To produce high-end products, clean vacuum is often required. Even small amounts of high-mass molecules can reduce product yield. The challenge is to timely detect the presence of relevant contaminants. Here is where MFIG can help1. The massfiltered ion gauge sensor (MFIG) continuously and selectively monitors the presence of high-mass contaminant molecules with a sensitivity down to 1E-13 mbar at total pressuresup to 1E-5mbar. This contribution presents field-test data to demonstrate the capabilities of the latest MFIG sensor in continuously and selectively detecting high-mass contaminant moleculesin (U)HVvacuum.