Question

If you are separating fatty acid methyl esters, what would expect to be the order of elution? (i.e., which would elute from the column first, second, third, etc.?). Explain your reasoning.

a. Butyric acid methyl ester

b. Palmitic acid methyl ester

c. Stearic acid methyl ester

d. Oleic acid methyl ester

Answer 1

All are methyl esters only. In normal-phase chromatography, the stationary phase is more polar than the mobile phase. So as polar molecules are retained in the column, your elution of molecules will go from non-polar to polar.

higher the chain length of hydrocarbon chain length - higher will be non-polar, we know hydrocarbons are non-polar (hydrophobic groups), in other words higher the molecular weight - higher the non-polar nature - elutes first.

Butyric acid - 4-carbon chain length

Palmitic acid - 16-carbon chain length

Stearic acid - 18-carbon chain length

Oleic acid - 18-carbon chain length, but one double bond present (unsaturated fatty acid)

polarity order: saturated < unsaturated.

elution order: non-polar > polar, this means non-polar compounds does not have any interaction with stationary phase and elutes first and polar compounds will elute last.

so, the order of elution will be:

c. Stearic acid methyl ester elute from the column first

d. Oleic acid methyl ester elute from the column second

b. Palmitic acid methyl ester elute from the column third

a. Butyric acid methyl ester elute from the column fourth

Hope this helped you!

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