Question

What are the surface functional groups on the stationary phase in normal phase LC and reverse phase LC. Write out the eluotropic series for both normal phase LC and reverse phase LC using the common solvents we discussed in class (acetonitrile, MeOH, diethylether, H₂O, pentane, toluene).  Why acetonitrile is a stronger eluting solvent than toluene in normal phase LC?

Answer 1

Stationary phase (adsorbent) is the substance which is fixed in chromatography within the column. It is solid and immobilized or coated on glass plate or any other support. It is used in separation in chromatography.

Type of substance or functional groups present in the substance characterizes the chromatography type.

Normal phase chromatography uses silica gel (commonly) as a stationary phase. It has OH or hydroxyl functional groups.thus stationary phase is polar here.

Reverse phase chromatography uses C13 silica gel oroctadecylsilane bonded phase or silica gel bonded with aliphatic chain, all these are giving non polar stationary phase

Elutropic series gives the listing of various compounds/solvent in order of eluting power for a given stationary phase.. Eluting power means how strongly solvent can pull off analyte from stationary phase

Normal phase adsorbent is polar, so non polar will have least elution power

Series as per elution power is Pentane<Toluene <Diethylether <Acetonitrile<Methanol< water

Reverse phase will have opposite series of elution asa adsorbent is non polar there.

Water<Methanol<Acetonitrile<diethylether<Toluene<pentane

Normal phase is polar hence polar solvent will have stronger elution power.

Since Acetonitrile is more polar than Toluene, hence stronger Eluting solvent