Typesetting of the retardation factor (Rf)
According to Nomenclature for Chromatography (IUPAC Recommendations 1993) [1, pp. 843, 845] (also listed in IUPAC Gold Book), retardation factor is denoted as follows:


Column chromatography: $R$ (capital $R$ in italics):


  3.7.13 Retardation Factor ($R$)
  
  The fraction of the sample component in the mobile phase at equilibrium; it is related to the retention factor and other fundamental chromatography terms: 
  
  $$R = 1/(k + 1)$$

Planar chromatography: $R_\mathrm{F}$ (capital $R$ in italics and upright capital $\mathrm{F}$ in subscript):


  3.8.04 Retardation Factor ($R_\mathrm{F}$) 
  
  Ratio of the distance travelled by the center of the spot to the distance simultaneously travelled by the mobile phase. Using the symbols of Fig. 2: 
  
  $$R_\mathrm{F} = b/a$$
  
  By definition the $R_\mathrm{F}$ values are always less than unity. They are usually given to two decimal places. In order to simplify this presentation the $hR_\mathrm{F}$ Values may be used: they correspond to the $R_\mathrm{F}$ values multiplied by 100. 
  
  Ideally, the $R_\mathrm{F}$ values are identical to the $R$ values (see 3.7.13).
  
  



Reference


Ettre, L. S. Pure and Applied Chemistry 1993, 65 (4) DOI: 10.1351/pac199365040819.