Discriminating organic carbon from endokarstic moonmilk-type deposits by LIBS. The case of a natural carbonated Martian analogue.

Moonmilk-type deposits exemplify carbonated Martian analogues existing in the subsurface of Earth, an endokarstic speleothem with a possible biochemical origin composed principally by carbonates, mainly huntite and dolomite. In this work, samples of moonmilk located in Nerja Cave (southern Spain) have been studied by LIBS with the aim of identifying carbon of biogenic origin by establishing a relationship between a molecular emission indicator, CN signal, and the organic carbon content. The characterization of this kind of carbonate deposit with a multiple mineralogical composition has been completed using scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and X-ray diffraction techniques for qualitative and semi-quantitative analysis. The information attained from LIBS regarding energy thresholds and time-resolved kinetics of CN emissions provides useful insight into the identification of different molecular emitters, namely organic and inorganic CN, depending on the laser irradiance and time settings conditions. These promising results are of application in the search and identification of biosignatures in upcoming planetary missions with astrobiological purposes.

Multi-analytical study of patination methods on steel substrates: a full insight into surface chemistry and morphology.

Patination of metals has been used for decorative or protective purposes, and several methods aimed to create coloured films on metal surfaces have been developed. This work describes a multi-analytical approach to characterize artificial blue patinas created on mild steel substrates by means of traditional recipes and methods for colouring ancient objects and artefacts. We suggest the combined use of secondary ion mass spectrometry, focused ion beam, X-ray diffraction spectroscopy, white light interferometry and reflectance spectroscopy to characterize blue patinas on steel substrates and to investigate the relationship between the developed colour and the patina layer microstructure and composition. Therefore, the analysis of the oxide films produced by either thermal or chemical colouring methods has been successfully performed, providing information about the film morphology, the surface composition and in-depth elemental distribution within the coloured layers, and the origin of the colour developed on the surface.

Size-dependent synergetic seeding effects in the inspection of airborne dry nanoaerosols by LIBS.

In the present paper, confined dry Cu nanoaerosols of controlled particle size are inspected under a time-resolved LIBS scheme to explore the effect of laser-particulate matter interaction upon the detection capability of airborne nanoparticulate material. Optically catapulted streams probed showed linear intensity vs mass correlation and similar signal stability which is linked to the seeding effect caused by smaller particles yielding hotter, albeit shorter plasmas. Seeding effect is demonstrated by hyperspectral time-resolved aerosol inspection, which exposes both, the interaction between multiple plasma nuclei and the discrete nature of the laser-particle interaction. Observed population/exhaustion cycles at the focal volume of the inspection laser explained the uncertainty values characteristic of LIBS inspection of aerosols. A thorough inspection of the emission in time evidenced a significantly different evolution of the intensity profile for commonly monitored Cu lines owed not only to the nature of the monitored transit and pulse energy, but also to particle size. These results suggest that the experimental settings for quantitative ultrafine aerosol inspection need to be tuned according to the target particle size and the particle density of the aerosol as seeding effects facilitates signal saturation, therefore this effect simultaneously contributes to and detracts from the analytical performance of LIBS on nanometric aerosols.

Standoff detection of explosives: critical comparison for ensuing options on Raman spectroscopy-LIBS sensor fusion.

In general, any standoff sensor for the effective detection of explosives must meet two basic requirements: first, a capacity to detect the response generated from only a small amount of material located at a distance of several meters (high sensitivity) and second, the ability to provide easily distinguishable responses for different materials (high specificity). Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) are two analytical techniques which share similar instrumentation and, at the same time, generate complementary data. These factors have been taken into account recently for the design of sensors used in the detection of explosives. Similarly, research on the proper integration of both techniques has been around for a while. A priori, the different operational conditions required by the two techniques oblige the acquisition of the response for each sensor through sequential analysis, previously necessary to define the proper hierarchy of actuation. However, such an approach does not guarantee that Raman and LIBS responses obtained may relate to each other. Nonetheless, the possible advantages arising from the integration of the molecular and elemental spectroscopic information come with an obvious underlying requirement, simultaneous data acquisition. In the present paper, strong and weak points of Raman spectroscopy and LIBS for solving explosives detection problems, in terms of selectivity, sensitivity, and throughput, are critically examined, discussed, and compared for assessing the ensuing options on the fusion of the responses of both sensing technologies.

Investigation of metallic interdiffusion in Al(x)Ga(1-x)N/GaN/sapphire heterostructures used for microelectronic devices by SEM/EDX and SIMS depth profiling.

Secondary ion mass spectrometry (SIMS) depth profiling has been applied to the study of the thermal annealing of ohmic contacts for high electron mobility transistors. The metallic stacks (Ti/Al/Ni/Au) were deposited over the Al(0.28)Ga(0.72)N/GaN/sapphire heterostructures and subjected to a rapid thermal annealing (850 degrees C for 30 s under N(2) atmosphere) to improve the contact performance. The surface morphology and the in-depth chemical distribution of the layered contacts were severely modified due to the treatment. These modifications have been analyzed by SIMS depth profiling and scanning electron microscopy-energy-dispersive X-ray microanalysis. The SIMS analysis conditions have been optimized to achieve simultaneously good sensitivity and to avoid ion-induced mixing effects produced by the primary beam sputtering.

Study on the effect of beam propagation through atmospheric turbulence on standoff nanosecond laser induced breakdown spectroscopy measurements.

We report on an experimental study of the effect of atmospheric turbulence on laser induced breakdown spectroscopy (LIBS) measurements. The characteristics of the atmosphere dictate specific performance constraints to this technology. Unlike classical laboratory LIBS systems where the distance to the sample is well known and characterized, LIBS systems working at several tens of meters to the target have specific atmospheric propagation conditions that cause the quality of the LIBS signals to be affected to a significant extent. Using a new LIBS based sensor system fitted with a nanosecond laser emitting at 1064 nm, propagation effects at distances of up to 120 m were investigated. The effects observed include wander and scintillation in the outgoing laser beam and in the return atomic emission signal. Plasmas were formed on aluminium targets. Average signal levels and signal fluctuations are measured so the effect of atmospheric turbulence on LIBS measurements is quantified.

Glow-discharge-assisted laser-induced breakdown spectroscopy: increased sensitivity in solid analysis.

A glow discharge operating in steady-state and pulsed temporal conditions is used to excite the material previously excited by a pulsed laser ablation system. The system provides a simple means by which to potentially excite the material ablated by the incident laser pulse by taking advantage of enhanced collisional excitation. In this way, one can effectively reduce laser pulse energies below the excitation and ionization thresholds to potentially those required solely for laser ablation of the material, reducing sample damage and improving the lateral resolution. Several critical parameters such as the gas pressure, gas type, and discharge voltage were evaluated, demonstrating the potential of the technique for spatially resolved analysis. The new dual glow-discharge laser-induced breakdown spectroscopy (GD-LIBS) synchronous scheme provides significant signal enhancements when compared to LIBS or GD under identical conditions.

Alternative statistical methods for spectral data processing: applications to laser-induced breakdown spectroscopy of gaseous and aerosol systems.

A new spectral data processing scheme based on the standard deviation of collected spectra is compared with the traditional ensemble-averaging of laser-induced breakdown spectroscopy (LIBS)-based spectral data for homogenous (i.e., pure gas phase) systems and with a LIBS-based traditional conditional spectral analysis scheme for non-homogenous (e.g., aerosol system) analyte systems under discrete particle loadings. The range of conditions enables quantitative assessment of the analytical approaches under carefully controlled experimental conditions. In the homogeneous system with gaseous carbon dioxide producing the carbon atomic emission signal, the standard deviation method provided a suitable metric that is directly proportional to the analyte signal and compares favorably with a traditional ensemble averaging scheme. In contrast, the applicability of the standard deviation method for analysis of non-homogenous analyte systems (e.g., aerosol systems) must be carefully considered. It was shown both experimentally and via Monte Carlo simulations that the standard deviation approach can produce an analyte response that is monotonic with analyte concentration up to a point at which the analyte signal starts to transition from a non-homogeneous system to a homogeneous systems (i.e., around a 50% sampling point for aerosol particles). In addition, the standard deviation spectrum is capable of revealing spectral locations of non-homogeneously dispersed analyte species without a priori knowledge.

Man-portable laser-induced breakdown spectroscopy system for in situ characterization of karstic formations.

This paper reports the development and field testing of a man-portable instrument based on laser-induced breakdown spectrometry (LIBS) for inspection and analysis of speleothems. The 50 mJ of a Q-switched Nd:YAG laser operating at 1064 nm was used to generate a plasma on the sample. Plasma emission was then guided using a fiber-optic cable to a 1/10 m spectrometer equipped with a charge-coupled device (CCD) array detector. Plasma light was automatically processed in order to obtain surface and in-depth information from the speleothems. A field campaign in the interior of Nerja Cave (a large karstic formation in the South of Spain) has been carried out, aimed at evaluating the analytical performance of the instrument when operating in an unfriendly environment. Identification analysis of the speleothems' alteration layers and depth profiles of Sr and Ca is carried out and reported.

Optical restriction of plasma emission light for nanometric sampling depth and depth profiling of multilayered metal samples.

Improvement in depth profiling capabilities of laser-induced breakdown spectrometry (LIBS) for multilayered samples has been attempted. For this purpose, in a typical LIBS experiment, an optical restriction consisting of a pinhole placed between the dichroic mirror and the collecting lenses has been used. This new optical approach allows observing only the light emission coming from the central region of the plume. The microplasma was created on the sample by a pulsed Nd:YAG laser operating at 1064 nm with a homogeneous distribution of energy across the beam. Light emitted by the microplasma was detected with an intensified charge-coupled device (iCCD) multichannel detector. The effect of pinhole diameter and the delay time influence on depth analysis have been assessed. An ablation range of only a few nanometers per pulse has been achieved. Depth profiles of various metals (Cr, Ni, Cu) from multilayered samples have been generated by LIBS and depth resolution at different delay times using various pinhole diameters have been calculated and compared.

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 m(2) and 650 m(2), 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.

Space and time-resolved laser-induced breakdown spectroscopy using charge-coupled device detection.

Space and time-resolved studies of laser induced plasmas in air at atmospheric pressure are presented. Photovoltaic solar cells have been used as samples. The second harmonic (532 nm) of a Nd : YAG laser at an irradiance of 18 x 10(12) W/cm(2) has been used. The precise focus of the beam allows a microanalysis at a 0.02 mm(2) surface area working in single-shot mode. The use of an intensified charge-coupled device (CCD) detector has allowed time-resolved studies in both imaging or spectroscopy modes. The two-dimensional capability of the CCD has enabled the study of atomic and ionic species distribution along the plume. Most data have been recorded using single-laser shot experiments. Spectral lines have been assigned to transitions in atomic components of the material under investigation in the neutral or ionic states of the corresponding atoms. Effects of delay in improving spectral resolution and some examples of spectral characterization of species as a function of its decay are shown.

Applications of laser-induced breakdown spectrometry (LIBS) in surface analysis.

The applicability of laser-induced breakdown spectrometry (LIBS) for surface analysis is presented in terms of its lateral and depth resolution. A pulsed N(2) laser at 337.1 nm (3.65 J/cm(2)) was used to irradiate solar cells employed for photovoltaic energy production. Laser produced plasmas were collected and detected using a charge-coupled device. An experimental device developed in the laboratory permits an exact synchronization of sample positioning using an XY motorized system with laser pulses. Multielement analysis with lateral resolution of up to 30 microm is feasible with the present system. Three-dimensional capabilities of the system are used for studies on the distribution of carbon impurities at the surface of the solar cells.

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 microm between adjacent craters and 4.8 microm along the microline.

Real-time monitoring of high-temperature corrosion in stainless steels by open-path laser-induced plasma spectrometry.

Open-path laser-induced plasma spectrometry (OP-LIPS) represents an appealing alternative for the real-time monitoring of high-temperature processes due to its inherent non-invasive and remote capabilities. In this work, stainless steel samples have been analyzed at 10 meters from the laser source. The effect of the high-temperature conditions to the protective anti-corrosion layer have been analyzed, as well as additional factors such as the type of steel and the exposure time. The number of pulses required to ablate the alteration layer has been found to follow a linear relationship with the square root of the exposure time, in excellent agreement with the off-line thermogravimetric measurements described in the literature.

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.

Sample illumination configurations for spatially resolved Raman spectrometry using a charge-coupled device detector.

In this paper two simple configurations for sample illumination using a charge-coupled device (CCD) detector have been shown. The choice of an appropriate sample illumination can be crucial to obtain spatial and spectral information of complex samples. It is demonstrated that simultaneous Raman spectra of a heterogeneous sample of three compounds can be obtained using a vertical sample illumination. Spatially resolved resonant Raman and surface-enhanced resonant Raman spectra of a complex (Ni-PAPH) have been observed with a low integration time. Dividing the CCD in two region; and with horizontal multiline sample illumination (argon-ion laser at 488 nm and HeNe laser at 632.8 nm) spatially resolved fluorescence spectra of a homogeneous mixture of dyes have been obtained. The total image was acquired in only 1 s.

Spectrophotometric reaction-rate method for the determination of nitrite in waters with pyridine-2-aldehyde 2-pyridylhydrazone.

A method for the kinetic determination of submicrogram amounts of nitrite has been developed, based on its acceleration of the rate of bromate oxidation of pyridine-2-aldehyde 2-pyridylhydrazone in acidic medium. The reaction is monitored spectrophotometrically at 372 nm. A comparative study with hydrochloric acid and perchloric acid media shows that the analytical parameters are affected by the type of acid used. Within-day precision, based on ten replicate determinations, was better than 0.011 mu/ml, which corresponds to 2.2-1.5% relative standard deviation at the concentrations examined. Application of this method in the determination of nitrite in water has been discussed. The recovery of nitrite from drinking waters ranges from 90 to 117% and the average relative standard deviation for nitrite determinations in polluted river water is 3.2%. Large amounts of nitrate and ammonium ions do not interfere. However, there is interference by Cu(2+), Pd(2+) and electroactive substances. Major advantages for the method are simplicity, absence of a reagent blank, and the wide determination range.

Laser-induced breakdown spectroscopy of silicate, vanadate and sulfide rocks.

Laser-induced breakdown spectroscopy (LIBS) in air at atmospheric pressure has been used to study four geological samples belonging to different structural families. Atomic emission spectra of vanadinite, pyrite, garnet and a type of quartz (compostela's quartz) are shown. The 532 nm line of a Nd:YAG laser at an irradiance of 18 x 10(11) W cm(-2) was used. The precise focus of the beam allowed microanalysis of a 0.02 mm(2) surface area working in single-laser shot mode. The use of an intensified gateable charge-coupled-device (CCD) detector permitted time-resolved studies. The spectral lines have been assigned to transitions in the neutral charge state of the corresponding atom of the material under investigation. The behavior of different transitions with time delay are shown. In experiments, minor components contained in several minerals have been detected. This fact has been used to demonstrate the applicability of the technique to characterize and identify similar minerals.

Surface-enhanced Raman spectrometry of triamterene on a silver substrate prepared by the nitric acid etching method.

Surface-enhanced Raman scattering (SERS) spectrometry of the diuretic drug triamterene is discussed. The SERS-active substrate used is a silver foil etched with nitric acid. The influence of solvent and sample doping method on sensivity, intercept and shape of the calibration graphs is discussed.