SPR is a physical method of latent print development that works especially well on difficult surfaces, notably wet or contaminated substrates. It's based on a simple principle: finely divided particles in suspension preferentially adhere to the fatty components of fingerprint residues.
SPR typically comes in two common forms:
- Black SPR: Contains molybdenum disulfide (MoS2) particles. Ideal for light-colored surfaces.
- White SPR: Contains titanium dioxide (TiO2). Designed for dark-colored surfaces.
SPR formulations can also have fluorescent variations for enhanced visualization under UV light.
SPR is a physical method of latent print development that works especially well on difficult surfaces, notably wet or contaminated substrates. It's based on a simple principle: finely divided particles in suspension preferentially adhere to the fatty components of fingerprint residues.
SPR typically comes in two common forms:
- Black SPR: Contains molybdenum disulfide (MoS2) particles. Ideal for light-colored surfaces.
- White SPR: Contains titanium dioxide (TiO2). Designed for dark-colored surfaces.
SPR formulations can also have fluorescent variations for enhanced visualization under UV light.
Equipment for Using SPR
- SPR Solution: Either commercially prepared or mixed in-house.
- Spray Bottle or Wash Bottle: For applying the SPR solution.
- Tray or Container: Large enough to submerge objects if needed.
- Rinse Solution: Distilled or deionized water in a wash bottle.
- Protective Gear: Gloves, safety glasses, and potentially a respirator mask.
Ingredients & Recipe for 1 Liter SPR
Here's a common recipe for black SPR. White SPR is essentially the same, substituting titanium dioxide for the molybdenum disulfide.
- Molybdenum Disulfide (MoS2): 5 - 10 grams
- Photo-Flo® or Similar Surfactant: 10 ml
- Distilled or Deionized Water: 1 Liter
Mixing Procedure
- Pour approximately 900 ml of distilled/deionized water into a clean, chemically resistant container.
- Add the Photo-Flo® or similar surfactant. Gently agitate to mix.
- Slowly add the molybdenum disulfide with continuous stirring. It's crucial to create a good suspension of fine particles.
- Add the rest of the distilled/deionized water.
- Thoroughly mix and transfer to a storage bottle or your sprayer.
Processing Latent Prints with SPR
- Preparation: Don appropriate safety gear. If your item is wet, don't allow it to dry.
- Application:
- Spraying: If possible, spray the item at a 45-degree angle across the surface, allowing the SPR to rinse over the area in question.
- Submersion: Submerge the object completely in a tray of SPR. Gently agitate, if practical.
- Rinsing: Use distilled/deionized water to gently rinse excess SPR from the surface. Observe for developed prints.
- Repeat: If the initial development is weak, repeat the SPR application and rinse steps.
- Documentation: After rinsing, photograph any developed prints. They are fragile and will degrade over time.
- Lifting (Optional): Lifts of SPR-developed prints can be attempted with standard lifting tape.
Storing and Shelf Life of SPR
- Store in an airtight, opaque container away from direct sunlight and heat to prevent premature settling and the breakdown of the surfactant.
- Refrigeration can lengthen the shelf life.
- Shelf life typically ranges from 6 months to a year, but this depends on storage conditions and variation in individual formulas. Always check for settling or visible signs of degradation before use.
Important Notes:
- Due to the finely divided particles, avoid excessive inhalation of SPR while mixing or spraying.
- SPR might leave residues. Test on an inconspicuous area of an item first.
- Developed latent prints must be photographed quickly, as the reaction product is fragile and easily dislodged.