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Scientific Approaches in Bloodstain Analysis (Chapter 1: Introduction…
Scientific Approaches in Bloodstain Analysis
Chapter 1: Introduction
Background
bloodstain analysis in question due to wrongful convictions
Purpose and Objectives
examine scientific methodology of bloodstain analysis
focus on physical properties of blood, velocities of spatter, angle of impact, and documentation
Rationale
blood a common type of physical evidence
stain interpretation aids reconstruction
Terms defined operationally for paper
Limitations
basic, subjective methods and findings
Hypothesis
1: angle of impact results in conclusive findings of the area of origin
2: misting is usually the result of a gunshot wound
3: photographic methods can reveal bloodstain causes
Chapter 2: Literature Review
Forensic Serology
forensic serology: analyze and study blood and other body fluids from crime scenes
blood class characteristics - blood type (ABO)
bloodstain analysis: study bloodstains to determine sequence of events
Biology of Blood
consists of plasma (55%), red blood cells (RBC), white blood cells (WBC), and platelets
RBC, aka erythrocytes: carry oxygen through body using hemoglobin; no nucleus
WBC, aka leukocytes
neutrophils surround and engulf bacteria
monocytes rebuild damaged tissue and protect proteins
eosinophils target parasites
basophils aid in inflammatory reactions
lymphocytes include B cells and T cells, which release antibodies or directly attack pathogens respectively
Blood types
ABO
A: has A Antigens and B antibodies
B: has B antigens and A antibodies
AB: has both antigens and no antibodies
O: has no antigens and both antibodies
Rh factor
Rh+: has type D antigens
Rh-: does not have type D antigens
antibodies react with their corresponding antigens (A with A), causing agglutination
function: transport CO2, urea, lactic acid, hormones, and immune system components, and to regulate core body temperature and pH levels
Viscosity
viscosity: resistance to shear or tensile stress in fluids; resistance to flow, or thickness
blood viscosity is based on hematocrit, or the ratio of hemoglobin to whole blood; higher hematocrit results in higher viscosity
shear stress = mass x acceleration = mass x (velocity/time) = momentum/time; difference in velocity between layers
viscosity = shear force / shear stress
blood 4x thicker than water
Blood Flight
forms tear drop shape due to surface tension when it leaves source, but molds into sphere due to surface tension
surfaces that are harder and less porous form more-contained patterns
impacting hard, smooth surfaces causes drop to break up into smaller satellite droplets surrounding parent stain
blood travels in a parabolic arc due to gravity and air resistance
spatter: blood mass broken up into droplets because fluid cannot be compressed; expiratory splatter if forced out of airway
smaller stain with smaller volume of blood and shorter falling distance
smaller drops have smaller terminal velocities
the direction of movement of a stain aligns with the long axis of the stain, with movement being in the direction of a tail if present
scallops, tails,a nd satellite spatter are more common on the side opposite where blood first impacted the target surface
point of origin: where blood began moving toward the target surface where stain is; determined using angles of impact and point of convergence
Chapter 3: Methodology
Angle of Impact
angle of impact =arcsin(w/l)
blood driped horizontally onto a surface creates circular stains, while non-horizontal drips create elongated stains
trace the long axis of the blood stains; where the lines intersect is the area of convergence
the distance from the stain to the area of convergence and the angle of impact can set up a right triangle to solve for the area of origin
Chapter 4: Results
Bloodstain Interpretation
classified as low, medium, or high velocity in terms of energy of the stain when it impacts a target surface
low velocity - dripping onto a target surface; no additional force beyond gravity; large and irregular in shape and size; less than 5 ft/sec
projected blood or arterial gushing result of blood leaving the body under presure
Medium Velocity
object produces force between 5 and 25 ft/sec
usually result from blunt or sharp force trauma, ex. weapon cast off
arterial spurt alternates between medium velocity during ventricular contraction and low velocity between contractions
High Velocity
misting normally a result of gunshot injuries
blood travels short distance due to air resistance acting against their small mass
gunshot wounds can be classified as high or low velocity depending on the firearm used
gunshot damage can be caused by direct impact of a bullet or by the shockwave of the bullet, stretching the surrounding tissue
Chapter 5: Discussion, Conclusions and Recommendations
Discussions
detailed photographic documentation and analysis are essential
dried blood should be packaged in paper; wet stains should be transported to the lab in plastic (to prevent contamination), dried, and then repackaged in paper
all evidence should be labeled with the collector's initials, the date and time of collection, a description of the evidence, where it was found, the agency name, and file number
PPE should be worn at all times
chain of custody documentation should be maintained at all times (description of evidence and package, where evidence was collected, date and time of collection, case number, collector, where evidence was delivered and stored)
Conclusion
blood pattern analysis is crucial to crime scene reconstruction
proper documentation confirms that evidence has not been contaminated its analysis
high impact patterns can indicate the weapon type, number of blows, movement and location of victim and assailant, injury type, time of the crime, and a sequence of injuries
Recommendations
make appropriate observations and document them accordingly
PCR now able to aid in DNA analysis
our ability to practice forensic science improves as technology does