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