Stable Isotope Geochemistry


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richard hughes <[log in to unmask]>
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Stable Isotope Geochemistry <[log in to unmask]>
Mon, 16 Feb 2004 20:43:49 -0600
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Another possibility is to reverse the polarity of the leads supplying
current to the filament.  (Suggested to me once by Gilles St-Jean and
Peter Stow).  I got some improvement once by doing this, then two days
later the sensitivity dropped again.  I reversed the leads again (back
to their original polarity) re-tuned and have had good sensitivity ever

Sometimes things just change.  I don't know why.

Good luck,
Richard Hughes

Ulrich Flenker wrote:
> On Mon, 16 Feb 2004, Chenery, Carolyn A wrote:
> > We recently replaced the filament and cleaned the source on our Delta Plus
> > XL continuous flow mass spec (with ConFlo III, a TC/EA and a Flash EA).
> > The sensitivity is now less than 1/2 of what we had before.  Previously we
> > had a signal on mass 45 of  5.06V for 1 bar CO2 now it is only 2.1V
> > During tuning I noticed the signal does not alter when the trap voltage is
> > changed and to get maximum intensity the X deflection, Electron energy, X
> > focusing voltage and Y1 are at maximum.  All the other settings are in the
> > middle range.
> > I have had the source out again and checked all the component positioning,
> > orientations and pin connections.  The relative intensities for mass 44, 45
> > and 46 are normal and peak shapes aren't bad.  The mass calibration has and
> > the HV DAC have not changed significantly.  The Ion gauge reading is 4.4e-6
> > as it always has with the needle valve open.  As far as I can tell there is
> > no problem with the gas inlet.
> > Has any one had similar problems and can you suggest the cause (and a cure).
> > Many thanks,
> > Carolyn
> >
> > Carolyn A. Chenery
> >
> Carolyn,
> some time ago we had a similar problem. The reason was that the filament
> (the wire itself, not the complete kathode) wasn't mounted in centre.
> Subtle deviations may cause severe problems here. Use a magnifying glass
> and carefully have a look at the filament.
> --
>         Uli Flenker
>         Institute of Biochemistry
>         German Sport University Cologne
>         Carl-Diem-Weg 6
>         50933 Cologne
>         +49(0)221/4982-5060