Touch DNA”, or “contact DNA”, refers to DNA samples that are deposited on a surface, or on an object, by touch or by contact. I’ve had good luck with touch DNA recovery from pistol triggers, abandoned gloves or ski masks, and once from the earpiece of a telephone.
Some investigators have successfully submitted samples from latent fingerprints that they developed; the prints didn’t have enough detail for a comparison, so they swabbed them to check for DNA. A big word of caution here, fingerprint brushes, and powder, used at one scene can transfer trace amounts of skin cells from one surface to another, leading to a big problem with cross contamination.
Bodily fluids, blood, hair, and/or saliva are going to have a better chance of yielding viable samples, mostly because of the potential volume load of genetic material deposited in them; viable cells containing DNA left by touch, or contact by a suspect is usually minimal in contrast, as touch DNA recovery is limited by the fact that it is almost exclusively carried by transferred skin cells.
There are small amounts of fragmented DNA present on the surface of the skin, and sweat may also contain fragmented DNA. (Quinones and Daniel 2011) Lower layers of skin have viable cells – both of these types of skin cells can be rubbed off, or knocked off the body, and deposited on a surface. These cells can contain DNA material, and they are what we are trying to collect.
There are three different collection techniques used in processing an area for touch DNA, those are using a swab (moistened and dry), by lifting (using adhesive tape), and by cutting out a sample for submission (cutting a section of material from a suspect’s garment).
Swabbing – a sterile cotton swab is rubbed on the object to be tested. The preferred method is to use one swab moistened with sterile water, and one swab used that is dry.
Lifting- adhesive tape is used to pick up cells with the sticky side of the tape. This method isn’t always easy, depending on the tape size vs the size and shape of the area you are using it on.
Cutting out a sample – using a sterile cutting tool, a section of an item is cut out, and the cut-out section is submitting for testing. This is done mostly on items like garments or hats.
Touch DNA samples, being small (in cell numbers) are amplified using polymerase chain reaction technology to create identical copies of the recovered sample, to make the sample large enough for analysis.
In touch DNA, to yield a good sample, the suspect has to touch the item for an extended period of time, or they have to repeatedly touch the item, to leave a sufficient amount of viable skin cells to generate a usable sample. I have seen studies that show that a 10 second contact time with an object produced a viable sample around 30% to 49% of the time in controlled lab studies. In the real world, that number will likely be much lower, depending on the surface and the environment. That being said, if an item is handled numerous times, or contacted by skin with pressure or friction, then the amount of viable cells deposited would be expected to be greater (Phipps and Petricevic – 2006).
Rough objects, like checkered pistol grips, and serrated triggers, along with objects with void spaces, tend to collect more viable skin cells compared to smooth objects.
When considering collecting touch DNA, the investigator must carefully chose the area to be checked. Check an area specific to the crime, say a knife handle used in an assault as apposed to a table it was found on. Sampling should be restricted to very specific areas to avoid obtaining a DNA profile not useful for the investigation.
Once the area is processed, collected samples will be secured in paper bags or envelopes – plastic bags should never be used because they retain moisture.