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