Forensic Chemistry: The Revelation of Latent Fingerprints

Forensic Chemistry: The Revelation of Latent Fingerprints

J. Brent Friesen *

Physical Sciences Department, Rosary College of Arts and Sciences, Dominican University, River Forest, Illinois 60305, United States

J. Chem. Educ., 2015, 92 (3), pp 497–504

DOI: 10.1021/ed400597u

Publication Date (Web): October 8, 2014

Copyright © 2014 The American Chemical Society and Division of Chemical Education, Inc.

ABSTRACT: http://pubs.acs.org/doi/abs/10.1021/ed400597u

This article seeks to fill in some gaps on information regarding chemical reactions and interactions taking place in the visualization techniques used to reveal fingerprints.

HISTORICAL

·         At the end of each finger is the volar pad consisting of skin ridges designed for gripping.  These skin ridges create a unique pattern for each person that allows matching the fingerprint with its origin.

·         Latent fingerprints are formed when the volar pad touches a surface and leaves a transparent thin layer of chemical residue on it in the distinct patter of the skin ridges.

·         Fingerprint analysis has been used in crime investigations for over 100 years.

·         Fingerprint analysis continues to be an important process despite the development of DNA analysis.  Fingerprints can distinguish between two people who have identical DNA.

·         The chemical methods used to visualize fingerprints fall into two categories:

o   Those that involve chemical reactions

o   Those which are based on intermolecular forces to create adhesion

·         Difficulties arise from

o   The quality of the surface

o   The quantity of the fingerprint residue

CHEMICAL COMPOSITION OF FINGERPRINT RESIDUE

·         Chemicals left behind as fingerprint residue can originate from exogenous and endogenous sources.

·         Exogenous sources:

·         Chemicals from anything handled by the fingers

·         Personal hygiene chemicals (commonly found in latent fingerprints)

·         Endogenous sources:

·         Sweat and/or body oil from eccrine, apocrine, apoeccrine, and sebaceous glands.  Each of these glands may have slightly different chemical compositions.

·         Eccrine glands general secrete “classic sweat, an aqueous solution of electrolytes and hydrophilic compounds such as urea”.

·         Other three glands general secrete “lipophilic fatty and waxy substances such as squalene and cholesterol”.

·         Almost all families of organic functional groups have been detected in fingerprints and/or sweat: alcohols, phenols, aldehydes, ketones, esters, carboxylic acids, amines, amides, and amino acids

·         Common endogenous components of fingerprints can be detected by GC-MS (e.g. squalene, cholesterol, and fatty acids) except for a few like inorganic and organometallic salts.  Amino acids are easily detected with ninhydrin but not GC-MS without prior derivatization.

·         The specific chemical composition of fingerprints may be a unique characteristic for each person as well.

·         Fingerprints likely change as a person ages.

CHEMICAL REACTIONS IN THE REVELATION OF LATENT FINGERPRINTS

REACTIONS OF FREE AMINO ACIDS: NINHYDRIN AND ITS MIMICS

·         Figure 1 shows a possible 4-step mechanism for the reaction of ninhydrin with amino acids to produce a visible product. The final product is the chromophore diketohydrindylidenediketohydrindamine or “Ruhemann’s purple (see Figure 1 for structure).

·         Chemical revelation of choice when trying to visualize prints on paper.

·         Dansyl chloride and Lawsone are two other reagents that have been developed to react with amino acids to produce a colored and/or fluorescent compound.  The highly fluorescent adducts can be more easily contrasted from the background than ninhydrin-treated fingerprints.  See Figure 2 for structures of these 2 and other ninhydrin-related compounds.

SILVER NITRATE

·         Has been used in law enforcement since the 1930’s.

·         In this method, an aqueous solution of silver nitrate is sprayed on the surface, left to dry, and then exposed to sun or UV light until dark ridges appear.

·         The series of reactions involves precipitation of silver chloride when the silver reacts with chloride ions in the fingerprint.  A disproportionation reaction in which both silver and chlorine are converted to their elemental forms occurs under UV light.

·         See paper for reaction equations.

PHYSICAL DEVELOPER

·         The solution contains, Ag+, Fe2+, and Fe3+ ions and citrate.

·         In one reaction, Ag+ + Fe2+ à Ag(s) + Fe3+. The Fe3+ and citrate stabilize the solution by suppressing the spontaneous redox reaction above.

·         In an alternate reaction, silver nanoparticles attached to an organic anion are formed.  These negatively charged silver nanoparticles are attracted to the positive charge of the latent fingerprint residue.  N-dodecylamine acetate is a cationic surfactant that prevents the attachment of the silver nanoparticles to the fingerprint surface thus allowing the nanoparticles to selectively attach only to the fingerprint residue. The presence of hypochlorite ion in bleach creates a darker print from the formation of silver oxide through the following reaction: OCl- + 2 Ag(s) àAg2O + Cl-.

GUN BLUE

·         This method was developed in 1995 to reveal fingerprints on brass shell casings.

·         Brass is an alloy of copper and zinc.  Gun blue is a solution of copper (II) sulfate and selenous acid.  The brass shell casing is immersed in this solution.

·         In the presence of these chemicals, both Cu and Se are produced by the following redox reactions:

·        

·         The metallic copper and selenium form on shell casing except on surfaces that have fatty fingerprint residues.

CHEMICAL REACTION AND PHYSICAL ADHERENCE: CYANOACRYLATE ESTERS

·         This method was discovered by a law enforcement agency in Japan. 

·         The fingerprint is revealed in this method by exposing the surface to cyanoacrylate ester fumes for a period of time until hardened, tan-colored fingerprint images form.  Methyl, ethyl, and n-butyl cyanoacrylate esters are the most commonly used.

·         Superglue is a cyanoacrylate ester discovered by Harry Coover in 1942.

·         Figure 3 in the article shows the reaction mechanism for the polymerization chain growth of cyanoacrylate ester using lactate anion for initiation.  Higher pH increases the amount of accumulated superglue polymer.

·         In an alternate mechanism scenario, a competing theory is that clumps of superglue form during the fuming process which then sticks to the oily fingerprint residue.

PHYSICAL ADHERENCE: INORGANICS

Iodine fuming:

·         Oldest form of fingerprint revelation best used for paper or cardboard surface.  The fingerprint residue is exposed to iodine fumes in a sealed chamber. A yellow-brown color forms which fades quickly once the print is removed from the fumes.  Spraying with starch enhances the color and preserves it with a blue-black color.  (Rigorous studies indicate that the formation of the I5- trapped in the amylose helix structure gives rise to this blue-black color.)

Ruthenium tetroxide (RuO4 or RTX)

·         A dark-colored ruthenium dioxide is formed when fingerprints are exposed to RuO4 fumes.  The mechanism is unknown.  See article for a description of the synthesis of water-insoluble RuO4.

Dyes:

·         Sudan black is a lysochrome azo dye especially useful for revealing fingerprints on wetted surfaces and waxy paper.  The object is immersed in a methanol solution of Sudan black that washes off the non-fingerprinted surface leaving the fingerprint image behind. a lysochrome is a dye used for staining lipids.  See Figure 4 for the structure of Sudan black.

·         Oil red O is a lipophilic dye already used for biological staining.  It works well on wet porous surfaces of paper or cardboard.  The dye is dissolved in methanol made basic with sodium hydroxide.  After the stain has developed, it is made neutral using a buffer solution.

·         Fluorescent dyes like Rhodamine adhere to the lipophilic surface of a fingerprint.  This adherence method requires a special light source for excitation of the fluorophores and/or photographing the fluorescent emission light.  An advantage is the ability to selectively enhance ridge details.  See paper and Figure 5 for other examples and structures of fluorescent dyes used for fingerprint revelation.

·         Gentian violet (crystal violet) is useful for revealing fingerprints on the stick side of adhesive tape.  Crystal violet is also used to fix and stain certain bacterial cells.

Powders:

·         Commercial powders are available but commonplace talcum powder and charcoal dust powder can be used to reveal fingerprints.  The powder must stick only to the fingerprint residue and not the surface around it.  These powders are categorized according to their appearance as metallic, photoluminescent, and regular:

o   Regular – finely divided polymeric resin with a colorant

o   Metal – metallic oxides

o   Photoluminescent – fluorescent or phosphorescent organic dyes or ground up fish scales

·         Sticky-side wet powders (dry powders suspended in an aqueous solution containing an anionic surfactant) are useful for revealing prints on the sticky side of adhesive tape, bonding through noncovalent interactions. These sticky side powders are commonly titanium oxide based with traces of aluminum and silicon; the role of the anionic surfactant is not clear (could be for better wetting ability or to facilitate powder bonding).  Figure 6 gives the structures of examples of surfactants.

Small particle reagent:

·         This method is useful for wet surfaces.  The small particle reagent is a suspension of molybdenum disulfide in a surfactant solution.  The powder adheres to the fatty fingerprint residue insoluble in the wet aqueous environment.  Other powders used are titanium dioxide, zinc oxide, magnetite (Fe3O4), graphite, or zinc carbonate.

Vacuum metal deposition

·         In this rather expensive but popular technique, evaporated gold is deposited on the fingerprint. Ridges and troughs are resolved by different gold nanoparticle formation. The image is enhanced by exposure to evaporated zinc resulting in a negative print that stands out better upon photography.  This method works well on hard-to-print surfaces and has potential for smooth surfaces like polyethylene.

ANTIGEN – ANTIBODY INTERACTIONS: IMMUNOASSAY BASED TECHNIQUES

In this newly developed method, certain substances left on the fingerprint (drugs, etc) are targeted for detection by antibodies attached to gold nanoparticles.