Physical Developer (PD) is a powerful silver-nitrate-based reagent designed to visualize latent fingerprints on porous substrates, predominantly paper and cardboard. Unlike reagents like ninhydrin and DFO, which target amino acids within fingerprint residue, PD works by binding to the fatty components (lipids) of fingerprints. This makes it especially valuable under these circumstances:
- Older Prints: Latent prints that have aged or degraded.
- Wet Surfaces: Items previously exposed to moisture, making amino acid reagents less effective.
- After Other Reagents: In a sequential process, PD is often used after ninhydrin or DFO to reveal additional latent prints.
Equipment
- Shallow Trays: Glass or chemically resistant trays are essential as PD reacts with some metals. Size will depend on the items being processed.
- Fume Hood: PD emits volatile components, making a fume hood crucial for safety.
- Forceps or Tongs: Non-metallic and non-reactive to handle and move items in the solution.
- Graduated Cylinders and Beakers: Accurate measurement of chemicals.
- Chemical-Resistant Gloves: Essential to protect your skin.
- Goggles or Face Shield: Protection from splashes.
- Respirator with Filter for Organic Vapors: PD mist and fumes should not be inhaled.
Physical Developer Formula (1 Liter)
- Distilled or Deionized Water: 950 ml
- Silver Nitrate (AgNO3): 15-20 grams
- Ferrous Ammonium Sulfate (Fe(NH4)2(SO4)2·6H2O): ~2 grams
- Ferric Nitrate (Fe(NO3)3·9H2O): ~2 grams
- Citric Acid (C6H807): ~5 grams
- Non-Ionic Detergent (Photo-Flo, Synperonic N, etc.): ~1 ml
Important Note: It's imperative to use high-purity chemicals to ensure the correct chemistry of the solution.
Mixing Procedure:
- Cleanliness: Scrub all trays and equipment before use. Residues can create nucleation sites, resulting in undesirable silver plating.
- Distilled Water: Add 950 ml of distilled or deionized water to a suitable container.
- Silver Nitrate: Dissolve the silver nitrate completely, stirring consistently.
- Ferrous/Ferric Salts: Separately dissolve both the ferrous ammonium sulfate and ferric nitrate in small amounts of water before adding them to the main solution.
- Citric Acid: Add and dissolve the citric acid completely.
- Detergent: Finally, add the non-ionic detergent while minimizing excessive foam generation.
Processing with Physical Developer:
- Pre-Wet: Submerge the evidence briefly in distilled water. This prepares the porous surface.
- PD Immersion: Completely immerse the evidence in the Physical Developer solution. Development time can range from minutes to hours – monitor progress carefully.
- Rinse: Once fingerprints are sufficiently developed, remove the item and rinse thoroughly with distilled water.
- Drying: Air dry the item or use forced warm air, but avoid exposure to strong light.
Storage and Shelf Life
- Light Sensitivity: Store PD in amber or completely opaque containers, in a cool location. It is extremely sensitive to light degradation.
- Contamination: Prevent cross-contamination with other chemicals.
- Shelf Life: Under ideal conditions, a freshly prepared PD working solution typically has a usable life of several weeks to a few months. However, signs of silver buildup on the tray bottom indicate it's time to prepare a fresh batch.