GAS CHROMATOGRAPHY
Purpose:
The purpose of this laboratory exercise is to introduce, explain and apply the
concept of gas chromatography.
Discussion:
It is much more common to encounter an organic substance (natural or synthetic)
as a component of a mixture than as a pure substance. The last half of the 20th century
witnessed remarkable advances in the separation and purification of such mixtures.
Classical methods such as fractional distillation of liquids and recrystallization of solids
have been adapted for large scale production in the pharmaceutical and chemical
industries, but other methods that are more appropriate to the separation of small amounts
of material in laboratory-scale work have been developed. Chromatography is such a
method, and its various forms have the ability to separate exceedingly small quantities of
mixtures.
The first step in chromatography involves absorbing the sample to be separated
onto some material called the stationary phase. Next, a second phase (the mobile phase)
is allowed to move across the stationary phase. Depending on the properties of the two
phases and the components of the mixture, the mixture is separated into its components
according to the rate at which each is removed from the stationary phase by the mobile
phase.
In gas chromatography (GC) the stationary phase consists of beads of an inert
solid coated with a high-boiling liquid. The mobile phase is a gas, usually helium.
Below is a diagram of the major components of a GC.
The sample is injected by syringe onto a heated block where a stream of helium
carries in onto a coiled column packed with the stationary phase. The components of the
mixture move through the column at different rates (different retention times). The
components then pass through a detector which feeds a signal to a recorder whenever a
substance (other than the pure carrier gas) leaves the column. Thus, one determines the
number of components in a mixture by counting the number of peaks on a strip chart.
Identification of the components is possible by comparing the retention times of the
components of the mixture with authentic samples of each component.
Carey’s “Organic Chemistry,” 1992, pages 554 – 555.
Objectives:
In this lab, you will…
1. run several pure samples of chemicals through the GC.
2. run a mixture of several unknowns through the GC.
3. identify the components of the mixture by comparing the pure sample
chart peaks with the peaks on the unknown chart.
Materials:
SRI Model 310 Gas Chromatograph
10 L syringe
6 small test tubes
Wood test tube rack
Chemicals
Methanol
Cyclohexanone
Pentane
Acetone
Cyclohexane
Ethyl acetate
Unknown
mixture
Safety:
 All chemicals are flammable. Do not use any open flame.
 Follow procedures for the GC very carefully to avoid dangerous situations and to
avoid damaging the GC.
Procedures:
1. Open the main valve on the helium tank. If necessary, adjust the carrier gas
output pressure to 30 psi. Turn on the GC. Verify that the carrier gas is passing
all the way through the machine by dropping soap on the end of the column in the
oven. If bubbles are produced, continue. If there are no bubbles, do not continue
and get instructional help. Verify that the pressure of the carrier gas flowing
through the GC is within one psi of the setpoint.
2. Turn the TCD amplifier to the low setting. Start the PeakSimple data acquisition
program. Zero the data system signal on the PeakSimple program by clicking on
the “zero” button between the zoom in and zoom out icons.
3. Set the PeakSimple parameters as follows:
o Initial Temperature: 25°C
o Hold Time:
0.5 minutes
o Ramp Time:
5 minutes
o Final Temperature: 50°C
4. Zero the data system signal on the PeakSimple program again.
5. Obtain the five known chemicals (standards), the unknown mixture and acetone.
Pour a small sample (<1 mL) of each chemical into separate small, labeled test
tubes.
Procedure (cont’d):
6. Using the 10 L syringe, draw 1 L of the chemical into the syringe.
7. Start collecting data on the PeakSimple program. Wait ~10 seconds, then
carefully insert the syringe into the injection port and inject the chemical. Leave
the syringe in the injection port for ~10 seconds. Remove the syringe and rinse it
three times with acetone.
8. Allow the GC to run the sample and PeakSimple to collect the data. Save and
print the chromatogram.
9. Repeat steps 6 – 8 for the rest of the known chemicals and the unknown mixture.
Cleanup:
 Rinse the syringe three times with acetone to remove all organics.
 Shut down the computer. This will cause the GC to return to factory settings.
 Turn the TCD detector switch to off.
 Turn off the GC. Close the carrier gas valve.
 Place any remaining organics in the Organic Waste container.
Analysis:
 Match the peaks from your unknown mixture chromatogram with the
chromatograms of the known standards to identify the components in your
unknown mixture.
 Ask your instructor for the true identities of the components of your unknown
mixture.
Error:
 Discuss sources of error in your experiment.