Joseph K. Myers
2-24-04
The gas chromatograph will be used to identify components in a mixture. The response graph of a pure compound can be matched to the response peaks in a graph of a mixture to determine the components.
In this experiment, this theory can be applied in many ways, depending for the most part on the facilities available. One way is by making a graph of the mixture, and also making a graph of a combination of the mixture with pure compound. Those portions of the graph which increase in magnitude correspond to the pure compound. Those portions on the original graph represent that compound within the mixture.
Principally, gas chromatography is analogous to other chromatographic methods, but exchange takes place between a gas phase and a solid or liquid phase. The separating column is made up with either a solid adsorbent or with a carrier impregnated with an involatile liquid. By changing the polarity of the stationary phase, different degrees of chromotographic separation can be achieved.
The mixture to be analyzed is volatilized in the presence of some carrier gas, often helium, nitrogen, or such, to provide transport through the column. Depending upon the compatibility of the differing substances to the stationary phase, they are to a lesser or greater degree retained by it and released again. Finally, these substances leave the column.
The separating efficiency is equivalent to that of a column having 5000 theoretical plates per meter, in ideal cases. Some columns have even made it possible to achieve complete separation of enantiomers.
There is great usefulness in gas chromatography for monitoring the continuous production process of chemicals. In some cases, depending on the characteristics of the stationary phase, an idea can be formed as to the constitution of unknown volatile substances.
See [1].
None.
| Compound | MW | mp(C) | bp | fp | d(g/cm3) |
|---|---|---|---|---|---|
| toluene | 92.139 | -94.95 | 110.63 | ? | 0.8668 |
| cyclohexane | 84.159 | 6.59 | 80.73 | ? | 0.7739 |
A mixture is formed from the products of former fractional distillation. See [3]. The sample is prepared of varying amounts of the separated fractions (i.e., 1 and 4). A variant is to obtain ingredients from acceptable ones made available by the instructor.
The injection of the sample is made with the proper gas-tight syringe.
The sample size must be 1-5 microliters.
From the GLC equipment, a gas chromatogram will be obtained.
In the same way, and with the same instrumental conditions, obtain a chromatogram of pure samples of the reagents that were used.
Analyze the sample:
Identify each component in the mixture by comparing the rention time for each component with the retention times of samples of pure compounds.
Verify this conclusion by preparing a series of new sample mixtures, each of which will contain one volume of the pure known compound with two volumes of the original mixture. Upon preparation of a gas chromatogram of this new mixture, that peak of the mixture which has been amplified is corresponds to the peak of the pure compound that has been added.
There is nothing observable inside of the machine.
The observations of output are given in the results.
| fraction | cyclohexane peak (position and approximate percentage) | toluene peak | other peak |
|---|---|---|---|
| 1 | 20.6 units, 9% | none | acetone: 6.7, 91 |
| 2 | 19.3, 95 | 33.2, 5 | none |
| 3 | 19.6, 88 | 33.1, 12 | none |
| 4 | 20.2, 35 | 32.5, 64 | acetone: 7.7, 1 |
This process was used to analyze the four fractions from the preceding experiment.
The first fraction was very small, and the initial analysis for it as well as the second fraction had to be rejected.
After this, the gas chromatograms were successful.
The first fraction produced its acetone peak (amplitude 60 units) at 6.7 units past the air mark. A small cyclohexane peak (amplitude 5 units) was produced at 20.6 units.
The second fraction produced a very strong peak (more than 100 units) of cyclohexane at 19.3 units. A small peak of toluene (7 units) was visible at 33.2 units. The third fraction made very little difference: cyclohexane (~100) at 19.6, toluene (22) at 33.1. The fourth fraction was a burst of acetone (1) at 7.7, cyclohexane (63) at 20.2, and toluene (64) at 32.5.
None of the fractions produced a strongly-weighed toluene sample.
1. Beyer H, Walter W. Organic Chemistry. 1997. Translated 22nd Edition. p3.
2. Gilbert JC, Martin SF. Experimental Organic Chemistry. 2002. 3rd Edition. p9.
3. Myers JK. Simple and Fractional Distillation. 2004. Second Edition. Detail.