Monthly Archives: May 2011
Luminol: The “Magical” Blood Reagent
Luminol Reagent
Luminol is a common chemiluminescent blood reagent used at crime scenes and in the laboratory. Chemiluminescence means that the blood will glow a blue-ish green color through oxidation as opposed to fluorescence (excitation) after being illuminated by ultra-violet light with an ALS. Luminol can detect trace amounts of blood as low as one part per million. There are a number of formulas used but most consist of the reagent Luminol, hydrogen peroxide (oxidant), and distilled water. It has been shown to be very effective in detecting cleaned up blood and older blood degraded through time. A few years ago a colleague of mine received permission to test the floor joists below the spot where Lizzy Borden’s father was murdered. After 112 years he was still able to clearly see the blood flow draining down through the floor boards into the cellar!
Luminol does have some limitations however. It is not specific to human blood (presumptive test). That means that it will react to the blood of other mammals the same as it will with humans. In fact, Luminol will react with substances other than blood such as iron, copper, certain vegetables, and cleaners like bleach. Criminalists call this reaction a “false-positive“. A well-trained criminalist can often tell the difference between a Luminol reaction with bleach (for example) versus one with blood. Bleach reacts with a glittering appearance in bright points of light throughout the reaction area. Blood will have a more subtle and uniform glow.
Luminol reaction of blood with fill-flash photography
Another limitation is the duration and conditions of use. Chemiluminescence is typically only visible in near to total darkness. This can present a real challenge for criminalists, especially when working an outdoor crime scene. Most of the time you may have to do your examination and photography at night. Items like cars, clothing, weapons, etc can be brought into the interior spaces of the laboratory and processed there. If you must process a room in the crime scene you’ll have to block the windows with black plastic sheeting because shades and drapes simply won’t cut out enough light. Some scientists also believe that Luminol is a “potential carcinogen” and should be used with the appropriate respiratory protection.
Luminol reaction in soil four years after deposition. Size of photo area is about 2ft. square
The Three Main Types of Crime Scene Photographs
Smash & Grab Burglary
The goal of photographing a crime scene is not just to document the items present on scene. Good crime scene photography establishes information relevent to the criminal act. Criminal acts are defined by criminal statutes. As such, there are typically a number of “elements” or conditions that must be met to show that a crime has been committed (let alone who committed it). While every crime scene is different and may require specilized photographs to be taken (like aerial), there are three main types of photographs that CSI’s generally take.
Overall Photograph
The “Overall” photograph depicts the location of the crime scene or area of interest. It may be a photograph of a building, a vehicle, a field, etc. It shows the jury where the crime was committed or a particular area of interest where evidence was found. It may also show things like security barriers (crime scene tape), people on scene, damage to a structure, or general weather conditions but those are incidental.
Mid-range photo showing damage on the wall relative to furniture
The “Mid-range” photograph is the second type taken by CSIs. Some people mistakenly believe that the mid-range photo is just a “closer” photograph than the overall one because sometimes they document the same areas. But the purpose of the mid-range photograph is to document relationships between items of evidence or areas of importance (such as how close is the gun to the victim or the location of bloodstains to the point of entry) . Mid-range photographs are the most common type of photograph shown to juries because they tend to support theories, reconstructions, or statutory elements of the crime.
Close-up photo showing condition of revolver cartridges
The last type of photograph is the “close-up”. These are also referred to as “identification” photographs because they are intended to “identify” a person, place, or thing to the jury or others not at the scene. This may include photographs of a victim’s face, a fingerprint, or a vehicle. In court, they may be used to establish that the item of evidence being introduced is the same item recovered at the crime scene. Sometimes they show the “condition” of the item as it was encountered by the criminalist. It may be a vehicle parked haphazardly, a gun with a reddish-brown stain later determined to be the victim’s blood, or a forged check.
Many of you may not want to get into this level of procedural detail in your novel but the distinctions between these photos may assist you in describing what your characters are seeing or noting. “Hey, look how the suspect parked his vehicle on top of his neighbor’s trash can…maybe he was drinking. We should get a picture of that”. You get the point.
Cleaning Blood Creates a Clean Spot…Duh!
Bloodstained Sink
Criminalists are sometimes called upon to detect evidence at crime scenes in which the suspect, or suspects, have made efforts to “clean-up” the evidence. Most of the time this involves the cleaning of blood evidence. Let me start by saying that criminals in these situation don’t usually do a great job. Most of the time they will focus on the blood that they can see. I know that must seem obvious but blood (especially spatter) can find its way into all kinds of places (like the back of a door) that the criminal doesn’t think to check. Sometimes the color of the surface containing the blood makes it difficult to see (like dark carpeting).
Secondly, criminals typically clean to the point that they no longer see the blood. The blood may still be detectable using a variety of devices or techniques (no I can’t tell you the best ways to clean up blood). A lot of the scenes I’ve processed were just cleaned with a water-soaked towel. This just dilutes the blood and spreads it around. Also, the suspect may clean the scene and then forget to clean the clothes they were in while cleaning. There are a lot of variables you could work into a storyline.
Regardless of how it is done there is one constant variable found on scenes that have been cleaned up (assuming you arrive within a reasonably short time after cleaning) and that is a clean spot. Actually, in some homes a clean spot stands out more than blood spatter. I know that sounds far-fetched but it’s true. It’s the first thing I look for after considering the information I have about the crime (i.e. he was stabbed in the living room). But if there isn’t a clean spot then it’s likely there was no clean-up. Sounds simple right? Not so.
I once was called to a scene of a shooting where the suspect/victim walked into his apartment and cleaned his wound while waiting for the ambulance. All indicators and evidence were that the shooting occurred outside. Police arrived quickly and secured the scene. There was a lot of visible (not cleaned up) blood in the apartment in all the places you would expect given the story. However, the ADA still wanted every single item, even the folded socks in the sock drawer, treated with Luminol. There was no explanation given as to why the suspect would wash his socks in the drawer and leave all the other blood visible, it was just one of those Nike moments…Just Do It. Is this a common thing? I don’t think so but it illustrates that not everyone values the conclusion that if something has been cleaned…it should look “clean”.
So if you’re writing a scene in which CSI’s are processing a scene for latent blood you might want to add some comments about how the area looks differently than the rest of the room, or house, or wherever they are.
Presumptive vs Confirmatory Tests in Forensics
Unknown substance
Forensic scientists love running tests. Love, love, love it! It’s how we arrive at, and support, various conclusions. Often times we are asked to determine the identity of a particular substance; be it suspected drugs, blood, semen, explosive residue, etc. To do this we follow various departmentally established protocols utilizing both presumptive and confirmatory tests or examinations. But understanding the difference between the two types of tests is sometimes lost on those outside the laboratory like attorneys, detectives, and jurors.
Presumptive tests are designed to do one of the following:
- Eliminate the identity of a particular substance (i.e. the stain is not blood)
- Establish the probability (presumptively positive) of a substance identity (i.e. stain is probably blood)
Presumptive tests are presumptive because there is either a chance of a false positive result (bleach reacts with Luminol) or the result is defined by a larger group (the sample is blood but not necessarily human). Common presumptive tests include Phenolphthalein, Luminol, Hemastix, and Leuco-crystal Violet (blood); Acid Phosphatase (semen); Duquenois-Levine spot test (Marijuana) as well as many other tests.
Gas Chromatograph Mass Spectrometer
Confirmatory tests on the other hand are used to specify the identity of a substance. With suspected drug or explosive samples the forensic chemist will likely use a Gas Chromatograph-Mass Spectrometer (GCMS) or Fourier Transform Infra-red Spectroscopy (FTIR). For blood, a common field test is the ABAcard HemaTrace test which will specify the blood as human (primate) followed by laboratory DNA analysis. Semen samples collected on glass slides at autopsy or in a rape kit will be examined microscopically for sperm cells.
Stained sperm cells
So when writing scenes in which your characters are conducting forensic tests it would be best to know whether the test is presumptive or confirmatory before they draw any conclusions. Presumptive test results can add a bit of conflict to a story in that they may reveal the presence of blood in which later exams prove it to be non-human. I remember years ago we had a large dumpster that had at least five gallons of liquid blood poured into it (Coroner estimate). We spent hours carefully screening the materials on tarps before tests confirmed it was pig blood. Until that test was complete we had to proceed as if it were human blood just to be safe.
False Positives, Flase Negatives, and “WTF” Positive Reactions
When criminalists run presumptive forensic tests there is a risk of obtaining results that may not support certain conclusions drawn from those results. In plain English; what you get may not be what you got. This is where analysis, interpretation, and integration with other findings comes into play. It’s part of the investigative process that all of us are used to. Actually, a false positive is not really “false” in terms of the reaction, merely the reaction one might expect. As a writer, however, keeping these terms in mind can be very helpful when developing a scene.
A false-positive reaction occurs when a chemical reagent reacts positively to a substance other than the one being tested for. As an example, if a CSI sprays the blood reagent Luminol at a crime scene and sees a small area that glows he or she might believe or presume that the reagent is reacting with blood. In fact, there are other substances that Luminol may react with. This scenario might require other tests before one could testify that it is, in fact, blood.
Similarly, a test may result in a false-negative finding. Again, the result may not be truly negative in as much as it is negative to the questions being asked. Fore example, if you were to test a stain for blood with a reagent not sensitive enough to detect the trace amount present then you would get a negative result, but it would still be blood. Sometimes a sample is simply “contaminated” or may have been cleaned and therefore unable to yield a result to the exam (other than negative that is).
Different test have different failure rates. Some tests are very specific with very low rates of false results. Other exams (like a nitrate (NO3) test for gunshot residue) may be much less specific as Nitrates can be found in a large number of commercial products like fertilizers, tobacco, and cosmetics (to name a few). Actually, if you want to add some suspense into a scene you may use a false-result exam to either arrest or free a suspect that deserves neither.
Lastly, we have the “WTF” positive reaction. As you might imagine, this is not a scientifically accepted term. Every once in a while however we may get a reaction with absolutely no idea how it occurred. There are literally thousands of potential substances that we may encounter at crime scenes that have absolutely nothing to do with the crime being investigated. Such is the nature of the world. This is why forensic scientists utilize confirmatory exams and incorporate other findings into a crime scene reconstruction.
The Sticky Side of Tape: Yeah, We Can Get Prints Off That
Roll of Grey Duct Tape
“A man without duct tape is not a man at all” Plato 465 BC. Alright, alright, that was my quote but it seems to be the reigning philosophy among those of us afflicted with the Y Chromosome. I dare say you’d be hard pressed to find a tool relied upon by NASA astronauts and DIYers alike. Criminals love it too. You can bind and gag victims, make an improvised grip on a cut down shotgun stock, hold explosives together, or hold the bumper together on your getaway vehicle. Seriously though, criminals utilize everyday items in the commission of crimes and duct tape is no exception. And it’s not just duct tape, any kind of adhesive tape or label can be processed for fingerprints. It may be an address label or stamp on an envelope containing a threatening letter, the sticky edge of a post-it note, cellophane tape sealing an envelope or packing tape sealing a box.
By it’s very nature of being sticky, the adhesive side of tape is a terrific receiving surface for latent fingerprints. The first step is getting the tape separated from whatever surface it’s adhering to. Sometimes you luck out and the threatening letter is taped to the clean smooth glass of the victim’s car. Usually it’s more complicated than that. Duct tape stuck together (sticky side to sticky side) is the worst! That’s because the adhesive tends to stick together so well you simply can’t get the “original” sticky surface containing the print separated cleanly.
Criminalists have tried all kinds of methods from freezing the tape to chemical separators but neither method works consistently well. Exposed areas of the adhesive side are usually the best bet. Also, duct tape may tear off fingertip portions of the latex gloves the suspect was wearing, giving another surface to process. Cellophane and packing tapes, labels, stickers, and stamps usually yield better results. Many criminalists utilize commercial tape separating products like Un-Do to remove the tape from whatever it’s sticking to. These products work very well in preserving the latent fingerprint.
Sticky-side Powder Print on Electrical Tape
Two of the most common methods used by criminalists are “sticky-side powders” and Gentian Violet (Crystal Violet). Sticky-side powders are mixed with water and Photo-flo 200 to make a paste that is “painted” onto the sticky surface and allowed to stand for 15-30 sec. before being rinsed under a gentle stream of water. Gentian Violet is a dye stain in which the tape is immersed and soaked for up to several minutes and then rinsed with a gentle stream of water or water bath. Both methods work well and often an examiner will select a method that will provide the best contrast with the color of the tape. Gentian Violet turns the fingerprint ridges purple while the Sticky-side powder turns them a light grey.
Gentian Violet Fingerprint on Cellophane Tape
Criminalist vs Criminologist: Understanding the Difference
If you really want to get under a criminalists skin, call them a criminologist. The two terms are not interchangeable. Criminologists study criminal behavior, causes, and control as it relates to culture and society. They study things like recidivism rates, socio-economic stressors on criminal behavior, and the consequences of criminal activity on society (to name a few). You won’t find them at active crime scenes or working in a crime laboratory. Typically you’ll find them in Universities, independent academia, and non-governmental organizations (NGO). Think of them as cultural or behavioral scientists (not a great description but for our purposes it works).
Criminalists on the other hand are physical scientists who use scientific methods and techniques to find and interpret physical evidence. This includes DNA, blood, fingerprints, ballistics, etc. We rely on scientific findings and techniques used in a particular field and apply them within a legal framework. For example, the methods used by a forensic entomologist are the same used by a medical entomologist, just applied to a criminal investigation.
Accurate descriptions are important. They tell the informed audience that you have done your basic homework. In contrast, calling a criminalist a criminologist knocks you down a few pegs on the ladder of credibility with those knowledgeable of the fields. It is an easy mistake to make and an easier one to avoid.
Chemically Tagging Criminals
Chemical Tagging on Subject's Face
Ever since Edmond Locard’s seminal work on the Analysis of Dust Traces in 1930 police have recognized the value of linking a suspect to a crime scene through the analysis of trace evidence such as hairs, fibers, soils, etc. At some point, police discovered that they could introduce special powders at certain locations at a potential crime scene that the suspect would carry away. Such was the birth of Theft Detection Powder. These fine powders were similar to fingerprint powder but were only visible under ultra-violet light. This way, the suspect could not see the powder on their hands, face, or clothing. Think of it as criminal tagging.
Police would coat the surfaces of areas that only the suspect would have access to (such as a dummy cash box, certain door handles, blank checks, etc.). When a suspect was developed or an individual was caught fleeing the scene, the police could use a simple black-light to check for the powder. If the susepct wasn’t developed for a few days or weeks the powders may have been washed from the sking but the police could still check areas like jacket pockets, ball caps, and vehicles, that aren’t cleaned often.
Soon after, another nifty invention came along. We called it Pilfa-powder but it has gone by a number of names. Like fluorescent powders, Pilfa-powder was invisible to the naked eye (although purple in large quantities) but when it comes in contact with water it turns a deep purple color. This purple staining can not be washed off and will stay on the suspects skin for weeks. Obvioulsy this makes it difficult for the offender to hide the staining from friends, family, and co-workers.
Now, researchers have developed the latest version of introduced trace evidence. This new technology uses an invisible, non-toxic chemical spray containing a unique chemical signature. A dispenser can be installed above the entry/exit door to a business and can be tripped by an employee as the suspect flees. This chemical can stay on the skin for up to six weeks and even longer on clothing. Studies in Europe show an 85% reduction in theft and a 100% conviction rate in court.
This spray could be a powerful clue in your novel. You might even consider finding it on a victim, or a ch9ild, and then have your characters trying to discern how they got it on them. There is no evidence to show that the chemical can be cross-contaminated or “rubbed off” onto a third party.
