Thin Layer Chromatography

[msamadia1231]

What a neat name, huh? Well, let's find out what makes this so neat.


Remember in Orgo lab they gave you a piece of paper that looked shiny? Well, that shiny part is called the stationary phase and in TLC would be polar. The shiny part for general purposes used in TLC is silicic acid. TLC is all about attraction of molecules. The attraction of molecules is so big on this topic that they have a formula that decribes it, Rf value. All the Rf value is a ratio of how much the solute travelled up the plate divided by the total length of distance (the vertical race tracks, if I may).

And so going back, you have the stationary phase, that's polar, and at the bottom of paper, with a pipette, make drops of your testing molecule onto the paper. What about the mobile phase? The solvent, the liquid in the beaker, is the mobile phase. Than using, UV lights you can see how far your drops travelled up. The stationary phase is polar, so remember the rule "polar attracts polar" and nonpolar attracts nonpolar. So if the solute being analyzed travells further up, that means it isn't interacting with stationary phase, and hence will have a higher Rf value (numerator increases). That tells us that our solute being analyzed is nonpolar.

Now most extreme cases, lets say it was interacting with the stationary phase and it wasn't moving up. How do we make it move up? Well, if we increase the polarity of the solvent, its forces (dipole-dipole interactions) will increase and "overpower", if I may, the stationary phase.


Now you know why TLC is so neat, and mostly tested on the MCAT. If we can appreciate the concepts, we will be more willing to allow us to learn it, and make long-term relationships come the test day.

[kfowler_117623]

It should be noted that common solvents are hexane elutions for simple organic experiments. This is because hexane, being a straight chain hydrocarbon is very nonpolar.