Ion transfer from multipole ion guides into multipole ion guides and ion traps

Abstract

A multipole ion guide is configured to improve the transmission efficiency of ions which traverse the length of one ion guide and enter either another multipole ion guide such as a quadrupole mass analyzer or a three dimensional ion trap. The ion transfer multipole ion guide radial dimensions are reduced such that the pole assembly and an appropriately shaped exit lens can be positioned within a portion of the internal space defined by the larger radius second multipole ion guide poles. Ions exiting the first ion guide of reduced size find themselves inside the second ion guide close to the centerline. In this manner ions can be efficiently transferred from one ion guide to another, even for those ions with low kinetic energies. In a second embodiment of the invention, the exit region of a multipole ion guide is configured such that the multipole ion guide poles can be extended into a counterbore of a three dimensional ion trap end cap electrode. With this configuration, ions (including those with low kinetic energies) can be transferred into a three dimensional ion trap with increased trapping efficiency.

Description

[0001] This application claims the priority of U.S. Nonprovisional Application Ser. No. 08 / 857,191 filed May 15, 1997, and the priority of U.S. Provisional Patent Application Ser. No. 60 / 017,619, filed May 14, 1996, the disclosures of which are fully incorporated herein by reference.[0002] The present invention relates to an apparatus and method for increasing the efficiency of ion transport from ion sources into a multipole ion guide, a quadrupole mass analyzer or a three dimensional ion trap. Multipole ion guides have been effectively used to capture and transport ions which are delivered into vacuum from Atmospheric Pressure Ion (API) sources such as Electrospray (ES) and Atmospheric Pressure Chemical Ionization (APCI). Ions whose mass to charge (m / z) values fall within the stability region of the multipole ion guide are transmitted through the length of the guide and delivered to the entrance region of a mass analyzer. Specifically, the present invention addresses the ion transf...

AI Technical Summary

Benefits of technology

[0012] A multipole ion guide has been configured with a reduced diameter such that the ion guide with the appropriately shaped exit lens can be positioned inside a larger diameter multipole ion guide. Ions exiting the smaller multipole ion guide pass through the exit lens and are focused to the centerline already inside the larger ion guide. Since the ions leaving the exit lens aperture of the multipole ion guide with reduced dimensions are already inside the larger ion guide, high ion transfer efficiencies can be achieved even with ions having low axial translational energies. Improved mass analysis resolution at higher sensitivities can be achieved with this ion transfer optic when ions are transferred into a quadrupole mass analyzer. The smaller ion guide can be configured to extend continuously through more than one vacuum stages or reside entirely within one vacuum stage. The smaller ion guide can be configured to reside in a different vacuum stage than that of the downstream larger ion guide with the smaller ion guide exit lens serving as the vacuum partition. Alternatively, all multipole ion guides can be configured to reside in the same vacuum pumping stage.

[0013] In a second embodiment of the invention, a multipole ion guide with reduced radial dimensions is positioned such that the rods of the multipole ion guide extend into a counterbore of the entrance endcap of a three dimensional quadrupole ion trap. The entrance aperture in the ion trap endcap as serves as the multipole ion guide exit lens. During the ion trap filling cycle, a portion of the ions rejected from entering the entrance aperture of the three dimensional ion trap due to unfavorable RF phase electric fields can be retrapped by the multipole ion guide. Ions ejected out of the entrance endcap by the three dimensional ion trap during mass analysis scanning can also be retrapped by the multipole ion guide. To increase duty cycle and sensitivity, the ion trap endcap voltage can be set higher than the kinetic energy of the ions exiting the multipole ion guide to trap ions in the multipole ion guide during a three dimensional ion trap mass analysis cycle. The multipole ion guide rod potentials can be set to reduce the m / z stability window. In this manner, ions with undesirable m / z values can be ejected from the multipole ion guide and prevented from entering the three dimensional ion trap, thus reducing space charge effects in the ion trap. The multipole ion guide can be configured to extend continuously into more than one vacuum pumping stage.

Problems solved by technology

The AC and DC voltages applied to the poles of a multipole ion guide can be selected to achieve the functions of selective m / z ion transmission and ion rejection for those ions within the ion guide; however, the fields created by the applied voltages can pose some difficulty for ions trying to enter the ion guide.

The more time an ion spends in the fringing fields while attempting to enter a multipole ion guide, the more cycles of AC voltage it can be exposed to and thus the more likely that it may be potentially driven into an unfavorable trajectory.

As was described above, ion losses can occur in the entrance region when transferring ions into a quadrupole ion guide or mass analyzers due to the electric fields which influence the ion trajectories as they approach and enter the quadrupole ion guide.

In addition, ions approaching the quadrupole entrance can enter unstable trajectories due to fringing field affects.

The more time an ion spends in the quadrupole fringing fields the more chance it has of being driven into an unstable trajectory.

An ion with lower kinetic energy will also spend more time in the fringing fields at the quadrupole entrance and consequently have an increased chance of being driven into an unfavorable trajectory.

When transferring ions from one multipole ion guide to another multipole ion guide, as occurs in triple "quadrupole" mass analyzers, losses can occur in the interface regions between each multipole ion guide.

Electrostatic entrance lens configurations do not fully compensate for the variations in entrance conditions encountered with quadrupole ion guide mass analysis operation.

Douglas, however, does not teach the configuration of extending the rods of a multipole ion guide into a counterbore of a three dimensional ion trap endcap to improve the trapping efficiency by recapturing ions within the ion guide that have been rejected by the ion trap entrance orifice.

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