Now that we have a way to witness the results of our separation, how do we make sense of them? Interpretation is accomplished through measurement of the retardation factor. This is a measure of the ability of an analyte to rise up a TLC plate. The retardation factor, or Rf value, is the ratio of the distance traveled by the analyte to the distance traveled by the solvent.
- Rf = distance traveled by
analyte / distance traveled by the solvent
If you recall the previous discussion of chromatograms, the Rf value is analogous to a retention time. It is characteristic of that particular sample for that particular solvent. Comparison of Rf values can provide evidence to help determine if two samples are identical or not. However, sometimes a particular solvent will produce similar Rf values for dissimilar compounds. This is yet another reason why one analysis of any type is never conclusive. A different solvent combination could very well yield varying results demonstrating that the two compounds are indeed different. Two TLC runs with two different solvent setups is beneficial in providing verification of your results.
Tips for taking Rf measurements