Trace Evidence

By Matt Jobbins, 3 minutes to read
Industry: Technology: Material: ,

The instruments commonly used in criminal or environmental forensic investigations and analysis include scanning electron and optical microscopes. These tools are used to characterize forensic evidence like fabrics, metals, textile or glass. Microscopic imaging can also aid in identifying scratches and indents from tool marks, blood, hair classification, particle analysis, or scrutinizing residues such as sand, mud, and diatoms.

Hair and Fibers

Hair and fiber samples are often collected as trace evidence. Fiber types can be determined using optical microscopy techniques. Human hair can be distinguished from animal hair using a Scanning Electron Microscope (SEM). Moreover, the detailed high magnification of the human hair sheds light on the ethnicity of the suspect and other details like whether the hair has been chemically treated or not. Different species of animals have different follicular structures as well.

Colorized Scanning Electron Microscope image of hair
Colorized Scanning Electron Microscope image of a human hair.

Microscope examination of fibers, like the cotton fiber shown here imaged by SEM, cannot provide quantitative characteristics of fibers, but is non-destructive and can be used for a qualitative comparison.

Colorized SEM image of cotton
Colorized Scanning Electron Microscope image of cotton.

Particulates

Particle analysis can apply to criminal or environmental forensics. Types of soil, dust, particles, and minerals can often be associated with specific locations.

Soil and mud are collected from shoes and boots of the suspects and analyzed with the SEM. The structure and composition of the minerals, sand etc. found in these samples can shed light on whether the suspect has been near the crime scene recently. Shown here is a colorized SEM image of the copper-containing mineral chrysocolla.

Colorized SEM image of chrysocolla
Colorized Scanning Electron microscope image of chrysocolla.

Paint Examination

Paint chips or flakes found in crime scenes or accidents are frequently analyzed to determine the colors, makes and models of the vehicles involved. Cars are usually painted with four to five layers of primer, color, and clear coats. The composition of these layers can be determined by EDS analysis in the Phenom SEM. Vehicle manufacturers use unique combinations of the paint layers for each model produced. An analyzed sample can be compared with a suspect vehicle or searched on a database of known layer compositions.

Paint layers analyzed using EDS in the SEM
Paint layers analyzed using Energy-Dispersive Spectroscopy (EDS) in the Scanning Electron Microscope.

Botany / Pollen

Seeds, pollen, and leaves found at crime scenes can lead detectives to locations where the crime could have occurred, or places where suspects and victims have been. Because pollen and seeds are produced at specific times of the year, these pieces of evidence also reveal to investigators when the crime occurred. Diatoms found in water also pinpoint where crimes were committed.

SEM image of corn pollen.
Scanning electron microscope image of corn pollen.

Entomology

Insects found on or in bodies can help determine the time and cause of death. Each species of fly, beetle and other decomposing insects tend to lay their eggs at different times after death. By identifying the larvae in the body and considering environmental conditions, such as temperature and exposure, investigators can estimate how long a victim has been dead. The insects and larvae can also be analyzed to determine whether poison, drugs or other toxins were involved in the case.

Colorized SEM image of ant leg
Colorized Scanning Electron Microscope image of an ant leg at 1450x magnification.