Identification of Minor Dye Components of Fibers via Integrating-Cavity-Enhanced Raman Spectroscopy

• NCJ Number: 305810
• Author(s): Hergen Eilers
• Date Published: June 2022
• Length: 107 pages
• Grant Number(s): 2018-DU-BX-0178
• Agencies: NIJ-Sponsored

Annotation

This research report examines the interaction of two acid dyes with nylon, demonstrated in the tetrahertz Raman spectra as red and blue shifts of the rotational C-C torsion band of nylon, and suggests that the results could help with the forensic identification of textile trace evidence.

Abstract

This document reports on experiments aimed at enhancing the analytical capabilities for forensic examinations of dyed fibers, more specifically, on evaluating the integrating-cavity-enhanced Raman spectroscopy (ICERS) for its potential to significantly enhance the signal strength while concurrently removing the fluorescence, and as a result, enhancing the signal-to-noise ratio (S/N) and allowing for the identification of minor dye components on fibers. The main objectives were to determine and compare the respective S/N of standard Stokes Raman spectra and ICERS anti-Stokes Raman spectra of three things: of red, blue, and black dyes, and mixtures of those dyes; of cotton and polyester fibers dyed with red, blue, and black dyes; and of cotton and polyester fibers dyed with mixtures of red, blue, and black dyes, and to determine the minimum detectable dye concentrations that could be identified. Challenges faced by researchers during this project included: the preparation of integrating cavities from highly reflecting fumed silica, which required the development of new procedures that would allow for cavities which are both mechanically stable and highly reflecting; machining of cavities to integrate optical components, which required the development of new processes; matching the optical properties of the cavities with those of optical fibers to collected Raman-scattered light, which required tradeoffs; and cavities that appeared to enhance residual fluorescence from some optical components, which overwhelmed the desired Raman signals. The researchers were able to successfully address the cavity processing and preparation challenges but were unable to make satisfactory Raman measurements inside the cavity that would have demonstrated the desired signal enhancement. However, their THz (tetrahertz) measurements showed that the structural properties of nylon are affected by the dying process.