volatile fatty acid analysis

[sepscientologist]

I'm given aqueous samples of VFA (volatile fatty acids) which are pH 6 ish. My standard used to be just about 5 mg/mL of the acids
in water and hence, about pH 2. When I adjust the pH of my standard to pH 6 the response factor goes way down! Duh I guess it
would.

Should I acidify all my real samples and if so, what acid and what pH? I'm using a PEG type column.

Or, can I esterify? I have tried a few alcohols but the alcohol always seems to coelute somewhere around the Propionic acid ester.

[chromatographer1]

I have seen in the literature, extractions of free acids into ether using nitric acid.

I have also seen direct aqueous injection using sulfuric acid.

I don't remember the details. Perhaps a trip to the library or a search on the internet might be of great benefit.

best wishes,

ROd

[Peter Apps]

VFAs in water are a hardy perrennial on the forum - have a look through the archives. What are you injecting ?, an extract or the aqueous matix ?, or headspace ?

Peter

[sepscientologist]

injecting aqueous samples containing short chain fatty acids. Results are useless because the pH
of the samples is sometimes below and sometimes above pKA'a.