Ninhydrin can reveal the criminal, can you?

What is it?

Ninhydrin is a chemical used to detect ammonia or primary and secondary amines. When reacting with these free amines, a deep blue or purple color known as Ruhemann's purple is produced

Uses
Ninhydrin can be used in analysis of solution that contain amino acids or ammonium ions.

• Most of the amino acids (except proline) are hydrolyzed and react with ninhydrin, producing a deep purple color known as Ruhemann's purple.
• In protein analysis thin layer chromatography (TLC) and ninhydrin are combined to separate and detect the protein's amino acids.
• A solution suspected of containing the ammonium ion, can be tested by dotting it onto a solid support (silica gel) and treated with ninhydrin, a purple color can confirm the existence of ammonium ions in the solution.
• A ninhydrin solution is commonly used by forensic investigators in the analysis of latent fingerprints, on porous surfaces such as paper.

Properties

It is a white solid which is soluble in ethanol and acetone at room temperature. Ninhydrin can be considered as the hydrate of indane-1,2,3-trione.

IUPAC name: 2,2-dihydroxyindane-1,3-dione.
Chemical Formula: C9H6O4
Molar Mass: 178.14 g/mol
Density: 0,862 gr/cm³
Melting Point: 250 °C
Solubility in water: 20 g/L

Discovery
Ninhydrin was discovered in 1910 by the German-English chemist Siegfried Ruhemann (1859–1943). In the same year, Ruhemann observed ninhydrin's reaction with amino acids.

After his death, Swedish investigators Oden and von Hofsten proposed that ninhydrin could be used to develop latent fingerprints.

Ninhydrin in crime scenes

Perhaps the most productive and cost-effective method of developing latent fingerprints on paper is treatment with Ninhydrin, something very usefull in a crime scene. 

Problem in use

The problem with spraying Ninhydrin solutions is that 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. This potentially dangerous exposure is minimized by dipping or painting.

Nitrile gloves are recommended as they are more resistant to chemicals, thus offering better protection, goggles, breathing mask, and a lab coat should be worn whenever working with Ninhydrin solutions.

Process
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 disadvantage of these solvents is that they dissolve or run most inks, so If you examine a document for fingerprints you should save a copy.

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.

For faster results

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, without touching the paper.

Additional it would be good to work in a dark place with room temperature (25 °C).

References: Wikipedia.com ~ Organic Chemistry by David Klein ~ Crimeandclues.com