Chromatography Forum

Hi everyone!

I've been asked to analyse the zinc ricinoleate content of a polyethylene film with it as an additive... I've only heard about this in cosmetic formulation....
Apart of the extraction from the matrix that I'll probably perform by soxhlet I'm quite lost...
I know I can analyse fatty acids with the GC-FID but I don't know how to transform the salt into the acid, 'cause in literature is found that the salt is not water soluble...

So if someone around has any idea it would be great!!
Thanks in advance.

I believe that a simple treatment of the sample with MeOH:BF3 will do the trick for you. I have used this technique to determine additives in plasticized pvc formulations. Calcium and zinc salts of fatty acids are great stabilizers for pvc. If I take a small amount of the pvc compound (that's what it's called when all of the ingredients are added), add a couple of milliliters of MeOH:BF3 (I get mine from Sigma-Aldrich in the U.S.), seal it in a vial and cook it at 100 °C for an hour or so, I get pretty much quantitative methylation of the fatty acids. I then add about 4 mL of a 14% sodium phosphate (tribasic, aqueous solution) to kill the acidic reagent. Dilute the solution further to about 20 mL with more water and then solid-phase extract them out. Reconstitute them off of the solid-phase extraction cartridge in something like methylene chloride for the GC analysis. You need to make sure that the MeOH is in large excess over the ricinoleate. You may even be able to treat the polypropylene directly with the MeOH:BF3 as I do the pvc. No soxhlet extraction required. You'll need to verify that with some recovery studies.

Everything has some solubility in a given solvent. Even rocks are somewhat soluble in water. If as the salt dissolves, one of the reaction products - in this case the fatty acid part of the salt - is converted to something else (like the fatty-acid methyl ester), the equilibrium for that dissolution reaction (good old Ksp) will shift to the right (Le Chatelier's Principle) forcing the solid to dissolve even more. I think you'll have no problem getting at the ricinoleate part of your sample.

Thanks a lot for your advice!!!

I'll try both aproaches as soon as I recibe the samples. One by soxhlet with petroleum ether; and other as you say, directly digesting with MeOH:BF3, and share my thoughts

Be sure to add an internal standard. I use pentadecanoic acid as it's not something you typically find in nature. Many times, the sources of fatty acids like this in industrial process are natural (animal fats, vegetable oils, etc.).

Does total zinc content work for you?

I see the biggest issue being extraction of the Zn ricinoleate, if it's imbedded inside the polyethylene. Polyethylene is not too soluble in stuff, need to get good contact with acidified methanol to extract it.

If extractable, I'd use sulfuric acid-methanol or BF3-methanol to simultaneously acidify and extract the Zn ricinoleate, heat 5 minutes to complete transformation into methyl esters, then cool and add some hexane and "float" the hexane out using saturated NaCl.

You find that the hydroxyl group on the methyl ricinoleate will require somewhat higher temperatures on the GC than does methyl oleate.