Never heard of introducing air into the system. You might have a contaminated corona discharge needle where you are not getting a good charge on your gas and the excess air is giving more charges. I would check my connection where the corona needle plugs into and also abrasively clean my needle.
BTW what kind of samples are you running?
Samples that work well by APCI are those that:
1. Are small molecules, polar to non-polar: PAHs, PCBs, Fatty Acids, Phthalates.
2. Contain heteroatoms: Ureas, benzodiazepines,carbamates.
Samples to avoid for APCI are those that:
1. Typically are multiply charged in solution: Proteins,peptides
2. Are very thermally unstable: