Rf Values
Guiedo the Magnificant (ottende@PLU.edu)
Fri, 27 Sep 1996 19:11:23 -0800 (PST)
An Rf value is a ratio of how far the solvent has travled vs. how
far the sample has traveled on the TLC. To do this first measure the
distance from the sample origin (your penciled line) to the midpoint of
the 'mass' of the dot. What I mean by mass, is you don't measure it in
the middle necessarily. Try to get the same surface area (or amount)
above your measuring point as below. If your dot is a nice circle, it
will be the middle, but in the case of mine (which was shaped more like a
comet) I had to measure a little closer to the top of the spot.
Next you measure the distance that the solvent travled FROM THE
PENCILED LINE. (If you measure from the bottom of the paper, then you
are giving the solvent a head start to get farther, no fair.)
Take these measurements for EACH sample dot. If your sample has
several dots traveling different distances (like the reference sample,
which has more than one compound in it) measure EACH of these dot
distances also. (This means that the reference will have a few Rf values
in the end, it's ok.)
Divide the distance that the dot traveled by the distance the
solvent traveled to obtain your Rf value (Rf stands for retardation
factor [how slow, or retarded, the dot is compared to the solvent
front). Here is the formula for you math minded people:
Distance dot traveled
Rf = ----------------------------
Distance solvent traveled
Now for those of you whose solvent traveled at the same speed all of the
way across the TLC paper, your Distance Solvent Traveled will be the same
or close to the same for each Rf value (each dot). However, for those of
you whose solvent traveled at different speeds (making the solvent front
look more like a mountain range than a straight line) you will have
different values for Distance Solvent Traveled for each sample (samples
with multiple dots, like the reference, will have the same D.S.T value
for those dots because the solvent in that 'lane' going up traveled the
same distance.) That's fine becuase you are looking for a ratio. If the
solvent traveled at a slower or faster rate in a particular 'lane' then
the samples also traveled at a proportionally slower or faster rate. It
all works out when you do the division...