A mass or mass to charge ratio selective ion trap comprising: a first device arranged and adapted either to: (i) generate a radially asymmetric pseudo-potential barrier or well which acts to confine ions in a first (y) and a second (x) direction within said ion trap, or to (ii) generate a pseudo-potential barrier or well which acts to confine ions in a first (y) direction and a DC potential barrier or well which acts to confine ions in a second (x) direction within said ion trap; a second device arranged and adapted to generate a substantially quadratic DC potential well which acts to confine ions in a third (z) direction within said ion trap; and a third device arranged and adapted to excite ions in said third (z) direction so as to mass or mass to charge ratio selectively eject ions in said second (x) direction and/or in said third (z) direction from an ion ejection region of said ion trap; wherein the trapping volume of the ion trap is extended in two spatial dimensions so that ions expand to fill a rectangular prism which is elongated in two spatial dimensions; and wherein the profile of said substantially quadratic DC potential well progressively varies along said second (x) direction so that an electric field is maintained along said second (x) direction, wherein the magnitude of the electric field in said second (x) direction varies with position in said third (z) direction so that said electric field electric field urges, channels or directs ions towards said ion ejection region wherein said electric field causes ions to experience substantially different acceleration fields in said second (x) direction dependent upon the relative position of said ions in said third (z) direction. a first device arranged and adapted either to: (i) generate a radially asymmetric pseudo-potential barrier or well which acts to confine ions in a first (y) and a second (x) direction within said ion trap, or to (ii) generate a pseudo-potential barrier or well which acts to confine ions in a first (y) direction and a DC potential barrier or well which acts to confine ions in a second (x) direction within said ion trap; (i) generate a radially asymmetric pseudo-potential barrier or well which acts to confine ions in a first (y) and a second (x) direction within said ion trap, or to (ii) generate a pseudo-potential barrier or well which acts to confine ions in a first (y) direction and a DC potential barrier or well which acts to confine ions in a second (x) direction within said ion trap; a second device arranged and adapted to generate a substantially quadratic DC potential well which acts to confine ions in a third (z) direction within said ion trap; and a third device arranged and adapted to excite ions in said third (z) direction so as to mass or mass to charge ratio selectively eject ions in said second (x) direction and/or in said third (z) direction from an ion ejection region of said ion trap; wherein the trapping volume of the ion trap is extended in two spatial dimensions so that ions expand to fill a rectangular prism which is elongated in two spatial dimensions; and wherein the profile of said substantially quadratic DC potential well progressively varies along said second (x) direction so that an electric field is maintained along said second (x) direction, wherein the magnitude of the electric field in said second (x) direction varies with position in said third (z) direction so that said electric field electric field urges, channels or directs ions towards said ion ejection region wherein said electric field causes ions to experience substantially different acceleration fields in said second (x) direction dependent upon the relative position of said ions in said third (z) direction.