Septa and Vacuum Cavity - Precision Issue

[syx]

Dear Members,

We use auto sampler with pull-to-fill injector. These days we have trouble with poor precision reproducibility in series injections from one vial.
We always fill the vial up to ¾ capacities.
The septa are bought from two suppliers. The original septa for the LC are thinner than the other and more fragile. In 2-3 injections it will be broken into 2 parts and down to the sample solution inside the vial.
Then we use other septa that be used in different LC. These septa are thick and more elastic; do not be broken after repeated injections. But new problem appears: poor precision.
After we inspected any possible reasons, we come in a conclusion that the cause of the problem is vacuum cavity in the vial after injection, so that the plunger can not draw the solution perfectly.
We tried to open the vial just a moment before injections, and we got good precision.
We tried to use the original septa for the LC, and we got good precision … and broken septa in the sample solution.
We tried to use the septa from other LC, and we got bad precision!

The question:
Is it safe to use the original septa that always be broken after several injections?
We just worry it will contaminate the solution and the debris can block the needle and the line.

What should we do to eliminate vacuum cavity when we use the other septa? Should we open-close the sample vial after each injection (it works and gives good precision)?

Regards,
SYX

[Tony]

Poor precision for pull-type loop filling can sometimes be due to a leaky plunger in the filling syringe. If the plunger is particularly worn, any slight vacuum encountered during filling will cause air to leak around the plunger and give irreproduceable volumes. This is especially the case with teflon-tipped plungers, which make good seals but have a limited lifetime and should be replaced regularly.

Assuming the syringe is OK, most injection sysytems should easily cope with mild cavitation caused by sampling a small volume (e.g. 5-20 uL) from a 1 mL vial.

Tony

[Ann]

You can buy pre-slit septa from Chromacol, this solved a similar problem in our lab.

Good luck

Ann

[MG]

Some autosamplers have a "vial vent" mode that blows some air into the vial before the injection, to address this problem. If yours doesn't have this setting, you might be able to write a custom injector program that accomplishes the same thing.

[Uwe Neue]

If you have a Waters columns and supplies catalogue, it will give you instructions on which septa types are right for particular applications. The bottom line is that preslit vials are recommended for the application that you are after. Alternatively, you can use PTFE septa, which do not reseal (but do not break either to land in your sample), but the PTFE vials do not reseal.

[syx]

Thanks to Ann, MG, and Mr. Neue for the replies.
Mr. Neue, in the catalogue is told that we should use pleslit PTFE/Silicone septa to prevent vacuum formation in sample vial. I will try it.