Ninhydrin (triketohydrindene hydrate) is a powerful chemical reagent that has been a cornerstone of latent print development for decades. Its primary function is to react with the amino acids present in fingerprint residue, producing a deep purple color known as Ruhemann's purple. This reaction makes latent prints on porous surfaces readily visible.
Equipment
- Shallow tray or dish: For dipping or immersing evidence.
- Spray bottle or atomizer: For controlled application.
- Fume hood or well-ventilated area: Essential for safety due to solvent fumes.
- Drying chamber or oven: To accelerate the development of latent prints.
- Personal Protective Equipment (PPE): Gloves, lab coat, eye protection, and a respirator mask if ventilation is inadequate.
Ninhydrin Solution Ingredients (1 Liter)
- Ninhydrin (powder): 5 grams
- Ethanol (or Isopropanol): 950 mL
- Petroleum Ether (or HFE-7100): 50 mL
- Acetic Acid (optional): A few milliliters may be used to adjust pH
Mixing Procedure
- Safety First: Work in a fume hood or well-ventilated area, and don full PPE.
- Dissolve Ninhydrin: Combine the ninhydrin powder with the ethanol (or isopropanol) in a clean glass container. Stir until fully dissolved.
- Add Solvent: Slowly add the petroleum ether (or HFE-7100) while stirring.
- Adjust pH (optional): If desired, add a small amount of acetic acid to slightly lower the pH. This can sometimes improve print development.
- Transfer to Storage: Pour the mixed solution into a dark glass bottle for storage. Label the bottle clearly with the contents, date, and hazards.
Latent Print Processing with Ninhydrin
- Preparation: Place the evidence item in a clean, shallow tray or on a non-porous surface.
- Application:
- Dipping: Submerge the item briefly in the ninhydrin solution.
- Spraying: Apply ninhydrin with a spray bottle, evenly coating the surface.
- Drying: Allow the item to air dry thoroughly in a well-ventilated area, away from direct light.
- Development:
- Room Temperature: Prints may develop over several hours or days at ambient temperature.
- Humidity Chamber: Place the item in a humidity chamber (70-80% humidity) to accelerate development.
- Heat: A steam iron or oven set at a low temperature (around 80°C) can further speed up the reaction.
- Documentation: Once prints are visible, photograph them immediately for documentation.
Storage and Shelf Life
- Storage: Store the mixed ninhydrin solution in a tightly sealed dark glass bottle, away from heat and light. It's ideal to refrigerate it for increased shelf life.
- Shelf Life: A properly mixed and stored ninhydrin solution can remain effective for several months. However, it's always best to test the solution periodically on a control surface to ensure potency.
Important Considerations
- Use ninhydrin primarily on porous surfaces (paper, cardboard, untreated wood).
- Exercise caution as ninhydrin can stain hands and surfaces.
- Always work with proper ventilation to avoid inhaling harmful fumes.