Gas Chromatography Technique Quiz
1) Study the chromatograph (below) of a mixture of Compounds A and B, run on the GCs in the teaching labs at CU Boulder. Compound A has the shorter retention time.
- What is the retention time of compound A? Compound B?
- Which compound is present in a larger amount?
- Which compound has the lower boiling point?
- What would happen to the retention times of compounds A and B if the column temperature were raised?
- You suspect that compound B is octane. What can you do to provide supporting evidence for this hypothesis?
Show answer
- The retention time of compound A is 1.11. this number is read both at the top of the peak and in the RT column. The retention time of compound B is 2.27.
- Compound A is present is a larger amount, since it makes up 56% of the mixture.
- Compound A has the lower boiling point since it comes off the column first.
- If the column temperature were raised, both compounds A and B would boil sooner and thus come off the column in a shorter time. Thus, the retention times of both compounds A and B would be smaller than before the temperature was raised.
- To support your hypothesis that the peak with a retention time of 2.27 is octane, first obtain a true sample of pure octane to use as a standard. Inject this standard octane on the same GC and the same volume of injection (allowing for the fact that the mixture is about 67% octane). If the standard shows a peak at a retention time of 2.27, you can be pretty certain that compound B is octane. If the standard octane shows a peak with a retention time of as low as 2.0 or as high as 2.5, it could still mean that compound B is octane, since the GCs vary in retention time measurement from run to run. You should try running an internal standard. Take a milliliter of the mixture and add a milliliter of octane to it. Run a GC of this mixture + standard octane; if the slower moving peak (RT might vary from 2.0 - 2.5) increases in size and percent composition, it is quite likely that compound B is indeed octane. (Since other compounds might also run with the same RT, it does not absolutely identify the compound.)
2) Consider the following compounds:
| Compound |
Boiling point (°C) |
| Methyl cyclohexane |
101 |
| Pentane |
36 |
| Octane |
126 |
| 2,3-Dimethyl octane |
165 |
| Heptane |
98 |
If the compounds in the table above are run on an OV-101 GC column, which compound will have the lowest RT? Highest?
Show answer
OV-101 GC columns are the ones used in the Gow-Mac columns; they are non-polar and separate essentially on boiling point, especially if the compounds are similar in structure. All of the compounds in the table are hydrocarbons, hence of similar structure. The one with the lowest boiling point, pentane, will have the lowest RT. The compound with the largest boiling point, 2,3-dimethyl octane, will have the highest RT.
3) Consider the following compounds:
| Compound |
Boiling point (°C) |
| Propionic acid |
141 |
| 2-Hexanol |
140 |
| Isoamyl acetate |
142 |
| 3,4-Dimethylheptane |
140 |
If they are run on an DEGS GC column, which column will have the lowest RT? Highest?
Show answer
DEGS columns are much more polar than OV-101 columns. When using DEGS columns, the polarity of the samples will have an effect on the rate of their movement through the polar column: polar compounds will be retained longer than non-polar compounds, especially if the boiling points are similar. Of the compounds in the table (note that they all have similar boiling points), propionic acid is the most polar, and hence will be retained the longest and have the largest RT. 3,4-dimethylheptane is the least polar, and therefore will have the lowest RT.
4) You inject your sample into the GC and then wait several minutes, but you see no peak. What should you do?
Show answer
If you do not see a peak when you inject a sample into the GC, probably you did not get any sample into the syringe. Start over, filling the syringe carefully making sure the tip is in the liquid when you draw it up. Look at the filled syringe from different angles to make sure that you do have sample in it. The syringes often become plugged with a small piece of the rubber septum in the injection port, and will no longer take up sample. If you suspect this is the case, get a different syringe and give the plugged one to your TA or the Lab Coordinator.
5) State the stationary and moving phases in each type of chromatography below:
- Thin layer
- Column
- Gas
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- In thin layer chromatography, the stationary phase is the adsorbent on the plate; the moving phase is the eluting solvent which moves up the plate.
- In column chromatography, the stationary phase is the adsorbent which fills the column; themoving phase is the eluting solvent which goes down through the column.
- In gas chromatography, the stationary phase is the high boiling liquid which is contained in the steel or glass coiled column; the moving phase is the carrier gas which flows through the column packing.
6) Rank the following compounds in terms of their expected retention times on GC (OV-101 column) and their Rf values on TLC.
Show answer
In GC, on a non-polar column, they are likely to separate by boiling point. Thus the first off (and lowest RT) would be methyl acetate, then methylcyclohexane, then hydroxymethylcyclohexanol and finally 2-hydroxymethylcyclohexanol. The strong polarity of methyl acetate might cause it to move slower than the methyl cyclohexane, so standards would need to be run.
On TLC, the most polar compound would move the slowest (and have the lowest Rf ), and the least polar the fastest. The most polar is 2-hydroxymethyl cyclohexanol, then hydroxymethylcyclohexanol, then methyl acetate, and finally methylcyclohexane.
(On a very polar GC column, the 4 components would probably be in the same order as given for the TLC.)
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