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Bloodstain pattern analysis: scratching the surface

Larkin, Bethany Alexandria Jane (2015) Bloodstain pattern analysis: scratching the surface. Doctoral thesis (PhD), Manchester Metropolitan University.


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Bloodstain Pattern Analysis (BPA) is a forensic application of the interpretation of distinct patterns which blood exhibits during a bloodletting incident, providing key evidence with its ability to potentially map the sequence of events. The nature of BPA has given the illusion that its evidentiary significance is less than that of fingerprints or DNA, relying solely on the interpretation of the analyst and focusing very little on any scientific evaluation. Recent preliminary literature studies have involved a more quantitative approach, developing directly crime scene applicable equations and methodology, which have established new ways of predicting the angle of impact, impact velocity, point of origin of blood and blood pattern type. Using these new equations and further improving on them to include a variation of impact surfaces, surface properties (i.e. porosity, roughness, manufacturing process etc.) and changes in blood properties is the principal focus of this work. The primary objective of this research is to expand the knowledge of blood and surface interactions and generate general equation/s or quantitative approaches that encompasses some of the possible conditions, in relation to Bloodstain Pattern Analysis (BPA), which may be encountered at a crime scene. Overall validating BPA and supporting a more reputable / respected scientific field giving credence to its usage within criminal trials. This thesis is presented in three parts: The first part explores blood, its characteristics and how manipulating the components of blood (i.e. packed cell volume, PCV), can alter the way a bloodstain forms and dries. Since packed cell volume is instrumental in the overall viscosity of blood, which ultimately determines the final bloodstain diameter via the natural fluctuation exhibited throughout the body and by the individual human characteristics, it was deemed necessary to investigate its effect on the interpretation of bloodstains. Packed cell volume was found to alter the size of bloodstains significantly, where increments in their diameter were experienced when PCV% was decreased; angled impacts were unaffected. The mechanism of drying blood was also analysed, the current understanding being that blood dries primarily by the Marangoni Effect. However this is found to be altered when PCV% is considered; low PCV% exhibits a strong Coffee Ring Effect where higher PCV% levels dry by the Marangoni Effect. Other drying characteristics considered were volume analysis, skeletonisation and the halo effect where PCV% was manipulated. Volume analysis methods were significantly affected by PCV%, where new drying constants were established and several established scientific methods were shown to be unreliable at determining the volume. The second part of this thesis investigates surface interaction, exploring the fundamentals of various common surface types, and how individual features (i.e. surface roughness) affect the interpretation of bloodstains; four common surfaces were considered (wood, metal, stone/tile and fabric). Blood drop tests were performed at different heights and angles where recently formulated equations were applied to the results to create new constants, which could be used to distinguish between surface types. Wood and fabric were found to alter the spread of blood most significantly, constants increased or decreased substantially, compared to the original value. The last part of this thesis expands the groundwork set forth in part two. Surfaces were manipulated, either by heat or cleaning. Since it is possible that blood may interact with a surface which may have been cleaned (to remove blood, or simply to clean surface prior to any blood impaction) or heated (i.e. radiators), it is important to fully explore surface alterations which commonly occur in an everyday environment and therefore are highly probable to be encountered at a crime scene. Surface manipulation is investigated in the form of a heated surface, where a blood boiling curve reminiscent of the water boiling curve was created establishing four visibly recognizable boiling regimes. Heat was found to decrease the resultant bloodstain diameter, separate blood into its components and create reduction rings as the temperature increased. An equation accounting for these changes was deduced, further showing how simple alterations to the surface, which have previously been overlooked, can interfere with the results. Further surface manipulation was implemented in the form of cleaning, since cleaning can be performed before blood impacts, therefore causing a surfactant layer, of after blood has impacted the surface, indicating crime evasion. Secondary analysis of blood on a heated surface in conjunction with cleaning was implemented, establishing the effectiveness of presumptive testing and the ability to extract valuable DNA. Initial presumptive testing and DNA extraction was found to be successful for all temperatures, however when various cleaning methods were applied (a common occurrence at crime scenes) DNA testing produced negative results at temperatures of 50oC onwards. Fabric washing, using various household detergents and methods of washing/drying were also evaluated. Detergents significantly increased the resultant diameters of bloodstains, secondary rings were experienced on all polyester and silk fabrics, establishing constants relating to the secondary ring produced. Repeated cycles of washing were found to produce a stable fabric after 6 cycles, for most fabric types.

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