Student Worksheet

Collecting Trace Evidence

Your group has been given a 'crime scene' to investigate. You need to use the instructions provided to collect and analyse the trace evidence to try to work out what occurred.

Procedure

Put on your gloves, lab coat and hairnet to avoid contaminating the crime scene.
Take a photo OR draw a picture of your crime scene before you 'enter' it. Place this in the results section.
Write a hypothesis below about what you think has happened at the crime scene, based purely on your observation.
Look for trace evidence.
1. Carefully place the trace evidence in an evidence bag or bottle, making sure to use your gloved hands or forceps.
2. Replace each piece of evidence with a numbered label. Record a description of each piece of evidence next to the appropriate number in the Results table.

Hypothesis:

.

Results:

A photo or picture of the crime scene (staple to this sheet as a separate page)

Analysing Trace Evidence

Blood analysis - Kestle-Meyer test
The Kastle–Meyer test is one of the two main classes of forensic tests commonly employed by crime labs in the chemical identification of blood. The chemical indicator phenolphthalein is used to detect the possible presence of haemoglobin. If haemoglobin is present, the result will be a bright pink colour.

Materials

3% Hydrogen peroxide in a dropper bottle
Cotton bud with animal blood on it as the murder weapon
Phenolphthalein in a dropper bottle

Procedure

Add a drop of phenolphthalein reagent to the presumed murder weapon (i.e. the cotton bud)
After a few seconds, add a drop of hydrogen peroxide to the murder weapon.
If the swab turns pink rapidly, it has blood on it. Note: Do not wait over 30 seconds as it will turn pink naturally due to the natural oxidation of the chemicals in the air.
Record in the results if blood was present or not.

Fingerprint analysis

The hairless parts of the body (i.e. the inner surfaces of the hands and the soles of the feet), are covered with patterns formed by raised ridges of skin known as friction or papillary ridges. The sweat glands, located in the outer layer of the skin (called the dermis), discharge sweat at the skin surface through sweat pores found at the top of the ridges. When we touch a surface with our fingers, this sweat on the ridges of our fingers leaves what we commonly call a fingerprint. They follow three fundamental principles:

A fingerprint is an individual characteristic; no two people have the same fingerprint pattern, not even identical twins!
A fingerprint pattern will remain unchanged for the life of an individual unless it gets damaged due to permanent scars or skin diseases.
Fingerprints have general characteristic ridge patterns that allow them to be systematically identified.

Latent fingerprints are impressions left on a surface, such as glass. Prints may be collected by revealing them with a dusting of black powder and then lifting them with a piece of clear tape. Investigators can compare them to fingerprints police have on file in a database, or from those taken by a suspect.

Materials

Clear tape
Glass from the crime scene
Fingerprint powder or any fine powder (such as talcum powder, cornstarch, or cocoa powder)
Fingerprint brush (or any small brush with very soft bristles)

Procedure

Sprinkle a little bit of powder on the glass, then gently wipe off the excess powder with the soft brush, being careful to leave the fingerprint/s intact. This may take some practice to get right.
Stick a piece of clear tape over the fingerprint/s firmly, and then lift it up; the print should adhere to the tape.
Either photograph the print and insert it in the results section or stick it in.
Compare the print to the suspects’ prints and see if you get a match.

Hint: If you are having issues lifting the prints, try researching 'fingerprint fuming' and see if you have more success with it.

Tyre tread analysis

Materials

Camera
Ruler

Procedure

Take a photo of the tyre prints present at the crime scene and insert the photo into the results section of this report OR draw them.
Measure the features on the tyre tread and include these measurements on the photo.
Visit the suspects’ cars. Take a photo of the tyres of each and insert it in the results OR use ink to make an impression of each.
Measure the features on the tyres and include them with photos or impressions.
Decide which, if any, car the tyre tread from the crime scene came from.

Hair analysis

Hair lasts longer than many other types of evidence and is often left behind unnoticed by the perpetrator of a crime. It can link a suspect to the scene of a crime, show contact with a victim, and identify clothing or shoes abandoned by a suspect.

Hair is composed of three parts – the cuticle (on the outside), medulla (next layer) and cortex (middle), all visible under a microscope. There are three common factors used to identify hairs: scale pattern, medulla patterns, and medullary index.

Materials

Forceps
Glycerin (or water)
Hair sample from the crime scene
Microscope slide and cover
Microscope slides of suspects’ hair

Procedure

Making a wet mount of your hair

Place a drop of glycerine (or water) on the hair.
Using the forceps, carefully place the hair from the crime scene on the microscope slide.
Gently lower the coverslip over the hair, trying to avoid making bubbles in the liquid as you lower it down.
- Hint: Use this video for a demonstration of the above process:
  Making a wet mount microscope slide with hair - Norman Price (youtu.be/p8Wc7ZBoIok)
Use the microscope to look at the hair and follow the instructions below for each of the characteristics.
- Hint: Watch the following clip if you need help using the microscope. The video might show a different microscope from the model you are using, but the skills are similar. BIOLOGY 10 - Basic Microscope Setup and Use - Fresno State (youtu.be/SUo2fHZaZCU)

Scale Patterns

Observe the outside of the hair under 100X and 400X. By carefully adjusting the lamp and diaphragm you should be able to see one of the scale patterns shown below.
Record the type of scale pattern on the data table.

Source

Medulla Pattern and Shape

The medulla is a collection of cells having the appearance of a central canal running down the centre of the hair. The medulla is generally classified as continuous, intermittent, fragmented, or absent. Human head hairs have fragmented patterns or are absent. Most animals have either a continuous or interrupted pattern.

Observe the medulla of the hair under 100X and 400X magnifications.
List the pattern on your data table.

Source

Humans, as well as many animals, have medullae that are cylindrical in appearance. Other animals exhibit medullae that have patterned shapes. For example, a cat has a medulla that can best be described as a string of pearls.

Describe the shape you see on your data table. The names below may be helpful or if necessary give your own description. If no pattern is visible say none.

Source

Medullary Index

If your microscope has a measurement feature, determine the diameter of the medulla and the diameter of the entire hair.
Use the formula below to determine the index and enter it into your data table. Generally, for humans, the index is less than ⅓. For animals, it is ½ or greater.

Medullary index = diameter of medulla ÷ diameter of hair

Repeat the steps above for each suspects’ hair and the victim's hair.
Record the results in your data table. Determine if any of the samples match the crime scene sample.

Results

Blood analysis (circle result)

Blood present

Blood absent

Fingerprint analysis

Insert fingerprint below

Which suspect’s fingerprint does it match (if any)?

.

Tyre tread analysis

Record your tread analysis data below.

Which suspect’s car does it match (if any)?

Hair analysis

Which suspect’s hair sample does it match (if any)?

Conclusion - Based on the evidence, the most likely suspect is:

Real-life CSI - Trace Evidence

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