Sue, Paul has a good point, in our case when mass 18 peak was higher than 2 V the reference peak values were unstable, and optimal m/z=18 was <1V (on the middle cup), cheers Dariusz Paul Eby wrote: > Sue, > > Another possibility: water. If mass 18 is high, you'll get a drift in > results for reference peak injections, quite separate from anything > happening in the GC. > > Paul > > > At 09:52 AM 28/04/2006, you wrote: >> Thanks Paul: >> >> I have cleaned the injector per manufacturers recommendation, but >> omitted the brushing step since it didn't look too bad. So perhaps it >> needs a more rigorous cleaning. >> >> It does seem as though the combustion column is to blame. I did get >> two decent runs of this standard before observing mass dependent >> fractionation. Prior to these last tests the new combustion oven was >> oxidized at 940C for 30 minutes followed by overnight at 540C. I'll >> go ahead and reoxidize at each temperature longer. Perhaps the >> reaction at 940 was not complete enough. >> >> Thanks again for your input! >> Sue >> >> On Apr 28, 2006, at 11:40 AM, Paul Eby wrote: >> >>> Sue, >>> >>> Two possibilities come to mind: your combustion oven, while new, >>> might not have enough oxygen. As the run progresses, oxygen is >>> depleted and results get worse. >>> >>> The other potential source might be your injector. If it isn't hot >>> enough, or it's dirty, you could get a mass dependent fractionation. >>> >>> Paul Eby >>> University of Victoria >>> >>> >>> At 09:29 AM 28/04/2006, you wrote: >>>> Hello All: >>>> >>>> I am seeking any possible hints to solve a problem that has developed >>>> in our GC-IRMS system. >>>> >>>> At the moment the suite of standards we run for d13C of FAMEs in >>>> order to check for consistency no longer exhibits a constant >>>> correction factor across all the compounds in each standard >>>> chromatogram which range from 14:0 through 23:0 in size. In fact we >>>> are seeing greater depletion in 13C for the longer (or later RT) >>>> FAMEs by as much as a few per mil relative to the first compounds >>>> (14:0). I have been trying to isolate the variables and this is what >>>> I've found so far: >>>> >>>> 1. Flow is constant throughout the entire GC run (2 mL/min). >>>> 2. CO2 injected throughout the run remains constant in it isotopic >>>> composition so does not exhibit the depletion see in compounds that >>>> must be combusted. >>>> 3. I figured it must be the combustion column so it has been replaced >>>> and we are again seeing the same problem. >>>> 4. Peak shape looks pretty good with widths remaining within 15-20s >>>> and fairly sharp ratio traces. The ratio traces do become a little >>>> broader with the last few more deplete compounds but not particularly >>>> worse than runs where we've gotten good consistent values. >>>> >>>> We are using an Agilent (HP) 6890 GC with a split/splitless injector >>>> in splitless mode. I have changed the septa (done frequently >>>> anyway), liner, and the gold seal and washer. The GC column is very >>>> new but also has been cut (`0.5m) to see if reactive sites may have >>>> been a problem. We are using the metal Valco cross piece and have >>>> been now for years without a problem, could this have developed >>>> active sites that may yield such symptoms? >>>> >>>> Thanks for any information anyone might have regarding this >>>> problem...I've been at this for some time now and am in danger of >>>> pulling out all my hair! >>>> >>>> Best regards, >>>> Sue >> >> Susan Ziegler >> Associate Professor >> Department of Biological Sciences >> University of Arkansas >> Fayetteville, AR 72701 >> 479.575.6944 (lab 6342) >> 479.575.4010 (fax) >> http://www.uark.edu/~susanz/ -- Dariusz Strapoc Department of Geological Sciences Indiana University, Bloomington 1001E 10th St. Bloomington, IN 47405 Tel.office (812) 856-4560 Tel.home (812) 331-0424 e-mail [log in to unmask]