Technical Note

Ninhydrin Processing

Introduction

Various solutions of Ninhydrin may be used to develop latent prints on porous surfaces such as paper, cardboard, and wood. Ninhydrin reacts with amino acids commonly found in latent print residue to form a purple compound, yielding latent prints visible in shades ranging from reddish-purple to brownish-purple.

Safety

Precautions should be taken to avoid contact with both the powder form and the liquid solutions of Ninhydrin. Nitrile gloves should be worn, as latex gloves deteriorate with some of the solvents used, most notably acetone. A dust mask and goggles should also be worn to prevent the chemical or solvent getting into the eyes. In addition, some of the solvents used are flammable and must be mixed and used in a fume hood, away from all sources of heat or spark. If using Ninhydrin mixed in a flammable liquid at a crime scene, a full-face breathing apparatus should be worn and avoid any source of heat or spark.

Mixing Instructions

• SOLUTION ONE (alternative to Freon formulation)
  3M suggests the following solution using 3M™ Novec™ Engineered Fluid HFE-7100 as a replacement solvent for Freon, which has been banned for being an ozone depleter.

  1. In a 250 ml beaker, dissolve 5 grams Ninhydrin crystals in 45 ml ethanol.
  2. Add 2 ml ethyl acetate.
  3. Add 5 ml acetic acid
  4. Continue mixing until all the Ninhydrin has dissolved into solution
  5. Transfer this "stock solution" to a one liter beaker
  6. Stir in one liter of 3M™ Novec™ Engineered Fluid HFE-7100. Mix unitl a milky yellow solution is formed.
  7. Cover and allow the solution to settle for approximately 30 minutes. A thin oily-looking film may form on the top of the solution. This film consists of water, excess ethanol and ninhydrin and must be removed prior to use. The film can be removed by any of the following procedures:
     1. If available, process the working solution through a separatory funnel. Again, allow the solution to settle for at least 30 minutes. Drain the bottom phase into a squirt bottle or storage container. Stop draining when the separate, clear-looking solution nears the bottom of the funnel, or approximately 50-100 ml of solution remains in the funnel. This remaining solution should be discarded in a proper waste container for flammable solvents as it consists of undissolved ethanol, water and ninhydrin.
     2. If a separatory funnel is not available, use a pipette to skim the oily film from the top, again discarding the waste in a proper waste receptacle.
     3. If neither a separatory funnel nor pipette is available, simply transfer teh solution into a squirt bottle. This will help insure a clean solution. When the solution level is below the straw in the squirt bottle, discard the remaining solution in a proper waste receptacle.
  8. When not in use, protect from direct light to preserve the shelf life of the solution.

  3M™ Novec™ Engineered Fluid HFE-7100 is safe to use without a fume hood and does not run most inks. This formula is recommended by the Home Office as an alternative to the Freon mixture.

• SOLUTION TWO
  Perhaps the simplest Ninhydrin solution to mix is to add 25 grams of Ninhydrin crystal to one gallon (or 4 liters) of acetone. This solution can be mixed in small quantities using the same proportions. This solution works reasonable well, but will cause many inks to run badly.

• SOLUTION THREE
  In a 500 ml separatory funnel, place 3.8 grams of Ninhydrin crystals and 20 ml of methyl alcohol (methanol). Shake the funnel, being careful to aim the two ends away from the face and eyes, and being certain to release the pressure in the funnel frequently. Add 480 ml of petroleum ether and again, shake the funnel to mix the two solutions together. Hold the funnel away from the face and eyes and release the pressure frequently. Allow the solution to stand for approximately 10 minutes.

  There will be two distinct layers of solution. The lower layer will be smaller in proportion and yellow in color. Shake the funnel again and allow it to stand for 5 more minutes. Drain the bottom layer into a beaker or other container and save it. The clear layer is working solution. Place it into a dark colored plastic or glass container and properly label it.

  Pour the saved yellow solution back into the separatory funnel and add 480 ml of petroleum ether to it. Shake it and allow it to stand for 10 minutes. Shake it again and let it stand for about 5 minutes. Drain the bottom yellow layer and discard it. The clear layer is additional working solution. Place it into the dark colored container and properly label it.

Processing Instructions

Because of the nature of some of the chemicals involved, any use of the solution should be done in a fume hood or while wearing a full-face breathing apparatus respirator. Just using the product outside in the "open air" is not sufficient protection against the potential hazards involved. It is recommended to apply Ninhydrin solution by painting or dipping. For dipping, glass, metal or plastic trays can be used. Metal, plastic or wooden tweezers or forceps can be used to move the paper items in and out of the tray of Ninhydrin. Immerse the item for a few seconds and remove it from the tray, allowing the excess liquid to drip back into the tray. To paint, hold a cotton ball with a pair of forceps, dip it into the Ninhydrin solution and lightly brush the surface to wet it. Do not scrub the surface hard.

After applying the Ninhydrin solution, allow the paper item to air dry inside the fund hood. Latent prints may develop within a few hours or may take a few days at room temperature. Additional applications of the Ninhydrin solution can be done each day. To speed up the development process, apply humid heat. This can be done in a humidity cabinet or chamber or by the use of a steam iron. If a steam iron is used, be cautious not to touch the paper directly with the iron as it can scorch the paper. Heat a steam iron to a temperature which will give steam. Pass it over the paper several times. If the latent prints are faint, the paper should be dipped again, allowed to dry, and steamed again.

Clearing Agent

The simplest and most readily available ingredients for use as a clearing agent are water and household bleach. Mix one part of bleach to two parts of water. Soak the paper in this solution. Blot and dry immediately. Active drying, such as using a hand-held hair dryer, is necessary. Although the Ninhydrin stains will be removed, the document will not appear unaltered. It will look as though it has been washed.

Sequential Processing

Treatment with Ninhydrin may be done after DFO and before Physical Developer. It is suggested to photograph any latent prints developed after each step before proceeding onto the next step.

Photography

Photography of latent prints developed with Ninhydrin may be enhanced by using a dark green filter with black and white film. This may improve the contrast.

Pre-mixed Ninhydrin

Pre-mixed Ninhydrin is available in an 8 ounce pump bottle. Because the cap can be unscrewed, the solution can be used for spraying, or poured into a tray and used for dipping. The solution of Ninhydrin does not contain acetone and should not run most inks.

Additional Reading

Advances in Fingerprint Technology edited by Dr. Henry Lee and Dr. R. E. Gaensslen
Friction Ridge Skin: Comparison and Identification of Fingerprints by James F. Cowger
Manual of Fingerprint Development Techniques by the British Home Office, second edition
Scott's Fingerprint Mechanics by Robert D. Olsen, SR

Ordering Information

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