Ninhydrin Processing

By Pat A. Wertheim

This article originally appeared in Minutiae, the Lightning Powder Co. Newsletter, #45, November-December 1997.

Perhaps the most productive and cost-effective method of developing latent fingerprints on paper is treatment with Ninhydrin. Freshly-mixed Ninhydrin solutions are less expensive and more dependable than premixed aerosol cans or pump spray dispensers. While the premixed containers are ready for instant use when purchased, safety experts today caution against spraying and instead encourage either dipping or painting to apply the solution.

The problem with spraying Ninhydrin solutions is that, even in a fume hood, airborne particles of Ninhydrin dust can form as the carrier evaporates. These microscopic particles may not be effectively removed from the lab by the fume hood, and may find their way back into the air you breathe. Since Ninhydrin reacts with amino acids, any exposure to your body, especially to your eyes or lungs, could have serious results. This potentially dangerous exposure is minimized by dipping or painting.

Protective gear should be worn when using Ninhydrin. Latex gloves are not adequate protection against Ninhydrin solutions. Latex allows some chemicals to pass through and can actually dissolve in other chemicals. Nitrile gloves are recommended as they are more resistant to chemicals, thus offering better protection. Chemical resistant gloves, goggles, breathing mask, and a lab coat should be worn whenever working with Ninhydrin solutions.

If a questioned document examination for handwriting or indented writing is desired, it should always be done prior to a latent print examination. Many of the subtle writing characteristics used by document examiners and any indented writing are lost during chemical processing. In addition, a document examiner may be able to testify that the suspect actually wrote the document, while a latent print examiner can only testify the suspect handled the paper. In the absence of suspects, however, a latent print may yield results through AFIS.

The simplest Ninhydrin solution to prepare is made by pouring 25 grams of Ninhydrin crystals into a gallon (or 4 liters) of solvent. The easiest solvent to obtain is acetone, which is available at any paint store. Methyl Alcohol (methanol) works well, also. The major drawback to these solvents is that they dissolve or run most inks. If the writing itself is important, the document should be photographed and photographed and photocopied prior to chemical treatment.

A more sophisticated solution may be prepared by dissolving 5 grams of Ninhydrin in 75 millilitres of ethyl alcohol (ethanol). To this, add 25 milliliters of ethyl acetate and 3 milliliters of acetic acid. Finally, add this solution to 1 liter of heptane. This formula is more effective on papers with calcium carbonate whiteners in them because it facilitates the reaction by maintaining a slightly acidic environment. The heptane formula is also less apt to run or dissolve inks.

To “dip” the document, choose a clean tray, such as a photographic developer tray, big enough to hold the document lying flat. Put enough Ninhydrin solution in the tray to cover the bottom and simply lay the document in the solution. Remove the item and lay the item in again, other side down. Remove the item again and allow excess solution to run off into the tray. Place the document on a piece of blotter paper or other clean paper to finish drying. Or construct a “clothes line” to hang up the papers inside the fume hood.

To “paint” the document, clamp a clean cotton ball in forceps, dip the cotton ball into the Ninhydrin solution, and dab or paint the document with the wet cotton. Make sure the entire surface is wetted with the solution. Allow the document to dry.

Development of prints may be expedited by application of moist heat. The simplest method of doing that is to hold a steam iron an inch or so above the document and allow the hot steam to lightly waft across the document. Do not apply the iron directly to the paper or allow droplets of water from the iron to contaminate the paper. While steaming will rapidly develop most prints on the paper, it may overdevelop the background and take away from the contrast of the prints. An alternate method is to simply allow several days for the prints to develop without the moist head, constantly monitoring the paper to catch the prints at their peak development and contrast.

Once developed, latent prints should be photographed, as they are prone to fade with time. Fading can be delayed by storing the item in an airtight plastic bag after it is dry and the prints have reached optimum development.

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