Gas chromatography stationary phases can be nonpolar, moderately polar, or polar
ID: 1002369 • 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. 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) When the stationary phase is polar, polar compounds would have greater affinity towards the polar stationery phase. So, hexanol (polar compound) would remain on the stationary phase for a longer time compared to the other two compounds. So, the order of elution : Heptane (elutes first-nonpolar)<Benzene(intermediate polarity)<Hexanol(elutes at the end - polar)
(b) On a non polar stationary phase like SE-30, non polar compounds would be present bound for a longer time compared to polar compounds. The order of elution will be 1. Hexanol follwed by 2. benzene followed by 3. Heptane
(c) In R-phase liquid chromatography, we have non polar stationary phase and polar mobile phase, Non polar compound remains with the stationary phase for a longer time and the polar compound is eluted out first. Here, hexanoic acid is more polar than hexanamine. Hence, hexanoic acid is eluted out first and hexanamine later.
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