Tag Archives: fingerprint

Ninhydrin Starring in CSI!



Every family has their own special traditions. For my mummy and I, we enjoy curling up on the couch on a Friday evening to watch marathons of our favourite TV-shows. One such show is CSI, specifically CSI Miami. I love Horatio’s character the most! He’s just so badass.


If you’re an avid CSI fan such as me, you may have noticed that in the field of forensics, fingerprint work makes an appearance in every episode. Have you ever wondered what they use in forensics to develop fingerprints?






Ninhydrin has become the most common method used to reveal prints on porous surfaces. Nearly all forensics labs use ninhydrin for this purpose, and some seldom use anything other than ninhydrin. Ninhydrin is cheap, sensitive, and commercially available in disposable spray cans. The developed prints are a high-contrast purple that’s readily visible on most paper backgrounds.



Ok, so how does the ninhydrin work exactly?

Dusting the crime scene is the commonest, simplest and oldest latent print developing technique. A white powder composed of the chemical ninhydrin is used to develop latent prints (prints invisible to the eye). Latent prints are formed by sweat, either from the hands themselves or by unconscious contact between the fingers and the face or other parts of the body. Even the swiftest of criminals find it difficult to escape without leaving behind the trace of a single fingerprint. The traces of amino acids present in perspiration bind with the ninhydrin and the prints begin to appear in about an hour. In the pH range of 4-8, all α- amino acids react with ninhydrin; a powerful oxidizing agent to give a purple colored product (diketohydrin) termed Rhuemann’s purple.




Ninhydrin degrades amino acids into aldehydes, ammonia, and CO2 through a series of reactions; the net result is ninhydrin in a partially reduced form hydrindantin. Ninhydrin then condenses with ammonia and hydrindantin to produce an intensely blue or purple pigment, sometimes called Ruhemann’s purple.

ninhydrin reaction

In our last lecture, we made a clear distinction between the two tests : Biuret test and Ninhydrin test. Mr. Matthew (my lecturer) made a video that clearly differentiates between the two tests in a clear understandable manner so if you are having trouble understanding their differences, I urge you to watch the vid! It’s really helpful and not too long.

Apparently students mix up their uses all the time, and if my lecturer didn’t stress on this I’m 99.9% sure I would’ve been one of those careless students as well.The Biuret test is used to test for proteins while the Ninhydrin test is used to test for amino acids.  Don’t forget!!!!

Mr. Matthew also posed a question to us. Do all amino acids give that lovely purple colour when the ninhydrin test is performed? From his tone I knew there had to be some exception.

Here’s what I found:

The color produced when the ninhydrin test is performed varies slightly from amino acid to amino acid, probably because the unreacted acids complex with the pigment.

Proline and hydroxyproline give a yellow color.  Proline has aliphatic side chains with a distinctive cyclic sturcture. The secondary amino (imino) group of proline  residues is held in a rigid conformation that reduces the structural flexibility of polypeptide regions containing proline. Proline does not give the ninhydrin reaction as this reagent requires free alpha amino group (-NH2) but proline has an imino group (-NH).  For the amino acids which have a free -NH2 (amino) group, ninnydrin test is positive but is negative for proline because it only has -NH (imino) group.

I hope this post was helpful! Can you find any more amino acids that don’t give the purple colour when the ninhydrin test is performed?