4) Unlabeled GC columns

Problem:

You are working in a laboratory. In a drawer, you find 2 old GC columns that have no labels. In order to find out the characteristics of the stationary phase, you inject standards of the following substances on both columns: o-xylene, benzaldehyde, and phenol. At 100°C (isotherm), you obtain the following retention times:

4a) Based on these findings, what can you say about the properties of the columns?

4b) You need to separate benzene from the three substances o-xylene, benzaldehyde, and phenol. Based on the above findings, would you expect that it is possible with one of the above columns? If yes, which one?

4c) You need to separate anisole from the three substances o-xylene, benzaldehyde, and phenol. Based on the above findings, would you expect that it is possible with one of the above columns? If yes, which one?

Help:

4a) You need to know the chemical structure of the given compounds. If you do not know them, look them up: www.chembiofinder.cambridgesoft.com.

4b) Think about the interactions that these compounds can undergo (see Chapters 3 and/or 4 in the script).

Answer:

4a) On column 1 the retention time appears to correlate with the size of the analytes. Thus, only van der Waals interactions and cavity energy appear to be important. The H-bonding ability of phenol does not result in an increased retention. The column therefore must be apolar. On column 2 the relative retention of benzaldehyde (a strong H-bond acceptor) is slightly increased over the one of xylene (a weak H-bond acceptor) while the retention of phenol (a H-bond donor and acceptor) is much stronger than the other compounds. Column 2 therefore must be a good H-bond acceptor. It might also be a weak H-bond donor but the slightly higher retention of benzaldehyde compared to xylene on column 2 as compared to column 1 can also result from differences in van der Waals interactions.

4b) Benzene is smaller than the other compounds (i.e. it has weaker van der Waals interactions) and should therefore be retained less strongly than the other compounds on column 1. However, the difference in retention of benzene and similarly sized phenol might be rather small. Separation on column 2, on which phenol is additionally retained by H-bond interactions, will certainly be the better choice.

4c) Anisole is similar to benzaldehyde in size and in the ability to accept H-Bonds. Thus, separation of anisole from benzaldehyde on both columns might be problematic.

Note: The answers given above are based on the very rough estimation methods for partitioning from Chapter 3 (see script). These methods only allow you to get a rough idea of wether compounds will elute close together or not. In gas chromatography, even compounds with very similar retention behaviour may still be completely separated. Therefore, compounds with similar estimated retention times may still be separable on a GC column.