14.2: Collecting Biological Evidence - DNA

Most crime laboratories can conduct DNA typing on blood and other human biological samples to determine genetic markers. DNA testing does not require a large sample of material; however, the samples must be collected in a manner to prevent contamination or degradation, which can render the DNA testing useless.

Types of biological evidence suitable for DNA testing include:

• Bloodstains/liquid blood
• Semen
• Vaginal fluid
• Hairs
• Saliva
• Mucus
• Bones, nails, teeth
• Fecal samples – large, harder samples will have a greater likelihood of cells on it than softer or loose samples
• Perspiration stains

As previously mentioned, feces and urine are not the best source of DNA owing to the low concentration of nucleated cells present in human waste. DNA also deteriorates quickly in urine if it is present. Mucus can be a good source of DNA, especially if there is bloody discharge with the mucus, but it may degrade if not processed within a month. Vomit is a poor source of DNA, but that does not mean it cannot be attempted. It is best to check with your laboratory before submitting urine or vomit for analysis.

The techniques used for collecting the specimen will depend on the stain itself and the surface upon which it was deposited. The absolute best method for collecting the specimen is to collect the item upon which the stain is found, like a shirt or pair of pants. Unfortunately, the specimen is usually found on a wall, door, bed, sofa, item of furniture, ceiling, or floor. When this happens, you have no alternative but to collect the specimen in situ. It is important to document the specimen’s location and relative position to the crime scene and other items of evidence with notes and photography prior to collection. The forensic technician must use universal precautions before handling any biological evidence to prevent contamination or the contribution of their own DNA to the specimen. Wearing proper personal protective equipment, especially an N95 mask and gloves, can also minimize exposure to possible infectious diseases and hazardous materials.

Gloves

Sweat from the collector’s bare hands will very likely contaminate evidence. Latex gloves provide a simple barrier against contamination for the evidence collector. The forensic technician should change their gloves each time a new piece of evidence is handled to prevent cross contamination of the evidence. Your author highly recommends to “double glove,” which is wearing one pair of latex gloves over another. This keeps the hands of the forensic technician covered at all times, even when changing gloves. Your author also recommends wearing cotton gloves underneath the first layer of latex gloves. This helps to collect perspiration and makes changing gloves much easier. The forensic technician should avoid wiping their forehead or scratching their noses or wiping their eyes while handling items of evidence. Fight the urge to handle a cellular telephone or adjust eyeglasses. Any time gloves have come in contact with such items, it is essential to change them before proceeding to handle items of evidence in the scene. Another thing to consider with latex gloves is the manner in which they are removed from the box. Obviously reaching into the box of gloves with bare hands has the potential to transfer the DNA of the forensic technician onto the very gloves that they hope will prevent contamination. The same applies to that pair of gloves you shoved in your trouser pockets at the beginning of your shift. The proper method of extracting a latex glove from the box is to look for the cuff, lift it with a hooked little finger, remove the glove from the box by the cuff, and carefully don the glove. Then lift the second glove with the gloved hand in the same manner. For gloves that you have prepared and placed in a pouch at the beginning of the shift, your author dons gloves in the manner prescribed, rolls out a clean sheet of butcher’s paper, and removes two gloves from the box. I place the gloves on the paper, roll them into a ball, and turn the ball inside out. This will keep the outer surface protected until used.

Masks

Excessive talking, sneezing, or coughing in the vicinity of the evidence may cause small, but detectable amounts of the DNA of the crime scene team members to be inadvertently transferred to the evidence. It is essential for all persons entering the crime scene to don a good mask, such as the N-95, in order to reduce the potential for contamination. Most of the contamination in a crime scene likely comes from the mouth or nose of the technician or detectives.

Collection devices

Disposable tweezers, scalpels and scissors are the best instruments to use while collecting human biological evidence. Your author uses a 10% bleach solution with a distilled water rinse to sterilize the instruments. Your author then places them in a down-draft workstation and subjects them to three minutes of short-wave ultra-violet light before placing them in a clear plastic zip-lock bag.

Touch DNA

Touch DNA is a “touchy subject” for the crime laboratories as many law enforcement agencies now submit hundreds of items of evidence for processing in cases ranging from petty theft to murder. Unfortunately, the laboratories are already overwhelmed with submissions and testing every possible item that a suspect may have touched or may not have touched is adding to an already burdensome backlog. Your author will try not to submit evidence unless he is certain that the item was handled by the suspect. For example, prior to processing an item for latent prints that was likely touched by the suspect, your author first inspects the item for the inherent luminescence or glistening of the undeveloped print. A clean white light used at an oblique angle can assist you in this examination. If it appears to be a partial print or smudge, your author will swab the area for touch DNA typing instead of developing it further for fingerprint impressions. In major cases, your author will use sterile powders and brushes to dust an area for potential latent prints. Smudges and partials that are developed can be collected for touch DNA typing. Cyanoacrylate fuming will develop prints that can later be typed for DNA. Unfortunately, most of our cyanoacrylate fuming chambers are not cleaned and sterilized between development and there may be the potential for cross-contamination. Your author uses shortwave UV light lamps that he places in the chamber prior to every use.

DNA extraction and swabbing

There are two basic methods for collecting DNA from items of evidence and from the crime scene, swatch collection and swabbing. If a biological stain is observed on a garment, like an undershirt or underwear, the stained area can be carefully cut from the garment with a pair of sterilized scissors, packaged in a pharmaceutical fold, and delivered to the laboratory for DNA extraction. The most common method used by forensic technicians to recover the DNA is through swabbing the item with either a sterile cotton swab or sterile foam swabs. The process is quite easy but requires some careful preparation by the technician in order to prevent swab contamination. Your author has a fool-proof method for the proper collection of swabbed DNA evidence. Your author first dons gloves and a face mask. He then selects a sterile cotton-tipped swab in sterile packaging. He writes the pertinent information from the case on the exterior of the packaging with a fine-tipped felt pen taking care not to pierce the packaging. He labels only one swab envelope and collects only one specimen from a specific location at a time as not to introduce confusion or a dodgy memory into the process. He opens the stick-side end of the packaging and cuts about ½” off the ends to facilitate ease in returning the swabs to the packaging later. He places a plastic applicator shield on the stick end of the swabs, generally getting two swabs into one applicator shield. With extreme caution not to touch the cotton-tips of the swab on anything, your author extracts both swabs at the same time with a quick motion. Holding a sterile water ampoule (your author does not recommend using saline) approximately ½” above the cotton tip, he drops one or two drops of water onto the swab. He then gently, with enough pressure to transfer DNA to the swab but not break the sticks, rubs the surface of the item. Once accomplished, your author pushes the applicator shield over the cotton tips and returns it back into the same sterile packaging from which the swabs were removed, because, why not, it is still clean evidence packaging and applicator shields will protect the swabs from contamination. Following this, the swabs are placed in a sterile drying cabinet to dry thoroughly. Once dried, the package containing the swabs is tape-sealed and placed into an evidence envelope with a corresponding evidence inventory label.

Figure \(\PageIndex{1}\): Swabbing a knife for DNA. Author's collection