GC-FID HeadSpace Help


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 you say, stay stable do you mean you're seeing results instability? I assume you peak RT's are also stable since you say your peak shape is also OK?

    A couple of questions, since headspace isn't my forte and since you've replaced the loop and that seemed to help, what material is your sample loop? When you say injector, do you mean the injection valve itself or an ALS turret, or an entire headspace head? 

    Are you sure that the sample isn't binding to the loop material / activating it? Are you certain there's no issue with the headspace heater which might prevent evaporation of the sample? Are you sure that you're adequately flushing the loop if you're seeing carry-over? Are you sure the temp profile of the GC run is sufficient to remove all of the sample from the column?

  • I should have said consistency, not stability. The retention times and peaks look fine. Sometimes the calibration looks really good, and QC to follow looks fine. Sometimes the calibration is way off (no change in set-up) or the calibration looks fine but the QC is super out of range. The loop is stainless steal, and the probe (the injector) are both deactivated. 

    I'm not sure what is happening, which is why I'm trying to figure it out. It could be that any one of those things are happening, but I'm not sure how to figure out if it is or not. What should I flush the sample loop with? I tried methanol and that completely messed up everything. DI doesn't seem to do much. I am in the process of trying a few different method changes (lower sample loop oven temp, slower ramp in the GC oven to temp), but wasn't sure if something was glaringly obvious to what we are running. 

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