A Novel LIBS Sensor for Sample Examinations on a Crime Scene.

In this work, we present a compact LIBS sensor developed for characterization of samples on a crime scene following requirements of law enforcement agencies involved in the project. The sensor operates both in a tabletop mode, for aside measurements of swabbed materials or taken fragments, and in handheld mode where the sensor head is pointed directly on targets at the scene. The sensor head is connected via an umbilical to an instrument box that could be battery-powered and contains also a color camera for sample visualization, illumination LEDs, and pointing system for placing the target in focus. Here we describe the sensor's architecture and functionalities, the optimization of the acquisition parameters, and the results of some LIBS measurements. On nano-plotted traces at silica wafer and in optimized conditions, for most of the elements the detection limits, in term of the absolute element masses, were found to be below 10 picograms. We also show results obtained on some representative materials, like fingerprints, swabbed soil and gunshot residue, varnishes on metal, and coated plastics. The last, solid samples were used to evaluate the depth profiling capabilities of the instrument, where the recognition of all four car paint layers was achieved.

Crime Light Imaging (CLI): A Novel Sensor for Stand-Off Detection and Localization of Forensic Traces.

Stand-off detection of latent traces avoids the scene alteration that might occur during close inspection by handheld forensic lights. Here, we describe a novel sensor, named Crime Light Imaging (CLI), designed to perform high-resolution photography of targets at a distance of 2-10 m and to visualize some common latent traces. CLI is based on four high-power illumination LEDs and one color CMOS camera with a motorized objective plus frontal filters; the LEDs and camera could be synchronized to obtain short-exposure images weakly dependent on the ambient light. The sensor is integrated into a motorized platform, providing the target scanning and necessary information for 3D scene reconstruction. The whole system is portable and equipped with a user-friendly interface. The preliminary tests of CLI on fingerprints at distance of 7 m showed an excellent image resolution and drastic contrast enhancement under green LED light. At the same distance, a small (1 µL) blood droplet on black tissue was captured by CLI under NIR LED, while a trace from 15 µL semen on white cotton became visible under UV LED illumination. These results represent the first demonstration of true stand-off photography of latent traces, thus opening the way for a completely new approach in crime scene forensic examination.

Profiling and imaging of forensic evidence - A pan-European forensic round robin study part 1: Document forgery.

The forensic scenario, on which the round robin study was based, simulated a suspected intentional manipulation of a real estate rental agreement consisting of a total of three pages. The aims of this study were to (i) establish the amount and reliability of information extractable from a single type of evidence and to (ii) provide suggestions on the most suitable combination of compatible techniques for a multi-modal imaging approach to forgery detection. To address these aims, seventeen laboratories from sixteen countries were invited to answer the following tasks questions: (i) which printing technique was used? (ii) were the three pages printed with the same printer? (iii) were the three pages made from the same paper? (iv) were the three pages originally stapled? (v) were the headings and signatures written with the same ink? and (vi) were headings and signatures of the same age on all pages? The methods used were classified into the following categories: Optical spectroscopy, including multispectral imaging, smartphone mapping, UV-luminescence and LIBS; Infrared spectroscopy, including Raman and FTIR (micro-)spectroscopy; X-ray spectroscopy, including SEM-EDX, PIXE and XPS; Mass spectrometry, including ICPMS, SIMS, MALDI and LDIMS; Electrostatic imaging, as well as non-imaging methods, such as non-multimodal visual inspection, (micro-)spectroscopy, physical testing and thin layer chromatography. The performance of the techniques was evaluated as the proportion of discriminated sample pairs to all possible sample pairs. For the undiscriminated sample pairs, a distinction was made between undecidability and false positive claims. It was found that none of the methods used were able to solve all tasks completely and/or correctly and that certain methods were a priori judged unsuitable by the laboratories for some tasks. Correct results were generally achieved for the discrimination of printer toners, whereas incorrect results in the discrimination of inks. For the discrimination of paper, solid state analytical methods proved to be superior to mass spectrometric methods. None of the participating laboratories deemed addressing ink age feasible. It was concluded that correct forensic statements can only be achieved by the complementary application of different methods and that the classical approach of round robin studies to send standardised subsamples to the participants is not feasible for a true multimodal approach if the techniques are not available at one location.

Forensic Analysis of Commercial Inks by Laser-Induced Breakdown Spectroscopy (LIBS).

Laser-induced breakdown spectroscopy (LIBS) was tested for all of the relevant issues in forensic examinations of commercial inks, including classification of pen inks on one paper type and on different paper types, determination of the deposition order of layered inks, and analysis of signatures and toners on one questioned document. The scope of this work was to determine the potential of a single LIBS setup that is compatible with portable instruments for different types of ink analysis, rather than building a very large database for inks and papers. We identified up to seven metals characteristic for the examined inks, which allowed to fully discriminate all eight black inks on one type of printing paper. When the inks were tested on ten different papers, the correct classification rates for some of them were reduced for reasons thoroughly studied and explained. The replicated tests on three crossing points, each one involving a pair of blue or black inks, were successful in five cases out of six. In the test simulating documents of forensic interest (questioned documents), LIBS was able to correctly identify the differences in three inks used for signatures on one of the three pages and the use of different printing inks on each page of the document.

Integrated Laser Sensor (ILS) for Remote Surface Analysis: Application for Detecting Explosives in Fingerprints.

Here, we describe an innovative Integrated Laser Sensor (ILS) that combines four spectroscopic techniques and two vision systems into a unique, transportable device. The instrument performs Raman and Laser-Induced Fluorescence (LIF) spectroscopy excited at 355 nm and Laser-Induced Breakdown Spectroscopy (LIBS) excited at 1064 nm, and it also detects Laser Scattering (LS) from the target under illumination at 650 nm. The combination of these techniques supplies information about: material change from one scanning point to another, the presence of surface contaminants, and the molecular and elemental composition of top target layers. Switching between the spectroscopic techniques and the laser wavelengths is fully automatic. The instrument is equipped with an autofocus, and it performs scanning with a chosen grid density over an interactively-selected target area. Alternative to the spectroscopic measurements, it is possible to switch the instrument to a high magnification target viewing. The working distances tested until now are between 8.5 and 30 m. The instrument is self-powered and remotely controlled via wireless communication. The ILS has been fully developed at ENEA for security applications, and it was successfully tested in two outdoor campaigns where an automatic recognition of areas containing explosives in traces had been implemented. The strategies for the identification of nitro-compounds placed on various substrates as fingerprints and the results obtained at a working distance of 10 m are discussed in the following.

Standoff monitoring of aqueous aerosols using nanosecond laser-induced breakdown spectroscopy: droplet size and matrix effects.

Nanosecond laser-induced breakdown spectroscopy has been examined for the analysis of suspended matter in a free stream of air. The real-time monitoring of this scenario poses major challenges for an accurate categorization due to its changing characteristics such as composition, size, and density of particles. The effects of particle size and matrix in the optical emission responses registered from such scenarios have been evaluated. Distant (10 m) plasmas of saline solutions, containing either NaCl or Na2SO4 at different concentrations, have been induced by nanosecond laser pulses at a wavelength of 1064 nm. The effects of the droplet size and its concentration on differences in the laser-induced breakdown probability, the intensity of the characteristic lines, and the plasma emission continuum have been discussed. The quantification of sodium in distant water droplets has been proved. However, an initial knowledge on the average droplet size is required. The average droplet size could be determined from the slope of H I and O I lines versus the continuum plasma emission, which is only weakly influenced by the salt content in the droplets.

Sample Preparation for Repeated Measurements on a Single Liquid Droplet Using Laser-Induced Breakdown Spectroscopy.

We studied changes in laser-induced breakdown spectroscopy (LIBS) signal intensity with the thickness of a liquid layer placed on a solid substrate, where an easily evaporating methanol sample was used. For a certain optimal liquid film thickness we obtained a manifold increase of the LIBS signal from methanol. Progressive liquid film thinning leads to a reduction and a successive disappearance of laser-induced splashes; the latter condition drastically reduces the sample consumption and allows measurements to be repeated many times on a single liquid droplet. In following, we developed two methods for actively controlled deformation, i.e., thinning of a liquid droplet (volume ∼10 µl) prior to its sampling by LIBS. Control of the droplet's height was achieved on a Si-SiO2 wafer substrate by electro-wetting in the case of water solutions or by target rotation in the case of viscous liquids. The chosen substrate also has the advantages of low cost, easy manipulation, and very high purity, thus minimizing interference with analytes. Through the droplet deformation, in a single-pulse excitation at moderate laser energy (70 mJ), we clearly detected Fe and Mn in peanut oil, which represent trace elements in edible oils (∼ 1 part per billion), according to results published in the literature.

Deep ablation and depth profiling by laser-induced breakdown spectroscopy (LIBS) employing multi-pulse laser excitation: application to galvanized steel.

The potential of a multi-pulse (MP) laser excitation scheme for deep stratigraphy of electrolytically galvanized steel using laser-induced breakdown spectrometry (LIBS) has been evaluated. For this purpose, a commercial electro-optically (EO) Q-switched Nd:YAG laser was employed, where by reducing the delay between the Q-switch opening and the flash lamp, a train of pulses (up to 11) separated by approximately 7.40 µs was generated during one lamp flashing. Plasma emission after each individual laser pulse of the MP sequence was detected by a spectrograph equipped with an intensified charge-coupled device (iCCD) detector. With MP excitation, the ablation efficiency was increased ten-fold on iron sample and 22.5-fold on zinc material with respect to dual-pulse or single-pulse excitation. The LIBS signal generated by MP excitation shows an analogous enhancement. Although the total energy per shot delivered to samples was only 60 mJ, it was possible using LIBS to measure the sample stratigraphy up to depths of 90 µm on zinc-coated steel sheets. A satisfactory agreement between the Zn thickness determined by the MP-LIBS system and data from the manufacturer has also been obtained.