I have two acid methods that work with small acids.
I have a chloroformate esterfication If you go splitless and SIM you can go much lower than I do (I have no need to).
Samples can be dissolved in isobutanol if it is fat based or 0.1 N NaHCO3 if it is aqueous/spray dry.
Dilute down to the range of 500 to 25 ppm of each fatty acid.
I make a 1000 ppm 4-methyl-valeric acid in isobutanol ITSD solution
I make a fatty acids std at 1000 ppm also in isobutanol.
The reaction I do in an autosampler tube and cap. Cap when vortexing but loosen it up after to allow gas generated to escape.
25ul pyridine
75ul isobutanol (25ul ITSD 50ul of sample or isobutanol)
200 ul 0.1 N NaHCO3
Syringe 40 ul of isobutyl chloroformate
Immediately vortex vigorously for at least 10 sec (critical)
Wait a few minutes
Add 33ul of 7N NaOH
Syringe in 40 ul of isobutyl chloroformate
Vortex immediately
Add 400ul of hexanes and 200ul of saturated NaHCO3 and NaCl to neutralize the derivatizing reagent (It makes a big peak of carbonic acid diisobutyl ester)
Transfer the organic layer to a new tube.
I inject 2ul 10:1 split at 280 deg inlet with a typical split liner with wool.
Ramp
Start 40 deg for 5 minutes (enough time to get isobutyl acetate ahead and separated from the big pyridine peak only bit before.
16 to 325 5 minutes
I’ve gone all the way up to stearic. It also does linoleic and oleic. IT doesn't work well on hydroxy acids.
Method II is esterfication with isopropanol.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2817593/
I follow the procedure in the paper but I omit the toluene and use isopropanol instead of methanol as the small methyl esters at very volatile and tail too much on db-5's. I use aqueous 35%HCl and dilute it in isopropanol to a few percent as they do in the paper so the water becomes insignificant. I then make a 10ml reaction mix of the sample in IPA/HCl and heat to 100 deg C for 2 hours then cool and add water saturated with NaCl NaHCO3 to neutralize the acid and inject the isopropanol layer (with high salt IPA and water don't mix) This method also does TCA cycle acids and lactic but also transesterfies the fats and other esters.
There is a headspace in-situ esterfication technique as well that works for the small acid and of course just plain old headspace with a free fatty acid column though it might not be sensitive enough for you.