Chromatography
# Can be qualitative or quantitative
# Also a separation technique
# Many different types: all involve a stationary phase and a mobile phase
Paper Chromatography
When the apparatus (below) is set up the solvent (water in this case, the mobile phase)
moves up the filter paper (the stationary phase). The ink is dissolved and the solvent
front continues to rise. As it does the components of the ink are adsorbed onto the
surface of the paper and then desorbed back into the solvent.
solvent front
paper (stationary phase)
origin
solvent: water (mobile phase)
Different components in the ink are adsorbed to differing degrees and therefore rise to
different levels and can be distinguished. The most strongly adsorbed components
rise the least distance.
adsorb: to bond to a surface (nb: this is very different to absorb - to take in)
desorb: to break the bonds at a surface
origin: point at which the sample to be tested is placed
chromatogram: the paper or stationary phase that displays the results of the test
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Rf Values
The components can be identified by calculating an Rf value.
Rf = distance moved from origin by component
distance moved from origin by solvent
Rf = 8 = 2 = 0.67
12 3
(no units)
solvent front
12cm
origin
8cm
Once the Rf value is obtained it can be compared to the Rf values of known
substances. Identical conditions must be used to obtain the Rf values for the known
and unknown substances.
The choice of solvent is vital to obtaining useful and meaningful results.
Why would you choose methylated spirits as a solvent when separating/testing
indelible ink?
How do chemists determine the distance travelled by colourless components of a
chromatogram?
Thin Layer Chromatography (T.L.C.)
Is similar to paper chromatography, however, a fine layer of silica gel or alumina, on
a glass or plastic slide acts as the stationary phase.
Compare paper and TLC, listing pros and cons of each.
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Column Chromatography
Paper and Thin-layer Chromatography have been super-ceded by various types of
column chromatography. In column chromatography a sample is introduced into
one end of the column and carried to the other end by a solvent. The stationary phase
in the column is a fibrous or granular solid such as alumina (Al2O3). Different
components travel faster than others and reach the end of the column sooner. A
detector at the end records the type and amount of each chemical species eluted from
the column.
The diagram above shows the basic features of a column chromatography apparatus.
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High Performance Liquid Chromatography (HPLC)
HPLC is a type of column chromatography. It is both qualitative and quantitative.
The time taken by a component to travel through the column is called the retention
time (Rt). This value is compared to the Rt of known compounds to determine the
unknown component. The detector at the end records how much of each component
is eluted.
The diagram above shows the major features of an HPLC in a schematic fashion.
List 4 ways in which HPLC differs from earlier forms of column chromatography.
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The diagrams above are examples of chromatograms (the results from a
chromatography experiment).
a) is normal coffee
b) is caffeine
c) is de-caff coffee
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Gas-Liquid Chromatography
Gas-liquid chromatography is another type of column chromatography. It is also both
qualitative and quantitative. The mobile phase is a gas and the stationary phase is a
viscous liquid which has been coated onto a porous or honeycomb solid.
The diagram above shows the major features of a GL chromatograph in a schematic
fashion.
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