The aim of this study is to investigate the mechanisms responsible for the increase in ablated mass and signal enhancement observed on multi-pulse excitation. Several experiments were designed to obtain evidence that confirms the laser–sample and/or laser–plasma interaction, with special attention to the role of the pulse width on these effects. A train of pulses, with a separation of a few microseconds between pulses, was used for laser-induced breakdown spectroscopy (LIBS) analysis. The signal emission of Si was improved by an enhancement factor of about 60 compared to conventional single-pulse LIBS (SP-LIBS). The number of spikes, their amplitude, and their pulse duration were found to be variable for different Q-switch delays. A temporal study was performed to determine whether or not a laser–plasma interaction took place. The effect of pulse width (as responsible of laser–sample interaction) was also evaluated. The results demonstrate that, although both interactions contribute to the observed effect, the process is predominantly governed by the pulse width.

Angle of Observation Influence on Emission Signal from Spatially Confined Laser-Induced Plasmas:

The present work focuses on the influence of the angle of observation on the emission signal from copper plasmas. Plasma plumes have been generated inside a home-made chamber consisting of two parallel glass windows spaced by 2.5 mm. This chamber allows observing plasma plumes from different collection angles throughout their perimeter, spanning from 20° to 80° with respect to the surface of the Cu target. In order to minimize the observed volume of the plasma, measurements were made from the closest distance possible through a metallic hollow tube. Single-pulse and collinear double-pulse excitation schemes with a Nd:YAG laser (1064 nm, 5 ns) have been investigated. The results have shown that the selection of the best angle to collect light from the plasma is related to the excitation mode. On the other hand, the shot-to-shot signal variability has been found to depend on the shape of plasma plumes. In single-pulse excitation, a good correlation between the observed laser-induced breakdown spectroscopy (LIBS) emission (from spatially confined plumes) and their integrated signal of plasma image has been ascertained. However, this fact was less evident in double-pulse LIBS, which could be due to a different mechanism involved in the ablation process.