and stability is thereby eroded in present practice the mass spectrometer must then be disassembled and the film removed either by mechanical means or chemical treatment this is undesirable as it is not only costly but it defeats the basic intention of having the mass spectrometer operate on a continuous basis over an extended period of time to solve this problem the bias on the extractor plate 48 is selected so that a preselected amount of electrons is allowed to pass with the ions into the filter chamber 46 because of the magnet 34 and the arrangement of filament 32 as described above relative to axis 60 a general direction of electron migration or flow toward and into the filter and toward the sensor will occur generally along the axis 60 this general direction of electron flow may be enhanced by an additional magnet 82 these electrons in the filter chamber combine with the ions other than those having the pre selected mass to charge ratio and in the area of the quadrupole filter 50 i e on the filter elements as described above so that the negatively charged electrons can combine with and thus neutralize the positive ions these neutralized ions will eventually leave the filter under the influence of the ion pump and will not interfere with the performance of the quadrupole filter 50 another magnet 84 is present at the far end 86 of the filter chamber 46 to remove.