Saturday
September 20, 2014

Homework Help: Difference?

Posted by Sheryl on Saturday, October 14, 2006 at 10:07pm.

What is the difference between these two questions? They look the same to me, like they're asking the same thing. Not likely.

Question: A student spots an unknown sample on a TLC plate and develops it in dichloromethane solvent. Only one spot, for which the Rf value is 0.95, is observed. Does this indicate that the unknown material is a pure compound? What can be done to verify the purity of the sample using TLC?

Question: You and another student were each given an unknown compound. Both samples contained colorless material. You each used the same brand of commercially prepared TLC plate and developed the plates using the same solvent. Each of you obtained a single spot of Rf = 0.75. Were the two samples necessarily the same substances? How could you prove unambiguously that they were identical using TLC.

Thanks for your input.

Sheryl


Think I found the answer. The difference is side by side differences:

Now it is possible to measure the distances that the solvent and the individual components traveled from the baseline. We need to be able to report the results in a manner which allows comparison from one student to another and from one day to another. Since not every person will have the same length plate or will allow the solvent to travel the same distance, direct side by side comparison of plates is meaningless. We will standardize our results by calculating the retention factor (Rf). This is a ratio of the distance that a component has traveled from the baseline divided by the distance that the mobile phase has traveled from the baseline. The Rf is characteristic of a compound under a certain set of conditions the same compound run on the same type of plate using the same eluting solvent should have the same Rf. However, even when performing a TLC under apparently identical circumstances, slight errors are involved. Moisture in the air, varying thickness of silica gel, amount of sample loaded on the plate, and other factors can all lead to variations. Therefore the acceptable experimental error on an Rf value is + 0.05. The best way to compare samples is to run them all on the same plate at the same time.


You have answered parts of your two questions. First, the two questions are not the same. The first one, even though only one spot appears, it isn't necessarily a pure one component material. You should use another solvent and see if there is more than one spot on the developed plate.

The second one, as you point out in your other post, does not necessarily mean that the two samples are identical. There are so many uncontrolled factors that the best way to check this one is to redevelop the two samples but put both on the same plate, side by side. Another way to do it is to double spot it;i.e., put the first spot on, let the solvent evaporate, then add the second spot over the first one, let the solvent evaporate and develop the plate. If only one spot appears on the developed plate you can be essentially certain that the two materials are the same compound.

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