Rf values and chromatogram interpretation

A chromatogram shows the substances in a sample that have separated as the solvent moved. Each separated substance appears as a spot. The number and positions of the spots provide information about the sample.

Calculating an Rf value

The Rf value compares how far a substance travelled with how far the solvent travelled. Both distances are measured from the origin, the line where the sample was placed.

Measure the substance’s distance from the origin to the centre of its spot. Measure the solvent’s distance from the origin to the solvent front, the furthest point reached by the solvent. Use the same units for both measurements; the Rf value has no units.

For example, if a spot moves $$3.2,\text{cm}$$ and the solvent front moves $$8.0,\text{cm}$$:

The measurements are given to two significant figures, so the result is reported as $$0.40$$, also to two significant figures. Measurements should reflect the precision of the measuring scale, and the final answer should be given to an appropriate number of significant figures.

Using Rf values to identify substances

Different compounds can have different Rf values in a given solvent. A measured Rf value can therefore be compared with known values to help identify a substance. The comparison must use the same solvent, because the same compound can have a different Rf value in a different solvent. A matching Rf value supports an identification but does not prove it: different substances may have similar values.

Interpreting the number of spots

A pure substance produces one spot in a solvent system. A mixture or impure sample can produce more than one spot when its substances separate. More than one spot therefore shows that the sample contains more than one substance.

A single spot alone does not prove that a sample is pure: two substances may not have separated in that solvent system.

Paper chromatography

Paper chromatography separates the substances in a mixture by allowing them to travel different distances through paper.

A small spot of the mixture is placed near the bottom of a strip of paper. The bottom of the paper is placed in a solvent, with the sample spot above the solvent level. The solvent moves up through the paper and carries substances from the sample with it.

Why substances separate

The paper stays in place, so it is called the stationary phase. The solvent moves through the paper, so it is called the mobile phase.

Each substance distributes differently between the solvent and the paper. A substance that spends more time in the solvent is carried further up the paper. A substance held more strongly by the paper spends less time moving with the solvent, so it travels a shorter distance.

As a result, the components of the mixture can form separate spots at different positions on the paper. These spots allow the mixture to be analysed further.