Fingerprint Forensics–Part 2
From Scott:
In the previous six installments of my police investigation series, I focused primarily on the investigation at death scenes for five of them. I covered a number of aspects of these cases, including how we determine if a death is accidental, murder, suicide, or natural. I examined ways to estimate how long it has been since the person’s death, and I covered the procedures we follow for each situation.
In a post three months ago (4/7/2014) I did “Forensic Fun with Fingerprints” (the Part 1 of this Part 2). At this point, I would like to go into more detail about the science of fingerprints and palm prints. Because it has been a while since my last post on this topic, some of what I present here will repeat part of the previous post in order to make the discussion clear, and so you don’t have to refer to the previous one to understand this one.
Fingerprints receive wide media coverage, both in the news and in movies and TV. But what is truth and what is Hollywood hype? In the crime shows you see on television, it seems that the investigators recover fingerprints at every scene they go to. In reality, recovering a suspect’s prints is a rare event. Usually, this happens during a crime of passion, such as a homicide. The suspect made no plans for committing the crime and took no precautions against leaving evidence behind.
But in most cases, the crime was planned ahead of time and precautions were taken accordingly. At burglary scene after burglary scene, we see finger-shaped marks in the dust, or on glass, but we come up empty when we process for fingerprints. This is a clear indicator that the person wore gloves. We continue looking for prints, even when we see evidence of the presence of gloves, because there is always the possibility that maybe the person took a glove off for a second and ended up touching something.
What leaves a fingerprint? What exactly is it that we find when we process a given surface? At its most basic level, a fingerprint is the residual deposit of oils, water, and secretions from your skin, left in the pattern formed by the shape of the friction ridges on the fingers or palms. The bottoms of the feet also have these friction ridges, and those patterns are as unique as fingerprints. The evolutionary purpose of this skin is to provide friction. Without the ridges, your skin would be too slippery to get a reliable grip on much of anything. Your fingerprints were fully formed sixteen weeks after you were conceived, and other than growing larger as the body grows, they never change.
If you receive a scar on friction ridge skin, that scar will become part of the pattern, and will in fact make identification that much easier. But the rest of the pattern will remain unchanged. If the skin on your finger is burned badly enough to blister, the skin will eventually fall off… and the new healthy skin beneath will have the same fingerprint pattern.
When cops talk about fingerprints, you may hear the term “latent prints.” There are actually three different types of prints, with latent being the most common. Latent prints are there, but they are hidden and require some type of processing to bring them out.
Patent prints are visible without processing. A great example of this is a bloody fingerprint. The blood forms the pattern of the friction ridges, leaving a visible fingerprint. This type is considered to be the smoking gun of fingerprint evidence. If the suspect leaves behind a fingerprint in the victim’s blood, he has some explaining to do!
The third type is plastic prints. These are prints that are embedded into the surface, or “substrate” as we call it. Think of a wad of Play-Doh. If you grip it in your hand and then lay it down, you’ll see the image of your fingerprints and your palm prints. This actually happened in a case in the East St. Louis, Illinois area. A burglar found the victim’s child’s Play-Doh and, with his bare hands, molded it into a phallic symbol with a note explaining his intentions. His prints were plainly visible all over his work.
We have different ways of processing for latent prints. The most common method involves black powder and a fingerprint brush. This technique is typically used first, and it seems to be most prominent when the processing is done at the scene. A more effective method is magnetic powder, which is applied with a magnetized wand. This powder is much finer, and therefore will attach to smaller bits of latent prints. The processing is basically the same as with a brush, but it has drawbacks. It’s much harder to do it on a vertical surface, and the very nature of it means it can’t be used on a metal substrate. If properly equipped, and investigator can use fluorescing powders, which require a light source with a certain wavelength (color) of light, and a certain color of goggles. They also need the ambient light to be at a much lower level. Like black powder, fluorescing powders come in the brush-on type and the magnetic variety.
In our lab, we have other methods of processing prints. One technique, glamorized in movies, is the use of cyanoacrylates, the “Superglue method.” The evidence is placed in an enclosed area (we use a fish tank with a lid). The glue is vaporized in the tank, which contains a beaker of boiling water to add humidity. The glue will adhere to the prints, producing a white friction ridge pattern (not black, as shown in the Beverly Hills Cop movies). This process typically takes about ten minutes. One major benefit to this method is that the glue forms a protective layer over the entire item, protecting the fingerprint. With a print recovered through the use of powders, the investigator has one chance to lift the print with tape and preserve it. If the tape tears, or becomes wrinkled, the print is lost forever (unless it was photographed first). But with a cyanoacrylate print, if the investigator errs in the recovery process, it can be tried again and again. The print is basically permanent.
If the Superglue developed a bit of ridge detail, but not enough to identify, there are other techniques at our disposal to further enhance the latent. Black powder and fluorescing powder can be attempted again after the Superglue process. We also have liquid chemicals, which are sprayed on and rinsed off with water. Like the fluorescing powders, these typically require special lighting and goggles to make a print visible.
On a side note, photography of fluoresced latent prints presents a special challenge. Like the human eye, the camera cannot detect the print without the correct wavelength of light and a colored lens cover. Photography of prints is a must, because in this age of technology almost all of our fingerprint identification work is done with computers. For this reason, fingerprint examiners tend to have an aversion to prints developed with fluorescing techniques.
For latent prints on paper, we have two options available. The first is a nasty chemical call ninhydrin. It’s alcohol-based, so it evaporates quickly. The paper is soaked in ninhydrin and allowed to dry before being heated and steamed with a clothing iron. The iron is not actually pressed against the paper, but it is brought close to the surface. The combination of heat and humidity causes the ninhydrin to react with amino acids in the latent prints to form a purple fingerprint.
Side note: If you really want your character to sound educated in this field, the color is actually referred to as “Ruhemann’s Purple.” Any ninhydrin that gets on your skin will leave that same color, and it can’t be washed off. It will fade on its own in a day or so. Over the course of the next few days and weeks, the ninhydrin prints will continue to develop, but the vast majority of the results are achieved quickly with an iron.
The other method for paper is achieved using physical developer. It looks for residue that isn’t water soluble, so paper that has been wet for quite some time can still be processed for latent prints. However, since there is some silver in the mix, this method is quite expensive. If anything metal is placed in the solution, the silver will immediately fall out, rendering it useless. And the paper will turn gray-black during the processing, completely ruining it.
Tape presents a special problem. The sticky side of tape, be it electrical, duct, packaging, or whatever, will retain tremendously clear latent prints by pulling the residue right off the skin. The problem becomes how to recover the print. If we use powders, they will stick to the tape and hide the print. What we have is a special powder that is mixed with water to form a paste. We brush it on the sticky side of the tape, then rinse it with water. We’ve recovered some of our best prints in this way. In case you’re wondering, the technical name for this is “sticky side powder.”
There are, of course, other methods available for the recovery of latent prints, but this is our primary group of techniques. Everything follows a set process. Before I close this entry, I’ll give you a real-world example of how this all comes together.
We had a homicide where we had a solid suspect, but we needed evidence to confirm our suspicions. Money had been taken from a coffee can, so when black powder failed to reveal anything, I took the can back to our lab. I used Superglue to lock any latent prints in place, then tried black powder again. This time, I saw a few scraps of ridge detail, but nothing we could work with. I tried our liquid chemicals, but still had only a small area of one finger. It was better, but not enough to work with. Once the can was dry, I decided to try some of our new fluorescing powder. Not only did the entire finger show up, but half of the fingers on each side of it did. We had our identification!
Next time, I’ll delve a little deeper into the topic of fingerprints, including an overview of just what goes into making a fingerprint identification. It isn’t quite as easy as they’ve made it look on TV, but with the constant improvements in technology, we’re getting better.
–Scott