I know that this topic was up for an Agilent and an Elan 9000 before, however i've tried all the suggestions posted with no success. I have a NexION 350d with high backgrounds on heavy masses (around 30 cps on mass 220). The background seems to increase with increasing mass. So far i ruled out the QID and the quad being faulty. I've tried the discriminator treshhold but it seems that it does not affect the background at all even when being at high values. Putting deflector voltage to -100 does not show any backround in low masses but increases when looking at higher masses. I've also replaced the detector, but this did not help at all. The background appeared from one day to another, did not see any slow increase over time.
Thanks in advance for all adivce on how to tackle this issue!