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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 $R_\mathrm{f} = 1- \frac ba$?

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    $\begingroup$ I always learned Rf as being Retention Factor, not Retardation Factor. Your question still stands though... $\endgroup$
    – Dennis Cao
    Jun 9 '18 at 20:48
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    $\begingroup$ They're often used interchangeably in TLC, but IUPAC uses Retardation factor (Rf) for planar chromatography , and Retention factor ( k ) for column chromatography. $\endgroup$
    – Ratharax
    Jun 9 '18 at 21:01
  • $\begingroup$ 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. $\endgroup$
    – Dr. J.
    Jun 10 '18 at 11:41
  • $\begingroup$ See also retention factor $k$ and retardation factor $R_\mathrm F$. $\endgroup$
    – user7951
    Jun 11 '18 at 13:10
  • $\begingroup$ Ah yes, it is inverted, thanks Dr.J. I'll fix that. $\endgroup$
    – Ratharax
    Jun 14 '18 at 0:40
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The clearest answer is that an analyte is retained by both the mobile and the stationary phase and the retention factor is a measure of the ratio between retention in the mobile phase and in the stationary phase. So you could think of it as a higher Rf value indicates a higher retention in the mobile phase.

(Also, in answer to some of the comments, the retardation factor is the inverse of the retention factor)

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  • $\begingroup$ That makes sense to me, but I thought that "retention factor" was reserved for discussing column chromatography? $\endgroup$
    – Ratharax
    Jun 14 '18 at 0:46
  • $\begingroup$ I'm not sure, I haven't come across that as a limitation, it may be a convention but I don't think it's a hard rule; I have definitely seen the retardation factor used in reference to TLC, for example, but very rarely. $\endgroup$
    – aml
    Jun 21 '18 at 15:41

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