María López-López

Raman spectroscopy for forensic analysis of inks in questioned documents.

The methods for perpetrating forgery and alteration of documents are becoming increasingly more sophisticated. Forensic examinations of questioned documents routinely involve physical and chemical analysis of inks. Raman spectroscopy is a very attractive technique for ink analysis because it combines chemical selectivity with ease and fast analysis and it does not require sample preparation nor leads to destruction of the evidence. However, some limitations of this technique include low sensitivity and the overwhelming phenomenon of fluorescence, which can be solved by resonance Raman spectroscopy and surface-enhanced Raman spectroscopy. This article aims to demonstrate the great potential of the Raman-based techniques by providing an overview of their application to forensic examinations of ink evidence from pens and printers. Moreover, it is also addressed the chemistry of ink-paper interactions and the problematic of intersecting lines.

Comparative analysis of smokeless gunpowders by Fourier transform infrared and Raman spectroscopy

Fourier Transform Infrared (FTIR) and Raman spectroscopic techniques were used to perform a comparative study of the spectral profiles of single-base, double-base and triple-base smokeless gunpowders. Preliminary results based on visual comparison of the spectra point out that spectra obtained by both vibrational techniques were useful for a rapid identification of gunpowders containing dinitrotoluene as one of the major components and triple-base gunpowders. Additionally, the Raman spectra of gunpowders with diphenylamine in its primary composition showed a characteristic band, assigned to 2-nitro-diphenylamine, allowing the identification of this type of gunpowders. Further differentiation among the spectra of different types of smokeless gunpowders obtained by both vibrational spectroscopic techniques was investigated by discriminant analysis. Different analyses were applied to spectral data considering the different composition of gunpowders. The presence or absence of different compounds (such as dinitrotoluene, diphenylamine or dibutyl phthalate) or the type of gunpowder according to the number of active components (single-base or double-base gunpowder) has been taken into account. FTIR and Raman spectroscopy in combination with discriminant analyses were successful tools of forensic interest for the classification of gunpowders and the possible identification of unknown samples of gunpowders.

Noninvasive detection of concealed explosives: depth profiling through opaque plastics by time-resolved Raman spectroscopy.

The detection of explosives concealed behind opaque, diffusely scattering materials is a challenge that requires noninvasive analytical techniques for identification without having to manipulate the package. In this context, this study focuses on the application of time-resolved Raman spectroscopy (TRRS) with a picosecond pulsed laser and an intensified charge-coupled device (ICCD) detector for the noninvasive identification of explosive materials through several millimeters of opaque polymers or plastic packaging materials. By means of a short (250 ps) gate which can be delayed several hundred picoseconds after the laser pulse, the ICCD detector allows for the temporal discrimination between photons from the surface of a sample and those from deeper layers. TRRS was applied for the detection of the two main isomers of dinitrotoluene, 2,4-dinitrotoluene, and 2,6-dinitrotoluene as well as for various other components of explosive mixtures, including akardite II, diphenylamine, and ethyl centralite. Spectra were obtained through different diffuse scattering white polymer materials: polytetrafluoroethylene (PTFE), polyoxymethylene (POM), and polyethylene (PE). Common packaging materials of various thicknesses were also selected, including polystyrene (PS) and polyvinyl chloride (PVC). With the demonstration of the ability to detect concealed, explosives-related compounds through an opaque first layer, this study may have important applications in the security and forensic fields.

New protocol for the isolation of nitrocellulose from gunpowders: utility in their identification.

In this work, a new approach for the isolation of nitrocellulose from smokeless gunpowders has been developed. A multistep solvent extraction method was needed to purify nitrocellulose contained in gunpowders. For single-base or double-base gunpowders six consecutive solvent extractions were selected: three extractions with methanol (to remove nitroglycerin, 2,4-dinitrotoluene, ethyl-centralite, diphenylamine, and diphenylamine derivatives); one extraction with dichloromethane (to remove colorants and plasticizers of organic nature); one extraction with methanol (to facilitate a final polar extraction); and one extraction with water (to remove ionic components) were necessary at 35 degrees C. For the triple-base gunpowder studied, eight solvent extractions were needed due to a high concentration of the water-soluble nitroguanidine was present. In addition to the same five initial phases used for the single-base and double-base gunpowders, three water extraction phases at a higher temperature (75 degrees C instead of 35 degrees C) were also needed. A final step to solubilize nitrocellulose in methyl ethyl ketone was used to remove inert components (mainly graphite). Nitrocellulose isolated from these propellants was characterized by Fourier-Transformed Infrared Spectroscopy (FTIR spectroscopy). The same FTIR spectra were observed for nitrocelluloses isolated from different types of gunpowders. A comparison of FTIR spectra of nitrocellulose samples of different nitration degree evidenced that the bands regions most affected by this factor were: 3600-3400cm(-1), corresponding to the stretching vibrations of residual hydroxyl groups; 1200-1000cm(-1), attributed to the valence vibrations nuCO of the glucopyranose cycle; and 750-690cm(-1), assigned to vibrations of the nitrate group. In both cases, the bands appearing in these regions were more pronounced in the spectra of nitrocellulose samples of low nitration degree.

Dynamite analysis by Raman spectroscopy as a unique analytical tool.

Apart from powerful explosives, dynamites are complex samples with an intricate analysis. These mixtures of compounds of diverse chemical nature present a challenge to the analyst, and as a result, several analytical techniques need to be applied currently for their analysis. Taking into account that presently there are almost no methods for dynamite analysis in the literature, it is crucial to develop analytical methods that could be applied for the analysis of these samples. This study introduces the use of Raman spectroscopy to analyze dynamites. Two different dynamites made up of ethylene glycol dinitrate and ammonium nitrate, among other minor components, were analyzed by Raman spectroscopy. First, confocal Raman spectroscopy allowed the identification of different components easily distinguished by eye (ammonium nitrate, ethylene glycol dinitrate, and sawdust). Then, Raman mapping was used to show the distribution of the main components throughout the dynamite mass. Finally, several minor components were identified after flocculation (nitrocellulose) or precipitation (sawdust, CaCO3, and flour). The results obtained demonstrate the huge potential of this technique for the analysis of such a complex and tricky sample.

Fast Analysis of Complete Macroscopic Gunshot Residues on Substrates Using Raman Imaging

Raman spectroscopy has emerged as a viable technique for the organic analysis of gunshot residues (GSRs), offering additional information to the well-established analysis using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX). In this article, a Raman imaging system with an electron-multiplying coupled-charged device (EMCCD) camera was used to analyze complete GSR particles from both conventional and nontoxic ammunition fired at different cloth targets. The same cloths were then stained with blood to mimic real evidence and measured. The direct analysis using Raman imaging of the GSR particles collected with the stubs used for SEM-EDX analysis (the frequent method used for GSR collection) was evaluated. Multivariate curve-resolution and chemical-mapping methods were applied to the spectroscopic data to identify and highlight the signal corresponding to the GSR particles and differentiate them from the substrates. It was confirmed that both measurement approaches (on the targets and the stubs) could be used for the identification of GSR particles, even under unfavorable conditions such as the presence of blood. The results obtained demonstrate the huge potential of Raman imaging for the fast analysis of complete GSR particles and prove its complementary usefulness in the analysis of the stubs used by the well-established SEM-EDX technique.

Determination of the nitrogen content of nitrocellulose from smokeless gunpowders and collodions by alkaline hydrolysis and ion chromatography

In this work, a method to determine the nitrogen content of nitrocellulose from gunpowders and collodions is proposed. A basic hydrolysis of nitrocellulose with 1.0% (m/v) NaOH at 150°C during 30 min was carried out for nitrocellulose from gunpowders (after its previous isolation by a protocol optimized by our research group) and from collodion samples. The concentration of nitrate and nitrite ions in the hydrolysate was determined by ion chromatography with suppression and conductimetric detection. The nitrogen content of nitrocellulose was calculated from the values of the concentration of both ions. The quantitative method was evaluated in terms of selectivity, sensitivity, robustness, limits of detection and quantification, and precision, measured as repeatability and intermediate precision. These parameters were good enough to demonstrate the validity of the method and its applicability to the determination of the nitrogen content of nitrocellulose contained in different types of gunpowders (single- and double-base gunpowders, manufactured from 1944 to 1997) and in commercial collodion samples. For gunpowders, the nitrogen content determined with the optimized method was compared with the values reported by the official label of the ammunition (obtained by a digestion/titration method) and errors, by defect, ranging from 1% to 15.2% (m/m) were calculated. The highest errors were obtained for the oldest gunpowders and could be attributed to the loss of nitro groups in the nitrocellulose molecule during aging. For collodion samples, errors could not be calculated since the real nitrogen content for these samples was not given in the label. In addition, the analysis time (2h for nitrocellulose isolation, 1.5h for nitrocellulose hydrolysis, and 0.2h for chromatographic separation) was about 10 times lower than in the digestion/titration method nowadays used for gunpowder samples.

Detection and identification of explosives by surface enhanced Raman scattering

ABSTRACT Surface Enhanced Raman Scattering (SERS) has undergone an important development over the last few years, particularly in the detection and identification of extremely low traces of explosives. The large number of studies and results generated by this increasing research makes a comprehensive overview necessary. This work reviews in detail that research focused on the identification of explosives by SERS, including TNT, DNT, RDX, PETN, TATP, HMTD, perchlorate, etc. either in bulk state, in solution or in vapor phase. In brief, TNT and DNT have been widely studied by SERS due to its aromatic structure and LODs down to 5–10 zg and 10−17–10−13 M have been achieved. The other explosives have been quite less researched; therefore, few results are available to be compared and a bit more modest LODs have been reached such as 10−13 M for RDX, 10−4 M for TATP, 5 pg for PETN, or 10−9 M for perchlorate. In addition, the challenges of detecting both explosives vapors and perchlorate anion by SERS are thoroughly discussed.

Raman imaging for determining the sequence of blue pen ink crossings

This manuscript presents a preliminary investigation on the applicability of Raman imaging for non-destructive and rapid analysis of blue crossing ink lines. The MCR method was used to facilitate visualization of the distribution of inks of the same colour and the most predominant Raman signature at the crossing was used to interpret the order of application of inks. Different pen ink types, different times separating the application of the two ink lines and different paper substrates were used. From the 90 Raman images examined, the correct order of application was determined in more than 60% by direct observation. The remainder cases were not as clear due to the uneven distribution of inks and the empty spaces similar to a net-like pattern observed at the crossing. This pattern was possibly caused by physical impediments (the first ink applied acting as a physical barrier) or chemical impediments (the two inks did not stick). Such impediments were more strongly observed in the crossings involving the U. Eye pen ink, causing the complete skipping of this ink line. Moreover, most crossings showed some mixing between the two inks and it was more accentuated when the times separating the application of the inks were shorter, since the ink was fresher. The use of white or certificate papers did not seem to influence in the inks distribution nor determining the order of the inks. Although this study provided useful insights regarding crossing ink lines, future statistic studies may be helpful for more objective examinations.

Recent non-chemical approaches to estimate the shooting distance.

Shooting distance estimation offers useful information for the reconstruction of firearm related incidents. The muzzle to target distance is usually estimated by examining the bullet entrance hole and the gunshot residue pattern. To visualize the pattern the forensic analyst usually uses presumptive tests based on color chemical reactions that are applied using long and tedious proceedings. Due to the drawbacks of the chemical tests recent developments for shooting distance estimation not based on color chemical tests were described in the literature. The present review covers the approaches for shooting distance estimation published in the last 10 years considering two types of target, clothing and skin.