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 >> > >