By Dena Weiss, professor of criminal justice at American Military University
The dynamics involved in crime scene examination have captured the hearts of Americans, often through exciting portrayals on television dramas and movies. Gathering forensic evidence has been portrayed by the media as being easily seen, quickly collected, and rapidly analyzed—resulting in solved cases in a matter of hours.
And while the science has evolved from the early days of Scotland Yard to the current interwoven web of state and federal forensic laboratories, the collection and analysis of criminal evidence is far from simple and rarely speedy. Criminalists must be well versed in criminal law as well as technically competent in many aspects of the sciences including mathematics, chemistry and biology, in order to be successful in the forensic field.
Partial finger and palm prints left at a crime scene are either collected at the scene, or the items they are believed to have been deposited on are collected and brought back to the laboratory for chemical processing. The type of process used to collect latent prints depends on whether the surface is porous or nonporous. Porous items are processed with chemical reagents such as ninhydrin and iodine which react with amino acids excreted in fingerprint residue (Lee & Gaensslen, 2001). Latent prints that are absorbed into porous items such as paper, pottery, and untreated wood will develop purple when processed with ninhydrin and brown when developed with iodine. The colored print that results will fade over time so it is necessary for the analyst to photograph the image.
Nonporous items consist of items that do not easily absorb liquid such as metals, plastics, Styrofoam and treated wood. Nonporous items can be processed using various powders and a brush. Black powder is a universal powder that can be used on most surfaces. Magnetic powder works well on surfaces such as Styrofoam, marble and tile. Bright fluorescent powders are typically used on human skin and the dashboards of cars. Items can also be processed using cyanoacrylate, which is fumed in a chamber, and causes a white residue to form on the ridges of latent prints. After giving the glue time to solidify, powders or dye stains can be used to further enhance the white print detail. Dye stains include Ardrox and Rhodamine 6G. Dye stains are most successfully used on surfaces such as plastic bags and firearms (International Association of Identification, 2007).
Comparison of unknown fingerprints to known inked prints involves analysis of class and individual characteristics seen within the ridge detail of the finger or palm print. If suspects are not provided to the analyst for comparison, latent prints of good quality may be entered into the Automated Fingerprint Identification System (AFIS). This database will pinpoint geometrical similarities between print characteristics and return a candidate list of possible matches from a database of arrested individual’s finger and palm prints. The fingerprint expert then examines all possible candidates to determine whether there is an identification among the listed candidates.
Firearm examinations include an attempt to determine if a bullet was fired from a specific weapon as well as matching casings to specific types of weapons. When a bullet is fired from a weapon, it leaves individual characteristics such as rifling, pin impression, breach face markings and extractor marks. These markings are a result of the individualizing characteristics produced during manufacturing of weapons.
In order to visualize these characteristics, a comparison microscope is used that allows an analyst to view both an unidentified spent bullet and/or casing to a known sample in the same field of view. The known sample is collected by firing standard ammunition in a controlled setting from a weapon collected in an investigation (Swanson, Chamelin, Territo, & Taylor, 2009).
Spent bullet evidence can provide information as to the type of firearm the ammunition was fired from, which would be considered class evidence or, in rare circumstances, individualizing evidence when the spent bullet or casing has clear striations and is not mangled from hitting a target.
If only spent bullets or casings have been submitted to the laboratory without a weapon then the evidence can be entered into a database known as the National Integrated Ballistic Information Network (NIBIN). The database will scan and compare the individual characteristics or striations on the evidence item to images obtained from recovered firearms in an attempt to make a match.
Serology involves the examination of evidence for body fluids such as blood, semen and saliva. Stains on clothing and bedding are tested using color tests such as Phenolphthalein for the presence of blood and the Acid Phosphatase test (AP) for seminal fluid. Phenolphthalein will result in a rapid red color reaction and the AP test results in a purple color if semen is present on the evidence item.
Confirmatory tests for the presence of spermatozoa involve extracting the sample and placing the extract on a slide to dry. The slide is then stained for easier viewing of sperm cells and the slide is viewed under a compound microscope. If spermatozoa are identified, the extract is used in a Polymerase Chain Reaction which is a DNA test that produces a DNA profile of the sperm cell and any other cell material within the stain such as epithelial cells found in vaginal fluid or saliva (Saferstein, 2009).
Blood samples immediately undergo DNA processing after preliminary color testing is done if the sample is not too degraded due to the environment or bacterial growth. Other items of serological significance that a DNA profile can be obtained from include saliva, tissues, teeth, bone, and fingernail scrapings. If a DNA profile has been successfully developed off of an item of evidence such as a vaginal swab from a rape kit and there is no suspect in the case, the DNA profile will be entered into the Convicted DNA Indexing System (CODIS) database. Once submitted to CODIS the profile is compared to known samples in the system obtained from suspects convicted of various felony crimes which may result in identification. Identification of body fluids through DNA profiling is individualizing except when it comes to identical twins whose identity can only be determined through fingerprint comparison
Drugs found at crime scenes or on suspects during arrest can be screened in the field using color tests. These color tests narrow down the drug class or eliminate the substance altogether of being an illegal substance. These tests include the Marquis test for heroin and morphine and the Duquenois-Levine test for marijuana.
Further testing involves confirmatory tests that are performed in the laboratory such as precipitin tests are microcrystalline tests. Precipitin tests can determine whether a bloodstain is human or animal and microcrystalline tests which are similar to color tests but instead of a color reaction result in crystal formation to identify a drug.
Instrumentation that is used to identify complex mixtures of narcotics is referred to as a Gas Chromatograph (GC). The GC separation technology can determine what cutting agents were used in drugs or specific ingredients used to make mixtures such as whether ephedrine or pseudoephedrine was used to create a methamphetamine batch.
When the GC is combined with a Mass Spectrometer, quantitative data can be calculated as well. Analysts run the samples through the instruments and receive a spectrum printout which can be compared to the known spectra of various illegal drugs such as heroin. Individualization can be accomplished for most drug samples although it is not always known what all the constituents of a mixture are (Saferstein, 2009).
This is just a brief overview of some of the technology involved in the forensic disciplines seen in most state forensic laboratories. As technology advances some of these forensic disciplines will become obsolete. Document examinations are very rarely requested in crime labs anymore. This is a result of society progressing to computers rather than handwritten or typed documents. Microscopic identification of hair is also not requested as often due to the advancements in DNA analysis and hair root extractions.
About the Author: Dena Weiss is a full-time professor at American Public University System, teaching courses in criminal justice and forensic science. She has been a crime scene investigator for more than 17 years and is currently a fingerprint expert for a central Florida police department. Prior to that position, she was a serologist for the Florida Department of Law Enforcement. Her court experience includes testifying in court cases in over 15 Florida counties in more than 200 federal and circuit court cases.
Dena has a bachelor’s degree in chemistry and sociology from Mary Baldwin College and a master’s degree in forensic science from Virginia Commonwealth University. She is currently working on her PhD in business administration with an emphasis in criminal justice.
Gaensslen, R., Harris, H.A. & Lee, H. (2008). Introduction to Forensic Science & Criminalistics. New York: McGraw Hill Higher Education.
International Association of Identification Chesapeake Bay Division. (2007). Latent fingerprint processing techniques – selection & sequencing guide.
Lee, H. &. Gaensslen, R.E. (2001). Advances in Fingerprint Technology. New York: CRC Press.
Saferstein, R. (2009). Forensic Science from the Scene to the Crime Lab. New Jersey: Pearson Prentice Hall.
Swanson, C., Chamelin, N.C., Territo, L. & Taylor, R. (2009). Criminal Investigation. New York: McGraw Hill Higher Education.