Chemistry and fingerprints

A variety of chemical reagents may be used to develop or enhance latent or partially visible prints. It is also possible to use various dyes or powders to make the prints visible to the unaided eye or to set the stage for producing fluorescence under certain wavelengths of light with lasers or forensic light sources. Because the chemical properties of latent fingerprints, if any, are unknown, the forensic fingerprint examiner must select the appropriate development method based on the substrate, the material on which a latent print may be deposited.

For instance, a number of reagents are specifically designed for use in developing prints on the adhesive side of tape. This is a very useful technology since criminals who immobilize victims often use tape, particularly duct tape. It is extremely difficult to get tape off a roll without touching the adhesive side, so the chances of finding the perpetrator's prints on the adhesive side of the tape are fairly good. Two common methods use gentian violet and sticky-side powder.

Gentian violet is a biological stain that is used to visualize latent prints on adhesive surfaces such as duct tape. The solution containing gentian violet is applied by spraying or dipping. It is a toxic substance and an irritant and as such must be handled with care and in a laboratory environment.

Sticky-side powder is a product specifically designed for use in developing latent prints on the adhesive side of tape. It is painted onto the adhesive side with a brush, allowed to remain there for 10 to 20 seconds, and then rinsed with water. A gray-black or white print will be developed. The product is nontoxic, and the surface can be reprocessed to further enhance the print. The development results from chemical adhesion of the powder particles to the fatty or oily deposits in the latent print.

Generally, however, the first step in latent print processing is to determine whether the substrate is porous or nonporous. The examiner can determine this merely by imagining what a drop of water would do if placed on the surface. If the surface would absorb water, the material is porous; if not, it is nonporous. For nonporous media powder (carbon black or iron filings), superglue, or a combination of powder and superglue will probably work best. For porous media the best chance for developing any latent prints that may be present involves sequential processing. It is also important to ascertain whether the prints were wet or dry when deposited. If the prints are the result of blood deposition, specialized development is required.

Treatments for substrates other than tape include small particle reagent and Sudan Black B. Small particle reagent is a suspension of molybdenum disulfide particles in water. This solution works as a kind of a liquid fingerprint powder by adhering to the fatty portion of the latent print residue. Small particle reagent works best on surfaces that are wet or have been wet. The particles form a gray coating on the fingerprint. The small particle reagent is applied by dipping or spraying. It is cheap, nontoxic, and easy to use.

Sudan Black B is a dye that stains the fatty components of latent prints to produce blue-black images. It can also be used to enhance superglue-developed images. Articles are treated by immersing them into the solution. After immersion the article is rinsed with water to remove excess dye. Sudan Black B is also cheap and nontoxic and can be used on contaminated surfaces.

The general processing of porous objects found at the crime scene involves a number of different methodologies. Whether the substrate being examined is paper, wood, leather, or some other porous material, a particular sequence must be followed for best results. This sequence begins with iodine fuming, followed by ninhydrin (alternatively, amido black or tetramethylbenzidine [TMB]), and then silver nitrate or physical developer.

Iodine crystals have the ability to go from the solid state directly into the gaseous state without ever becoming a liquid. This process

Physical Developer Fingerprints
Print developed with iodine fumes (Courtesy of the author)

is known as sublimation. Iodine reacts with starch to produce a dark brown deposit. Fingerprint examiners started taking advantage of these two properties many years ago when they developed the technique of iodine fuming of porous materials such as paper. Iodine crystals are placed into a glass tube with plugs of glass wool on either end. A piece of rubber tubing is connected to one end to blow into. By holding the tube firmly in the hand during the process the examiner transfers sufficient heat to the iodine crystals to cause sublimation. When the iodine fumes contact the paper, they penetrate it deeply. If starch from a latent print is present, it reacts to produce a brown residue. This is not a permanent effect, so it must be photographed immediately. Iodine fuming is a useful technique for developing old fingerprints that have "soaked into" the paper over time. A fingerprint developed with iodine is shown in the photograph.

Ninhydrin is a protein dye that may be applied after iodine fuming has been attempted. Latent prints frequently contain amino acids. Nin-hydrin reacts with amino acids and proteins to produce a deep violet color. (If one happens to get some of the ninhydrin solution on their

Ninhydrin Reaction
Print developed with ninhydrin (Courtesy of the author)

hands, they will have purple stains for several days.) The ninhydrin is usually applied as a solution in alcohol or petroleum ether. It is sprayed on, or the item of interest is immersed in the solution. The use of steam or heat speeds up the development process, which can sometimes take overnight. The prints must be photographed to preserve the results. Ninhydrin will also enhance prints in blood since protein will be present. Ninhydrin must be handled with care although it is not highly toxic. A print developed with ninhydrin is shown in the photograph.

Two other chemical reagents that are also effective in the enhancement of bloody prints are amido black and TMB. Like ninhydrin, amido black is a protein stain. TMB, on the other hand, reacts with the hemoglobin molecule in the blood to produce a colored product.

Silver nitrate reacts with salt to produce a dark colored residue. Since salt is often part of the latent print residue, this reaction allows yet another way of development or enhancement of prints. It must follow ninhydrin if sequential enhancements will be applied. If salt is present in the substrate, the entire object will darken. Silver nitrate reacts with light to darken also. In fact, silver nitrate is the principal reactive material in

General Enhancement Methods

General Enhancement Methods

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© Infobase Publishing

General approach for enhancing latent fingerprints black-and-white film, in which the brightest areas show up darkest on the film (hence the term negative).

The last reagent in the sequence is physical developer. Physical developer often replaces silver nitrate in the sequential processing scheme since it is silver nitrate based and thus reacts with salts. Use of physical developer is the only technique to show adequate results on paper items that have gotten wet. It also can be used to detect footwear impressions on paper items. Some of the various approaches to developing latent prints are illustrated in the accompanying flow charts.

The use of chemical properties to develop latent prints has its limits. Developing fingerprints on bodies, for example, is a topic that always generates tremendous interest. Strangulation and other crimes involving physical contact between the victim and the perpetrator raise the question of whether identifiable fingerprints can be located on human tissue. Different methods, such as dusting with powder and using iodine fuming, have been tried over the years. Although success rates remain very low, the current method of choice is superglue fuming followed by dusting with magnetic powder that has been stained with Rhodamine. A laser or other light source is then used to visualize the print. Hairless areas of the body are more likely to yield identifiable prints.

Rhodamine is a stain that is used primarily to enhance the luminescence of latent prints that have been treated with the cyanoacrylate (superglue-fuming) procedure. Rhodamine 6G is very useful for this purpose because it produces fluorescence in the presence of lasers and forensic light sources. This fluorescence aids in the visualization, especially when background interference is a problem. A spray or rinse bottle is filled with methyl alcohol or distilled water and a

Bloody Fingerprint Enhancement

Ninhydrin

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G Infobase Publishing

Enhancing bloody fingerprints

Nonporous Surface Sequence

Nonporous Surface Sequence

Ninhydrin Wavelength
Developing fingerprints on nonporous substrates

small amount of Rhodamine 6G added. The Rhodamine solution is then applied to the surface of interest. When light of the appropriate wavelength (450-525 nanometers) is introduced, areas that have absorbed the dye will fluoresce. Typically fingerprint residue that has been "coated" with superglue will absorb more dye than the surrounding substrate and result in enhanced visualization. Rhodamine 6G is thought to be relatively safe in small amounts. It must never be ingested or inhaled, however.

Development Sequence for Adhesive Tape

Development Sequence for Adhesive Tape

Superglue Enhanced Fingerprint
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Developing fingerprints on adhesive tape

Developing latent fingerprints on porous surfaces

Porous Surface Sequence

Porous Surface Sequence

Fingerprint Ridge Characteristics
© Infobase Publishing

The fingerprint examiner has no way of knowing which of a variety of possible fingerprint residue components may be present and in what amount. The availability of numerous fingerprint development techniques increases the chance that the examiner will isolate the latent prints.

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Responses

  • MARCO
    What reagents can be used for the sticky side of tape?
    7 years ago
  • agenore
    Is fingerprint residue toxic?
    7 years ago
  • lena
    Can one lift fingerprints from paper over 50 years old with a physical developer reagent?
    7 years ago
  • Brigitte
    How to make small particle regeant fingerprint solution for wet material?
    7 years ago
  • paula
    CAN Fingerprint dusting powder BE DANGEROUS IF INGESTED?
    6 years ago
  • MARIE
    How is chemical and powders useful in latent prints?
    6 years ago
  • nora
    Why paper is the best porous material that can be located or identified at a crime scene?
    6 years ago
  • elizabeth
    How to make wet wop solution at home for latent prints?
    6 years ago
  • maria schwarz
    What are the reagents used in fingerprints?
    5 years ago
  • protasio
    Why is paper not a good substrate for holding latent prints?
    5 years ago
  • SINIKKA VIRRANKOSKI
    How long do latent prints that are developed using reagents such as ninhydrin last?
    5 years ago
  • LIZZIE
    How long to iodine print enhancers last on paper?
    4 years ago
  • Yorda
    Why is chemical enhancement fingerprint?
    4 years ago
  • lucas hertzog
    How chemistry applies to fingerprinting?
    4 years ago
  • jens ebersbacher
    How useful is ninhydrin in forsenci science?
    4 years ago
  • roman johnstone
    Why are the powder and chemical in fingerprinting dangerous?
    4 years ago
  • Fethawit
    How is fingerpints us for forensic science?
    4 years ago
  • fre-swera selam
    How chemistry and fingerprints are related?
    4 years ago
  • yorda
    How our chemicals used in forensic scientists blood and fingerprinting?
    3 years ago
  • maksymilian
    How chemistry involves the use of fingerprints?
    3 years ago
  • TONI
    Can you use fingerprint powder after rhodamine 6G?
    3 years ago
  • melanie
    How to darken ninhydrin prints?
    3 years ago
  • Abrha Alem
    What science involves fingerprints?
    2 years ago
  • Sophie Ostermann
    What type of crimes can you solve using gentian violet stains?
    2 years ago
  • aatos
    Why is fingerprint detection related to chemistry?
    2 years ago
  • debra
    How to classify the chemicals in fingerprints?
    1 year ago
  • lyla
    What are chemical reagents?
    8 months ago
  • HABTE
    How does the chemistry of fingerprinting work?
    3 months ago
  • jennifer
    Which chemical developer is not permanent, so it must be photographed.?
    2 months ago
  • lotta
    What chemicals conceals you fingerprint?
    1 month ago
  • Mariano
    Is fingerprint proccessing chemistry?
    4 days ago

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