Gas chromatography stationary phases can be nonpolar, moderately polar, or polar
ID: 879401 • Letter: G
Question
Gas chromatography stationary phases can be nonpolar, moderately polar, or polar, with many in between phases. (a) An example ofa polar phase is carbowax (polydiethylene glycol). You wish to separate the following substances most polar to least polar)hexanol. benzene, and heptane. Assuming equal volatilities, what order of elution would you expect? Why? (b) 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 c) Common stationary phases for liquid chromatography are R-phase ODS (nonpolar) and N phase silica (polar). N-phase mobile phase is nonpolar and ase mobile phase is polar Hexamine (CH (CHz)4CH2NH2) and hexanoic acid (CH3(CH2.cooH) are subjected to R-phase chromatography at pH 11 using acetonitrile: water 60:40 as mobile phase (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
Answer:
a) if the stationaty phase is polar then there will be dipole- dipole interactions in between the molecules and stationay phase.
Among the given compounds most polar is hexanol, then benzene then heptane. More the polarity of the compound is , stronger will be the interaction with stationary phase and less ease of eluting out.
Therefore hexanol being most polar binds most strongly and elutes out at the end.
The one that will elute first is heptane followed by benzene and lastly hexanol.
b) If we use non polar stationary phase the order of elution gets reversed hexanol elutes first followed by benzene and then heptane.
c) at pH 11 (basic pH) hexanoic acid will be in ionic form and hexamine will be in it's normal state. In such case hexanoic acid is more polar than hexamine. Hence in non polar R-phase with polar mobile phase hexanoic acid comes first followed by hexamine.
at pH 3 (acidic pH) hexamine will get protonated and is in ionic form. that means more polar than hexanoic acid which is in normal state at the pH.
Therefore in nonpolar R-phase with polar mobile phase at pH 3, hexamine being ionic and most polar elutes first followed by hexanoic acid.
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