Purpose of the Experiment
In this laboratory you will perform a quantitative analysis of a liquid mixture in order to determine its percent composition. This is possible by using gas chromatography (GC), an analytical chemistry technique common in the organic chemist's lab. The method for using the instrument and interpreting the data is independent of the identity of the sample. Consequently, we will simply refer to our mixture as a solution of some compound A mixed with some other compound B.

In GC a small sample of the liquid mixture is vaporized and pushed into a long metal column which is coated with a liquid. The vapor will dissolve in the liquid and later evaporate moving it further down the column. The polarity of the molecules will cause A and B to have different affinities for the liquid in the column. Thus, A may stay dissolved for a long time, while B may evaporate quickly. This will cause B to be pushed out the other end of the column before A; in other words, we have separated the two compounds.

A GC can be used for two main purposes. The most common use is to analyze the mixture to determine how much of the mixture is A and how much is B. This analytical method of GC uses a chart recorder to measure a "peak" associated with each of the molecules. The area beneath the peak is proportional to the amount of the compound. For example, if a mixture of two compounds gives two peaks with integrated areas of 100 units and 400 units, we can proceed as follows:

Total area =	100 units + 400 units = 500 units
First compound percentage =	100 units ÷ total units = 20%
Second compound percentage =	400 units ÷ total units = 80%

Next we need to know which peak is associated with which molecule. This can be determined easily if one knows in advance what the compounds are. For example, if we know that the sample consists of molecules A and B, we can "spike" the original sample by adding some A to the mixture and repeating the GC to see which peak gets larger. The peak that gets larger comes from A. In this way one can continue until all of the peaks have been identitified.

A second use for GC is to actually purify a sample. Since analytical techniques use very small quantities of sample a modified GC which can handle large samples must be used. This method, call preparative GC, is outside the scope of this laboratory.

General Procedure

1. Make sure that the GC is warmed up and helium is flowing. Write down the the inlet and column temperature settings, and the attenuator setting. Also record the column temperature.
2. Clean the syringe by drawing 3-6 µL of sample into the barrel and emptying onto a paper towel or into a small test tube. Repeat two more times to insure that any residue in the syringe comes from the sample to be analyzed.
3. Draw about 1-2 µL air, 1 µL sample, and another 1-2 µL air into a syringe.
4. Position the syringe needle against the rubber septum at the injection port
   

   CAUTION! Injection port is very hot!

5. In one quick yet gentle motion, push the needle through the septum, empty the syringe and remove the needle.
6. Immediately press the START button on the integrator.
7. Wait until all peaks have been recorded, then press the STOP button on the integrator.
8. If any peak is off-scale on the integrator (evidenced by a peak which is flat at the top), repeat the experiment using either less sample in the syringe or higher attenuator setting on the GC.
9. Once a chromatogram is obtained with reasonably sized peaks, use the data provided by the integrator to calculate percentage composition. Note that the first peak is from the air in the syringe and should not be included in any of your calculations. When reading the data, RT stand for "retention time" which simply cross-references each peak to its area. The AREA numbers are the integrated area beneath each peak. Write down the areas for the sample peaks. Add them together to obtain the total area. Divide each peak's area by this total to calculate percent composition.

   Note: The printed values for TOTAL AREA and AREA% reported by the integrator are not correct since the integrator does not disregard the air peak at the beginning^*.

10. When finished with all samples, the paper can be ejected from the printer by holding the CTRL button and pressing L on the keyboard.