Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Gas Chromatography - Organic Chemistry

  1. Jan 27, 2010 #1
    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.
  2. jcsd
  3. Jan 19, 2017 #2


    User Avatar
    Science Advisor
    Gold Member

    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.

    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.
  4. Jan 20, 2017 #3


    User Avatar

    Staff: Mentor

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted