Gas Chromatography - Organic Chemistry

[singinglupine]

Why do ten molecules of toluene that enter a GC column at exactly the same time not reach the detector at the same time?

Why are London Dispersion forces of the sample molecules strongly influencing GC measurements? And why are they negligible in TLC experiments on silica gel?

I have no idea how to answer the first question. The second question I'm thinking deals with the fact that the stationary phase in TLC, silica gel is polar, while the stationary phase in GC is relatively non polar. So in GC there are more weak interactions, nonpolar to nonpolar and in TLC there are stronger interactions, polar-polar aka dipole-dipole interactions.

 

[TeethWhitener]

Why do ten molecules of toluene that enter a GC column at exactly the same time not reach the detector at the same time?

At the very least, remember diffusion: if you drop dye into a glass of water, the dye molecules will diffuse to eventually homogeneously fill the glass of water. Same thing with toluene molecules in a GC column. They spread out based on the statistical distribution of their velocities.

The second question I'm thinking deals with the fact that the stationary phase in TLC, silica gel is polar, while the stationary phase in GC is relatively non polar. So in GC there are more weak interactions, nonpolar to nonpolar and in TLC there are stronger interactions, polar-polar aka dipole-dipole interactions.

This is a good answer. I might mention also that the mobile phase in GC is generally also nonpolar (helium or argon), whereas the mobile phase in TLC can be either polar or nonpolar.

 

[DrClaude]

What slows down the molecules in the column is the constant adsorption/desorption process. That process is random, so the time a given molecule will take to go down the column is variable.