Lets start about fingerprints and its related stuffs. Moreover, we see about how fingerprints are identified.
Introduction to fingerprints
Fingerprints : Prints of finger ridges whose details are known. It is a god quality evidence.
Fingermarks : Finger ridges on the surfaces whose details/sources is unknown.
Finger ridges found on fingers, palms, sole of feet and evolved in primates alone.
Also DNA profiling can’t distinguish between identical twins but fingerprints can.
Finger ridges are also referred as friction ridges which won’t change in lifetime except injury or decomposition after death.
Types of fingermarks
Patent : Recorded using photography. No enhancement needed. Contaminants deposited are paints, blood, dust.
Latent : Enhancement needed to visualize the marks. Numerous contaminants may be deposited in the latent. Eg: cosmetics, perfumes, food residues, other glands.
Major glands responsible for deposition as contaminants are
- Eccrine : present / deposit throughout the body but highest densities found in palm and soles.
- Apocrine : Primarily found in axillary region. Eg: Armpit.
- Sebaceous glands : localized to regions contain hair follicle, face, scalp.
Eccrine and Sebaceous glands are the most deposition contribution to latent.
Alternate Light Source : places like doors, doorknobs, windows, railings, etc. uses laser or LED devices that emit a particular wavelength, or spectrum, of light for examining latent prints. Some devices have different filters to provide a variety of spectra that can be photographed or further processed with powders or dye stains. Eg blue light with an orange filter to find latent prints on desks, chairs, computer equipment or other objects.
Smooth surface : prints are detailed or uninterrupted. Eg: Glass, Paper.
Porous substances like paper absorb water-soluble part of the fingermarks that deposits within a second. This can’t be simply rubbed away.
Non-Porous substances like wood none of latent fingermarks of water-soluble part is absorbed. Because dry non-porous surfaces may submerged in aquatic environments at different time intervals. So we need to analyze the effects of the nature of aquatic environment and the length of submerged time. This is difficult and may result nothing.
Semi-porous substances like varnish wood can absorb water-soluble part of fingermarks deposition and can takes hours rather than seconds.
Rough / Roughed- textured surface : fingermarks or finger ridges are not detailed or interrupted. Eg: Wooden door, textured wallpaper.
Time : Time of contact influences fingermark’s quality. Longer the contact time, better the quality for contaminated fingermark.
Time deposition of contaminants like dust or others increases which may decreases the quality.
Pressure : Pressure of contact which may harder the contact, better the quality.
Dust Accumulation, exposure to water, light and heat also have detrimental effects.
Selection of chemical enhancement reagent is surface in which prints are deposited on.
Physical detection : Common physical detection method is powder. Traditionally, black / silver powder is used but now supplemented by colored fluorescent powder.
By dusting nonporous surface with black granular, aluminum flake, black magnetic, etc. and any prints are appear they are photographed.
Also, lifted from the surface with clear adhesive tape. This tape is then placed on a latent lift card to preserve the prints.
Optical detection : High intensity light sources like laser or fluorescent chemical powder which applied by brush with powder physically adhering to visualize it.
Chemical Detection : Chemical reagents are used to visualize the fingermarks.
Powder suspension : To enhance fingermarks in uPVC (unplasticised polyvinyl chloride). Used for rigid PVC, pipe work and window frames. Mostly used for non-porous surfaces.
1,8-Diazafluoren-9-one is used to locate latent fingerprints on porous surfaces and it causes fingerprints to glow, when they are illuminated by blue-green light.
Powders : For non-porous surfaces, different metals are used to make powders. Metallic dust which are applied using special powder brush and magnetic applicator are used to find prints.
Ninhydrin used for porous surfaces like paper, cardboard. Surface is washed with this chemical solution and left to dry before placed in 70 deg centigrade humid over 15 minutes. Ninhydrin also known as 2,2-Dihydroxyindane-1,3-dione.
Ninhydrin react with amino acid present in the latent prints to produce purple colored mark.
Cyanoacrylate / super glue fuming used for non-porous surface. This is used before applying powders or dye stains which involves exposing the object to cyanoacrylate vapors. Then fumes (vapors) will adhere to any prints present on the object allowing them to be viewed with oblique ambient light or a white light source.
Cyanoacrylate are not applied directly on the surface instead they placed on a special chamber where items and small droplets of this glue are kept separately and under some conditions it gets vaporized to produce crystalline on the surface. This white crystalline substances on latent prints stabilizes the marks (prints).
Amido Black is a non-specific protein stain that reacts with any protein present which is typically used for enhancing bloody impressions on human skin.
We talked about latent prints or fingerprints on objects then how they analyze for prints present in a human body?
Here is the answer , fingerprint examiners use a small magnifier called a loupe to view minute details of a print. A pointer called a ridge counter is used to count the friction ridges.
When a latent prints or fingerprints are recovered then they are photographed. This image is uploaded to a database which holds details about prints from known individual as well as from other crime scenes which have not been identified.
If database produces a match this is further verified by trained fingerprint examiner. This practice is called subjective comparison.
There are many analysis methods are there but this is a common one.
Blood, semen, vaginal secretion, saliva are also collected.
Average human body contains 4 to 5 liters of blood and approximately 45% of consists of cells with aqueous solutions like salt, protein and etc.
Leukocytes are the source of DNA and used for DNA profiling from blood stains.
Happy Learning !
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Image Credit : Google Images, forensic science simplified.