Chromatography Forum

Hello. 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.

"When a brand new sample loop and injector are installed, things look great." Is the clue - you have surface contamination. Sadly, it is a real problem to get rid of. You have nothing to lose by flushing hot water through the valve and tubing to try to wash soem of it out.

Peter

Yeah, I know I have some surface contamination. I am looking for ways to mitigate that at all possible through my method. I lowered the sample loop oven temp to 85 to see if I could not cause as much foam/boiling in the sample, but my calibration/qc looked pretty bad.

I believe your loop and transfer line need to stay over 100C to prevent condensation staying in the lines. HS Oven could probably be less to lower amount of water vapor injected in the first place. What is GC inlet temp? Easy to overload liner with water vapor. Normal 2mm headspace liner could be too small?

Agilent has a steam cleaning procedure, HS temps are 125,140,150. But only using 1mL water per vial, column disconnected. With dirty samples possibly running the steam clean every few months would be helpful. Or flow path PM.

Yeah, I know I have some surface contamination. I am looking for ways to mitigate that at all possible through my method. I lowered the sample loop oven temp to 85 to see if I could not cause as much foam/boiling in the sample, but my calibration/qc looked pretty bad.

Reduce the sample equilibrium temperature - if you have foaming and boiling of the samples that is the cause of the surface contamination. Only headspace, not boiling sample liquid is supposed to pass into the sample loop. The sample loop oven temperature needs to be about 10C higher than the sample equilibirum temperature to ensure that liquid does not condense.