I'm just curious to know if reseachers have found a link between vial septa used and extranuous peaks, now that the LOD of instruments is getting better. This seems to occur mostly in water/environmental testing using the "purge and trap" method.

People have described the peaks as volatiles from the silicone septa. Any thoughts as to why its not seen due to injection port septa? Would the typical grey rubber Head Space(HS) septa be better? Or is it a factor that the HS LOD is not capable of detecting in the PPB range?

Any thoughts, info, articles about the above would be appreciated.

Thanks,
Doug

There is definitely a relationship between vial septa and extraneous peaks in a chromatogram. If you use an Archon or similar vial autosampler for soils in purge and trap, there are special low bleed septa that must be used to avoid extra peaks in the chromatogram. If you do repetitive injections from the same vial using a liquid autosampler you may begin to see extra peaks in the chromatogram.

The polymers used for vial septa are significantly different from injection port septa and headspace vial septa are different from purge and trap vial septa. Vial and purge and trap septa are a softer, low temperature polymer that the injection port or headspace septa. Bleed from the low temperature septa is much higher.

If you look at the peaks with a mass spectrometer you will see the peaks are low molecular weight siloxanes. I don't remember any articles about bleed, but it is more of an issue as detectors get more sensitive.

Both vial and inlet septa can cause extraneous peaks in a chromatogram. Generally speaking, injection port septa are more thermally stable than septa manufactured for VOA and autosampler vials. However, I have seen all kinds of septa generate bleed profiles. These are usually evident in a chromatogram as a homologous series of peaks.

I have a few documents here on that topic that I could e-mail or fax to you. If you're interested, please send me a personal e-mail and I'll send them off.

Regards,
Jason Ellis
Agilent Technical Support