More Fluoresce-science

The key utility of using fluorescent dyes in revealing latent prints is because these dye maximize the contrast between the ridge details of the print and the surface on which the print was placed. Besides Nile Red, Rhodamine 6G, and Basic Red mentioned in the previous post, other fluorescent dyes commonly used in forensics are Basic Yellow 40, Ardrox, Safranine O, and 4-(4-methoxybenzylamino)-7-nitrobenzofurazan (MBD).  The Chesapeake Bay Division of the International Association for Identification has a comprehensive website on latent fingerprint processing techniques. All of these can be used to enhance cyanoacrylate fumed prints to help them stand out from the background. They can be incorporated into fingerprint powders or applied directly in a solution. Interestingly, there are several custom mixtures of these dyes available commercially such as “RAM” (Rhodamine 6G, Ardrox, and MBD), and“RAY” (Basic Red 28, Ardrox, and Yellow Dye 40) as seen at the Arrowhead Forensics store. Apparently, the dye mixtures can be visualized under different wavelengths to produce a variety of effects.

Fluorescing Fingermarks

Several fluorescent dyes are used for the direct detection of fingerprints and/or the enhancement of superglue fumed fingerprints. All of these dyes fall into the category of lysochrome dyes. In general, they adhere to lipophilic surfaces with Van der Waals forces. The great advantage of using fluorescent dyes is that the fingerprint ridge detail can be selectively enhanced. And thus, the surface on which the fingerprint is deposited can be completely ignored. Fluorescent compounds have an amazing diversity of structural features, the one commonality being an extended pi system. According to Wikipedia, Rhodamine dyes are commonly used for biotechnology applications such as fluorescence microscopy, flow cytometry, fluorescence correlation spectroscopy, and enzyme-linked immunosorbent assays. The disadvantage of fluorescent dyes is that specialized light sources are needed to excite the fluorophores and/or photograph the fluorescent emission light. For example, Nile Red has an excitation band of 515-530 nm and an emission band of 525-605 nm (OBCresources). Rhodamine 6G an excitation wavelength of 526 nm and an emission wavelength of 555 nm (OBCresources). A variety of alternative light sources (ALS) are sold by forensic vendors such as Sirchie Labs.     The sources can be as compact as a small flashlight or as big as a vacuum cleaner.

Suddenly Sudan

Oil Red O, the subject of a previous post, is just one of several fat soluble (lysochrome) dyes that are used in fingerprint revelation. Sudan Black is another popular lysochrome dye which is especially useful in developing prints on waxy paper and wetted surfaces. Apparently, Sudan Black refers to Sudan Black B, a member of the Sudan family of azo dyes. It is typically dissolved in an alcohol and applied by immersing the object in the solution. The dye washes off the non-fingerprinted surfaces and reveals the latent print. I looked into the curious name for the dyes. I could not find the exact reason why a chemical compound would be named after an African nation. The best explanation I could come up with is that a British chemist (or manufacturer) may have thought that this was an exotic name for a new commercial chemical product. Then the chemists got to work and produced dozens of related compounds and called them Sudan I, Sudan III, Sudan IV, Sudan Black, Sudan Orange G, Sudan Red 7B, Sudan II Blue, etc… Rather recently the Sudan dyes have been banned from food products due to their possible carcinogenic properties. The BBC reported that the country of Sudan was not amused by this turn of events: “Sudan outraged at namesake dye.”

As Good as Gold

Vacuum metal deposition (VMD) is a technique that has found some popularity despite its rather great expense. Initially, a sample of gold metal is evaporated in a chamber containing the object to be fingerprinted. The gold atoms are deposited on the latent fingerprint residues preferentially to the surface on which the fingerprint lies. This is somewhat reminiscent of the I₂ vapor method. Understandably, only enough gold is used to form a thin layer of adhered metal. The gold deposition is followed by evaporating zinc metal under the same conditions. The zinc binds to the gold and enhances the print to the point where it can be photographed. A recent article in Forensic Science International entitled “Visualisation of fingermarks and grab impressions on fabrics. Part 1: Gold/zinc vacuum metal deposition” studies the efficacy of VMD on white cotton, nylon, polyester, polycotton fabrics. The method worked well on these hard-to-print surfaces and has other potential applications to smooth surfaces such as polyethylene. Amazingly, this technique has been in use since 1976 in the U.K.

http://scienceillustrated.com.au/blog/science/fingerprints-can-now-be-taken-from-fabric/

Oil Red Oh

A hydrophilic dye called Oil red O has attracted some attention recently as a good compound to reveal latent fingerprints on difficult surfaces. Alexandre Beaudoin in a Journal of Forensic Identification article entitled “New technique for revealing latent fingerprints on wet, porous surfaces: Oil Red O” proposes that oil red O is a good alternative for the silver ion based “physical developer” in revealing lipidic latent prints on wet porous surfaces such as paper and cardboard. This has been followed up with at least two more papers by the same author exploring the virtues of this dye. This particular lysochrome has been used in biological staining for many years. The method involves dissolving oil red O in a methanol solution made basic with sodium hydroxide. After the stain has taken hold, the pH of the surface is adjusted to neutral with a buffer. This rather recent discovery shows that the science and art of latent fingerprint revelation is a field open to ongoing investigation. It also points to a interesting link between biological stains and fingerprint analysis.

A Crimefighter Called Ruth

Ruthenium Tetroxide (RuO₄) is an inorganic oxide that has found its way into fingerprint residue revealing literature. Ruthenium Tetroxide even has a racy alias: RTX. As far as I know, it is not widely used. I ran across it in a recent Patricia Cornwell novel: Book of the Dead. Subsequent to this discovery I found a handful of scientific articles that report on the use of RuO₄ fuming or direct application as a method to reveal latent prints. Apparently RuO₄ can be produced in situ by mixing aqueous RuCl₂ and ammonium cerium nitrate NH₄Ce(NO₃). The resulting tetroxide is evidently not soluble in water and is released. RTX may also be applied in a halogenated hydrocarbon solvent. Evidently, RuO₄ is attracted to the fatty fingerprint residues in much the same way as iodine and superglue fumes are. An article entitled “Latent Fingerprint Processing by the Ruthenium Tetroxide Method” in the Journal of Forensic Identification reports that RuO₄ reacts to form dark colored ruthenium dioxide in the presence of fatty fingerprint residues. An article entitled “Fingerprint recovery from human skin surfaces” in the Science & Justice Journal used RTX to develop fingerprints left on dead bodies. Ruthenium tetroxide is reportedly less toxic than its close relative Osmium tetroxide.

Underwater Fingerprint Detection

Small Particle Reagent is used to detect latent fingerprints left on wet surfaces. The reagent is made of a suspension of fine molybdenum disulfide in a surfactant solution. Apparently, the particles adhere to the water insoluble fatty residues of fingerprints. Other materials besides molybdenum disulfide can be used as well: titanium dioxide, zinc oxide, magnetite (Fe₃O₄), graphite, or zinc carbonate. It is rather like the wet version of fingerprint powders. It is unclear whether the detergent functions simply to spread out the suspension on a surface or if the detergent molecules interact with the fingerprint residue to facilitate the attraction between the particles and hydrophobic materials. The reagent is sprayed on the object and the excess material can be gently washed off. One detergent that is used is niaproof 4, an anionic surfactant with an akylsulfate group. In an article in the Journal of Scientific Research, Chulalongkorn University entitled “Latent Fingerprint Detection by Various Formulae of SPR on Wet Non-Porous Surfaces” the authors explored 11 different small particle reagent formulations  by varying the particle composition, surfactant concentration, addition of choline chloride, and pH. While molybdenum disulfide consistently gave the best images, the other factors did not seem to effect the outcome in any significant way.

http://www.evidentcrimescene.com/cata/chem/chem.html

Super Cyanoacrylate Esters

The cyanoacrylate ester fuming method of revealing fingerprints was discovered by the Criminal Identification Division of the Japanese National Police Agency in 1978. It was soon practiced all over the world. Amazingly, fingerprint residue exposed to cyanoacrylate ester fumes for brief periods of time become harden tan-colored fingerprint impressions. There are three different esters that are popularly marketed: methyl, ethyl, and n-butyl cyanoacrylate. Superglue itself was first described by Dr. Harry Coover on 1942 while working for Kodak Research Laboratories to develop a clear plastic for gunsights. Coover finally realized that cyanoacrylate was a useful adhesive in 1958 while he was working for Eastman Kodak. It is tempting to conjecture that superglue attaches itself to the surface of the fingerprint residue with its well-known gluing mechanism. However, an article in the Journal of Forensic Science entitled “A mechanistic model for the superglue fuming of latent fingerprints” suggests that clumps of superglue are absorbed into the oily fingerprint residue. This interesting article was written as a result of an undergraduate research project in which the authors describe a series of experiments to determine the affinity of superglue fumes for various substances. Long chain fatty acids absorb the cyanoacrylate fumes very well. In fact, the longer the hydrocarbon chain the faster the deposition of the superglue. A subsequent article in the same journal entitled “Understanding the chemistry of the development of latent fingerprints by superglue fuming” measured the chain growth of cyanoacrylate polymers initiated by lactate. The increase in mass of 45 microL blobs of solution was measured over time. As you might imagine increasing the pH increased the accumulation of superglue mass. These two articles carefully present competing theories to explain the chemistry of superglue fuming of latent fingerprints.

The Sticky Side of Life

Given, the importance of developing latent prints on the sticky-side of tape, several methods for revealing them. Much like the “dusting” method of latent print revelation, a series of sticky-side wet powders are available. Generally, these are dry powders which are suspended in an aqueous solution with an anionic surfactant. Very likely the forces to adhere the grains of powder to the fingerprint reside are non-covalent interactions. However, it is not clear what role the detergent plays. The surfactant may simply act as a wetting agent ant allow the powder to spread out over the surface of the tape. On the other hand, the surfactant may also facilitate the interaction between the powder and the oily fingerprint residue. A recent article in “Science and Justice” determined that the powder materials are generally titanium dioxide based with traces of aluminum and silicon. A popular surfactant solution used in commercial sticky side powders is called “Photo-Flo 200” which is an aqueous solution of propylene glycol and p-tert-octylphenoxy polyethoxyethyl alcohol (octoxynol 9). An article in the Southern California Association of Fingerprint Officers suggests that a regular fingerprint powder can be suspended a dish detergent solution to create a home-made sticky side powder. In my experience this is a quite dramatic experiment. You brush this black goo on the tape, wash it off, and presto – a nice print!

I Got This on Tape

There a several methods to reveal latent prints on the sticky side of tape. Why is this important? 1) Tape seems to play an important part in some crimes: abductions, for example. 2) It is difficult to tape something without leaving a fingerprint on the sticky side at some point. 3) It is almost impossible to handle tape with gloves. How do sticky side latents vary from other surfaces? Possibly the contact with adhesive leaves a somewhat different cocktail of fingerprint residues than contact with a smooth surface. I could not find any information on this. I can speculate that skin (epithelial) cells are more likely to left on tape than on other surfaces. Gentian (also called crystal) violet stain has the remarkable property of revealing fingerprints on the sticky side of tape. Gentian violet is a synthetic purple dye with antifungal and antibiotic properties that can be used for cleaning open wounds. In biology, solutions of crystal violet can simultaneously fix and stain certain cells such as bacteria. I am not certain of how gentian violet stains fingerprint residue. It is tempting to propose that it interacts with the DNA of skin cells. It evidently has little attraction to adhesives used in tape manufacturing. It is very easy to use – just dip the tape in an aqueous gentian violet bath for 30 seconds or so then wash off the excess dye under running water. Gentian dye will stain your hands, clothes, etc… so you have to be careful about that. http://www.bvda.com/images/b810_use.jpg