Dear all members,
I am wondering if anyone has the experience working on the GCNCIMS (Agilent). Recently I find a sudden sensitivity drop of my instrument. It looks fine in the PCI and NCI tune reports. I also checked for air leak by following the procedures laid down in the hardware manual and it's okay. The ion source was just cleaned a month ago. Does anyone has any suggestions?

I would suspect a dirty repeller, especially if you are using samples with a complex matrix or samples that have been derivatized. I no longer use Agilent GCs, but I remember from when I did that there could be deposits on the repeller that could not be seen with the naked eye but that would affect performance. One of the unfortunate "features" of the Agilent software is that there are no real passing critera for a PCI or NCI tune. If the instrument tunes it passes, even though the performance may be substantially degraded.