Flask Logo Organic Chemistry at CU Boulder

Gas Chromatography

In gas chromatography (GC), the stationary phase is a high-boiling liquid and the mobile phase is an inert gas. In the organic chemistry teaching labs at CU Boulder, GC is used as an analytical tool to find out how many components are in a mixture. It can also be used to separate small amounts of material.

The process of gas chromatography is carried out in a specially designed instrument, called a gas chromatograph. A very small amount of liquid mixture is injected through a rubber septum into the instrument and is volatilized in a hot injection chamber. Then, it is swept by a stream of carrier gas through a heated column which contains the stationary, high-boiling liquid. As the mixture travels through this column, its components go back and forth at different rates between the gas phase and dissolution in the high-boiling liquid, and thus separate into pure components. Just before each compound exits the instrument, it passes through a detector. When the detector sees a compound, it sends an electronic message to the recorder, which responds by plotting a peak. For similar compounds, the area under a GC peak is roughly proportional to the amount of compound injected. If a two-component mixture gives relative areas of 75:25, you may conclude that the mixture contains approximately 75% of one component and 25% of the other.

Retention Time (RT)

The retention time, RT, is the time it takes for a compound to travel from the injection port to the detector; it is reported in minutes on our GCs. The retention time is measured by the recorder as the time between the moment you press start and the time the detector sees a peak. If you do not press start at the same time you inject your sample, the RT values will not be consistent from run to run.

Efficient separation of compounds in GC is dependent on the compounds traveling through the column at different rates. The rate at which a compound travels through a particular GC system depends on the factors listed below:

Generally the number one factor to consider in separation of compounds on the GCs in the teaching labs is the boiling points of the different components. Differences in polarity of the compounds is only important if you are separating a mixture of compounds which have widely different polarities. Column temperature, the polarity of the column, flow rate, and length of a column are constant in GC runs in the Organic Chemistry Teaching Labs. For each planned GC experiment, these factors have been optimized to separate your compounds and the instrument set up by the staff.

Descriptions and Use of Gas Chromatographs

Two types of gas chromatographs are available in the Organic Chemistry teaching labs, the Gow-Mac and the Vernier Mini GC. Explanations (including pictures) of how use each type of apparatus are linked on this website.

Gas Chromatography Movie

CU Organic Chemistry has produced a movie about gas chromatography. The chromatographs in this movie are not the same as the ones currently in use in the CU labs, but the general principles are still the same.

Gas Chromatography Technique Quiz

See how well you understand gas chromatography by taking the online Gas Chromatography Technique Quiz!