Patent ID: 8766171

Claim:
A method of processing ions in a linear ion trap, the method comprising: a) establishing and maintaining a two-dimensional substantially quadrupole field, the field comprising a quadrupole harmonic of amplitude A 2 and an octopole harmonic of amplitude A 4 , wherein A 4 is greater than 0.01% of A 2 , A 4 is less than 5% of A 2 , and, for any other higher order harmonic with amplitude An present in the field, n being any integer greater than 2 except 4, A 4 is greater than ten times An; wherein the quadrupole mass filter comprises a first pair rods, a second pair of rods and two auxiliary electrodes interposed between the first pair of rods and the second pair of rods, 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 two auxiliary electrodes at an auxiliary frequency equal to the first frequency and in the first phase, iv) a DC voltage to the two auxiliary electrodes and b) introducing ions to the field.