GC/LC caps

Discussions about GC and other "gas phase" separation techniques.

8 posts Page 1 of 1
Hi all,

I am setting up a LC-MS/GC-MS lab and want to know the types of caps used in your lab and why. For example, in my previous lab we used to use red rubber/PTFE septa caps as we were not doing multiple injections from the same vial.

Thanks for your help in advance!
I generally use standard screw caps because they are cheap and I don't have crimping tools. I've used screw/crimp/ and snap on caps with a split (for UPLC) and in my experience if you are going to do multiple injections or use volatile solvents crimp caps are superior in resealing. Screw caps have a lot of variability in terms of how tight the cap is closed (most people tend to overtorque them) and the septa is thinner.

I do use crimp caps for headspace and find they hold the seal especially after puncture much better. I really should switch to them for ALS as well.

When I ran an Agilent 1290 I used the snap on caps with the preslit and found they held their seal pretty well too. Probably because there is less risk of under or over-torquing them.
Thanks MSCHemist that is quite informative for the LC..

So then you would definitely prefer to go with the crimp caps for multiple injections or volatile solvents, but would also recommend snap on caps?

Has anyone had extensive cap experiences with GC who could comment?
For GC - I would also recommend crimp caps for volatiles especially if the samples are going to be sat on the autosampler for a few days. You do not need crimp cap for multiple injections other than with volatiles. I use the screw caps daily for routine analysis and as long as they are correctly tightened (I personally don't feel these can be over tightened) then they will be fine. All GC PTFE septa lids are generally fine even the cheaper ones, I don't get carry over of the septa or damaged needles.
We switched from crimp to screw caps for both GC and LC, even for the samples using Methylene Chloride as solvent.

Of course we went with the least expensive ones and the only thing I don't like is that if you don't immediately recap or make multiple injections before recapping I begin to see siloxane peaks showing up. These have a very thin PTFE liner on the inside and silicon rubber septa, probably would be better to have natural rubber instead, but those seem to be the more expensive option now. Once the teflon liner is punctured the bleed begins.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
We switched from crimp to screw caps for both GC and LC, even for the samples using Methylene Chloride as solvent.

Of course we went with the least expensive ones and the only thing I don't like is that if you don't immediately recap or make multiple injections before recapping I begin to see siloxane peaks showing up. These have a very thin PTFE liner on the inside and silicon rubber septa, probably would be better to have natural rubber instead, but those seem to be the more expensive option now. Once the teflon liner is punctured the bleed begins.


Yes I suppose that is always the risk in going for the cheapest caps.. Would be good to know if anyone has compared with more expensive caps?
I would recommend not using cheap crimp top vials as you will see a lot of interfering compounds with the more expensive caps giving much cleaner chromatograms. Personally, for the screw top vials, cheaper vs expensive has no difference so it makes sense to go cheaper.
The reason I switched from screw to crimp cap on HPLC is when reinjecting a check standard of Limonin and Nomilin I noticed both would decrease with each injection and form a third peak and it got worse and worse with every injection (sitting without injection did not have this effect). I then tried crimp and snap caps and found both fixed the issue. These weren't cheap screw caps either.

I watched a webinar and indeed screw caps can be overtorqued just as easily as the GC septa nut can be with similar effects. You greatly reduce the resealing ability of the septa. Using snap or crimp caps eliminates that variability.
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