Criminalistics is the application of various sciences to answer questions relating to the examination and comparison of biological, trace evidence and impression evidence. This includes fingerprint analysis, footwear and tire impressions, and toolmark evidence as well as drug analysis, ballistics and firearm examination.
Numerous chemical reagents are available to enhance and locate the presence of blood deposits. The Phenolphthalin (Kastel-Meyer) test is performed by rubbing a cotton swab that has been moistened in a saline solution on the suspected blood stain. A drop is added to the swab followed by a drop of hydrogen peroxide 3%. A positive reaction will turn the swab pink to red within 15 seconds. Leucomalachite Green (LMG) is a test that’s performed the same way. A positive reaction is indicated by a greenish-blue color that will appear almost immediately.
Luminal is a test where a reagent is sprayed onto the object to be checked. It must be viewed in total darkness. In a positive reaction, it will luminesce violet within 5 seconds. The Ortho-Tolidine test is performed by rubbing a cotton swab that has been moistened in a saline solution on the suspected blood stain. A drop is added to the swab, and then a drop of hydrogen peroxide 3% is added. A positive reaction is indicated by an intense blue color.
The Tetra-Methyl Benzidine (TMB) is another in a series of presumptive tests that is specific for blood. TMB is an enhancement reagent. The Tetra-Methyl Benzidine reacts with the HEME in the blood. Spray the surface lightly two to three times. The bloody imprint pattern should turn a greenish-blue. Overspray may give a very dark blue pattern and mask ridge patterns. Hemaglow is a protein reactant, which does not ordinarily react with household cleaners like Luminol does. Hemaglow also glows brighter and can be photographed with a flat plane camera.
Leucocrystal Violet (LCV) is a relatively new positive blood identifier that turns permanently violet when in contact with blood. Leucocrystal violet can be used in testimony in court as a blood identifier at the scene without the need for further testing to identify the stain as blood.
In the Fluorescein test, the chemical mixture of fluorescein causes a catalytic reaction to occur between the hemoglobin in blood and oxygen. This produces a luminescent stain, which will luminesce in the dark when excited by Alternate Light Sources (ALS). The major advantage is that it will continue to luminesce for hours under UV or ALS after the initial application, and without additional applications of the reagent. Forensic Toxicology
For centuries, poisoning had been a popular method of murder. Poison mimics common medical diseases, leading physicians to believe a victim died of natural causes. Arsenic was very popular as a poison, as well as other heavy metals such as thallium. Many cases of arsenic poisoning had been attributed to gastric conditions in the 19th century. Metal-based poisons soon became very detectible with standard toxicology tests.
Plant-based toxins have also been used in murder. In fact, any natural substance in the right dose can be poisonous. Determining whether or not a poison was used requires a toxicological examination. Scientists and researchers continue to improve and discover new means of separating, analyzing and identifying chemical substances. Techniques are becoming more specialized, and technologies are being combined to create ever more sensitive and sophisticated tests.
Two increasingly important approaches to chemical detection and identification are gas chromatography, a method of separating substances, and mass spectrometry, a method of measuring the mass of molecules. These techniques allow investigators to identify with reasonable certainty (admissible in a court of law) minute amounts of toxic substances found in the bodies of victims or in trace evidence collected at crime scenes.
Image Enhancement Methodology
Advances in image-enhancement technology are helping police visualize evidence, such as imprints and impressions. Imprints are patterns left on hard surfaces. Impressions are three-dimensional patterns or indentations made in a softer medium such as mud, sand or snow.
Technology has also improved in the ability to detect latent fingerprints, which are among the most valuable types of physical evidence in criminal investigations. Fingerprints are accepted by the courts as good evidence for personal identification. In addition to dusting with powder for detecting and visualizing latent prints at a crime scene, there are techniques using chemicals such as cyanoacrylate (“superglue”), iodine fuming, ninhydrin reagents, silver nitrate and fluorescent reagents. Furthermore, there have been some remarkable results in developing latent prints by use of the laser. These techniques have expanded the capabilities of investigators.
Use of the Automated Fingerprint Identification Systems (AFIS) has significantly advanced the ability of law enforcement to identify and apprehend criminals whose prints are in the various databases. Several AFIS technologies are in use across the United States, which can be problematic because the investigator may have to submit a questioned latent to more than one AFIS database. However, AFIS has revolutionized the manner in which police departments search latent prints for matches.
Entire databases containing hundreds of thousands of fingerprints can be searched within minutes. Latent prints recovered from a crime scene can be computer matched to a criminal fingerprint database. The success of these systems is due to the large databases maintained by a number of state law enforcement agencies as well as the military and other government agencies, which maintain fingerprint databases on all personnel and employees.
New Advance in “Bullet Fingerprinting” Technique
“Bullet fingerprinting” technology, developed at the University of Leicester in the United Kingdom, may revolutionize how authorities obtain fingerprints off bullets or metal casings, including bombs. The technique involves studying the chemical and physical interactions occurring between the metal and the fingerprint sweat deposit.
Using advanced surface imaging techniques, such as an atomic force microscope, nanoscale observations of fingerprinted brass samples can identify optimum conditions to promote the natural enhancement of the fingerprint, vastly improving its recovery rate. It has also proven that components of the sweat deposit survive washing and wiping of the surface.
According to researcher Alex Goddard, “Once a finger has touched the metal surface, a residue remains behind. This starts to react with the metal, and an image of the fingerprint can be developed by use of elevated temperature and humidity, with the resultant image becoming a permanent feature on the surface of the metal.” Currently, fingerprint recovery from bullets is very low, i.e., less than 1 percent. This new technique uses a natural process, and even if it only leads to a small increase in success rate, then that would be significant.
Previous recovery methods include applying powder to the material which can actually damage the evidence. This new technique promotes a naturally occurring process which does not involve adding anything to, or damaging, the evidence. Instead, it employs heat and humidity to promote the enhancement of the fingerprint image. There are also indications that it could be used after other techniques have failed, perhaps as a last resort.
Forensic Lasers and Alternate Light Sources
Forensic lasers produce an extremely high-energy beam of light that is capable of causing fluorescence in certain materials. The fluorescent effect is usually viewed through goggles or lenses. They are quite expensive and require accessory cooling systems. Forensic lasers, because of their size, are usually confined to laboratories.
The Alternate Light Source (ALS) is more portable and less expensive today. Initially, the original units were so heavy and cumbersome they had to be wheeled into the crime scene on a hand truck. Today, the ALS units are hand-held with twice the amount of power. ALS is a high intensity white-light source which has multiple filters and provides several wavelengths of visible light for examining the scene. In addition, you can use infrared, polarized or oblique lighting. Also, ALS units can detect microscopic trace evidence, such as hairs.
Both lasers and ALS units detect body fluid stains, fibers and various materials that contain chemical substances capable of fluorescence. These units are also useful in locating fingerprints when fluorescent fingerprint powder is applied. Most of the wavelengths in an ALS are in the visible range of the spectrum. A UV source can be built into an ALS unit. The usefulness of UV depends on the fluorescence capability of the trace evidence.
Vernon Geberth, M.S., M.P.S., is a homicide and forensic consultant and the former commander of NYPD’s Bronx Homicide. He can be reached at www.practicalhomicide.com.