Extraction of stearic/palmatic acid

[Mattias]

I am trying to develop an LC method for the determination of stearic and palmatic acid in tablets.

The method itself seems to work fine. The standards dissolve easiliy in THF and I get nice peaks and separation with a standard C18 column and detection at 212 nm.

However, when I try to extract the stearate/palmitate from the tablets I get terrible recovery (5-10%). I have mortared the tablets and then stirred the powder with THF for >1 hour. I have also heated the bottles to about 40°C without any improvement.

I need some good ideas!

Thanks
/Mattias

[HW Mueller]

Acidify the material before you extract, a pH of ~6 has done good. Why do you use THF?

[Bill Tindall]

have you acidified the extraction solvent? These acids may be present as salts. Add a bit of phosphoric acid and see if you see imporvement.

[Mattias]

Thanks for you answers!

THF is the only water-compatible solvent I have tested that dissolves both palmatic and stearic acid (it doesn't work with ACN or MeOH, at least).

Isn't there some safety issues regarding mixing THF with acids? Or is that just a problem with diethylether?

[Mattias]

Results:

I added just one drop of phosphoric acid and the recovery went up to almost 100% after just a couple of minutes of stirring! And the lab didn't explode either

Thanks a lot for your input.
/Mattias

[Bill Tindall]

Dang, I wish I could get paid for all this good advice!

I don't know where you got the idea that ethers would react violently with acid. Ethers are about the most inert class of chemicals there is.

[HW Mueller]

Mattias was probably thinking about peroxides, they can explode if you distill an old unprotected ether, no acids required. But I don´t understand why you need a "water compatible solvent", there are miscible proportions of CHCl3, MeOH, H2O (famous article: Bligh, Dyer, Canh J Biochem Ohysiol37, 911 (1959), etc.

[HW Mueller]

Ups, that´s Can J Biochem Physiol, 37 ....

[Mattias]

Thanks, I´ll have a look and see if I can change my solvent!

I am not so used to work with other solvents than MeOH or acetonitrile, so I got a bit cautious from the warning text on the bottle.. I mixed a larger portion of phosphoric acid directly in the THF (5%) and it got very hot. Now I use diluted acid instead, and it works fine!