代写CHE 113: Forensic Science Experiment: Blood Spatter Pattern Analysis代写Web开发

CHE 113: Forensic Science

Experiment: Blood Spatter Pattern Analysis

Online (At Home) Laboratory

Introduction

“The success or failure of any criminal investigation often depends on the recognition of physical evidence left at a crime scene and the proper analysis of that evidence.  Crime scenes that involve bloodshed often contain a wealth of information in the form. of bloodstains…” (Wiid, Antoinette Bedelia (2016) The use of blood pattern analysis to reconstruct a crime scene, University of South Africa, Pretoria)

The pattern, size, shape, and the location of such stains may be very useful in the reconstruction of the events that occurred.” (William G. Eckert and Stuart H. James Interpretation of Bloodstain Evidence at Crime Scenes, Second Edition 2nd Edition, CRC Press; 2 edition, July 14, 1998)

“Art in the blood is liable to take the strangest forms.” (Sherlock Holmes – the Greek Interpreter)

In forensic analysis, we often look for patterns: we seek patterns that indicate that something unusual has happened or is somehow “out of line” with normal patterns.  Once found, we analyze these patterns carefully to see if we can find some process that formed the observed pattern.  Making the link between the pattern and the mechanism that formed it is often fundamental to any good forensic analysis.

Bloodstain Pattern Analysis is a good example of the type of process linking observation with mechanism and typically involves the examination of the shapes, locations, and distribution bloodstains in an effort to try to provide an interpretation of the physical events which gave rise to their origin.  The underlying idea is that all bloodstains and bloodstain patterns are characteristic of the physical forces that have created them.  The determinations made from bloodstain patterns found at the scene or left on the suspect and victim of a case may be used to confirm or refute assumptions concerning events and their sequence, including:

· The relative position of victim and the attacker (e.g., standing, sitting, lying) before, during and after the crime;

· The locations and movements of the victim and the attacker;

· The possibility of a struggle (e.g., blood smears, blood trails);

· What types of weapons may have been used and how they were used;

· Any information about the identity of the attacker (blood typing)

Blood-based evidence may also be used to confirm or refute statements in the case by determining if:

· the stain patterns on a suspect’s clothing are consistent with their reported actions;

· the stain patterns on a victim or at a scene are consistent with accounts given by witnesses, victim or the suspect?

Objective

The goal of this laboratory exercise is to allow you to become familiar some of the basics of blood pattern analysis by exploring the patterns formed by droplets of blood when they impact various surfaces at various angles.  You will also learn about the effects that various surface textures can have on the geometry of blood droplets along with the basic concepts necessary to recognize, document and interpret bloodstain pattern evidence.

Background

Apart from its chemical composition and biological function, blood is also a fluid whose behavior. can be understood by considering how any fluid behaves in response to certain forces and conditions.  As a fluid, blood behaves according the laws of physics and concepts of a field called fluid dynamics – a field dealing with the flow and motion of liquids.  In this lab, we will first explore several of the fluid properties of blood that are particularly relevant to forensic investigations and then exmine how blood behaves when acted upon by various physical forces.

Blood Volume:  On average, blood volume accounts for ~8 % of our total body weight, 5 to 6 liters of blood for males and 4 to 5 liters of blood for females.  A blood loss of only 1.5 liters, internally or externally, typically causes incapacitation while a 40 percent blood volume loss typically produces irreversible shock (death).  

Surface Tension:  The elastic-like property of the surface of the liquid that makes a droplet contract into a spherical shape, caused by the forces of attraction between the molecules of the liquid, is called its surface tension.  The cohesive force of surface tension holds the droplets together so that they tend to resist breaking up into smaller droplets.  It is this force that primarily turns a continuous stream of blood into a spray of individual droplets.  The size and distribution of these droplets depends upon the force exerted on the fluid, causing it to break up into the droplets: the greater the force the smaller the droplets tend to be.

Bloodstains: There are main three categories of bloodstains, based upon how they are distributed:

· Passive

· Transfer

· Projected

Passive Bloodstains are droplets whose motion is determined solely by the force of gravity.  This category can be further subdivided into drops, drip patterns, pools, and clots.  Passive bloodstains are most often caused by the simple dripping of the blood from a stationary object, such as a weapon or person standing still, and fall vertically downward.

Transfer Bloodstains can be created whenever a wet, bloody surface comes in contact with a second surface.  This process is very similar to a printing process where a stamp containing ink is impressed upon a piece of clean paper to leave an image of the design on the stamp.  In transfer bloodstains, a recognizable image of all or a portion of the original object’s surface may be seen in the pattern, as in the case of a bloody hand or footwear leaving a smudge on the floor or window.  Transfer bloodstains can be further subdivided into contact bleeding, swipe or smear, wipe, and smudge. 

Projected Bloodstains are created when an exposed blood source is subjected to an action or force in addition to the force of gravity, whether produced from inside the body of by an outside agent.  The size, shape and number of resulting stains will depend primarily on the amount of force utilized to strike the blood source.  This category can be further subdivided into arterial spurt/gush and cast-off stains. Arterial spurt/gush bloodstain patterns result from blood exiting the body under pressure from a breached artery due to the person’s blood pressure, a significant force that is necessary to move blood within the body [normal blood pressure is ~2.3 psi and, as a comparison, a balloon has an internal pressure of ~1 psi).  Cast-off stains occur when blood is released or thrown from a blood-bearing object, such as a weapon, in motion.  

When the pattern results from an injury, the type of pattern is often classified by how fast the force impacted the blood pool (usually the person).  A low velocity impact spatter is found when the impact occurs at speeds up to about 5 ft/sec. (1.5 m/sec.).  The droplets observed are rather well formed and typically are larger than 3 to 4 mm in diameter.  This might arise from a bleeding person who was walking or running, from a relatively slow assault with someone’s fists, or from a very short fall.  Medium velocity impact spatters arise when the velocity of the object is between 5 and 25 ft/sec. (7.5 m/sec.).  The droplets found here typically range in size between 1 and 4 mm in diameter.  These types of patterns usually come from blunt and sharp force trauma injuries, most often from assaults with weapons (e.g., bats, hammers, clubs, etc.) and accidents.  The final type of pattern, high velocity impact, is usually observed when the object is moving at speeds of 100 ft./sec. (30 m/sec.) or faster.  The resulting bloodstain patterns usually appear as a fine “mist” with droplet sizes smaller than 1 mm.  These types of patterns arise from bullet wounds, explosives, injuries from high-speed machinery or expelling blood by sneezing or coughing.

Angle of Impact Determination 

When blood is projected from a source, the droplet typically lands on a surface (e.g., floor, wall, ceiling, etc.) at an angle <90°.  The angle of impact of a droplet is the acute angle formed between the trajectory of the blood droplet as it moves through the air and the plane of the surface it strikes (q) as shown in Figure 6.  By utilizing simple trigonometric calculations, it’s possible to determine the impact angle for any given blood droplet by measuring the width and length of the droplet using the equation:

       (Eq. 1)

As an example, the “opposite side A” is the line between points A and B of the triangle in Figure 6 while the “hypotenuse side B” is line between points C and A.  Measuring the width of a droplet will give you the length of line A-B while measuring the length of the(width droplet will give the length of side A-C.  Knowing these two values allows the use of equation (1) to determine the angle that the droplet made when striking the surface.  For example, using the measurements for the length and width of a bloodstain as 1.5 cm wide (side AB) and 3.0 cm long (side AC), the impact angle in this example can be calculated at 30° using the sine formula below:

By measuring the angle of impact of many droplets in a blood pattern, it should be possible to “work backwards” and find the location where the blood pattern originated.

Region of Convergence and Origin Determination

The area or region where a distributed blood spatter initiated is called the region of convergence (or area, sometimes incorrectly called “point of convergence”).  This location can often be determined by first finding the direction of travel (Figure 7) and impact angles (Figure 6) for many blood droplets and then tracing each backward to a common location since each must have originated from some point along the calculated line of travel (Figure 8).  

When many droplets need to be considered in this fashion, the point of intersection of all the lines of travel must be the approximate place where all the droplets originated – the location where the impact caused the blood to be projected from.

Combining the determination of the area of convergence for many droplets allows for a determination of the region of space that the blood spatter originated as shown in Figure 9.

Experimental Method

In this experiment, you will explore several basic ideas in understanding and interpreting blood spatter patterns.  This will include examining blood droplets falling from different heights and angles on several surfaces.  Additionally, you will explore finding convergence points from a blood spatter patter.

Materials Needed:

· You will need to make a sample of ‘artificial blood’ (100 mL will probably be enough).  Any liquid that is red with a bit of “thickness” will work reasonably well (probably good enough for this lab) although just colored water runs way too fast to mimic blood well.  It’s a little like Goldilock’s porage – you want an artificial blood that’s not too think and not too thin.  To get a good substitute, you can use sample “recipe” below for a better “blood” substitute to use for this lab to get more realistic patterns and better results.  A simulated blood can be made by mixing together the following ingredients:

o 1 cup water;

o 1 tablespoon cocoa powder;

o 8 tablespoons corn syrup;

o 1 to 2 teaspoon red food coloring and 4 drops yellow food coloring (although you can use any color).

· Eye dropper (or something to drop some “blood” droplets).

· Whiteboard (or simply sheets of moderately heavy white paper – just not thin paper since the “blood” would “bleed” through the paper, no pun intended).

· Various textured surfaces to drop blood droplets onto such as wood, cardboard, carpet sample, etc.

· Simple measuring devices: meter stick, ruler, and protractor (a way to measure angles).

· Clear adhesive tape.

· Calculator (or computer with internet) to calculate arcsin values.

· Graph paper and a straight edge.

Procedure I:  Creating Known Blood Spatters

Angle Measurements:

1. Use the simulated “blood”, placed in an eyedropper to form. the blood droplets.

2. Lay a sheet of paper or whiteboard (similar to a tiled floor) flat on the floor.  Drop several droplets of ‘blood’ using a dropper at a height of 1 inch, 12 inches, 36 inches and 60 inches above the floor.  This will represent blood dropping from an object/person that is not moving horizontally (standing still) at an angle of impact of 90° to the paper (passive blood spatter).  Be sure to label these drops appropriately.

3. Repeat step 2 (use additional sheets of paper) but adjust the angle of the paper to the floor (for example, use a piece of cardboard as a backer and prop up one side with a book), using the protractor to make an angle of 15° above flat (75° angle of impact).  This will represent blood dropping from an object/person who is moving at a slow to moderate pace.  Make droplet at this angle at a height of 1 inch, 12 inches, 36 inches and 60 inches above the floor.

4. Repeat this process again but adjust the angle of the whiteboard to 30°, 45°, 60°, and 75° degrees and run each angle with drops from 1 inch, 12 inches, 36 inches and 60 inches above the floor.  This will represent blood dropping from an object/person who is moving at an increasingly quicker pace.  You should have a total of 24 different samples of blood drops from repeating steps 2 thru 3 four times (six angles and four different heights).  Record your data in the Table on the Data Sheet.

5. For each of the 24 different blood samples that you made from step 4, measure the length and width of your bloodstain samples (width is the shorter of the two measurements).  Use the following formula below to calculate the angle of impact for each sample and then compare your calculated angle with the “known” angle (the angle that you used to set up the experiment with the protractor).  Make sure your calculated angle is entered into Table 1.

Note: To use this formula, you need to divide the width of the droplet by the length of the droplet.  This value is the sinq, where q is the angle of the impact that you are looking for.  To find just q, find the arcsine of the answer you got from the division above (arcsine is usually a function in a calculator or use one of many arcsine calculators on the internet, such as: http://www.rapidtables.com/calc/math/Arcsin_Calculator.htm).

6. Compare your calculated angles of impact (step 5) to the measured bloodstain angle from step 2 (angle and height).  

Surface Differences

7. Lay the paper on the cardboard flat on the floor.  Drop a sample of ‘blood’ using a eyedropper at a height of 24 inches.  Repeat this step but replace the paper with a wood surface (the rougher the better) to see how this changes the shape of the droplet formed.  Try other surface materials, as available, such as cement, glass, carpet, etc.

8. Create a chart that summarizes the similarities and differences between the wood, cardboard, whiteboard and other surfaces.  Measure the width and length of the droplet pattern on the paper and other surfaces.  Pay close attention to size, shape, thickness, spatter patterns, etc. and use the space after Table 1 to describe what it looks like.  A photograph of the droplet will also be useful (you can use phone camera or similar – it doesn’t need to be a fancy photo).

Unknown Blood Droplet

9. Have another person make a droplet using an angle unknown to you (don’t look but make sure that they know the angle that they used) but known to them – this will be your unknown droplet.  Measure the length and width of your unknown bloodstain sample (width is the shorter of the two measurements).  Use the following formula to determine the angle of impact (same as in step 5 above).

10. Compare your determined angle of impact (step 4) to the appropriate bloodstain from step 1 (angle and height).  In the space provided after Table 1, draw and label (height, angle, length, width) a copy of the unknown bloodstain sample into your data section and explain why there are any similarities/differences.

11. Clean off all surfaces and rinse out all glassware appropriately.  

Procedures II:  Determining the Point of Origin in Three-Dimensional Space

 

1. Select a given blood stain pattern.  You can either make your own 3D blood pattern, as described below, or use the one attached at the end of this lab procedure:

(1)  create your own by first laying a large piece of paper on the floor and hitting a small sample of your artificial blood at least a foot or more above the paper with your hand or an object (e.g., a stick or bottom of a glass) to create a distributed blood spatter pattern.  After allowing the pattern to dry, then analyze it using the procedure below and compare to see if your calculated results correspond with your known setup used to create the spatter]

(2) Use the spatter pattern attached at the end of this lab exercise.

2. Find and draw lines through several well-formed blood droplets, indicating the direction the spot came from (towards the point of origin) – these lines should roughly intersect at a common location, known as the Point of Convergence  [Note: you should select a few spots (3-5) that give the clearest data].  [The term point of convergence is usually more accurately described as area of convergence, since a point is usually too specific to be determined.]

3. Measure the length and width of each blood spot (at least 5 spots but more are fine), and put your data in Table II.  Then, using the arcsin function on your calculator (sometimes shown as sin^-1 or use an online arcsin calculator), determine the angle of impact that created that spot.

4. On graph paper, determine an accurate scale that will allow you to graph the distance from the center of each blood spot to the now labeled point of convergence on the x - axis.  Similarly, label the y-axis with a scale that will allow you to graph on the paper the impact angles for each spot back to the origin (x = 0), this may take a bit of trial and error.  A sample of the end result is shown in Figure 8.

5. Graph the distance to the point of convergence for each blood spot directly on the x – axis, labeling on data points.

6. Using a protractor, measure up from the x axis to the calculated angle of impact and draw a line at this angle that intersects the y axis for each point.  Repeat this process for each blood spot that you are using for your analysis.

7. You should have lines drawn back to the y axis that, with luck, have converged at particular point on the y axis.  This represents the height above the ground where the blood originated (putting you into three-dimensional space…)

8. Average all the values obtained on the y – axis, and list this as your height above the ground for the point of convergence.

Procedures III:  Creating your own blood spatter for analysis.

1.  Using your artificial blood from Procedure I, create a low velocity blood spatter pattern to be analyzed by “throwing” some blood at a large piece of paper on the floor using a dropper.  Record your distance above the ground that you threw the blood.

2. Circle four blood drops you perceive to be the best representation of the overall pattern, that you can analyze, and label them A, B, C and D.

3. Repeat the Procedure II directions to analyze the created pattern.