Ninhydrin: 2,2-Dihydroxyindane-1,3-dione, a Reagent Sensitive to Amino Acids
Like many of the techniques used in crime labs and crime scenes, ninhydrin (also known as 1,2,3-indantrione monohydrate) was not specifically used for developing latent prints. The first mention of ninhydrin in chemical literature was in 1910. Ninhydrin was a chemical used to detect ammonia or primary and secondary amines (amino acids). When reacting with these free amines, a deep blue or purple color known as Ruhemann's purple is produced. In 1955 a British patent was issued covering its use in developing latent fingerprints.
Several very effective formulations appeared since the patent was issued. Most were highly flammable and some quite hazardous from a health standpoint. Today one of the most popular formulas uses the 3M™ solvent—NOVEC™ Engineered Fluid, which is far down the hazardous scale.
This reagent is packaged commercially in both aerosol and pump spray containers. Many crime labs mix their own solutions and use tray development .Regardless of the application method, both sides of the document or object is saturated with the solution, and then it is allowed to air dry. Development may be accelerated by the careful application of live steam from a steam iron. Development without acceleration of any kind can take 24 hours or longer at room temperature.
Ninhydrin produces a purple color on most porous surfaces. Several factors, including the amount of amino acids that were contained in the latent print, influence the clarity of the resulting developed latent print. Quite often the visible ridges may be fragmented, while some print donors produce a brilliantly colored print with excellent ridge detail.
Ninhydrin prints may be enhanced by the addition of a zinc chloride treatment. This will produce fluorescence under UV or blue alternate light sources.
In many cases, lab technicians will process porous surfaces with iodine (PrinTips-02) fuming followed by DFO (PrinTios-03). If these processes fail to produce usable latents, then ninhydrin development should be used as a follow up.
Regardless of the chemical process used, good practice is to secure the Material Safety Data Sheets (MSDS) for the product. Here you will find a list of potential health hazards, first aid treatment and the precautions needed to safely use the product.
Learn more from the Technical Bulletin: “Overview of Latent Print Development Techniques”
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