On LCQ Advantage (Finnigan) we observe a dramatical loss of sensitivity (intensity of parent ion) when we narrow down the Isolation Width to a certain value. Example: Reserpine - intensity 1000times lower when changing IW from 1 to 0.9. Other substance - intensity 100times lower when changing IW from 1.2 to 1.1...
Drop in sensitivity due to not effective isolation of ions by a non optimal isolation width is mentioned in the instrument manual. However I would expect loss of intensity by a much lower value of IW...? The instrument was repeatedly recalibrated and serviced with no improvement..