Gas chromatography stationary phases can be nonpolar, moderately polar, or polar
ID: 1002382 • Letter: G
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, c) 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
Substances to be separated : Hexanol, Benzene, Heptane.
Increasing order of polarity for these substances is Heptane < Benzene < Hexanol.
Like dissolves like is the basic principle. It means polar solvent dissolves polar substance and non-polar one to non-polar substances.
As solvation of substance increases it’s elution decreases i.e. more time requires for elution. Dissolution in turn determines elution. As dissolution i.e. solvent – solute association increases more time it takes to elute.
Hence we can say that,
i) From polar stationary phase non-polar substance elutes first and polar one at the last.
ii) From non-polar stationary polar substance will elute first and non-polar at the last.
Based on this let us tackle with question:
a) Carbowax (Polydiethylene glycol) – a polar stationary phase:
Hence most non-polar will elute first and polar at the last.
I.e. Heptane elute first.
The order of elution will be Heptane > Benzene > Hexanol.
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b) SE:30 Methyl silicon gum – Non polar stationary phase:
Hence most polar will elute first.
Elution order is : Hexanol > Benzene > Heptane.
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c) At pH 11 Hexanoic acid (pKa = 4.88) will exist in ionized i.e. charged form (–vely charged form) and hexamine (pKa > 10) will be in unprotonated form i.e. neutral foms.
Charged species are relatively more polar that uncharged neutral species.
Hence at pH = 11, Hexanoic acid is more polar than Hexamine.
In R-Phase chromatography stationary phase is non-polar and owing to above explanation we can say that polar substance will elute first.
At pH = 11 Hexanoic acid being more polar will elute first from R-Phase chromatogram.
d) At pH 3 now Hexanoic acid will be in unionized i.e. neutral form and hexamine will exist in it’s protonated form (R-NH3+) and hence now hexamine being more polar will elute prior to hexanoic acid.
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