The invention relates to ion transfer assemblies for directing ions from an atmospheric pressure ion source into a mass analyzer for analysis.
Atmospheric pressure ion sources coupled to mass spectrometers by an ion transfer assembly often produce random noise which can severely limit the signal-to-noise ratio in the mass spectra. This noise is believed to be caused by charged particles or clusters of ions and solvent molecules which reach the detector region at random times. The abundance of the noise can be affected by several parameters related to the ion source including spray stability, involatile buffer concentration, solvent flow, and sampling configuration. A variety of techniques have been devised to reduce the affect of such noise, as described, for example, in U.S. Pat. Nos. 6,392,225, 5,750,993, and 5,171,990, each of which is incorporated by reference herein.
The invention provides ion transfer assemblies and methods for directing ions from an ionization source to a mass analyzer. In general, in one aspect, the assemblies include a first partition element (e.g., a skimmer) separating a viscous flow region from a transition flow region, a second partition element (e.g., a skimmer) separating the transition flow region into a first transition flow chamber and a second transition flow chamber, and a focusing element defining a cavity shaped to direct a portion of a gas flow including ions entrained in a background gas from a first aperture in the first partition element towards a second aperture in the second partition element based on gas dynamics, without requiring the application of external electrostatic fields. In some embodiments, vents or slits can be provided in or between the first partition element and the focusing element to provide for expansion of the gas flow in the transition flow region.
In general, in another aspect, the invention features an ion transfer assembly for directing ions from an ionization source to a mass analyzer. The assembly includes a gas dynamics focusing element located in a transition flow region. The gas dynamics focusing element is configured to receive from the ionization source a gas flow including ions and a background gas and transmit a portion of the gas flow for sampling into a molecular flow region. The gas dynamics focusing element includes one or more surfaces defining a cavity shaped to direct a portion of the gas flow along a center line based on local gas dynamics.
In general, in another aspect, the invention features methods for directing ions from an ionization source to a mass analyzer. The methods include receiving in a transition flow region a gas flow including ions entrained in a background gas, introducing at least a portion of the gas flow into a focusing element located in the transition flow region, and directing a portion of the gas flow along a center line for sampling into a molecular flow region. The focusing element including one or more surfaces defining a cavity. The gas flow is directed based on a local gas dynamics effect resulting at least in part from the shape of the cavity.
The invention can be implemented to provide one or more of the following advantages. Ions can be focused in the transition flow region, and the number of collisions between ions and background gases in that region increased, based on gas dynamics, without requiring the application of electrostatic potentials to increase ion kinetic energy. As a result, signal to noise ratios can be enhanced by reducing noise, enhancing signal, or a combination of both.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.
One embodiment of an ion transfer assembly is illustrated in FIG. 1. An atmospheric pressure ionization source 100 is connected to receive a sample from an associated apparatus such as a liquid chromatograph or syringe pump. The atmospheric pressure ionization source can be an electrospray ionization source, an atmospheric pressure chemical ionization source, an atmospheric pressure matrix assisted laser desorption source, a photoionization source, or a source employing any other ionization technique that operates at pressures substantially above the operating pressure of the mass analyzer (e.g., from about 1 torr to about 760 torr). The source 100 forms ions representative of the sample, which ions are transported from the ion source to the mass analyzer by an ion transfer assembly. In particular, the ions are entrained in a background gas and transported from the ion source through a capillary 105 into a viscous flow chamber 110 which is maintained at a lower pressure (e.g., 0.5 to 10 torr) than the atmospheric pressure of the ionization source 100. Due to the differences in pressure, ions and gases are caused to flow through capillary 105 into the viscous flow chamber 110, where the ions and gases expand to form a supersonic free jet. The end of the capillary is opposite a first skimmer or partition element 115 that separates the viscous flow chamber 110 from a transition flow chamber 140, which is maintained at a lower pressure (e.g., from about 2 to about 400 millitorr) than viscous flow chamber 110. A tube lens 125 surrounds the end of capillary 105 and provides an electrostatic field that focuses the ion stream leaving capillary 105 through the aperture 130 in first skimmer 115, which is preferably displaced from the axis or cavity of the capillary 110. The operation of tube lens 125 is described in U.S. Pat. No. 5,157,260 which is incorporated by reference herein.
A second partition element or skimmer 135 is located proximate to first skimmer 115 and separates transition flow chamber 140 from a molecular flow chamber 145 that is preferrably maintained in the neighborhood of about 10−4 torr. Second skimmer 135 samples the ion stream exiting from aperture 130 in first skimmer 115. First skimmer 115 and second skimmer 135 are formed and positioned such that ions and gases transitioning from viscous to transition flow are focused based on the gas dynamics in the immediate vicinity. A portion of the background gases and a corresponding portion of the entrained ions are allowed to expand into transition flow chamber 140 where it is pumped away. However, the gas dynamics in the vicinity are such that a portion of the gases and ions are directed along center line 150. In particular, the first skimmer 115 is contoured to define a cavity formed to control the evacuation of background gases enabling gas flow focusing and allowing for the declustering process to continue beyond the electrostatic centering influence of the tube lens. Thus, the ion stream is focused along center line 150 without requiring any electrostatic potential to be applied to first skimmer 115 or second skimmer 135 (i.e., they can simply be grounded to the instrument chassis). Ions traveling through aperture 155 in second skimmer 135 are then directed by additional guide elements 160 (e.g., a multipole ion guide) into mass analyzer 165 disposed in high vacuum chamber 170, and, ultimately, to detector 175 whose output can be displayed as a mass spectrum. Preferably, the contoured back end of the first skimmer is formed to allow for just enough pumping so as not to allow a molecular gas beam to subsequently enter the high vacuum region, but to restrict the evacuation of background gases so as to allow for the influence of gas flow focusing to occur. Appropriate additional guide elements 160 can include additional skimmers or lenses, and preferrably include RF-only optics, such as RF/DC quadrupoles, other multipoles or other optical devices. Mass analyzer 165 can be any mass analyzer or hybrid combination of mass analyzers, including quadrupole mass analyzers, ion trap mass analyzer (3D or linear 2D ion traps), time of flight mass analyzers, fourier transform mass analyzers, sector mass analyzers, orbitrap mass analyzers, or the like.
A different embodiment is illustrated in FIG. 3. Like the embodiment of
Still another embodiment is illustrated in
Focusing element 500 can be a separate element, as illustrated in
Without wanting to be bound by theory, it is believed that the combination of first skimmer 115 and second skimmer 135 enhances ion transmission along the center line and reduces noise by effectively disrupting gas expansion in the transition flow region. More specifically, it is believed that the focusing element's interior surface focuses the ion beam by directing both ions and background gas along the center line, thereby both increasing the number of ions transmitted through the transition flow region (i.e., decreasing the number of ions that would otherwise be pumped away in the transition flow region) and increasing the number of collisions between ions and gas in that region, which is believed to disrupt adducts or clusters that may contribute to chemical noise. Ions exiting the cavity formed by the focusing element continue in their stream lined paths in the core gas flow and are collisionally dampened to the center since the lighter background gases (e.g., predominantly nitrogen gas molecules) scatter to define the outer core boundary. As the restriction decreases over the length of the assembly due to the influence of the external cone angle of the second skimmer, the background gases are pumped away while the core of the beam enriched in ions is sampled by the second skimmer. This lowers the required aperture size of the second skimmer, thereby restricting the gas load into the subsequent optics.
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, while no electrostatic potential is applied to the partition and focusing elements in the described embodiments (and no such potential is required to achieve the focusing benefits of the invention), in some embodiments it may be desirable to apply a voltage to some or all of these elements to provide for further ion acceleration in the transition flow region. Accordingly, other embodiments are within the scope of the following claims.
This application claims the benefit of Provisional Application No. 60/384,649, filed on May 31, 2002, which is incorporated by reference herein.
Number | Name | Date | Kind |
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5157260 | Mylchreest et al. | Oct 1992 | A |
5171990 | Mylchreest et al. | Dec 1992 | A |
5432343 | Gulcicek et al. | Jul 1995 | A |
5750993 | Bier | May 1998 | A |
6060705 | Whitehouse et al. | May 2000 | A |
6392225 | Schwartz et al. | May 2002 | B1 |
Number | Date | Country | |
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60384649 | May 2002 | US |