Hello Paul,

	This sounds exactly like two partially resolved ion beams. 
We use a similar peak shoulder to measure Fe isotopes with ICP-MS. 
What I can't figure out is what fragment is creating a beam with 
m/z=2.  I don't think an EI source can strip both electrons from a 
4He atom...

good luck,

Charles



>Merry Christmas, everyone. For those not already on holidays:
>
>Running dD on our Delta XL, I first noticed that I had a weird shape 
>when peak centering. Mass 2 shows shoulders on either side, with 
>flat tops. Mass 3 and all masses on the universal collectors seem 
>fine.
>
>H3 correction came in at an astonishing 0.7 (compared to the usual 
>7.0 to 8.0).
>
>Almost as astonishing is that I was able to make measurements. No 
>problems with drift, fluctuations, etc. Calibrations and multiple 
>standards all checked out. No loss of sensitivity. Good 
>repeatability.
>
>My first thought was the energy filter, but I think that is on the 
>mass 3 cup. My second thought was for the electron suppressor, as 
>that can give odd peak shapes, but I'm not convinced (I don't think 
>it explains the flat tops of the shoulders). No tuning parameters 
>seem to have an effect. Mass spec wasn't opened up for cleaning or 
>anything. Vacuum also fine.
>
>I can send a peak scan to those interested. The normal peak is 2000 
>to 2400 steps HV. There is a flat top shoulder (90% height) from 
>1800 to 2000 steps, and another (10% height) from 2400 to 2600 
>steps. Saw a similar shape with a magnet scan over the peak.
>
>Paul Eby
>
>
>[log in to unmask]
>Biogeochemistry Lab (E-Hut, room 102)
>School of Earth and Ocean Science
>University of Victoria
>
>PO Box 3055
>Victoria, B.C.
>Canada
>V8W 3P6
>
>phone: 250-721-6183
>fax:   250-472-4620

-- 

Charles Knaack
GeoAnalytical Laboratory
Geology Department
Washington State Univ.
Pullman, WA 99164-2812
(509)335-6742, FAX (509)335-7816
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