The regular mass range of the monopole mass spectrometer can be extended to very high mass numbers if at the end of the regular mass range, the maximum ac rod voltage is kept constant and the dc rod voltage is reduced. During this extended mass range the mass is inversely proportional go this dc voltage. If the ion energy is kept constant the sensitivity stays essentially constant over the whole mass range and the resolution is inversely proportional to the mass. If the ion energy is reduced in proportion to the dc voltage the resolution stays constant, but the sensitivity is reduced. This method of mass scan permits with excellent sensitivity the detection of vacuum system contaminants of very high molecular weight. The total scanning time necessary to cover this wide mass range is much shorter than the time required for a regular scan over the same mass range.