Hi,

I am really hoping there is someone out there who might help me.

We have been running GC-IRMS (Delta XP) analyzing hydrogen.  All was going 
well until we noticed that the Box and Trap currents were noisy and 
fluctuating while we were set to Carbon for daily instrument checks.  They 
remained stable as long as Isodat was set to Hydrogen.

Because it was stable for Hydrogen I continued to run samples.  I figured I 
was seeing a little leakage or arcing and I would need to clean the source 
before I switched back to Carbon in a few weeks.

Then the filament blew.  I didn't want to take the time to clean the source 
so I swapped in a new filament (actually a rebuild which we have used 
before with good results).  Once things were back up the Trap and Box 
currents were stable, but where I normally saw currents for the Box at 0.7 
mA and Trap at 0.78 mA, the new readings were Box at 1.3 mA and Trap at 
0.19 mA.  I did have sample peaks and signal, but the sensitivity was only 
about one third of what I had before.

So I pulled the source and cleaned it (it was quite dirty), and installed a 
new Thermo filament.  Now I have a Box current of 1.5 mA and no Trap 
current at all.  The LED on the front of the Delta tells me there is no 
emission (obviously with no trap current there's no emission).

The folks at Thermo are sending me a new Source Controller Board to try 
out, but I replaced this board in Nov 2004.  A board a year is going to get 
pretty darned expensive!

Does anyone have any thoughts or something I might give a try before the 
board arrives?  Oh, because the symptoms were there before I took the 
source apart, teh folks at Thermo and I are relatively sure that I haven't 
put it together oddly, although if it seems to be a consensus I'll take it 
apart one more time....

Thanks,
Carolyn




_______________________________________
Carolyn Colonero
Isotope and Organic Geochemistry Laboratory
MIT/EAPS
E34-546
42-44 Carleton Street
Cambridge, MA 02142
lab: 617-324-4002
office: 617-253-7850