Blood Pattern Analysis Laboratory Report

Aims

The objective of this laboratory report is to comprehensively investigate various aspects of bloodstain patterns, including:

1. Describing the morphology of bloodstains when dropped at different angles.
2. Exploring the effects of drop height on bloodstain diameter.
3. Determining the angle of impact and calculating percentage errors.
4. Investigating the influence of different surface types on bloodstain shape and diameter.

Introduction

Bloodstain pattern analysis (BPA) is a systematic approach that utilizes the principles of fluid dynamics and physics to assess bloodstain patterns at crime scenes.

It involves the analysis of bloodstains based on their shape, size, and distribution (Brodbeck, 2012). BPA helps answer critical questions about how blood traveled within a given space to produce stains on various surfaces. The applications of BPA are diverse, with its primary purposes being the reconstruction of events at crime scenes and the identification of areas with a higher likelihood of offender movement for the prioritization of DNA samples (Brodbeck, 2012).

BPA relies on the principles of physics, including mechanics and the physics of fluids, as well as other fields of knowledge, such as biology, chemistry, and medicine (Brodbeck, 2012).

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When examining perpendicular impacts (i.e., when the angle of impact is 90°), the contact diameter between the blood drop and the target surface expands until it reaches a maximum, a phenomenon known as spreading (Attinger et al.

, 2013). Experimental data indicates that the height from which the blood drop falls is directly related to the diameter of the stain, with higher falls resulting in larger spreading and stain size (Attinger et al., 2013). Beyond a certain fall height, the edges of round stains become disrupted, forming spines that increase in number as the impact fall height rises (Attinger et al., 2013). Importantly, the type of surface also influences these characteristics, leading to variations in spreading, spine formation, and stain morphology (Attinger et al., 2013).

For oblique impacts, bloodstains typically exhibit an elliptical shape with potential elongation in the forward direction (Adam, 2012; Attinger et al., 2013). Previous experiments have shown that the ratio of width to length (W/L) increases monotonically with increasing impact angle, and this relationship is described by the equation W/L = sin(α) (Attinger et al., 2013). Thus, measuring the ellipticity of a stain provides valuable information about the angle of impact (Attinger et al., 2013).

Method

The experiment consisted of two main parts:

1. Part 1: Bloodstain Diameter and Drop Height (90° Angle)

In the first part of the experiment, we investigated the relationship between bloodstain diameter and drop height at a 90° angle of impact. Three different target surfaces were used: Target 1 (tile, glass), Target 2 (carpet, cloth), and Target 3 (newspaper, printing paper).

The following procedure was followed for each target:

1. The target was labeled with the angle of impact (90 degrees) and the dropping heights (10cm, 20cm, 30cm, 50cm, and 1m).
2. A large piece of brown paper was placed underneath to collect satellite spatters.
3. The target was positioned perpendicular to the dropper using a tape measure as a plumb bob.
4. The dropper was filled with blood, and air bubbles were expelled from the dropper tip.
5. A drop of blood was allowed to fall freely onto the target for each specified height.
6. The process was repeated for all dropping heights.
7. Targets were moved to a secure location to dry.
8. The diameter of each stain at its widest point was measured in millimeters, and the results were recorded for all targets and drop heights.
9. The point at which the drop height no longer affected stain diameter for each target was determined.

1. Part 2: Angle of Impact (Oblique) at 50cm Height

In the second part of the experiment, we investigated the effect of the angle of impact on bloodstains at a fixed height of 50cm. Two different target surfaces were used: Target 1 (paper) and Target 2 (fabric).

The following procedure was followed for each target and angle:

1. The target was labeled with the angle of impact, the dropping height (50cm), and an arrow to indicate the stain's direction of travel.
2. Target 1 was affixed to the angle apparatus.
3. The dropper was positioned 50cm above the target surface using the case as a plumb bob.
4. The dropper was filled with blood, and excess blood was removed from the dropper tip.
5. A single drop of blood was allowed to fall freely onto the target.
6. The target was slightly moved to the side, and additional drops were allowed to strike the target, ensuring that two or three stains suitable for measurement were obtained.
7. The target was moved to a secure location for drying.
8. Width and length measurements of each stain were taken in millimeters.
9. The angle of impact for each stain was calculated using the formula: ^{sin⁻¹(Width/Length)}.
10. The percentage error for each stain was calculated using the formula: ^{|Actual Angle - Calculated Angle| / Actual Angle} × 100%.
11. The values obtained were recorded.

Results

Part 1: Bloodstain Diameter and Drop Height (90° Angle)

The results of the first part of the experiment are summarized in Table 1 below:

Part 2: Angle of Impact (Oblique) at 50cm Height

The results of the second part of the experiment are presented in Table 2:

Discussion

The results obtained from the first part of the experiment, as shown in Table 1, indicate that bloodstain diameter increases with the height of the drop, which aligns with the findings of Attinger et al. (2013). However, it is noteworthy that this behavior is not consistent across all types of target surfaces, as observed with the carpet, where the drop height no longer influenced stain diameter after 30 centimeters. This demonstrates the significant influence of the target surface on stain characteristics, including diameter.

In the second part of the experiment, as presented in Table 2, it is evident that bloodstains assume an elliptical shape as described in previous studies (Attinger et al., 2013; Adam, 2012). The ratio of width to length (W/L) increases with the angle of impact, allowing for the determination of the angle of impact. However, the results also reveal a level of inaccuracy, indicated by the percentage error exceeding 5% in some cases. This suggests that not all results can be considered highly reliable.

During the experiment, several limitations were observed, including the use of a dropper that may lead to variations in blood drop volume.

Conclusion

In conclusion, this experiment provided valuable insights into the characteristics of bloodstain patterns. In the first part, it was demonstrated that both the drop height and the choice of target significantly impact the form and diameter of bloodstains. The results indicated that an increase in drop height leads to an increase in stain diameter, but this behavior is not consistent across all types of target surfaces, such as carpet.

In the second part of the experiment, the relationship between the angle of impact and the width and length of bloodstains was explored, allowing for the determination of the angle of impact. However, the presence of a notable percentage error in some results highlights the potential for inaccuracies in laboratory settings.

Overall, this experiment underscores the importance of understanding the dynamics of bloodstain patterns and the need for careful consideration of experimental limitations in forensic investigations.