[DeltaForce]

Hi,

I've recently done an experiment in the analysis of fatty acid methyl derivatives where it was ok to use a solvent of hexane:ether for tlc analysis of these compounds.

But, when it came to analysing fatty acids produced by PLA₂ hydrolysis, it was necessary to modify the solvent (above) with the addition of a little bit of formic acid for the tlc analysis.  Why was this?  I can't understand the theory behind like the interactions otherwise I'd have worked it out by now 

Help appreciated!!!

DeltaForce

[movies]

Think about what is different about the molecules you are trying to elute on the TLC plate.  Also, think about the nature of the solid phase on the TLC plate.  How might it interact differently with the different molecules you are dealing with?

[DeltaForce]

Think about what is different about the molecules you are trying to elute on the TLC plate.  Also, think about the nature of the solid phase on the TLC plate.  How might it interact differently with the different molecules you are dealing with?

All I think of is the capillary action of the eluent as it progresses up the tlc plate but how is it different for this type of compound?  I know fatty acids are water insoluble is that what the formic acid did, maybe make them less hydrophobic?  The solid phase in this case was made from Silica Gel-G.   

[movies]

Okay, think abouyt the actual interactions between the atoms themselves.  Would you say that silica gel is polar or non-polar?  What about your eluent, polar or non-polar?  Then think about the molecules you are eluting.

[DeltaForce]

Okay, think abouyt the actual interactions between the atoms themselves.  Would you say that silica gel is polar or non-polar?  What about your eluent, polar or non-polar?  Then think about the molecules you are eluting.

Silica Gel is polar, a metal oxide with (OH) groups on surface.  The solvent (hexane:ether) is, well, I presume it's non-polar otherwise what would be the point as it would adhere to the OH on the tlc surface, no?

As for the free fatty acids, maybe they aren't as polar, or are somehow unstable??  Therefore addition of formic acid to the solvent was required , but what does formic acid (HCO₂H actually do?  There is a lack of info in my textbook!

[movies]

Good, so the amount that your molecule "sticks" to the silica gel will determine how far it moves up the plate.  So what else can you say about a metal oxide or a hydroxyl group in the presence of your fatty acid?  If the two reacted somehow, would that change the polarity of the molecules you were dealing with?  Could you prevent this reaction with some other additive, possibly one that would be more reactive in the same way?