<P> Developing agents depend on the presence of organic materials or inorganic salts for their effectiveness, although the water deposited may also take a key role . Fingerprints are typically formed from the aqueous - based secretions of the eccrine glands of the fingers and palms with additional material from sebaceous glands primarily from the forehead . This latter contamination results from the common human behaviors of touching the face and hair . The resulting latent fingerprints consist usually of a substantial proportion of water with small traces of amino acids and chlorides mixed with a fatty, sebaceous component which contains a number of fatty acids and triglycerides . Detection of a small proportion of reactive organic substances such as urea and amino acids is far from easy . </P> <P> Fingerprints at a crime scene may be detected by simple powders, or by chemicals applied in situ . More complex techniques, usually involving chemicals, can be applied in specialist laboratories to appropriate articles removed from a crime scene . With advances in these more sophisticated techniques, some of the more advanced crime scene investigation services from around the world were, as of 2010, reporting that 50% or more of the fingerprints recovered from a crime scene had been identified as a result of laboratory - based techniques . </P> <P> Although there are hundreds of reported techniques for fingerprint detection, many of these are only of academic interest and there are only around 20 really effective methods which are currently in use in the more advanced fingerprint laboratories around the world . </P> <P> Some of these techniques, such as ninhydrin, diazafluorenone and vacuum metal deposition, show great sensitivity and are used operationally . Some fingerprint reagents are specific, for example ninhydrin or diazafluorenone reacting with amino acids . Others such as ethyl cyanoacrylate polymerisation, work apparently by water - based catalysis and polymer growth . Vacuum metal deposition using gold and zinc has been shown to be non-specific, but can detect fat layers as thin as one molecule . </P>

When is a fingerprint formed on the fingertip