Cyanoacrylate fuming, often recognized by its common name, superglue fuming, is a forensic technique used to visualize latent fingerprints at crime scenes. This method helps investigators develop otherwise invisible friction ridge patterns left on various surfaces, making them visible and stable for collection and further analysis.
The Chemical Makeup of Latent Fingerprints
Latent fingerprints are complex residues transferred from the friction ridges of fingers to a surface. These residues are primarily composed of secretions from the human skin’s glands. Eccrine glands, found across most of the body, contribute water, amino acids, urea, lactic acid, and various salts to the print residue.
Sebaceous glands, concentrated in areas like the face and scalp, produce an oily substance called sebum, which is also transferred to fingerprints. Sebum contains fatty acids, triglycerides, waxes, and squalene. These lipid-based components contribute to the overall composition and durability of the print. Apocrine glands, located in specific areas like the armpits, contribute proteins and lipids, adding to the diverse chemical makeup of a latent print. This mixture provides the necessary reactive sites for the fuming process.
The Cyanoacrylate Polymerization Reaction
Cyanoacrylate monomers are volatile molecules that convert into a gaseous state when heated or placed in a fuming chamber. When these gaseous monomers encounter a latent fingerprint, they undergo anionic polymerization. This reaction is initiated by compounds present within the fingerprint residue, such as water, amino acids, and fatty acids. Moisture from the fingerprint, along with ambient humidity, acts as a catalyst for this reaction.
During the polymerization, the cyanoacrylate monomers link together end-to-end, forming long chains of polycyanoacrylate. This newly formed polymer is a hard, white substance that adheres directly to the fingerprint ridges. As more layers of the polymer accumulate, the previously invisible friction ridge patterns become clearly visible as white, solidified structures. This solidification effectively “fixes” the print, making it robust for subsequent handling and analysis.
Environmental Factors Affecting Fuming
The effectiveness of cyanoacrylate fuming is influenced by environmental conditions, particularly humidity and temperature. An optimal humidity level, typically between 80% and 90%, is important because water acts as an initiator for the polymerization reaction. Too little humidity can slow or prevent the fuming process, while excessive humidity can lead to over-fuming, obscuring ridge detail.
Temperature also plays a role by affecting the volatilization rate of the cyanoacrylate monomers. Higher temperatures generally increase the rate at which the liquid cyanoacrylate converts into a gas, accelerating the fuming process. The age of a latent print also impacts fuming success, as older prints may have fewer volatile or reactive components remaining. Over time, some of the water and other reactive substances in the print can evaporate or degrade, potentially reducing the number of available initiation sites for polymerization.
Enhancing Fumed Fingerprints
Once cyanoacrylate fuming has made a latent print visible, further enhancement is often required to improve its contrast and visibility for photography or analysis. The white polycyanoacrylate polymer can be challenging to distinguish on light-colored or patterned surfaces. Forensic technicians often apply fluorescent dyes, which stain the developed polymer. Common fluorescent dyes include Ardrox, Basic Yellow 40, and Rhodamine 6G.
These dyes absorb light at specific wavelengths and then re-emit it at a longer wavelength, making the fingerprint ridges glow when viewed under an appropriate light source. This fluorescence enhances the contrast between the print and the background, allowing for clearer imaging. Powder dusting, similar to traditional fingerprint development, can also improve the visibility of the fumed print, particularly on darker surfaces.