Oil Red O

Oil Red O (ORO) is a lipophilic dye. That means it has a strong affinity for fats and lipids – components abundant in fingerprint residue. Due to its ability to bind to these fat/lipid components, ORO is remarkably effective for developing latent prints on porous surfaces such as paper and cardboard. It's an especially important technique when these surfaces have been wet, since water degrades the amino acid residues targeted by other traditional reagents.

Equipment

Here's the basic equipment you'll need when working with Oil Red O:

• Shallow Trays: Large enough to accommodate your evidence. Glass or chemically resistant plastic work well.
• Fume Hood: For mixing and processing. ORO solution has a strong odor of methanol.
• Gloves: Nitrile or similar chemical-resistant gloves.
• Graduated Cylinder: For accurate measurement of ingredients.
• Stirring Rod: Glass or inert plastic.
• Rinse solution bottles: For rinsing excess ORO.

Oil Red O Solution (1 Liter)

• Oil Red O dye: 0.5 - 1.5 grams (Adjust based on the desired intensity)
• Methanol (Methyl Alcohol): 750 ml
• Distilled Water: 250 ml
• Sodium Hydroxide (NaOH): An amount for making a basic solution (approximately a few pellets if your methanol is pure).

Mixing Procedure

1. Safety First: Work in a fume hood, wearing gloves and eye protection. Methanol is flammable and toxic.
2. Dissolve NaOH: In a small amount of distilled water, add a few pellets of NaOH. Keep in mind, this reaction gives off heat.
3. Combine Solution: In a clean vessel, add the methanol. While stirring continuously, slowly add the dissolved NaOH solution to the methanol.
4. Add Dye: Carefully add the Oil Red O powder and stir continuously until completely dissolved.
5. Top Up: Slowly add the remaining distilled water, stirring all the while. Do this slowly as additional heat can be generated here.

Processing Latent Prints with Oil Red O

1. Submerge Evidence: Carefully submerge the item of evidence in the ORO working solution. Agitation may help in some cases.
2. Develop Prints: Allow the evidence to soak until latent prints develop a reddish-orange color. Check at intervals to monitor progress. Development times can range from minutes to hours.
3. Rinse: Thoroughly rinse the evidence with distilled water to remove excess dye. Be gentle, as rinsing can cause blurring, especially with heavily saturated prints.
4. Second Rinse (Optional): A second rinse using a phosphate buffer solution may neutralize alkalinity and help stabilize the dye, but distilled water alone can be sufficient.
5. Drying: Air dry the evidence in a well-ventilated area, ideally shielded from strong light.

Storage and Shelf Life

• Light Sensitivity: Store your ORO working solution in a dark glass bottle to protect it from excessive light exposure. Light promotes photo-oxidation, degrading the reagent.
• Cool Conditions: Ideally, store your solution in a cool environment to extend its shelf life.
• Shelf Life: Even under ideal conditions, Oil Red O is not the most stable of reagents. Generally, a six-month shelf life is expected. Degradation will show as faded orange prints, or the reagent will turn dark (like used motor oil).

Important Note: The exact process can vary slightly between laboratories. Be sure to refer to your laboratory's specific standard operating procedures. Let me know if you'd like additional insights or clarifications on anything above!