Chloe Zhang
Plastic fingerprints are three-dimensional prints left on soft surfaces such as soap, wax, wet paint, or fresh caulk. They are classified as such due to the nature of the surface on which they are found. To lift a plastic fingerprint, analysts may use a brush, pour from a container, or use a pipette/eye dropper to apply a polymer solution. Other methods include the use of dark powder, which sticks to the prints and can then be lifted with clear adhesive tape.
| Characteristics | Values |
|---|---|
| Type of surface | Soft surfaces (soap, wax, wet paint, fresh caulk, etc.) or hard surfaces |
| Visibility | Visible prints are formed when blood, dirt, ink, paint, etc. are transferred to a surface; latent prints are invisible to the naked eye and are formed when the body's natural oils and sweat are deposited onto another surface |
| Collection methods | Dusting surfaces with powder, using alternate light sources, cyanoacrylate (superglue) fuming, AccuTrans®, Amido Black, DFO, ninhydrin reagent |
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What You'll Learn
Using powders and dyes
To lift fingerprints, you can use either a powder or a dye. Powders are often used in conjunction with tape to lift the print, whereas dyes are used to make the print visible to the naked eye or under certain light conditions.
Powders
Powders can be bought or made at home. To make your own, you will need corn starch, a lighter or matches, a candle, a ceramic bowl, a knife or paintbrush, and a mixing bowl. Light the candle and hold the bottom of the ceramic bowl over the flame to burn a layer of soot onto it. Scrape the soot into a mixing bowl and mix it with corn starch. You can also use baby powder, baking powder, cocoa powder, or flour.
To apply the powder, sprinkle a small amount over the print and gently blow to cover the whole print. Then, use a brush to gently dab and remove any excess powder. Place a piece of clear tape over the print, holding onto one corner, and slowly peel it away. Place the tape onto a contrasting piece of paper—if you used white powder, use black paper, and vice versa.
Dyes
Dyes are used to process surfaces before powders are applied. For non-porous surfaces, investigators often use cyanoacrylate (superglue) processing, exposing the surface to vapours that will adhere to any prints. These can then be viewed under oblique ambient light or a white light source.
For porous surfaces, chemicals such as ninhydrin and physical developers are used, reacting with amino acids and inorganic salts in the latent print residue. DFO (1,2-diazafluoren-9-one) is another chemical used to locate latent fingerprints on porous surfaces, causing them to fluoresce, or glow, under blue-green light.
Alternate light sources (ALS) are often used in conjunction with powders or dye stains. For example, investigators may use a blue light with an orange filter to find latent prints, with a fluorescent dye stain helping the print appear clearly.
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Cyanoacrylate processing
To perform cyanoacrylate processing, a fuming chamber is required. This can be a commercial chamber or a DIY setup using a glass aquarium or a clear plastic container. The chamber should have an airtight seal to contain the toxic cyanoacrylate fumes and be set up in a well-ventilated area. Inside the chamber, an aluminium dish or tray is placed on a warming plate, and a few drops of liquid cyanoacrylate are added. The items with potential fingerprints are then suspended in the chamber, ensuring they are positioned to be fully exposed to the fumes.
The warming plate is then plugged in, and the chamber is sealed. The liquid cyanoacrylate is heated to around 80-100°C to produce vapours. This heating process reduces the time required for development. It is important to monitor the development regularly to avoid over-development, which can obscure the fingerprint pattern. The vapours react with the traces of amino acids, fatty acids, and proteins, and moisture in the fingerprint residue, resulting in the formation of a visible, sticky white material along the ridges of the fingerprint.
Once the fingerprints have fully developed, the items are removed from the chamber. The resulting prints may be used as they are or further enhanced through various techniques. For instance, regular or magnetic fingerprint powder can be dusted onto the prints to make them stand out for easier identification. Additionally, the prints can be photographed and analysed using photo editing software to adjust brightness and contrast for better visibility.
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Alternate light sources
Fingerprints on plastic are classified as latent prints, which are formed when the body's natural oils and sweat are deposited on another surface. While they are not readily visible, they can be enhanced using alternative light sources.
In the case of plastic fingerprints, investigators often use oblique ambient light or a white light source to visualize the prints. This is often done in conjunction with cyanoacrylate (superglue) processing, where the object is exposed to cyanoacrylate vapors, which adhere to any prints present. This allows the prints to be viewed and captured for further analysis.
Other methods for visualizing latent fingerprints on porous surfaces include using chemical developers such as ninhydrin and DFO (1,2-diazafluoren-9-one). DFO causes fingerprints to fluoresce when illuminated by blue-green light, making them visible under specific light sources.
The choice of method depends on the nature of the surface and the availability of equipment and resources.
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Chemical treatments
Recently, scientists have developed a new type of adhesive tape that can be used with an instrument called an ion mobility spectrometer, which can detect chemical residues. This instrument can help investigators determine whether the suspect handled narcotics or explosives before leaving the print, as well as preserving the print image for traditional fingerprint analysis.
Another chemical treatment for lifting fingerprints is the use of mercuric iodide, which is applied as fumes to the material containing the latent impressions. Ruthenium tetroxide is a similar fuming method that can be used to develop latent fingerprints on a variety of surfaces, including plastic, doors, and human skin. This process involves the oxidation of ruthenium chloride, which is carried out by mixing a solution of 0.1% ruthenium(III) chloride hydrate and 11.3% ceric ammonium nitrate solution at room temperature in a closed container. Latent marks that come into contact with these fumes are developed within 10 to 20 minutes, depending on the substrate.
In cases where fingerprints are suspected to be present in blood and may not be clearly visible, chemical techniques can be applied to enhance their appearance. This typically involves the use of aerosol analogs, such as Leuco-Malachite Green, Leuco-Crystal Violet, Amido Black/Sudan Black, and Luminol. When sprayed on the suspected area, these chemicals cause the prints to turn distinct colours, making them more visible. However, it is important to note that the use of fingerprint powders may contaminate or ruin the evidence, limiting the ability to perform other techniques.
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Adhesive tape
Scientists have developed a new type of adhesive tape that can provide additional insights. When used with an ion mobility spectrometer, this tape can help determine if the suspect handled narcotics or explosives before leaving their fingerprints. This instrument analyzes chemical residues left on the tape, providing valuable information about the suspect's activities.
When processing adhesive tapes for fingerprint examination, the non-adhesive side is typically treated according to the tape's porous or non-porous nature. For removing the tape, products like Un-du adhesive remover are recommended, although freezing can also be effective for certain types of tape like duct tape.
The adhesive side of the tape can then be processed using various techniques. One method, referred to as "ABP" or "alternate black powder," involves creating a mixture of distilled water, soap, and black fingerprint powder (though any color should work) to achieve a shaving cream-like consistency. This mixture is then painted onto the adhesive side, rinsed, and preserved by mounting it on white photo paper or clear plastic.
Another approach, described by a user, involves using super glue fume and lighting to capture images. They then suggest applying products like UnDo or heptane to remove the tape, followed by a commercially available powder suspension called WetWop, and rinsing again. The final step is to apply a dye stain, such as Basic Yellow 40, to both sides of the tape, capturing additional images.
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Frequently asked questions
Plastic fingerprints are three-dimensional prints found on soft surfaces such as soap, wax, wet paint, and fresh caulk.
Plastic fingerprints are visible to the naked eye as they are formed when a substance like blood, dirt, ink, or paint is transferred from a finger or thumb to a soft surface.
First, dust the surface with a dark powder that sticks to the print. Then, use clear adhesive tape to lift the print. Alternatively, you can use a liquid casting compound like AccuTrans® to fill in the nooks and crannies of rough or textured surfaces.
Other methods include using chemical reagents such as DFO (1,2-diazafluoren-9-one) or cyanoacrylate (superglue) processing, as well as alternate light sources and filters to enhance the appearance of latent prints.
