Ghost peaks. Siloxanes accumulate in the column overnight

[Igor]

Hello!

I'm new to chromatography, so my question might seem stupid. The GC-MS I'm using now had been working well for four years untill recently when we started to observe a lot of siloxanes accumulating in the instrument after some hours of not using it. The performance of the column was gradually worsening and especially it was while analyzing polar compounds. We changed the column and changed the liner and syringe, but it didn't help us, we didn't change a septum, but it is from the same box as the ones used before. From the analysis of the GC-MS data it seems like a septa bleed occurs, becuse m/z 73 coincides with the peaks on the chromotogram and it is dominating in the baseline profile. Also after just several injections of solvents (chloroform and ethyl acetate) with new syringe and liner we observed some dirt in the syringe and inside the liner as well. Moreover, I noticed some metallic particles in the liner and on the surface of the septum. How come this happend? Maybe the syringe touches some metallic parts in the injection port and damages the septum? I appreciate if anyone could help me with the advice on the account, maybe someone encountered something similar.

Thanks!

[kubowicz.tomasz]

Hello

To narrow problem down I'd recommend:
1.Run blank (no injection) - if problem disappears it means it is syringe or vials/septa
2.Use empty vial with lid and septa and inject - you'll check if it is a solvent issue (possible extraction)
3.use vial without septa (just plastic lid) and inject (try with/without solvent) - you'll check if it is septa issue

Good luck

Regards

Tomasz Kubowicz

[Peter Apps]

Hi Igor

Welcome to the forum.

What is the stationary phase in the column ?. If it is a siloxane it might be an additional source of siloxane peaks.

What has changed recently ? - analyses that were working well do not go bad for no reason.

Dirt and metallic particles in the inlet from injections of solvents are very puzzling. What type and make of syringe are you using ? Are you sure that the autoinjector is properly aligned with the inlet port ? Were the metallic particles on the top or bottom surface of the septum ? What make and grade of solvents are you using ?

What instrument do you have, what are you analysing, is there any packing in the inlet liner ?

Peter

[Igor]

Thanks for the replies!

Tomasz,
1. We ran the blank, but we still had this problem, probably because somethig had already been in the liner. Maybe we should change the liner and the septum and try again without any injection. If everything is going to be fine, so it is likely to be the septum.
2,3. We checked the affect of the vial septa, but they are ok. The problem does not disappear when we're not using them.

Peter,
The stationary phase are siloxanes, but even changing the column doesn't help and fragmentation of siloxanes on the mass spectra look different to those from the column.
The strange that nothing has changed. The solvents are ok, they all are of good quality, we tried different ones, but they are not the problem. We use Shimadzu and a liner with a glass fiber packing.
However, you gave me a clue...I suspect that one part that fixes the septum, which is on top of it, is a little bit bent. Could it be the problem if the syringe needle and the septum are not perfectly aligned?

Thanks again for your help!

[Peter Apps]

Hi Igor

A bent inlet top could make the syringe needle rub on metal somewhere, but I would be surprised if it would produce a visible amount of metal particles. You might have cross threaded a nut - that will give you lots of particles, misalignment, and possibly a leak. Misalignment can make the needles tear the septum, which exposes more septum to carrier gas, and makes septum particles fall into the liner.

The glass wool in inlets s usually deactivated by silanization, and that can also give siloxane peaks if it deteriorates.

Also check your septum purge flow - it should be at least 3 ml/min.

Peter

[Igor]

Thanks! I'll follow your advice.

[Himar]

You can also try manually injecting by hand to see if you feel any obstruction.