Retention time and peak area shift for random injections

[skooma]

Hi!

I'm detecting acetic acid in nonvolatile API using GC. It is not ideal method for this kind of analysis, but I'm not allowed to switch to HPLC.
The method has been set up on two different instruments and on both of them some injections in the middle of the sample set have higher peak area for all peaks, this is also accompanied with longer RT. On the next run everything looks normal again.
Personally I think the problem is in the needles, since after the API has been injected few times, the needle is not moving smoothly anymore. The sample is 25mg/ml of API dissolved in DMSO, I can't go much lower due to sensitivity issues.
I have tried to add more washes for the needle(wash solvent is also DMSO) and added different solvent combinations (Isopropanol, methanol), but these do not seem to help. DMSO is also the best solvent for this API, so changing the wash solvent doesn't seem reasonable to me.
Currently I'm using the 10ul "gold" agilent syringes, but I have ordered two types of the "blue" ones. One which has smaller volume (5ul) and other which uses PTFE tip.
Am I overconcentrating on the needle and can this be something else?
Method parameters:
Inlet: 250C
Detector: FID 250C
Split: 10
Injection volume: 1ul
Start temp 40, final 240
Solvent: DMSO
Wash solvent: DMSO
Nr of washes 6pre, 6post
Plunger mode: fast
Viscosity delay: 6 sec (This was increased from 3 to 6, but didn't give any effect)
Instrument is Agilent 7890, with FID detector using the standard injection tower.

Example chromatogramhttps://imgur.com/a/Ee50Qr7

[dblux_]

Hi!

Injecting 1 µL from 10 µL syringe isn't the best solution regarding accuracy.
5 µL syringe would be better.

But the problem may be caused by carrier gas flow control.

[James_Ball]

Not sure how long you have been running the analysis on these instruments but it might be a good idea to clean or replace the split vent line and the split vent trap. If some of the API is being deposited in there since it is solid at room temperature, then it could be causing random problems. I have removed the copper line from the inlet to the trap before and found it to be terribly contaminated, easier to just put in a fresh length of copper tubing instead of trying to rinse it out.

[skooma]

Not sure how long you have been running the analysis on these instruments but it might be a good idea to clean or replace the split vent line and the split vent trap. If some of the API is being deposited in there since it is solid at room temperature, then it could be causing random problems. I have removed the copper line from the inlet to the trap before and found it to be terribly contaminated, easier to just put in a fresh length of copper tubing instead of trying to rinse it out.

We do this maintenance annually, but haven't replaced the copper tubing, just rinsed it. I will try if this improves the situation, because smaller syringe didn't help. The RT shift was smaller though.

Thanks

[James_Ball]

Not sure how long you have been running the analysis on these instruments but it might be a good idea to clean or replace the split vent line and the split vent trap. If some of the API is being deposited in there since it is solid at room temperature, then it could be causing random problems. I have removed the copper line from the inlet to the trap before and found it to be terribly contaminated, easier to just put in a fresh length of copper tubing instead of trying to rinse it out.

We do this maintenance annually, but haven't replaced the copper tubing, just rinsed it. I will try if this improves the situation, because smaller syringe didn't help. The RT shift was smaller though.

Thanks

Recently I have heard of a few people having trouble with the Agilent Gold syringes and sticking plungers. We have always used the Hamilton 10ul syringes without any problems on our Agilent autosamplers.