Rotation 2 – Latent Print Processing, Trace Evidence Support, and Digital Evidence Handling

Rotation 2 – Latent Print Processing, Trace Evidence Support, and Digital Evidence Handling

This rotation puts you inside the agency's print lab and evidence processing areas. You will develop latent prints, assist with trace evidence packaging, and receive digital evidence items. Each task carries chain-of-custody obligations and safety requirements. Errors here are not academic—they can compromise a prosecution.

Latent Print Development: Core Concepts

A latent print is invisible to the naked eye. It is formed when sweat and oils from friction-ridge skin transfer to a surface. Your job is to make that deposit visible, document it, and preserve it without altering or destroying it.

Your choice of development method depends on the substrate (the surface you are examining) and the environment the item came from.

• Aluminum powder brushing – best on dark, non-porous surfaces such as dark plastic or metal. Apply with a fiberglass brush using light, circular strokes. The aluminum particles adhere to the fatty residue of the print and reveal ridge detail.
• Black powder brushing – best on light-colored non-porous surfaces. Technique is identical to aluminum powder; choose contrast over the surface color.
• Cyanoacrylate (CA) fuming – used in a dedicated fuming chamber. Cyanoacrylate vapors polymerize on the amino acids and proteins in print residue, leaving a white, hard deposit. This method works on most non-porous surfaces and fixes the print before powder is applied for additional contrast or fluorescence work.
• Ninhydrin – a chemical reagent that reacts with amino acids in sweat residue on porous surfaces such as paper, cardboard, or unfinished wood. It produces a purple compound called Ruhemann's purple. Ninhydrin requires adequate dwell time (often 24–48 hours at room temperature, or accelerated with gentle heat).
• Alternate light source (ALS) – used in conjunction with fluorescent powders or post-fuming to cause print deposits to fluoresce. Goggles with the correct filter for the wavelength in use are mandatory before activating the lamp.

Document everything. Photograph each surface before development, after development, and after lifting. Include an ABFO scale in every photograph. Complete the agency's latent print examination worksheet immediately—do not reconstruct notes from memory.

Lifting. On non-porous surfaces, clear or frosted lifting tape is applied over a powder-developed print and then transferred to a backing card of contrasting color. Label the card with the case number, item number, your initials, and the date. If the print cannot safely be lifted—such as on curved glass or painted surfaces—photograph it in place and document why lifting was not attempted.

Trace Evidence Support

Trace evidence includes hair, fiber, glass fragments, soil, and similar microscopic or small-scale material. Your role is to assist the examiner; you are not conducting the analysis. Your responsibilities are:

1. Contamination prevention. Change gloves between items. Use clean instruments (forceps, scalpel) for each transfer. Work on clean butcher paper or a clean collection surface.

2. Packaging. Paper packaging is preferred for biological trace because it allows moisture to escape and prevents mold. Plastic is acceptable for non-biological trace (glass, soil) when paper is not adequate. Seal, label, and initial every package before it leaves your hands.

3. Documentation. Record every handling step in your externship journal. The supervising examiner's initials confirm your technique was correct.

Never combine trace items from different locations or items into a single package.

Digital Evidence Handling

Digital evidence is fragile and legally sensitive. A single write operation to a storage device can alter timestamps and metadata, potentially making the evidence inadmissible or introducing doubt at trial.

Write-blocking is mandatory. Before connecting a mobile device, USB drive, or SD card to any workstation, attach a hardware write-blocker. A hardware write-blocker is a physical device that allows read commands to pass through while blocking any write commands from the forensic workstation to the evidence device. Software write-blockers exist but are considered less reliable in many agency protocols—follow the agency's specific policy.

The workflow is:

1. Receive the item and document its physical condition on the digital evidence intake form (model, serial number, battery state, whether the device is on or off).

2. Connect the write-blocker first, then connect the evidence device to the write-blocker, then connect the write-blocker to the workstation.

3. Have the supervisor verify the write-blocker is in place and operational before any imaging or preview begins. This verification must be recorded on the intake form with the supervisor's initials.

4. Begin imaging or preview only after supervisor sign-off.

Never bypass the write-blocker under any circumstance. Doing so in this program triggers immediate corrective review, and in a real case it can destroy evidence integrity.

Safety Throughout Rotation 2

• Chemical-resistant gloves and goggles are required whenever handling ninhydrin, cyanoacrylate, or operating the CA fuming chamber. The chamber must be used in a ventilated area or under a fume hood.
• ALS eye protection must be rated for the specific wavelength of the lamp in use. Standard safety glasses are not sufficient.
• Powder inhalation is a hazard; if you are working with powder in quantity or in a confined space, wear an appropriate particulate respirator.
• Treat every case item as a potential biohazard; wear gloves and handle items as if bloodborne pathogen exposure is possible.