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.
The key difference between the two questions lies in the scenarios being presented and the specific experimental conditions.
In the first question, a student observes only one spot with an Rf value of 0.95 after developing an unknown sample on a TLC plate using dichloromethane solvent. The student is questioning whether this indicates that the unknown material is a pure compound. To verify the purity of the sample using TLC, the student can perform additional tests. One option would be to use a different solvent system to check if any additional spots appear, which could indicate impurities or other components in the sample.
In the second question, two students each have an unknown compound that appears colorless. They both use the same brand of TLC plate and the same solvent to develop the plates. Both students obtain a single spot with an Rf value of 0.75. The question here is whether the two samples necessarily contain the same substance. To prove unambiguously that the samples are identical using TLC, the students can perform a couple of methods. One approach is to run both samples on the same plate side by side. If only one spot appears on the developed plate, it would strongly suggest that the two materials are the same compound. Another method would be to double spot the samples, which involves placing the second spot directly over the first one on the TLC plate, allowing the solvent to evaporate, and then developing the plate. If only one spot appears on the developed plate, it would provide further evidence that the two samples are identical.
In summary, while the questions may appear similar at first glance, the experimental conditions and approaches for verifying the purity or identity of the unknown compounds are different in each case.