Latent Print Analysis
The Latent Prints section conducts examinations on a variety of evidence recovered from crime scenes. The term "latent" refers to a print, whether from a finger, palm or foot, that is not yet fully developed and visible. Latent print examiners use a variety of chemical and physical procedures to make unseen prints visible.
Latent prints are composed of various substances including salts and amino acids present in perspiration, which comes to the surface of the skin through pores in the palms of the hands and soles of the feet. Prints can also be transferred onto objects as the result of contact with substances such as grease, oil, paint and blood.
Though still referred to as latent prints, or latent impressions, evidence submitted to the laboratory often bears prints that are visible before any chemical or physical development. Therefore, a thorough visual examination of all evidene is conducted prior to any processing. Under ordinary light evidence is examined with a magnifier. Any visible prints are preserved photographically. Because latent prints are fragile and can evaporate, be easily wiped or washed away, any impressions of value present upon submission to the lab or developed at any stage of processing must be preserved photographically.
For nonporous surfaces, such as a mirror, exposing the item to the fumes created by heating Super Glue allows the compounds in the glue to bind with the print residue. This makes the print's ridge detail visible and also helps make it more durable. After Super Glue-fuming, other techniques to visualize latent impressions, including fluorescing dyes, fingerprint powders and lift tape, can then be employed.
For porous evidence, including paper, cardboard and untreated wood, chemical processes are required. Two common chemical processing procedures used at the Vermont Forensic Laboratory are ninhydrin and physical developer. Ninhydrin, which stains latent impressions purple, reacts with amino acids present in perspiration. Following ninhydrin processing is physical developer, a process in which silver selectively deposits onto fatty, oily residues. Impressions developed from this process are typically dark grey on a light grey background.
Because every living thing is biologically unique, and because fingerprint, palm print and footprint patterns, formed during the fetal stage of human development, remain unchanged throughout an individual's lifetime, latent prints can be used to identify individuals to the exclusion of all others. Even identical twins' prints differ from one another. Thus, it is possible to determine the identity of a person who came in contact with evidence from a crime scene. Equally important is the use of latent prints to confirm (or disprove) the identity of the deceased, which can also be an integral part of a criminal investigation.