A fingerprint is the unique impression left by the friction ridges on the fingers, palms, or soles of the feet. Impressions are categorized as patent or latent. Patent prints are visible, often formed by the transfer of substances like ink or grease. Latent prints are invisible, created by the natural transfer of body residues from the skin onto a surface. This residue is a complex chemical mixture of organic and inorganic compounds.
The Biological Origin of Latent Prints
The chemical composition of a latent print is directly linked to the glands embedded in the skin. The skin on the palms and soles is rich in eccrine glands, which produce sweat. Eccrine sweat is mostly water, containing dissolved salts and small organic molecules. Since fingers constantly secrete this sweat, eccrine residue is a universal component of all latent prints.
The second major source of residue comes from the sebaceous glands, which produce an oily substance called sebum. Although sebaceous glands are not on the palms and soles, their secretions are easily transferred to the hands when a person touches other parts of their body. This transfer mixes the water-soluble eccrine sweat with the oily sebum, creating the full chemical profile found in a latent print.
The Three Primary Molecular Classes
The latent fingerprint residue is composed of three primary classes of molecules, each targeted by different forensic techniques. The first class consists of inorganic salts and water, primarily derived from eccrine sweat. Water evaporates quickly, leaving behind dissolved salts, mainly sodium chloride and potassium chloride, in the pattern of the friction ridges.
The second class is made up of water-soluble organic non-lipid compounds, also secreted by the eccrine glands. This category includes small biological molecules like amino acids, urea, uric acid, and lactic acid. Amino acids are of particular interest because they are relatively stable and react predictably with certain chemical developers. These organic compounds are essential for visualizing older prints on porous surfaces like paper.
The third molecular class comprises lipids and fatty materials, which are hydrophobic and come mostly from sebaceous secretions. This group includes complex organic molecules such as triglycerides, free fatty acids, waxes, cholesterol, and squalene. These oils do not evaporate quickly and are responsible for the print’s ability to adhere to non-porous surfaces, such as glass or plastic.
How Chemical Knowledge Aids Detection
Forensic science exploits the unique chemical properties of these three molecular classes to visualize the print. Visualization techniques are chosen based on the surface type and the expected chemical composition of the residue.
The chemical reagent Ninhydrin is a classic targeted approach, reacting with amino acids (Class 2) to produce a distinct purple-blue compound known as Ruhemann’s purple. A related chemical, 1,8-Diazafluoren-9-one (DFO), also targets amino acids but yields a fluorescent product under an alternate light source.
Lipid and fatty materials (Class 3) are targeted by physical methods like powder dusting, where fine particles adhere mechanically to the oily residue. Dyes such as Sudan Black are also used to stain the lipids directly on non-porous surfaces. For the inorganic salts (Class 1), Silver Nitrate reacts with chloride ions to form silver chloride, which darkens when exposed to light, revealing the ridge pattern.