Zinc Chloride (ZnCl₂) is a fluorescent latent print development reagent used primarily on porous surfaces (paper, cardboard, raw wood). Here's why it's important in our field:
- Post-Ninhydrin Enhancement: Zinc Chloride is typically employed after processing with reagents like Ninhydrin or its analogs (e.g., DFO, 1,2-Indanedione).
- Fluorescence: Zinc Chloride reacts with the amino acid residues left by fingerprints, causing them to fluoresce a vibrant yellow when viewed under an alternate light source (ALS) with wavelengths in the violet-blue range.
- Legacy Technique: While still valuable, it's worth noting Zinc Chloride is a somewhat older technique now often supplemented with more modern fluorescent reagents.
Equipment
- Fume Hood: Zinc Chloride fumes can be irritating and preparation/use must be conducted within a fume hood.
- Personal Protective Equipment (PPE): Gloves, lab coat, chemical splash goggles.
- Spray Bottle: For even application of the reagent.
- Forceps: For handling evidence.
- Humidity Chamber or Steam Iron: (Optional) To accelerate development.
Zinc Chloride Solution (1 Liter)
Several variations exist, but here's a common HFE-7100 based recipe:
- Ingredients:
- Zinc Chloride (30 grams)
- HFE-7100 (500 mL)
- Anhydrous Ethanol (20 mL)
- Glacial Acetic Acid (10 mL)
- Petroleum Ether (500 mL)
Mixing Procedure
- Safety First: Don PPE and work within the fume hood.
- Dissolve Zinc Chloride: Combine the zinc chloride with HFE-7100 and use a magnetic stirrer for approximately 20 minutes until completely dissolved.
- Add Other Components: Gradually add the anhydrous ethanol and the glacial acetic acid.
- Dilute: Stir in the petroleum ether.
- Labeling: Create a label with reagent name, date prepared, expiration date, your initials, and safety information.
- Transfer to Storage: Carefully transfer the solution into a dark, shatter-proof container.
Latent Print Development with Zinc Chloride
- Previous Development: Ensure the item has been processed with Ninhydrin (or analog) and latent prints have been allowed to develop fully.
- Fume Hood: Work within the fume hood for the entire process.
- Application: Lightly and evenly spray the processed item with Zinc Chloride, keeping the bottle in motion.
- Air Drying: Allow the item to air dry thoroughly. Repeat spraying and drying for a second application.
- Optional Acceleration: Process the item in a humidity chamber (80-100°C, 65% humidity for 20 minutes) or carefully use a household steam iron to hasten development.
- Visualization: Examine the surface under an ALS with excitation in the violet-blue range. Developed prints should fluoresce yellow.
- Documentation: Photograph any developed latent prints immediately.
Storage and Shelf Life
- Dark Containers: Store Zinc Chloride in dark, shatterproof bottles to minimize light exposure.
- Cool Location: Opt for a cool storage location away from direct heat.
- Shelf Life: Properly stored Zinc Chloride solutions typically have a shelf life of about 6 months. Always label the bottle with the preparation and expiration dates.
Important Considerations
- Zinc Chloride can have variable results based on surface type, environmental factors, and the age of the latent print.
- Always run a test print concurrently with evidence to ensure reagent quality.
- Background fluorescence can be an issue, so careful consideration of the excitation wavelengths of the light source is necessary.