Comparison of stubbing and the double swab method for collecting offender epithelial material from a victim's skin

doi:10.1016/j.fsigen.2011.04.019

Forensic Science International: Genetics

Volume 6, Issue 2, March 2012, Pages 219-223

https://doi.org/10.1016/j.fsigen.2011.04.019 Get rights and content

Abstract

After manual strangulation, epithelial cells originating from the offender can often be found on the skin of the victim. In order to obtain a conclusive DNA profile, it is important to secure as many epithelial cells from the offender and as few epithelial cells from the victim as possible. In this study, two methods for securing offender DNA were compared: the double swab method and an adapted tape-lifting method, so-called stubbing. 50 male volunteers were asked to simulate manual strangulation on the forearm of a female volunteer. After securing the epithelial material, DNA profiles were generated. The contribution of both donors to the samples was determined from the number of detected alleles, specific for each donor, and the average peak height of the donor-specific alleles. For the offender, in all cases except one, partial or full profiles were obtained and no difference between the double swab and the stubbing method was observed. For the victim, fewer alleles were detected by means of double swab than by means of stubbing. In conclusion, the double swab method performs slightly better than the stubbing method. However, from a practical point of view, the stubbing method may be preferred over the double swab technique.

Introduction

In cases of manual strangulation, strong physical contact occurs between offender and victim. Often, the offender leaves epithelial cells of himself on the victim's skin during this process. After securing these cells from the victim's skin, a DNA profile may be generated [1], [2] containing genetic information of the offender.

The double swab technique [3] is a common method for securing epithelial cellular material from objects [4], [5]. This method first applies a single, wet swab onto the surface of interest in order to loosen the cells, followed by a dry swab to secure the loosened cells. Apart from the double swab method, several studies describe tape-lifting as a method for securing epithelial cells from objects [6], [7], [8]. The tape-lifting method consists of a tape to which the epithelial cells adhere. It is mostly carried out by placing the tape over the surface of interest by a gloved finger, pressing it onto the surface and subsequently lifting it. Some publications report on securing epithelial material from the skin, instead of from objects, by means of tape-lifting [9], [10], [11]. However, these publications do not consider securing epithelial material of an offender from the victim's skin. Instead, they focus on collecting cell material from the person that is examined.

In a previous study performed at the Netherlands Forensic Institute, the tape-lifting method was adapted to create more distance between object and researcher (data not published). This so-called stubbing method is based on SEM (scanning electron microscope) stubs, for which a double-sided adhesive DNA-free tape was stuck to a SEM holder. In this way, contamination risks are reduced due to a greater distance between user and object, and pressure can be applied more evenly. The stubbing method was directly compared to the double swab method, which has been tested before for securing offender epithelial cells from a victim's skin [12]. Fifty independent samples were obtained for the stubbing as well as the double swab method by simulating manual strangulation using volunteers with known DNA profiles. To our knowledge, this is the first time that the stubbing method is investigated for securing epithelial cells from the skin of a victim and, in addition, both double swab and stubbing results are compared for such a large population.

Section snippets

Experimental set-up

50 couples, each consisting of one man and one woman, were matched based on their DNA profiles. The DNA profiles of each couple had a maximum of eight overlapping alleles out of a total of 30 alleles. Volunteers were asked to not wash their hands 2 h prior to the experiment. Before simulating strangulation, the man rubbed his hands for 5 s to loosen epithelial cells. Then, the ‘offender’ (in our study always male) placed his dominant hand on the forearm of the victim (in our study always female)

Results

The optimal method for securing offender epithelial cells from the skin of a victim should satisfy the following conditions. (1) It should secure as much offender DNA as possible. (2) The ratio offender DNA:victim DNA should be as high as possible to minimize the disturbing influence of the victim's DNA. In case of manual strangulation, securing epithelial cells of the victim can hardly be avoided. Consequently, peaks corresponding to the victim's DNA will also be visible in a DNA profile. In

Discussion

In this study, an adapted tape-lifting method to secure offender DNA from a victim's skin was examined. This method was compared with the double swab method in a large random population. It was found that both methods perform equally well in securing offender DNA, both regarding the number of detected alleles and the peak heights. For securing victim DNA, small differences between both methods were observed. Concerning the number of detected offender alleles, no significant difference between

Acknowledgements

We would like to thank all volunteers for their willingness to simulate strangulation. We also would like to thank Fleur Beemster and Wiljo de Leeuw who developed the stubbing method and Ivo Alberink who helped us with the statistical analysis of the results.

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The analysis of DNA mixtures can be problematic, especially when in trace quantities such as when a biological sample is deposited onto a substrate which contains background DNA (for example, in the case of touch DNA deposited onto a garment containing the wearer’s DNA). We conducted a preliminary investigation into the possibility of removing such multi-donor deposits layer by layer using a differential tape-lifting method. Two types of tape were tested using two different numbers of applications for sampling layered deposits of touch DNA/touch DNA and touch DNA/saliva, both on the same polyester–cotton plain woven material. The data showed that there was no significant increase in the ratio of secondary to primary deposit when sampled in this manner, compared to direct extraction from cuttings of the touched fabric. A similar result was also obtained even when the deposits were on opposing surfaces of the fabric and the sampling was carried out on the secondary deposit side. These findings indicate that biological material, whether touch DNA or saliva, does not predominantly remain on the side of the fabric on which it is deposited (at least for plain-woven polyester–cotton). They also highlight the importance of considering substrate properties when making assumptions as to the resulting location of biological materials from a deposition event, and the necessity to conduct further research on the interactions between substrates and deposits.
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It should be noted that, just as with other adhesive tape based methods, stubbing is less or not suitable for textiles that are wet (no adhesive bonding), that have so extremely been exposed to the elements that they fall apart (textile and traces both getting lifted), or are covered in dirt (sample will contain mostly dirt, which could of course be a trace itself). Even though various collection methods have been compared with each other [10–14], only a few studies considered the effect of variations within the stubbing technique on the final sample: the effect of tape brand [9,15] and the number of tape-liftings [9] have been studied. In the stubbing procedure that is currently used at our forensic institute, the ‘stubbing force’ (i.e.: maximum normal force applied on the stub during trace collection) is applied manually, which makes it hard to control.
DNA is a highly valuable lead to identify people who were possibly involved in a crime. Even by small contact events, minute amounts of DNA (‘trace DNA’) can be transferred from a DNA source to an evidentiary item, which can be enough for a successful DNA analysis. The focus of this research is to get more insight in the collection of trace DNA from textiles by ‘stubbing’, which is a tape-lifting method using double-sided tape placed on a stub. The relation between the ‘stubbing force’ (the normal force that is applied during stubbing) and the collection efficiency of microspheres is investigated.
Microspheres (Ø25 μm) were used as mock traces to mimic DNA-containing micro-traces. The particles were applied to textile substrates in a suspension of ethanol that was left to evaporate before sampling. Experiments were performed on three different polyester substrates. Traces were collected by stubbing while using 5 different stubbing forces. The number of microspheres placed on each substrate was counted before sampling and all stub-tapes were analysed after sampling to count how many of the microspheres were picked up, both by using stitched images from a digital light microscope. Custom-made image recognition software was used to automatically count the microspheres.
On all tested polyester substrates, the mean efficiency of the collection of microspheres increased with increasing stubbing force in a concave down increasing function. The increase of collection efficiency stagnated around 3–12 N, depending on the substrate material. The theoretical maximum collection efficiencies varied between 38% and 78%, depending on substrate material as well.
Stubbing with a force higher than 12 N does not notably influence the collection efficiency from the variety of textiles that were tested. However, because the theoretical maxima of the collection efficiencies were far from 100%, it is highly likely that stubbing multiple times on the same spot of a substrate increases the total collection efficiency. The gained knowledge will help to standardize and improve the effectiveness of stubbing.
Targeting relevant sampling areas for human biological traces: Where to sample displaced bodies for offender DNA?
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Sampling strategy is one of the deciding factors in DNA typing success rates. Small amounts of bodily fluid traces and (skin) contact traces are currently not visualized in standard forensic practice. Trace recovery is usually based on the information available in a particular case and on the experience and ‘forensic common sense’ applied by the trace recovery expert. Interactions between an offender and a victim may have characteristic features, resulting in specific trace patterns. Understanding these interactions, and their resulting trace patterns, might improve crime related trace recovery as well as DNA typing success rates.
In this study, we examined the interactions between offender and victim when a body has been relocated from one position/location to another. The contact between the hands of the offender and the body of the victim was visualized using a fluorescent dye in a lotion that was applied to the hands of the individual undertaking the relocation. The contact locations were scored and patterns were analyzed based on both victim and offender characteristics (height, weight, age, gender). The resulting patterns were compared to current trace recovery practices in the Netherlands. The results of this large-scale study facilitate evidence-based sampling supporting both investigative and evaluative forensic examinations.
DNA transfer in forensic science: A review
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Understanding the variables impacting DNA transfer, persistence, prevalence and recovery (DNA-TPPR) has become increasingly relevant in investigations of criminal activities to provide opinion on how the DNA of a person of interest became present within the sample collected. This review considers our current knowledge regarding DNA-TPPR to assist casework investigations of criminal activities. There is a growing amount of information available on DNA-TPPR to inform the relative probabilities of the evidence given alternative scenarios relating to the presence or absence of DNA from a specific person in a collected sample of interest. This information should be used where relevant. However, far more research is still required to better understand the variables impacting DNA-TPPR and to generate more accurate probability estimates of generating particular types of profiles in more casework relevant situations. This review explores means of achieving this. It also notes the need for all those interacting with an item of interest to have an awareness of DNA transfer possibilities post criminal activity, to limit the risk of contamination or loss of DNA.
Appropriately trained forensic practitioners are best placed to provide opinion and guidance on the interpretation of profiles at the activity level. However, those requested to provide expert opinion on DNA-related activity level issues are often insufficiently trained to do so. We advocate recognition of DNA activity associated expertise to be distinct from expertise associated with the identification of individuals. This is to be supported by dedicated training, competency testing, authorisation, and regular fit for purpose proficiency testing.
The possibilities for experts to report on activity-related issues will increase as our knowledge increases through further research, access to relevant data is enhanced, and tools to assist interpretations are better exploited. Improvement opportunities will be achieved sooner, if more laboratories and agencies accept the need to invest in these aspects as well as the training of practitioners.
Detection of offender DNA following skin-to-skin contact with a victim
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Rutty [2] also demonstrated that partial offender profiles can be observed on the skin of a victim up to 10 days after contact. Other studies [3–5] have also demonstrated the transfer of offender DNA onto skin. The majority of these studies used profiling methodologies that are less sensitive than those currently in common use.
Forensic medical examiners are frequently asked to examine persons who claim to have been assaulted. If the suspect is unknown and there has been contact between his or her skin and the alleged victim, there is an expectation that DNA can be collected from the victim’s skin. In this way, the retrieved suspect DNA from the skin of the victim can be used to support the proposition that places the suspect at the scene.
This study investigated the transfer and persistence of offender DNA on a victim following a mock physical assault and further transfer of the offender DNA onto clothing worn over the assaulted area. Mock assault scenarios were conducted with the offender using medium pressure without friction and heavy pressure with friction on the wrist and upper arm of the victim. Samples were taken at either 0 h, 3 h or 24 h post assault, with 18 assault scenarios conducted at each time point. Samples from the victim’s skin where the assault had taken place and, where applicable, clothing worn over the assaulted area were collected using the double swabbing method.
Offender DNA was observed on the victim’s skin at 0 h in the majority of samples with a higher transfer rate observed where heavy pressure and friction was used. The presence of the offender profile was detected in the samples collected at 3 h and 24 h, with 25% and 12% of samples respectively producing a LR in support of Hp. Transfer of offender DNA to clothing worn over the assaulted area was demonstrated, with 19% of samples producing a LR in support of Hp. Samples taken from clothing were complex mixtures, with 23% of samples producing four or more person mixtures. Indirect transfer of other DNA was also observed, with background DNA on the offender’s skin observed in the clothing swabs of the victim on a number of occasions.
Our data suggests that sampling from clothing worn over the assaulted area may be an additional or better avenue for the recovery of offender DNA post assault where there has been significant time between assault and sampling.
Combined recovery of biological and fibre traces
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We present a method in which DNA and fibre traces are jointly recovered by taping. The DNA traces are isolated by standard laboratory procedures. Fibre traces are isolated afterwards in order to improve efficiency.
Two tests have been carried out to evaluate the suitability of the presented method. In the first test, possible changes in appearance of fibres due to the DNA isolation procedures are investigated. In the second test, the recovery of fibres from a contaminated surface and their possible loss due to the DNA isolation procedure are investigated.
It is concluded that polyester fibres are hardly affected by the DNA isolation procedure. In contrast, a relatively large number of the investigated cotton fibres were altered. The observed differences do not indicate a structural damage to the fibre or the dyes, but rather the washing-out of some components. The observed changes may require that fibres from a known source are also exposed to the DNA isolation procedures to assess the induced changes, but do not prevent a meaningful comparison. The recovery of fibres is slightly lower than the routine procedures for fibre recovery. Therefore, it was decided to perform extra taping of the recipient in cases where fibre investigation is requested.
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Comparison of stubbing and the double swab method for collecting offender epithelial material from a victim's skin

Abstract

Introduction

Section snippets

Experimental set-up

Results

Discussion

Acknowledgements

Legal Med.

Forensic Sci. Int.

Sci. Justice

Forensic Sci. Int.

Forensic Sci. Int.

Forensic Sci. Int. Genet. Suppl. Ser.

DNA typing of epithelial cells after strangulation

Int. J. Legal Med.

An investigation into the transference and survivability of human DNA following simulated manual strangulation with consideration of the problem of third party contamination

Int. J. Legal Med.