A method of mass filtering ions comprising: mass filtering ions using a first mass filter so as to mass selectively transmit only ions having a first range of mass to charge ratios; and mass filtering the ions transmitted by the first mass filter using a second mass filter, wherein the second mass filter only transmits ions having a second range of mass to charge ratios that is a sub-set of the first range of mass to charge ratios; wherein the first mass filter and the second mass filter are located in a single vacuum chamber; characterised in that: at least one electrode of the first mass filter comprises an aperture extending entirely through the electrode and/or comprises a recess extending only partially through the electrode, wherein the aperture and/or recess is arranged and configured such that ions that are unstable in the first mass filter pass into or through the aperture and/or into the recess such that they are not transmitted by the first mass filter; and the ions transmitted by the first mass filter are guided into the second mass filter using a RF-only ion guide arranged between the first and second mass filters, wherein the RF-only ion guide does not comprise slotted apertures or grooved recesses, and wherein the RF-only ion guide is also located in said single vacuum chamber. mass filtering ions using a first mass filter so as to mass selectively transmit only ions having a first range of mass to charge ratios; and mass filtering the ions transmitted by the first mass filter using a second mass filter, wherein the second mass filter only transmits ions having a second range of mass to charge ratios that is a sub-set of the first range of mass to charge ratios; wherein the first mass filter and the second mass filter are located in a single vacuum chamber; characterised in that: at least one electrode of the first mass filter comprises an aperture extending entirely through the electrode and/or comprises a recess extending only partially through the electrode, wherein the aperture and/or recess is arranged and configured such that ions that are unstable in the first mass filter pass into or through the aperture and/or into the recess such that they are not transmitted by the first mass filter; and the ions transmitted by the first mass filter are guided into the second mass filter using a RF-only ion guide arranged between the first and second mass filters, wherein the RF-only ion guide does not comprise slotted apertures or grooved recesses, and wherein the RF-only ion guide is also located in said single vacuum chamber. at least one electrode of the first mass filter comprises an aperture extending entirely through the electrode and/or comprises a recess extending only partially through the electrode, wherein the aperture and/or recess is arranged and configured such that ions that are unstable in the first mass filter pass into or through the aperture and/or into the recess such that they are not transmitted by the first mass filter; and the ions transmitted by the first mass filter are guided into the second mass filter using a RF-only ion guide arranged between the first and second mass filters, wherein the RF-only ion guide does not comprise slotted apertures or grooved recesses, and wherein the RF-only ion guide is also located in said single vacuum chamber.