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I have identified carvacrol and thymol via thin-layer-chromatography and via gas chromatography On a normal silica plate the thymol has a higher Rf-value then carvacrol. The explanation for this would be the position of the hydroxyl-group and the difference size of the nearby methyl or isopropyl group. I think thymol should be more hydrophobic then carvacrol, explaining the less interaction with the silica gel resulting in a higher Rf-value.

The problem is, that the analysis by gas chromatography with a non-polar column shows a different elution profile. I would expect that thymol, according to the higher hydrophobicity, should have a higher retention time then carvacrol. This isn't the case. Carvacrol (27.08 min) elutes after thymol (26.51 min). Is there any explanation I'am not aware or is my thinking simple wrong?

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  • $\begingroup$ Thymol has a lower boiling point (232 °C) than Carvacrol (237.7 °C, both from Wikipedia), I'd bet this is the reason because AFAIK the volatility has a more important effect on the separation than the column polarity in gas chromatography. $\endgroup$ – Molx May 10 '15 at 23:27
  • $\begingroup$ @Molx You are right, gas chromatography's separation is more influenced by volatility (related to boiling point) of the constituents of a mixture than their adsorption to the stationary phase. In addition -- as already stated in the original question -- the column deployed in GC was a non-polar one. Except explicitly stated differently, thin layer chromatography more often uses a polar stationary phase (silica, acid/neutral/basic alumina, for example) where different strength of adsorption is important. On contrast, HPLC more often deploys reverse phase (less/apolar) stationary phases. $\endgroup$ – Buttonwood May 11 '15 at 16:48
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For gas chromatography, boiling point is one of the most important determinants of elution time. For thin-layer chromtagraphy, that isn't true.

The comments from @Molx and @Buttonwood on this question expound on this point quite well.

@Molx:

Thymol has a lower boiling point (232 °C) than Carvacrol (237.7 °C, both from Wikipedia), I'd bet this is the reason because AFAIK the volatility has a more important effect on the separation than the column polarity in gas chromatography.

@Buttonwood:

... gas chromatography's separation is more influenced by volatility (related to boiling point) of the constituents of a mixture than their adsorption to the stationary phase. In addition -- as already stated in the original question -- the column deployed in GC was a non-polar one. Except explicitly stated differently, thin layer chromatography more often uses a polar stationary phase (silica, acid/neutral/basic alumina, for example) where different strength of adsorption is important. [...]

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