Fingerprints are made from sweat. Within the sweat are a number of chemical compounds:

• Inorganic ions (Na⁺, Cl^-)
• Proteins, amino acids
• Lipids
• Water

While the water and other components in the fingerprint will evaporate or degrade rather quickly, it is the lipids (fats and oils) and amino acids that allow fingerprint residues to last long enough for collection.

Lipids

Lipids are also called fats or oils. Lipids represent one of the four major classes of biological molecules. Lipid molecules are all based on the presence of fatty acids, which are long carbon tails (16-20+ Carbons long) with a carboxylic acid (-COOH) group at one end.

The carbon tails may be fully saturated (no double bonds) or may be mono- or poly-unsaturated (containing one or more carbon-carbon double bonds). When combined with glycerol (a 3-carbon alcohol), the resulting compounds are called triglycerides.

This is the form in which most lipids are transported in the blood. Phospholipids are modified triglycerides, in which one of the fatty acid groups is replaced with a phosphate group (PO₄^3-), making a polar head group.

Because of the lipid content in fingerprints, many of the processes used to "raise" the prints are based on lipid reactivity. One such process is Iodine Fuming.

Forensic Fingerprinting I: Iodine Fuming
TheHomeScientist (YouTube)

Iodine (I) is an element in Group 7 in the periodic table and therefore a Halogen. Iodine is a solid at room temperature but sublimes (direct solid to gas phase change) to form a gas at relatively low temperatures. As a technique Iodine Fuming is one of the simplest methods only requiring a hood or isolated area with good ventilation as the fumes are toxic and a heat source to sublime the iodine. The fumes when passed over the material containing the latent prints with slowly turn a reddish brown color as the Iodine binds to the lipids in the print.

Prints Raised with Iodine

The reaction between iodine and the lipid components of fingerprints was originally thought to be chemical in nature with iodine reacting to double bonds located on the fatty acids but more recently the thinking has become that the iodine fumes simply become physically stuck to the lipids. This thinking is supported by the temporary nature of the iodine fixation. Prints raised by iodine fuming should be photographed immediately as the iodine releases over time and the print "fades".

Amino Acids

Amino acids are the building blocks of proteins, the biomolecules that make up the muscles and enzymes in our bodies, and as such, serve as one of the more important biochemical molecules. There are 20 common amino acids, all of which contain the same core structure:

The 20 Amino Acids

The identification and characterization of these molecules can be done numerous ways depending upon what you are trying to prove. One such method to determine the presence of amino acids within a fingerprint is accomplished with Ninhydrin.

The steps necessary for the colorimetric identification of amino acids in a sample is quite complex, but the overall reaction can be generalized as follows:

α-amino acid + 2 ninhydrin → CO₂ + 3 H₂O + aldehyde + final complex (Deep Purple)

In summary, ninhydrin, which is originally yellow, reacts with the amino acid and turns deep purple, and it is this purple color that is detected within this method. Despite being successful for indicating the presence of amino acids, the ninhydrin technique cannot uniquely identify individual amino acids.

Forensic Fingerprinting II: Ninhydrin
TheHomeScientist (YouTube)