I gave an answer this question about TLC. I now realize that I am blundering my way through this question.

The problem statement doesn't specify if the compounds to be eluted are polar or non-polar. So this seems to leave a door open.

Question 1 - Could compounds be either relatively polar or relatively non-polar?

Question 2 - If an absolutely non-polar solvent like hexane were used, and some other more polar solvent, wouldn't a reversal of the order of the elution of the compounds happen?

Question 3 - Have I in fact, like a blind squirrel, finally found the nut?


The problem specifies that "normal phase TLC" is being used so the stationary phase is polar.

The problem only states that the elutants have different polarity and not that any is actually non-polar like hexane.


With Question 3 I was trying to be funny and ask if my two answers to the original question are right.


Question 1

Yes, of course! Something that is "relatively" non-polar would travel further with a non-polar mobile phase because it would have less attraction to the stationary phase. While a polar substance would not travel as far with the mobile phase because it would have a greater attraction to the stationary phase.

Question 2

Now, that depends. Are you also changing the stationary phase? Exactly how polar is the new polar-mobile phase? It would really depend on which phase the solutes have a greater attraction to.

Question 3

You are on your way. Quantitative HPLC is the bane of my existence. There's a million factors to consider, such as column temperature, flow rate of the solvent, particle size, polarity and charge. Usually, it takes a bit of experimentaton to reach a perfect combination that allows complete separation of whatever it is you're interested in. Usually, your best bet in identifying a compound at a particular point is to run a little bit of pure sample on its own. In the end, above all else, you need to find a combination that doesn't result in chemicals overlapping with each other.

In qualitative chromatography, as long as you can resolve all possible signals, you've done well enough.

  • $\begingroup$ In answering the problem am I correct in believing that because the compounds could be relatively polar, or relatively non-polar that the most polar phase could flip-flop between plates 2 and 3? $\endgroup$ – MaxW Feb 26 '17 at 21:33
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    $\begingroup$ Assuming all compounds are polar, the most polar mobile phase would be on plate #3. Because, the mobile phase would be able to "push" compounds out of their binding sites by binding to the silica. The least polar compound is compound Z, because the mobile phase can more easily break it's bond to the silica. That means the least polar solvent is on plate #2 - because the mobile phase can not replace the binding sites as easily. $\endgroup$ – Bob Feb 26 '17 at 22:08
  • $\begingroup$ They could indeed flip-flop, but I think you'd need to change both mobile and stationary phases. $\endgroup$ – Bob Feb 26 '17 at 22:10
  • $\begingroup$ To be completely honest with you though, chromatography confuses the hell out of me. There's too much theory involved lol. Hence why it's the bane of my existence. But, in applied sciences, I'm pretty sure most of us probably just fiddle around with chemicals til we find something that works. $\endgroup$ – Bob Feb 26 '17 at 22:14
  • $\begingroup$ Understand what you mean about chromatography. Factor A makes it go up, Factor B makes it go down, factor C makes it go left. Factor D makes it go right. So which way does it go?!? $\endgroup$ – MaxW Feb 26 '17 at 22:19

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