SPME and siloxanes

[Rocknroll99]

Hello,

I have been getting more and more siloxane in my SPME analysis of volitiles. I presume they come from the SPME fibre but am not 100% coninved.

I dont do many of these analysis so I am confident with the SPME.

Anyway the compunds I seem to be getting (and they seem to be getting larger) are:

Cyclopentasiloxane
Cyclotrisiloxane
Cyclohexasiloxane
3-Isopropoxy-1,1,1,7,7,7-hexamethyl-3,5,5-tris(trimethylsiloxy)tetrasiloxane
and others

My system is a Shimadzu GCMS. I am using a Restek colum RTx-5Sil MS

I am using 60um PDMS/DVB SPME fibres

Temperature programme is 40oC to 240oC at 12 oC/min Inject temp is 220oC

Can anyone tell me where the siloxanes are coming from and how I can limit them

Cheers for anyhelp you can give me on this

Matt

[chromatographer1]

"I am using 60um PDMS/DVB SPME fibres"

Note the PDMS.

PMDS = Polydimethylsiloxane

Heating the fiber in AIR, or coating the fiber with oxygenated compounds ?

PMDS is reactive. It is not inert. I did not say VERY reactive, but PDMS is NOT inert 100%.

Decompose PDMS and what do you get?

"Cyclopentasiloxane
Cyclotrisiloxane
Cyclohexasiloxane
3-Isopropoxy-1,1,1,7,7,7-hexamethyl-3,5,5-tris(trimethylsiloxy)tetrasiloxane
and others"

Also D4 as well as D3 and D6.

Solution?

Condition the fiber after each sample in an inert gas stream (injector for example)

You will get similar decomposition in time from any silicone column like Restek colum RTx-5Sil MS and many other of other brands as well.

But I suspect the majority of your peaks are from the fiber. Try not to heat the fiber in air.

Do not expect an infinite lifetime for the fiber.

Good luck,

Rod

[Rocknroll99]

Rod

Cheers for that mate. Exactly what I wanted to know. Thanks for letting me tap into you brain.

Regards and thanks from New Zealand

Matt

[Ultimate Science 101]

Although the SPME itself is a strong contender for the siloxanes you are detecting, the septum must also be considered (unless using a Merlin microseal). Standard methyl silicone septa are likely to core out and leach components into the liner over repeated rejections. A Restek ice blue septum is more amenable to SPME injections and may provide a better platform for injections over time.