A mass spectrometer comprising: an ion source configured to generate ions, from a sample of interest, in a high pressure region; a first vacuum chamber extending between an inlet aperture, configured to receive the ions generated by the ion source from the high pressure region, and an exit aperture, positioned downstream from the inlet aperture and configured to transmit at least a portion of said ions from the first vacuum chamber to a second vacuum chamber maintained at a lower pressure relative to the first vacuum chamber; at least one ion guide (106, 506) disposed within the first vacuum chamber between the inlet aperture and the exit aperture, the at least one ion guide comprising: a plurality of rods (130, 530) comprising at least a first pair of rods (130a, 530a) and a second pair of rods (130b, 530b), extending along a central longitudinal axis from a proximal end disposed adjacent the inlet aperture to a distal end, the plurality of rods being spaced apart from the central longitudinal axis and configured to define an internal volume within which the ions received through the inlet aperture are entrained by a flow of gas; and a plurality of auxiliary electrodes (140, 540) extending along at least a portion of the ion guide, each auxiliary electrodes being interposed between a single rod of the first pair of rods and a single rod of the second pair of rods; and a power supply (193, 195, 197) coupled to the ion guide; the power supply being configured to provide a first RF voltage at a first frequency and a first phase to the first pair of rods and a second RF voltage at the first frequency and a second phase to the second pair of rods for radially confining the ions within the internal volume; characterized in that: the first vacuum chamber is maintained at a pressure above about 500 mTorr; and the power supply is further configured to provide an auxiliary electrical signal to at least one of the auxiliary electrodes to overlay an asymmetric DC field, thereby to radially deflect from the internal volume at least a portion of low mass-to-charge ratio (m/z) ions from their average trajectory along the central longitudinal axis so as to prevent transmission of said low m/z ions through the exit aperture. an ion source configured to generate ions, from a sample of interest, in a high pressure region; a first vacuum chamber extending between an inlet aperture, configured to receive the ions generated by the ion source from the high pressure region, and an exit aperture, positioned downstream from the inlet aperture and configured to transmit at least a portion of said ions from the first vacuum chamber to a second vacuum chamber maintained at a lower pressure relative to the first vacuum chamber; at least one ion guide (106, 506) disposed within the first vacuum chamber between the inlet aperture and the exit aperture, the at least one ion guide comprising: a plurality of rods (130, 530) comprising at least a first pair of rods (130a, 530a) and a second pair of rods (130b, 530b), extending along a central longitudinal axis from a proximal end disposed adjacent the inlet aperture to a distal end, the plurality of rods being spaced apart from the central longitudinal axis and configured to define an internal volume within which the ions received through the inlet aperture are entrained by a flow of gas; and a plurality of auxiliary electrodes (140, 540) extending along at least a portion of the ion guide, each auxiliary electrodes being interposed between a single rod of the first pair of rods and a single rod of the second pair of rods; and a plurality of rods (130, 530) comprising at least a first pair of rods (130a, 530a) and a second pair of rods (130b, 530b), extending along a central longitudinal axis from a proximal end disposed adjacent the inlet aperture to a distal end, the plurality of rods being spaced apart from the central longitudinal axis and configured to define an internal volume within which the ions received through the inlet aperture are entrained by a flow of gas; and a plurality of auxiliary electrodes (140, 540) extending along at least a portion of the ion guide, each auxiliary electrodes being interposed between a single rod of the first pair of rods and a single rod of the second pair of rods; and a power supply (193, 195, 197) coupled to the ion guide; the power supply being configured to provide a first RF voltage at a first frequency and a first phase to the first pair of rods and a second RF voltage at the first frequency and a second phase to the second pair of rods for radially confining the ions within the internal volume; characterized in that: the first vacuum chamber is maintained at a pressure above about 500 mTorr; and the power supply is further configured to provide an auxiliary electrical signal to at least one of the auxiliary electrodes to overlay an asymmetric DC field, thereby to radially deflect from the internal volume at least a portion of low mass-to-charge ratio (m/z) ions from their average trajectory along the central longitudinal axis so as to prevent transmission of said low m/z ions through the exit aperture. the first vacuum chamber is maintained at a pressure above about 500 mTorr; and the power supply is further configured to provide an auxiliary electrical signal to at least one of the auxiliary electrodes to overlay an asymmetric DC field, thereby to radially deflect from the internal volume at least a portion of low mass-to-charge ratio (m/z) ions from their average trajectory along the central longitudinal axis so as to prevent transmission of said low m/z ions through the exit aperture.