GCMS Sensitivity loss and retention time shift

[Riverfish]

I wonder if anyone can suggest what is wrong.

I am analysing 16 PAHs by GCMS using a method we have been using unmodified for some time now. In our latest run I noticed that the peak areas of my standard were decreasing from run to run. also the retention time of the first couple of compounds shifts about 5 seconds over the course of 11 injections of the same standard (different vials) although the latter peak times don't shift at all.

Can anyone think of a reason?

Thanks in anticipation of a reply.

[Ron]

A decrease in area with a longer retention time is usually a sign of a leak in the inlet system. This affects early eluting compounds more than late eluters, but usually all retention times are affected to some extent.

[Ron]

A decrease in area with a longer retention time is usually a sign of a leak in the inlet system. This affects early eluting compounds more than late eluters, but usually all retention times are affected to some extent.

[Riverfish]

Thanks for that, we've done an air-water check but no evidence of a leak, however, I know how fiddly these inlets are,I 've inherited three septumless head, cryo-cooled PTV injectors and I've still to get used to them.