Method of operating quadrupoles with added multipole fields to provide mass analysis in islands of stability

1. A method of processing ions in a quadrupole rod set, the method comprising

a) establishing and maintaining a two-dimensional substantially quadrupole field for processing the ions, the field having a quadrupole harmonic with amplitude A 2 and a selected higher order harmonic with amplitude A m wherein m is an integer greater than 2, and the magnitude of A m is greater than 0.1% of the magnitude of A 2 ;

b) introducing the ions to the two-dimensional substantially quadrupole field and subjecting the ions to both the quadrupole harmonic and the higher order harmonic of the field to radially confine ions having Mathieu parameters a and q within a stability region defined in terms of the Mathieu parameters a and q;

c) adding an auxiliary excitation field to transform the stability region into a plurality of smaller stability islands defined in terms of the Mathieu parameters a and q; and,

d) adjusting the two-dimensional substantially quadrupole field including the auxiliary excitation field to place ions within a selected range of mass-to-charge ratios within a selected stability island in the plurality of stability islands to impart stable trajectories to the selected ions within the selected range of mass-to-charge ratios for transmission through the rod set, and to impart unstable trajectories to unselected ions outside of the selected range of mass-to-charge ratios to filter out such ions.

2. The method as defined in claim 1 wherein the selected higher order harmonic with amplitude A m is one of (i) a hexapole harmonic such that A m is A 3 , and (ii) an octopole harmonic such that A m is A 4 .

3. The method as defined in claim 2 wherein the rod set comprises a plurality of substantially cylindrical rods.

4. The method as defined in claim 2 further comprising passing the selected ions through the quadrupole rod set and subsequently detecting the selected ions.

5. The method as defined in claim 2 wherein the magnitude of A m is greater than 1% and is less than 20% of the magnitude of A 2 .

6. The method as defined in claim 2 wherein the magnitude of A m is greater than 1% and is less than 10% of the magnitude of A 2 .

7. The method as defined in claim 2 , wherein the auxiliary excitation field is an auxiliary quadrupole excitation field.

8. The method as defined in claim 7 , wherein the rod set comprises:

a quadrupole axis;

a first pair of rods, wherein each rod in the first pair of rods is spaced from and extends alongside the quadrupole axis; and

a second pair of rods, wherein each rod in the second pair of rods is spaced from and extends alongside the quadrupole axis.

9. The method as defined in claim 8 , wherein step (a) comprises providing an at least partially-AC potential difference between the first pair of rods and the second pair of rods at a selected frequency to provide the two-dimensional substantially quadrupole field.

10. The method as defined in claim 9 , wherein step c) comprises i) determining an excitation frequency of the auxiliary excitation field as a function of the selected frequency, and ii) providing the auxiliary excitation field at the auxiliary excitation frequency.

11. The method as defined in claim 10 wherein step c) i) comprises determining the excitation frequency to be N/M times the selected frequency, N and M being different integers.

12. The method as defined in claim 11 wherein step d) comprises determining the selected stability island to have a highest magnitude of the Mathieu parameter a in the plurality of stability islands.

13. The method as defined in claim 12 wherein step d) comprises determining a tip in the selected stability island, the tip being selected to have the highest magnitude of the Mathieu parameter a in the selected stability island, and then adjusting the two-dimensional substantially quadrupole field to place the selected ions within the tip.

14. The method as defined in claim 12 wherein step d) comprises determining a tip in the selected stability island, the tip being selected to have a lowest magnitude of the Mathieu parameter a in the selected stability island, and then adjusting the two-dimensional substantially quadrupole field to place the selected ions within the tip.