Negative polarity shows no signal

Hi,

We have a QSTAR XL system which seems to be working well in positive mode. We get nice signal with more than 8000 resolution. But when we try the system with negative polarity, no signal goes through at all. The TOF mass scan runs and we get like 5-10 counts per second at 2300 CEM. Q1 scan gets more counts per second but still no signal gets through. Tuning parameters do not seem to be doing much and the instrument panel in the analyst service diagnostics shows no fault. The only warning we get from time to time is Q3 current out of range on the event viewer which repairs itself. The negative polarity used to work fine until now.

Any thoughs on the matter would be greatly appreciated.

Thank you

Carlos

PD.- we have a QSTAR MS for parts so we can try different things.