Axial ion source with magnetic field adjustment

Abstract

Systems and methods taught herein generate a non-uniform magnetic field in an ionization region of an ion source to improve robustness in electrical and chemical ionization processes, particularly negative chemical ionization (CI) processes. The non-uniform magnetic field within the ionization volume spatially separates electrons and anions such that anions primarily pass through an ion exit aperture in the ionization chamber, while electrons are directed to strike a sidewall or end wall of the ionization chamber remote from the ion exit aperture. As a result, a greater number of ions exit from the ion source towards the mass analyzer. The systems and methods taught herein also increase the lifetime of the instrument by avoiding damage that may be caused by electrons impacting the surface around the hole.

Description

Background Art

[0001] Mass spectrometry can be used to perform detailed analysis of samples. In addition, mass spectrometry can provide both qualitative (is compound X present in the sample) and quantitative (how much compound X is present in the sample) data for a large number of compounds in a sample. These capabilities have been used for a wide variety of analyses, such as testing drug use, determining pesticide residues in food, monitoring water quality, etc.

[0002] The sensitivity of a mass spectrometer may be limited by the efficiency of the ion source, ion losses in the mass spectrometer and mass analyzer, and detector sensitivity. Increasing the efficiency of the ion source (e.g., the number of ions produced per unit sample or per unit time) can significantly improve the detection limit of a mass spectrometer, thereby enabling the detection of lower concentrations of compounds or the use of smaller amounts of sample. In addition, increasing the stability of the ion source and the number of ions produced over time can improve quantitative comparisons between runs and samples. Summary of the invention

[0003] It should be understood that the drawings are not necessarily drawn to scale, nor are the relationships between objects in the drawings necessarily drawn to scale. The drawings are intended to be depictions of various embodiments of the devices, systems, and methods disclosed herein for clarification and understanding. Wherever possible, all drawings will use the same reference numerals to refer to the same or similar parts. In addition, it should be understood that the drawings are not intended to limit the scope of the present teachings in any way. BRIEF DESCRIPTION OF THE DRAWINGS

[0004] To easily identify the discussion of any particular element or act, the most significant digit(s) in a reference number refers to the drawing number in which the element is first introduced.

[0005] Figure 1 is a block diagram of a mass spectrometry platform according to various embodiments taught herein.

[0006] Figure 2A Illustrated is a cross-sectional view of an ion source according to various embodiments taught herein.

[0007] Figure 2B Illustrated Figure 2A Exploded perspective view of the ion source.

[0008] Figure 3A A simulation of electron trajectories in an ion source when used for positive chemical ionization (CI) is illustrated.

[0009] Figure 3B A simulation of electron trajectories in an ion source when used for negative CI is illustrated.

[0010] Figure 4 Aspects of the subject matter according to one embodiment are illustrated.

[0011] Figure 5A A simulation of the magnetic field lines of an ion source comprising only paramagnetic components is illustrated.

[0012] Figure 5B A simulation of magnetic field lines and magnetic field strength in an ion source including a combination of paramagnetic and ferromagnetic elements according to various embodiments taught herein is illustrated.

[0013] Fig. 6A A simulation of electron trajectories within a conventional ion source having predominantly or exclusively paramagnetic components is illustrated.

[0014] Figure 6B A simulation of electron trajectories within an ion source having a combination of paramagnetic and ferromagnetic components is illustrated.

[0015] Fig. 7A is a photograph of the filament side of the ion exit aperture 236 and end wall 234 from a conventional system including only paramagnetic elements.

[0016] Figure 7B is a photograph of the filament side of the ion exit aperture 236 and end wall 234 from an ion source including both paramagnetic and ferromagnetic elements 242 as taught herein.

[0017] Fig. 8A Illustrated is a simulated xy plot of the distribution of electron positions in the plane of the ion exit aperture for a conventional source having only paramagnetic components.

[0018] Figure 8B Illustrated is a simulated xy plot of the distribution of electron positions in the plane of an ion exit aperture of an ion source having a combination of paramagnetic components and ferromagnetic element 242 according to embodiments taught herein.

[0019] Fig.9A A simulated z-theta plot of the distribution of electron positions along the inner surface (ie, the inner bore) of a conventional ionization chamber is illustrated.

[0020] Fig. 9B Illustrated is a simulated z-theta plot of the distribution of electron positions along the inner surface (ie, inner bore) of an ionization chamber for an ion source having a combination of paramagnetic and ferromagnetic elements 242 according to embodiments taught herein.

[0021] Fig.10 Several plots are shown relating to the magnetic field of various sources as a function of distance from the magnetic field generator.

[0022] Fig.11A A simulation of the magnetic field lines of a conventional ion source comprising only paramagnetic components is illustrated.

[0023] Fig. 11B A simulation of magnetic field lines and magnetic field strength in an ion source including a combination of paramagnetic and ferromagnetic elements according to various embodiments described herein is illustrated.

[0024] Fig. 12A A simulation of electron trajectories within a conventional ion source having predominantly or exclusively paramagnetic components is illustrated.

[0025] Fig. 12B A simulation of electron trajectories within an ion source as taught herein is illustrated, wherein a ferromagnetic element 242 is located proximate to the electron lens.

[0026] Fig.13A Illustrated Fig. 12A Magnified portion of the middle view.

[0027] Fig. 13B Illustrated Fig. 12B Magnified portion of the middle view.

[0028] Fig.14A Illustrated is a simulated xy plot of the distribution of electron positions in the plane of the entrance aperture of a conventional source having only paramagnetic components.

[0029] Fig. 14B Illustrated is a simulated xy plot of the distribution of electron positions in the plane of an entrance aperture of an ion source having a combination of paramagnetic components and ferromagnetic elements according to various embodiments taught herein.

[0030] Fig.15 Several curves are shown relating the magnetic field of various sources as a function of distance from the magnetic field generator.

[0031] Fig.16A A simulation of the magnetic field lines of a conventional ion source comprising only paramagnetic components is illustrated.

[0032] Fig. 16B A simulation of magnetic field lines and magnetic field strength in an ion source including a combination of paramagnetic and ferromagnetic elements according to various embodiments described herein is illustrated.

[0033] Fig.17A A simulation of electron trajectories within a conventional ion source having predominantly or exclusively paramagnetic components is illustrated.

[0034] Fig. 17B A simulation of electron trajectories within an ion source according to embodiments taught herein is illustrated, wherein a ferromagnetic element is placed in or near a tube lens located behind an end wall along the central axis.

[0035] Fig.18A Illustrated is a simulated xy plot of the distribution of electron positions in the plane of the ion exit aperture for a conventional source having only paramagnetic components.

[0036] Fig.18B A simulated xy plot of the distribution of reflected electron positions on the exit face of the ion exit aperture (ie, the face opposite the electron source) is illustrated. DETAILED DESCRIPTION

[0037] Embodiments of systems and methods for ion sources having improved performance and robustness are described herein and in the accompanying presenting figures.

[0038] The section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described in any way.

[0039] In the detailed description of various embodiments, for the purpose of explanation, many specific details are set forth to provide a thorough understanding of the disclosed embodiments. However, it will be understood by those skilled in the art that these various embodiments may be practiced with or without these specific details. In other examples, structures and devices are shown in block diagram form. In addition, it will be readily appreciated by those skilled in the art that the specific order of presenting and executing methods is exemplary, and it is contemplated that the order may be varied (unless otherwise explicitly stated) and still remain within the spirit and scope of the various embodiments disclosed herein.

[0040] All literature and similar materials cited in this application, including but not limited to patents, patent applications, articles, books, treatises, and internet web pages, are expressly incorporated by reference in their entirety for any purpose. Unless otherwise described, all technical and scientific terms used herein have the meaning commonly understood by one of ordinary skill in the art to which the various embodiments described herein belong.

[0041] It should be understood that there is an implicit "about" before the specific temperatures, concentrations, times, pressures, flow rates, cross-sectional areas, etc. discussed in the present teachings, so that there are slight and insubstantial deviations within the scope of the present teachings. In this application, unless otherwise specifically provided, the use of the singular includes the plural. In addition, the use of "comprising", "including" and "containing" is not intended to be limiting. It should be understood that both the foregoing general description and the following detailed description are merely exemplary and illustrative, and are not limitations of the present teachings.

[0042] As used herein, "a" or "an" may also mean "at least one" or "one or more." In addition, the use of "or" is inclusive, such that the phrase "A or B" is true when "A" is true, "B" is true, or both "A" and "B" are true. In addition, unless otherwise required by context, singular terms shall include pluralities and plural terms shall include the singular.

[0043] "System" describes a set of real or abstract components comprising a whole, in which each component interacts or relates to at least one other component within the whole.

[0044] In some embodiments, the systems and methods taught herein use the strategic placement of ferromagnetic elements to produce non-monotonic changes in the magnetic field in the ionization volume of the ionization chamber to improve the robustness in the chemical ionization process (particularly negative chemical ionization (CI) process). In practice, since it is very difficult to establish an electrostatic potential in the ion source to allow analyte anions to pass through the ion source to reach the analyzer in the presence of electrons, negative CI may have poor robustness. Because anions and electrons have the same polarity, the source optics that attract or repel anions also attract or repel electrons. In the case of operating an axial source in the negative CI mode, an electron beam can be limited along the central axis of the source. This limited electron beam can form a space charge field. This space charge field affects the transmission of anions through the hole of the source by repelling anions radially away from the central axis and therefore further away from the ion exit hole, wherein the ion is intended to pass through the ion exit hole to leave the source and generate a signal at the detector. By employing a non-uniform magnetic field within the ionization volume as taught herein, electrons and anions can be spatially separated such that anions primarily pass through an ion exit aperture in the ionization chamber while electrons are directed to impact side or end walls of the ionization chamber away from the ion exit aperture.

[0045] Some embodiments of the systems and methods taught herein solve the problems caused by the accumulation of electrons on the surface near the ion outlet hole. In particular, the systems and methods taught herein improve the ion throughput and increase the life of the instrument by avoiding damage that may be caused by the surface around the electron impact hole. Electrons with sufficient kinetic energy can interact with the material on the surface of the ion source to form a dielectric layer (sometimes referred to as "burning" or "stitching" by those skilled in the art). Then, the electrons can impact and reside at the dielectric layer or the combustion layer to cause charging, which can then repel anions and cause the transmission of analyte anions through the source to be reduced. When additional electrons impact the dielectric layer, the surface is charged to the initial potential of the electron, which is typically -70V. The surface charge repels anions. Another problem caused by combustion is that the combustion surface needs to be cleaned regularly. By using ferromagnetic elements to modify the magnetic field to produce a local non-monotonic magnetic field in the ionization volume, the disclosed system and method reduce combustion, which reduces maintenance costs and reduces the resulting electron accumulation near the ion outlet hole. In addition, the use of ferromagnetic materials strategically placed adjacent to the ionization volume can improve the negative CI process or negative EI process by reducing the burning or stitching of electrons near the ion exit hole. In conventional ion sources using only paramagnetic materials, electrons strike the end wall around the ion exit hole, resulting in burning. In negative CI or negative EI mode, the accumulated electron charge at the burn repels negative ions, thereby reducing ion transmission. The embodiments described herein can prevent electrons from burning onto the area around the ion exit hole to improve ion transmission.

[0046] In some embodiments, the systems and methods taught herein improve robustness in electron ionization (EI) processes and CI processes by improving the injection of electrons into the ionization chamber via the entrance aperture. By introducing a locally increased magnetic field density in the region of the entrance aperture, the electron beam from the filament can be tightened closer to the central axis at the entrance aperture. Therefore, burning and charging at the entrance aperture are reduced, and the electron flux through the entrance aperture is increased. Therefore, EI processes and CI processes are enhanced by the presence of a greater number of electrons in the ionization chamber. The reduction in burning also increases the life of the instrument and reduces the requirements for cleaning or repair.

[0047] In some embodiments, the systems and methods taught herein can mitigate the impact of reflected electrons that have left the ion exit hole but are returned by subsequent lens elements on the exit face of the ion exit hole. In an EI process or a positive CI process, the final tube lens in the ion source can be set to repel electrons. Some of the repelled electrons are thus reflected back to the source, and a large number of electrons collide with the ion exit hole on the exit face (i.e., the side facing the exit is opposite to the side facing the ionization volume) in a conventional source. By adopting a non-uniform magnetic field, the reflected electrons can be focused toward the central axis to return to the ionization volume through the ion exit hole, and thus avoid impacting the exit face of the ion exit hole. Focusing electrons on the central axis also improves the efficiency of electron reflection, and improves system robustness by reducing the intensity of burning at the ion exit hole.

[0048] Various embodiments of the mass spectrometry platform 100 may include, for example, Figure 1 In various embodiments, Figure 1 The elements of may be incorporated into the mass spectrometry platform 100. According to various embodiments, the mass spectrometer 100 may include an ion source 102, a mass analyzer 104, an ion detector 106, and a controller 108.

[0049] In various embodiments, the ion source 102 generates a plurality of ions from a sample. The ion source 102 may include, but is not limited to, an electron ionization (EI) source, a chemical ionization (CI) source, or a combination of both an EI source and a CI source.

[0050] In various embodiments, the mass analyzer 104 may separate the ions based on their mass-to-charge ratio. For example, the mass analyzer 104 may include a quadrupole mass filter analyzer, a quadrupole ion trap analyzer, a time-of-flight (TOF) analyzer, an electrostatic trap (e.g., an orbital trap) mass analyzer, a Fourier transform ion cyclotron resonance (FT-ICR) mass analyzer, etc. In various embodiments, the mass analyzer 104 may also be configured to fragment the ions using collision induced dissociation (CID), electron transfer dissociation (ETD), electron capture dissociation (ECD), photoinduced dissociation (PID), surface induced dissociation (SID), etc., and further separate the fragmented ions based on mass-to-charge ratio.

[0051] In various embodiments, the ion detector 106 can detect ions. For example, the ion detector 106 can include an electron multiplier, a photomultiplier, an avalanche diode, a silicon photomultiplier, a Faraday cup, etc. The ions leaving the mass analyzer can be detected by the ion detector. In various embodiments, the ion detector can be quantitative so that the exact count of ions can be determined. In various embodiments, such as when an electrostatic mass analyzer is used, the functions of the mass analyzer 104 and the ion detector 106 can be performed by the same component.

[0052] In various embodiments, the controller 108 can communicate with the ion source 102, the mass analyzer 104, and the ion detector 106. For example, the controller 108 can configure the ion source or enable / disable the ion source. In addition, the controller 108 can configure the mass analyzer 104 to select a specific mass range to be detected. In addition, the controller 108 can adjust the sensitivity of the ion detector 106, such as by adjusting the gain. In addition, the controller 108 can adjust the polarity of the ion detector 106 based on the polarity of the detected ions. For example, the ion detector 106 can be configured to detect positive ions or configured to detect negative ions.

[0053] Figure 2A and Figure 2B 2 is a diagram illustrating an ion source 200 that can be used as the ion source 102 of the mass spectrometry platform 100. The ion source 200 can include an electron source 202, an electron lens 204, an ionization chamber 206, lens elements 208, 210 and 212, and an RF ion guide 214. In addition, the ion source 200 can include a body 216, an insulator 218, spacers 220 and 222, a retaining clip 224, one or more ferromagnetic elements 242, and a magnetic field generator 240. In various embodiments, the ionization chamber 206, the lens elements 208, 210 and 212, and the RF ion guide 214 can be aligned so that the ions generated by the ion source form an ion beam. The alignment of the ionization chamber 206, the lens elements 208, 210 and 212, and the RF ion guide 214 and the direction of the ion beam define a central axis 244 of the ion source. The positioning of the ferromagnetic element 242 inside, around, or outside the ion source adjusts the magnetic field generated by the magnetic field generator 240. In some embodiments, the positioning of the ferromagnetic element 242 can divide the ionization volume 230 into a paramagnetic section 248 and a ferromagnetic section 250. The trajectories of electrons having a low relative mass to the analyte ions will be more affected by changes in the magnetic field or the lack of a magnetic field. Therefore, adjustments in the magnetic field, together with adjustments in the electrostatic field within the source, can focus, disperse, or manipulate the trajectories of electrons while minimally affecting the analyte ions. For example, in the systems and methods described herein, the magnetic field generator 240 and the ferromagnetic element 242 are positioned relative to the ionization chamber 206 to focus the electrons closer to the central axis 244 or to defocus or deviate the electrons from the central axis 244 depending on the particular application.

[0054] The electron source 202 may include a thermionic emitter 226 for generating electrons. In various embodiments, the electron source 202 may include more additional thermionic emitters for generating redundant or increased electrons. In alternative embodiments, the electron source 202 may include a thermionic filament, a field emitter, an electron multiplier, a photoelectric effect emitter, or other electron source. Electrons may travel axially along the ion source 200 through the entrance aperture 246 of the electron lens 204 and enter the ionization chamber 206 to ionize gas molecules. The electron lens 204 may be used to prevent ions from traveling in the reverse direction toward the electron source.

[0055] The ionization chamber 206 may include a gas inlet 228 for directing a gas sample (e.g., neutral molecules) into an ionization volume 230 defined by the ionization chamber 206. The gas molecules within the ionization volume 230 may be ionized by electrons from the thermionic emitter 226. The efficiency of this interaction between neutral molecules and electrons increases with increasing electron density, and a high electron density is preferred to ensure a sufficient level of interaction to produce a robust output signal. A high density of electrons may be achieved by confining the electrons to a central axis 244 in a paramagnetic segment 248. In this case, electron "confinement" means that the magnetic field generated by the magnetic field generator 240 has sufficient strength to limit the movement of electrons away from the central axis 244 of the ionization volume, thereby achieving a target electron density. The confinement of electrons near the central axis 244 in the paramagnetic segment 248 may form a high density electron region 254, where, due to the concentration of electrons, the electrons interact with neutral molecules from the gas sample to form ions (e.g., analyte ions in electronic ionization or reagent ions in chemical ionization) at high speed. In the ferromagnetic section 250, the magnetic field is reduced, and the electrons are not as well confined near the central axis 248. That is, the electrons deviate away from the central axis 248 within the ferromagnetic section 250 (due in part to Coulomb repulsion and the lower magnetic field).

[0056] The lenses 208 and 210 can define a lens volume 232. The lens volume 232 can include a lens region where some electrons may be present. In various embodiments, the lens volume can also include an ionization volume and an area outside the lens. An end wall 234 can limit the flow of gas from the ionization volume 230 to the lens volume 232, thereby forming a large pressure difference between the ionization volume 230 and the lens volume 232. In some embodiments, the ferromagnetic element 242 can include the end wall 234. An ion exit aperture 236 can provide a path through the end wall 234 for ions to exit the ionization chamber 206.

[0057] In various embodiments, the ionization chamber 206 and the lens element 208 may be joined to form an extended ionization element that defines at least a portion of the ionization volume 230 and the lens volume 232. In such embodiments, the lens element 208 may be electrically coupled to the ionization chamber 206. In other embodiments, the joined ionization chamber 206 and lens element 208 may be electrically isolated such that different voltage potentials may be applied to the ionization chamber 206 and the lens element 208.

[0058] The lenses 210 and 212 and the RF ion guide 214 can assist in the axial movement of ions from the ionization volume 230 to additional ion optical elements and the mass analyzer 104 of the mass spectrometry platform 100. In various embodiments, the ion guide assembly 238 can include the lens 212 and the RF ion guide 214. The ion guide assembly 238 can include additional insulating portions to electrically isolate the lens 212 from the RF ion guide 214. In addition, the insulating portions can include standoffs to prevent electrical contact between the lens 210 and the lens 212.

[0059] When assembled into body 216, insulator 218 may prevent electrical contact between lens element 208 and lens element 210. Spacer 220 may prevent electrical contact between electron lens 204 and ionization chamber 206. Spacer 222 may be indicated to prevent electron source 202 from rotating, and retaining clip 224 may hold other components within body 216.

[0060] The ferromagnetic element 242 can adjust (i.e., increase or decrease) the magnetic field formed by the magnetic field generator 240 at a local position in or near the ionization chamber 206 to control the movement of electrons. The magnetic field generator 240 may include one or more magnetic field generating sources, such as electromagnets and permanent magnetic field generators. Although the magnetic field generator 240 is illustrated as being positioned at the outside and end of the ion source, one or more magnetic field generators 240 may also be positioned at other positions relative to the ion source, including, for example, the outer wall or inner wall of the ionization chamber 206. For example, a magnet located at the opposite side of the ion source may generate a magnetic field that slowly decreases and then begins to increase again across the length of the ionization zone. The resulting field generated by one or more magnets is modified by the ferromagnetic element as described in the embodiments herein to produce a local and sharp change in the magnetic field to affect the movement of electrons or ions. In some embodiments, the magnetic field generator 240 may be positioned approximately at a single radial distance from the central axis 244 and extend along the central axis 244. In the presence of a magnetic field, electrons emitted from thermionic emitter 226 (e.g., a heated filament) undergo a helical motion perpendicular to the magnetic field lines as the electrons are accelerated and decelerated by the electrostatic field potential of the source optics (e.g., electron lens 204, lens element 208, lens element 210, lens element 212). By introducing a ferromagnetic element 242 adjacent to or as part of one or more lens elements, the magnetic permeability through the source becomes dynamic and results in variations in magnetic flux at different longitudinal locations along central axis 244.

[0061] The ferromagnetic element 242 can be capable of reducing electron collisions with the end wall 234 of the ionization chamber 206, and can reduce the electron outflow through the ion outlet hole 236. With the electrostatic potential set to transmit analyte anions during negative CI analysis, most electrons either pass through the source or collide near the ion outlet hole 236 of the ion source 200 (e.g., on the end wall 234). The high-density electron collision at the end wall 234 near the ion outlet hole 236 increases the rate of generating the dielectric layer (i.e., "burning"), thereby causing the charging and repulsion of the analyte anions intended to be transmitted outwardly to the detector through the ion outlet hole 236. Similarly, the electrons leaving the ionization chamber 206 can collide with other surfaces that are not so easy to clean or are not designed to be regularly cleaned (and eventually cause charging). Both of these problems may lead to a lack of robustness in negative CI analysis.

[0062] The ferromagnetic element 242 is positioned to adjust the magnetic field produced by the magnetic field generator 240 inside, around or outside the ion source. The electron trajectory with low relative mass to the analyte ion will be more affected by the change of the magnetic field or the lack of the magnetic field. Therefore, adjusting the magnetic field together with the electrostatic field in the source becomes an additional tool for focusing, dispersing or manipulating the electron trajectory to minimally affect the analyte ion. In particular, the ferromagnetic element can cause the appearance of spatial separation between a group of ions and a group of electrons, and the group of ions will be less affected by the change of the magnetic field and will tend to keep gathering near the central axis, and the group of electrons will be more affected by the change of the magnetic field and will tend to separate away from the central axis.

[0063] The ferromagnetic element 242 can be positioned within the ionization chamber. In an exemplary embodiment, the ferromagnetic element 242 can include a cylinder positioned axisymmetrically about the central axis 244. In some embodiments, the ferromagnetic element 242 is permanently or removably secured or attached to the outer wall of the ionization chamber using fasteners or adhesives. In some embodiments, a friction fit is used to hold the ferromagnetic element 242 within the outer wall of the ionization chamber. Although the ferromagnetic element is Figure 2A and Figure 2B 242 is illustrated as being positioned or disposed within the ionization chamber, but alternative positioning of the ferromagnetic element 242 is contemplated. For example, in some embodiments, the ferromagnetic element 242 can be embedded directly into the outer wall of the ionization chamber. Alternatively, the outer wall of the ionization chamber can be formed by joining (e.g., welding) a length of paramagnetic material and a length of ferromagnetic material end to end.

[0064] In other embodiments, the ferromagnetic element 242 may be disposed entirely outside (i.e., external) the ionization chamber or ionization volume. In some embodiments, the ferromagnetic segment 250 may be considered to be the volume enclosed by the ferromagnetic element 242, particularly for cylindrical ferromagnetic elements having an internal bore. In some embodiments, the ferromagnetic element 242 may be configured to move to allow adjustment of the position of a non-uniform magnetic field within or adjacent to the ionization chamber. For example, the ferromagnetic element may include or may be connected to a translation aid, such as a track, motor, slide, or other structural feature. The translation aid may be manually operated or motorized. In some embodiments, the movement of the ferromagnetic element may be controlled by a controller, for example, Figure 1 A controller 108 controls other aspects of the operation of the ion source. In some embodiments, the controller 108 can adjust the position of the ferromagnetic element based on modeling data, off-line feedback, or real-time feedback based on measured parameters such as ionization efficiency or analyte ion throughput.

[0065] Typically, paramagnetic materials are used to construct the structural elements of the ion source. Paramagnetic materials may include 300 series stainless steel, aluminum and aluminum alloys, certain superalloys such as Inconel(R) (Special Metals Corporation, New Hartford, NY) or Hastelloy(R) (Haynes International, Inc., Kokomo, IN), Nichrome (nickel-chromium alloy), titanium, or any other suitable material having a relative magnetic permeability close to 1. For the purposes of this disclosure, paramagnetic materials may also include materials that are considered non-magnetic, such as polymers.

[0066] The ferromagnetic element may include a ferromagnetic material. The ferromagnetic material may include iron, steel, cobalt, nickel, alloys of these metals, permalloy, nickel-iron high permeability alloy, 400 series stainless steel, or any other material with a relative magnetic permeability greater than 1.1. It should be understood by those skilled in the art that the ferromagnetic element may include multiple separated or joined ferromagnetic elements or materials, and such a configuration including multiple elements may be referred to as a ferromagnetic segment. Additional paramagnetic segments may follow the ferromagnetic element of the source. In some embodiments, the surface of the ferromagnetic element 242 may be coated with a different material (e.g., a coating) to reduce the chemical reactivity of the surface or make the surface easier to clean. For example, the coating may be applied as an additional layer, or by modifying an existing surface layer using a chemical vapor deposition, thermal deposition, thermal or electron beam evaporation or sputtering process. In some embodiments, a paramagnetic element (such as in the form of stainless steel) may be inserted into or around the surface of the ferromagnetic element to block chemical interactions between the composition of the ionization chamber and the ferromagnetic element. For example, the paramagnetic element may be a foil or thin cylindrical insert located between the ferromagnetic element 242 and the central shaft 244. In some embodiments, the paramagnetic insert may be removable or replaceable when it becomes dirty, thereby facilitating permanent installation of the ferromagnetic element while still maintaining the ferromagnetic element surface clean.

[0067] Figure 3A is an illustration of a simulation of electrons in the ion source 200 using positive chemical ionization (CI). Figure 3A The potentials used for the simulations are shown in and Table 1. In positive CI, ionized electrons form reagent ions in the ionization volume. These reagent ions then interact with analyte neutral species to form positive analyte ions.

[0068] Table 1: Positive chemical ionization

[0069]

[0070] In the ion source 200, the electrons may be located along with the ion beam on the central axis 244. This may have the advantage of using the negative space charge from the electron beam to focus the positive ions to the central axis 244. In addition, the negatively charged ion exit aperture may help extract the positive ions. These features may also be beneficial when used for positive CI.

[0071] Figure 3B is a diagram of a simulation of electrons in the ion source 200 performing negative CI. Figure 3B The potentials used for the simulation are shown in Table 2. In negative CI, ionized electrons can form reagent ions in the ionization volume. The outer shell electrons released during this ionization can be at thermal energy. The ionized electrons can also lose kinetic energy when they collide with the reagent gas. Ultimately, the ionized electrons can lose kinetic energy and reach thermal energy. These various thermal energy electrons can then interact with analyte neutral species and can be captured to produce negative analyte ions.

[0072] The electrons that hit the area around the end wall 234 can cause the accumulation of the insulating layer around the ion exit hole, thereby changing the potential to a potential close to that of the electron (-70V). In various embodiments, neutral molecules from the analyte of the matrix can temporarily fall on the surface of the ionization chamber 206. The molecules usually leave the surface. However, if the electrons hit the neutral molecules on the surface, the electrons can be attached to the surface in the form of inorganic carbon, silicon dioxide or other insulating materials, depending on the composition of the molecules. This can form an insulating layer on the surface of the metal. When a charged particle (such as an electron) hits the insulating layer, the charge of the charged particle cannot be quickly dissipated by the metal below, and the opposite charge can accumulate on the insulating layer. Once this happens, the ion exit hole can become a potential barrier for electrons and negative ions. This reduces the number of negative ions that leave the ionization volume 230 and travel to the ion detector 106 to be detected.

[0073] Table 2: Negative chemical ionization

[0074]

[0075] Figure 4 Magnetic field density curves are illustrated as a function of distance from the magnetic field generator along the central axis. Curve 406 represents a conventional system having only or predominantly paramagnetic components, and illustrates an undisturbed magnetic flux density that decreases monotonically with distance from the face of the magnetic field generator.

[0076] Curve 404 represents a system including a ferromagnetic element as taught herein. Due to the presence of the ferromagnetic element, the magnetic flux density along the length of the ionization volume (e.g., as a function of distance from the magnetic field generator 240) shown in curve 404 is non-monotonic due to sharp fluctuations in density at local locations along the central axis 244. In a system corresponding to curve 404, the initial section of the ionization chamber (e.g., paramagnetic section 248) is composed of paramagnetic material, thereby allowing the magnetic field generated by the external magnetic field generator 240 (such as a permanent magnetic field generator) through the source section to be unaffected. The paramagnetic section is followed by a ferromagnetic element, which forms a ferromagnetic section 250 and causes a sharp and local increase in the magnetic flux density (B) just before the ferromagnetic element 242 (due to the field lines being pulled toward the central axis to preferentially pass through the more permeable ferromagnetic material as opposed to air / vacuum or paramagnetic material). The magnetic flux density then drops to the center of the ferromagnetic section 250. In this embodiment, after the magnetic flux density within the ferromagnetic segment decreases dramatically, the magnetic flux density increases dramatically and locally again to exceed the ferromagnetic element (again due to the field lines that have been drawn into the magnetic isolation material). A higher magnetic flux density is obtained upon exiting the material than would occur at the same location without the ferromagnetic element. This increased magnetic flux density may occur, for example, when the second paramagnetic segment is disposed after the ferromagnetic segment 250 along the central axis 244.

[0077] Figure 4 The dashed curve 402 in exemplifies the percentage change in magnetic flux density between curve 404 and curve 406. Curve 402 rises from 0% to about +40% and then drops to a low point of -100%. Curve 402 then rises again to a height of about +80% (exemplifying a greater magnetic flux density at the exit of the ferromagnetic element) before slowly dropping back to a change of 0%. In various embodiments, the ferromagnetic element may cause the percentage change in magnetic flux density within the ionized volume relative to the same ionized volume without the ferromagnetic element to be in the range of 5% to 100%, in the range of 10% to 50%, in the range of 20% to 80%, in the range of 40% to 60%, in the range of 50% to 100%, or in any other suitable range. In various embodiments, the ferromagnetic element may cause the percentage change in magnetic flux density within the ionized volume relative to the same ionized volume in the absence of the ferromagnetic element to be in the range of -5% to -100%, in the range of -10% to -50%, in the range of -20% to -80%, in the range of -40% to -60%, in the range of -50% to -100%, or in any other suitable range.

[0078] The trajectories of electrons, which have a low mass relative to the analyte ions, are affected to a greater extent by changes in the magnetic field (or lack thereof) than are the ion trajectories. Therefore, adjusting the magnetic field together with the electrostatic field within the source becomes an additional tool to focus, disperse or manipulate the electron trajectories while minimally affecting the analyte ions.

[0079] Figure 5A The simulation of the magnetic field lines of an ion source including only paramagnetic components is illustrated. To simplify the simulation, the magnetic field generator and the ion source are considered to be axisymmetric, and only the portion on one side of the central axis 244 is drawn. The simulation is performed using pyFEMM (a Python interface for finite element method magnetism). As shown, the magnetic field lines start at the front of the magnetic field generator 240 and wrap around and terminate at the opposite side. The field lines pass through the components of the ion source with almost no turning or bending. In addition, as shown in FIG. Figure 5A The field strength represented by the background shading in is uniform in all components of the ion source and shows no concentrations in or near any component of the ion source.

[0080] Figure 5B 1 illustrates a simulation of magnetic field lines and magnetic field strength in an ion source 102, 200 including a combination of paramagnetic and ferromagnetic elements according to various embodiments taught herein. Figure 5A Compared to , the field lines in this figure are slightly deviated. Figure 5A The field strength in the ferromagnetic element 242 of the ion source is maintained to a greater extent than in the same element formed of paramagnetic material.

[0081] Fig. 6A A simulation of electron trajectories in a conventional ion source having predominantly or only paramagnetic components is illustrated. The inner surface 604 and the ion exit aperture 602 are indicated. Using a pyFEMM-based (Scientific Instrument Services) calculated electron trajectories. In these calculations, the electrostatic lens potential was kept the same for comparison between different configurations, and only the magnetic field was varied between simulations. As expected, when electron motion is confined by the magnetic field, the trajectories of the electrons tend to be parallel to the central axis 244. The accumulation of electrons at the central axis introduces a space charge that can repel anions away from the central axis. As a result, the repelled anions are less likely to be in a position to pass through the ion exit hole. In addition, most of the electrons hit the end walls surrounding the ion exit hole.

[0082] Figure 6B A simulation of electron trajectories within an ion source having a combination of paramagnetic and ferromagnetic components is illustrated. The inner surface 604 and the ion exit aperture 602 are indicated. Fig. 6A Compared with the electronic simulation results shown, Figure 6B The motion of electrons in FIG. 24 shows the electron flow away from the central axis 244 and toward the inner hole of the ionization chamber and the portion of the end wall 234 away from the ion exit hole located at the central axis 244. In the volume adjacent to the ferromagnetic element 242, the electron motion is not restricted to spiral around the main direction of the magnetic field lines, and the electrons are more free to move along trajectories that are not parallel to the central axis 244. In effect, the ferromagnetic element 242 acts as a shield for the magnetic field generated by the magnetic field generator 240.

[0083] Fig. 7A It is a photo of the filament side of the ion outlet hole 236 and the end wall 234 from a conventional system including only paramagnetic elements. Using the electrostatic potential set to transmit analyte anions during negative CI analysis, most electrons either pass through the ion outlet hole or collide with the filament side of the end wall near the ion outlet hole of the ion source. The high-density electron collision near the outlet hole increases the rate of producing a dielectric layer on the surface of the filament side, thereby causing charging and repulsion of the analyte anions. Therefore, the ratio of the analyte anions transmitted to the detector from the ion outlet hole is reduced. It is also undesirable for electrons to leave the source, because electrons may collide with other surfaces that are not easy to clean or are not designed to be regularly cleaned (and eventually cause charging). Both may lead to lack of robustness in negative CI analysis.

[0084] In this particular embodiment, due to the asymmetric nature of the thermionic emitter, Fig. 7A The intense localized combustion 702 seen is asymmetric. One of ordinary skill in the art will appreciate that some systems may produce more Fig. 7AThe combustion mode shown is a more symmetrical combustion mode or a more asymmetrical combustion mode. In this case, the thermionic emitter is an asymmetric filament that produces a greater electron flux through some portions of the entrance hole 246 in the electron lens 204 than through other portions. When the electrons have enough energy to interact with the material on the surface within the ion source to cause the dielectric layer to form, a combustion 702 is caused. Then, the dielectric layer can accumulate more electrons at the location because the electrons cannot be easily sucked away to ground due to the low conductivity of the dielectric layer. The combustion 702 located on the end wall 234 adjacent to the ion outlet hole 236 presents special difficulties because the negative surface charge generated by the accumulated electrons may repel the analyte anions that attempt to leave through the ion outlet hole. The removal of the combustion 702 requires cleaning at shorter intervals (causing more frequent instrument downtime), and the cleaning process is more complicated for intense combustion. Offset combustion (e.g., caused by source asymmetry) presents a particularly difficult situation because the combustion tends to be concentrated rather than dispersed. The concentration of electrons at the end wall near the ion exit aperture can increase the intensity of combustion more quickly and more stringently exclude desired analyte anions from the flow.As described below, the improved systems and methods taught herein can reduce the severity of combustion.

[0085] Figure 7B is a photograph of the filament side of the ion exit aperture 236 and end wall 234 from an ion source including both paramagnetic and ferromagnetic elements 242 as taught herein. Figure 7B The end wall 234 is depicted at Fig. 7A The depicted workpiece was operated under similar working conditions and the sample was injected. Fig. 7A The intensity of combustion in this sample was significantly reduced compared to the sample in , and the combustion was not clearly confined to one or a few discrete locations.

[0086] Fig. 8A A simulated xy plot of the distribution of electron positions in the plane of the ion exit aperture 602 of a conventional source having only paramagnetic components is illustrated. The area between the black circles represents the end wall 234, and the area within the inner black circle represents the ion exit aperture. In this example, the ion exit aperture has a diameter of 1.2 mm. In this simulation, electrons are emitted from a thermionic emitter. Then, data is collected from the electron population within a distance of a few millimeters from the lens. About 45.2% of the electron population hits the end wall 234, and an additional 16.1% of the electron population passes through the ion exit aperture.

[0087] Figure 8BA simulated xy plot of the distribution of electron positions in the plane of the ion exit aperture 602 of an ion source having a combination of paramagnetic components and ferromagnetic element 242 is illustrated. The area between the black circles represents the end wall 234, and the area within the inner black circle represents the ion exit aperture. In this example, the ion exit aperture has a diameter of 1.2 mm. In this simulation, approximately 15.9% of the electron population within a few millimeters of the lens strikes the end wall 234, and an additional 7.2% of the electrons pass through the ion exit aperture. The number of electrons that hit the end wall or pass through the exit aperture is proportional to the number of electrons that hit the end wall. Fig. 8A In addition, the distribution of electron positions on the end wall 234 is Fig. 8A More dispersed, that is, Fig. 8A As can be seen, there is no concentration of electron positions near the ion exit aperture.

[0088] Fig.9A A simulated z-theta diagram of the electron position distribution along the inner surface 604 (ie, the inner bore) of the ionization chamber is illustrated. In this simulation, approximately 39.8% of the electrons impact the inner surface of the ionization chamber.

[0089] Fig. 9B A simulated z-theta diagram of the distribution of electron positions along the inner surface 604 (i.e., inner bore) of the ionization chamber for an ion source having a combination of paramagnetic and ferromagnetic elements 242 is illustrated. In this simulation, approximately 77.5% of the electrons impact the inner surface (inner bore) of the ionization chamber. Fig.9A The comparative experiment illustrated shows a significant increase in the number of impacts on the inner surface of the ionization chamber.

[0090] In the above-described embodiment, the introduction of the ferromagnetic element 242 enables the number of electrons passing through or near the ion exit hole 236 to be reduced and the fouling of the end wall 234 to be reduced. The ferromagnetic elements described herein may also be used to achieve other advantages. In some embodiments, an electron lens including an entrance hole separates the electron source from the ionization chamber. As described below with respect to Figures 10 to 14B As described, the use of ferromagnetic elements within or adjacent to the ionization chamber may improve the flux of electrons entering the ionization chamber 206 through the entrance aperture 246 of the electron lens 204 .

[0091] Fig.10Several plotted lines are shown for various sources related to the magnetic field along the central axis as a function of distance from the magnetic field generator 240. Curve 1002 represents the magnetic flux density as a function of distance for a conventional ion source that includes only paramagnetic materials. Curve 1002 shows a monotonic decrease in the magnetic flux density within the ionization volume from one end of the ionization chamber to the other end. Curve 1004 represents the magnetic flux density as a function of distance for an ion source according to certain embodiments described herein, which includes a ferromagnetic element to form an improved electron throughput through the entrance aperture of the electron lens. Curve 1004 shows the non-monotonic change in the magnetic flux density within the ionization volume as a function of distance along the ionization chamber (e.g., distance from the magnetic field generator). Curve 1006 represents the percentage change in the magnetic flux density value of curve 1004 compared to curve 1002. A useful characteristic of a ferromagnetic element is that the magnetic flux density increases sharply at local locations at the front and rear of the ferromagnetic element. Curve 1006 illustrates such an increase 1008 in magnetic field density, and in some embodiments, the increase 1008 may be spatially located adjacent to the electron lens. By tailoring the proximity of the ferromagnetic segment 250 (front paramagnetic segment 248 described below) to the electron lens 204, the magnetic flux density through the entrance aperture 246 of the electron lens 204 may be increased. Such an increase in the magnetic field will tighten the radius of the electron beam passing through the aperture (i.e., confine the electrons more tightly to the central axis 244). The tightening of the electron beam may reduce burning and charging near the entrance aperture 246 of the electron lens 204, and increase the electron flux through the entrance aperture 246. In some embodiments, the electron lens 204 may be separated from the ferromagnetic element 242 by a distance such that the location of the magnetic flux density peak in front of the ferromagnetic element 242 overlaps with the location of the electron lens 204.

[0092] Fig.11A The simulation of magnetic field lines of an ion source including only paramagnetic components is illustrated. Figure 5A and Figure 5B The same simulation parameters and constraints as described above. As shown, the magnetic field lines start at the front face of the magnetic field generator 240 and wrap around to terminate at the opposite face. The field lines pass through the components of the ion source with almost no turns or bends. In addition, as Fig.11A The field strength represented by the background shading in is uniform in all components of the ion source and shows no concentrations in or near any component of the ion source.

[0093] Fig. 11B 1 illustrates a simulation of magnetic field lines and magnetic field strength in an ion source 102, 200 including a combination of paramagnetic and ferromagnetic elements according to various embodiments described herein. Fig.11A In contrast, the field lines in this figure deviate, especially near the entrance aperture 246. In addition, compared to Fig.11AThe field strength in the ferromagnetic element 242 of the ion source is maintained to a greater extent than in the same element formed of paramagnetic material.

[0094] Fig. 12A illustrates a simulation of electron trajectories in a conventional ion source having primarily or exclusively paramagnetic components, and Fig.13A Illustrated Fig. 12A A magnified portion of the image. FIG. 12A to FIG. 13B The plotted lines in the Fig. 6A and Figure 6B The same parameters are produced as described above. Fig. 12A The electrons are confined to some extent along the central axis 244 along the entire length of the ion source, but the confinement is not particularly tight near the entrance aperture. Fig. 6A As in the simulation in , electrons are confined to the central axis 244 to form a space charge problem, and the electrons are allowed to escape through the ion exit hole. Fig.13A As can be seen in the enlarged view of FIG, several electron trajectories terminate on the wall of the entrance hole, and a lower density of electron trajectories can be observed near the central axis.

[0095] Fig. 12B illustrates a simulation of electron trajectories within an ion source as taught herein, wherein a ferromagnetic element 242 is located near an electron lens, and Fig. 13B Shows Fig. 12B Magnified portion of the image in . The ferromagnetic element 242 is offset from the electron lens by a distance 1208. In various embodiments, the distance 1208 between the electron lens 204 and the ferromagnetic element may be in the range of 0.1 mm to 0.5 mm, in the range of 0.25 mm to 1.0 mm, in the range of 0.5 mm to 1 mm, in the range of 0.75 mm to 2 mm, in the range of 1 mm to 10 mm, in the range of 5 mm to 20 mm, in the range of 10 mm to 50 mm, in the range of 0.1 mm to 50 mm, or any suitable distance to position the local magnetic flux density peak at an appropriate location near or on the electron lens 204. In some embodiments, the distance 1208 may be a location in front of the ferromagnetic element at which the concentration of magnetic field lines entering the ferromagnetic element forms a peak in the magnetic field density. As Fig. 13B The electrons are better confined to the central axis, as evidenced by the darkness caused by overlapping paths through the entrance aperture, and a greater number of electrons pass through the entrance aperture. The greater confinement of the electrons occurs because the magnetic field lines are highly coalesced or concentrated to form an increased magnetic field at a distance 1208 away from the end of the ferromagnetic element 242. Note that Fig. 12A and Fig. 12BThe difference in ion trajectories between 240 and 241 can be attributed to a change in the material type of the structure (ie, a change from paramagnetic to ferromagnetic for the ferromagnetic element 242) since there is no change in the shape of the structure in the ionization chamber.

[0096] In other embodiments, the ferromagnetic element 242 may be placed near or adjacent to a source filament (such as thermionic emitter 226) located at a distance 1208 in front of the electron lens 204. This configuration is similar to the above and Fig. 12B and 13B The configuration shown differs in that the ferromagnetic element 242 is located at a distance 1208 behind the electron lens 204. Because the magnetic field lines are also highly concentrated at the rear (or exit) of the ferromagnetic element 141 (see Fig.10 ), so the magnetic flux density also experiences a local intensity peak at a distance 1208 behind the end of the ferromagnetic element 242. The placement of the ferromagnetic element 242 a distance 1208 in front of the electron lens 204 can increase the magnetic field density through the entrance aperture 246, which more tightly confines the electrons to the central axis 244 at the entrance aperture 246. This confinement reduces electron collisions near the entrance aperture 246 and increases the electron flux through the entrance aperture 246 and into the ionization chamber.

[0097] Fig.14A A simulated xy plot of the distribution of electron positions in the plane of the entrance aperture of a conventional source with only paramagnetic components is illustrated. The area between the black circles represents the electron lens, while the area inside the inner black circle represents the entrance aperture. In this example, the entrance aperture has a diameter of about 1.6 mm. In this simulation, within a distance of a few millimeters from the lens, about 88.8% of the electron population passes through the entrance aperture and enters the ionization chamber. Similar to Fig. 6A , electrons still have a relatively large number of collisions with the end wall near the ion exit hole.

[0098] Fig. 14B A simulated xy plot of the electron position distribution in the plane of the entrance aperture of an ion source having a combination of a paramagnetic component and a ferromagnetic element 242 is illustrated. The area between the black circles represents the electron lens, while the area within the inner black circle represents the entrance aperture. In this example, the entrance aperture has a diameter of approximately 1.6 mm. In this simulation, within a distance of a few millimeters from the lens, approximately 95.1% of the electron population passes through the entrance aperture and enters the ionization chamber. Therefore, increasing the magnetic field near the entrance aperture using the ferromagnetic element 242 increases the electron flux entering the ionization chamber. For the same power level in the thermionic emitter, a greater electron flux produces greater efficiency in the ionization of analyte molecules.

[0099] In some embodiments, an ion source including a ferromagnetic element as taught in the embodiments described herein can also reduce the number of reflected electrons that return from outside the ionization chamber and hit the end wall. Such embodiments are particularly useful for ion sources used in specific electron ionization (EI) mode configurations, in which electron reflection is used to increase the probability of ionization events in the ionization volume. In this mode, the final tube lens outside the ion exit hole and outside the ionization chamber has a voltage set to repel electrons so that at least some of the electrons are reflected back into the ionization chamber, where the electrons have more opportunities to interact with analyte substances. In a conventional ion source operating in EI mode, a large number of reflected electrons may collide with the exit face (i.e., the face opposite to the thermionic emitter). In contrast, in the embodiments of the ion source taught herein, the ferromagnetic element may be placed at a distance from the end wall (e.g., as a final tube lens) to increase the magnetic field density at the ion exit hole of the end wall. As a result, fewer electrons collide with the end wall, and the electron reflection efficiency (i.e., the percentage of reverse-traveling electrons that re-enter the ionization chamber through the ion exit hole) increases.

[0100] Fig.15 Several curves are shown for various sources relating to magnetic flux density along the central axis as a function of distance from the magnetic field generator 240. Curve 1502 represents the magnetic flux density as a function of distance for a conventional ion source comprising only paramagnetic material. Curve 1502 shows a monotonic decrease in flux density as a function of distance from one end of the ionization chamber to the other end. Curve 1504 represents the magnetic flux density as a function of distance for an ion source according to some embodiments taught herein that includes a ferromagnetic element to prevent reflected electrons from hitting the exit face (i.e., the face opposite the thermionic emitter) of the end wall 234. Curve 1504 shows a non-monotonic change (i.e., increase and decrease) in magnetic flux density from one end of the ionization chamber to the other end (e.g., as a function of distance from the magnetic field generator 240). Curve 1506 represents the percentage change in the magnetic flux density value for curve 1502 compared to curve 1504. In this embodiment, the location of the ferromagnetic elements is selected to form a high magnetic field density at or just beyond the end wall 234, thereby confining electrons on the central axis 244 and preventing reflected electrons from impacting the exit face of the end wall, which causes charge accumulation and fouling / burning on the end wall surface.

[0101] Fig.16A A simulation of magnetic field lines for an ion source including only paramagnetic components is illustrated. As shown, the magnetic field lines begin at the front face of the magnetic field generator 240 and wrap around to terminate at the opposite face. The field lines pass through the components of the ion source with little turning or bending. In addition, as shown in FIG. Fig.16A The field strength represented by the background shading in is uniform in all components of the ion source and shows no concentrations in or near any component of the ion source.

[0102] Fig. 16B The simulation of magnetic field lines and magnetic field strength in an ion source 102, 200 including a combination of paramagnetic elements and ferromagnetic elements according to various embodiments described herein is illustrated. In particular, the ferromagnetic element is part of a tube lens located along the central axis and adjacent to the end wall of the ionization chamber. Fig.16A The field strength in the ferromagnetic element 242 of the ion source is maintained to a greater extent than in the same element formed of paramagnetic material.

[0103] Fig.17A Simulations of electron trajectories within a conventional ion source having primarily or only paramagnetic components are illustrated. In these simulations, the pressure is reduced and the electrostatic potential on the components is set to be in EI mode, where the final tube lens is set to repel electrons. When the electrons are reflected by the tube lens back into the ionization chamber, the electron trajectories are not well confined to the central axis and impact the exit surface of the end wall 234.

[0104] Fig. 17B A simulation of electron trajectories within an ion source as taught herein is illustrated, wherein the ferromagnetic element 242 is placed in or near the tube lens along the central axis at a distance 1706 away from the end wall 234. In various embodiments, the distance 1706 between the end wall 234 and the ferromagnetic element can be in the range of 0.1 mm to 0.5 mm, in the range of 0.25 mm to 1.0 mm, in the range of 0.5 mm to 1 mm, in the range of 0.75 mm to 2 mm, in the range of 1 mm to 10 mm, in the range of 5 mm to 20 mm, in the range of 10 mm to 50 mm, in the range of 0.1 mm to 50 mm, or any suitable distance to position the local magnetic flux density peak at a suitable location near or on the end wall 234. Because the electrons are reflected by the negative potential applied in the tube lens, they turn to re-enter the ionization chamber. The ferromagnetic element 242 creates a peak in magnetic flux density at a distance 1706 (i.e., at the end wall) to confine the electron trajectory to near the central axis 244, so that fewer electrons strike the exit surface of the end wall 234. The greater confinement of the electrons occurs because the magnetic field lines are highly coalesced and concentrated to form an increased magnetic field near the end of the ferromagnetic element 242.

[0105] Fig.18A A simulated xy plot of the distribution of electron positions in the plane of the ion exit aperture of a conventional source having only paramagnetic components is illustrated. The area between the black circles represents the exit face of the end wall 234, while the area within the inner black circle represents the ion exit aperture. In this example, the ion exit aperture has a diameter of 4.0 mm.

[0106] Fig.18BA simulated xy plot of the distribution of reflected electron positions on the exit face of the ion exit aperture (i.e., the face opposite the electron source) is illustrated. The increase in magnetic field density near the ion exit aperture increases the confinement of electrons near the central axis. As a result, fewer electrons collide with the end wall of the source. Fig.18A Compared to the paramagnetic source, fewer electrons hit the end wall, while more electrons pass through the ion exit hole and return to the ionization chamber. In this simulation, the number of electrons hitting the end wall was reduced by 33% when using ferromagnetic materials compared to only paramagnetic sources (52.0% of the electrons hit the lens vs. 68.3% of the electrons hit the lens). Similarly, embodiments including ferromagnetic elements improve the signal in analyte detection due to increased electron reflection efficiency (which increases the chance of ionization of the analyte). Finally, the ion source taught herein has increased robustness because the intensity of the burning produced by reflected electrons near the ion exit hole is reduced.

[0107] Although the present teachings are described in conjunction with various embodiments, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications, and equivalents, as will be understood by those skilled in the art.

[0108] In addition, when describing various embodiments, the specification may have presented the method and / or process as a specific sequence of steps. However, insofar as the method or process does not rely on the specific order of the steps set forth herein, the method or process should not be limited to the specific sequence of steps described. As will be appreciated by those of ordinary skill in the art, other sequences of steps are also possible. Therefore, the specific order of the steps set forth in the specification should not be interpreted as a limitation on the claims. In addition, claims relating to methods and / or processes should not be limited to performing their steps in the order written, and those skilled in the art can easily understand that the sequence can be varied and still remain within the essence and scope of the various embodiments.

[0109] The embodiments described herein may be practiced with other computer system configurations including hand-held devices, microprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers, etc. Embodiments may also be practiced in distributed computing environments where tasks are performed by remote processing devices connected through a network.

[0110] It should also be understood that the embodiments described herein may employ various computer-implemented operations involving data stored in a computer system. These operations are those requiring physical manipulation of physical quantities. Typically, but not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. Additionally, the manipulations performed are often explicitly referred to as such as generating, identifying, determining, or comparing.

[0111] Certain embodiments may also be embodied as computer readable code on a computer readable medium. A computer readable medium is any data storage device that can store data that can thereafter be read by a computer system. Examples of computer readable media include hard drives, network attached storage (NAS), read-only memory, random access memory, CD-ROM, CD-R, CD-RW, magnetic tape, and other optical and non-optical data storage devices. The computer readable medium may also be distributed on a network coupled computer system so that the computer readable code is stored and executed in a distributed manner.

Claims

1. An ion source, comprising: an electron source configured to generate electrons; an ionization chamber having an entrance aperture through an electron lens, an ion exit aperture through an end wall, and a central axis through an ionization volume within the ionization chamber, the ionization chamber being configured to generate ions; and A ferromagnetic element is disposed adjacent to the ionization volume such that the electrons are confined to the central axis within a paramagnetic section of the ionization volume and the electrons deviate away from the central axis within the ferromagnetic section of the ionization volume.

2. The ion source of claim 1, further comprising a magnetic field generator adjacent to an end of the ion source closest to the electron source.

3. The ion source of claim 1, wherein the paramagnetic section comprises a high density electron region in which the electrons interact with neutral molecules introduced through a gas inlet to form analyte ions or reagent ions. 4 . The ion source of claim 1 , further comprising a second paramagnetic segment disposed after the ferromagnetic segment along the central axis.

5. The ion source of claim 1, wherein the ferromagnetic element comprises the end wall. The ion source of claim 1 , wherein the ferromagnetic element is disposed within the ionization chamber.

7. The ion source of claim 1, wherein the ferromagnetic element is attached to or embedded in a portion of an outer wall of the ionization chamber.

8. The ion source of claim 1, wherein the ferromagnetic element is disposed outside the ionization chamber.

9. The ion source of claim 1, wherein the ferromagnetic element is configured to be moved to adjust a position of the ferromagnetic segment within the ionization volume.

10. The ion source of claim 1, wherein the ferromagnetic element produces a non-monotonic variation in the magnetic field along the central axis from the entrance aperture to the ion exit aperture.

11. A method of operating an ion source, the method comprising: generating a magnetic field in an ionization volume of an ionization chamber of the ion source using a magnetic field generator; passing electrons through a paramagnetic segment of the ionization volume, wherein the magnetic field confines the electrons to a central axis of the ionization volume; as well as The electrons are caused to pass through a ferromagnetic segment of the ionization volume produced by a ferromagnetic element disposed within or adjacent to the ionization chamber, the electrons being deflected away from the central axis within the ferromagnetic segment of the ionization volume.

12. The method of claim 11, further comprising introducing neutral molecules into the ionization chamber through a gas inlet, and Wherein causing the electrons to pass through the paramagnetic segment includes causing the electrons to interact with the neutral molecules in a high density electron region in the paramagnetic segment to form analyte ions or reagent ions.

13. The method of claim 11, further comprising passing ions through the ferromagnetic section of the ionization volume to spatially separate the ions and the electrons.

14. The method of claim 11, wherein the ferromagnetic element is disposed within the ionization chamber.

15. The method of claim 11, wherein the ferromagnetic element is attached to the ionization chamber or the ferromagnetic element is embedded within a portion of an outer wall of the ionization chamber.

16. The method of claim 11, wherein the ferromagnetic element is disposed outside the ionization chamber.

17. An ion source, comprising: an electron source configured to generate electrons; an ionization chamber having an entrance aperture through an electron lens, an ion exit aperture through an end wall, and a central axis through an ionization volume within the ionization chamber, the ionization chamber being configured to generate ions; and A ferromagnetic element is located at a distance from the end wall or the electron lens so that the magnetic field lines are concentrated at the ion exit hole or the entrance hole, respectively.

18. The ion source of claim 17, wherein the distance is the length of a paramagnetic segment in the ionization volume.

19. The ion source of claim 17, wherein the ferromagnetic element comprises a final tube lens disposed after the end wall along the central axis.

20. The ion source of claim 17, wherein the surface of the ferromagnetic element is protected by a paramagnetic insert or by a coating to prevent chemical reaction with the electrons or the ions.

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