Lab Exercise



Lab 1: Recovery of Latent Fingerprints


1 coffee cup
2 oz. (two ounces) of talcum powder or corn starch
2 oz. (two ounces) of cocoa powder
1 small soft bristle faint brush or preferably a small makeup brush (The softer the brush—the better for the
1 small roll of packaging tape
Several pieces of light paper and dark paper cut into approximately 3 x 5 cards
1 inexpensive magnifying glass

Instructions for Lab:

  1. Place the talcum or corn starch into the coffee cup.
  2. Find an object that had a dark smooth surface.
  3. Rub your fingers across your forehead to get a good deposit of natural oil. Or instead, should you

have dry skin, you can utilize an oil based hand cream.

  1. Once you have a fair deposit of oil on your fingers, touch the dark surface that you have selected,
    placing fingerprints on the surface.
  2. Next, dip the tip of your soft brush into the white talcum powder or corn starch. Tap the brush
    lightly on the side of the coffee cup to remove excess residue.
  3. Now that you have the brush lightly powdered, gently move the tip of the brush over the dark
    surface on which you have deposited your latent fingerprints. Move in a circular motion around
    the surface. You should see the prints begin to show. Do not dust the prints too much, as they will
    fill up with powder and make a smudge instead of a print. Likewise, too much powder on your
    brush will “clog” the print by filling in the space between the ridges.
  4. Once you have developed the prints to where they are visible, take about a two inch section of
    tape, lay it gently on top of the print, moving from one edge of the tape to the other so as not to
    smudge the print, and carefully rub the tape, sticky side down, onto your fingerprint. Move your
    finger in a circular motion over the tape applying gentle pressure until you feel that the print has
    stuck to the tape.
  5. Pull the tape up from one side and lift it off of the surface. You should see the fingerprint on the
    tape. Take one of your dark 3 x 5 pieces of paper and adhere the tape to the paper by attaching
    one side of the tape and smoothing the entire section of tape to your paper.
  6. The white colored fingerprint should stand out on the black or dark paper background and be
    easily visible. Look at your fingerprint with a magnifying glass. Then examine your fingers with the
    glass and see if you can determine which of your fingers left the latent fingerprint that you have
    just recovered. Now, you are ready to send your recovered trace evidence—the fingerprint, to the
  7. Repeat the experiment utilizing the cocoa powder; but this time utilize a light colored surface and
    attach your print to the light colored 3 x 5 paper.

Lab 2: Chromatography


2 coffee filters
1 medium size drinking glass
1 pair scissors
1 bottle of rubbing alcohol
5 different types of ink pens (about 5/ball point) of the same color ink (blue or black) each manufactured

by a different company
1 inexpensive magnifying glass
1 pencil
1 roll of Scotch tape (any brand)

Instructions for Lab:

  1. Cut the coffee filters into approximately 5 inch single ply strips.
  2. Place about an inch or two of alcohol in the glass.

C. Place a large “dot” of ink on a strip of coffee filter with one of your ball point pens approximate 3⁄4
inch from one bottom of the strip. Set the strip aside, as this will serve as your item of “evidence.”

  1. Next, take all of the ball point pens, and place a single dot with each different pen about 3⁄4 of an
    inch from the bottom of your other coffee filter strips. So, you will have several strips of coffee
    filters with one good sized dot of ink from each pen. Each pen will then have a single dot
    represented on a single strip. These strips represent “suspect” pens. With reasonable luck, only
    one will likely match the original dot that you sat aside on the original strip of coffee filter. Our goal
    is to identify the original pen from all of the others by use of chromatography.
  2. We will begin our forensic search for the “guilty pen” that we first used.
  3. Take your pencil and tape one end of a marked coffee filter to the pencil, allowing one end, the
    end with the dot, to dangle freely.
  4. Place the pencil across the top of the glass and allow the free end of the coffee filter to dangle
    into the glass. The tip of the filter should touch the alcohol that you poured into the glass. You can
    do this by rolling the pencil to adjust the “depth” of the strip. Make sure that it touched the alcohol
    just enough so as to allow the alcohol to absorb upward on the filter. Your ink dot should be about
    1⁄2 to 3⁄4 inch above the alcohol.
  5. Observe the filter strip as the ink absorbs upward and over your ink dot.
  6. The ink on the filter should begin to separate. Various color bands should begin to radiate
    outward from the dot as the ink begins to separate.
  7. Repeat this process with each strip of coffee filter, setting the filter strips aside on a soft surface
    to dry.
  8. After allowing each of the “suspect” strips to absorb alcohol and separate, take your original
    coffer filter strip that you set aside and allow it to absorb alcohol and separate.
  9. Once all of the strips have dried (a short time is all that it will take), take your magnifying glass
    and compare your original strip (the evidence strip) to all of the others (suspect strips). Can you
    identify which pen made the original mark (dot) on the evidence strip by comparing the strips to
    each other?

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