hI Simon--

Do you have a leak opening up at high temp?  Just wondering, you didn't
mention looking at Ar while ramping the GC, or using Ar to check for
leaks.  Also doing small CO2 injections (or Ar) at low and high temp can
help you find the problem.

Do you have enough He pressure to maintain flow rates?  We need ~80 psi
from the tank to maintain 2 mL/min flow rates at 320.  Occassionally the
regulator gets bumped when working on other tanks, and if it's not
brought back up to pressure, flow rates drop at high temp and peaks come
out late.

Back flush valve problems (one inside oven)?  I'm assuming the setup is
similiar to our GCC III.  We had one problem with a leak through the
valve stem (teflon seal) that was hard to trace, but I believe that was
an across the board leak.  Also check the bleed out line to make sure He
is actually bleeding out.  The connections can loosen over time.

Press fit connectors?  I used them in the past, and they've been
great--until recently.  Make sure they haven't opened up on you
(methanol on the outside or Ar leak check).

good luck

gerry

www.geology.yale.edu/escsis/ESCSIS-web1.html



Simon Eaton wrote:

>We are experiencing great problems with a high-eluting (320
>degrees C) compound by GC-IRMS in hydrogen mode. Analysis by
>GCMS is fine, and the same derivative (propyl-chloroformate /
>methanol) works fine on GC-IRMS for lower temperature eluting
>amino acids. We get almost nothing on exactly the same sample for
>which we get a very good response in GCMS.
>
>I presume the problem is one of activity somewhere in the system
>(Delta XP with Agilent 6890 and high-temperature conversion
>interface). We are using Restek Siltek liners (which work well on
>GC-MS) so I expect the problem is either (i) the metal cross piece
>between the column and the interface or (ii) the junction betweeen
>the fused silica and the ceramic reactor tube. We have tried BGB
>analytik deactivated press fit connectors instead of the metal T-piece
>(Valco cross piece) but this does not improve matters. We are
>currently conditioning the ceramic reactor via a methane back flush,
>and the union with the fused silica is made exactly as recommended
>by Thermo (i.e. burning off 5mm of the polyimide coating).
>
>Any ideas?
>
>Simon
>
>--
>Simon Eaton,
>Senior Lecturer in Paediatric Surgery and Metabolic Biochemistry
>Unit of Paediatric Surgery
>Institute of Child Health
>30 Guilford Street
>London WC1N 1EH
>Phone + 44 (0)20 7905 2158 (Office, Answer)
>      + 44 (0)20 7905 2396 (Laboratory)
>Fax   + 44 (0)20 7404 6181
>Mobile + 44 (0) 7762 180877
>
>
>