Hi Sue,
Did you check the reduction furnace? We had a similar issue before we replaced 
our reduction furance.  This seems to have fixed our precision issue.

Cheers,

Zheng-Hua


>===== Original Message From Stable Isotope Geochemistry 
<[log in to unmask]> =====
>Hi, Sue,
>
>  The  940C @ the oven may be high enough for organic compounds. As Paul
>suggested, O2 should be enough in your combustion oven, otherwise the
>later eluted compounds may not have enough O2 to be completely combusted
>and that will result the depletion in 13C for your later RT compounds.
>
>  You can try manual integration, but if the peak shape is good, the
>result may be as the same as the auto integration.  By the way, are you
>using the Finnigan or Isoprime IRMS?
>
>
>Wendy Zhang
>Air Quality Research Division
>Environment Canada
>4905 Dufferin Street
>Toronto, Ontario M3H 5T4
>Tel: (416) 739-5792 (office)
>Tel: (416) 739-5822 (lab)
>Fax: (416) 739-4664
>Email:[log in to unmask]
>
>
>
>-----Original Message-----
>From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
>Behalf Of Susan Ziegler
>Sent: April 28, 2006 11:52 AM
>To: [log in to unmask]
>Subject: Re: [ISOGEOCHEM] GC-C-IRMS issues
>
>
>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/