Organic Chemistry, Separation & Analysis

Rf is the distance a spot has moved from the baseline divided by the distance the solvent front has moved from the baseline, $R_{\mathrm{f}}=\frac{\text{distance moved by spot}}{\text{distance moved by solvent front}}$; the denominator is the FRONT, never the length of the paper or a fixed scale.

Trigger: any chromatogram, a table of Rf values, or the words 'solvent front'.

Instances:
(i) read the front height first, then divide each spot height by it;
(ii) if the front sits at 10 grid units, each unit is $R_{\mathrm{f}}=0.1$, so counting squares gives Rf directly;
(iii) a spot on the baseline has $R_{\mathrm{f}}=0$ and one at the front has $R_{\mathrm{f}}=1$, so every Rf lies between 0 and 1.