Mass spectrometer collision/reaction cell multipole and method. The multipole may have first and second portions and an intermediate portion therebetween, the first and second portions operating at first and second q values lower than a third q value at the intermediate portion. A low-mass cut-off of the multipole may be controlled by varying a q value from a first to at least a second value. The multipole may have multipole electrodes disposed about a central axis and having a respective first portion, second portion, and intermediate portion therebetween which is radially closer to the central axis. This offers relatively high acceptance and ion transmission, while providing low-mass cut-off for removing undesired/interfering ions and helping reduce background count.