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