Forensic Science
T. Trimpe 2006 http://sciencespot.net/
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.
We can use chromatography to separate the components of
inks and dyes, such as those found in pens, markers,
clothing, and even candy shells. Chromatography can also be
used to separate the colored pigments in plants.
http://members.shaw.ca/vict/chemistry_test3.htm
A few basics ...
Mixture – Two or more substances that are mixed together, but not chemically
combined.
Examples of mixtures ...
Air – mixture of gases
Bowl of cereal – mixture of cereal and milk
Soda pop – mixture of soda syrup, water, and CO2 gas
Fog –water suspended in air
Kool-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 chemically
Water –Hydrogen and oxygen combined chemically
Carbon Dioxide – Carbon and oxygen combined chemically
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 ...
Solution
Solute
Solvent
Lemonade
Soda pop
Ocean water
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.
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.
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.
RF
=
Distance pigments traveled
Distance water traveled
Directions
• Obtain the supplies you’ll need from your teacher.
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Tape – Label with marker
Pencil
Paper
Beaker with a small amount of water
Paper strips
Pencil
Markers for testing
Masking tape
Timer
Ink
•
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
don’t 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.