Self-Calibrated Laser-Induced Breakdown Spectroscopy for the Quantitative Elemental Analysis of Suspended Volcanic Ash

Real-time analysis of fine ash in volcanic plumes, which represent magma fragments expelled from the crater during explosive eruptions, is a valuable tool for volcano monitoring and hazard assessment. To obtain the chemical characterization of the juvenile pyroclastic material emitted in volcanic plumes, many analytical techniques can be used. Among them, laser-induced breakdown spectroscopy (LIBS) is the one that can most easily be adapted to advanced applications in extreme environments. In this paper, LIBS experiments based on self-calibrated approaches are used to determine the elemental composition of suspended volcanic ash. To simulate the conditions of dispersed volcanic ash in the atmosphere, different sizes of volcanic ash samples are suspended in the air by laser-induced shockwaves in a dedicated chamber, and a parametric study is carried out to establish the optimal experimental conditions for recording usable plasma emission spectra for each ash size. The quantitative analysis is performed using a self-calibrated analytical method, including calibration-free LIBS, which is based on the calculation of the spectral radiance of a uniform plasma in local thermodynamic equilibrium. The method accounts intrinsically for self-absorption since it modifies the intensity of spectral lines and thus leads to an underestimation of the elemental fraction. An intensity calibration of the spectra based on the measurements of Fe lines intensities was also used in this work to deduce the apparatus response from the spectrum itself and avoid the use of standard calibration lamps. Results demonstrate the potential of real-time measurements of elemental fractions in volcanic ash with good agreement with the literature composition.

Electron Probe Microanalysis of Transition Metals using L lines: The Effect of Self-absorption

Electron microprobe-based quantitative compositional measurement of first-row transition metals using their L$\alpha$ X-ray lines is hampered by, among other effects, self-absorption. This effect, which occurs when a broad X-ray line is located close to a broad absorption edge, is not accounted for by matrix corrections. To assess the error due to neglecting self-absorption, we calculate the L$\alpha$ X-ray intensity emitted from metallic Fe, Ni, Cu, and Zn targets, assuming a Lorentzian profile for the X-ray line and taking into account the energy dependence of the mass absorption coefficient near the absorption edge. We find that calculated X-ray intensities depart increasingly, for increasing electron beam energy, from those obtained assuming a narrow X-ray line and a single fixed absorption coefficient (conventional approach), with a maximum deviation of $\sim$15% for Ni and of $\sim$10% for Fe. In contrast, X-ray intensities calculated for metallic Zn and Cu do not differ significantly from those obtained using the conventional approach. The implications of these results for the analysis of transition-metal compounds by electron probe microanalysis as well as strategies to account for self-absorption effects are discussed.

The Processing of Visual Signals in Major Depressive Disorder

The balanced processing of the internal mental world and the external world is a crucial aspect of everyday well-being. An extensive control of the internal emotional and cognitive world that often results in an internal expression of distress is a common feature of internalizing disorders. However, how depression affects the processing of the external world is still an open question. We, therefore, tested the processing of visual signals in major depressive disorder (MDD). To this end, we recorded the electroencephalogram of 38 MDD patients and 38 controls, while they performed a response-choice task with informative feedback and a passive viewing task. MDD patients differed significantly from controls in the early information processing of visual stimuli. The vertex positive potential (VPP) evoked by feedback in the response-choice task and pictures in the passive viewing task were smaller in MDD patients than in controls. This outcome suggests that depression might subtract attentional resources from external signal processing, with potential consequences in various cognitive domains.

Branching Ratio Method for Assessing Optically Thin Conditions in Laser-Induced Plasmas

The branching ratio method is usually used to evaluate the optical thinness conditions in laser-generated plasmas, which are important for the application of analytical methods such calibration free laser induced breakdown spectroscopy (CF-LIBS). In this communication, we warn on the possibility that in some circumstances, the branching-ratio method might give results close to the one characterizing optically thin plasma conditions, even in the presence of a substantial self-absorption for the transitions considered.

Determination of Stark Shifts and Widths Using Time Resolved Laser-Induced Breakdown Spectroscopy (LIBS) Measurements

Stark broadening parameters have been estimated for resonant lines of Al(I) using time resolved measurements. The relation between the various emission line characteristics at different phases of opacity have been utilized to obtain the value of plasma temperature and Stark width parameters from the experimental data. The observed value of the center line intensity and Lorentzian component of the line width are compared against a simulated value of these parameters for optically thin case. The plasma temperature and Stark broadening parameters are obtained for the best fit condition by matching the experimentally observed and the simulated values of intensity and line widths. The time resolved measurements result in much better estimates for Stark parameters by allowing multiple points for fitting keeping the number of variables limited. The Stark shift parameters are also obtained from the slope of the plot of observed central wavelength shifts versus observed electron number density which is measured as a function of time. Hence, a method utilizing multiple-time observation data to obtain the Stark broadening parameters for lines showing self-absorption has been demonstrated.

Solving self-absorption in fluorescence

One of the most common types of experiment in X-ray absorption spectroscopy (XAS) measures the secondary inelastically scattered fluorescence photon. This widespread approach has a dominant systematic of self-absorption of the fluorescence photon. The large impact of self-absorption compromises accuracy, analysis and insight. Presented here is a detailed self-consistent method to correct for self-absorption and attenuation in fluorescence X-ray measurements. This method and the resulting software package can be applied to any fluorescence data, for XAS or any other experimental approach detecting fluorescence or inelastically scattered radiation, leading to a general solution applicable to a wide range of experimental investigations. The high intrinsic accuracy of the processed data allows these features to be well modelled and yields deeper potential insight.

Detecting Temporal Changes of Self-Absorption in a Laser-Induced Copper Plasma from Time-Resolved Photomultiplier Signal Emission Profiles

This work reports an investigation on the feasibility of using a photomultiplier tube (PMT) to follow the time evolution of self-absorption of copper resonance transitions at 324.7 nm and 327.4 nm. The plasma was obtained by focusing a Nd:YAG laser, operated at 1064 nm, on a series of aluminum alloy standard disks containing different copper concentrations. The results described have been obtained at different times and with different set-ups. These set-ups consisted of a Paschen-Runge polychromator, a LIBS 2000 spectrometer, and a spectrometer equipped with both an intensified charge-coupled device (ICCD) and PMT. Both PMT signals and time-resolved spectra were obtained and the ratio of the two Cu resonant lines was calculated, compared, and discussed. By selecting different delay times and integration gates of the PMT signals, the self-absorption effect of the Cu resonant lines was found to be changing, implying that, by careful selection of the integration window of PMT signals, the self-absorption may be minimized, thus improving the calibration linearity of the technique.

Calibration-Free Laser-Induced Breakdown Spectroscopy (CF-LIBS) with Standard Reference Line for the Analysis of Stainless Steel

In this work, calibration-free laser-induced breakdown spectroscopy (CF-LIBS) is used to analyze a certified stainless steel sample. Due to self-absorption of the spectral lines from the major element Fe and the sparse lines of trace elements, it is usually not easy to construct the Boltzmann plots of all species. A standard reference line method is proposed here to solve this difficulty under the assumption of local thermodynamic equilibrium so that the same temperature value for all elements present into the plasma can be considered. Based on the concentration and rich spectral lines of Fe, the Stark broadening of Fe(I) 381.584 nm and Saha-Boltzmann plots of this element are used to calculate the electron density and the plasma temperature, respectively. In order to determine the plasma temperature accurately, which is seriously affected by self-absorption, a pre-selection procedure for eliminating those spectral lines with strong self-absorption is employed. Then, one spectral line of each element is selected to calculate its corresponding concentration. The results from the standard reference lines with and without self-absorption of Fe are compared. This method allows us to measure trace element content and effectively avoid the adverse effects due to self-absorption.

Evaluation of Optical Depths and Self-Absorption of Strontium and Aluminum Emission Lines in Laser-Induced Breakdown Spectroscopy (LIBS)

Laser-induced breakdown spectroscopy (LIBS) is a widely used laser spectroscopic technique in various fields, such as material science, forensic science, biological science, and the chemical and pharmaceutical industries. In most LIBS work, the analysis is performed using radiative transitions from atomic emissions. In this study, the plasma temperature and the product [Formula: see text] (the number density N and the absorption path length [Formula: see text]) were determined to evaluate the optical depths and the self-absorption of Sr and Al lines. A binary mixture of strontium nitrate and aluminum oxide was used as a sample, consisting of variety of different concentrations in powder form. Laser-induced breakdown spectroscopy spectra were collected by varying various parameters, such as laser energy, gate delay time, and gate width time to optimize the LIBS signals. Atomic emission from Sr and Al lines, as observed in the LIBS spectra of different sample compositions, was used to characterize the laser induced plasma and evaluate the optical depths and self-absorption of LIBS.

Photon source term after single collision in targets of silicon, copper and lead for 50-500 keV X-ray beams

Single-scattered X-ray doses at 1 m from silicon, copper and lead targets were calculated using an analytical point-kernel method considering the self-absorption, and the calculated values were compared with detailed results of a Monte Carlo calculation with respect to the emission angle. In the calculations, a slab slanted at 3° to the beam axis was used for silicon in addition to the cylindrical targets for the three materials, and the slab geometry showed the largest doses. The analytical calculations were underestimated compared with the Monte Carlo calculations by less than 24% for silicon and 40% for copper, particularly at large-angle scattering, which was attributable to the buildup effect of the single-scattered X-rays in the targets. By considering the buildup effect, the difference from Monte Carlo results decreased to less than 20%. For lead, the influence of fluorescent X-rays produced by the source beam was dominant in the backward direction, which was also calculated analytically. The simple analytical program can be applied to any target size and shape by considering self-absorption and the buildup effect, both of which inform the simple dose estimation method.

Self-consciousness concept and assessment in self-report measures

This study examines how self-consciousness is defined and assessed using self-report questionnaires (Self-Consciousness Scale (SCS), Self-Reflection and Insight Scale, Self-Absorption Scale, Rumination-Reflection Questionnaire, and Philadelphia Mindfulness Scale). Authors of self-report measures suggest that self-consciousness can be distinguished by its private/public aspects, its adaptive/maladaptive applied characteristics, and present/past experiences. We examined these claims in a study using 602 young adults to whom the aforementioned scales were administered. Data were analyzed as follows: (1) correlation analysis to find simple associations between the measures; (2) factorial analysis using Oblimin rotation of total scores provided from the scales; and (3) factorial analysis considering the 102 items of the scales all together. It aimed to clarify relational patterns found in the correlations between SCSs, and to identify possible latent constructs behind these scales. Results support the adaptive/maladaptive aspects of self-consciousness, as well as distinguish to some extent public aspects from private ones. However, some scales that claimed to be theoretically derived from the concept of Private Self-Consciousness correlated with some of its public self-aspects. Overall, our findings suggest that while self-reflection measures tend to tap into past experiences and judged concepts that were already processed by the participants’ inner speech and thoughts, the Awareness measure derived from Mindfulness Scale seems to be related to a construct associated with present experiences in which one is aware of without any further judgment or logical/rational symbolization. This sub-scale seems to emphasize the role that present experiences have in self-consciousness, and it is argued that such a concept refers to what has been studied by phenomenology and psychology over more than 100 years: the concept of pre-reflective self-conscious.