Dear all,


Our Finnigan Delta Plus XL mass spectrometer is connected to an EA and 
a TC/EA via a Finnigan MAT Conflo III split interface. We ran sulfur 
isotopes last week, and everything went alright until we switched back 
to CN.  When all the
references are turned off, mass 28, 29, and 30 backgrounds are very 
high, (~ 3-3.5 V).But mass 44 and 2 are normal (<0.01 V for mass 44, 
and ~0.2 V for mass 2). The high background goes to normal when the 
needle valve, where the gas comes into the Delta Plus, is turned off.  
It looks like air is getting into the He somewhere upstream of the 
Delta Plus needle valve. Our EA and TC/EA have no leaks, neither do the 
needle valve nor the joints where gases come in and out of the Conflo 
box.  The gauges on the ion sources are normal (10^-3 and 10^-6 mbar 
with He flowing).  I double checked the collector and the resistor on 
Channel 3 is set properly. We switch this resistor to run sulfur 
isotopes regularly every year, and haven’t had trouble before.  The 
Helium tanks are still the same ones we used before and during the 
sulfur isotope run.

So if anyone here knows what is going on, or have any suggestions, 
could you please let me know? Thanks a lot.


Majie Fan

Department of Geosciences
University of Arizona
Tucson, AZ 85721


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