We see reduced sensitivity in APCI source when polarity is negative. Technical support advised to loosen the findings that hold the nebulizer in place to introduce air into the source. The rational is that the gases are to pure going into the source causing charge ground in the source - no ions detected. There is no signal at all. The solution of leaving the nebulizer untightened does work; however, this is not a good permanent fix because the nebulizer housing is not secure and could cause inconsistency in results. In general the APCI methodology we use has a long way to go. Please post tips and tricks on this specific problem and general suggestions about APCI methodology.