Question

The following data was collected from a TLC plate for 4 different compounds. The distance's traveled by each is shown below:

Spot A: 8.2 cm

Spot B: 0.7 cm

Spot C: 6.5 cm

Spot D: 1.3 cm

The spotting line was placed 1.5 cm above the base and the solvent traveled 10.5 cm from the bottom of the plate. Calculate the Rf value for each compound.

My answers to this were

A: 0.78 B: 0.067 C: 0.62 D: 0.12

I got this question wrong, can someone explain why?

Answer 1

TLC stands for Thin Layer Chromatography; it is an analytical technique in which we exploit each sample's affinity toward different polar/nonpolar solvents.

We use a stationary phase: a phase which does NOT moves, as the name implies. It is typically the "thin layer" or the "plate". It is commonly to use silica gel plates, which are polar, but any species could be used, such as paper.

We also use a mobile phase: as the name implies, this phase will "move" and will carry along the path all the components from the solvent. Many times, you will be able to split in several components the "mixture" of the sample. Therefore, they can be seen in the stationary phase (plate). Typically, we allow the mobile phase to go up to the "Rfinal" mark, or final reference. This is the distance travelled by the solvent (mobile phase). It starts at solvent level, it is R0, therefore.

Rf = distance traveled by sample / distance traveled by solvent

Then, "Rf" value allows us to understand the affinities of anaytes towards the solvent.

As Rf increases, this implies higher affinity toward the mobile phase ( since it is carried all along the mobile phase)

If Rf is low, this implies a low affinity toward the mobile phase; therefore higher affinity toward the stationary phase, since it sticks to the plate rather than travel upwards.

now...

Solvent Front = (10.5-1.5) = 9 cm

so

Rf-A = 8.2/9 = 0.911

Rf-B = 0.7/9 = 0.077

Rf-C = 6.5/9 = 0.722

Rf-D = 1.3/9 = 0.144

your probable error --> you are using 10.5 as a basis, not 10.5-1.5 M which is the actual solvent distance