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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 GC Instrument

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.

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).


		inside of the injector port
		the septum
		the column
		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.)


	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.


		reading temperatures on the Gow-Mac
		reading temperatures on the Varian

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.

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.

Factors which affect GC separations

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:

Volatility of compound: Low boiling (volatile) components will travel faster through the column than will high boiling components
Polarity of compounds: Polar compounds will move more slowly, especially if the column is polar.
Column temperature: Raising the column temperature speeds up all the compounds in a mixture.
Column packing polarity: Usually, all compounds will move slower on polar columns, but polar compounds will show a larger effect.
Flow rate of the gas through the column: Speeding up the carrier gas flow increases the speed with which all compounds move through the column.
Length of the column: The longer the column, the longer it will take all compounds to elute. Longer columns are employed to obtain better separation.

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.

Procedure for GC

Gas Chromatography: Procedure

(1) Add the sample to be injected to the syringe.

A 25µL glass Hamilton syringe is used to inject the GC samples. Only 2-4 µL of sample is injected onto the column, which means that you fill only a small part of the barrel with sample. Examine the syringe carefully before you fill it. The divisions are marked "5 - 10 - 15 - 20 - 25".

Place the tip of the needle in the liquid. Slowly draw up a small amount of liquid by raising the plunger, then press on the plunger to expel the liquid back into the liquid. This serves to “rinse” the syringe with your sample, ensuring that what you will measure in the GC run is the composition of your mixture. Repeat the rinse process one or two times. Then, draw up the plunger slowly again while the needle is in the liquid and carefully fill the syringe with liquid about halfway to the “5”.



		It is often hard to see the liquid in the syringe. If the syringe is clogged, the plunger will be in the correct position but the barrel of the syringe will be filled with only air, as in the bottom syringe in the photo to the left. The best thing to do is to carefully examing the syringe after you think that you have filled it. Hold it up to the light to get a better view. Small air bubbles in the syringe will not affect the GC run (middle syringe in the photo to the left). As long as there is enough liquid in the syringe, the GC run will work fine. If you keep getting bubbles, just pull the plunger up a bit past the "halfway to the 5" mark to compensate. If you have a VERY large air bubble, you will not have enough liquid to show a reading on the GC (e.g., the bottom syringe in the photo).

(2) Inject the sample into the injector port.

You are need to do two things sequentially and quickly, so make sure you know where the injection port is and where the start button on the recorder is.
Push the needle of the syringe through the injection port and immediately press the plunger to inject the sample, then immediately press the start button on the recorder.
You will feel a bit of resistance from the rubber septum in the injection port; this is to be expected and you should be prepared to apply some pressure to the syringe as you force the needle into the instrument all the way to the base of the needle.

(Click on each of the photos below for a larger view.)

Push the needle of the filled syringe through the injector (as far as it will go) and quickly push the plunger.

Remove the syringe immediately . . .

. . . and quickly press the start button on the integrating recorder or the start recording button on the computer (ask your TA which device is connected to the GC that you are using)

Here's a close-up of the integrating recorder.

Here's a series of pictures showing how to run the computer program.

(3) Sit back and wait.

(4) End the GC run.

When you have seen all of the peaks which you suspect are in the mixture, or when the recorder has shown a flat baseline for a few minutes or so, press stop on the recorder.
When you press stop, the recorder will print out the peaks, the retention times, and the areas under the peaks. When it is done printing, you can press “enter” a couple times to advance the paper.
Carefully tear the paper off the recorder. The paper is not perforated, so do not try to pull up and expect it to pop out of the recorder. Instead, pull it down to start a tear from one edge, and then continue the tear until the paper is cut and free.



		This may seem trivial -- showing you how to tear the paper. But too many times a student has tried to yank the paper out instead of starting a tear and tearing it neatly. Yanking the paper can result in the paper being torn below the plastic cutting surface on the recorder, and the paper gets jammed down inside the recorder. If this happens, the entire recorder has to be dis-assembled, a process which takes about 15 minutes, thus putting the entire GC out of service until it can be fixed.

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