Thermionic Detector

[Ashraf_Khan]

I'm using a 6890 GC with an NPD and replaced the ceramic bead yesterday. But the stabilized output signal drops from 25 to 5 pA or lower after a while. Even when the Adjust Offset and Bead Voltages are set to zero, the output is 1.5 pA. Several 'pop' sounds are heard before the signal drops again to 1.5. When the detector is stabilized, I inject 1ul of methanol solvent with 40 ppm of nitrobenzene, but there is a big 'lump' after 30 sec (due to quenching' effect ?) and no nitrobenzene peak at all (DB-Wax, 30m x 0.53 mm, 1.5 u; splitless inlet 250 C, He Linear Velocity 42 cm/sec and Oven 40 - 200 C @ 10C/min). Agilent sent some troubleshooting guidelines, but it didn't fix the problems. Any suggestion would be appreciated.

AZK

[Ashraf_Khan]

Is a GC with a thermionic detector (NPD - Nitrogen Phophorous) more sensitive, selective, and reliable than a GC with a Chemiluminescence detector (Nitrogen Chemiluminescence) when it comes to analyze organic nitroso-compounds ? Or the opposite ?