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How To Become A Blood Spatter Analyst – Taken from the 2019 October issue of Physics World. Members of the Institute of Physics can enjoy the entire issue via the Physics World app.
Analyzing bloodstains after a shooting can be critical to finding a criminal, but it’s a questionable area of forensic science. Sidney Perkowitz explains how understanding the physics of blood distribution can help uncover the truth
How To Become A Blood Spatter Analyst
Joe Bryan, once a popular and respected high school principal in a small Texas town, has been in prison for more than 30 years. He is serving a 99-year sentence for shooting and killing his wife in 1985. The incriminating evidence was the victim’s bloodstains found on a portable flashlight. A witness qualified as a forensic blood analysis (BPA) expert interpreted the stains as showing Bryan cowering next to his wife when she was shot, testimony at the forefront of Bryan’s sentencing. It was counter-proof that he was actually attending a conference 120 miles away, an alibi that made it nearly impossible for him to shoot his wife, since he would have to leave the event, go home, commit the murder, and return. At home. conference within a certain time frame. Bryan maintains his innocence to this day.
Chart: Blood And Blood Spatter, C.8 Teacher Copy
Such evidence, based on the physical behavior of blood at a crime scene, dates back to late 19th century Europe. She became famous in the United States during the famous murder trial of Sam Sheppard in 1955. and has since played an important role in other murder trials, including that of football player and actor OJ Simpson (1994-95, not guilty ) and musical cases. producer Phil Spector (2007-2009, reconviction – guilty).
Police investigators use BPA to work on bloodstains at crime scenes to reconstruct the locations and actions of the people and weapons involved. Traces are droplets, stains and splatters that result when drops of blood radiate from the impact of a projectile or blunt instrument before hitting and staining a surface. However, according to the astonishing 2009 According to a report by the US National Academy of Sciences (NAS) that still resonates today, BPA lacks scientific rigor and valid accreditation of its practitioners. This is a serious concern because the BPA results showed that people were later innocent, as many believe Bryan; and why not trusting BPA analysis can allow the guilty to go free. This made it necessary to reevaluate the physics of BPA.
While the United States leads the world in firearm ownership, with 120 guns per 100 people and 64% of homicides in the United States involve firearms, other countries also have high rates of shootings. For example, 30% of homicides in Canada involve firearms, and a dozen countries, including Brazil, surpass the United States in gun deaths per 100,000 people. Determining the scientific validity of BPA could therefore have international implications for addressing the 250,000 annual firearm-related deaths worldwide, classifying them as homicides or suicides and, in the former case, bringing those responsible to justice.
In terms of physics, BPA reconstruction is a complex fluid mechanics problem that involves tracking the behavior of blood under various forces and environmental conditions. The challenge is compounded by the fact that blood is a complex fluid that contains both liquid (plasma) and solid (blood cells) components. Furthermore, blood properties such as pH or the number of red blood cells vary from person to person.
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But this work is more than just academic research. According to Alicia Carriquiry of Iowa State University in the US, this could have real consequences. As a statistician and director of the Center for Statistics and Applications of Forensic Evidence (CSAFE), funded by the US National Institute of Standards and Technology (NIST), Carriquiry has a broad view of forensic science. “BPA is one of those areas where science has a lot to say,” she says. “Unlike other forensic disciplines, in BPA we actually have physical and fluid dynamic models that can help answer questions related to trajectory, point of origin, and the like.
However, according to the 2009 NAS report, titled “Strengthening Forensic Science in the United States,” found that basic science was not suited to BPA. It was co-chaired by a prominent US federal judge and an academic statistician. He involved collaborators from relevant scientific disciplines, including physics. Overall, with the exception of DNA analysis, the report found deficiencies in nearly all forensic techniques, including hair, fibers, fingerprints and bite marks. “The interpretation of forensic evidence is not always based on scientific research to determine its validity,” she says. “It’s a serious problem.” This criticism was repeated in a separate 2016 Report of the Council of Science and Technology Advisors to the President of the United States, written by the director of the Office of Science and Technology Policy together with a group of scientists.
For BPA in particular, the NAS report noted the complexity of fluid dynamics and indicated that BPA analysts should understand the physics involved. But in the absence of strict training requirements to become certified as BPA experts (they are trained only to follow predefined procedures) the report concludes that “the opinions of bloodstain pattern analysts are more subjective than scientific… The uncertainty surrounding [BPA] is huge.” “In 2018, the Texas Commission on Forensic Science reached similar conclusions in the Bryan case, calling BPA’s interpretation of the evidence “inaccurate” and “scientifically unsound.”
However, when used correctly, BPA can provide valuable clues to understanding the circumstances of a shooting. For example, drops of blood hitting the floor at a certain angle will create a series of elliptical dots whose width-to-length ratio determines the angle of impact (Figure 1). BPA analysts are trained to plot straight trajectories that follow the long dimension of each ellipse at that impact angle. These pathways converge to provide the site from which the blood originates. While this correctly represents the location of the gunshot wound on the floor, the straight-line procedure overestimates the height of the wound because real gravity paths are parabolas modified by aerodynamic drag. The error is usually large enough to mistakenly position the victim in a standing position rather than a sitting position.
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When drops of blood hit the floor at an angle, elliptical spots are formed, where their width-to-length ratio gives the angle of impact. Traditional blood testing practitioners draw a straight line from the stain to the angle of impact to reveal the origin of the blood. While this correctly maps paths along the floor (gray lines), straight trajectories (dashed lines) overestimate the vertical height of impact because the blood would have taken an altered parabolic path (blue) due to gravity and drag.
This is one of the established BPA methods that can be improved by deeper physical analysis. in 2011 Washington State University physicists Christopher Varney and Fred Gittes put the equations of projectile motion, including gravity and drag, into a form that uses all the specific data from a series of splattered bloodstains (Am. J . Phys. 79,838). They found that a plot of dot impact angles and the inverse of their horizontal distances from the vertical impact axis provides a valid height result if the droplet launch angles are not distributed too widely. In a test in which a viscous blood substitute was sprayed, the researchers used this method to calculate the effective launch height of 88 cm with an accuracy of 8%. In comparison, linear trajectories overestimate launch height by 100%.
In 2015 Nick Laan of the University of Amsterdam and the Netherlands Institute of Forensic Science and colleagues used the liquid properties of blood to determine the height of a gunshot wound (Scientific Reports 5 11461). Previous work has derived an equation that relates the impact velocity of a drop of liquid blood to its volume and angle of impact and to the width of the dried spot, as determined by the known capillary and viscous behavior of blood . To apply this method, the researchers created models of human blood spatter under controlled conditions. For each of the 40 individual bloodstains, they determined its width and angle of impact and, using a commercial 3D surface scanner, measured the volume of the stain, from which they determined the volume of the original drop. These parameters produced the impact velocity, providing enough information to solve the gravity motion and drag equations. The average value of the results for the height from which each fall originated was 58.5 cm, only 8% less than the actual height of 63.7 cm. While the straight line method gave 91.1 cm, a much larger error of 42%.
These two papers and others analyze the behavior of blood droplets after their formation to improve on standard BPA. However, mechanical engineers and fluid dynamics specialists Alexander Yarin and Patrick Comiskey of the University of Illinois at Chicago, together with Daniel
