# 2.3C: The Retention Factor

A convenient way for chemists to report the results of a TLC plate in lab notebooks is through a "retention factor",$$^2$$ or $$R_f$$ value, which quantitates a compound's movement (Equation \ref{2}).

$R_f = \dfrac{\text{distance traveled by the compound}}{\text{distance traveled by the solvent front}} \label{2}$

To measure how far a compound traveled, the distance is measured from the compound's original location (the baseline marked with pencil) to the compound's location after elution (the approximate middle of the spot, Figure 2.14a). Due to the approximate nature of this measurement, ruler values should be recorded only to the nearest millimeter. To measure how far the solvent traveled, the distance is measured from the baseline to the solvent front.

The solvent front (Figure 2.14b) is essential to this $$R_f$$ calculation. When removing a TLC plate from its chamber, the solvent front needs to be marked immediately with pencil, as the solvent will often evaporate rapidly.

The $$R_f$$ value is a ratio, and it represents the relative distance the spot traveled compared to the distance it could have traveled if it moved with the solvent front. An $$R_f$$ of 0.55 means the spot moved $$55\%$$ as far as the solvent front, or a little more than halfway.

Since an $$R_f$$ is essentially a percentage, it is not particularly important to let a TLC run to any particular height on the TLC plate. In Figure 2.15, a sample of acetophenone was eluted to different heights, and the $$R_f$$ was calculated in each case to be similar, although not identical. Slight variations in $$R_f$$ arise from error associated with ruler measurements, but also different quantities of adsorbed water on the TLC plates that alter the properties of the adsorbent. $$R_f$$ values should always be regarded as approximate.

Although in theory a TLC can be run to any height, it's customary to let the solvent run approximately $$0.5 \: \text{cm}$$ from the top of the plate to minimize error in the $$R_f$$ calculations, and to achieve the best separation of mixtures. A TLC plate should not be allowed to run completely to the top of the plate as it may affect the results. However, if using a saturated, sealed TLC chamber, the $$R_f$$ can still be calculated.

$$^2$$Sometimes the $$R_f$$ is called the retardation factor, as it is a measurement of how the movement of the spots is slowed, or retarded.