Trace Evidence Scrapings: A Valuable Source of DNA? by Stouder, Reubush, Hobson, and Smith (Forensic Science Communications, October 2001)
October 2001 - Volume 4 - Number 4
Research and Technology
Trace Evidence Scrapings:
A Valuable Source of DNA?
Stacy L. Stouder
DNA 1 Unit
Kimberly J. Reubush
Trace Evidence Unit
Deborah L. Hobson
DNA 1 Unit
Jenifer L. Smith
DNA 1 Unit
Federal Bureau of Investigation
Collection and analysis of trace evidence (e.g., hairs and fibers) from evidentiary materials may indicate an association with a suspect, a victim, or both to the evidence. The collected trace evidence debris may also contain sufficient cellular material removed from an item to permit identification of the wearer of that item through DNA analysis. To test this hypothesis, T-shirts and hosiery were worn by FBI Laboratory personnel for a period of time and then scraped for trace evidence. The pillboxes used to collect the scrapings were swabbed with applicators moistened with sterile water and processed alongside a friction swab of the item. The amount of DNA obtained from trace evidence scrapings was compared to the amount of DNA obtained from a friction swab of an item. Samples were then amplified by the polymerase chain reaction (PCR) using the AmpFlSTR® Profiler Plus™ Amplification Kit (PE Applied Biosystems 1998), and the results were compared. This study demonstrates that trace evidence debris can provide a sufficient quantity and quality of DNA to potentially identify the wearer of an item.
In general, evidence from violent crimes submitted to the FBI Laboratory is currently processed for trace evidence, screened for biological fluids and tissues, and then analyzed by DNA typing methods, if appropriate. One method of trace evidence collection in the Laboratory involves scraping items to remove hairs, fibers, and other debris adhering to the item. Because skin cells are constantly shed, it is likely that the collected debris contains cells from the individual who wore the garment. These skin cells contain nuclear DNA and may have evidentiary value for DNA analysis. Minute amounts of biological material may yield sufficient quantities of DNA. For example, in a study of dandruff and its potential use in forensics, investigators found that 1–1.5 mg of dandruff yielded 30–40 ng of DNA (Lorente et al. 1998). Studies show that limited quantities of DNA are sufficient to obtain a profile from several polymorphic short tandem repeat (STR) loci (Moretti et al. 2001). Thus, skin cells obtained from scraping an item of clothing could contain sufficient DNA to potentially determine the source of the wearer by STR analysis (Budowle et al. 2000).
In a bank robbery, for example, when only masks or gloves are recovered, cellular debris may be the only biological material available for forensic DNA analysis. Currently, evidentiary items that have been previously screened for trace evidence are swabbed to collect DNA from skin cells or cells present in saliva or sweat along friction ridges (i.e., collars and cuffs) or other surfaces where cells may be deposited (i.e., mouth and nose areas of a ski mask). However, substantial cellular debris may be removed during the scraping process prior to the item being analyzed for DNA evidence. In this study, the amount of DNA recovered and the resulting DNA profile from the pillbox scrapings from an item of clothing to that obtained by swabbing the garment were compared.
Eleven employees wore a freshly laundered item of clothing, with the exception of one participant who wore new panty hose, for a period of time, generally one day. Females wore hosiery, and males wore T-shirts. After the workday, the items were collected and stored in clean paper or plastic bags and were maintained at room temperature until analysis. All items were processed for trace evidence by scraping the inside and outside of the items, which were hung from a metal rack over a table covered in clean paper. Debris was collected, transferred to, and stored in a pillbox. The processing room was cleaned with Cavicide® (Micro-Aseptic, Palatine, Illinois), and the collection paper was changed after each item was scraped to avoid potential contamination among items (FBI Laboratory 2000; SWGMAT 2000).
DNA analysis was performed on the items, along with their corresponding pillboxes containing the trace evidence debris, for all study participants, their cohabitants (primarily spouses), if appropriate, and the personnel conducting scraping and DNA analysis (FBI Laboratory 1999). Potential cellular material was collected from each item using a sterile swab moistened with sterile water. Full-length panty hose were swabbed on the inside trunk and foot areas, knee-high nylons were swabbed entirely on the inside and outside, and T-shirts were swabbed around the neckline. Hereinafter these are referred to as friction swabs. A second set of swabs was used to collect the material found on the inside of the pillbox (excluding any hairs and fibers). Following an organic extraction (Comey et al. 1994), DNA was quantified using the human specific slot blot hybridization assay ACES™ Human DNA Quantification Probe Plus Kit (Life Technologies, Gaithersburg, Maryland; Budowle et al. 1995). Samples were amplified using the AmpFlSTR® Profiler Plus™ Amplification Kit (PE Applied Biosystems, Foster City, California) and typed by capillary electrophoresis on an ABI Prism™ 310 Genetic Analyzer (PE Applied Biosystems, Foster City, California; PE Applied Biosystems 1998) using the GeneScan® and GenoTyper® software.
|Figure 1. The quantity of DNA recovered from friction swabs of an item and pillbox swabs. Item 1 was worn on multiple occasions, which may account for the increased level of DNA recovered from that item. Note that the knee-high nylons (Items 5, 6, and 8) yielded a minimal amount of DNA from both the friction and pillbox swabs. Click here to view enlarged image.|
The results of this study demonstrate that DNA recovered from friction swabs and trace evidence debris may contain a suitable quantity and quality of DNA to conduct DNA analysis. However, for 9 of the 11 item pairs analyzed, the quantity of DNA recovered from the pillbox swab was equal to or greater than that from the friction swab (see Figure 1 and Table 1). The average amount of DNA recovered was approximately 4 ng from the friction swabs and 21 ng from the pillboxes. In this study, on average, more DNA was recovered from T-shirts than from hosiery, with knee-highs yielding the least quantity of DNA. One of the T-shirts was worn on several occasions without laundering (Item 1). Not surprisingly, the quantity of DNA recovered from this item (100 ng from the pillbox) exceeded the DNA recovered from the other T-shirts in this study (20 to 40 ng).
Table 1. Quantity of DNA Recovered From the Friction Swab of an Item Compared to the Corresponding Quantity Recovered From the Trace Evidence Debris Collection (Pillbox)
|Friction swab||Pillbox swab|
|# of donors1||Source of DNA||DNA
|# of donors1||Source of DNA||DNA
|5||3||Wearer||Spouse, unknown||1.55||Insufficient profile|
|6||4+||Wearer||Spouse, unknown||< 1.55||3+||Wearer||Spouse, unknown||< 1.55|
|8||3+||Wearer||Spouse, children?2||4.55||3+||Wearer||Spouse, children?2||1.55|
|Average for friction swab collection||3.55||Average for pillbox
1Number of donors determined by number of alleles per locus. Peak height (in relative fluorescence units) information also used.
2Although the spouse and children cannot be excluded as potential contributors to these mixtures, they cannot account for all the non-wearer’s DNA present.
|Figure 2. Single-source DNA profiles obtained from DNA extracted from the friction swab of a T-shirt and a pillbox swab of trace evidence debris. The top panel (Q7-1) represents the profile from the friction swab, whereas the lower panel shows the profile from the pillbox swab. x = size in base pairs; y = relative fluorescence units. Click here to view enlarged image.|
|Figure 3. The results from Item 11, which was a new pair of panty hose, show a mixture from the friction swab (upper panel) and a single-source profile from the pillbox swab (lower panel). These findings suggest that the additional type may have originated from the manufacturing site or the wearer’s environment. x = size in base pairs; y = relative fluorescence units. Click here to view enlarged image.|
|Figure 4. An example of a single-source STR profile obtained from DNA from a pillbox containing trace evidence and a multi-donor profile obtained from the friction swab of the same item. The top panel (Q2-1) shows the profile of the friction swab containing DNA from three or more individuals including the wearer and her spouse. The second panel illustrates the single-source profile obtained from the pillbox swab of that same item. Click here to view enlarged image of the top two panels.) Panels 3 and 4 are the DNA profiles of the wearer and her spouse, respectively. Click here to view enlarged image of Panel 3 and Panel 4. x = size in base pairs; y = relative fluorescence units.|
A summary of the results is listed in Table 1. In general, the STR profiles derived from pillbox debris contained no DNA or less DNA from a nonwearer source than those profiles from the friction swabs. Single-source profiles were obtained from 50% of the pillbox profiles (i.e., 5 of 10; 1 item contained insufficient DNA, and no STR profile was obtained) compared to 27% of the friction swabs. An example of a single-source profile from both the pillbox and friction swabs can be seen in Figure 2. Unlike the donors of the other T-shirts, the donor of Item 7 is unmarried. Two of the hosiery donors (Items 9 and 11) are also unmarried. The DNA profile from the pillbox and friction swabs from Item 9 are from a single source (the wearer). Figure 3 depicts the STR profile results from Item 11. Whereas the DNA recovered from the pillbox was a single source, the friction swab contained a major (the wearer) and an unknown minor contributor. The hosiery was removed from the original packaging and worn for an afternoon prior to testing. During this time, the only individual to come in contact with this item was the donor. These results suggest that the extraneous DNA profile may have originated at the manufacturing site or was transferred from the wearer’s environment (Locard 1930). Nevertheless, the wearer of this hosiery is clearly identified as the major contributor of DNA in the STR profile.
Figure 4 shows the DNA results from a married individual. A single-source profile of the wearer was obtained from the pillbox. However, the friction swab produced a mixture of DNA from at least three individuals—both the wearer and her spouse are included as contributors. In four sample pairs, a mixture was present in both specimens. However, there was less contribution from the minor contributor(s) (i.e., the spouse or an unknown) in the pillbox profile than the friction swab profile. Furthermore, there was only one sample pair in which the profile from the pillbox was a mixture (the wearer and their spouse) and a single-source profile was obtained from the friction swab (Item 10). In all profiles containing a mixture of DNA, the individual who wore the item is either the major contributor (12 of 13) or one of the major contributors (1 of 13). To account for the minor contributors in samples with mixtures, the cohabitants’ (primarily spouses) STR profiles were compared to the results. In most cases, these individuals accounted for the other source of DNA in the mixture (Table 1). Additional DNA contributors were found in some samples, and their source remains unknown. However, all DNA Unit personnel and the trace evidence technician participating in the study were excluded.
This study demonstrates that trace evidence scrapings can provide a source of material for forensic DNA analysis. On average, the pillbox scrapings yielded greater quantities of DNA, which increases the chance for obtaining a result and for preserving the sample for possible future retesting (National Research Council 1996). Furthermore, the DNA profiles tend to be less likely to exhibit mixtures from the scrapings than from the friction swab, which facilitates profile interpretation. Additional studies are being conducted to test the possibility of DNA carryover in washing machines during the laundering process in an effort to explain the presence of cohabitants’ DNA on items of clothing.
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