At Gilles and others suggestion, I checked to see if the amplifiers  
were zeroes with the source off. they were/are. I ran the amplifier  
test in the diagnostics program and came back with zeroes, but it was  
only finding one gas configuration (CO) not all of them (N2 and CO2  
being left out).
Then I tried a mass calibration as Sergey suggested. The mass  
calibration cannot be completed because instead of shifting the mass  
44 BDAC value to a lower magnet field (from start at 11637) as it is  
supposed to (and does for masses 12 and 28), it shifts to a higher  
magnet field (19595) after the second cycle and then cannot find the  
peak on the third cycle.
We have a newly installed (in June and July) Delta V Advantage running  
with Isodat 2.5.
Any more suggestions or do I need to call in the Thermo service  
engineer? It is under warranty afterall!
-- 
Charlotte Lehmann
Research Technician
Bates College
Department of Geology
206A Carnegie Science Building
44 Campus Avenue
Lewiston, ME 04240
Phone: 207-786-6485
FAX: 207-786-8334


Quoting "Oleynik, Sergey A." <[log in to unmask]>:

> Charlotte,
> 1. I think you have checked amplifiers with ion source turned off and they
> had zeros.
> 2. It seems your 44 beam is very close to your 45 cup. Some 44 ions hit
> the edge of 44 cup and reflect to 45 cup. The remedy is mass calibration.
> Hope it will help.
> Sergey.
>
> --
> Sergey Oleynik, PhD
> Dept. Earth and Atmospheric Sciences
> Purdue University
> 550 Stadium Mall Drive
> West Lafayette, IN 47907
> Phone: 765-494-3274
>
> On Thu, August 31, 2006 11:20 am, Charlotte Lehmann wrote:
>> I am wondering if anyone can suggest why the Mass45 background (BGD)
>> value would be negative, and how to correct for this. I am also
>> getting very depleted isotopic values to go along with the negative
>> BGD value.
>> --
>> Charlotte Lehmann
>> Research Technician
>> Bates College
>> Department of Geology
>> 206A Carnegie Science Building
>> 44 Campus Avenue
>> Lewiston, ME 04240
>> Phone: 207-786-6485
>> FAX: 207-786-8334
>>
>
>