Forensic Differentiation of Black Shoe Polish Stains using Attenuated Total Reflection- Fourier Transform Infrared Spectroscopy and Chemometrics

Abstract

Shoe polish smears or stains are encountered as evidentiary material on a variety of substrates, particularly in cases related to sexual and physical assault, murder, theft, and robbery. This trace evidence can be used as proof of contact to narrow down the search in a criminal investigation. The identification and differentiation of samples with similar composition and the same hue can be a difficult task for the investigator.

In the present work, the scope of Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy for discriminating shoe polish samples of 15 different brands is investigated in combination with chemometric tools that is principal component analysis. The discriminating power of visual analysis is 99.04 %, while in principal component analysis it is 98.09 %, which is highly significant. The blind test is carried out with unknown samples to the analyst, to predict their respective sample source. Intra-brand variation, homogeneity and the effect of different substrates on sample analysis are also studied. The reported method provides considerable results for the identification and differentiation of shoe polish samples of different brands. The stains on substrates could be identified and linked to their parent source as well.