Ninhydrin and silver nitrate
As all of us know that latent fingerprints are developed by using chemical methods.
One of the technique is Ninhydrin method .
Development of latent fingerprints by Ninhydrin - This technique developed by "Ruhemann" in 1911
and the reaction color of this technique referred to his name "Ruhemanns purple" .
The mechanism - Our fingerprints have sweat residues and amino acids are a part of
it. Ninhydrin chemically react with amino acids and resulted in development of
latent fingerprints.
Procedure - For development of fingerprints you will have to mix an organic solvent
with ninhydrin and followed by adding supplements like acetic acid. You can use
organic solvent like acetone(usually) , rubbing alcohol (ethanol or isopropanol) ,
petroleum ether etc. This mixture is sprayed by sprayer or dip the exhibit in to the
solution. The exhibit is then left to dry. The development of fingerprints is slow on
room temperature however after applying the solution some stains appear but
complete development takes 1 to 2 days. We accelerated the process by increasing
temperature and humidity. For this we just use steam iron , the exhibit (paper) placed
between two paper towels and steamed. Prints were developed on paper. You can
also heated the exhibit in a cabinet at a temperature of 80 to 120 degree centigrade
for 15 to 20 minutes. A pink to purple color print appeared. Companies also provide
Customize formulation of ninhydrin in disposable spray cans.
Ninhydrin is sensitive and its very cheap but it has toxic effects. It developed fingerprints on porous surfaces like paper , newspaper , card boards etc and on non porous surfaces (glass , plastic etc). Ninhydrin treatment is best suited for latent fingerprints visibility especially who are older one.
Its always used before silver nitrate and before physical developer.
now we move to silver nitrate technique -
now we move to silver nitrate technique -
Silver Nitrate (AgNO3) is a toxic , strong oxidizer and corrosive inorganic compound used in treatment of wounds.
Procedure for development of latent fingerprint with silver nitrate ----
A 3% silver nitrate solution is used , generally 3% solution is used for latent fingerprints development. Silver nitrate solution can be applied by dipping of exhibit in it , spraying on the exhibit and by brushing.
Spray the solution on the exhibit and make sure that it completely covered the exhibit. Dry the exhibit fifteen to thirty minutes (in shadow). After drying of exhibit expose it to ultraviolet lamp or to sunlight. Complete ridges appearing from five minutes to an hour and a black color(or grey) fingerprint appeared. Avoid exposing too long as it results in staining of the exhibit (silver nitrate reduced to metallic silver). For removing the excess silver nitrate bathe the exhibit in water. Dry it and take photograph. If you want to preserve it than place it between black sheets of paper.
If no prints were found on paper and you want to restore it to original condition than dip it in a solution of mercuric chloride. It resulted in a white product and it removed by rinsing in water.
The mechanism - Sodium chloride present in latent prints react with silver nitrate and insoluble silver chloride formed. Exposing this silver chloride to sunlight reduced it to metallic silver and a black color print appeared.
Precautions - Always put gloves , wear protective clothing and put splash goggles.
The exhibit always put on paper towel or any old newspaper for protecting the surface from over spray.
Development by silver nitrate must be used after other methods as ninhydrin , iodine fuming didn't interfere with it. Due to high cost of silver nitrate , it can not make its position in forensic kits. This technique is useful on plastics , card boards , paper unvarnished wood etc.
MR. DEEPAK PAWAR has been working as Finger Print Expert / Police Officer in Finger Print Bureau , Delhi Police at New Delhi since 2001 . He is the senior most Finger Print Expert , attending / visited several scene of crime and solved many crime cases .He has passed M.Sc (Chemistry) and he is the Member of International Association of Identification . He is the Member of Delhi Police welfare association . He is supervising the Forensic , New Delhi District , Mobile Crime Team . He is one of the Expert in the Special Investigation Team held for investigation of important cases . He has been giving fingerprint proficient training to Delhi Police personnel's . He has been Presented / giving lectures on topics related to forensic science . He is also making examination paper of Finger Print proficient course . He is awarded with Police Medal by MHA and Delhi Police.