People generally know what fingerprints are. They know that the tips of the fingers and toes have "grooved" areas of skin, and when they touch a smooth surface, it can leave a a print which can be developed and observed.
They've watched TV shows like CSI and know that these prints can be analyzed to determine the identity of the person who left it.
Lets look at how the ridges on your skin are formed, and then lets look at how these prints are left and collected at a scene...
Ridges develop during the fetal stage of development. At around the 6 weeks period, the hand area begins to form, and the fatty area which will later form the palm and fingers is termed the "volar pad" or volar area.
The volar pads continue to develop, and around the 12 week point, the "friction ridges", (or the grooved areas which leave the fingerprint impressions) begins to form.
Definition of a fingerprint: An impression left on a surface by the friction ridges of the skin on the fingers.
These friction ridges can be classified as primary, secondary, and incipient ridges.
These ridges also form patterns, which are classified as whorls, arches, and loops.
Loops are the most prevalent in the population and account for 60%. They look just like they sound- a loop.
A whorl is second at about 30% and looks like a bulls eye in the centre of the finger (not exactly, but thats the best way to describe it for now. I'll try to add pictures or photos
when I can.)
The arch is the least common at 10%, and it makes a little arch or an almost triangle like pointy pattern.
There are also combinations/variations
Benefits to Law Enforcement
These ridges on the fingers begin in the fetal stage and continue to develop. Once developed, these ridges do not change and because of that, prints that are located at a crime scene can be catalogued and stored for future identification purposes.
However, fingerprints can be altered to a certain degree. If an individual cuts his finger deep enough, it will scar on the finger and become permanent.
There have been cases where people have actually tried to cut their fingerprints off, or use chemicals to try and "smooth" out
the ridges so that no distinct detail can be found in a print.
Finding and Collecting
Not every crime scene will be suitable for collecting prints. Some places will be so dirty that it may be impossible to locate one. Or the surface may be inadequate.
They are best located on a smooth clean surface. Of course, criminals aren't the most thoughful bunch, and they don't generally look for a clean smooth to purposely place a print to help make your job easier...so you have to hunt for them in places where you think it's likely that they have touched.
There may be witnesses in the area who observed something that the suspect was handling. Or if you're investigating a break and enter, a homeowner may notice something that is out of place and handled by the suspect.
After initial photographs are completed, the investigator can begin dusting the area.
Depending on the surface that is to be dusted, an investigator can elect to use different types of powders. Some of these include:
Magnetic powders. They are comprised of iron fillings which are ground down into a powder form. These have chemical compounds which are combined with it. These powders are generally used on a non metallic surface or on a surface that will not become magnetized.
These powders are delivered using a magnetized "wand" and gently passed over top of the surface.
The idea is that the metallic portion of the powder will then become lifted away from the surface with another pass of the wand, leaving only the chemical compounds adhering to the
prints, exposing them.
Metallic powders. These are made of finely ground metals. These are able to adhere to the impressions
well. They are commonly made of aluminum and bronze.
Granular powders. They consists of powdered chemical compounds which come in various colors.
They are most commonly found in black, white and grey. These powders can be easily brushed off the impression, thereby ruining it unless care is taken.
Once the area is dusted carefully, the area is examined carefully. Lighting conditions play a large part in this. Having a magnifying glass on hand can also be a help as light prints
can go undetected if it just glanced over quickly.
If a print is detected, it is labeled, recorded and photographed using a digital camera.
The photographs are important here as they will be used for a variety of purposes.
They will be used as evidence in court to show where the prints were located at the scene. They can also be analyzed using computer software should the physical lifting of the print
be unsuccessful with less than stellar results.
The digital photos of the prints can be used to make a match with a suspect.
Once the photos are taken, the prints are "lifted". Using a clear tape, the investigator presses down on the print. The print (which is now highlighted by the powder) is attached
to the sticky side of the tape. This sticky side is now pulled off the surface slowly and the print is attached to the tape.
The tape is now carefully pressed back down on to a clear piece of plastic or acetate. The fingerprint is now preserved and can be analyzed in an attempt to make an identification.
Why do powders stick and reveal fingerprints?
Everyone sweats to a certain degree, and there are many factors which will come into play. Age, sex, genetics, diet, stress levels all play a part in the amount you sweat.
The palmar and plantar surface have sweat glands, and these particular ones are termed eccrine sweat glands. Although eccrine glands are found all over the body, the plantar and palmar surfaces have the greatest density.
The sweat from these glands are composed of the following:
0.5% - 1.5% inorganic salts (chloride, iodine etc..) and organic substances (fatty acids, amino acids, ammonia and urea).
98.5% - 99.5% water.
Humans also have sebaceous glands which are only located on areas which are covered by hair. These glands secrete saturated fats, and waxes. (If you reach up and touch your forehead up at your hairline, and rub your fingers together, you will feel a slight amount of that waxy oil on your fingers.)
Even areas around your nose will have oil and wax. Consider how often you may touch your face or hair in a day. It's easy to see that sebaceous materials will easily end up on your fingertips.
Powders will stick very well to the wax and fats secreted by the sebaceous glands. These usually get transferred to the fingertips by touching hair or scratching the forehead, etc.
When a surface is touched, the inorganic and organic substances secreted by both the eccrine and sebaceous glands get left on that surface. Once the powder is brushed onto the surface, it sticks to the inorganic and organic substances, revealing the print.
Of course, the surface being dusted will play a role on whether or not the fingerprint will reveal. A clean, smooth surface is best, but that condition is not always found in crime scenes...
Analysis of the fingerprint
Movies and television shows often depict fingerprints identifying suspects by having actors sit in front of a computer screen.
Numbers and ridge patterns flash before the screen for a few seconds, and then BOOM! Red lettering says MATCH and the full dossier of a criminal shows up on screen. They have their man!
Unfortunately, it's not that easy. Yes, computer software does get used, but it's only a useful tool which gathers certain fingerprints on the database which have points of interest in the same area of the collected fingerprint.
It's now up to the technician to analyze the gathered prints on the database with the collected print and determine if there is a match.
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