Headspace repeatability issue

[chem111]

Hello everyone.

I have a strange problem with a GC-FID method for the determination of tert-Butanol using DMSO as solvent.
The instrument is Agilent 7890 with a gas tight syringe headspace sampler.
I have run the method before without any problems but now i have a repeatability issue.
The tert-Butanol peak area decreases every two or three injections by about 30% and i have noticed that at the same time that the area of tert-Butanol decreases there is suddenly a peak at about 29 minutes (tert-Butanol at 4.5 minutes) which is missing or very small in area in injections thet the area of tert-Butanol is O.K.

I can't figure it out and will appreciate any insights on the problems.

Thank you.

[rb6banjo]

Is the 29 minute peak much broader than anything else in your chromatograms? If yes, it's likely coming out from a previous injection. My feeling is that its appearance is probably unrelated to the sudden loss in your analyte signal. In my experience (25 years now), I've never seen a situation where these 2 types of things were directly related.

Please list all of your sampling/analysis conditions (times, temperatures, injection volumes, inlet liner type, column, etc.). The more information you can give, the better suggestions you will get from the participants on this board.

[kubowicz.tomasz]

Hello

I'd recommend:
1.Check syringe and sampler (tubing with gas, seals)
2.Check inlet for leaks (septa, liner, split vent trap)

You didn't specify sampler (Agilent or CTC/PAL)

Regards

Tomasz Kubowicz

[Peter Apps]

Leaky vial caps most likely. Check you sealing technique by putting a bit of pentane or dichloromethane in a vial, cap it and then immerse it completely in hot water - a leak will show as a stream of bubbles.

Peter

[chem111]

The column is -Agilent J&W DB-624 30m x 0.32mm x 1.8um
Temp. program- 40c for 20 min than at 20c/min to 240c for 20 min (60 minutes total)
incubation temp-100c
incubation time-45 min
syringe temp-110c
injection volume-1ml
inlet temp-140c
detector temp-250c
split-1:5 with 1.0 mm ID Straight Inlet Liner.
The sampler is Agilent GC injector 80 (previously CTC/PAL)

I will check the vials for leaks.

[krickos]

Additional hint:

Compare syringe temp vs boiling point of DMSO. Any chance of condensation and drop on sensitivity especially for polar high boilers?

This is one limitation of the syring technique. The loop/valve Agilent uses can suffer from the same issue, but if you keep temps in valves/looops/transferline above the bp of all solvents including sample solvent (very often forgotten) you work around the issue.

kind regards Chris