Gas chromatography stationary phases can be nonpolar, moderately polar, or polar

Question

Gas chromatography stationary phases can be nonpolar, moderately polar, or polar, with many in between phases. (a) An example of a polar phase is carbowax (polydiethylene glycol). You wish to separate the following substances: hexanol, benzene, and heptane. Assuming equal volatilities, what order of elution would you expect? Why? SE-30 is a methyl silicon gum, which is very nonpolar. What would be the order of elution for these same compounds? For partial credit, explain your selections. Common stationary phases for liquid chromatography are R-phase ODS (nonpolar) and N phase silica (polar). N-phase mobile phase is nonpolar and R-phase mobile phase is polar. Hexamine (CH_3(CH_2)_4CH_2NH_2) and hexanoic acid (CH_3(CH_2)_4COOH) are subjected to R-phase chromatography at pH 11 using acetonitrile.water 60:40 (acetonitrile is less polar than water). Which will elute first? Why? If we changed the pH in part (c) to 3, which would elute first? Why?

Explanation / Answer

A) Polar stationary phase binds strongly with polar molecules effectively since the chemical interaction is strong. In case of Carbowax (polydiethylene glycol), it effectively binds with molecule containing groups capable of forming hydrogen bonds. Hexanol with 1 hydroxyl group in it binds with the Carbowax by H bonds. Thus on elution, Hexanol is eluted out at last whereas the non polar molecules which are loosely bound to stationary phase are eluted first.
Now comparing the order of polarity for benzene and heptane, heptane with the polarity index of 0, would elute out first and benzene with the vale of 2.7 after the former.

Hence the order of elution for the above gas chromatography is
Hexanol< Benzene < Heptane

B) Now in the case of SE-30 which is a very non polar solvent acting as the stationary phase, the order almost get reversed since the interaction would be strong in case of like molecules as far as polarity is concerned. The molecule with least polarity interacts with methyl silicon gum strongly whereas the one being polar would get eluted out first.

Hence the order of elution in this case is:
Heptane< Benzene < Hexanol

C) In the R phase chromatography, at pH = 11
Hexamine exist in its amine form but Hexanoic acid exist in the form of its conjugate base as Carboxylate ion due to the basic medium that extracts acidic proton from carboxyl group.
In this situation, since Hexanoic acid with a full fledged negative charge is more polar than Hexamine, the latter binds strongly with stationary phase and eluted out last.
Hence the order for elution is:
Hexanoic Acid> Hexamine

At pH=3,
the order get reversed since now in the acidic medium, hexamine exist in the form of its conjugate acid, i.e, ammonium ion form with full fledged positive charge and hence is more polar than hexanoic acid.
Hence the order of elution in the R phase chromatography is:
Hexamine > Hexanoic acid

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