Thank you all for your suggestions yesterday.
I finally got a good mass calibration. The Mass 45 BGD was still negative.
So, I tried a very simple thing: I changed the method ever so slightly  
to make the gas switch and He dilution come on 8 sec earlier. I think  
what happened is that even though I measured the He flow with a flow  
meter and reset it after my leak checks to the same value, the flow  
must have been slightly higher from changing the He tank. That is the  
only thing that had changed between a good run and the problem with  
the negative Mass 45 BGD values coming up.
Thanks again.
-- 
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