X-Ray spectroscopy in forensics
X-ray spectroscopy is used widely in forensic science. Main strengths are its non-destructive nature, thus preserving crime evidences, its ability to identify chemical compounds, to determine elemental chemistry and, in some cases, elemental speciation. Moreover the versatility of the X-ray techniques permits the analysis of very diverse materials, -inorganic, organic, metals-, in powder, solid or liquid forms.
Different X-ray spectroscopy based tools, just as many other analytical techniques, have been used in forensic science for several decades. Classical X-ray based techniques used in forensic work are X-ray powder diffraction (XRD), X-ray fluorescence (XRF), X-ray imaging and Energy dispersion X-ray Emission linked to an electron microscope (SEM-EDX). These complementary techniques are mainly used in micro- and macrotrace analysis. Conventional XRF, whilst attractive for the forensic analyst, sometimes cannot be applied because in the majority of cases crime scene specimens are microscopic in nature. A common bench-top XRF system has an analysis spot of perhaps 2–4 cm, and is unsuited to perform, for instance, analysis of a 100 μm fragment.
During the last twenty years, noticeable development was made in the instrumental aspects of X-ray spectrometry, especially in the improvement of X-ray optics and detection systems. All this resulted in a wide variety of instrumentation becoming available today. Significant advances in focusing optics (development of collimators and polycapillary lenses) have promoted the design of micro beam sources or the analysis of small regions by X-ray instrumentation employing conventional X-ray tubes as the source of primary radiation. The use of automatized XYZ stages allows the possibility to do point, line profile or mapping analyses. A microscopic particle from a crime scene can be directly analysed without any sample preparation, simply located using optical cameras, and subsequently characterized for elemental content.
Along the presentation a selection of real forensic evidence types and their analysis by XRS methods (XRD, XRF, micro-EDXRF) will be described, such as glassy fragments, cosmetics, healthcare products, gunshot residues and counterfeit currency.