Materials Included In Kit

Additional Materials Required

Prelab Preparation

Use the crime scene unknown masters to make copies of the unknowns for student use in Part C. Collect newspapers to spread on work areas in the laboratory.

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Simulated blood can be disposed of according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. The activity can reasonably be completed in two 50-minute class periods.

• Be sure to shake the simulated blood right before use.
• With a toothbrush or other stiff brush, blood spatter patterns can be created with the simulated blood to show larger spatter patterns from multiple-sized flying droplets and from various angles. If this is done on very large pieces of paper (butcher paper), the actual point of origin can be determined by drawing lines through the lengths of the elongated drops. This is a logical and fun extension of this activity but it can be messy.
  {10519_Tips_Figure_3}

• Six bloodstain pattern “unknowns” are included in the kit. Duplicate these as needed for your classroom. Student answers may vary with the unknown data provided in the key.
• Another logical extension of this lab is to test alternative surfaces for spatter patterns. At specified heights unknown surfaces could actually be identified by spatter patterns if comparison patterns are established. This can be done and unknown crime scenes created.
• Several very prominent criminal cases have been decided based upon blood spatter evidence. Students might enjoy researching some of these cases on the Internet and reporting on them to the class.
• Students can make unknown crime scene spatters, let them dry, and then swap with other student groups to analyze as further “unknowns.”

Sample Data

Part A. Free Fall Data

Part B. Angle of Impact Data

Answers to Questions

1. Describe the changes to the diameter of the spattered drops as the height was increased? Explain your answer in terms of speed and energy.

As the height increased the time of the fall increased and the speed increased. The longer the drop falls, the faster it will be traveling just before it hits a surface. The more energy it has, the more energy that needs to be dissipated when it hits a surface, so the drop spatters more.

2. Describe any other patterns in the spattered drops in addition to the diameter changes that might help in identifying the height of an unknown drop.

With the greater force with increasing height, the result was a greater flattening of the blood drops. They shot outward upon impact causing “spiked” edges to the drops. The greater the height, the longer the spikes.

3. Graph your results plotting height vs. diameter on the grid.

Part B. Angle of Impact Data

1. When a blood spatter droplet becomes elongated, how can you tell which direction the blood is moving?

The pointed or elongated end is pointing in the direction of travel, the blunt end of the droplet is nearer the source.

2. Graph results for length and width on the grid. Graph length vs. angle and width vs. angle on the same grid. Describe the relationships under the graph.

As the angle of impact gets larger, the length of the drop increases and the width gets smaller.

Teacher Handouts

10519_Teacher1.pdf

Introduction

Blood and bloodstains are common at many crime scenes. In addition to the blood type, the stain patterns can often reveal key elements in reconstructing the crime. In this lab, simulated blood will be used to collect data on blood stain patterns.

Concepts

• Bloodstain pattern analysis

• Surface tension
• Control

Background

It is important a crime scene investigator examine the location, distribution and appearance of blood stains and spatters may be useful in interpreting and reconstructing a crime. The actual analysis at the crime scene can become very complex and usually requires three-dimensional thinking and sophisticated measuring techniques. The International Association of Blood Pattern Analysts has developed a list of blood pattern terminology and standards for measurement. Only a few of these terms and standards will be tested in this experiment.

The average-sized male (150–160 lbs.) has a blood volume of about 5 L. Blood is slightly denser and 3 to 4 times more viscous than water. It has a liquid portion (plasma) and many other cellular components held loosely in suspension. If a blood sample is allowed to stand in a tube, the cells will become separated from the plasma and settle to the bottom. A typical blood sample is about 55% plasma and about 45% cells.

The liquid contained in a drop of blood is held together by a cohesive force referred to as surface tension. This cohesive force tends to keep a drop of blood in a spherical shape as it falls. When a drop of blood is released and is free to fall under the influence of gravity, it is governed by the same physics principles as any free-falling object. Recall that the force of a free-falling object can be expressed by the formula:

F = ma

where

F = force
m = mass
a = acceleration

(Remember: acceleration due to gravity is 9.8 m/s²)

In free fall, the longer the object is falling the greater the speed of descent it will have and thus the greater the kinetic energy it will have before it hits a surface. (You would rather fall from a six-foot ladder than a six story building!) This principle can be witnessed in blood drop spattering patterns. If a drop falls a greater distance, it falls longer and thus with a greater total energy (splat!).

In actual crime scene analysis, the most important variable to consider and try to control is the contact surface itself. When striking a surface, the blood will leave a pattern that is very much dependent on the type and nature of the landing surface. The harder and less porous the surface, the less the blood drop will break apart. The softer and more porous the surface, the more the blood droplet will break apart. For example, blood falling onto a surface of smooth glass will remain fairly intact. Blood falling onto a sidewalk (rough concrete) will tend to break apart. When conducting controlled laboratory experiments with blood, it is critical to use the same surface materials for all tests.

Materials

Safety Precautions

The simulated blood in this activity is a nontoxic mixture but it can be messy and will stain items red. Use newspapers or paper towels to cover work areas and assist in cleanup. Clean up spills immediately with soap and water. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water upon completion of laboratory work.

Procedure

Part A. Free Fall

1. Cover the work area with newspapers or paper towels.
2. Mount a Beral-type pipet perpendicular to the floor with a ring stand and clamp (see Figure 1).
3. Place paper (recycled, half-pieces of 8½" x 11" paper are a good choice) directly beneath the pipet on top of the newspapers.
4. Secure approximately 5 mL of simulated blood in a small beaker. Fill the pipet with simulated blood and place it in the clamp on the ring stand.
5. Adjust the tip of the pipet so that it is exactly 20 cm from the flat surface below. (For short distances place the ring stand directly on the floor.)
6. Squeeze out a single drop to spatter on the paper below. Slide the paper over slightly and squeeze another drop out of the pipet. Repeat for a total of 5 drops. Try to keep the drop size consistent and avoid air bubbles in the tip of the pipet. A little practice coordinating with a partner will yield consistent drops.
7. Write “20 cm” in the corner of the paper. Record any observations about the splatter on the Bloodstain Worksheet. Hold a ruler over each drop and measure its diameter to the nearest mm. Record the measurements on the Bloodstain Worksheet.
8. Set the paper aside where it will not be disturbed and allow it to air dry.
9. Calculate the average size of the five drops and record it on the Bloodstain Worksheet.
10. Repeat steps 1–9 six more times, each time raising the height of the pipet by 20 cm until a total height of 140 cm is reached.
11. Answer the questions and complete the graph for Part A on the Bloodstain Worksheet.

Part B. Angle of Impact

1. Use a piece of cardboard to create an inclined plane. Place a piece of target or collection paper on top of the cardboard.
2. Use a protractor to measure the angle between the inclined plane and the table top or floor (see Figure 2).
3. Set the cardboard so that the angle of inclination is 75° with respect to the floor.
4. Elevate the pipet to a point 20 cm above the top of the incline and squeeze a drop of simulated blood onto the target paper on the inclined plane.
5. Move the paper slightly and squeeze another drop onto a clear portion of the paper.
6. Repeat step 5 for a third drop at the 75° angle.
7. Label the paper “75°” and lay the paper flat and examine the spattered drop. Use a ruler to measure the width and length of the spattered drops to the nearest mm and record the measurements on the Bloodstain Worksheet. Record any other observations about the drops and set the paper aside to dry.
8. Repeat steps 1–7 for 60°, 45°, 30° and 15° angles.
9. Record all data and observations on the Bloodstain Worksheet.
10. Answer the questions for Part B on the Bloodstain Worksheet.

Part C. Unknowns

1. Secure one or more crime scene blood stains from your instructor.
2. For each unknown stain pattern use your data from Parts A and B to infer the height and angle of impact.
3. Once an inference has been made, use the experimental equipment to try to duplicate the blood stain pattern in the lab. When the spatter pattern has been duplicated, dry the evidence and write a defense for your analysis.

Student Worksheet PDF

10519_Student1.pdf