Quadrupole mass filter for charged particles

A mass filter for charged particles includes a cylindrical conductive housing at ground potential and an array of linear conductors arranged in parallel in the housing and divided equally into four subarrays. The conductors of each subarray lie in one of four planes having a tubular arrangement with a square cross section on the longitudinal axis of the housing. The conductors have a substantially uniform distribution in the planes. A dedicated voltage is applied to each conductor in each subarray, the voltages being selected cooperatively and with the dimensions of the housing and the square cross section, such that a quadrupole type of electric field is generated within the tubular arrangement. Equations are disclosed for determining the voltages.