Methods of operating ion optics for mass spectrometry
View Patent ↗In various embodiments, provided are methods for focusing ions for an ion fragmentor, and methods for operating an ion optics assembly. In various embodiments, the present teachings provide methods that substantially maintain the position of the focal point of the an incoming ion beam over a wide range of collision energies, and thereby provide a collimated ion beam for a collision cell over a wide range of energies. In various embodiments, the present teachings provide methods that facilitate decreasing ion transmission losses over a wide range of collision energies.
1. A method for focusing ions for a collision cell using an ion optics assembly comprising a first ion lens disposed between a retarding lens and an entrance to a collision cell, comprising the steps of:
providing sample ions formed at a source electrical potential;
establishing a first electrical field to decelerate sample ions entering the retarding lens and establishing a second electrical field between the retarding lens and the first ion lens to accelerate sample ions from the retarding lens and into the first ion lens, wherein sample ions are substantially focused to a focal point within the first ion lens and form a substantially collimated ion beam after the focal point and before the entrance to the collision cell; and
establishing a third electrical field between the first ion lens and the entrance of the collision cell to decelerate sample ions from the first ion lens.
2. The method of claim 1 , wherein:
the retarding lens comprises a first electrode, a second electrode and a third electrode, the step of establishing the first electrical field comprising applying a first electrical potential to the second electrode; and
the first ion lens comprises said third electrode, a fourth electrode and a fifth electrode, the step of establishing the second electrical field comprising applying a first second electrical potential to the fourth electrode; and
the step of establishing the third electrical field comprising applying a third electrical potential to the fifth electrode.
3. The method of claim 2 , wherein sample ions are substantially focused to a focal point between the third electrode and the fourth electrode.
4. The method of claim 2 , wherein the electrical potential on the first electrode is substantially the same as the electrical potential on the second electrode.
5. The method of claim 2 , wherein the electrical potential on the third electrode is substantially the same as the electrical potential on the fifth electrode.
6. The method of claim 2 , wherein the electrical potential on the fifth electrode is substantially the same as the electrical potential on the entrance of the collision cell.
7. The method of claim 2 , wherein the difference between first electrical potential and the second electrical potential establishes the second electrical field.
8. The method of claim 2 , wherein the difference between second electrical potential and the third electrical potential establishes the third electrical field.
9. The method of claim 1 , wherein when the sample ions of interest are positive ions:
the second electrical potential is more negative than the first electrical potential;
the third electrical potential is more positive than the second electrical potential;
and the third electrical potential is less than or equal to the source potential.
10. The method of claim 1 , wherein when the sample ions of interest are negative ions:
the second electrical potential is more positive than the first electrical potential;
the third electrical potential is more negative than the second electrical potential;
and the third electrical potential is greater than or equal to the source potential.
11. The method of claim 1 , wherein the difference between the source electrical potential and the third electrical potential is in the range between about 250 volts to about 5000 volts.
12. A method for operating an ion optics assembly comprising a first ion lens disposed between a retarding lens and an entrance to a collision cell, comprising the steps of:
substantially focusing sample ions to a focal point in the first ion lens and forming after the focal point in the first ion lens and before the entrance to the collision cell a substantially collimated ion beam of sample ions at a first collision energy by:
establishing a decelerating electrical field to decelerate sample ions entering the retarding lens by applying a first electrical potential to an electrode of the retarding lens;
establishing an accelerating electrical field between the retarding lens and the first ion lens to accelerate sample ions from the retarding lens and into the first ion lens by applying a second electrical potential to an electrode of the first ion lens; and
establishing a decelerating electrical field between the first ion lens and the entrance of the collision cell to decelerate sample ions from the first ion lens by applying a third electrical potential to the entrance of the collision cell;
changing the first collision energy to a second collision energy different from the first collision energy;
substantially focusing sample ions to the focal point in the first ion lens and forming after the focal point in the first ion lens and before the entrance to the collision cell a substantially collimated ion beam of sample ions at the second collision energy by:
establishing a decelerating electrical field to decelerate sample ion entering the retarding lens by applying a fourth electrical potential to an electrode of the retarding lens, the fourth electrical potential being substantially equal to the first electrical potential;
establishing an accelerating electrical field between the retarding lens and the first ion lens to accelerate sample ions from the retarding lens and into the first ion lens by applying a fifth electrical potential to an electrode of the first ion lens; and
establishing a decelerating electrical field between the first ion lens and the entrance of the collision cell to decelerate sample ions from the first ion lens by applying a sixth electrical potential to the entrance of the collision cell.
13. The method of claim 12 , wherein the focal point in the first ion lens is a distance F from an entrance to the retarding lens and the distance F varies within less than about ±4% when the difference between the first collision energy and the second collision energy is less than about 5000 electron volts.
14. The method of claim 13 , wherein the distance F varies within less than about ±2% when the difference between the first collision energy and the second collision energy is less than about 5000 electron volts.
15. The method of claim 13 , wherein the distance F varies within less than about ±1% when the difference between the first collision energy and the second collision energy is less than about 5000 electron volts.
16. The method of claim 12 , wherein:
the retarding lens comprises a first electrode, a second electrode and a third electrode, the first electrical potential being applied to the second electrode; and
the first ion lens comprises said third electrode, a fourth electrode and a fifth electrode, the second electrical potential being applied at least to the fourth electrode.
17. The method of claim 16 , wherein sample ions are substantially focused to a focal point between the third electrode and the fourth electrode.
18. The method of claim 12 , wherein when the sample ions of interest are positive ions:
the second electrical potential is more negative than the first electrical potential;
the third electrical potential is more positive than the second electrical potential;
the fifth electrical potential is more negative than the fourth electrical potential; and
the sixth electrical potential is more positive than the fifth electrical potential.
19. The method of claim 12 , wherein when the sample ions of interest are negative ions:
the second electrical potential is more positive than the first electrical potential;
the third electrical potential is more negative than the second electrical potential;
the fifth electrical potential is more positive than the fourth electrical potential; and
the sixth electrical potential is more negative than the fifth electrical potential.
20. The method of claim 12 , wherein the first collision energy and the second collision energy are both in the range between about 250 electron volts to about 5000 electron volts.
21. The method of claim 12 , wherein the fourth electrical potential is within about ±5% of the first electrical potential.
22. The method of claim 12 , wherein the fourth electrical potential is within about ±2.5% of the first electrical potential.
23. The method of claim 12 , wherein the step of changing the collision energy comprises substantially fixing one of a source potential at which sample ions are formed or the electrical potential on the entrance to the collision cell; and changing the other.
24. The method of claim 23 , wherein the step of changing the collision energy comprises substantially fixing a source potential at which sample ions are formed and changing the electrical potential on the entrance to the collision cell.
25. A method for operating an ion optics assembly comprising a first ion lens disposed between a retarding lens and an entrance to a collision cell, comprising the steps of:
focusing sample ions at a focal point within the first ion lens a distance F from an entrance to the retarding lens and forming before the entrance to the collision cell a substantially collimated ion beam of sample ions at a first collision energy of the sample ions with respect to a neutral gas in a collision cell; and
maintaining the focal point substantially at the distance F for collision energies different from the first collision energy by substantially maintaining the all the electrical potentials on the retarding ion lens and changing one electrical potential on the first ion lens.