Radiant energy – Ionic separation or analysis – With sample supply means
Reexamination Certificate
2000-05-05
2002-06-11
Anderson, Bruce (Department: 2881)
Radiant energy
Ionic separation or analysis
With sample supply means
C250S292000
Reexamination Certificate
active
06403952
ABSTRACT:
FIELD OF INVENTION
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 transfer from a multipole ion guide into either a subsequent multipole ion guide such as a quadrupole mass analyzer or a three dimensional ion guide mass analyzer. Atmospheric Pressure Ion Source mass spectrometry (API-MS) has emerged as a sensitive method for detecting sample ion solutions with both discrete sample and on-line sample introduction methods. The invention improves performance with quadrupole mass and ion trap mass spectrometers for both on-line and off-line applications. In addition, the apparatus and methods described can be configured to improve quadrupole and ion trap mass analysis performance with ion sources other than API sources.
BACKGROUND OF INVENTION
Multipole ion guides have been used to efficiently transfer ions through vacuum or partial vacuum into mass analyzers. In particular, multipole ion guides have been configured to transport ions from an Atmospheric Pressure Ion (API) Source through one or more vacuum pumping stages and into a mass analyzer. Quadrupole, magnetic sector, Fourier Transform (FTMS), three dimensional ion trap and Time-Of-Flight (TOF) mass analyzers each have different entrance ion optics criteria which must be satisfied by any ion source ion transport or focusing system. The present invention addresses optimization of the transfer of ions from one multipole ion guide into a subsequent multipole ion guide, quadrupole mass analyzer or a three dimensional quadrupole ion trap. Multipole ion guides and ion traps operate with sinusoidal voltages and separate or combined DC voltages applied to one or more electrodes. The sinusoidal voltage wave forms are usually referred to as AC or RF because the frequency of these wave forms generally fall within the radio frequency range. The combination of AC and DC voltages applied to the rods of a multipole ion guide or the endcaps and ring electrode of a three dimensional quadrupole ion trap can be selected to establish stable ion trajectories for some mass to charge (m/z) values while rejecting others. Mass selection for mass analysis can be achieved in this manner, or ions can be trapped while colliding with background gas to achieve Collisional Induced Dissociation (CID) ion fragments from trapped ions or from ions traversing the length of the ion guide. Ions whose m/z values do not have a stable trajectory for the AC and DC potentials applied to the rods of a multipole ion guide will be rejected from the ion guide before reaching the ion guide exit. 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. AC and DC voltages applied to the poles of a commercial analytical quadrupole can reach hundreds of volts and even kilovolt potentials. Similarly, the trajectories of ions attempting to enter a three dimensional quadrupole ion trap are greatly influenced by the RF fields produced from voltages applied to the ring electrode appearing at the ion guide endcap entrance orifice. Ion transport into a multipole ion guide will be considered first.
For a geometrically ideal multipole ion guide, there is no net electric field at the very centerline of the ion guide except for the common DC offset potential applied equally to all ion guide poles. Ions of a given polarity attempting to enter a device whose electrodes have an AC voltage applied can encounter a retarding or rejecting electric field gradient during a portion of the AC voltage phase. Multipole ion guides with an even number of symmetrically spaced parallel poles or rods ideally have no net AC (or RF) field at the centerline or axis of the assembly. Ion beams, however, have a finite cross section and most ions will enter a multipole ion guide such as a quadrupole mass analyzer at some radial distance off the centerline. Consequently, the trajectory of these ions will be influenced by an AC and an asymmetric DC field. Depending on the phase of the AC field, the asymmetric DC field off the centerline and the ion kinetic energy in the axial direction, an approaching ion may successfully enter the ion guide and maintain a stable trajectory, or may be rejected from entering the multipole ion guide or may enter the ion guide with an unstable trajectory. 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. For a given average ion energy, the higher an ion m/z value, the lower its velocity. Consequently, the larger the m/z value of an ion, the more time an ion will spend traversing the entrance region of a multipole ion guide while entering the rod assembly. Similarly, if the average ion kinetic energy is reduced, ions of a given m/z value will spend more time traversing the fringing fields of the multipole ion guide as they enter the ion guide. The AC voltages applied to the rods of a multipole ion guide with an even number of poles generally have equal RF amplitude but opposite phase for each adjacent rod or pole. For example, the opposing rods of a quadrupole ion guide have the same phase, which is itself 180 degrees out of phase from the AC voltage applied to each neighboring rod or pole.
One means used to achieve quadrupole mass analyzer m/z selection, is to apply RF and positive and negative polarity DC voltage to the rods with a selected RF to DC amplitude ratio. The DC voltage is equal in amplitude but opposite in polarity on adjacent rods. When quadrupole mass analyzers are scanned in this mass selective mode to acquire a mass spectrum, the AC and DC amplitudes increase proportionally with selected m/z during a scan. Consequently, an ion with a higher m/z value and a slower velocity than a lower m/z value, moves more slowly through the entrance fringing fields and must traverse a higher AC and DC fringing field amplitude in entering the quadrupole in scan mode. Ion transmission efficiency in quadrupole mass analyzers can decrease with increasing m/z, due in part to a decreased efficiency of ions entering the quadrupole. The positive and negative DC voltage components may be added to form a common offset voltage. This DC offset potential can be set to aid in accelerating ions into the quadrupole. In some applications, an additional low amplitude AC wave form, which has a lower frequency than the RF voltage component, is capacitively added to the RF voltage. This additional low amplitude AC voltage of a selected frequency or frequency set is added to the RF voltage to provide resonant frequency excitation for specific ion m/z rejection or fragmentation. With the exception of the DC offset voltage component, the effective AC and DC field strength decreases the closer an ion is positioned to the ion guide centerline. The invention improves the ion transport into a multipole ion guide such as a quadrupole mass analyzer by minimizing the fringing field effects and insuring that ions are delivered close to the multipole ion guide centerline with angular trajectories within the acceptance window of the multipole ion guide.
A quadrupole is the most commonly used multipole ion guide configuration for con
Gulcicek Erol
Whitehouse Craig M.
Analytica of Branford, Inc.
Anderson Bruce
Levisohn, Lerner, Berger & Langsam LLP
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