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Dry Ninhydrin

The use of ninhydrin crystals carried within a solution and then applied to paper or other porous surfaces is considered a laboratory "workhorse" for many friction ridge analysts. This chemical formulation reacts with trace amino acids deposited on porous surfaces from friction ridge skin, ultimately producing a color termed "Ruhemann's Purple."

Ninhydrin solutions can be mixed and carried within several different solvents—from alcohols, ketones, ethers, and alkanes, to synthesized organic-fluorine solvents—each having their own history of effective latent print development. If the friction ridge analyst is not concerned with the destruction or "bleeding" of various inks is due to their absence on the item, an acetone carried ninhydrin solution may be effective. If the analyst must preserve the mechanical/handwritten ink on a document from destruction or running, effective ninhydrin formulations within relatively non-polar solvents have been documented to achieve this goal.

A non-destructive alternative to applying ninhydrin to a susceptible evidentiary item is the dry-ninhydrin method. This technique can be utilized when the physical condition of the item can be easily damaged (thermal paper for example), or when mechanical/handwritten in is observed and a non-polar ninhydrin solvent is not readily available. In addition, this method allows for the item to be processed without being soaked in a bath of ninhydrin; thus, eliminating direct contact between the items physical characteristics and many of the caustic chemical solvents used as carriers in traditional ninhydrin formulations.

The analyst begins the dry ninhydrin process similar to how one would utilize the traditional ninhydrin dip method:
  1. Two paper sheets are saturated within a concentrated ninhydrin formulation*. White printer paper is ideal; however, kitchen paper towels, or Kimwipes, found in many forensic laboratories could also be used.
  2. Allow time for both paper sheets to completely dry—typically less tan two minutes.
  3. "Sandwich" the evidence item between the two ninhydrin-saturated paper sheets. A known latent print control should also be included. Make sure that both sheets completely cover the evidentiary item and control, use more paper sheets if necessary.
  4. If available, place both ninhydrin sheets and "sandwiched" evidence item into a vacuum-sealed bag. The vacuum forces contact between the ninhydrin sheets and evidence (required for appropriate reaction between the ninhydrin crystals and secreted amino acids). Perhaps a more practical method I to place the ninhydrin sheets and evidence into a plastic bag or sheet protector. Seal the bag and place firmly between two additional items—just as one would do to press flowers. This again will force the required contact between the ninhydrin sheets and the evidence item.
  5. At room temperature, place all items in a dark area for a minimum of 48 hours.
  6. Once time has elapsed, both the evidence and known control should be examined for visible latent impressions.
  7. If satisfied with the results, normal procedures for documenting visible impressions should be conducted. If impressions are visibly faint, allow more time to develop and check periodically (every 24 hours). Over time, if no friction ridge impressions are developed using this method, it is highly recommended that traditional ninhydrin methods be conducted.

As previously described with traditional ninhydrin solutions, there are different methods and formulation that can be used to meet the goal of developing latent friction ridge impressions. The dry ninhydrin method is no exception. Pierre McMahon recently wrote in Identification Canada (July/August/Sept. 1996) of a very similar technique to the one mentioned above, with its primary focus on its application with thermal paper. Within Scott's Fingerprint Mechanics, Robert Olsen Jr. wrote of two similar techniques—one describing the use of applying ninhydrin crystals directly on the evidence and then placing the item in a humidity chamber. In a Journal of Forensic Identification (Vol. 45 No. 2, 1995) technical report, Robin Bratton and John Juhala also wrote of "dry" methods and techniques, but with DFO (1,8-diazafluoren-9-one).

Jon T. Stimac
Dactell innovations
PO Box 4831
Sunriver, Oregon 97707

*Concentrated Dry Ninhydrin Formulation
1.2 g ninhydrin crystals
100 mL acetone

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