Trace Evidence

Trace Evidence
At a crime scene tiny fragments of physical trace evidence like hair, fibers, paint, or glass can be obtained. Trace evidence from a crime scene is either gathered directly or indirectly from the clothing of a victim or suspect. As a forensic scientist, you examine the physical, optical and chemical properties of a questioned sample using microscopes and compare those either against known standards or to a reference sample . Such evidence is often used to reconstruct an event and to indicate that a certain person or thing was present at the crime scene.

Preliminary Examination
A preliminary examination using stereo microscopes provides quick, accurate and generally non-destructive means to characterize trace evidence. Items to examine can vary from adhesive tape, to clothing. In order to identify individual microscopic traces it might be necessary to use different light conditions such as UV or to use special boom stands. You need stable instruments that offer the required flexibility.

Hair Examination
You examine if hair is animal or human, from which body part it originates, and if it exhibits racial characteristics. Therefore, you study the inner and outer morphology using transmitted light microscopes. Shape, color, texture and other visual aspects need to be taken into account when you compare hair found at the crime scene with hair of e.g. a suspect. High reproducibility and comparable results are key to your successful examination.

In the forensic lab fibers are generally classified as natural, synthetic and inorganic. The fiber under investigation is compared with a known sample. Typically, fibers are mounted in special liquids with known index of refraction and are characterized with respect to various parameters such as relative refractive index, color, and morphology. Especially synthetic fibers are examined with polarization contrast in order to characterize the birefringent properties of the material. Fluorescence microscopy or micro-spectrophotometry often complement your examination to determine the spectrum of dyed fibers. Using scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) allows you to visualize the morphological details of the fibers in high resolution and to obtain their chemical composition.

As a forensic paint examiner you characterize the color, number of layers, layer sequence and thickness of the paint by mainly using light microscopes. For further physical and chemical characterization of a paint chip with FT-IR microscopy, micro-spectrophotometry or SEM with EDS the different layers are generally dissected individually under a stereo or zoom microscope. The data obtained is particularly useful in automotive paint analysis, where databases are used to identify possible brands, models and years of manufacture of motor vehicles.