A mass spectrometer having a multipole ion guide device, comprising: an electronic controller; a plurality of main electrodes (105, 106, 107, 108) operably connected to the electronic controller and an RF power source for applying RF voltages in the multipole ion guide device under operation of the electronic controller; at least one auxiliary electrode (111, 112, 113, 114) connected to a DC voltage source via the controller, the at least one auxiliary electrode (116, 117, 118, 119) disposed between at least two adjacent ones of the main electrodes (105, 106, 107, 108), characterised in that the at least one auxiliary electrode (111, 112, 113, 114) comprises: one or more curved thin plates forming one or more curved substrates (116, 117, 118, 119), which are positioned between curved ones of the at least two adjacent main electrodes (105, 106, 107, 108); electrical elements including at least one array of finger electrodes and a plurality of resistors interconnecting respective finger electrodes of the at least one array, the at least one array of finger electrodes being disposed upon the one or more curved thin plates so that the finger electrodes and the resistors are supported by the one or more curved substrates (116, 117, 118, 119); wherein the voltage source applies a static DC voltage to the electrical elements such that the finger electrodes present a monotonically progressive voltage gradient on respective finger electrodes of the array along a length of the auxiliary electrode (111, 112, 113, 114), an electronic controller; a plurality of main electrodes (105, 106, 107, 108) operably connected to the electronic controller and an RF power source for applying RF voltages in the multipole ion guide device under operation of the electronic controller; at least one auxiliary electrode (111, 112, 113, 114) connected to a DC voltage source via the controller, the at least one auxiliary electrode (116, 117, 118, 119) disposed between at least two adjacent ones of the main electrodes (105, 106, 107, 108), characterised in that the at least one auxiliary electrode (111, 112, 113, 114) comprises: one or more curved thin plates forming one or more curved substrates (116, 117, 118, 119), which are positioned between curved ones of the at least two adjacent main electrodes (105, 106, 107, 108); electrical elements including at least one array of finger electrodes and a plurality of resistors interconnecting respective finger electrodes of the at least one array, the at least one array of finger electrodes being disposed upon the one or more curved thin plates so that the finger electrodes and the resistors are supported by the one or more curved substrates (116, 117, 118, 119); wherein the voltage source applies a static DC voltage to the electrical elements such that the finger electrodes present a monotonically progressive voltage gradient on respective finger electrodes of the array along a length of the auxiliary electrode (111, 112, 113, 114), one or more curved thin plates forming one or more curved substrates (116, 117, 118, 119), which are positioned between curved ones of the at least two adjacent main electrodes (105, 106, 107, 108); electrical elements including at least one array of finger electrodes and a plurality of resistors interconnecting respective finger electrodes of the at least one array, the at least one array of finger electrodes being disposed upon the one or more curved thin plates so that the finger electrodes and the resistors are supported by the one or more curved substrates (116, 117, 118, 119); wherein the voltage source applies a static DC voltage to the electrical elements such that the finger electrodes present a monotonically progressive voltage gradient on respective finger electrodes of the array along a length of the auxiliary electrode (111, 112, 113, 114),