Asymmetrical features of mass spectral peaks produced by quadrupole mass filters

The individual mass spectral peaks produced by a quadrupole mass spectrometer (QMS) are asymmetric; they exhibit a 'tail' on the low mass side. In some cases a definite structure is observed in the tail. We show that the tail structure is a consequence of the use of circular electrodes. An extreme case of an experimentally observed QMS mass peak with a distinct tail structure is shown and the general form is reproduced using our numerical model. The effect of instrument resolution, length, operating frequency, ion energy, mass and ion source aperture upon the tail structure are considered. Results show that extensive long tails originate mainly from ions that enter the mass filter at a relatively large distance from the QMS axis; also no significant tail is produced in the case of ideal hyperbolic form electrodes of finite length.