Separation Science
Differences in IMFs can be used to
separate chemical substances for
further analysis.
Applications: forensics,
atmospheric chemistry, research,
environmental chemistry, food
science, etc.
Separation Science
How CAN we separate and identify a
mixture of alkanes and their isomers?
Which of these isomers of pentane has the
highest boiling point?
Gas Chromatography
Uses differences in IMF to separate
compounds and isomers.
Used to determine the composition of a
mixture:
qualitatively (what is present) and
quantitatively (how much of each
component is present).
Gas chromatography using modern
instrumentation and columns requires only a
trace of sample dissolved in a volatile solvent.
Gas Chromatography
A
E
Gas
Chromatograph
D
B
C
Sample: mixture of
volatile liquids (~1L)
Gas Chromatogram
B
E
C
Abundance
A
D
0
5
10
Time (minutes)
15
20
Each
substance will
elute from the
chromatograph
after a
characteristic
time – and
recorded on a
chromatogram.
Gas Chromatograph
Injection Port
Detector
Capillary Column
Data System
or Recorder
Carrier Gas
Supply
Oven
Gas Chromatography (GC)
A mixture (liquid or gas) is injected into a thin,
heated column packed with a solid coated with a
viscous liquid. The mixture vaporizes and is
carried through the column by a carrier gas
(generally helium).
column
Carrier gas
+ mixture
Packing material
covered with a viscous
liquid
detector
Gas Chromatography (GC)
Capillary Tube
Liquid Stationary Phase
A ABA
A
A
A A
BA B B
B
He Carrier gas
AA B
A BA
B
He Carrier gas
A
A
A A
B
B
B
B
B
B
B B
0
Immediately after injection
After several minutes
Compounds A and B interact with the stationary phase through
intermolecular forces: (van der Waals or dipole-dipole forces, including
hydrogen bonding).
A interacts more strongly with the stationary liquid phase and is
retained relative to B, which interacts weakly with the stationary phase.
Thus B spends more time in the gas phase and advances more rapidly
through the column and has a shorter retention time than A.
Gas Chromatography (GC)
A
A
A A
He Carrier gas
A
A
A A
B
B
B
B
B
B
B
A
B B
Time
0
After several minutes
Resulting chromatogram
out first
out last
Typically, components with similar polarity elute in order of
volatility.
Thus alkanes elute in order of increasing boiling points;
lower boiling alkanes will have shorter retention times than
higher boiling alkanes.
Concept Test
Which hydrocarbon has the highest boiling
point?
Concept Test
Which hydrocarbon will come out of the GC
column first?
Gas Chromatograms
tR = “retention time”
Peak due to hexane
Air peak
ConcepTest
Which chromatogram on the
next slide is due to heptane,
and which to octane?
A
B
heptane
octane
octane
heptane