Emily C. Lennert
touch, DNA, fingerprint, recovery, surfaces, glass, plastic, metal, paper, storage
- Ostojic, L.; Wurmbach, E. Analysis of fingerprint samples, testing various conditions, for forensic DNA identification. Science and Justice. 2017, 57, 35-40.
The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.
When an individual touches a surface, fingerprints may be deposited. The deposited fingerprint contains “touch” DNA, i.e. trace amounts of DNA deposited during interaction with an object or surface. Due to the small quantity of DNA present and external factors affecting DNA recovery, touch DNA samples often results in incomplete DNA profiles. External factors may include the effect of the surface from which the sample is collected, the amount of time between deposition and collection, the length of time that the depositor made contact with the surface, environmental degradation, and more. In addition, mixtures of DNA from multiple individuals may also be present. These factors contribute to difficulty in touch DNA sample interpretation. The authors of this study examined the effect of storage time, depositing hand, surface type, and mixtures to examine some of the challenges in touch DNA interpretation.
Six volunteers participated in this study. Samples were deposited on the chosen surface by pressing the thumb on the surface for 3 seconds. The surface was then swabbed with a sterilized cotton swab moistened with 4 μL of 5% Triton X-100. DNA was then extracted from the swabs using a prepGEM® Tissue extraction kit, according to manufacturer instructions. Following extraction, the quantity of DNA was determined using polymerase chain reaction (PCR), then amplified with an AmpFISTR® Identifiler® PCR Amplification Kit. DNA profiles were then acquired using a 3130xl Genetic Analyzer.
Effects of Storage Time
The effect of storage time was examined by depositing samples on glass microscope slides. Samples were stored in cleaned boxes, either closed or open, i.e. no lid, for 1, 3, 10, 20, and 40 days. In closed boxes, more than 25% of samples produced DNA profiles that were at least 70% complete after 40 days of storage. Examining the effect of time period, no significant difference in DNA profile completeness was seen between 1 and 3 days. At 10 days, a significantly lower number of DNA profiles showed high levels of completeness compared to 1 and 3 days. Shorter storage times were associated with more complete DNA profiles. Comparing open and closed boxes, at 10 days closed boxes showed more complete profiles compared to open boxes; however, this difference became insignificant at the 20 day mark. The differences between open and closed storage were determined to be negligible; however, the authors note that this is based on storage within a laboratory.
Effects of the Depositing Hand
Three right handed volunteers were selected for the determination of the effect of depositing hand, e.g. which hand left the fingerprint. The purpose of this trial was to determine whether the dominant hand would deposit more or less DNA. Each volunteer deposited a sample from their right and left thumbs, and the samples were compared. The DNA profiles obtained from the left hands, i.e. non-dominant hands, were determined to be more complete than those obtained from the right hands, i.e. dominant hands. The difference was significant for 2 of 3 volunteers.
Effects of Surface Type
Surfaces were tested to determine the effect of substrate, i.e. material, on DNA recovery. “Touch” samples were deposited on glass, plastic, paper, and a U.S. quarter dollar coin, i.e. metal. Samples were stored for 3 days prior to swabbing. Following sample processing, the quantity of DNA recovered from each substrate was compared. Full profiles were obtained for a number of glass, paper, and plastic samples; however, no full profiles were obtained for metal samples. Over half of the glass samples produced profiles at least 70% complete. Over 25% of plastic samples produced profiles that were at least 70% complete. Fewer than 25% of paper samples produced profiles that were at least 70% complete, and nearly no profiles were obtained from metal samples. The authors theorized that the lack of profiles from the metal samples may be attributable to DNA degradation due to the presence of copper and nickel in the U.S. quarter dollar coin.
Effects of DNA Mixtures
Three person mixtures of DNA were then examined. Samples were deposited on an empty, cleaned beer bottle with the label removed. The trunk of the bottle was etched into 6 equally sized sections for sampling. Six bottles were collected, with the order of volunteers changing between samples, e.g. ABC, CBA, BAC, etc. Each volunteer held the bottle for approximately 60 seconds. Each section of the bottle was swabbed separately to produces 36 samples of bottle sections. Overall, 26 of the 36 bottle sections showed 3 distinct DNA profiles. An additional 8 samples indicated 2 distinct profiles with the indication of a possible third. In the remaining 2, only 2 distinct profiles were identified. Volunteer C was the major contributor in 5 of the 6 whole bottle samples, even when they were not the last person to touch the bottle, indicating that the order of touching may not be as important as the amount of cellular material, i.e. DNA, each individual deposits.
- Time showed a significant effect on the completeness of the profiles starting at day 10 and beyond. No significant difference was observed between day 1 and day 3.
- After 40 days of storage, over 25% of fingerprint samples allowed for the recovery of profiles that were at least 70% complete, with some complete profiles recovered.
- In the comparison of dominant and non-dominant hands, a significant difference was observed for two of three volunteers, with non-dominant hand samples resulting in more complete profiles.
- The degree of completeness for profiles varied depending on surface, with glass producing the most complete profiles. Sample obtained from a U.S. quarter, i.e. metal, produced almost no profiles.
- Samples containing a mixture of DNA were obtained. From these samples, it was determined that the major contributor was not dependent on the individual that last touched the surface, but was dependent on the individual that shed the most cellular material. In other words, it was not the order that mattered, but the individual.
Touch DNA is based on the collection of small amounts of DNA to generate full or partial profiles. Therefore, it is important to understand the effect of time, surface, and the individual on the recovery of DNA profiles from touch samples.
- Touch DNA samples are susceptible to degradation over time; however, complete profiles may still be recovered after up to 40 days of storage.
- The individual’s cellular material deposition may have more of an impact on the major contributor in a mixture than the order in which the object was touched.
- Surface may have a significant effect on the recovery of touch DNA profiles.