Trace Evidence: Collection, Preparation, and Microscopic Examination

What Is Trace Evidence?

Trace evidence consists of small or microscopic materials transferred between people, objects, and environments during contact. Edmond Locard's exchange principle states that every contact leaves a trace. In crime scene work, that principle is the foundation for collecting fibers, hair, glass fragments, and soil — materials that can link a suspect to a scene or a victim to a location. Because these materials are tiny and easily lost, collection technique and documentation discipline are critical from the first moment you approach an item.

Safety Before You Begin

Wear nitrile gloves at all times when handling evidence items. Change gloves between different items to prevent cross-contamination. Safety glasses are required whenever you are working with glass fragments, immersion oils, or solvents. A lab coat protects your clothing from becoming an unintended source of fiber contamination. Never pipette immersion oil by mouth. If a glass fragment punctures a glove, stop, remove the glove carefully, inspect your hand, and report the incident to the instructor. Dispose of broken glass in a puncture-resistant sharps container, not a regular waste bin.

Tape-Lift and Pick Technique

Tape lifts capture loosely adhering fibers, hair, and particulates from fabric and other surfaces. Press a loop of clear adhesive tape firmly onto the surface in overlapping rows, then mount the tape sticky-side-down onto a clear acetate sheet. Label the acetate immediately: item number, area sampled, date, and your initials. Acetate sheets are easier to scan and photograph than individual tape segments.

For individual hairs or coarse fibers that may be obscured or folded under a tape, use fine-tipped forceps. Grip the hair near its midpoint — not the root or tip, which are the most fragile and most informative zones — and transfer it directly onto a glass slide. Add one drop of mounting medium, lower a cover slip at an angle to exclude air bubbles, and label the slide on the frosted end with a diamond-tipped marker or pencil. Never use ink that can smear under oil or solvent.

Microscopic Hair Examination

Hair examination under the compound microscope focuses on four morphological descriptors:

• Scale pattern: Coronal (crown-shaped, common in small mammals), spinous (petal-like, common in cats and some other species), or imbricate (overlapping, flat — typical of human hair).
• Color and pigment distribution: Describe the overall color and whether pigment is evenly distributed, stippled, or absent (unpigmented or bleached).
• Medullary pattern: The medulla is the central canal or core of the hair shaft. It may be absent, fragmented, discontinuous, or continuous. In humans, the medulla is often absent or fragmented.
• Medullary index: The diameter of the medulla divided by the total diameter of the hair shaft. In humans this ratio is generally less than one-third; in many animals it exceeds one-half. You will measure both diameters using the microscope's calibrated ocular micrometer and record the ratio on your worksheet.

A comparison microscope allows two slides to be viewed simultaneously in a split field, making side-by-side comparison of questioned and known hairs straightforward. If only a photomicrography adapter is available, photograph each field at the same magnification and compare images. Sketch or photograph at least two fields of view per mount and annotate each image with scale information.

Important limitation: Microscopic hair comparison is a screening tool, not identification. It cannot individualize a hair to a single person the way nuclear DNA analysis can. Document this limitation on every worksheet.

Glass Fragment Examination: Refractive Index Screening

Glass has a characteristic refractive index (RI) — the ratio of the speed of light in a vacuum to its speed in the glass. When a glass fragment is immersed in oil of the same RI, the fragment's edges become invisible (the Becke line disappears). This is the basis of the immersion method.

In this exercise, your instructor demonstrates the concept and you sort fragments from two simulated items using a series of oils of known RI. For each fragment, note which oil causes the edges to become least visible and record that oil's RI as the apparent RI of the fragment. Group fragments that share the same apparent RI.

Limitations to document: The immersion method is a screening comparison. Small RI differences within a single pane of glass are normal due to manufacturing variation. Two fragments sharing an apparent RI does not prove common origin; it shows they cannot be excluded. Confirmation requires instrumentation such as a glass refractive index measurement (GRIM) system. Note these limitations explicitly in your results.

Documentation Requirements

Every preparation must be labeled before it leaves your hands. Every observation must be recorded in ink on the worksheet at the time of observation, not reconstructed afterward. If you make an error, draw a single line through it, initial it, and write the correction. Do not use correction fluid. Your chain-of-custody habits in this lab exercise are the same habits you will use on real evidence.