Hi Herb, 

Check the mass 46 background...if it's high (e.g. 20-100mV) than there is an
air leak causing NO2 (mass46) formation in the IRMS.

Mark

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
Of Herbert Tobias
Sent: Monday, October 15, 2007 1:01 PM
To: [log in to unmask]
Subject: [ISOGEOCHEM] Dramatic Sensitivity Decrease

Hello All,

I have a question about a MAT 252 IRMS.  I vented the instrument to replace
the sampling capillary.  I let it pump down for ~2hrs.  When testing CO2 gas
pulses from a dual inlet volume and injected CO2 from a GCC interface, the
signal was 10x lower than normal.  This means its the IRMS itself.  I tuned
the lenses, and now got 4x lower signal than normal, better but not good.
The voltages and currents on the filament are normal.
Has anyone seen this problem and know how to resolve it?

Thanks in advance...

cheers,
-Herb
Cornell University