Thanks for the idea... I'll give that one a shot.

fingers crossed,
Greg

Gilles St-Jean wrote:
> Hi Greg 
>
> The likely culprit is a broken bleed capillary at the end of the backflush valve.  Is is usually found near the oven door.  It is often coilled to the side and hanging.  If you unroll it you will probably find it broken.  Replace it and your peaks should return to normal.  The dimentions are in the GC-C interface manual.
>
> Regards
>
> Gilles
>
>  
>
> Hi all,
> I'm working with a GC-C-IRMS system that has not been in use for some
> time ( a number of months), attempting to run amino acid samples for N15
> determination. Our system uses an Agilent 6890 GC, a Thermo GC III
> interface, and a Delta Plus XL mass spec. I'm using a Chirasil-val
> column for the GC separation (50 m x 0.25mm ID). The signal amplitude
> I'm getting is off the expected value by a bit better than a order of
> magnitude (factor of 20, give or take). I'd initially figured my either
> my preparatory chemistry was bad or there was an issue with the
> reactors, so I tried the argon test outlined in the GC III manual (ie,
> inject a couple of microliters of air and observe mass 40); the
> amplitude on this was off by approximately the same factor. My next
> guess was a dead volume, so I sequentially have tried bypassing
> different fittings and observing the response. Strangely, the thing that
> makes the big difference is the GC column itself. If I bypass the column
> with a couple of meters of deactivated silica capillary, the argon test
> comes out in the acceptable response range, so at first glance, at least
> all the fittings downstream of the column appear good. Also, the problem
> does not seem column-specific- I've tried other columns of the same
> physical dimensions but with a different stationary phase and I seem the
> same problem.
>
> The only thing I can think of is that the issue has something to do with
> operating pressure. I'm running the GC in constant flow mode, so in
> order to maintain that flow, the pressure through a 50 meter, small-bore
> column must be substantially greater than when the short bypass
> capillary is used. I have little personal GC-C-IRMS background (I'm
> learning by doing) and am running out of ideas to resolve this. Has
> anyone ever run into a situation like this before, or does anyone have
> sufficient expertise to hazard a guess as to the cause? Any thoughts are
> greatly appreciated. Thanks very much,
>
> Greg Ellis
> Paleoceanography/Biogeochemisty Lab
> University of South Florida College of Marine Science
>
>