CN116453933A

CN116453933A - Ion activation and fragmentation at sub-ambient pressure for ion mobility and mass spectrometry

Info

Publication number: CN116453933A
Application number: CN202211287496.6A
Authority: CN
Inventors: R·T·库鲁鲁加马; K·R·牛顿
Original assignee: Agilent Technologies Inc
Current assignee: Agilent Technologies Inc
Priority date: 2021-10-22
Filing date: 2022-10-20
Publication date: 2023-07-18

Abstract

An ion source may include an ionization chamber for maintaining at atmospheric pressure. The ion source may further comprise a pressure drop chamber for maintaining at sub-atmospheric pressure, and ion transfer means comprising an inlet in the ionization chamber and an exhaust in the pressure drop chamber. The ion transfer means may define an ion path from the inlet to the outlet. The ion transfer device may be positioned to emit ions and neutral gas molecules from the discharge outlet as an expanded beam comprising a low gas density region surrounded by a high gas density region, the high gas density region having a higher gas density than the low gas density region. The ion source may be used, for example, in Ion Mobility Spectrometry (IMS), mass Spectrometry (MS), and hybrid IM-MS.

Description

Ion activation and fragmentation at sub-ambient pressure for ion mobility and mass spectrometry

Priority

The present application claims priority from commonly assigned and co-pending U.S. provisional application serial No. 63/271,070 filed on 10 months 22 of 2021, the disclosure of which is incorporated herein by reference in its entirety.

Technical Field

The present invention relates generally to Ion Mobility Spectrometry (IMS), mass Spectrometry (MS) and ion mobility mass spectrometry (IM-MS), and more particularly to the development and implementation of ion activation in IMS systems, MS systems and IM-MS systems.

Background

Mass Spectrometry (MS) systems typically include an ion source for ionizing components of a sample under study, a mass analyzer for separating gas phase ions based on their different mass-to-charge ratios (or m/z ratios, or simply "masses"), an ion detector for counting the separated ions, and electronics for processing output signals from the ion detector as needed to produce a user-interpretable mass spectrum. In general, a mass spectrum may include a series of peaks that indicate the relative abundance of detected ions as a function of their m/z ratio. Mass spectrometry can be used to determine the molecular structure of a sample component, thereby enabling qualitative and quantitative characterization of a sample. One type of MS may include a time-of-flight mass spectrometer (TOF MS). TOF MS can utilize a high resolution mass analyzer (TOF analyzer). Ions may be transported from the ion source to the TOF inlet region by a series of ion guides, ion optics and various types of ion processing apparatus. The TOF analyzer may include an ion accelerator that injects ions in packets (or pulses) into a flight tube (flight tube) without an electric field. In a flight tube, ions of different masses may travel at different speeds and thus be separated (e.g., dispersed) according to their different masses, thereby achieving a time-of-flight based mass resolution.

Ion Mobility Spectrometry (IMS) may represent a gas phase ion separation technique in which ions generated from a sample in an ion source are separated based on their different mobilities through a drift cell (drift cell) of known length filled with an inert gas of known composition and maintained at a known gas pressure and temperature. In low electric field drift type IMS, ions are pushed forward through the drift cell under the influence of a relatively weak, uniform Direct Current (DC) voltage gradient, for example in the range 10V/cm to 20V/cm. The mobility of ions may depend on their Collision Cross Section (CCS) (and hence their size and conformation or shape), charge state (e.g., +1, +2 or +3), and to a relatively small extent on their m/z ratio. Thus, ion separation by IM may be relatively different from ion separation by MS. Ions from the drift cell may eventually reach the ion detector and the output signal from the ion detector may be processed to generate peak information that is used to distinguish between the different analyte ion species detected. If the time taken for the ions to spend in the drift tube region is known, and the pressure and voltage across the drift tube is also known, the filter CCS may be determined for any ion of interest. CCS parameters can be specific for a given molecule and instrument independent and can therefore be used as the only parameter for compound identification. Thus, CCS parameters can be related to structural characterization of molecules and theoretical molecular dynamics modeling, as well as to some other scientific disciplines.

The IMS system may be coupled online with a mass analyzer, which may be a TOF analyzer. In a combined IM-MS system, ions may be separated by mobility before being transferred to a mass analyzer, where they are then mass resolved. Due to the difference between IM-based and MS-based separations, performing these two separation techniques in tandem can be particularly useful in the analysis of complex chemical mixtures, including high Molecular Weight (MW) biomolecules (e.g., biopolymers), such as polynucleotides, proteins, carbohydrates, and the like. For example, the increased dimensions provided by IM separation may help separate ions that are different from each other (e.g., different in shape) but exhibit overlapping mass peaks. On the other hand, the increased dimension provided by MS separation may help separate ions of different masses but similar CCS. Such hybrid IM-MS separation techniques may be further enhanced by combining them with Liquid Chromatography (LC) or Gas Chromatography (GC) techniques. The IM-MS system may thus be capable of acquiring multidimensional IM-MS data from a sample characterized by an acquisition time (e.g., chromatographic time or retention time), ion abundance (e.g., ion signal intensity), ion drift time through an IM-drift cell, and an m/z ratio categorized by MS.

The ions may be activated by collisions with neutral gas molecules having a sufficiently high collision energy to cause collision heating of the ions, rather than collision cooling. At sufficiently high Collision Energy (CE), ion activation can fragment ions. This mechanism of ion fragmentation can be implemented in a collision cell (collisioncell) and can be referred to as Collision Induced Dissociation (CID) or Collision Activated Dissociation (CAD). Ion activation may also be utilized to cause folding protein ions or other large biomolecular ions to unfold, which may be referred to as Collision Induced Unfolding (CIU). Depending on the magnitude of the collision energy, the CIU may or may not be accompanied by fragmentation/dissociation. Ion activation followed by ion mobility separation can be used to identify closely related ions that are difficult to identify using only other techniques including IM or MS.

Two techniques for generating gas phase ions may include electrospray ionization (ESI) and Matrix Assisted Laser Desorption Ionization (MALDI), both of which may represent Atmospheric Pressure Ionization (API) techniques. For MS and IMS, gas phase ions may need to generate minimal or no solvent molecules and may need to attach adducts to analyte ions. Endogenous ion activation methods combined with ESI or MALDI techniques can provide completely desolventized gas phase ions. Both MS and IMS can benefit from endogenous ion activation and fragmentation techniques performed prior to mass and/or ion mobility analysis, such as by enhancing structural identification of gas phase ion structures. For folded molecules, the above CIU technique induced by ion activation may be able to improve molecular structure analysis. Thus, endogenous ion activation and fragmentation can be used for MS, IMS, and hybrid IM-MS applications.

MS, IMS, and hybrid IM-MS instruments may not include ion activation mechanisms in the ion source that can gain enough energy to collapse larger biomolecules or to de-cluster larger biomolecules. Commercial mass spectrometers may be equipped with a capillary-interceptor interface (capillary-skimmer interface) that couples the atmospheric pressure ionization region of the ion source with a first vacuum region in the mass spectrometer to allow for modest ion activation. Such capillary-interceptor interfaces may not be able to provide high enough energy for collision activation or fragmentation of larger biomolecules. Typical pressures in the capillary-interceptor interface may be less than 1 torr. At higher pressures, such capillary-interceptor interfaces may not be able to provide a sufficiently high collision energy prior to discharge. Thus, larger biomolecules may not be activated, fragmented, or unfolded using a capillary-interceptor interface. In some applications, a dopant gas (e.g., nitrogen as a dopant into the helium buffer gas) may be added to allow slightly higher collision energy prior to discharge, but this approach may not be good enough. The problem of achieving high collision energy while avoiding discharge is addressed in U.S. patent No. 9,916,968 to Kurulugama et al, the entire disclosure of which is incorporated herein by reference, and which relates to endogenous ion activation.

Mass spectrometers that employ an ion funnel interface to couple an atmospheric ionization region with a high vacuum region do not have a capillary-interceptor interface. Instead, the capillary tube may be directly connected to a sub-atmospheric pressure region of a vacuum chamber containing the ion funnel device. Here, the capillary may be in line with or orthogonal to the ion funnel axis. When the capillary is orthogonal to the ion funnel axis, an ion deflector plate may be used to direct ions into the ion funnel. For capillary-ion funnel interfaces, it may be more challenging to achieve ion activation due to the high pressures at which the ion funnel is operated and due to the mechanical design.

There remains a need to provide improved endogenous ion activation, unfolding and fragmentation in high pressure regions of mass spectrometers or additional analysis devices, such as stand alone ion mobility spectrometers, or in hybrid IM-MS instruments. There is also a need to provide improved desolventization and declustering of analyte ions prior to mass spectrometry.

Disclosure of Invention

To address the foregoing needs, in whole or in part, and/or other needs that may have been observed by those skilled in the art, the present disclosure provides methods, processes, systems, devices, apparatuses, and/or devices, as described by way of example in the implementations set forth below.

The present disclosure provides apparatus and methods for ion activation, including molecular ion refolding and/or fragmentation for use in IMS systems, MS systems, and IM-MS systems. The examples disclosed herein may allow for higher excitation levels than previously known devices and methods. The examples disclosed herein may achieve this by enabling the deposition of an increased amount of internal energy in the ions as compared to previously known apparatus and methods.

In accordance with examples disclosed herein, an ion source may include an ionization chamber for maintaining at atmospheric pressure. The ion source may further comprise a pressure drop chamber for maintaining at sub-atmospheric pressure, and ion transfer means comprising an inlet in the ionization chamber and an exhaust in the pressure drop chamber. The ion transfer means may define an ion path from the inlet to the outlet. The ion transfer device may be positioned to emit ions and neutral gas molecules from the discharge outlet as an expanded beam comprising a low gas density region surrounded by a high gas density region, the high gas density region having a higher gas density than the low gas density region.

According to examples disclosed herein, an ion source may include: an atmospheric pressure ionization chamber, a pressure drop chamber configured to maintain a sub-atmospheric pressure therein, and an ion transfer device comprising an inlet in the ionization chamber and an exhaust in the pressure drop chamber and defining an ion path from the inlet to the exhaust. The ion transfer device may be configured to emit ions and neutral gas molecules from the discharge outlet as an expanded beam comprising a low gas density region surrounded by a high gas density region, the high gas density region having a higher gas density than the low gas density region. An electrode may be located in the pressure drop chamber at a gap distance from the discharge port. The discharge port and the electrode may be configured to generate an electric field therebetween to accelerate ions emitted from the discharge port to collision energy effective to induce ion activation of ions in the pressure drop chamber without voltage breakdown. Further, the exhaust and the electrode may be configured to position the electric field in overlapping relation with the low gas density region.

For the ion sources described above, the pressure drop chamber may be configured to maintain the sub-atmospheric pressure in the range of about 0.5 torr to about 30 torr.

The ion source may further comprise a vacuum system configured to reduce the pressure drop chamber to the sub-atmospheric pressure.

For the ion source described above, the gap distance may be in the range of between about 0.5mm to about 5 mm.

For the ion source described above, the exhaust may be positioned on an exhaust axis and the electrode may include an aperture positioned on the exhaust axis.

For the ion source described above, the electrode may comprise a cylindrical section defining the aperture.

For the ion source described above, the exhaust port and the electrode may each comprise substantially equal inner diameters.

For the ion source described above, the exhaust and the electrode may be configured to control the expanded beam such that the low gas density region transitions to a mach disk located at or in the aperture.

For the ion source described above, the ion transfer means may comprise a main bore having an inner diameter less than an inner diameter of the discharge port, and a tapered section fluidly coupling the main bore to the discharge port. The inner diameter of the tapered section may increase from the inner diameter of the main bore to the inner diameter of the discharge port.

For the ion source described above, the ion transfer means may comprise: a capillary tube through which the primary bore extends; and a cap mounted to or as part of the capillary tube. The cap may include the tapered section and the discharge port.

For the ion source described above, the tapered section and the exhaust port may be configured to control the expanded beam such that the low gas density region extends from the exhaust port to the electrode.

The ion source may further comprise a voltage source configured to apply a potential difference between the discharge outlet and the electrode to generate the electric field.

For the ion sources described above, the voltage source may be configured to apply a potential difference in the range from about 0V to about 1000V.

For the ion source described above, the potential difference across the electric field may be specified to be sufficiently high to increase the collision energy to a value effective to promote desolvation of solvated ions emitted from the discharge outlet. Alternatively or additionally, the potential difference across the electric field may be specified to be sufficiently high to increase the collision energy to a value effective to promote de-clustering of cluster ions emitted from the discharge outlet. Alternatively or additionally, the potential difference across the electric field may be specified to be sufficiently high to increase the collision energy to a value effective to cause folding (bio) molecular ions emitted from the discharge outlet to be unfolded by collision-induced unfolding. Alternatively or additionally, the potential difference across the electric field may be specified to be sufficiently high to increase the collision energy to a value effective to collapse folded (bio) molecular ions emitted from the discharge outlet by collision-induced collapse without dissociating the (bio) molecular ions. Alternatively or additionally, the potential difference across the electric field may be specified to be sufficiently high to increase the collision energy to a value effective to dissociate ions emitted from the discharge outlet by collision-induced dissociation.

The ion source may further comprise an ion guide positioned along an ion guide axis in the pressure drop chamber.

For the ion sources described above, the ion guide may be configured to generate a radio frequency electric field effective to limit radial movement of ions relative to the ion guide axis.

For the ion sources described above, the ion guide may be configured to generate a direct current potential gradient along the ion guide axis.

For the ion source described above, the ion guide may comprise an ion guide inlet along the ion guide axis and an ion guide outlet spaced from the ion guide inlet. The ion guide inlet may surround at least a portion of the electrode.

For the ion sources described above, the ion guide may comprise an ion funnel.

For the ion sources described above, the ion guide may comprise a plurality of ion guide electrodes spaced apart from one another along the ion guide axis. The ion guide electrode may comprise a plurality of respective ion guide apertures.

For the ion source described above, the discharge opening may be located on a discharge opening axis that is radially offset from the ion guide axis.

For the ion source described above, the ion guide may be designated as a first ion guide, and the ion guide axis may be designated as a first ion guide axis. The ion source may further include: a second ion guide positioned along a second ion guide axis and configured to receive ions from the first ion guide.

For the ion sources described above, the second ion guide may be configured to generate an electric field effective to trap ions in the second ion guide for a controlled period of time.

For the ion source described above, the second ion guide may comprise an ion funnel.

For the ion source described above, the second ion guide may comprise a plurality of ion guide electrodes spaced apart from one another along the ion guide axis. Further, the ion guide electrode may comprise a plurality of respective ion guide apertures.

For the ion source described above, the second ion guide axis may be radially offset from the first ion guide axis.

The ion source may further comprise ionization means for generating ions from the sample in the ionization chamber by atmospheric pressure ionization.

For the ion source described above, the pressure drop chamber may not include an skimmer.

In accordance with examples disclosed herein, a spectrometry system can include an ionization chamber for maintaining at atmospheric pressure. The pressure drop chamber may be maintained at sub-atmospheric pressure. The ion transfer means may comprise an inlet in said ionization chamber and an exhaust in said pressure drop chamber. The ion transfer means may define an ion path from the inlet to the outlet. An electrode may be located in the pressure drop chamber at a gap distance from the discharge port.

According to examples disclosed herein, a spectrometry system can include an ion source according to any of the examples disclosed herein. Further, the spectrometry system can include a vacuum housing configured to receive ions from the pressure drop chamber, and an ion analyzer in the vacuum housing.

For the spectrometry system described above, the ion analyzer may comprise an ion mobility cell or a mass analyzer.

For the spectrometry system described above, the ion analyzer may represent a first ion analyzer. The spectrometry system can further comprise a second ion analyzer configured to receive ions from the first ion analyzer.

For the spectrometry system described above, the first ion analyzer may be an ion mobility cell and the second ion analyzer may be a mass analyzer.

For the spectrometry system described above, the first ion analyzer may be an ion mobility cell. Further, the second ion analyzer may be a mass spectrometer comprising a first mass analyzer, a collision cell configured to receive ions from the first mass analyzer, and a second mass analyzer configured to receive ions from the collision cell.

In accordance with examples disclosed herein, a method for analyzing a sample may include performing atmospheric pressure ionization in an ionization chamber to generate ions from the sample. The method may further comprise transferring the ions from the ionization chamber to a pressure drop chamber maintained at sub-atmospheric pressure. Further, the method may include subjecting the ions emitted into the pressure drop chamber to an electric field that accelerates the ions to collision energy effective to induce ion activation of the ions without voltage breakdown.

In accordance with examples disclosed herein, a method for analyzing a sample may include performing atmospheric pressure ionization in an ionization chamber to generate ions from the sample. The method may further comprise transferring the ions from the ionization chamber to a pressure drop chamber maintained at sub-atmospheric pressure. The ions and neutral gas molecules may be emitted into the pressure drop chamber as an expanded beam comprising a low gas density region surrounded by a high gas density region having a higher gas density than the low gas density region. Further, the method may include subjecting the ions emitted into the pressure drop chamber to an electric field that accelerates the ions to collision energy. The collision energy may be effective to induce ion activation of the ions without voltage breakdown. The electric field may be positioned in overlapping relation with the low gas density region.

For the above method, the method may further comprise maintaining the pressure drop chamber at a pressure in a range between about 0.5 torr and about 30 torr.

For the above method, said transferring of said ions may comprise ejecting said ions from an ejection outlet of an ion transfer device. Further, subjecting the ions to the electric field may include applying a potential difference between the discharge outlet and electrodes in the pressure drop chamber to accelerate the ions to the collision energy.

For the above method, the transferring of the ions may comprise controlling the expanded beam such that the low gas density region transitions to a mach disk located at or in the aperture.

The above method may further comprise applying the electric field at a potential difference in the range from about 0V to about 1000V.

For the above method, the collision energy may be selected from: an impact energy effective to promote desolvation of solvated ions emitted from the discharge outlet, an impact energy effective to promote declusation of clustered ions emitted from the discharge outlet, an impact energy effective to cause folding biomolecular ions emitted from the discharge outlet to be unfolded by impact-induced unfolding without dissociating the biomolecular ions, and/or an impact energy effective to cause ions emitted from the discharge outlet to be dissociated by impact-induced dissociation.

For the above method, the transferring of the ions may include emitting the ions into an ion guide located in the pressure drop chamber.

For the above method, the ion guide may comprise an ion funnel.

The method may further comprise transferring the ions into an ion analyzer after transferring the ions into the pressure drop chamber.

For the above method, the ion analyzer may comprise an ion mobility cell or a mass analyzer.

For the above method, the ion analyzer may represent a first ion analyzer. The method may further comprise transferring the ions from the first ion analyzer to a second ion analyzer.

For the above method, the first ion analyzer may represent an ion mobility cell and the second ion analyzer may represent a mass analyzer.

The method may further comprise transferring the ions into an ion mobility cell and subsequently into an ion detector after transferring the ions into the pressure drop chamber.

The method may further comprise measuring a respective arrival time of the ions at the ion detector relative to a time at which the ions are transferred into the ion transfer cell.

The method may further comprise calculating a time of arrival distribution of the ions, and/or calculating a collision cross section of the ions, based on the measured time of arrival.

For the above method, the ions transferred into the pressure drop chamber may comprise folded biomolecular ions. The collision energy may be effective to unfold the folded biomolecular ions, and the measuring the respective arrival times may comprise measuring the respective arrival times of the unfolded ions.

For the above method, the collision energy may be effective to generate fragment ions by collision induced dissociation, and the measuring of the respective arrival times may include measuring the respective arrival times of the fragment ions.

The method may further comprise transferring the ions into a mass analyzer and subsequently into an ion detector after transferring the ions into the pressure drop chamber to produce a mass spectrum of the ions.

For the above method, the collision energy may be effective to generate fragment ions by collision-induced dissociation. The method may further comprise generating a mass spectrum of the fragment ions.

The method may further comprise transferring the ions to an ion mobility cell after transferring the ions to the pressure drop chamber, and subsequently transferring the ions to the mass analyzer.

The method may further comprise generating an ion mobility drift time spectrum and a mass spectrum of the ions after transferring the ions to the ion detector.

For the ion sources and methods described above, the overlapping relationship may correspond to 50% or more, or 60% or more, or 70% or more, or 80% or more, or 90% or more, or 100% overlap of the electric field with the low gas density region.

According to examples disclosed herein, a spectrometry system can include a memory and at least one processor for performing all or part of any of the methods disclosed herein.

In accordance with examples disclosed herein, a spectrometry system can include a controller, and an ion detector in communication with the controller. The spectrometry system can be configured to perform all or part of any of the methods disclosed herein.

According to examples disclosed herein, a non-transitory computer readable storage medium may include instructions for performing all or part of any of the methods disclosed herein.

According to examples disclosed herein, a system may include the computer-readable storage medium.

Other devices, apparatuses, systems, methods, features and advantages will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.

Drawings

The invention may be better understood by reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the drawings, like reference numerals designate corresponding parts throughout the different views.

FIG. 1 is a schematic diagram of an example of a spectrometry system or instrument according to examples disclosed herein.

Fig. 2A is a schematic perspective view of a discharge port end of an example ion transfer device according to examples disclosed herein.

Fig. 2B is a schematic cross-sectional view of the discharge end of the ion transfer device illustrated in fig. 2A and an electrode for ion activation/fragmentation also in accordance with examples disclosed herein.

Fig. 2C is another schematic cross-sectional view of the ion transfer device and the discharge port end of the electrode illustrated in fig. 2B, also schematically illustrating an example of the structure of the gas/ion flow and the electric field applied between the discharge port end and the electrode.

FIG. 3 is a schematic diagram of an example of a spectrometry system or instrument according to another example disclosed herein.

Fig. 4A shows an ion mass spectrum of tuned mixed ions obtained by operating an IM-qTOF instrument with an internal source ion activation voltage set to 0V.

Fig. 4B shows fragment ion mass spectrometry data of a sample obtained by operating an IM-qTOF instrument with a previously known configuration, in which the endogenous ion activation voltage was set at 400V.

Fig. 4C shows fragment ion mass spectrometry data for the same sample as associated with fig. 4B, obtained by operating an IM-qTOF instrument having a configuration as disclosed herein, wherein the endogenous ion activation voltage is set at 400V.

Detailed Description

In this disclosure, when a particular numerical value is modified, the term "about" is considered to include a numerical range of +/-10% of that numerical value.

The present disclosure describes apparatus and methods for improved ion activation and fragmentation of molecules, including proteins and other biomolecules, and Collision Induced Unfolding (CIU) for structural analysis in conjunction with Mass Spectrometry (MS), ion Mobility Spectrometry (IMS), and mixed ion mobility mass spectrometry (IM-MS) instruments. The devices and methods described herein provide high ionic activation energy in the high pressure region (e.g., about 0.5 torr to about 30 torr), which allows for the unfolding of macromolecules such as proteins and biomolecules, including large native proteins such as monoclonal antibodies. According to an aspect of the present disclosure, such high ion activation energy may be achieved in a high gas pressure region while avoiding voltage breakdown and further without the need to add a dopant gas to the ion source region. Ion activation and refolding may be accomplished, for example, prior to ion mobility separation with or without mass spectrometry, or prior to mass spectrometry without prior ion mobility separation. Ion activation may also be used to improve desolvation and declustering of gas phase ions prior to ion mobility separation with or without mass spectrometry, or prior to mass spectrometry without prior ion mobility separation. Ion activation may also be achieved prior to ion migration separation to enable determination of the time-of-arrival distribution or Collision Cross Section (CCS) change of the ion unfolding pattern accompanying the (bio) molecule.

Fig. 1 is a schematic diagram of an example of a spectrometry system or instrument 100 according to an example. The operation and design of the various components of mass spectrometry systems, including Mass Spectrometry (MS) systems, ion Mobility Spectrometry (IMS) systems, and hybrid ion mobility mass spectrometry (IM-MS) systems, are generally known to those skilled in the art and, therefore, need not be described in detail herein. Rather, certain components are briefly described to facilitate an understanding of the presently disclosed subject matter.

In a series of ion processing flows, the spectrometry system 100 can generally include: an ion source 102 configured to generate gas phase ions 108 from a sample 110 introduced into the ion source 102; and a spectrometer 106 configured to receive ions from the ion source 102 and process the ions as needed to produce analytical data describing the ions and thereby produce components of the original sample 110. The horizontal arrows in fig. 1 indicate the general or synthetic direction of ions through the spectrometry system 100.

The ion source 102 may generally include a housing 112 enclosing an ionization chamber 104 in which ions 108 are generated, and an ion source-spectrometer interface 120 configured to receive and transfer ions into a spectrometer 106. One or more ionization devices 124 may be configured (and positioned) to ionize components of the sample 110 in the ionization chamber 104. The ionization chamber 104 may be maintained at (or about) atmospheric pressure. The interface 120 may include one or more pressure drop chambers 128 (or chambers having one or more pressure drop regions) configured to reduce the gas pressure relative to the ionization chamber 104 and collect and compress ions into a beam in preparation for transferring ions into the spectrometer 106. The one or more inner walls 130 may provide a physical boundary between the ionization chamber 104 and the (first) pressure drop chamber 128. The pressure drop chamber 128 may be maintained at a reduced pressure, also referred to herein as a (high) sub-atmospheric pressure. In this context, high sub-atmospheric pressure may refer to a pressure that is lower than the vacuum level of the pressure maintained in the ionization chamber 104, but higher than the pressure maintained in Yu Puyi. As one non-limiting example, the high sub-atmospheric pressure may be in the range from about 0.5 torr to about 30 torr.

The ion source 102 may further include an ion transfer device 132 configured (in position) to transfer ions (and neutral gas molecules) from the ionization chamber 104 to the pressure drop chamber 128. To this end, ion transfer device 132 may include an inlet 136 in fluid communication with ionization chamber 104 and an exhaust 138 in fluid communication with pressure drop chamber 128. The ion transfer device 132 may extend from an inlet 136, through one or more inner walls 130 between the ionization chamber 104 and the pressure drop chamber 128, and to an exhaust 138. The ion transfer device 132 may thus define an ion path from the inlet 136 to the outlet 138 and emit a flow of ions and neutral gas molecules from the outlet 138. The ion source 102 may further include an electrode assembly 140 positioned in the pressure drop chamber 128. A voltage source 142 provided by suitable electronics of the spectrometry system 100 can be in electrical communication with the exhaust 138 of the ion transfer device 132 and the electrode assembly 140. Representative examples of ion transfer devices 132 and electrode assemblies 140 are described in more detail below.

The spectrometer 106 may generally include a housing (or vacuum enclosure) 144 configured to receive ions 128 from the pressure drop chamber. The vacuum housing 144 may enclose one or more vacuum chambers 146. The ion analyzer 116 and the ion detector 150 may be located in at least one of the vacuum chambers 146. In one example, the spectrometer 106 may be a Mass Spectrometer (MS) configured to generate an ion mass (m/z) spectrum, in which case the ion analyzer 116 may include at least one mass analyzer. In another example, the spectrometer 106 may be an Ion Mobility Spectrometer (IMS) configured to generate an ion drift spectrum and calculate an ion Collision Cross Section (CCS), in which case the ion analyzer 116 may include at least one Ion Mobility (IM) drift cell. In fig. 1, the ion analyzer 116 may also schematically represent other ion processing apparatus, which may include additional ion analyzers. Thus, in another example, the spectrometer 106 may be a hybrid ion mobility mass spectrometry (IM-MS) instrument configured to generate two-dimensional IM-MS spectral data. In this case, the ion analyzer 116 may include a first ion analyzer followed by a second ion analyzer configured to receive ions from the first ion analyzer. The first ion analyzer may be an IM drift cell and the second ion analyzer may be a mass analyzer.

In another example, an ion fragmentation device can be positioned between the first ion analyzer and the second ion analyzer, thereby causing the spectrometer 106 to produce a fragment ion spectrum. In this case, the first ion analyzer may be a mass analyzer (e.g., a mass filter) configured to select precursor ions for fragmentation, and the second ion analyzer may be a (final) mass analyzer configured to mass resolve product ions generated from the precursor ions in the ion fragmentation device. In another example, the spectrometer 106 may comprise an IM-drift cell, followed by a mass analyzer, followed by an ion fragmentation device, and then a (final) mass analyzer. In another example, the IM-drift cell may be configured as a Trapped Ion Mobility Spectrum (TIMS) tunnel, which may be configured to selectively release ions from the ion funnel according to its mobility. In another example, the configuration of the IM portion of the instrument may be a structure based on a non-destructive ion manipulation (SLIM) technique. The SLIM device may include one or more linear segments, each segment defined by a pair of planar plates (e.g., printed circuit boards or PCBs). Each plate may include a combination of flat DC guard electrodes and RF/DC electrodes (or, alternatively, RF-only electrodes) configured to confine ions and push ions forward through the SLIM device. One or more SLIM segments may be operated to facilitate collision activation and/or create ion accumulation (storage and release) regions within the SLIM device. The plurality of SLIM segments may be arranged in a serpentine fashion to increase the ion path length through the SLIM device. See, e.g., tolmarhev et al Characterization of Ion Dynamics in Structures for Lossless Ion Manipulations, anal. Chem.2014,86,9162-9168; may et al Resolving Power and Collision Cross Section Measurement Accuracy of a Prototype High-Resolution Ion Mobility Platform Incorporating Structures for Lossless Ion Manipulation, J.Am.Soc.Mass Spectrom.2021,32,1126-1137; arndt et al, high-Resolution Ion-Mobility-Enabled Peptide Mapping for High-Throughput Critical Quality Attribute Monitoring, doi.org/10.1021/jasms.0c00434, J.Am.Soc.Mass Spectrom (2021), the entire contents of each of which are incorporated herein by reference. Other High Resolution IM (HRIM) devices now known or later developed may also be suitable for use in the examples disclosed herein.

The at least one interior wall 148 may provide a physical boundary between the (last) pressure drop chamber 128 of the ion source 102 and the (first) vacuum chamber 146 of the spectrometer 106. Depending on the type of ion processing equipment operating in the spectrometer 106 and the number of different vacuum chambers 146 provided, different vacuum levels may be maintained in different areas of the vacuum housing 144. For example, the IM drift cell may be "pressurized" to a drift gas pressure in the range from, for example, 1 to 10 torr. More generally, the IM drift cell may be configured to operate at pressures up to atmospheric pressure. Thus, an IM drift cell located in the spectrometry system 100 can operate in a range from about 1 torr to about 760 torr. On the other hand, the mass analyzer may be used in a range from, for example, 10 ^-4 To 10 ^-9 Operating at pressures in the range of torr. The spectrometry system 100 can include a vacuum system configured to maintain various regions of the spectrometry system 100 at a desired pressure level and remove non-analytical neutral molecules from the ion path, as schematically represented in fig. 1 by arrows 154 and associated ports in communication with corresponding chambers. To this end, the vacuum system may include various components (ports, conduits, pumps, etc.), as will be appreciated by those skilled in the art.

An opening 156 through the wall 148 may provide a path to transfer ions into the vacuum chamber 146. Various ion optics may define the opening 156 or be located adjacent to the opening 156. For example, a skimmer cone (or sampling cone) may be positioned at the opening 156 or define the opening 156. While a skimmer may be provided in the examples disclosed herein, a skimmer may not be necessary, as will become apparent from the further description herein.

In general, ion transfer arrangement 132 may take a variety of forms. In typical examples contemplated for this disclosure, ion transfer device 132 may be or include a capillary tube. The geometry of the capillary tube may be desirable for various reasons. The small diameter holes of the capillaries may act as gas conduction barriers that help maintain the pressure differential between the higher pressure ionization chamber 104 and the lower pressure drop chamber 128 and may reduce the amount of gas molecules transferred into the pressure drop chamber 128 with ions. In addition, the length of the capillary may provide opportunities for ion desolvation and declustering and neutral droplet evaporation to occur in the capillary. This mechanism may be enhanced by providing a heating device (not shown) in thermal contact with the capillary tube. In some examples, the capillary tube may be composed of glass. In some examples, the capillary tube may include resistive or conductive elements at or near the inlet 136 and the outlet 138 to enable a potential difference to be applied across the capillary tube.

In some examples and as described further below, ion transfer device 132 may include an end cap structure mounted at the outlet end of the row of capillaries. The end cap structure may be electrically conductive and receive a (typically electrostatic) voltage potential from a voltage source 142. The end cap structure may also be configured (e.g., in shape, size, position) to regulate or control the flow of gas/ions emitted from the ion transfer device 132 in a manner described below.

The electrode assembly 140 may include at least one electrode 158 (or counter electrode, also referred to herein as an ion activated electrode or a fragmenting electrode) positioned in the pressure drop chamber 128 a predetermined axial gap distance (or vent-electrode distance) from a vent 138 (e.g., capillary outlet) of the ion transfer device 132. In this context, the term "axial" may relate to a longitudinal axis along which the ion transfer device 132 is arranged, which also generally corresponds to an axis along which ions travel from the discharge port 138. The electrode 158 may include an electrode aperture 174 coaxially positioned at a predetermined axial gap distance. As a non-limiting example, the gap distance (axial distance between the exhaust port 138 and the electrode 158) may be in the range of about 0.5mm to about 5 mm. Electrode assembly 140 may also include one or more structural components (e.g., electrically insulating components) as desired for mounting electrode 158 in a fixed position in pressure drop chamber 128, routing wiring to electrode 158, etc., as will be appreciated by those skilled in the art.

The voltage source 142 may be configured to apply a predetermined potential difference between the ion transfer device 132 (e.g., its exhaust port 138) and the electrode 158 to accelerate ions emitted from the exhaust port 138 to a predetermined collision energy at which the ions collide with neutral gas molecules in the pressure drop chamber 128. The magnitude of the potential difference may be selected for a given pressure and discharge-electrode distance such that the collision energy effectively causes collisional heating/activation of ions in the pressure drop chamber 128 without voltage breakdown. As non-limiting examples, the voltage source 142 may be configured to apply a potential difference in the range from about 0V to about 1000V, or from about 0V to about 500V.

The magnitude of the potential difference may be selected such that the collision energy is effective to achieve the desired ion activation pattern. As an example, the collision energy may be increased or adjusted to a value effective to promote desolvation of solvated ions emitted from the discharge port 138, and/or to a value that promotes declustering of clustered ions emitted from the discharge port 138. Additionally, the collision energy may be increased or adjusted to a value effective to dissociate ions emitted from the discharge port 138 by Collision Induced Dissociation (CID). Additionally, the collision energy may be increased or tuned to a value effective to cause folding (bio) molecular ions emitted from the discharge port 138 to unfold while dissociating or not dissociating the bio molecular ions by Collision Induced Unfolding (CIU), as desired in a particular application. According to an aspect of the present disclosure, electrode assembly 140 may be configured such that all such modes may be performed in a high pressure environment (e.g., in the range from about 0.5 torr to 30 torr as specified elsewhere herein) without causing undesirable discharge due to voltage breakdown. The vent-electrode distance may be set or adjusted as needed to prevent voltage breakdown and/or to optimize conditions for a particular ion activation mode, taking into account the pressure range and collision energy range contemplated for a given application.

The configuration of ion transfer device 132 and electrode assembly 140 may allow a relatively high electric field to be achieved at the discharge port 138 (e.g., capillary outlet) of ion transfer device 132, improving some collision-based activities compared to conventional ionization-spectrometer interfaces, and making other collision-based activities impractical or impossible in conventional ionization-spectrometer interfaces. The ion transfer device 132 and electrode assembly 140 may operate at a higher pressure regime than conventional capillary-interceptor interfaces. An interceptor may not be required in examples of the present disclosure.

The ion transfer device 132 and the electrode assembly 140 may work in conjunction with other ion processing devices provided in the pressure drop chamber(s) 128, such as ion guides and ion funnel-based devices, as described below in conjunction with fig. 3. The ion guide in the pressure drop chamber may be configured to generate a radio frequency electric field effective to limit radial movement of ions relative to the ion guide axis and/or to generate a direct current potential gradient along the ion guide axis. The ion guide may include an ion guide inlet along an ion guide axis and an ion guide outlet spaced apart from the ion guide inlet. The ion guide inlet may surround at least a portion of the electrode assembly 140. The discharge port 138 of the ion transfer arrangement 132 may be located on a discharge port axis that is radially offset from the ion guide axis. The ion guide may comprise a plurality of ion guide electrodes spaced apart from one another along an ion guide axis and comprise a plurality of respective ion guide apertures. The ion guide may comprise or be configured as an ion funnel, or be configured as another type of stacked ring ion guide, such as an S-lens. Another example may be a combined ion guide, which may comprise two stacked ring ion guides of different diameters. The axes of the two stacked ring ion guides may be parallel but offset from each other such that one stacked ring ion guide is above the other stacked ring ion guide. The ring electrodes of the two stacked ring ion guides may be slotted, e.g., they are not complete rings, but rather have open gaps. The gap of the lower stacked ring ion guide may be facing upward and the gap of the upper stacked ring ion guide may be facing downward and thus toward the gap of the lower stacked ring ion guide. Under the influence of the DC potential difference, ions may enter the lower stacked ring ion guide and move upward through the gap and into the upper stacked ring ion guide.

The pressure drop chamber(s) 128 may include a plurality of ion guides, such as a first ion guide positioned along a first ion guide axis, and a second ion guide positioned along a second ion guide axis and configured to receive ions from the first ion guide. The second ion guide may be configured to generate an electric field effective to trap ions in the second ion guide for a controlled period of time. The second ion guide may include or be configured as an ion funnel. The second ion guide may include a plurality of ion guide electrodes spaced apart from one another along the ion guide axis and include a plurality of corresponding ion guide apertures. The second ion guide axis may be radially offset from the first ion guide axis.

In addition, the ion transfer device 132 and the electrode assembly 140 may work in conjunction with operating a separate collision cell in the spectrometer 106. Methods of ion activation using both the electrode assembly 140 and the collision cell may be developed to produce additional information about the ions that is not possible or readily determinable from the use of either the electrode assembly 140 alone or the collision cell.

The spectrometry system 100 can also include a controller (or system controller, or computing device) 176 configured to control or monitor the various components and functions of the spectrometry system 100. For example, the controller 176 may control or perform preprogrammed operations of the voltage source 142 and thus control the electric field and collision energy implemented in the pressure drop chamber 128 of the ion source 102.

An example of a method for analyzing a sample will now be described with reference to fig. 1. The ion source 102 may be operated to perform atmospheric pressure ionization to generate ions from a sample in the ionization chamber 104. Ions may be transferred via ion transfer device 132 into pressure drop chamber 128, which may be maintained at a relatively high sub-atmospheric pressure. In the pressure drop chamber 128, the ions may be subjected to an electric field that accelerates the ions to collision energy through operation of the voltage source 142 and the electrode assembly 140. The collision energy may effectively cause collision heating of ions in the pressure drop chamber 128 without voltage breakdown. The collision energy may be set to a value effective to perform a desired treatment on ions emitted from the discharge port 138 of the ion transfer device 132 and into the pressure drop chamber 128. Examples may include facilitating desolvation of solvated ions, facilitating decolonization of clustered ions, fragmenting ions by collision-induced dissociation, and unfolding folded biomolecular ions (with or without fragmenting ions) by collision-induced unfolding. The collision energy may be set by controlling an electric field, which may be generated by applying a potential difference between the ion transfer device 132 and the electrode assembly 140. The potential difference may be, for example, in the range of about 0V to about 1000V.

In one example, after transferring ions into the pressure drop chamber 128, the ions may be transferred into an ion mobility cell of the spectrometer 106 to separate the ions according to mobility. The separated ions may then be transferred to an ion detector 150. The ion detector 150 may be used to measure the respective arrival times of ions at the ion detector 150 relative to the time at which the ions were transferred into the ion transfer cell. Based on the measured arrival times, the arrival time distribution of the ions and/or the collision cross section of the ions may be calculated. In the case of folded (bio) molecular ions, these ions may first be unfolded in the pressure drop chamber 128 as described above, and the arrival time of the unfolded ions may be measured. Also as described above, fragment ions may be generated in the pressure drop chamber 128 and the arrival time of the fragment ions may be measured.

In another example, after transferring ions into the pressure drop chamber 128, the ions may be transferred into a mass analyzer of the spectrometer 106 to separate the ions according to a mass to charge ratio (m/z). The separated ions may then be transferred to an ion detector 150. The signal output from the ion detector 150 may be used to generate a mass spectrum of ions, which may be fragment ions generated in the pressure drop chamber 128 as described above.

In another example, after transferring ions into the pressure drop chamber 128, the ions may be transferred into an ion mobility cell and then into a mass analyzer of the spectrometer 106. In this way, both ion mobility drift time spectra and mass spectra of ions can be generated.

Fig. 2A is a schematic perspective view of a discharge end of an example of an ion transfer device 232 according to an example. Fig. 2B is a schematic cross-sectional view of the discharge end of the ion transfer device 232 and also of the electrode 258 for ion activation/fragmentation. These components are shown as they would be positioned relative to one another in the pressure drop chamber 128 of the ion source-spectrometer interface 120, as described above in connection with fig. 1.

The ion transfer device 232 may include a discharge port 238 located on the discharge port axis 203. Electrode 258 may also be located on discharge port axis 203 and may be axially spaced from discharge port 238 by a gap distance G (also referred to herein as a discharge port-electrode distance). Specifically, electrode 258 may include electrode aperture 274 located at gap distance G on discharge port axis 203. In one non-limiting example, the gap distance G may be in the range from about 0.5mm to about 5 mm. In operation, a stream (or spray) of ions and neutral gas molecules may be emitted from the discharge outlet 238 and through the electrode aperture 274 toward the inlet of the spectrometer 106 (e.g., the opening 156 of fig. 1). Ions and gas molecules may be driven through ion transfer device 232 under the influence of a pressure differential between ionization chamber 104 and pressure drop chamber 128 (fig. 1). Ions may also be directed into the inlet 136 of the ion transfer means 232 by an appropriately positioned electric field applied in the ionization chamber 104 (fig. 1), and may be forced through the ion transfer means 232 by an electric field applied across the length of the ion transfer means 232. Upon exiting the discharge outlet 238, the ions may be accelerated toward and through the electrode aperture by an electric field applied between the discharge outlet 238 and the electrode 258 (e.g., by the voltage source 142 shown in fig. 1).

In this example, the ion transfer device 232 may include a main bore 207 that fluidly interconnects the inlet 136 (in the ionization chamber 104 of fig. 1) with the exhaust 238. The main bore 207 may extend straight along the discharge outlet axis 203. The inner diameter of the main bore 207 may range from 0.3mm to 1.5 mm. Also in this example, ion transfer device 232 may include a tapered section 211 that fluidly couples main bore 207 and drain 238. The inner diameter of the inlet of the tapered section 211 may be equal or substantially equal to the inner diameter of the main bore 207 (e.g., +/-0.3 mm). The tapered section 211 may taper or diverge outwardly in a direction toward the electrode 258. That is, the inner diameter of tapered section 211 may increase from the inner diameter of main bore 207 to the inner diameter of drain 238 in a direction toward electrode 258. The inner diameter of the discharge outlet 238 may range from 2mm to 5 mm. The tapered section 211 and the discharge outlet 238 may act as a divergent nozzle. As shown in FIG. 2B, the inner diameter of the exhaust port 238 may be equal or substantially equal to the inner diameter of the electrode aperture 274 (e.g., +/-0.3 mm). With this configuration, tapered section 211 and exhaust port 238 may optimize the fluid mechanics of the gas/ion stream emitted from exhaust port 238, as further described below.

In this example, the ion transfer device 232 may include a capillary 215 and a capillary cap 219. The main bore 207 may be formed by the capillary 215 and terminate at a main bore exit 223, which may be the exit of the capillary 215. The capillary cap 219 may be mounted at the discharge end of the capillary tube 215 in any suitable manner. The tapered section 211 may be formed in the capillary cap 219 and the discharge port of the capillary cap 219 may be the discharge port 238 of the ion transfer device 232. Capillary cap 219 may include a cap inlet 227 in fluid communication with main bore outlet 223. The capillary cap 219 may also include a straight bore section 231 fluidly interconnecting the cap inlet 227 with the tapered section 211. The inner diameter of the straight bore section 231 may be equal or substantially equal to the inner diameter of the main bore 207 (e.g., +/-0.6 mm). The capillary cap 219 may be composed of a metal or metal alloy or other suitable conductive material, and may be composed of a material different from that of the capillary 215. In use, the capillary cap 219 may be electrically coupled to the voltage source 142 (fig. 1) such that an electrostatic potential may be applied to the capillary cap 219 to generate an electric field between the drain 238 and the electrode 258.

In another example, capillary cap 219 may be part of capillary tube 215 or integrated with capillary tube 215, e.g., as a single piece construction. In other words, the discharge end of capillary 215 may be configured to include features of capillary cap 219 (e.g., tapered section 211, ion transfer device discharge 238), and a separate capillary cap 219 may not be used.

Fig. 2C is another schematic cross-sectional view of the discharge port end of ion transfer device 232 and ion activation electrode 258, also schematically illustrating the structure of the gas/ion flow and the example of the electric field applied between discharge port end 238 and electrode 258. In an example, the gas/ion stream 235 may exit the main bore 207 (or straight bore section 231) and the exhaust 238 at supersonic (M≡1) or near supersonic and at a pressure different from the pressure of the pressure drop chamber 128. Upon exiting main bore 207 (or straight bore section 231), the gas/ion stream may undergo supersonic expansion and its supersonic increase. Thus, the gas/ion stream 235 may exit the discharge outlet 238 in the form of an expanded (or expanded) beam 239. Depending on the pressure of the exiting gas/ion stream 235 and the pressure maintained in the pressure drop chamber 128, the expanded beam 239 may be over-expanded or under-expanded as will be appreciated by those skilled in the art. During the emission of the gas/ion stream 235, an electric field E may be applied between the discharge outlet 238 and the electrode 258 to accelerate ions toward the electrode 258 and achieve a desired collision energy effective to induce ion activation of the ions without voltage breakdown (discharge), as described above.

The expanded beam 239 may include a low gas density region 243 (also referred to as a "silent region" or "no sound region") located on the discharge port axis, and one or more high gas density regions 247 coaxially surrounding or enclosing the low gas density region 243. The structure of the high gas density region(s) 247 may be in the form of a "barrel shock" as will be appreciated by those skilled in the art. Here, the terms "low" and "high" may be relative to each other. That is, the high gas density region 247 may have a higher gas density than the low gas density region 243. The gas density of the high gas density region(s) 247 may also be higher than the gas density of the surrounding interior of the pressure drop chamber 128. In one non-exclusive example, the gas density of low gas density region 243 may be in the range of 0.1 torr to 2 torr and the gas density of high gas density region(s) 247 may be in the range of 1 torr to 30 torr. The velocity of the gas in low gas density region 243 may be the highest (M > > 1), while the velocity of the gas in high gas density region(s) 247 may be lower but still supersonic (M > 1). The low gas density zone 243 may abruptly transition (or terminate) at another high gas density zone whose impingement structure can be oriented generally in a normal relationship (perpendicular) to the discharge outlet axis and the net direction of gas/ion flow. The high gas density region may be characterized as a "mach disk", "impact diamond" or "forward (wave)" 251, as will be appreciated by those skilled in the art. Outside the mach disc 251, the gas velocity may be subsonic (M < 1).

The low gas density region 243 may facilitate ion activation (with or without ion fragmentation) as determined based on analysis of the spectrometry system 100. Ions exiting the discharge outlet 238 of the ion transfer device 232 may be accelerated through the low gas density region 243 by the applied electric field E. Because of the low gas density in low gas density region 243, ions may be accelerated to a higher translational energy between each collision with a flowing gas molecule. As a result, the collision energy achieved by the presently disclosed examples may be higher compared to previously known ion transfer apparatus and associated counter electrode configurations, and thus the ion excitation (with internal energy states) may be significantly higher compared to previously known configurations. Further, by locating the electric field E in overlapping relation with the low gas density region 243 (or equivalently, locating the low gas density region 243 in overlapping relation with the electric field E), bulk excitation and fragmentation efficiency may be enhanced. Increasing the extent of this overlap may maximize the benefit gained by accelerating ions through low gas density region 243, i.e., allowing for increased internal energy to be deposited in the ions at the available pressure and voltage. Depending on the examples disclosed herein, the electric field E may completely overlap the low gas density region 243 (i.e., the low gas density region 243 may be completely immersed in the electric field E, or the electric field E may overlap the low gas density region 243 100%) or substantially overlap the low gas density region 243. In this context, "substantially overlapping" may refer to the electric field E overlapping the low gas density region 243 by more than 50% (or at least a portion of the low gas density region 243 is located outside of the discharge port 238, assuming that the low gas density region 243 may begin inside the tapered section 211 of the ion transfer device 232). In other examples, the electric field E may overlap the low gas density region 243 by 60% or more, or 70% or more, or 80% or more, or 90% or more.

Thus, in an example, the discharge outlet 238 (and associated tapered section 211, if any) and electrode 258 can be configured (e.g., in position, shape, size, etc.) to position the electric field E in overlapping relation with the low gas density region 243. In particular, the gap distance G between the exhaust port 238 and the electrode 258 (fig. 2B) may be set to maximize the overlap. Further, the discharge outlet 238 (and associated tapered section 211, if any) may be configured (e.g., in position, shape, size, etc.) to control the shape or configuration of the expanded beam 239 to help maximize overlap with the electric field E. In a further example, as schematically depicted in fig. 2C, the exhaust port 238 and the electrode 258 may be configured to position the mach disk 251 at the electrode aperture 274 or in the electrode aperture 274, thereby ensuring that the low gas density zone extends completely from the exhaust port 238 to the electrode 258. At and beyond mach disk 251, the gas density may suddenly increase (e.g., as compared to low gas density region 243). Thus, this configuration may create a highly efficient ion activation region 255 at or within the electrode aperture 274, wherein the probability of collisions between the accelerated ions (which may also be high energy due to interactions occurring in the low gas density region 243) and gas molecules may be greatly increased.

Fig. 3 is a schematic diagram of an example of a spectrometry system or instrument 300 according to another example. The spectrometry system 300 can generally include an ion source 302 and a spectrometer 306, which in this example can be an IM-MS spectrometer and more particularly an IM-qTOF spectrometer. As shown in fig. 1, the general direction of the ion process flow may be left to right.

In this example, in a series of ion processing flows, the ion source 102 may include an ionization chamber 304 and an ion transfer device in the form of a capillary 332 leading to an ion source-spectrometer interface. A capillary cap (not shown) as described above in connection with fig. 2A-2C may also be provided. The interface may include a first pressure drop chamber containing a high pressure ion funnel 368 and a second pressure drop chamber containing an accumulating/pulsing ion trap 334. As one non-limiting example, the high sub-atmospheric pressure at interface operation may be in the range from about 0.5 torr to about 30 torr. As another example, the high pressure ion funnel 368 in the first pressure drop chamber may operate at a pressure in a range from about 2 torr to about 30 torr and the ion trap 334 in the second pressure drop chamber may operate at a pressure in a range from about 1 torr to about 20 torr. As a further example, the ion funnel 368 may operate at a pressure of approximately 5.0 torr and the ion trap 334 may operate at a pressure of approximately 4.0 torr.

In this example, the high pressure ion funnel 368 and ion trap 334 may be configured as an ion funnel comprising a corresponding series of inter-axis spaced apart perforated rings or plate-shaped funnel electrodes, as will be appreciated by those skilled in the art. A Radio Frequency (RF) potential may be applied to the funnel electrode in a manner that constrains radial movement of ions and thereby compresses the ion beam along the respective longitudinal axes of the high pressure ion funnel 368 and ion trap 334, and a Direct Current (DC) potential may be applied to the funnel electrode thereby creating an axial DC voltage gradient to keep the ions moving forward, as also understood by those skilled in the art. The ion trap 334 may include a converging inlet region 378 and a diverging/constant diameter/converging trap region 346. The electrostatic grid electrodes 352 in the trap region 346 may be used to alternately trap ions in the trap region 346 and transfer the ions into the spectrometer 306 in a pulsed manner (periodically releasing ions in packets or pulses). The high pressure ion funnel 368 may be oriented non-coaxially with the ion trap 334, with the axis 368 of the high pressure ion funnel offset (as shown) or angled from the axis of the ion trap 334. This configuration may be used to reduce the amount of neutral species entering the well region 346 and improve ion transport into the well region 346. A similar dual ion funnel system is further described in U.S. patent No. 8,324,565, which is incorporated herein by reference in its entirety.

The ion source 102 further can include an electrode assembly 340 positioned near the discharge of the capillary 332 and configured in accordance with any of the examples described herein. The capillary 332 may extend a small distance into the inlet end of the high pressure ion funnel 368 and thus the electrode assembly 340 may be positioned in the inlet end of the high pressure ion funnel 368. The voltage between the capillary outlet and the first funnel inlet electrode of the high voltage ion funnel 368 may be about 50V. Based on this mechanical design, it can be challenging to obtain a sufficiently high electric field at the capillary outlet to cause collision-induced ion activation of larger biomolecules. However, the electrode assembly 340 may operate in the inlet region of the high pressure ion funnel 368 to easily achieve collision-induced ion activation as described herein.

In this example, in a series of ion processing flows, the spectrometer 306 may include an IM analyzer (drift cell) 342, a rear ion funnel 360 immediately following the drift cell 342, one or more linear multipole ion guides 362 and 364 (e.g., hexapole, octapole, etc.) and/or other ion optics behind the rear ion funnel 360, a quadrupole mass filter 418 for selecting ions, a linear multipole-based collision cell 422 for producing fragment ions, an ion beam compressor 426, inlet optics 402, a time-of-flight (TOF) analyzer 316, and an ion detector 350. Alternatively, the filter 418 (or another filter) may precede the IM drift cell 342.

The cell 342 can include a plurality of cell electrodes 314 spaced apart along a longitudinal axis of the cell 342. In one non-limiting example, the length of the drift cell 342 may be 0.78m, operate at a drift gas (e.g., nitrogen) pressure ranging from about 1 torr to about 10 torr (e.g., about 4 torr), and apply a typical uniform drift axial DC electric field gradient of 20 v/cm. In the presence of a drift gas, the axial field gradient may move ions through the drift cell 342, whereby the ions are separated in time based on their different Collision Cross Sections (CCS), as will be appreciated by those skilled in the art. The controller 176 (fig. 1) may calculate the "drift time" taken for each ion to traverse the length of the drift cell 342 based on the ion arrival time measured at the ion detector 350. The time scale of IM separation may typically be milliseconds (ms). The rear ion funnel 360 may include a plurality of axially spaced apart funnel electrodes 318 that apply RF and axial DC fields as described above. The rear ion funnel 360 may receive the IM-separated ions and forward the IM-separated ions into the spectrometer 306.

Multipole ion guides 362 and 364 may include respective sets of axially elongated guide electrodes 370 and 372 that are circumferentially spaced about respective longitudinal axes of multipole ion guides 362 and 364. As described above, the director electrodes 370 and 372 may apply an RF field to focus ions along an axis. As a non-limiting example, multipole ion guides 362 and 364 can be at 10 ^-3 To 10 ^-5 Operating at pressures in the range of torr.

The quadrupole mass filter 418 can comprise a set of four parallel rod-shaped electrodes positioned at a radial distance from the central axis of the filter 418 and circumferentially spaced from one another about the central axis so as to surround an axially elongated inner filter volume leading from an ion inlet end to an axially opposite ion outlet end of the filter 418. The mass filter 418 may apply a tuned composite RF/DC field to allow selected ions to pass through its ion exit end and further into the spectrometer 306. The filter 418 may thus operate as a bandpass filter, with the operating parameters of the RF/DC field specifying the width of the m/z passband (Δm/z), as well as the low m/z cutoff and the high m/z cutoff of the m/z passband. During some sample runs, or during some periods of a given sample run, the filter 418 may operate as an RF-only ion guide without actively filtering ion transport.

The collision cell 422 may comprise a linear multipole electrode configuration and may be pressurized with a collision gas (e.g., argon, nitrogen, etc.) to a pressure effective for CID, e.g., about 10 millitorr. RF potentials applied to the collisional cell electrodes can focus ions to the central axis of the collisional cell 422, while axial DC voltages applied across the length of the collisional cell 422 can push ions forward through the collisional cell 422. Precursor ions (or "parent" ions) that collide with collision gas molecules of sufficient energy may fragment into fragment ions (or "product" or "daughter" ions). As described above, in addition to operating the electrode assembly 340 in the ion source 302, the collision cell 422 may also be actively operated as an ion fragmentation device. During some sample runs, or during some periods of a given sample run, the collision cell 422 can operate as an RF-only ion guide without actively inducing ion fragmentation.

The ion beam compressor 426 may include a set of multipole electrodes converging toward an axis to enhance beam compression and provide efficient ion transport.

In this example, the TOF analyzer 316 may include an ion accelerator 406, a vacuum (e.g., 10) oriented orthogonal to the inlet optics 402 and the field-free TOF flight region ^-4 To 10 ^-9 A torr) TOF flight tube (not shown), an ion detector 350, and an electrostatic reflector (or ion mirror, or mamorin mirror) 410. The reflector 410 may provide 180 deg. reflection in the ion flight path in the flight tube between the ion accelerator 406 and the ion detector 350, thereby extending the length of the flight path and correcting the kinetic energy distribution of the ions. The area containing the inlet optics 402 may be pumped to the vacuum level of the flight tube. In operation, the ion accelerator 406 may accelerate (e.g., inject) discrete packets of ions into the flight tube at a predetermined pulse rate (or emission rate). TOF injection pulses typically occur on a much faster time scale (microseconds (μs)) than IM injection pulses (milliseconds (ms)). Since the TOF injection rate (frequency) can generally be relatively higher than the IM injection rate (frequency), many TOF injection pulses can occur during the period between two consecutive IM injection pulses. Each ion packet injected into the flight tube may include a range of ion masses depending on how the front filter 418 and collision cell 422 operate. In each ion packet, ions of different masses (m/z ratios) can travel through the flight tube at different speeds and thus have different total flight times, e.g., ions of smaller masses travel faster than ions of larger masses. Thus, each ion packet may be spatially dispersed (e.g., dispersed) according to a time-of-flight distribution. The ion detector 350 may detect and record the time each ion arrives (e.g., impacts) at the ion detector 350. The data acquisition process implemented by the controller 176 (fig. 1) may correlate the recorded time of flight with the m/z ratio.

It will be appreciated that fig. 1-3 may be high-level schematic depictions of examples of spectrometry systems and associated components consistent with the present disclosure. Other components may be included, such as additional structures, vacuum pumps, gas lines, ion optics, ion guides, electronics, and computer-related or electronic processor-related components, as desired for a practical implementation.

Examples

Fig. 4A shows an ion mass spectrum of tuned mixed ions obtained by operating an IM-qTOF instrument with an internal source ion activation voltage set to 0V. This may be the case where the ions do not undergo ion fragmentation. The endogenous ion activation voltage may be a voltage applied to the capillary discharge and the fragmenter lens that increases the Collision Energy (CE) and thereby allows the ions to accelerate to induce ion activation and fragmentation. Fig. 4B shows a fragment ion mass spectrum of the same tuned mixed ion obtained by operating an IM-qTOF instrument with a previously known configuration, with the endogenous ion activation voltage set at 400V. Fig. 4C shows a fragment ion mass spectrum of the same tuned mixed ion obtained by operating an IM-qTOF instrument with a configuration as disclosed herein, wherein the endogenous ion activation voltage was set at 400V. Fig. 4B and 4C show ion Fragmentation Efficiencies (FEs) for several high mass ions and demonstrate that the presently disclosed configurations exhibit improved FEs. For example, an FE of m/z=1522 using the previously known configuration may be about 28%, whereas an FE of m/z=1522 using the presently disclosed configuration may be about 93%. FE may be determined by the following formula:

Fe= [1- (ion signal intensity at 400V/ion signal intensity at 0V) ] x 100

In a typical example, the ionization device used in the ion source as disclosed herein may be an Atmospheric Pressure Ionization (API) device. Examples of API ionization devices may include, but are not limited to, spray ionization devices (e.g., devices for electrospray ionization (ESI), probe electrospray ionization (PESI), desorption electrospray ionization (DESI), solvent Assisted Ionization (SAI), matrix Assisted Ionization (MAI), thermal spray ionization, sonic spray ionization, ultrasonic Assisted Spray Ionization (UASI), etc.), atmospheric Pressure Chemical Ionization (APCI) devices, atmospheric pressure photo-ionization (APPI) devices, atmospheric pressure laser desorption ionization (AP-LDI) devices, atmospheric pressure matrix assisted laser desorption ionization (AP-MALDI) devices, atmospheric pressure plasma-based devices, ambient ionization devices, etc. The sample to be ionized and analyzed may be introduced into the ion source by any suitable means, including a combination technique, wherein the sample is the output of a pre-ionization analytical separation instrument, such as a Gas Chromatography (GC), liquid Chromatography (LC), or electrophoresis (e.g., capillary electrophoresis, CE) instrument.

In addition to the funnel-based ion traps described above, examples of other ion traps that may be used in spectrometry systems as disclosed herein may include, but are not limited to, ion traps based on two-dimensional (linear) and three-dimensional multipole electrode arrangements. Alternatively, the provided ionization apparatus and ionization chamber may be configured to provide ion accumulation and pulsing functions, in which case a separate ion trap may not be provided.

The ion fragmentation device provided in the spectrometry system as disclosed herein may comprise a collision cell as described above, or may have a configuration other than a CID-based device. For example, the ion fragmentation device can be configured to perform Electron Capture Dissociation (ECD), electron Transfer Dissociation (ETD), infrared multiphoton dissociation (IRMPD), and the like.

In an example, a spectrometry system as disclosed herein can include a quadrupole mass filter as a first mass analyzer and a TOF analyzer as a second mass analyzer. More generally, however, various types of mass analyzers may be used in spectrometry systems. Examples may include, but are not limited to, multipole electrode structures (e.g., quadrupole mass filters, linear ion traps, three-dimensional Paul traps, etc.), electrostatic traps (e.g., kingdon traps, knight traps, and so forthTraps), ion Cyclotron Resonance (ICR) traps or Penning traps such as those used in fourier transform ion cyclotron resonance mass spectrometry (FT-ICR or FTMS), electric field sector instruments, magnetic field sector instruments, etc.

The ion detector provided in the spectrometry system as disclosed herein may be, for example, an Electron Multiplier (EM), a microchannel plate (MCP) detector, a faraday cup, or the like.

As will be appreciated by those skilled in the art, the spectrometry systems as disclosed herein may include various other ion optics positioned along the ion path, which optics are not specifically described above or shown in the figures. These ion optics may be configured to control or manipulate the ion beam (e.g., focus, shape, steer, cool, accelerate, decelerate, slice, etc.), as will be appreciated by those skilled in the art.

The controller 176 schematically depicted in fig. 1 may represent one or more modules, control units, components, or the like configured to control, monitor, and/or time the operation of various devices that may be provided in a spectrometry system as disclosed herein. As described above, the controller 176 may control or perform preprogrammed operations of the voltage source 142 or 242 and thus control the electric field and collision energy implemented in the pressure drop chamber 128 of the ion source 102 or 302 (fig. 1-3). The controller 176 may be in communication with and control other devices that may be associated with the ion source 102 or 302 and the spectrometer 106 or 306, such as an ionization device, an ion funnel and other ion guide, an ion trap, an IM analyzer (e.g., a drift cell), a mass filter, a collision cell or other ion fragmentation device, a TOF analyzer or other mass analyzer, an ion detector, a vacuum system, ion optics, a sample introduction device, upstream LC, GC or CE instrumentation, etc. One or more modules of the controller 176 may be, for example, a computer workstation, desktop computer, laptop computer, portable computer, tablet computer, handheld computer, mobile computing device, personal Digital Assistant (PDA), smart phone, or the like, or may be embedded therein.

The controller 176 may also schematically represent all electronic components not specifically shown in fig. 1-3 that may be required for the actual operation of the spectrometry system, such as voltage sources, timing controllers, clocks, frequency/waveform generators, processors, logic circuits, memory, databases, etc. The controller 176 may also be configured to receive ion measurement signals from the ion detector and perform tasks related to data acquisition and signal analysis as needed to generate chromatograms, drift spectra, CCS spectra, and mass spectrometry characterizations of the analyzed sample. The controller 176 may also be configured to provide and control a user interface that provides a screen display of spectral data and other data with which a user may interact. The controller 176 may also be configured to execute data processing algorithms (such as feature discoverers). The controller 176 may include one or more reading devices on or in which a non-transitory or tangible computer readable (machine readable) medium including instructions for performing all or part of any of the methods disclosed herein may be loaded. For all these purposes, the controller 176 may be in electrical communication with the various components of the spectrometry system via a wired or wireless communication link (as represented in phantom between the controller 126 and the ion detector 150 in fig. 1). Also, for these purposes, the controller 176 may include one or more types of hardware, firmware, and/or software, as will be appreciated by those skilled in the art.

It should be understood that one or more of the processes, sub-processes, and process steps described herein may be performed by hardware, firmware, software, or a combination of two or more of the foregoing on one or more electronically or digitally controlled devices. The software may reside in a software memory (not shown) in a suitable electronic processing component or system, such as the controller 176 schematically depicted in fig. 1. The software memory may include an ordered listing of executable instructions for implementing logical functions (i.e., the "logic" can be embodied in digital form, such as digital circuitry or source code, or in analog form, such as an analog source, such as an analog electrical, acoustic, or video signal). The instructions may be executed within a processing module, which may include, for example, one or more microprocessors, general-purpose processors, a combination of processors, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), or a Field Programmable Gate Array (FPGA). Further, the diagram depicts a logical division of functionality with physical (hardware and/or software) implementation that is not limited by the physical layout of the architecture or functionality. Examples of the systems described herein may be implemented in various configurations and operated as hardware/software components in a single hardware/software unit or in separate hardware/software units.

The executable instructions may be implemented as a computer program product having instructions stored thereon that, when executed by a processing module (e.g., the controller 176 shown in fig. 1) of an electronic system, direct the electronic system to execute the instructions. A computer program product may be selectively embodied in any non-transitory computer readable storage medium for use by or in connection with an instruction execution system, apparatus, or device, such as an electronic computer-based system, processor-containing system, or other system that can selectively fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this disclosure, a computer-readable storage medium can be any non-transitory means that can store a program for use by or in connection with an instruction execution system, apparatus, or device. The non-transitory computer readable storage medium can be, for example, selectively an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device. A non-exhaustive list of more specific examples of the non-transitory computer readable medium includes: an electrical connection having one or more wires (electrons); portable computer magnetic disk (magnetic); random access memory (electronic); read only memory (electronic); erasable programmable read-only memory, such as flash memory (electronic); optical disk storage such as CD-ROM, CD-R, CD-RW (optical); and digital versatile disk storage, i.e., DVD (optical). Note that the non-transitory computer-readable storage medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner, if necessary, and then stored in a computer memory or machine memory.

It will be further understood that the terms "in signal communication" or "in electrical communication" as used herein mean that two or more systems, devices, components, modules or sub-modules are capable of communicating with each other via signals propagating on some type of signal path. A signal may be a communication, power, data, or energy signal that may communicate information, power, or energy from a first system, device, component, module, or sub-module to a second system, device, component, module, or sub-module along a signal path between the first and second systems, devices, components, modules, or sub-modules. Signal paths may include physical, electrical, magnetic, electromagnetic, electrochemical, optical, wired, or wireless connections. Signal paths may also include additional systems, devices, components, modules or sub-modules between the first and second systems, devices, components, modules or sub-modules.

More generally, terms such as "communicate" and "in communication with … …" (e.g., a first component "in communication" or "in communication with a second component") may be used herein to indicate a structural, functional, mechanical, electrical, signal, optical, magnetic, electro-magnetic, ionic, or fluid relationship between two or more components or elements. Thus, the fact that one component may be considered to be in communication with a second component is not intended to exclude the possibility that additional components may be present between and/or operatively associated with the first and second components.

It will be understood that various aspects or details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation, as the invention is defined by the claims.

The invention also comprises the following items:

1. an ion source, comprising:

an ionization chamber for maintaining at atmospheric pressure;

a pressure drop chamber for maintaining a sub-atmospheric pressure; and

an ion transfer arrangement comprising an inlet in said ionization chamber and an exhaust in said pressure drop chamber,

wherein the ion transfer arrangement defines an ion path from the inlet to the outlet, and wherein the ion transfer arrangement is positioned to emit ions and neutral gas molecules from the outlet as an expanded beam comprising a low gas density region surrounded by a high gas density region, the high gas density region having a higher gas density than the low gas density region.

2. The ion source of item 1, further comprising:

an electrode located in the pressure drop chamber at a gap distance from the discharge port, wherein the electrode is for:

generating an electric field between the discharge port and the electrode to accelerate ions emitted from the discharge port to collision energy effective to induce ion activation of the ions; and

The electric field is positioned in overlapping relation with the low gas density region.

3. The ion source of item 2, wherein the exhaust port is located on an exhaust port axis and the electrode comprises an aperture located on the exhaust port axis.

4. The ion source of item 1, further comprising a vacuum system for reducing the pressure drop chamber to the sub-atmospheric pressure.

5. The ion source of item 1,

wherein the ion transfer apparatus comprises:

a main hole having an inner diameter smaller than an inner diameter of the discharge port; and

a tapered section fluidly coupling the main bore to the discharge port, and

wherein the inner diameter of the tapered section increases from the inner diameter of the main bore to the inner diameter of the discharge port.

6. An ion source according to item 5,

wherein the ion transfer apparatus comprises:

a capillary tube through which the primary bore extends; and

a cap mounted to or as part of the capillary tube, and

wherein the cap comprises the tapered section and the discharge opening.

7. The ion source of item 1, further comprising an ion guide positioned in the pressure drop chamber and along an ion guide axis.

8. The ion source of claim 7, wherein the ion guide is configured to generate a radio frequency electric field effective to limit radial movement of ions relative to the ion guide axis.

9. The ion source of claim 7, wherein the ion guide is configured to generate a direct current potential gradient along the ion guide axis.

10. The ion source of item 7,

wherein the ion guide comprises an ion guide inlet along the ion guide axis and an ion guide outlet spaced apart from the ion guide inlet, and

wherein the ion guide inlet surrounds at least a portion of an electrode located in the pressure drop chamber.

11. The ion source of claim 7, wherein the ion guide is a first ion guide and the ion guide axis is a first ion guide axis, the ion source further comprising:

a second ion guide positioned along a second ion guide axis to receive ions from the first ion guide.

12. The ion source of item 1, further comprising an ionization device for producing ions from a sample in the ionization chamber by atmospheric pressure ionization.

13. A spectrometry system, comprising:

an ionization chamber for maintaining at atmospheric pressure;

a pressure drop chamber for maintaining a sub-atmospheric pressure;

an ion transfer arrangement comprising an inlet in the ionization chamber and an exhaust in the pressure drop chamber, wherein the ion transfer arrangement defines an ion path from the inlet to the exhaust, and

an electrode is positioned in the pressure drop chamber a gap distance from the discharge port.

14. The spectrometry system of item 13, further comprising:

a vacuum housing for receiving ions from the pressure drop chamber; and

an ion analyzer in the vacuum housing.

15. The spectrometry system of item 14, wherein the ion analyzer comprises an ion mobility cell or a mass analyzer.

16. A method for analyzing a sample, the method comprising:

atmospheric pressure ionization in an ionization chamber to generate ions from the sample;

transferring the ions from the ionization chamber to a pressure drop chamber maintained at sub-atmospheric pressure; and

the ions emitted into the pressure drop chamber are subjected to an electric field that accelerates the ions to collision energy effective to induce ion activation of the ions without voltage breakdown.

17. The method of claim 16, wherein the ions and neutral gas molecules are emitted into the pressure drop chamber as an expanded beam comprising a low gas density region surrounded by a high gas density region having a higher gas density than the low gas density region, the method further comprising:

18. The method of claim 17, wherein transferring the ions further comprises controlling the expanded beam such that the low gas density region transitions to a mach disk.

19. The method according to item 16,

wherein transferring the ions further comprises ejecting the ions from an ejection outlet of an ion transfer device, and

wherein subjecting the ions emitted into the pressure drop chamber to the electric field further comprises applying a potential difference between the discharge outlet and electrodes in the pressure drop chamber to accelerate the ions to the collision energy.

20. The method of claim 16, wherein transferring the ions further comprises emitting the ions into an ion guide located in the pressure drop chamber.

Claims

1. An ion source, comprising:

An ionization chamber for maintaining at atmospheric pressure;

a pressure drop chamber for maintaining a sub-atmospheric pressure; and

wherein the ion transfer means defines an ion path from the inlet to the outlet, and

wherein the ion transfer device is positioned to emit ions and neutral gas molecules from the discharge outlet as an expanded beam comprising a low gas density region surrounded by a high gas density region, the high gas density region having a higher gas density than the low gas density region.

2. The ion source of claim 1, further comprising:

3. The ion source of claim 2, wherein the discharge port is located on a discharge port axis and the electrode comprises an aperture located on the discharge port axis.

4. The ion source of claim 1, further comprising a vacuum system for reducing the pressure drop chamber to the sub-atmospheric pressure.

5. The ion source of claim 1,

wherein the ion transfer apparatus comprises:

a tapered section fluidly coupling the main bore to the discharge port, and

6. The ion source of claim 5,

wherein the ion transfer apparatus comprises:

a capillary tube through which the primary bore extends; and

a cap mounted to or as part of the capillary tube, and

wherein the cap comprises the tapered section and the discharge opening.

7. The ion source of claim 1, further comprising an ion guide positioned in the pressure drop chamber and along an ion guide axis.

8. The ion source of claim 7 wherein the ion guide is configured to generate a radio frequency electric field effective to limit radial movement of ions relative to the ion guide axis.

10. The ion source of claim 7,