Chromatography Forum

Dear all!

I have to analyse fatty acids (c12-c20) in soil and i have problem how to prepare my sample. I would extract it with hexane then add potassium hydroxid in methanol to derivatize them. Then i would analyse it with GC-MS.

If anybody could help me with any suggest, or perhaps methods or literature i would be really thankfull.

Thank u very much!
Erdno

Search the forum for FAME methods, we've beaten them to death over the years. Not sure about the soil matrix effects, but I suspect you'll find BF3/MeOH to be a more popular derivitizing agent...

I agree

Regards

Ralph

Are you interested in only the free fatty acids, or do you want the fatty acid profile of the lipid content of the soil ?

Peter

Are you interested in only the free fatty acids, or do you want the fatty acid profile of the lipid content of the soil ?

Yes, that's the real question, I've done thousands of FAME assays. If just looking for composition of free fatty acids or their salts, simply extract with methanol and heat 5 minutes after adding some methanol-H2SO4 or BF3-methanol, then cool and extract into hexane, adding NaCl solution to help separate the layers.

If you want to include triglycerides composition, I'd saponify with KOH-methanol, then add my methanol-H2SO4 or BF3-methanol, then cool and extract into hexane, adding NaCl solution to help separate the layers.

For simple free fatty acids I do chloroformate esterfication. I use isobutyl chloroformate and isobutanol because I can just barely get isobutyl acetate separated from the catalyst for the reaction pyridine. I quench the reaction with 1N NaHCO3 to remove excess chloroformate.

For total FAMES I like the mild methanolysis procedure here
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2817593/
You can make the reagents out of common lab chemicals like aqueous 35%HCl. It gets diluted in the final reaction mix to 1.2% HCl and 2.4% H20 which is insignificant. I very occasionally get asked for a basic fatty acid profile.

I can also to lactic, malic, tartaric, citric, succinic, and fumaric using a deriviative of the above method though I replace the methanol with isobutanol and heat it at 90 deg for 2 hours then add 1ml of 1N NaHCO3 to quench the HCl.

In addition to the above helpful comments and observations I have found that the following website by W W Christie (The Oily Press) to be an excellent summary of lipids, their form, hydrolysis, derivatization methods and mass spectra

http://lipidlibrary.aocs.org/Analysis/c ... mber=19210

Regards

Ralph