The retention factor rf of a particular material is the ratio of the distance the spot moved above the origin to the distance the solvent front moved above the origin. Retention factors are useful in comparing the results of one chromatogram to the results of another. If the conditions in which the chromatogram are run are unchanged the retention factor for a given material should remain constant. A solvent front traveled for 0.
Calculate the rf value. You had initially spotted two compounds. We'll call them A and compound B. And then what you saw on the plate was that your mobile phase had traveled up to about here, A had traveled to about here, and B had traveled this far. But what does that really mean? How can we even report these values? The way we'd report them if we were writing up a lab report or writing a manuscript, you'd need something known as the retardation factor, also known as the retention factor or RF for short.
RF is equal to the distance traveled by solute over the distance traveled by the solvent. So the first step you need to do is measure these distances for the different compounds and also for the solvent, also known as the mobile phase. So let's put a ruler next to our TLC plate, much like you would if you were sitting in lab. We'll say that this is 1 unit, 2 units, 3 units, and 4 units. So we can measure the distance that A has traveled, and that's from the starting line to the center of the spot.
That's two units. And for compound B, again from the starting line to the center of the spot, that's 3 units. And for the solvent, the starting line to this finish line, that is 4 units. So let's plug that into our equation. If we wanted to solve RF of A, you need the distance traveled by compound A over the distance traveled by the solvent, so let's say A over S. Here, that would be equal to 2 over 4, and the convention is to report these values as decimal points, so we'll say that this is 0.
Now, we'll do the same for compound B. RF of B is equal to distance traveled by B over distance traveled by S. In this case, that's equal to 3 over 4, or 0. So what can we tell about these two compounds? If we remember from talking about the mobile phase and stationary phase, compounds that travel really far must be more attracted to the mobile phase, and therefore are less polar. So we can say that compound B is less polar and travels faster.
These techniques involve passing the mixture from a mobile phase to a stationary phase. Each substance in the mixture does this at a specific rate causing it to become separated as it moves into the stationary phase. The retardation factor Rf is the relative amount of time that a particular substance spends in the mobile phase.
The first step is to refine the retardation factor mathematically. It is the ratio of the time that a substance spends in the stationary phase to the time that it spends in the mobile phase.
Retardation factor values are helpful in the identification in comparing unknown and known compounds. Compounds can have the same Rf value for a singular solvent but not likely for multiple solvents. Using more solvents helps to obtain more Rf values for more accurate identification.
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