Mass spectrometer that consists of an array of four parallel rod electrodes whose centers form the corners of a square and whose opposing electrode pairs are connected. The electric potential applied between the connected pairs of rods is a superposition of a static potential and a sinusoidal radio frequency potential. The motion of an ion in the \(x\mbox{-}y\) plane perpendicular to the length direction of the rod electrodes is described by the Mathieu equation whose solutions show that ions in a particular m/z range can be transmitted along the \(z\mbox{-}\rm{axis}\).