Question

Normally benzoic acid is more strongly retained on the C18 column than is caffeine (the benzoic acid peak appears later than the caffeine peak). Explain why that is so in terms of elution chromatography theory. Hint: Caffeine is more soluble in water than benzoic acid.
please answer #1

Answer 1

According to Chemical Synergies, a mixture of two or more components can be segregated and this process is called Chromatography.  The hydrophobic part is the one which gets eluted first in case of ordinary liquid chromatography. As it is a mobile part which gets adhered with the non polar part. The ability of mobile part which get out of the column.

Elution Chromatography employs with the column, includes the migration of the solute. The Solute which gets emerged from the column is gets detected. In Contrast to the ordinary liquid Chromatography, alkyl non polar hydrophobic is the stationary phase, where as polar liquid is a mobile phase. C18 Column is a stationary phase which is a long alkyl non polar hydrophobic.

The first eluent will comes out which is more solubility in water. As per the Hint, Caffiene is more soluble in water than Benzoic Acid. This indicates Caffiene is the first eluent and it will comes out of the column first.

Benzoic Acid gets retained in the C18 Column. In case of Caffiene, the amine group present is basic. In highly acidic medium, the nitrogen atoms in basic amine groups gets protonated. Thus Caffiene is more polar than Benzoic Acid. The miscibility of Caffiene in water is more, thus it is more polar it comes out first, and the least polar Benzoic acid gets retained in the C18 Column.