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.
Movie on how to run a GC. On GoogleVideo - choose "smoothing" and "original size" from the lower right pull-down menu for best video.
The process of gas chromatography is carried out in a specially designed instrument. A very small amount of liquid mixture is injected into the instrument and is volatilized in a hot injection chamber. Then, it is swept by a stream of inert 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 printing a peak on a piece of paper.
The two brands of GCs used in the organic chemistry teaching labs are shown below: Gow-Mac series 350 on the left, Varian Aerograph Model 920 on the right. Click on each photo for a detailed enlargement.
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The GC consists of an injection block, a column, and a detector. An inert gas flows through the system. The injection chamber is a heated cavity which serves to volatilize the compounds. The sample is injected by syringe into this chamber through a port which is covered by a rubber septum. Once inside, the sample becomes vaporized and is carried out of the chamber and onto the column by the carrier gas.
The large photo below is of the inside of a GC, showing the column in the oven and the insulated chamber tht houses the detector. Click on the thumbnails to see larger photos of the column and detector, as well as the inside of the injector port (showing the septum).
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inside of the injector port | |||||
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the septum | |||||
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the column | |||||
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the detector inside the housing | |||||
| On the Varian 920 and Gow Mac 350 chromatographs, detection of the compounds is achieved with a thermal conductivity (TC or hot wire) detector. | ||||||
The column (see the photo above) is an integral part of the GC system. On the outside, all you see is a long stainless steel tube, 1/8 to 1/4 inch in diameter and 4-5 feet long, which is coiled to fit inside the instrument. Inside the column is the important component: the stationary phase composed of the high-boiling liquid. The liquid is usually impregnated on a high surface area solid support like diatomaceous earth, crushed firebrick, or alumina. The liquid can be applied in various concentrations: the more liquid, the more sites it has to interact with the compounds.
All of our GCs have columns which are five feet long and 1/8" or 1/4" in diameter and contain a methyl silicone polymer liquid phase (OV-101, 1.5%) on a diatomaceous earth support (chromosorb G). Methyl silicone is a liquid phase of intermediate polarity, and non-polar compounds such will separate according to their respective boiling points.
The carrier gas is an inert gas, helium. The flow rate of the gas influences how fast a compound will travel through the column; the faster the flowrate, the lower the retention time. Generally, the flow rate is held constant throughout a run. (The GCs at CU Boulder are set at a flow rate of 55 mL/min.)
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| This is where the carrier gas enters the Varian GCs and where the gas flow rate can be adjusted. Click on the photo above for details. | ||||
In a professional laboratory, the GC conditions would be critical for another experimenter trying to duplicate your observations. All of our GCs have the same columns (1.5% OV-101 on Chromasorb G) and the same flow rate (55 mL/minute) and detector bridge current (150 mAmps). Each instrument will have a different setting for:
It is a good practice to write down some of the settings on the instrument. The values for these temperaturs are viewed by turning the knob on the instrument below the gauge -- click on the thumbnails below to see detailed photos of how to do this.
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reading temperatures on the Gow-Mac | ||
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reading temperatures on the Varian |
Recorders
Two devices are used to record the GC traces/areas under peaks:
Each type of device records the messages sent to them by the detector as peaks, calculates the retention time, and calculates the area under each peak; all of this information is included in the printout. 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.
An integrating recorder is pictured below. Click on the photo for a detailed picture and the location of the start button (press when you inject), the stop button (press when you have seen your peaks, it tells the recorder to do the calculations and to print), and the enter button (paper feed).
The screen of one of the computers is pictured below. Click on this image to link to screen shots of how to start, stop, calculate, and print a GC trace using the computer program.
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.
