Forensic ScienceUnit 3 Part 3

T. Trimpe 2007 http://sciencespot.net/

3 Types of Fingerprint Evidence

• Visible: Left by the transfer of blood, paint or another fluid or powder onto a surface that is smooth enough to hold the print; evident to the naked eye

• Molded (plastic): Left in a soft medium like soap, putty or candle wax, forming an impression

• Latent: Left by the transfer of sweat and natural oils from the fingers onto a surface that is smooth enough to hold the print; not visible to the naked eye

Latent prints are impressions left by friction ridge skin on a surface, such as a tool handle, glass, door, etc. Prints may be collected by revealing them with a dusting of black powder and then lifted with a piece of clear tape.

Did you know? Camel hair is the most common animal hair used to make fingerprint brushes. Now many brushes (like the one above) are made out of fiberglass.

Tools for recovering fingerprints include brushes, powders, tape, chemicals, lift cards, a magnifying glass and Super

Glue.

• A crime lab can use fingerprints to identify the victim or identify or rule out a suspect.

• There are several types of prints a CSI might find at a crime scene.

How does a CSI obtain latent fingerprints?

Some investigators use fluorescent powder and UV lights to help them find latent prints on multi-colored or dark surfaces.

Magnetic powder can also be used to reveal latent prints. This type of powder works better on shiny surfaces or plastic baggies or containers.

The cyanoacrylate fuming method (often called the super glue method) is a procedure that is used to develop latent fingerprints on a variety of objects.

http://www.aetv.com/crime-360/evidence-locker/

Click the icon to view the Crime 360Super Glue

Video

Ninhydrin is a chemical that bonds with the amino acids in fingerprints and will produce a blue or purple color. It is used to lift prints from surfaces such as paper and cardboard.

Grab a Laptop and a Partner

A perpetrator might leave prints on porous or nonporous surfaces. Paper, unfinished wood and cardboard are porous surfaces that will hold a print,

and glass, plastic and metal are non-porous surfaces.

• A CSI will typically look for latent prints on surfaces the perpetrator is likely to have touched. For instance, if there are signs of forced entry on the front door, the outside door knob and door surface are logical places to look for prints.

• Breathing on a surface or shining a very strong light on it might make a latent print temporarily visible. When you see a TV detective turn a doorknob using a handkerchief, she's probably destroying a latent print. The only way not to corrupt a latent print on a non-porous surface is to not touch it.

Proper methods for recovering latent prints include:

• Powder (for non-porous surfaces): Metallic silver powder or velvet black powderA CSI uses whichever powder contrasts most with the color of material holding the print. He gently brushes powder onto the surface in a circular motion until a print is visible; then he starts brushing in the direction of the print ridges. He takes a photo of the print before using tape to lift it (this makes it stand up better in court). He adheres clear tape to the powdered print, draws it back in a smooth motion and then adheres is to a fingerprint card of a contrasting color to the powder.

• Powders and brushes at the CBI latent-fingerprint lab

Other Methods• Chemicals (for porous surfaces): Iodine,

ninhydrin, silver nitrateThe CSI sprays the chemical onto the surface of the material or dips the material into a chemical solution to reveal the latent print.

• Cyanoacrylate (Super Glue) fuming (for porous or non-porous surfaces)The CSI pours Super Glue into a metal plate and heats it to about 120 F. He then places the plate, the heat source and the object containing the latent print in an airtight container. The fumes from the Super Glue make the latent print visible without disturbing the material

Directions:

1 - Cover your table with white butcher paper or newspaper. You must dust everything on the paper!

2 - Get a lifting kit from your teacher that contains black powder, brushes, and clear tape.

3 - Press the pad of your right thumb on a CD or glass slide to make a print. Place on the paper covering your table.

4 – Dip a brush lightly into the container of black powder and then tap off the extra on the lid. You only need a very small amount of powder to dust the print.

5 – Hold the brush over the print and rotate it between your thumb and fingers. Use the brush to remove any extra powder.

6 - Use a small piece of clear tape to lift the print and place it in the box on your worksheet.

CAUTION: The black powder will be messy and isn’t easy to clean up. Don’t dust anything without permission!

Clean Up

1 – Clean off the CDs or glass slides and put them back in the kit with the brushes and tape.

2 – Have someone help you fold the paper in half and tap it to return the extra black powder to the container.

3 – Put the black powder in the box and have it checked in by your teacher.

4 – Get a towel and “dry” wash the table – especially the edges that weren’t covered with paper.

5 – Get a wet towel to wash off the table and then wipe it with some dry towels. Keep cleaning until all the black powder is off the table!