TLC plates separate molecules on the basis of _ whereas SEC separates molecules
ID: 491506 • Letter: T
Question
TLC plates separate molecules on the basis of _ whereas SEC separates molecules on the basis of _. Why should a pencil be used when preparing the TLC plates? How is the R_f value calculated? What are the possible maximum and minimum values for the R_f? What physical situation will lead to each? Draw the structure of PC and PE, and predict which molecule will have a higher R_f value. If an amino acid moved 5.11 cm on the chromatogram and the solvent front reached 10.34 cm, what is the R_f value of an amino acid? Show your calculations.Explanation / Answer
1]
TLC plates separate molecules on the basis of polarity whereas SEC separates molecules on the basis of size
Polar a molecule, the higher affinity it will have for the more polar silica plate and will therefore spend less time in the mobile phase. As a result, it will move up the plate more slowly. Conversely, a less polar molecule will spend more time in the mobile phase and will therefore move up the plate more quickly. The speed at which the molecules will move up the plate thus depends on the relative difference in polarity between the stationary and mobile phases, and will vary depending on the nature of the stationary and mobile phases used for separation. Size-exclusion chromatography (SEC), also known as molecular sieve chromatography, is a chromatographic method in which molecules in solution are separated by their size, and in some cases molecular weight.
2]
Why should a pencil be used when preparing the TLC plates
The pencil marks are made of inert carbon that will not move with the eluent.
3]
How is Rf value calculated? Possible maximum and minimum values for Rf. What is physical situation will lead to each?
Rf a/b
a = The distance from the starting point to the gravity center of the sample spot.
b = The distance from the starting point to the front of the developing solvent
Minimum value for Rf is 0 and maximum value is = 1.
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