Experiment: Latent Fingerprinting

EXPERIMENT: DUSTED! Operating Guide Dusted!

Visitors press their fingertips onto a clean Plexiglas sheet. Their fingerprints are then revealed as visitors dust over the print with fingerprint powder.

OBJECTIVES: Visitors will learn that every person has a unique set of fingerprints. Visitors will understand how fingerprints are revealed on surfaces.

SCIENCE TOPICS PROCESS SKILLS VOCABULARY

Properties of Matter Observing Fingerprint Properties of Electrons Comparing/Contrasting Latent

Procedure:

1. Always wear safety goggles. 2. Use the towel to clean and dry the plastic Plexiglas. 3. Firmly press one of your fingertips, fingerprint side down, anywhere on the Plexiglas. Try not to smudge your print. Can you see your print? 4. Take the same finger and dab it gently onto the oil sponge, then press firmly onto the Plexiglas near your other print. Can you see your print? 5. Pull the brush out of the powder container. 6. Carefully brush over where you left your fingerprints until you see clear prints. What do you see? Is one print more visible than the other? 7. Push the brush back into the powder container. 8. Take a piece of tape and press it down onto one of your prints and rub firmly. 9. Lift the tape off and place it onto a square of the black paper.

Why do we leave fingerprints behind? How can we collect them?

A Closer Look: In this experiment, you left your fingerprint on a Plexiglas surface. This was possible because your fingers produce a special mixture of amino acids, oil, and sweat that sticks to the plastic. It sticks because the amino acids and sweat have slightly charged particles that are attracted to other charged particles in the plastic. The same process causes our fingerprints to stick to metals, glass, and painted surfaces.

After depositing your fingerprint, you “dusted” it slightly with a fine fingerprinting dust. The dust was attracted to the charged particles in the fingerprint oil, too, but it wasn’t as attracted to the Plexiglas surface. When you brushed away the dust, it came off of the Plexiglas but kept on sticking to the fingerprint.

Fingerprints are the unique, permanent pattern of ridges and valleys on the underside of our hands.

UNIT 7 CRIME SCENE CHEMISTRY U7.3 EXPERIENCING CHEMISTRY ©2006 OMSI EXPERIMENT: DUSTED!! Operating Guide Because fingerprints can uniquely identify people, forensic scientists and police officers use fingerprints to understand who was present at a crime scene.

(with amounts to have on hand)  two clear Plexiglas sheets (approximately 8”x14”)  one sheet black construction paper (same size as Plexiglas)  one sheet white construction paper (same size as Plexiglas)  dry cloth towel  latent fingerprint brush (order information in Materials Prep)  spill-proof paint dispenser  talc  sponge  petri dish  glue gun  squeeze bottle (30 mL)  mineral oil (10 mL)  clear tape and dispenser (two rolls of tape) (order information in Materials Prep)  black and white construction paper (several sheets of each)  sealing plastic bags  small box  black, white, and dual fingerprint powders (for extensions)

Setup/Takedown Procedures

ORIGINAL SETUP

 Make the Plexiglas fingerprint sheets. Place the black construction paper on one side of one Plexiglas sheet. Firmly tape all edges of the paper to the Plexiglas. Label the sheet “Plexiglas” and “Use dry towel only, no water”. Repeat this procedure with the white sheet of construction paper and the other Plexiglas sheet.  Label the spill proof container “powder”. Add talc to the container so it just covers the bottom. Make certain the lid is secure.  Label the fingerprint brush “brush”. Insert into powder container.

UNIT 7 CRIME SCENE CHEMISTRY U7.5 EXPERIENCING CHEMISTRY ©2006 OMSI EXPERIMENT: DUSTED!! Operating Guide  Cut squares of black and white construction paper 2”x2”. Place the black squares in the small box. Save the cut white paper in a sealed plastic bag for Extensions.  Cut the sponge so that it fits snugly into the petri dish. Use a glue gun to adhere the sponge to the petri dish. Add mineral oil to the sponge (see Materials Prep).

WEEKLY SETUP

 Place the visitor instructions in a Plexiglas holder.  Take bin from cabinet.  On a tray, set out the following items: o Plexiglas sheet with black construction paper o tape and dispenser o powder container and brush o small box with black construction paper o dry cloth towel o sponge in petri dish  Turn off the water for the sink(s) near the experiment. Tape a sign saying “Please use next sink” on the faucet(s).  Take the talc from the stockroom and place under counter.

DAILY SETUP

 Set out powder and brush on tray.  Check paper supply. Replenish as needed.  Check tape supply. Replenish as needed.  Check sponge for oil. Replenish if needed.  Check talc supply. Replenish if needed.

DAILY TAKEDOWN

 Put powder and brush under counter.  With a dry towel, wipe up all powder from counter and tray.  Using a well-wrung sponge, wipe entire counter to remove powder. Dry the counter with a fresh, clean, dry towel.

WEEKLY TAKEDOWN

 Return talc to stockroom  Return all materials to the bin. Make certain all containers are closed and upright.  Return bin to the cabinet.  With a dry towel, wipe up all powder from counter and tray.  Using a well-wrung sponge, wipe entire counter to remove powder. Dry the counter with a fresh, clean, dry towel.

RUNNING SUGGESTIONS

 Make certain a dry cloth towel is always available. If towel becomes “dusty” replace with a clean, dry, cloth towel.  If the counter seems overly dusty, follow the Daily Takedown procedures to clean and dry the counter.  If visitors’ fingerprints are not clear it is usually for one of the following reasons. 1. Left a blurry print. Visitors should only press fingertip onto Plexiglas. Do not roll or jiggle finger on Plexiglas. 2. No oils on visitor’s finger. If visitors have dry fingers, use a little mineral oil from the sponge to moisten. 3. Too much oil left by visitor’s finger. Wipe fingers with a dry towel to remove excess oil. 4. Lack of ridges on fingertips. This can happen! See extensions.

EXTENSIONS

. Use the white Plexiglas sheet, white paper squares, and black fingerprint powder to demonstrate lifting fingerprints from different colored surfaces. You can also use the white powder on the white Plexiglas sheet to show that the fingerprints don’t show up. Then use the dual fingerprint powder on the black and white Plexiglas sheets to show how one powder could be used for many surfaces. CAUTION! The black powder and the dual powder are persistent and will stay around long after you’ve finished the demonstration. Be very sparing!

. Test different surfaces to determine how easy it is to lift prints. Suggestions include: the faucet handle, counter, windows, or stools. Visitors can use the white talk powder and dust for fingerprints all over the lab! Please don’t let them do this with the black or dual powders.

. Pineapples have an enzyme called bromelain that breaks down protein. Primarily, it is used to tenderize meat, which is why it is added to ham while cooking. Instructions for Jello® say “do not add fresh pineapple” because the pineapple dissolves the gelatin in the Jello®. Thus people who pick pineapples, work in a facility that cuts pineapples, or just use a lot of pineapple with their fingers, can have decreased ridges on their fingertips since the bromelain breaks down the protein in skin. http://www.uga.edu/fruit/pinapple.htm

Safety & Disposal

Health: Talc is not considered hazardous. Follow standard lab procedures. Flammability: See MSDS for further information. Reactivity: Unused talc may be thrown in the trash. Follow standard lab Contact: procedures.

MATERIALS PREP

To prepare mineral oil sponge 1. Use only a SMALL drop of mineral oil on the sponge. 2. Gently massage the oil into the sponge. 3. Test the sponge by placing a clean fingertip on the sponge. A faint oil residue should remain on your fingertip. If not, add another SMALL drop of mineral oil. 4. Test your fingerprint on the Plexiglas. Press your oily finger onto the Plexiglas. Use the brush and powder to reveal you print. If clear prints are not evident, you have too much oil on the sponge. 5. Remove excess oil from the sponge by wiping with a dry towel.

Tape 1. Order tape with “clear gloss” finish. Translucent tape is too blurry to see fingerprints well. 2. Success has been found with this brand: Office Depot, Item #305324, tape, 1” core, clear gloss finish, ¾” wide.

Care of latent fingerprint brush. 1. Always return brush to case handle first. 2. Never try to put brush in case by inserting the bristles first. 3. Always remove the brush by opening the end of the case that has the handle. Pull the brush out of the case by is handle. 4. Never try to remove the brush by pulling it out by its bristles. 5. To straighten the bristles, spin the brush.

For Latent print supplies: http://www.evidentcrimescene.com/cata/latent/latent.html

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