Rf (Retention Factor)
Definition
Distance travelled by the analyte divided by the distance travelled by the solvent front, on a TLC or HPTLC plate. A unit-less number between 0 and 1, characteristic of an analyte in a defined solvent system. Used to compare an unknown spot with a reference standard.
- Field
- TLC and HPTLC
- Formula
- Distance travelled by analyte / distance by solvent front
- Range
- 0 to 1, unit-less
- Use
- Compares unknown spot to a reference standard
Common questions
Why must Rf be interpreted with caution when comparing runs on different plates?+
Rf is sensitive to plate activation, chamber saturation, temperature, and exact solvent composition, so laboratories run the questioned sample and reference standard side by side on the same plate rather than comparing Rf values obtained on separate occasions.
Is a matching Rf value sufficient to identify a compound?+
No, an Rf match narrows the possibilities but is not specific enough alone, so it is typically followed by a colour reaction, UV visualisation pattern, or a confirmatory technique such as mass spectrometry before an identification is reported.
What causes two different compounds to show the same Rf value?+
Compounds with similar polarity and comparable interaction with the stationary and mobile phases can migrate similar distances, producing coincidentally matching Rf values despite being chemically distinct substances.
Related terms
- Limit of Detection (LOD)
- The lowest concentration of analyte that produces a signal reliably distinguishable from the instrument noise, conventionally three times the standard deviation of...
- Mobile Phase
- The solvent or mixture that flows over the stationary phase and carries analytes along. Polarity, viscosity and volatility set the separation; switching...
- Resolution (Rs)
- Rs = 2(tR2 − tR1) / (W1 + W2). Above 1.5 means baseline separation; below 1.0 means peaks overlap and quantitation is...
- Retention Time (tR)
- Time from sample injection to peak maximum at the detector in HPLC or GC. Characteristic of an analyte under a defined column...
- Stationary Phase
- The immobile material the mixture flows past. In paper it is water inside cellulose fibres; in classical column work and TLC it...