Two students developed TLC plates using the same solvent system ...

[amal]

Two students developed TLC plates using the same solvent system and each saw a single, UV-active spot with an Rf = 0.42. At first they assumed that their two samples must be the same compound. To confirm this, they co-spotted their samples on one TLC plate. Upon developing the plate and looking at it under a UV lamp, they observed the results shown below. Explain why they now see two spots, even though the Rf values on their previous plates were the same.

Question 2

One spot with an Rf of 0.62 is observed after you develop the TLC of a reaction mixture. Does this prove that you have only one product present? Explain.

[efwsefaw]

Answer to Question 1

There are several sources of error in measuring Rf values. The most common error is in the marking of the solvent front. That is, if each student is not diligent about marking the front immediately upon removing the plate from the developing chamber, the Rf value will be variable from experiment to experiment. Other sources of error in reported Rf values include:

Was the center of the spot determined in the same way?
Was the developing solvent the same from run to run? That is, was there some selective evaporation from a mixed-solvent system due to negligence in keeping the TLC chamber covered?
Was too much of one of the compounds spotted?

Answer to Question 2

No, it does not prove that you have one product present. There could be another compound(s) that has the same Rf value. Another compound(s) could also be present with a different Rf value that was not visualized by any of the methods used to detect the compound with an Rf = 0.62. Another compound(s) could also be present in sufficiently low concentrations so that it is not easily detected.