Autosampler septums and SPME

[spiderylu]

Hi all,

So I've been running some experiments for the past few weeks (info here: viewtopic.php?f=5&t=22454) and what we've ended up with is a bunch of silica saturated GC outputs. Rather frustrating. So our best guess is that there is material from the septum getting into the sample or onto the fibre/holder.

Currently my method is to add the bacteria onto agar in the vial, clamp the lid on, pierce the lid with an 18G canular and then insert the holder into the vial. Am I making some enormous rookie error? Are there specific vials/septums suitable for SPME? We're going to run a pilot study next week to determine the best way to collect so we don't get silica in the sample. So far we've got three methods - I'd be grateful to hear any other methods people have.

EDIT: Just for clarification: when I say septum, I mean the vial septum lid, rather than the septum in the GC!

Thanks

Lue

[rb6banjo]

Those siloxanes could also be coming from your GC inlet septum. Many of the SPME fibers contain PDMS as well. I'm afraid that it's difficult to completely eliminate them. Generally, they're small components in my chromatograms and I just ignore them. I have yet to have them interfere to a large extent with my components of interest.

You're probably not doing anything wrong. I have found some vial septa are worse than others for siloxanes. I genrally use the teflon-backed, butyl rubber septa from Sigma-Aldrich:

http://www.sigmaaldrich.com/catalog/pro ... &region=US

They show some low concentrations of light hydrocarbons in blank runs but they don't show massive amounts of siloxanes. They're the best I've found.

Good luck!

[spiderylu]

Thanks for the advice! Perhaps we ought to try some different septums

[Peter Apps]

I had exactly the same problem with siloxane (NB not silica) contamination when trying SPME of wild dog urine in headspace vials with ordinary Teflon-faced silicone septa. In the end I abandoned it - the siloxane peaks were bigger than anything coming from the sample itself. You could try using aluminium foil in place of the septum - the vial is not pressurized so it does not have to be leak tight. Poke the SPME needle itself through rather than making a hole with another needle- you want as snug a fit as possible to keep room air contaminants out.

Please let us know how it goes.

Peter

[k.schmidt]

Hello.
I have the same problem. I use screw-top 10 ml headspace vials fitted with a 1 mm thin blue silicone/clear PTFE septum, (Chromacol, SGE, UK).
I found this information in Wercinski's book (Solid Phase Microextraction – A practical Guide) “The GC system used in the study was quite clean, and no ghost peaks occurred when the column was temperature-programmed without injection. When the fiber sampled a blank vial, many peaks resulted; moreover, some of them were very large. Most of these peaks were identified by GC/MS as siloxanes and were found to be derived from the vial septum. Vial septa from the same manufacturer and from the same lot gave very consistent patterns; furthermore some brands of septa were much worse than others. Most of the contamination was removed by baking the vial septa overnight in a laboratory oven at 150 C. The remaining extraneous peaks were identified as nitrogen-containing residues from the glue used to hold the SPME fiber in the fiber assembly”.
I tried it and baking septa in the oven at 150 ºC helps to get rid of the siloxanes. But siloxanes appeared again if septa after baking were not kept in the oven (very frustrating as not always a rule)... Now just got a new batch. Wonder if the same problem will appear...