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Chemistry - A-level OCR A
Module 6
Module 6.3.1 - Chromatography and qualitative analysis
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Tests and observations for organic compounds
Chemistry - A-level OCR A > Module 6 > Module 6.3.1 - Chromatography and qualitative analysis
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Stationary
phase
The phase that does not
move
in chromatography-usually a
polar
solid
Mobile phase
The phase that
moves
in chromatography and is normally a
liquid
or
gas
Thin
Layer
Chromatography
(TLC)
A type of chromatography that uses
silica
gel or
alumina
on a card as the medium for the
stationary
phase
Retention factor (Rf) formula
distance
moved by the
component
/
distance
moved by the
solvent front
The more soluble a solute in the mobile phase ...
the
higher
the
Rf
value or the
lower
the
retention
time
Adsorbent
substance
An agent capable of
holding
other
molecules
onto its
surface
by
physical
or
chemical
means. In this case it is usually
silica
Adsorption
The process of
binding
or
sticking
to a surface. It's where the solid
silica
holds the
different
substances in the
mixture
to its surface
Can a component be identified from its Rf / Retention time
Yes
, by
matching
it with
reference
Rf
/
retention
times conducted with the same mobile and
stationery
phases
Gas Chromatography
A technique used to
separate
substances in a
gaseous
state or
volatile
compounds (have low boiling points and vapourise easily)
Stationary
phase for gas chromatography
liquid
(using
relative
solubility
) or
solid
(using
adsorption
)
Mobile phase for gas chromatography
inert
carrier gas e.g. helium or neon
Retention
time
amount of time a compound spends in
stationary
phase
time for a
component
to pass from the column inlet to the
detector
The longer a compound is present in the column (longer retention time), the...
more
strongly
it is
adsorbed
o rthe
higher
the
relative
solubility
(interaction between stationary and mobile phases could be either adsorption or relative solubility)
TLC vs Gas chromatography
Separates
soluble
compounds vs
volatile
compounds
Solid
stationary phase vs
Liquid
or solid stationary phase
Solvent
mobile phase vs
Inert
carrier
gas
mobile phase
Rf
value vs
Retention
time
How can you determine the
concentration
of a sample in gas chromatography
Peak integrations
(or area under each peak)
Procedure of determining the concentration of a sample in gas chromatography (peak integration)
1) Prepare
standard
solutions of known
concentrations
of compound being investigated
2) obtain
gas
chromatograms for each solution
3) plot a
calibration
curve of relative
peak
area against
concentration
4) obtain gas
chromatogram
of compound being investigated (under same conditions)
5) use
calibration
curve to measure the
concentration
of the compound
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