Application of total X-Ray fluorescence to gunshot residue determination.

Currently the great majority of the criminal acts have involved the use of firearms, for these reasons the evidences generate from these are one of the fundamental pillars of a forensic investigation. The firearm leaves evidence known as gunshot residue (GSR), which is principally composed of burnt and unburnt particles from the detonation, as well as fragments of the bullet, cartridge case, and the firearm. Gunshot residue (GSR) is produced when a firearm is discharged and large quantities of it can be transferred to an individual who has fired. SEM-EDX is the common technique used in the forensic laboratories, the analysis consists in detecting the particles and its elements. In this work we propose the use of X-ray Spectrometry by Total Reflection (TXRF) for the analysis of metals present in related samples in ballistic cases. The analysis was focused in the relationship of three elements present in GSR. A series of experiments with different persons firing gun of 9 mm was performed in a shooting range. Analytical XRFS signals corresponding to K line of Copper and L lines Barium and Lead were employed as the best discriminating variables. Machine Learning techniques, such as discriminant analysis, supported vector machines and partial least squares - discriminant analysis, enable the correct classification of all samples analyzed. A hundred samples were analyzed so far, this method has demonstrated a very high classification performance for detecting gunpowder residues in the skin.

Determination of toxic and potentially toxic elements in rice and rice-based products by inductively coupled plasma-mass spectrometry.

A study was undertaken to quantify the concentration of toxic and potentially toxic elements in samples of rice, rice crackers, rice noodles, infant cereals and rice vinegar available in the Argentine market. The determination of nine elements, namely, As, Cd, Cr, Hg, Ni, Pb, Sb, Se, and Zn in 29 samples was performed by inductively coupled plasma-mass spectrometry. Element concentrations spanned the range (ng g-1): As, 67-858; Cd, <0.2-24.0; Cr, 36.6-937; Hg, <50; Ni, 38.6-1040; Pb, <2.0-139; Sb, <3.0-24.7; Se, <8.4-178 and Zn, 129-32400. Mercury was not detected in any of the analyzed samples. The highest concentration (32.4 µg g-1) was found for Zn in infant cereals that according to the label were added by the manufacturer. To assess accuracy, NIST 1568a rice flour was analyzed and results were in good agreement with certified values.

Multiway analysis through direct excitation-emission matrix imaging.

In this work, a direct in-flow methodology for the acquisition of excitation-emission fluorescence matrices is presented. The system is particularly suited for measurements in the order of tens of milliseconds. A light source operated in continuous mode is dispersed through a grating and focused onto a square-section capillary. Under the spatially resolved excitation, the emission is collected, dispersed through a second grating and further focused onto a CCD array sensor. To allow the wavelength accuracy, a spectral calibration was performed registering the scattering signal of a dispersive element using interference filters ranging from 340 nm to 740 nm. The theoretical performance of the method was analyzed and second-order data obtained for different analyte mixtures are presented and discussed. PARAFAC was applied to evaluate the trilinearity of the obtained data. Mathematical evaluation by means of the criterion of similarity corroborates the agreement between experimental pure spectra and spectral profiles retrieved from PARAFAC. Moreover, the feasibility of the spectrometer to obtain second-order data for analyses with quantitative aims was demonstrated. Finally, fast data acquisition was proved by monitoring a chromatographic analysis of dye mixtures for the generation of third-order LC-EEM data. Here, an improvement in the resolution of the different instrumental modes was demonstrated.

Robotic plasma probe ionization mass spectrometry (RoPPI-MS) of non-planar surfaces.

In this report, we present a robotic sample introduction/ionization system for mass spectrometry (MS) for spot analysis and imaging of non-planar surfaces. The system operates by probing the sample surface with an acupuncture needle, followed by direct plasma chemical ionization time-of-flight MS.