SPME DVB/Carboxen/PDMS

[WK]

I have started use the fibres above this year and have had baseline noise problems with GC-MS. The noise encompasses all masses scanned which makes me think its multiplier noise.
I use an injector temperature of 250degC which is below the 270degC maximum temperature stated. I leave the fibre in for 2mins for conditioning and injection.
Has anyone had similar problems?
I don't use wool in the liner of course.
Thanks
WK

[ingochrist]

What type of liner do you use? You should be using a 0.75mm liner (max 1mm). It could be that your analytes just "bleed" out and seem like background. Also make sure that you've conditioned the fiber as descried in the instructions.

If you give me your e-mail address, I can forward you a 5-page instructional outline on how to improve your results with SPME. That guide was written for Combi PAL autosamplers, but it is also useful if you use the technique manually.

Best Regards,
Ingo Christ
ichrist@chromsys.com

[WK]

Thanks Ingo,
I should have made my post clearer.
I do not have an analysis problem. My peak shapes are good (Varian 1079 3.4mm id). Its just the baseline for subsequent liquid analyses and blank ramps.
Perhaps I just have a noisy multiplier.

[ingochrist]

In that case I can only recommend to bakeout your system. One suggestion (from my own experience), don't forget to increase the temperature at the MS transferline. I believe the Varian also allows changing that temp.

Best Regards,
Ingo Christ

[WK]

Ingo
One thing I have noticed today is that the noise is cyclical - ie its low for approx. 4mins then gets progressively higher to a max. after another 4 mins and then lower again over another 4 mins. This is continual.
Forgive my lack of physics but it looks like the seismic trace prior to and after an earthquake like in the movies.
I am baking out with transfer line at 290degC at the moment.
WK

[ingochrist]

This doen't sound like any background to me. I guess something is shot and needs replacement. I would also point to the multplier, but if you have two filaments, try using the other one. Also make sure that the signal cable isn't stuck somewhere or you see a visable defect on it.

Otherwise a source cleaning sometimes does wonders, too. I used to work for a company which made portable MS systems. If we had no clue what the source for a weired problem is, we took the MS apart and cleaned it. Most of the time, we were able to get it working after the cleaning and several tunes.

Regards,
Ingo

[WK]

Thanks Ingo,
I'm never really sure what the "source" is. I guess it refers in this case to the trap and filament?
I will shutdown the GCMS for Christmas today and will clean it up,change oil,change column,replace multiplier and replace filaments in the New Year. I will let you know what happens (if the thread doesn't lock up).
I would like to do these one at a time but its not good turning on/off so often - all need doing soon anyway.
Merry Christmas
PS I had a look at your good website - very interesting.

[Ron]

In an ion trap with internal ionization the trap is the ion source. I don't think you have a bad multiplier, at least I have never seen one fail in that manner. To me it sounds like either an injector or column problem, as it seems like the high background is related to a particular portion of the temperature program, and does seem like it could be something coming off the column.

[WK]

Hello all,
Hope you had a nice break and happy new year.
I have had the GC-MS shutdown for 10days and today I have come in to replace:
vacuum oil
multiplier
filaments
column
ferrules (siltite on interface now and vespel on injector).

I have swapped over to another injector port which I know is clean.
I have cleaned the silchrom trap in water then sonicated in dichloromethane.
I have sonicated quartz spacers in methanol.

Now on bakeout - let you know in the morning!!

WK

[WK]

Sorry for the delay.
I have a working GC-MS now. I don't know the precise cause since I made too many changes at once. My guess would be dirty injector/column.
I've now run out of calgas - grrrhhh! So I will top it up.
Thanks for all posts
WK