# Why is Rf called Retardation Factor?

I understand that $$R_\mathrm{f} = \frac{\text{distance traveled by center of analyte spot } (b)}{\text{ distance travelled by solvent front } (a)}$$ What I do not understand is why this is called retardation factor; an analyte that travels further in the mobile phase seems to me to be less retarded by the stationary phase, while one that doesn't travel as far seems more retarded. If we're using this terminology why doesn't $R_\mathrm{f} = 1- \frac ba$?

• I always learned Rf as being Retention Factor, not Retardation Factor. Your question still stands though... Jun 9 '18 at 20:48
• They're often used interchangeably in TLC, but IUPAC uses Retardation factor (Rf) for planar chromatography , and Retention factor ( k ) for column chromatography. Jun 9 '18 at 21:01
• You have the Rf inverted. It should be a/b or distance traveled by the spot divided by distance traveled by the solvent front. Nonetheless, a low value is more retarded or retained, so your question is still a valid one. It seems to be a matter of definition. Jun 10 '18 at 11:41
• – user7951
Jun 11 '18 at 13:10
• Ah yes, it is inverted, thanks Dr.J. I'll fix that. Jun 14 '18 at 0:40