8th Grade Forensic Science

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Chromatography

• 8th Grade Forensic Science

T. Trimpe 2006 http//sciencespot.net/
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What is chromatography?
From Wikipedia ... Chromatography (from Greek
word for chroma, colour) is the collective term
for a family of laboratory techniques for the
separation of mixtures. It involves passing a
mixture which contains the analyte through a
stationary phase, which separates it from other
molecules in the mixture and allows it to be
isolated.
Which means ... Chromatography is the physical
separation of a mixture into its individual
components.
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A few basics ...
Mixture Two or more substances that are mixed
together, but not chemically combined.
Examples of mixtures ...Air mixture of
gasesBowl of cereal mixture of cereal and
milkSoda pop mixture of soda syrup, water, and
CO2 gasFog water suspended in airKool-Aid
mixture of water, sugar, and flavor crystals
Compounds Two or more elements that are
chemically combined.
Examples of compounds ...Salt Sodium and
chlorine combined chemicallyWater Hydrogen and
oxygen combined chemically Carbon Dioxide
Carbon and oxygen combined chemically
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Solutions
Solutions are mixtures in which one substance is
dissolved in another. Solutions have two parts
solute and solvent The solute is the substance
that is dissolved. The solvent is the substance
that does the dissolving
Identify the solute and solvent in each solution
...
Solubility - A measure of how much of a given
substance will dissolve in a liquid. A
substance that does not dissolve in water is
called insoluble. A substance that does
dissolve in water is called soluble.
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Types of Chromatography
Liquid Chromatography is used in the world to
test water samples to look for pollution in lakes
and rivers. It is used to analyze metal ions and
organic compounds in solutions. Liquid
chromatography uses liquids which may incorporate
insoluble molecules.
Gas Chromatography is used in airports to detect
bombs and is used is forensics in many different
ways. It is used to analyze fibers on a persons
body and also analyze blood found at a crime
scene. Helium is used to move a gaseous mixture
through a column of absorbent material.
Thin-layer Chromatography uses an absorbent
material on flat glass or plastic plates. It is
used to detect pesticide or insecticide residues
in food. Thin-layer chromatography is also used
in forensics to analyze the dye composition of
fibers.
Paper Chromatography is one of the most common
types of chromatography. It uses a strip of paper
and capillary action is used to pull the solvents
up through the paper to separate the solutes.
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RF Factor
Because molecules in ink and other mixtures have
different characteristics (such as size and
solubility), they travel at different speeds when
pulled along a piece of paper by a solvent
(water). For example, black ink contains several
colors. When the water flows through a word
written in black, the molecules of each one of
the colors behave differently, resulting in a
sort of rainbow effect with the colors
separating out at different rates.
We can determine the RF (or retardation factor)
by measuring the distance the pigments traveled
up the strip and the distance the water traveled.
You will need to calculate the RF for each of the
colors that are present in a sample.
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Directions

• Obtain the supplies youll need from your
  teacher.
• Beaker with a small amount of water
• Paper strips
• Pencil
• Markers for testing
• Masking tape
• Timer
• Cut the strip so that it will be at the correct
  height long enough to reach the top level of
  the water. Write the pen number on the strip
  using a pencil or permanent marker.
• Use one of the testing markers to place one line
  on the paper strip at about ¼ inch from the
  bottom. You dont want the ink line to touch the
  water.
• Tape the top of the strip to a pencil and hang in
  the water for 5 minutes.
• Remove the strip from the water and use a pencil
  to mark the line between the wet and dry portions
  of the strip.
• Use the ruler to make the measurements of each
  color for RF data and then allow the strip to dry.

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Let's give it a try ...