Residual gas analyzer

Hi, I am having issues with Residual gas analyzer. The response of the Faraday cup detector to the residual gases inside the vacuum chamber
( <1X10-6 Torr) shows no variation for Ar, N2, O2 and H2O. All show the same intensity. When Ar is let in at <5 mTorr through the metering valve, the detector shows a jump in the intensity of all the gases. Again all gases are shown to have same intensity. And when the valve in closed the intensity drops to the vacuum level. I am unable to find a solution to this discrepancy. I am using a VG Fison system. However, I dont have an operating manual for trouble shooting. Can somebody help? Thanks.
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