Development of advanced Raman spectroscopy methods and databases for the evaluation of trace evidence and the examination of questioned documents - Phase II

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Abstract:

Researchers from the City University of New York, West Virginia University, the New York Police Department Crime Lab, and The Metropolitan Museum of Art have formed a multidisciplinary collaboration to further the application of Raman spectroscopy and non-destructive surface-enhanced Raman scattering (SERS) analysis techniques as applied to the evaluation of trace evidence and the examination of questioned documents. Through the sharing of instrumentation, reference materials, and their respective research expertise the team has assembled a reference database of spectra for forensic applications and optimized new techniques for non-destructive SERS analysis.

Raman spectroscopy is an established and increasingly utilized technique for the rapid and non-destructive analysis of paints, inks, fibers, mineral residues, pharmaceuticals, and controlled substances; however, the development of underlining reference databases for identifications has not kept pace. In particular, high quality, comprehensive databases of pigments, dyes, and inks are not available commercially. Moreover, while the technique is applicable to a variety of substances, analytes of forensic interest such as some natural and synthetic dyes found in textiles, inks, and paints display an excessive fluorescent background limiting Raman efficiency in most situations, leading to poor analytical results. Recent work in SERS has demonstrated that organic colorants present in inks, paints, and textile fibers can be easily identified from microscopic samples by treatment of the sample with silver nanoparticles prior to analysis.

The work in Phase I of the project successfully demonstrated that SERS can be applied to the identification of organic colorants present in inks, paints, and textile fibers. The techniques we developed are especially suited for handling microscopic samples: textile dyes were successfully identified from samples as small as a one-millimeter section of a single silk fibril of fifty-micrometer diameter and even from textiles severely degraded by burial. Analytical procedures for SERS of a number of representative dyes were developed, the core of a high quality spectral database was assembled as a proof of concept experiment, and innovative non-destructive approaches were investigated.

The research conducted in Phase II aimed to further explore the application of Raman spectroscopy to the evaluation of trace evidence and the examination of questioned documents, with the aim of solving the problems that have so far limited the applicability of this technique to the identification of organic colorants and other materials in trace amounts.

First of all, we optimized a highly reproducible method of production of silver nanoparticles based on a microwave-assisted reduction of silver sulfate with glucose in the presence of sodium citrate as a capping agent. We continued developing the SERS database, to which spectra of several additional natural and synthetic dyes, as well as controlled substances, were added. In particular, initial studies on problematic tattoo inks systems were performed with the aim of creating a combined normal Raman / SERS approach to their characterization. In addition, we investigated the coupling of SERS with separation techniques such as thin layer chromatography (TLC), as well as emerging methods for completely non-destructive SERS analysis of documents and textile fibers. We refined a non-destructive sampling approach based on the contact transfer of dyes to a gel support to improve its applicability to non-destructive analysis of gel inks. Finally, we conducted validation studies on the methods developed.

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