Chromatography
Learning objectives
By the end of the lesson you will be able to:
•State that chromatography is a method for separating a
mixture of soluble substances
•Describe some of the uses of chromatography.
•Explain separation by chromatography in terms of the
solutes being more or less soluble in the solvent.
Keywords
• Solvent- The liquid that dissolves the solute.
• Solute- The solid or liquid that dissolves in a
given solvent to form a solution.
Chromatography - Definition
A method of separation and analysis of a
mixture of soluble chemical substances.
Paper Chromatography – how does
it work?
Chromatography lets us separate inks
and dyes according to the size of
their particles.
As the solvent (water) rises through
the paper it dissolves the sample
mixture, which will then travel up
the paper.
Small particles travel further than
large particles, due to the
differences in solubility and their
attractions with the paper.
Retention factor (Rf) values
The Rf factor is used to compare the
components of various samples. The
Rf values of suspect samples can be
compared with known samples.
Rf =
distance from the base line to the spot
distance from the base line to the solvent front
If two substances
have the same Rf
value, they are likely
(but not necessarily)
the same compound.
If they have different
Rf values, they are
definitely different
compounds.
Solvent front
the point at which the water
stopped moving up the paper
Spot
the point at which a band
or spot of colour is
Base line
the line where the original
sample was placed
What can chromatography be
used for?
- identify additives in foods
- compare fibres found at a crime scene
- identify drugs and alcohol
- test water samples for pollution
- detect bombs in airports
- detecting pesticides or insecticides in food
- fingerprinting
What are food additives and why are
they put in our food?
• We need to make food last longer – so we find
ways to preserve it.
• A substance used to preserve food is known as a
food additive. Food additives can also be used to
improve taste or appearance of food.
• Additives that have been approved for use in
Europe have been given E numbers to identify
them.
Detecting Additives
• Food additives can be detecting by a technique known
as chromatography.
• This technique separates out the different components
within the food additives based on how well they
dissolve in a particular solvent.
• Their solubility determines how far they travel across a
surface (chromatography paper).
• Once the compounds have been separated out using
chromatography, they can be identified by comparing
with known substances. Alternatively a mass
spectrometer can be used.