Effects of Molecules on Retention Times

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University of Pittsburgh at Bradford
Science In Motion
Chemistry Lab 038
Effects of Molecules on Retention Times
Introduction:
There are several factors that effect the retention time of a molecule during a gas chromatography
experiment. Some of these factors include the size of the molecule, the shape of the molecule, and the
polarity of the molecule. By keeping any two of the three constant, the effect of the third factor on the
retention time can be examined.
Purpose:
The purpose of this experiment is to determine the effects of size, shape, and polarity of a
molecule on the retention time using gas chromatography.
Equipment/Materials:
Gas Chromatograph
Computer
Printer
Syringe
Sample vials with different volatile compounds
Safety:



Always wear an apron and goggles in the lab.
DO NOT misuse the syringes. These are very dangerous, especially when filled with hazardous
chemicals.
The gas chromatograph is like an oven, so the ports and top of the instrument may be hot. Use
with caution.
Procedure:
1. Obtain the structures for the samples to be tested.
2. Obtain a microliter syringe and a sample vial containing one of the samples to be tested.
3. Rinse the syringe 3 times with the sample to be tested. Then obtain 2 microliters of the sample in the
syringe.
4. Insert the needle on the syringe into the injection port. At exactly the same time that one member of
the group injects a sample, a second group member will press the space bar on the computer to begin
the data collection.
5. After this peak is recorded, end the run by pressing the end button on the computer.
6. Using a pen or pencil, record the identity of the sample on the chart. Be sure to record the conditions
of the gas chromatograph as well.
7. Repeat ALL of steps 2-7 for the other samples.
1
Data Tables:
Table 1:
Name of Substance Structure
Retention Time
methanol
ethanol
n-propanol
n-butanol
n-pentanol
n-hexanol
n-heptanol
Table 2:
Name of Substance Structure
Retention Time
n-butanol
2-butanol
iso-butanol
tert-butanol
Table 3:
Name of Substance Structure
Retention Time
pentane
n-pentane
hexane
n-hexane
heptane
n-heptane
2
Questions:
1.
Compare and contrast the structures in table 1.
2.
What conclusions can be made concerning the retention time and molecules in table 1?
3.
Compare and contrast the structures in table 2.
4.
What conclusion can be made concerning the retention time and molecules in table 2?
5.
Compare and contrast the structures in table 3. (Compare the first two molecules, the second two
molecules, and the third two molecules with each other.)
6.
What conclusion can be made concerning the retention time and molecules in table 3?
7.
Predict which molecule would have a longer retention time and explain why:
a. octane or octanol
b. decane or octane
c. n-propanol or 2-propanol
d. 2-proplanol or ethanol
8.
What other factors could effect retention times of molecules?
3
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