Assessment of detection limits for dyed and mounted textile fibers using Raman spectroscopy

Trace evidence in the form of textile fibers can be used to link objects and places during an investigation. Raman spectroscopy is a well-established technique that has been used for the examination of various pigments, paints, inks, and dyes. The objective of this study was to determine the capability of Raman spectroscopy to detect several different dye classes and colors on a variety of textile fibers. To test this, four categories of dyes, reactive, disperse, acid, and direct were examined with Raman microscopy while applied to one of five fiber types (cotton, polyester, nylon, wool, and rayon). Each dye category was tested using four colors, black, blue, red, and yellow, while at four concentrations of dye (w/w), 4% (black only), 1%, 0.5%, and 0.05% (blue, red, and yellow). Finally, each dye, fiber, color, and dye concentration combination were examined with Raman using one of two laser excitation sources (532 nm and 780 nm) while mounted in one of two mounting media, Permount™ and Entellan® new, as well as unmounted. Raman spectroscopy could detect some dyes at low concentrations (0.5% and 0.05%) even when mounted in mounting media and covered with a glass coverslip. Excitation source, dye category, dye concentration, fiber type, and mounting method all influence the ability to detect any given dye. These results support the continued study of Raman as a tool for the examination of fiber dyes as it has shown the potential to be effective even under constraints experienced by forensic examiners.