# Rf value?#@$%^& 1. Nov 9, 2004 ### xLaser rf value!?#@$%^&

Hey guys,

Before doing a lab tomorrow, we need to find out what Rf values are, i can't seem to find a defintion about it online so some help would be great. Basically we are doin this BIO but the teacher wont tell us exactly about the lab but it is about chlorophylls and pigments.

thx alot.

m!ke

2. Nov 9, 2004

### UrbanXrisis

There is no such thing as a table of Rf values. Chromatographic retention times are sensitively and unpredictably dependent on the conditions; if you run chromatography on the same sample three consecutive times (using the "same conditions") you will get different retention values (Rf or whatever) for the same substance each time!
That's not to say that the values will be wildly different; in sensitive techniques such as gas chromatography you get almost the same values for the same substance on consecutive runs. But with TLC the only way to unambiguously identify something is to run an authentic sample on the same plate as the substance to be identified. Not "another plate, right afterwards." The same plate.

That's why you will never find a table of Rf values. Or of GC retention times, for that matter.

3. Nov 9, 2004

### UrbanXrisis

Oh, for the definition: The retention factor, or Rf, is defined as the distance traveled by the compound divided by the distance traveled by the solvent.

4. Nov 9, 2004

### xLaser

ah ok i c, the last part about the definition was all i needed, thx alot dude, i'll update u on what happens tomorrow. =)