Identification of dyes on fabric exposed to lake and ocean water using near-infrared excitation Raman spectroscopy.

Forensic analysis of dyes present on fabric can provide a wealth of information about the relationship between the suspect and a crime scene. Such analysis requires analytical approaches that provide confirmatory information about the chemical structure of the dyes. A growing body of evidence indicates that near-Infrared excitation Raman spectroscopy (NIeRS) can be used for non-invasive and non-destructive detection and identification of colored fabric. However, it remains unclear how environmental factors such as water exposure could alter the accuracy of dye identification by NIeRS. To answer this question, we exposed cotton fabric colored by blue and magenta dyes to ocean and lake waters for 14 weeks. During this time, fabric was analyzed biweekly using a hand-held NIeRS spectrometer. Chemometrics was used to process and learn from the acquired NIeRS spectra. Our results indicate that both ocean and lake water cause substantial fading of dyes. NIeRS revealed that fading rates are greater for lake than ocean water. Such water-triggered fading substantially lowered accuracy of NIeRS-based dye identification. We observed a dye-dependent decrease in prediction accuracy from 100% at week 0 to 30-80% by week 6. Nevertheless, built spectroscopic libraries enabled ∼98% accurate identification of both dyes on fabric. These results indicate that water exposure possesses a great risk to NIeRS-based identification of dyes on fabric. However, a combination of NIeRS and artificial intelligence allowed for overcoming these limitations.