Quadrupole ion optical device

1 . A quadrupole ion optical device, for arrangement in a path of each of a plurality of ion beams exiting from a mass analyser towards a plurality of detector elements of a mass spectrometer, the plurality of ion beams being laterally separated at an exit from the mass analyser, the separation between the plurality of ion beams being proportional to the massto-charge ratio of ions in each of the plurality of ion beams, the quadrupole ion optical device comprising:

a plurality of electrodes, arranged around a central axis and configured to generate a quadrupole potential through which the path of each of the plurality of ion beams can be passed, an application of voltages to the plurality of electrodes generating a quadrupole potential in a region bounded by the plurality of electrodes; and

electrical circuitry configured to supply at least a first set of voltages or a second set of voltages to the plurality of electrodes, each voltage of the first or second set of voltages to be applied to one or more electrodes of the plurality of electrodes;

wherein application of the first set of voltages generates a first quadrupole potential having a saddle point at a first position in a plane normal to the central axis that optimises an alignment of a first group of ion beams of interest, which are included in the plurality of ion beams, into a first respective set of detector elements of the plurality of detector elements by minimising an angle of incidence between each of the ion beams of interest of the first group and a direction normal to a detection surface of the respective detector element of the first set of detector elements at which the ion beam is received;

wherein application of the second set of voltages generates a second quadrupole potential having a saddle point at a second position, different from the first position, in the plane normal to the central axis that optimises an alignment of a second group of ion beams of interest, which are included in the plurality of ion beams, into a second respective set of detector elements of the plurality of detector elements by minimising an angle of incidence between each of the ion beams of interest of the second group and a direction normal to a detection surface of the respective detector element of the second set of detector elements at which the ion beam is received;

wherein the first and second groups of ion beams of interest differ in mass-to-charge ratio and the first and second respective sets of detector elements are oriented at different angles;

wherein the quadrupole ion optical device is configured to provide a continuous flow of the plurality of ion beams from the mass analyser to the plurality of detector elements, without trapping ions within the quadrupole ion optical device.

2 . The quadrupole ion optical device of claim 1 , wherein the second position of the saddle point of the second quadrupole potential in the plane normal to the central axis upon application of the second set of voltages is displaced from the central axis.

3 . The quadrupole ion optical device of claim 1 , wherein the plurality of electrodes comprises six or more electrodes.

4 . The quadrupole ion optical device of claim 1 , wherein the electrodes are arranged such that, in the plane normal to the central axis, the region bounded by the plurality of electrodes extends further in a first direction than in a second direction, wherein the first and the second direction are orthogonal.

5 . The quadrupole ion optical device of claim 1 , wherein in the plane normal to the central axis each of the plurality of electrodes has an equal width.

6 . The quadrupole ion optical device of claim 1 , wherein in a plane normal to the central axis at least two of the electrodes of the plurality of electrodes have a different width, wherein the width of each electrode of the plurality of electrodes is configured to generate at a first predetermined location the saddle point of the first quadrupole potential in the plane normal to the central axis upon application of the first set of voltages, and generate at a second predetermined location the saddle point of the second quadrupole potential in the plane normal to the central axis upon application of the second set of voltages.

7 . The quadrupole ion optical device of claim 1 , wherein a size of each electrode of the plurality of electrodes and a spacing between pairs of electrodes of the plurality of electrodes are selected to provide a deviation of the electric potential of less than a threshold amount from an ideal quadrupole potential in a first area around the saddle point of the first quadrupole potential in a plane normal to the central axis upon application of the first set of voltages; and

to provide a deviation of the electric potential of less than a threshold amount from the ideal quadrupole potential in a second area around the saddle point of the second quadrupole potential in the plane normal to the central axis upon application of the second set of voltages;

wherein the first area is 50% to 150% of the second area.

8 . The quadrupole ion optical device of claim 1 , wherein the electrical circuitry is configured to permit simultaneous supply of a different voltage to each electrode of the plurality of electrodes.

9 . The quadrupole ion optical device of claim 1 , wherein the electrical circuitry comprises a first voltage divider arrangement, a second voltage divider arrangement and one or more voltage supplies;

wherein the first voltage divider arrangement is configured to supply the first set of voltages when the first voltage divider is electrically coupled to at least one of the one or more voltage supplies and the plurality of electrodes, each voltage of the first set of voltages to be supplied to one or more of the plurality of electrodes; and

wherein the second voltage divider arrangement is configured to supply the second set of voltages when the second voltage divider is electrically coupled to at least one of the one or more voltage supplies and the plurality of electrodes, each voltage of the second set of voltages to be supplied to one or more of the plurality of electrodes.

10 . The quadrupole ion optical device of claim 9 , wherein the electrical circuitry further comprises:

at least one switching relay to selectively electrically couple either the first voltage divider arrangement or the second voltage divider arrangement to the plurality of electrodes, or to selectively electrically couple at least one of the one or more voltage supplies to either the first voltage divider arrangement or the second voltage divider arrangement.

11 . The quadrupole ion optical device of claim 1 , wherein each voltage of the first or the second set of voltages is a direct current (DC) voltage.

12 . A mass spectrometer, comprising:

the mass analyser;

the plurality of detector elements; and

the quadrupole ion optical device according to claim 1 , wherein the quadrupole ion optical device is arranged between the mass analyser and the plurality of detector elements, such that the plurality of ion beams exiting from the mass analyser towards the plurality of detector elements pass through the quadrupole potential generated by the plurality of electrodes at the quadrupole ion optical device, without ions in the plurality of ion beams being trapped within the quadrupole ion optical device;

wherein the central axis of the quadrupole ion optical device is aligned with a direction of travel of ions in at least one of the plurality of ion beams towards the plurality of detector elements.

13 . The mass spectrometer of claim 12 , wherein in a plane normal to the central axis of the quadrupole ion optical device the region bounded by the plurality of electrodes extends further in the direction of lateral separation of the plurality of ion beams at the exit from the mass analyser than a direction in the same plane that is orthogonal to the direction of lateral separation of the plurality of ion beams at the exit from the mass analyser.

14 . The mass spectrometer of claim 12 , wherein the central axis of the quadrupole ion optical device is aligned with an optical axis of the mass spectrometer, the optical axis extending between the centre of the exit of the mass analyser and a detector element among the plurality of detector elements.

15 . The mass spectrometer of claim 12 , wherein the mass spectrometer is an isotope ratio mass spectrometer.

16 . A method of mass spectrometry, comprising:

passing a plurality of ion beams exiting from a mass analyser through a quadrupole potential generated by a quadrupole ion optical device and towards a plurality of detector elements, without trapping ions within the quadrupole ion optical device;

adjusting a position of a saddle point of the quadrupole potential to a first position, to optimise an alignment of a first group of ion beams of interest, which are included in the plurality of ion beams, into a first respective set of detector elements of the plurality of detector elements by minimising an angle of incidence between each of the ion beams of interest of the first group and a direction normal to a detection surface of the respective detector element of the first set of detector elements at which the ion beam is received;

adjusting the position of the saddle point of the quadrupole potential to a second position, to optimise an alignment of a second group of ion beams of interest, which are included in the plurality of ion beams, into a second respective set of detector elements of the plurality of detector elements by minimising an angle of incidence between each of the ion beams of interest of the second group and a direction normal to a detection surface of the respective detector element of the second set of detector elements at which the ion beam is received;

wherein the first and second groups of ion beams of interest differ in mass-to-charge ratio and the first and second respective sets of detector elements are oriented at different angles.

17 . The method of claim 16 , wherein adjusting the position of the saddle point to the first or second position minimises an overall deflection experienced by the ion beams of interest of the first or second group, respectively, passing through the quadrupole potential generated by the quadrupole ion optical device.

18 . The method of claim 16 , wherein adjusting the position of the saddle point to the first or second position minimises the sum of the angles between the ion beams of interest of the first or second group, respectively, and the directions normal to the detection surfaces of the respective detector elements or the first or second sets of detector elements, respectively, at which the ion beams of interest are received.

19 . The method of claim 16 , wherein:

the plurality of ion beams passing through the quadrupole potential are laterally separated from one another in a plane normal to a central axis of the quadrupole ion optical device; and

adjusting the position of the saddle point of the quadrupole potential to the first or second position comprises moving the position of the saddle point away from the central axis and along the plane normal to the central axis.

20 . The method of claim 19 , wherein adjusting the position of the saddle point to the first or second position reduces an angle between the path of one of the ion beams of interest of the first or second group, respectively, and the central axis of the quadrupole ion optical device.

21 . The method of claim 16 , wherein:

adjusting the position of the saddle point to the first or second position comprises switching from supplying a first set of voltages to supplying a second set of voltages to a plurality of electrodes of the quadrupole ion optical device;

the position of the saddle point is at a central axis of the quadrupole ion optical device when the first set of voltages are supplied to the plurality of electrodes; and

the position of the saddle point is moved away from the central axis to the first or second position when the second set of voltages are supplied to the plurality of electrodes.