Forensic characterization and differentiation of automotive lubricating greases using Fourier Transform Infrared spectroscopy and Scanning Electron Microscopy-Energy Dispersive X-Ray Spectroscopy

Abstract

Automotive lubricating greases may be recovered as stains on the ground at vehicular accident scenes or on the garments of persons of interest (i.e., suspects or victims) allegedly involved in events under investigation. Numerous studies have been performed differentiating automotive motor oil for forensic applications, however little research has been conducted to characterize and differentiate automotive greases for forensic purposes. This study aims to determine physical and chemical properties of greases to help with their characterization and differentiation in a forensic setting. Thirty-five commercially available grease samples were obtained from major US hardware stores and were initially characterized using light microscopy methods; then they were subsequently examined using attenuated total reflection (ATR)-Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). The initial microscopical assessment showed the presence of microscopic particles that were part of the original formulation of the grease samples. In terms of shapes, sizes, colors, optical properties, and concentration, these particles differed considerably between the grease samples. Three classes of particles were identified and consequently labelled as opaque, isotropic, and anisotropic. ATR-FTIR spectroscopy enabled obtaining a common infrared profile characteristic of all for grease samples. Close inspection of the spectra also highlighted the presence of minor infrared absorption bands that were useful to the differentiation of different grease samples. Because of the high interference of the lubricating medium of the grease samples during SEM-EDS analyses, an oil-solid separation procedure to extract the observed microscopic particles from the grease medium was developed based on the investigation of three solvents – two petroleum solvents, hexane, and pentane, and one green solvent, D-limonene – which led to the selection of D-limonene. The developed extraction method enabled the successful isolation of the detected particles for conducting SEM-EDS analyses. The opaque particles were identified as solid lubricant additives, containing various amounts of molybdenum and niobium. Grease samples that did not contain opaque particles were missing these heavy elements. These differences in composition offered increased discriminating capabilities for the differentiation of automotive greases, also considering that such particles are not present in other automotive lubricating fluids like motor oils or brake fluids.