A method of processing ions in a linear ion trap, wherein the linear ion trap comprises a first pair of rods, a second pair of rods and four auxiliary electrodes interposed between the first pair of rods and the second pair of rods, and wherein the auxiliary electrodes of a diagonally oriented pair of auxiliary electrodes (12a, b) are closer to the other auxiliary electrodes (12b, a) than to each other, the method comprising: a) establishing and maintaining a two-dimensional substantially quadrupole field, wherein the field comprises a quadrupole harmonic of amplitude A2 and an octopole harmonic of amplitude A4, wherein A4 is greater than 0.01 % of A2, A4 is less than 5% of A2, and, for any other higher order harmonic with amplitude An present in the field, n being any integer greater than 2 except 4, A4 is greater than ten times An; wherein establishing and maintaining the field comprises providing: i) a first RF voltage to the first pair of rods at a first frequency and in a first phase, ii) a second RF voltage to the second pair of rods at a second frequency equal to the first frequency and in a second phase, opposite to the first phase, iii) an auxiliary RF voltage to the four auxiliary electrodes at an auxiliary frequency equal to the first frequency and in the first phase, and iv) a DC voltage to the four auxiliary electrodes; b) introducing ions to the field and axially transmitting a selected portion of the ions having a selected m/z from the field by providing a dipolar excitation AC voltage to either the first pair of rods or a diagonally oriented pair of auxiliary electrodes (12a, b) at a lower frequency than the first frequency to radially excite the selected portion of the ions having the selected m/z; c) detecting the selected portion of the ions to provide a measured mass signal peak centered about a measured m/z ratio; and d) adjusting at least one of the auxiliary RF voltage and the DC voltage provided to the four auxiliary electrodes to slide the measured m/z ratio toward the selected m/z. a) establishing and maintaining a two-dimensional substantially quadrupole field, wherein the field comprises a quadrupole harmonic of amplitude A2 and an octopole harmonic of amplitude A4, wherein A4 is greater than 0.01 % of A2, A4 is less than 5% of A2, and, for any other higher order harmonic with amplitude An present in the field, n being any integer greater than 2 except 4, A4 is greater than ten times An; wherein establishing and maintaining the field comprises providing: i) a first RF voltage to the first pair of rods at a first frequency and in a first phase, ii) a second RF voltage to the second pair of rods at a second frequency equal to the first frequency and in a second phase, opposite to the first phase, iii) an auxiliary RF voltage to the four auxiliary electrodes at an auxiliary frequency equal to the first frequency and in the first phase, and iv) a DC voltage to the four auxiliary electrodes; i) a first RF voltage to the first pair of rods at a first frequency and in a first phase, ii) a second RF voltage to the second pair of rods at a second frequency equal to the first frequency and in a second phase, opposite to the first phase, iii) an auxiliary RF voltage to the four auxiliary electrodes at an auxiliary frequency equal to the first frequency and in the first phase, and iv) a DC voltage to the four auxiliary electrodes; b) introducing ions to the field and axially transmitting a selected portion of the ions having a selected m/z from the field by providing a dipolar excitation AC voltage to either the first pair of rods or a diagonally oriented pair of auxiliary electrodes (12a, b) at a lower frequency than the first frequency to radially excite the selected portion of the ions having the selected m/z; c) detecting the selected portion of the ions to provide a measured mass signal peak centered about a measured m/z ratio; and d) adjusting at least one of the auxiliary RF voltage and the DC voltage provided to the four auxiliary electrodes to slide the measured m/z ratio toward the selected m/z.