The Impact of Water Exposure on Touch DNA Persistence

Enhancing DNA Analysis: The Impact of Water Exposure on Touch DNA Persistence

Introduction

Criminals often dispose of incriminating evidence, including biological materials, in bodies of water to avoid detection. Water can significantly affect the preservation of touch DNA, which is left behind through physical contact. While previous studies have explored the impact of water immersion on blood, saliva, and semen, there is limited research on touch DNA. This article presents a comprehensive investigation into the persistence of touch DNA after prolonged exposure to water.

Effect of Water Exposure on Touch DNA Persistence

To assess the longevity of touch DNA cells after water exposure, researchers conducted experiments using different substrates and water types. Plastic, metal, and ceramic substrates were submerged in seawater or tap water. Diamond™ Nucleic Acid Dye, a staining agent, was used to visualize the touch DNA cells. Cell counts were compared before and after water exposure, ranging from 6 hours to 5 days, to analyze the cell loss rate over time. Combining the data for different substrate and water-type conditions revealed a logarithmic increase in the percentage of cells lost over time.

Influence of Substrate and Water Type

The material on which touch DNA cells are deposited significantly impacts their persistence. The study tested three substrates (plastic, metal, and ceramic) in tap water and seawater. It was found that metal substrates retained cells for a longer time compared to plastic or ceramic substrates. The influence of water type depended on the substrate, as cell persistence varied on metal surfaces. However, plastic and ceramic substrates showed similar cell loss over time regardless of water type.

In tap water, all substrates showed insignificant cell loss between 6 and 24 hours. However, after 24 hours, differences between substrates became apparent. Metal samples reached a plateau in cell loss after 6 hours, while plastic and ceramic substrates had a faster decrease in cell loss. In seawater, the trends of cell loss varied between substrates. Plastic samples gradually increased, while plastic and ceramic samples exhibited more significant variations. Metal and ceramic substrates showed minor differences in cell loss, suggesting only minor variations between nonporous substrates.

Advantages of Visualizing Cells

The ability to visualize cells after water exposure holds excellent potential for forensic laboratories in triaging evidence and conducting targeted sampling. Using techniques like Diamond™ Nucleic Acid Dye, forensic examiners can identify and prioritize items likely to yield valuable information for DNA testing. This approach can significantly improve efficiency by avoiding unnecessary testing on items that are unlikely to provide valuable insights for forensic investigations.

The Impact of Water Exposure on DNA Persistence

Previous research has shown that increased water exposure has a detrimental effect on the persistence of DNA across various aqueous conditions. Offenders have exploited this knowledge by subjecting assault victims and their clothing to water exposure. Factors such as substrate type, water type, temperature, and movement have been identified as additional variables influencing DNA persistence. While the magnitude of the effect may differ for different biological materials, the overall impact on DNA persistence remains consistent.

Long-term Exposure to Aqueous Environments

Extended exposure to aqueous environments has reduced the persistence of cellular material from various biological materials. Most studies on different biological materials have observed that DNA profiles become unobtainable after a certain number of water exposure days, even under optimal conditions. However, some studies have reported successful DNA profiling after prolonged exposure, highlighting the influence of substrate type. For instance, bedsheets exposed to river water yielded informative DNA profiles after six weeks, while bloodstains on nonporous substrates required sampling within 24 hours to achieve successful profiling.

The data for different timeframes were combined to assess the overall impact of water exposure on cell persistence. The results showed a logarithmic loss of cells with increased water exposure, supporting previous studies on DNA persistence. The percentage of cells lost increased over time, with the most significant loss occurring between 6 and 18 hours. The similarity in cell loss percentages between timeframes of 2, 3, and 5 days could be attributed to either a plateau in cell loss or a lower-than-predicted loss after five days. Similar loss trends were observed in the literature, emphasizing the importance of considering cell count in forensic investigations.

Influence of Water Type

The type of water to which biological materials are exposed also affects DNA persistence. Freshwater, particularly tap water, has been found to negatively impact cell and DNA persistence less than other water types like seawater. Seawater’s increased microbial and ionic interactions could contribute to higher cell loss, mainly on metal substrates. Cold, stagnant tap water provides the most favorable conditions for preserving DNA underwater. Exposure to other aqueous conditions results in increased loss of cellular material, further reducing the window of time in which useful biological material can be successfully recovered.

The trends of cell loss between water types differed for each substrate. Plastic samples showed similar trends, with statistically significant differences observed only between the 6-hour and 2-day timeframes. Metal samples exhibited less variation over time and had significantly fewer cells lost in both water conditions compared to plastic and ceramic. The difference between tap water and seawater was significant for metal and ceramic substrates at various timeframes.

Visualizing Touch DNA Using Diamond™ Nucleic Acid Dye

Over the years, researchers have tested various fluorescent dyes to visualize touch DNA, achieving varying levels of success. They reported Diamond™ Nucleic Acid Dye (DD) coupled with a Dino-Lite fluorescence microscope as a portable in situ method for visualizing DNA. DD, an external groove-binding dye, has the capability to permeate cell membranes and bind to both double and single-stranded DNA. Previous studies have demonstrated a correlation between the number of cells observed using DD and the quality of the DNA profile. Consequently, DD has been suggested as a tool for triaging evidence in forensic laboratories and screening swabs and tape-lifts before downstream processing. However, it is essential to note that background autofluorescence of substrates presents a limitation to the application of DD, as it makes differentiating cells from the background challenging. It is advisable to adopt a more careful approach and use DD only for certain types of casework samples instead of using it as a screening test for all samples.

Conclusion

This proof-of-concept study has provided insights into the persistence of touch DNA cells after water exposure. The findings indicate that over 50% of cells can remain viable on various submerged items for at least five days, justifying DNA analysis. The study highlights the impact of water exposure on cell persistence, showing a logarithmic loss of cells with increased exposure. Substrate type plays a crucial role, with textured surfaces exhibiting higher cell loss than smooth surfaces. Water type also influences cell persistence, with seawater causing increased cell loss due to microbial and ionic interactions. Understanding these factors is vital in forensic investigations, where the identification and preservation of cellular material are critical for DNA analysis.

By considering the influence of substrate type and water type on cell persistence, forensic examiners can make informed decisions when selecting items for DNA testing. Visualizing touch DNA using techniques like Diamond™ Nucleic Acid Dye provides a valuable tool for triaging evidence and conducting targeted sampling. It allows for more efficient utilization of resources by focusing on items likely to give helpful information to forensic investigations.

The results of this study contribute to the body of knowledge on DNA persistence in aqueous environments, particularly regarding touch DNA. By understanding the factors that affect cell persistence, forensic scientists and investigators can enhance their ability to extract valuable DNA evidence and answer critical questions in criminal cases.