So, that's about 88 ppmv (µg/mL) butyric acid on the low end of your concentration range. That's still a lot of material.
I would see what I can get by merely sampling the headspace above the liquid. What equipment do you have available to you? If you have a static headspace sampler, you could try that. It's simple but sometimes getting analytes with active hydrogens to the head of your GC column can be on the irreproducible side.
I'm certain that solid-phase microextraction will work. To run those acids without some sort of derivatization, you'll need a polar GC column (SupelcoWax, DB-Wax, etc.). I've had good success running organic acids on that type of phase. Set your split ratio to suit the sensitivity that you need/want. Have you ever used SPME? All you need is a split/splitless inlet on your GC. Change the inlet sleeve to one that's narrow bore (designed for SPME) and inject like you normally do with a syringe. The folks at Supelco (Sigma-Aldrich) will be very happy to help you.
I just came up with a method for determination of isovaleric acid in a substantially water matrix using the carboxen/pdms/divinylbenzene SPME fiber. I was able to detect down to 0.2 ppmv without working too hard.
Good luck!
