I'm hoping to seek some help for a problem I've been experiencing. We currently run a GC-FID with a headspace sampler testing for Acetic Acid, Propionic Acid, and Butyric Acid. I've been having issues reading the acetic acid consistently. Any suggestions would be extremely helpful. Our matrix is wastewater, samples ranging from Influent and primary clarifier to digesters and VFA feed.
For setup of my samples, I measure out 0.5g of NaCl, pipette 3mL of sample, 0.1mL of internal standard (2-ethylbutyric acid 1800mg/L), 0.2mL 2M H2SO4.
The method of running - Headspace oven temp - 100C, Sample Loop Temp - 185C, Transfer Line Temp - 190C. 15min equilibration, 0.5min injection. Starting GC oven temp 60C, Ramp to 190 in 15min and hold for 2min. Post run is 230C for 2min.
We are running a DB WAX UI column. I calibrate from 5ppm-300ppm. When a brand new sample loop and injector are installed, things look great. After about 1month of running, I start to see some carry-over occasionally. Now it's been over 3months and my acetic acid peaks look all over the place. I have tried running the calibration from high to low (I usually go from low to high), extra DI blanks. Nothing seems to be helping. The peak shapes themselves look fine, I just can't seem to get anything to stay stable.
Any help would be greatly appreciated! Thank you.