Geoff, I have a MAT 251 and Delta+ XL, each instrument has different tuning characteristics and isotopic / sample size linearities. You should tune each instrument for maximum linearity, or know what the voltage range is for an isotopic linearity that is acceptable for you. The new Isodat software versions have linearity measuring utility programs, we generally can get fairly good linearity from 0.5 to 7 volts in continuous flow on the Delta, and even lower voltage ranges on the 251 with a dual inlet system. Linearities will change over time as the source gets dirty and the filament wears, you may want to check this periodically, especially if your calibration standards shift or get more variable over time. It was not clear from your message, are you measuring voltages from 10 to 40 volts? If so this is well above any voltages we have tried to measure. You might repeat the measurements in the 1-7 volt range and see if you get better results. You are doing it the right way with tank CO2, but you might want to make sure you are not contaminating the CO2 by what ever method you get it from the tank to your IRMS. You also have to be careful of the isotopic difference between your machine standard and samples. If you are using tank CO2 which is generally fairly negative, and are trying to measure carbonate samples which are near 0 per mil, the isotopic difference between the two gases may be large and this will decrease your precision. Hope this is useful, Bill At 03:10 PM 5/14/2003 -0700, you wrote: >We recently installed a new Finnigan MAT253. After the install, we >discovered that the isotopic results being reported by ISODAT were varying >as a function of sample size. I would normally chalk this up to incomplete >sample combustion or acidification, but these samples were coming directly >from our CO2 standard tank. Finnigan has been totally unable to even >suggest what the problem might be or how to pursue it. I think the problem >is in how ISODAT (ver. 1.5) is processing the output from the 253, but its >not at all clear from the manual how to troubleshoot such a problem. There >is a -0.6 per mil reduction in d18O and a +0.3 per mil increase in d13C on >CO2 samples from the same tank ranging between 10K mV and 40K mV in size. >Any advise or tales of similar experiences would be greatly appreciated. > > > > > >Thanks, > >Geoff > > > > > >Geoffrey Garrison, Associate Researcher > >Department of Earth and Space Sciences University of Washington >63 Johnson Hall, Box 351310 > >Seattle, WA 98195 > >(206)616-8763, office > >(206)543-6327, lab > >[log in to unmask] > > Dr. William J. Showers Dept of Marine, Earth & Atm Sciences Box 8208 North Carolina State University Raleigh NC 27695 (919) 515 - 7143 Office (919) 515 - 7802 Fax (919) 515 - 3689 Lab (919) 515 - 7911 Field Lab [log in to unmask] Visit our RiverNet Web Site http://rivernet.ncsu.edu If entropy is increasing, where does it come from?