Chromatography Forum

Dear all,
I have to analize a mixture of fatty acid Magnesium salts (main stearic and palmitic acid). Can you suggest me a simply way to prepare a methyl esther or other kind of derivative to perform GC/FID analysis ?
Thank you!

Add 10-20mg amount of sample to a glass reaction vessel (I used a culture tube with a teflon lined cap) add 1mL of 14% BF3 in methanol, add methylene chloride to desired concentration and seal and heat @ 110°C for 10-15 min. Add several ml of NaCl saturated water and extract the methylene chloride layer by adding an equal amount of hexane so the non-polar fraction sits atop the salt water. Use a pipette to remove the upper layer and inject (after concentrating if needed).

I also did zinc and calcium salts as well.

What Chromatographer1 states should work fine. We do slightly different here: sample + 10 ml of the BF3-methanol (available from Aldrich), heat for 5 minutes on steam bath in volumetric flask (no stopper), then stopper, cool, add hexane, then add saturated NaCl to float the hexane layer to the flask neck, then transfer to an autosampler vial. This works fin on fatty acids and their metal soaps. Alternatively, 15% H2SO4 in methanol also works, same procedure.

Consumer Products Guy also has a good method (as always his posts are quite helpful) . The problem I did see sometimes in that making somewhat more concentrated solutions the BF3 in methanol did not always dissolve the salts of some fatty acids. (I used 1 mL, his method uses 10mL) That is why the methylene chloride was added. (With the free acids this is not needed) Low yields of the ester seem to follow low solubility of the salt.

If you can derivitize the FA salt to an ester without it, do so, as Mr. CPG suggests.

As a point of possible interest to you or to others on the forum, I once tested unsaturated oleic acid to see if any acid was not esterified by the method I used. After 6 min. less than 1000ppm was not esterified, and after 8 minutes essentially 100% conversion to the methyl ester was achieved. Heating longer than 10 min will begin to produce noticeable amounts of decomposition of the oleic acid. This is not a problem with the saturated fatty acids.

For bar soap samples we dissolve the sample in the BF3-methanol at ambient in a stoppered volumetric flask using a stir bar. Then the stopper comes out, and we proceed to the steam bath step listed above.

You may want to consider using an internal standard as your recoveries from the above FAME derivitizations may be quite variable until you dial in a procedure that works well for your samples & have practiced it a few times. Any fatty acid that is available in fairly high purity (and isn't among your analytes) will work. Capric acid can be had in >99% purities I think.