Maya Ellison
Fingerprints can be found on practically any solid surface, including plastic. There are three types of fingerprints: those on soft surfaces, such as soap or wax, which are likely to be three-dimensional; those on hard, non-porous surfaces, which are either patent (visible) or latent (invisible); and those on porous surfaces, such as paper or fabric. Latent fingerprints, which are formed when the body's natural oils and sweat are deposited on another surface, can be revealed on non-porous surfaces by exposing the object to cyanoacrylate (superglue) vapours, which adhere to the prints, allowing them to be viewed under light. Latent fingerprints on porous surfaces can be revealed using chemical reagents, such as ninhydrin, which reacts with amino acids and produces a dye.
| Characteristics | Values |
|---|---|
| Classification of fingerprints | Soft surfaces (soap, wax, wet paint, fresh caulk, etc.), hard surfaces (patent or latent prints) |
| Visible prints | Occur when blood, dirt, ink, paint, etc. are transferred from a finger or thumb to a surface |
| Patent prints | Can be found on smooth or rough, porous or nonporous surfaces |
| Latent prints | Occur when the body's natural oils and sweat are deposited on another surface; may require fingerprint powders or chemical reagents for detection |
| Fingerprint powders | Black granular, titanium oxide, aluminum, black magnetic, etc. |
| Non-porous surfaces | Exposed to cyanoacrylate (superglue) fumes before powder or dye application |
| Porous surfaces | Treated with chemicals like ninhydrin or physical developer to react with amino acid and salt residues |
| Alternate light sources | Laser or LED devices emitting specific wavelengths or spectra, often used with filters for photography |
| Photography | High-resolution photography with red or orange filters to enhance quality |
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What You'll Learn
Using fingerprint powders
Fingerprints are one of the most significant types of physical evidence at a crime scene. They are unique from person to person and remain unchanged during an individual's lifetime. The easiest and fastest method for fingerprint development is the use of conventional fingerprint powders.
Fingerprint powders are used to dust non-porous surfaces. Powders are generally used to develop latent fingerprints as they easily adhere to the sweat, oil, etc. present in the fingerprint. When the powder is applied to the print, it makes it visible.
The powder dusting technique involves the application of a fine powder on the impression of the print with the help of a brush such as a glass fiber or camel hair brush. The powder makes the ridges of the fingerprint visible. Once the fingerprint becomes visible, it can be lifted from the surface with clear tape and transferred to another surface to be taken to the laboratory for further analysis.
There are various types of fingerprint powders, such as black granular, titanium oxide, aluminium, black magnetic, and unconventional, non-toxic powders that are cheaper and commonly available alternatives to the expensive, toxic ones.
It is important to note that fingerprint powders can contaminate the evidence and ruin the opportunity to perform other techniques. Therefore, investigators may examine the area with an alternate light source or apply cyanoacrylate (superglue) before using powders.
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Alternate light sources
Latent fingerprints are invisible prints formed by the accumulation of sweat and natural oils from the skin on a surface. They are found on a wide range of surfaces but are not immediately visible, requiring special techniques for detection. The use of ALS can reveal these latent fingerprints by causing the prints to fluoresce, or glow, under specific light wavelengths.
One method of enhancing fingerprint detection with ALS involves the use of fluorescent chemicals. Fluorescent latent print powders can be applied to surfaces, causing the latent fingerprints to fluoresce under the ALS. This technique minimises the impact of the background, resulting in clearer photographs of the fingerprints. Fluorescent liquids and dyes can also be used for the same purpose, particularly on porous surfaces such as paper and cardboard.
Another technique employed with ALS is cyanoacrylate (superglue) fuming. Non-porous surfaces are exposed to cyanoacrylate vapours, which adhere to any latent fingerprints present. The vapours allow the fingerprints to be visualised under oblique ambient light or a simple white light source. This method is particularly useful for rough or highly textured surfaces.
The choice of light wavelength used with ALS is crucial. While blue or visible light may be used, it often requires additional filtration to block the intense light and allow visualisation of the fluorescence. UV light, on the other hand, produces visible fluorescence without the need for filtration, but eye protection is recommended. Additionally, the wavelength must be tuned to ensure that the processed print fluoresces without causing the background to glow.
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Cyanoacrylate (superglue) processing
Cyanoacrylate, also called superglue, is a chemical method for detecting latent fingerprints on non-porous surfaces such as plastic, glass, rubber, and varnished wood. This method is non-destructive and can be used to develop clear, stable, white-coloured fingerprints. The process involves placing the item with the potential fingerprint into a sealed chamber and exposing it to superglue vapours. The vapours react with the amino acids, fatty acids, and proteins in the fingerprint residue, turning it into a visible, sticky white material that outlines the ridges of the fingerprint.
Steps for Cyanoacrylate Fuming:
- Place the object with the potential fingerprint into a sealed chamber. This can be a simple homemade chamber made from a plastic bag and PVC pipe or a commercial chamber with features like temperature and humidity control.
- Place a small amount of liquid cyanoacrylate (superglue) into a container within the chamber. The amount of superglue used will depend on the size of the chamber and the object being processed.
- Heat the cyanoacrylate to a temperature between 80°C and 100°C. The heating process helps to speed up the development of fingerprints. However, it is important to avoid overheating the cyanoacrylate, as it can produce toxic hydrogen cyanide if heated above approximately 220°C.
- Monitor the development of the fingerprints. The vapours will react with the fingerprint residue, and the process should be regularly inspected to avoid over-development. The duration of exposure will depend on the intensity of the vapours and the level of residue present.
- Remove the object from the chamber once the fingerprints have developed. The white residue of the fingerprints should be visible on the object.
- Post-treatment procedures can be used to improve the contrast of the developed fingerprints. This can include powdering and staining techniques.
Precautions:
- Cyanoacrylate fumes are eye and respiratory irritants, so proper ventilation and safety precautions are crucial.
- The development of fingerprints should be monitored to avoid over-development, which can obscure the pattern of the fingerprints.
- The heating of cyanoacrylate should be carefully controlled to prevent the production of toxic hydrogen cyanide.
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Chemical developers
Plastic is a non-porous surface, and fingerprints on non-porous surfaces are often latent (invisible) prints formed when the body's natural oils and sweat are deposited on the surface. To reveal latent fingerprints on plastic, investigators can use cyanoacrylate (superglue) processing or fuming. This involves exposing the plastic object to cyanoacrylate vapours, which adhere to any prints present, allowing them to be viewed under light.
Another method for processing non-porous surfaces like plastic is the use of fingerprint powders, such as black granular, titanium oxide, aluminium, or black magnetic powder. These powders are dusted onto the surface, and once the prints appear, they are lifted with adhesive tape and transferred to a latent lift card for preservation.
For porous surfaces, such as paper, fabric, untreated wood, and sponges, different chemical developers are used to react with the amino acid and salt residues present in latent fingerprints. One such chemical is ninhydrin, a powder soluble in acetone or ethanol. When reacted with amino acids or amines, it produces a deep blue or purple dye called Ruhemann's purple. DFO is another chemical similar to ninhydrin that reacts with amino acids, and prints developed using DFO glow when exposed to blue-green light.
Luminol is a chemical used to detect fingerprints that came into contact with blood. Surfaces are activated by wiping them with a bleach and luminol solution, and the prints will then glow, indicating the oxidation process. A blacklight is then used to enhance the visibility of the print.
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High-resolution photography
When dealing with latent fingerprints, which are invisible prints formed by sweat and natural oils, other methods may be necessary to make the prints visible before photographing them. One approach is to use fingerprint powders, such as black granular, titanium oxide, aluminium, or black magnetic powder, which are dusted onto non-porous surfaces to make the prints appear. However, this method carries a risk of contaminating the evidence.
Alternate Light Sources (ALS), such as lasers or LED devices, can also be used to examine surfaces for latent fingerprints. These devices emit specific wavelengths of light that can be photographed or further processed with powders or dye stains. For example, a blue light with an orange filter can be used to find latent prints, and chemicals like ninhydrin or DFO can be applied to make the prints fluoresce under blue-green light.
Once the fingerprints are visible, high-resolution photography can be employed. This involves using a high-resolution device to capture detailed images of the fingerprints on the plastic surface. Adequate lighting is crucial, as fingerprints and their shadows are minuscule, and the light source must be positioned perfectly to highlight these details.
After capturing the photographs, image-enhancing techniques may be applied to improve the visibility of the fingerprints. This can include using computer algorithms to analyse the images and identify potential fingerprints within them. However, these algorithms typically require clear, high-resolution, and well-lit images to be effective.
Overall, high-resolution photography plays a vital role in collecting and analysing fingerprint evidence, especially when combined with other techniques to enhance the visibility of latent fingerprints on plastic surfaces.
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Frequently asked questions
Fingerprints on plastic can be either patent (visible) or latent (invisible). Patent prints can be found by simply looking for visible marks. Latent prints can be found by using fingerprint powders, such as black granular, titanium oxide, aluminium, or black magnetic. Alternatively, exposing the plastic to cyanoacrylate (superglue) vapours will make any latent prints visible under a light source.
Expose the plastic to cyanoacrylate (superglue) vapours by applying the superglue to a cloth and placing it in a sealed container with the plastic item. Once the vapours have adhered to the fingerprints, they will be visible under any light source.
Other methods for finding fingerprints on plastic include using an alternate light source (ALS), such as a blue light with an orange filter, or applying chemicals such as ninhydrin or luminol.
Ninhydrin is a chemical that reacts with the amino acids and salt residues present in fingerprints. Dissolve ninhydrin powder in acetone or ethanol to form a solution, then apply the solution to the plastic surface. This will produce a deep blue or purple dye (Ruhemann's purple) over the fingerprints.
