Question

Explain why we should or should not calculate an R_f for a given chromatography run. What is the point of the R_f? When does it make no sense to use one?

Answer 1

In chromatography usually we separate components of sample on thier solubility in particular solvent and how much it travels in along the chromatography paper

To make this technique more scientific we use Retention Value,(Rf) . Every compound has a unique Rf value for a particular solvent at particular solvent concentration.

It is defined as the ratio of distance traveled by the solute to the distance travelled by the solvent(called as solvent front) from the basline(its a point on the chromatography paper where the sample is initially spotted ).

As we know that solvent front always more than the distance tarveled by the solute the Rf value has a range of 0-1.

It makes sense to use the Rf value when we compare the position of bands in the sample to the position of the standard , in order to make guess whether the sample is same as standard.

When the Rf value is high it means the substance is non polar and has moved appreciable diatbced in comparison to solvent whereas it is polar if it has moved less distance as cimpacom to solvent.

It does not make any sense to use retention value in High performance liquid chromatography ( HPLC ) or in Gas chromatography (GC) since the elution cannot be visually examined. So with the help of suitable detector elutions are monitored w.r.t to the Elution Time.(ET)