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Forensic Science plays a very important part in the judicial system of any Country. Being an applied science, it is imperative to have a strong foundation in the natural sciences and also the development of practical skills in the application of these sciences to a particular discipline. Examination, analysis, interpretation, reporting, and testimonial support of physical evidence, being an essential part of Forensic Science, a Forensic scientist must be adept at incorporating knowledge and skills required in these jobs. Since eyewitness accounts could be compromised in truth or biased because of any number of reasons; physical evidence has gained enormous importance in criminal investigations during the past few decades. Physical evidence comprising elements like DNA, fingerprints, and trace evidence is capable of independently and objectively linking a suspect/victim to a crime, disproving an alibi, or developing important investigative leads. Not only nailing the offender, physical evidence can also save the innocent, disproving the charges leveled against him.
The most important stages of physical evidence examination are the initial stages. If conducted properly, they prove to be decisive in the successful resolution of criminal investigations. The recognition, collection, and preservation of physical evidence, such as DNA, should be done properly as these have been rigorously scrutinized and challenged in court. The advancement of applying DNA typing methods to Forensic applications over the past decade and half is a major one in the examination of biological evidence. Due to its astonishing sensitivity and power of discrimination, DNA analysis has become a critical part of forensic science, forensic medicine, and anthropology and in solving cases of paternity disputes. The physical evidence commonly submitted to forensic science laboratories for examination varies drastically. In the early days, DNA analysis could be suitably conducted only from evidence that had some biological substances having nucleated cells. This constraint has now been dispensed within the last few years with the invention of mitochondrial DNA sequencing in the forensic arena. Now, DNA is successfully isolated and typed from common biological specimens such as bones, blood and bloodstains, semen and seminal stains, tissues, organs, teeth, hairs, fingernails, saliva, urine, and other biological fluids. Only the quantity of DNA extracted varies. An important point to note is that compared to laboratory conditions, crime scene samples may contain significantly less usable amounts of DNA depending upon the prevailing environmental conditions. Gastric fluids and fecal stains can also give DNA strains but it can prove difficult to generate a complete profile from these samples due to significant degradation. Several factors affect the ability to obtain a DNA profile. The first and the foremost factor is sample quantity. Polymerase chain reaction-based (PCR) DNA typing methods are efficient and their sensitivity is noteworthy but still limited. The second issue of concern is sample degradation. Even a substantially large blood stain exposed for a prolonged period of time to the environment or to bacterial contamination can render the DNA in it useless and unsuitable for further analysis. The third issue of concern is sample purity. Impurities like dirt, grease, some dyes in fabrics, and other substances can cause serious doubts over the results of DNA typing. Although DNA allele “A” cannot turn into allele “B” by environmental degradation of the samples, but it is certain that the chances of getting a complete DNA profile from the sample are reduced.
Evidence Collection, Transfer & Deposit
Linkages or associations (e.g. person-person, person-other physical evidence, or person-crime scene) can be made using DNA evidence. Usually, biological evidence can be transferred by direct deposit or by secondary transfer. Blood, semen, body tissue, bone, hair, urine, and saliva, in short, any biological evidence, can be transferred to the crime scene by direct deposition through an individual’s body/clothing or any object. Biological fluids once they are deposited can adhere to the surface becoming stains. Tissue or hair can also be transferred by direct contact. Physical evidence when transferred through an intermediary (person or object) is called secondary transfer. Blood, semen, tissue, hair, saliva, or urine can be transferred to a person, object, or location through an intermediary. There is no direct contact between the original source (donor of the biological evidence) and the target surface. A secondary transfer is not conclusive evidence. It may, but does not necessarily, establish a direct link between an individual and a crime. The role of secondary transfer on the veracity of DNA results has been debated. In the more sensitive DNA typing methods, such as mitochondrial DNA sequencing and low-copy number PCR, secondary transfer is a significant concern.
Selecting the samples which warrant further examination is the first step in a criminal examination. This selection could make or mar the whole investigation. It is very challenging and pivotal as crime scenes are a far cry from the controlled environment of the laboratory, being very complex and chaotic. In such circumstances, an investigator with thorough experience of crime research would be very systematic in his evaluation of the crime scene and hence is a necessity. His ability of recognizing things so as to identify probative evidence both at the crime scene and the laboratory amongst a huge pile of randomly scattered redundant, irrelevant, or unrelated items is called Recognition. For example, even after collecting multiple bloodstains from the periphery of a stabbing victim, one might not be able to point to the person who has committed the crime. Certain forensic principles are employed in the recognition process such as pattern recognition and analysis and physical properties observation.
All biological evidence undergoes degradation. If it is carefully collected and stored then, it will ensure that all the useful information can be gathered from it by analyzing it. All biological evidence is subject to deterioration. The careful collection and storage of this evidence will help ensure that this evidence is preserved so that useful information can be obtained from its analysis. Impurities like dirt, grease, some dyes in fabrics, and other substances can seriously reduce the efficiency of the DNA typing process.
The physical evidence commonly submitted for examination to forensic science laboratories is countless in variety. However, only evidence that is biological in nature could be subjected to DNA analysis. Biological materials like blood and bloodstains; Semen and seminal stains; Tissues and cells; Bones and organs; Hairs with follicles and Urine and saliva (with nucleated cells) are the major sources of DNA samples. Other types of biological evidence, such as tears, perspiration, serum, and other body fluids without nucleated cells are not conducive to DNA analysis. Liquid biological specimens, like semen, when deposited, stick to the cloth or substratum and become stains. Non-fluid biological evidence, such as tissue, bone, or hair, can also be transferred by direct contact and deposited. There might even be an intermediate medium, facilitating the transfer of blood, semen, body tissue, hair, saliva, or urine to a victim, suspect, witness, object, or location. In a secondary transfer, there is no direct contact between the original source (donor of the DNA evidence) and the target surface. The role of the transfer intermediary could be played by a person, object, or a location. A secondary transfer does not provide very conclusive or concrete positive proof of a direct link of a person with a specific crime. The biological evidence remains on the target surface by absorption or adherence once it has been transferred through direct or secondary transfer. Liquid biological evidence is usually absorbed and solid state evidence adheres to the surface.
The method applied for collection generally depends upon the physical state and condition of the biological evidence. The following general guidelines apply for the collection of biological evidence for DNA analysis. It’s always advisable to collect the maximum possible quantity of material so that it is ensured that sufficient DNA for the testing purposes would be recovered. But, in doing so, it should be kept in mind and taken care of that the minimum possible impurities like additional dirt, grease, fluids, and other material from the surrounding area are collected as they are known to adversely affect the DNA typing process. Due care should be taken to ensure that each biological specimen is packaged according to the suitable forensic techniques. Prompt delivery to the forensic laboratory should be ensured as soon as the specimens are packed. A cool, dry environment is a must before they are submitted for testing to minimize specimen deterioration. Blood and seminal fluid can be conclusively matched back to an individual with the use of DNA profiling method. Otherwise, blood and some other body fluids can be said, with some degree of certainty, from a certain population group to which the individual belongs, by using the using the PCR method of analysis or the conventional serological techniques.
Liquid Blood Specimens at Crime Scenes
A clean and sterile syringe or disposable pipette is used to collect liquid blood and then transferred to a clean and sterile test tube. Blood clots can be transferred with the help of a clean spatula to a test tube. A clean cotton cloth can be used to soak up liquid blood or a blood clot but care should be taken to avoid areas containing only serum.
Wet blood samples, if they are collected, should be preserved in a suitable anticoagulant refrigerated. These specimens should be submitted to the laboratory as soon as possible.
Proper labeling of the samples with the case number, item number, date, time, location, and evidence with the collector’s name is a must.
Wet bloodstains especially those on smaller evidence should be allowed to air dry before being taken for collection and laboratory analysis. The quantity itself makes it valuable enough for care and be preserved in such a way that the integrity of any bloodstain patterns during the procedures like packaging and transportation remains intact. Large objects, which are physically not possible to be moved from the crime scene may have wet bloodstains on them, the wet blood should be transferred onto clean cotton cloth. The stained cotton cloth must be allowed to air dry before packaging in a paper container. Labeling of each and every sample is of utmost importance.
Small objects like weapons, garments, and other movable objects, having dried bloodstains on them should be collected as whole but separately packed so as to not compromise on their authenticity. Each article should be placed in a separate paper container and sealed and labeled properly. A detailed sketch of the bloodstain should be taken. A very efficient method to get stain samples is tape lifting them scraping off the object onto clean piece of paper. The tape lifter or the paper with the blood should then be put into a “druggist fold”, and put into a sealed envelope. Proper labeling of each item should be done. When the bloodstain is such that it can neither be scraped off nor the support object can be cut, then the stain should be eluted onto a clean switch, and a cotton switch should be rubbed to moisten the stained area. The switch is then placed in a paper fold packet after being given ample time to dry. The packet then goes into an envelope, sealed and properly labeled. The procedure is repeated on an adjacent but unstained area of the surface containing the bloodstain to serve as the control. A portion of the item containing the bloodstain can be removed with the help of a clean sharp instrument if the stain is located on an object that can be cut. Care should be taken that each cutting is packed and labeled separately. To serve as control, an unstained portion of the item should also be collected.
Notes, photography, videotape, and sketching are the best methods to comprehensively document. To transfer the liquid semen to a clean, sterile test tube, a clean syringe or disposable pipette should be used. The case and item number, date, time, location, and name of the collector, should be affixed on the tube on a label. The specimen should be submitted to the laboratory at the earliest and kept refrigerated till then. Absorption can also be used to transfer liquid semen onto clean cloth. The cloth should then be air dried, packaged, sealed, and labeled properly. Panties, clothing, bed sheets, pillows, and other movable objects having seminal stains should be collected in the condition they are in.
A wet stain must always be allowed to air dry before the article is collected. A scalpel or scissors can be used to cut off the stained part if the stain is on a large object. Similarly, a scalpel can be used to scrap of a stain if the stain is on an immovable, nonabsorbent surface. Separate paper containers should be used to pack each item. It should also be properly sealed and labeled. The packed items should be promptly submitted to the laboratory and refrigerated in the interim.
In the case of sex crimes, the victim should be rushed to the hospital immediately and examined as soon as possible. Any injury received by the victim should be photographed. Oral, vaginal, and/or anal swabs should be taken from the victim and air-dried for one hour in a moving air source as soon as possible. This should be quickly because the body begins breaking down the various components in seminal fluid through drainage, enzyme activity, pH, etc. This can either be accomplished by the doctor at the hospital, or, upon collecting the kit from the doctor, the investigator should bring it immediately to a secure place and air dry it. The moisture in the swabs allows microorganisms to grow, hence destroying the evidentiary value.
Detailed notes, photography, sketches, and videotaping should be employed to describe each item of evidence. A clean pair of forceps should be used to pick up such kind of evidence. A clean container without any added fixatives should be used to place these items. Each item should be placed in a clean container without any added fixatives. The containers should be stored in refrigerators after proper sealing and labeling. No delay should be done in submitting the evidence to the laboratory. When collecting reference samples from decomposed bodies, in addition to the blood sample, the following should also be collected in addition to the blood samples, a portion of deep muscle tissue, certain organ tissue like heart or brain but not liver or kidney, A few intact molar teeth, after ensuring mouth x-rays have been taken if identification is a problem. and a sample of compact bone like femur. Care should be taken to collect samples from a part that is away from the site of injury. Say, if there is a head injury, then sample of brain tissue should not be taken. The specimens should be freezed and preservatives like formalin should not be used.
The root sheath is very vital in examination of hair. If a root sheath is attached, then DNA analysis using PCR technology can say that this hair came from a certain percentage of the population to which the suspect belongs. Without root sheath, a microscopic analysis can say that the hair has the same characteristics as the suspect’s hair and is similar to his or her hair. Hair found at the scene should be put in a paper packet and then placed in an envelope. 15-20 hair samples are required if a microscopic examination is being done.
Transport and Storage of Evidence
To preserve DNA, it is important to keep the evidence dry and at room temperature when transporting it. Once the evidence has been secured in paper bags or envelopes, it should be sealed, labeled, and transported in a way that ensures proper identification of where it was found and proper chain of custody. Plastic bags retain damaging moisture; therefore never place evidence that may contain DNA in plastic bags. Direct sunlight and warmer conditions also may be harmful to DNA, so avoid keeping evidence in places that may get hot, such as a room or police car without air conditioning. For long-term storage issues, local laboratory should be contacted. Refrigerate liquid blood samples (do not freeze). Air-dry all wet blood and other body fluid stains on evidence items (do not subject to heat). Until submission to the crime laboratory, freeze all stained items except for any metal or glass items (e.g. knives, bottles). Metal or glass items should be stored at room temperature and submitted to the laboratory as soon as possible. Evidence from the suspect and victim must be handled and packaged separately.
Contamination issues are today of utmost importance because extremely small samples of DNA can be used as evidence. Therefore, great attention is necessary when identifying, collecting, and preserving DNA evidence. Evidence can be contaminated when DNA from another source intermingles with DNA relevant to the case. A sneeze or cough over the evidence or touch of mouth, nose, or other part of the face on the area that contains the DNA to be tested can lead to contamination. Since PCR replicates DNA in the evidence sample, it will also copy the contaminants or unintended DNA leading to compromised results. With such minute samples of DNA being copied, extra care must be taken to prevent contamination. PCR will copy whatever DNA is present in the sample, it cannot distinguish between a suspect’s DNA and DNA from another source; if a sample of DNA is submitted for testing which is contaminated, and then all of the DNA will be replicated.
To avoid such situations, there is a method to find contaminations in the DNA sample. Examination of DNA band patterns in the laboratory easily reveals the presence of contamination. For example with an STR, a two-band pattern is observed. If one observes more than two bands, it becomes apparent that one could be dealing with a mixture of DNA from different sources.
DNA research and testing has heralded a new dawn in law enforcement. The crime scene workers need to be aware of the possibilities available “in the field” to ensure that proper evidence collection techniques are followed. The initial collection is of the evidence which is most fragile or most easily lost. Evidence, which needs to be moved is also given due care. Biological evidence is only valuable when they can be compared to known DNA samples collected from victims and suspects. The kind of specimens collected from a crime scene and the way they are preserved determine largely whether successful DNA analysis can be performed. Therefore, to sum up, The technique used to collect and document such evidence, the quantity and type of evidence that should be collected, the way the evidence should be handled and packaged, and how the evidence should be preserved, are some of the critical points for a forensic DNA testing program.
Bibliography
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Inman, K., Rudin N., An Introduction to Forensic DNA Analysis, (New York: CRC Press, 1997)
Ladd C, Adamowicz MS, Bourke MT, Scherczinger CA, and Lee HC. A systematic analysis of secondary transfer. J Forensic Sci 1999;44:1270-2.
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Lee HC, Palmbach T, Miller M. Henry Lee’s crime scene handbook. New York (NY): Academic Press. In press 2001.
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