High-performance ion source and method for generating an ion beam

Abstract

 An ion source, in which a divergent skimmer electrode is placed, spatially separated from, but in short spatial distance downstream from the tip of the tip electrode, and in which a sheath gas is supplied to flush around the tip electrode, allows achieving a stable and high-flux ion beam. The dimensions of the skimmer electrode may be chosen to adjust the pressure difference across the skimmer electrode, and may support the formation of two Mach cones. This combination of features allows to greatly enhance the ion current, a significant improvement for deposition applications in particular.

Description

Field of the Invention

[0001] The present invention relates to an ion source and a method for generating an ion beam by electrospray ionization or by atmospheric pressure chemical ionization, in particular for applications in high-flow deposition of surface structures and for desorption electrospray applications.

Background and Related Prior Art

[0002] The forming of thin layers of various compositions has gained a large importance in many areas of science and industry, ranging from semiconductor fabrication to the making of sensors or high throughput screening in medical diagnostics. Depending on the substrate and on the material that is deposited, a variety of different techniques may be employed in the formation of these layers, ranging from vapour deposition to sputtering, plasma spraying, or soft landing by electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI).

[0003] ESI and APCI were originally developed to generate an ion beam for analysis in mass spectrometry from an electrolyte solvent, but is now gaining increasing importance in the formation of thin films, coating and deposition as well. Both methods may be employed to form layers from thermally or structurally unstable and/or reactive molecules.

[0004] In ESI-mode the ion beam is generated as follows: A strong electrical field is applied between a spray needle and a counter electrode. The spray needle is provided with a fluid conduit, typically with an inner diameter of 10 µm to 500 µm, which supplies a liquid solution comprising the molecules that shall be analyzed or deposited. The high electrical field at the tip of the spray needle, typically in the range of 5 to 10 kV/mm, leads to an electrical force to the electrolyte solution, so that the liquid surface forms a Taylor cone. When the electrical force predominates the surface tension and the gas pressure, charged droplets are formed, and the electrolyte solution is dispersed from the spray needle into a fine aerosol. A neutral sheath gas such as air or nitrogen may be supplied to the needle to support the formation of the aerosol and to facilitate the evaporation of the liquid solution. Compounds that ensure the conductivity, such as acetic acid, are customarily added to the solvent. The average droplet size shrinks when the solvent evaporates. This leads to an increase in the charge density at the surface of the droplet until the droplet reaches the Rayleigh limit and bursts into multiple smaller droplets in a process known as Coulomb fission. These smaller droplets will again undergo evaporation and become unstable, until finally a beam of single ions is formed that is drawn towards the counter electrode.

[0005] In APCI-mode a non-electrolytic liquid may be used. An additional needle at a high voltage of some kV is used to generate a corona discharge. Thereby the molecules of the sheath gas are ionized; these ions charge the droplets of the non electrolytic liquid.

[0006] The ion beam emanating from the spray needle may be used for analysis in mass spectrometers, but may likewise be employed for deposition on substrates in defined atomic layers by means of soft landing. The ion beam may be focused, and mass filters may be employed to remove neutral particles, or to purge the beam, which allows producing particles of well-defined mass and energy. Hence, very fine structures may be formed in a way similar to ion beam lithography. ESI and APCI are particularly useful in producing charged macromolecules, since it overcomes the propensity of these molecules to fragment by conventional ionization or thermal evaporation. The technique is ideally suited to deposit highly sensitive peptides and proteins in microarrays at a precision down to single molecular layers. Applications range from the design of sensors to high throughput screening in medical diagnostics. The ion beam can also be employed to purge proteins, or in the manufacturing of highly specific catalysts by defined deposition of clusters or metal-organic complexes.

[0007] ESI and APCI systems for analytical applications with a special emphasis on mass spectrometry are described in US patents US 4,144, 451, US 4,861,988 and US 6,294,779, and European patent specifications EP 0 511 961 B1 and EP 2 254 142 A2.

[0008] Ion beam deposition is a very versatile and precise technique, but has so far been costly and rather elaborate, difficult to implement, and time-consuming. The low beam intensity is the most limiting factor, and with current devices it takes hours or even days to deposit a few molecular layers. What is required is an ion beam source system and method that provides higher beam intensity and throughput as well as a high yield for analytical purposes.

Overview of the Present Invention

[0009] This objective is achieved by means of an ion source according to independent claim 1 and a method for generating an ion beam according to independent claim 12. The dependent claims relate to preferred embodiments.

[0010] An ion source according to the present invention comprises a first vacuum chamber and a second vacuum chamber in fluid communication with said first vacuum chamber through a first nozzle, said first nozzle having an inlet with a first cross-sectional area and an outlet with a second cross-sectional area, wherein said second cross-sectional area is larger than said first cross-sectional area and a diameter of said inlet is at least 1 mm. The ion source further comprises an ion emitter unit provided in said second vacuum chamber, said ion emitter unit comprising a tip electrode, and further comprising a first fluid channel adapted to supply a fluid to said tip electrode. Said tip electrode has a spray outlet facing said inlet of said first nozzle and is adapted to spray and direct said fluid in a direction normal to an inlet surface of said first nozzle, wherein a tip of said tip electrode is spaced 0.5 mm to 20 mm from said inlet of said first nozzle.

[0011] A tip electrode facing a first nozzle with widening cross-section (in the direction of the ion flow), but spaced apart by a distance of between 0.5 mm to 20 mm from the inlet of said first nozzle, wherein the first nozzle establishes a fluid connection between said second vacuum chamber and said first vacuum chamber so that said first vacuum chamber may be evacuated via said first nozzle allows a significant increase of the ion throughput over conventional electrospray devices. With the recited combination of features, an unfiltered ion current of 1.5 µA or above can be achieved at a yield (ratio of current in the first vacuum chamber and current emanating from the tip electrode) of exceeding 85 % at low needle current and exceeding 50 % at high needle current.

[0012] In a preferred embodiment, said tip of said tip electrode is positioned no more than 15 mm, and particularly preferably no more than 10 mm from said inlet of said first nozzle.

[0013] In a further preferred embodiment, said tip of said tip electrode is positioned no less than 1 mm, and particularly preferably no less than 5 mm, from said inlet of said first nozzle.

[0014] These distances may be measured along an axis of symmetry of said first nozzle, or in a direction normal to an inlet surface of said first nozzle.

[0015] The first nozzle may have an interior that is divergent from the second vacuum chamber to the first vacuum chamber, in particular a truncated cone.

[0016] In a preferred embodiment, an inner diameter of said first nozzle increases continuously between said inlet and said outlet.

[0017] Said first nozzle may serve as a so-called skimmer nozzle, and may skim outwardly lying portions of the ion beam. When positioned at the right distance of between 0.5 mm and 20 mm from the tip of the tip electrode, the skimmer nozzle forms the ion flow from the ion emitter to said first vacuum chamber through said first nozzle. In particular, the inventors found that the skimmer supports the formation of a Mach cone at the transition from the second vacuum chamber to the first vacuum chamber.

[0018] In particular, said tip electrode may lie on an axis through a centre of said first nozzle, or may deviate from said axis by a distance of no more than 30 %, preferably by no more than 10 %, and in particular by no more than 5 % of the diameter of said inlet of said first nozzle.

[0019] In the sense of the present invention, a tip electrode that is adapted to spray and direct a fluid in a direction normal to an inlet surface of said first nozzle may be understood as a tip electrode that is configured to spray said fluid, in particular as an aerosol, directly towards or in the direction of or into said inlet of said first nozzle. In particular, the tip electrode and/or spray outlet may be configured and/or positioned relative to said inlet of said first nozzle in a way that the fluid spray emitted from said spray outlet is directed substantially entirely and directly towards said inlet of said first nozzle. A certain divergence of the fluid spray when leaving said spray outlet is desired and may also be unavoidable in practice. But the centre of mass of the fluid stream emitted from said spray outlet should preferably be directed towards said inlet of said first nozzle, and in particular should preferably lie on a trajectory that traverses or runs through said inlet of said first nozzle.

[0020] In the sense of the present invention, a fluid may be understood to encompass liquids, gases and suspensions of solid particles or liquid droplets in a gas, such as aerosols. Typically, in an ion source according to the present invention the analyte or source comprising the ions will be supplied through said first fluid channel in the form of a liquid, and will be dispersed and sprayed from said tip electrode as an aerosol.

[0021] An axis of said tip electrode may be positioned along a direction normal to said inlet surface of said first nozzle, or said axis may be inclined with respect to said direction normal to said inlet surface of said first nozzle by an angle of no more than 10°, and preferably by an angle of no more than 5°.

[0022] Said spray outlet may be positioned to lie on said axis.

[0023] Said tip electrode may be positioned along a direction normal to an inlet surface of said first nozzle. In particular, said tip electrode may be inclined with respect to an axis through a centre of said first nozzle by an angle of no more than 10°, and preferably by an angle of no more than 5°.

[0024] In a preferred embodiment, said tip electrode is centred on said inlet of said first nozzle.

[0025] The axis according to the preceding embodiments may be an axis of symmetry of said first nozzle, preferably an axis of rotational symmetry of said first nozzle.

[0026] A configuration in which said tip electrode is centred on said inlet of said first nozzle and emits in a direction normal to said inlet of said first nozzle allows to further increase the yield and throughput of the ion current.

[0027] In a preferred embodiment, said first vacuum chamber comprises a gas outlet for connection with a pumping means.

[0028] The first vacuum chamber may be adapted to be evacuated to a first sub-atmospheric pressure. In particular, the ion source may comprise evacuation means or pumping means adapted to evacuate said first vacuum chamber to a first sub-atmospheric pressure, preferably of no more than 100 mbar, particularly preferably of no more than 10 mbar.

[0029] The second vacuum chamber may be adapted to be evacuated to a second sub-atmospheric pressure, said second sub-atmospheric pressure being higher than said first sub-atmospheric pressure. In a preferred embodiment, said second sub-atmospheric pressure is no higher than 500 mbar, preferably no higher than 150 mbar.

[0030] In a preferred embodiment, said first nozzle is the only gas drain of said second vacuum chamber. Hence, said second vacuum chamber may be adapted to be evacuated to said second sub-atmospheric pressure only via contact with said first vacuum chamber through said first nozzle. Preferably, said second vacuum chamber does not comprise a further outlet for connection with a pumping means.

[0031] The pressure level in said second vacuum chamber may be adjusted by selecting the pressure level in said first vacuum chamber, and by adjusting the dimensions, cross-sectional surface area and inner diameter of said first nozzle.

[0032] Preferably, said first nozzle has an inlet with a circular or oval opening.

[0033] In a preferred embodiment, an inlet diameter of said first nozzle is at least 1 mm, preferably at least 2 mm.

[0034] Said inlet diameter may be measured across a diagonal of said inlet opening of said first nozzle, preferably across the longest diagonal of said inlet opening of said first nozzle.

[0035] Preferably, a ratio of said second sub-atmospheric pressure in the second vacuum chamber and said first sub-atmospheric pressure in the first vacuum chamber is adjusted to lie in the range of 5 to 50, preferably in the range of 10 to 30.

[0036] The inventors found that a suitable choice of pressure ranges in the first and second vacuum chamber allows to greatly enhance the yield of the ion generation and the throughput. The choice of these pressure ranges and the structural adaptations to attain these pressure ranges constitute an independent aspect of the invention.

[0037] In this aspect, the invention relates to an ion source with a first vacuum chamber adapted to be evacuated to a first sub-atmospheric pressure of no more than 100 mbar, and preferably no more than 10 mbar, as well as a second vacuum chamber in fluid communication with said first vacuum chamber through a first nozzle, said first nozzle having an inlet with a first cross-sectional area and an outlet with a second cross-sectional area, wherein said second cross-sectional area is larger than said first cross-sectional area and a diameter of said inlet is at least 1 mm. Said second vacuum chamber is adapted to be evacuated to a second sub-atmospheric pressure via said first nozzle, said second sub-atmospheric pressure being no lower than said first sub-atmospheric pressure and being no higher than 500 mbar. An ion emitter unit is provided in said second chamber, said ion emitter unit comprising a tip electrode facing said inlet of said first nozzle, and further comprising a first fluid channel adapted to supply a liquid to said tip electrode.

[0038] Preferably, said second sub-atmospheric pressure is no higher than 150 mbar.

[0039] In a preferred embodiment, an inlet diameter of said first nozzle is at least 2 mm, and preferably at least 3 mm.

[0040] Said first nozzle may be a nozzle with some or all of the features described above, in particular a skimmer nozzle.

[0041] Said ion emitter unit may be an ion emitter unit with some or all of the features described above. In particular, said tip electrode may comprise a spray outlet facing said inlet of said first nozzle and may be adapted to spray and direct a fluid in a direction normal to an inlet surface of said first nozzle. Preferably, a tip of said tip electrode may be spaced 0.5 mm to 20 mm from said inlet of said first nozzle.

[0042] In a preferred embodiment, the ion source comprises a metal plate or metal shielding at least partially surrounding said inlet of said first nozzle. Said metal plate or metal shielding may serve as an impingement plate adapted to discharge any impinging ions. The metal plate or metal shielding may be electrically connected to said first nozzle.

[0043] In a preferred embodiment, said ion emitter unit further comprises a second fluid channel that is connectable to a gas reservoir.

[0044] Said second fluid channel may at least partially enclose said tip electrode, and may have an opening facing said inlet of said first nozzle.

[0045] Preferably, said opening of said second fluid channel comprises a second nozzle at least partially surrounding or enclosing said tip electrode.

[0046] The second fluid channel may serve to supply a sheath gas from said gas reservoir so said tip electrode.

[0047] In a preferred embodiment, said inlet of said first nozzle protrudes from said metal plate or shielding by at least 1 mm, preferably by at least 3 mm in the direction towards the tip electrode.

[0048] The inventors found that a nozzle that substantially protrudes from the impingement plate allows a better focus of the electrical field and the ion stream, and prevents ions from straying off and impinging on the inner head and sidewalls of the second vacuum chamber.

[0049] In a preferred embodiment, said opening of said second fluid channel is positioned and shaped such that a gas flow from said opening at least partially flushes around said tip electrode. In particular, said opening may be positioned and shaped such that said gas flow is directed coaxially with said tip electrode and towards said inlet of said first nozzle.

[0050] In a preferred embodiment, said tip electrode is placed in said second fluid channel, preferably concentrically with said second fluid channel.

[0051] The inventors found that the ion throughput and yield may be significantly enhanced when the sheath gas is fed coaxially with said tip electrode such that it flushes around said tip electrode.

[0052] Said second nozzle may assist to direct the sheath gas flow around said tip electrode and towards said inlet of said first nozzle, thereby further increasing the ion throughput.

[0053] Said second nozzle may be a nozzle that is pinched in the middle, comprising (in the direction of flow of the sheath gas) a first convergent section followed by a second divergent section.

[0054] Preferably, said second nozzle is a de Laval nozzle.

[0055] The ion source according to the present invention provides a high-flux and stable ion flow that may be employed both for analytical applications in mass spectrometry, as well as for layer deposition. Possible applications comprise conventional ESI and APCI as well as the more recent desorption electrospray ionization (DESI) technique, a combination of electrospray and desorption ionization.

[0056] The first fluid channel may be employed to supply a stream of electrolytic or non-electrolytic solution or solvent to the tip electrode from an external fluid reservoir. The solution or solvent may serve as the source of atoms, molecules or particles to be electrosprayed and analyzed in a mass spectrometer, or to be deposited on some substrate.

[0057] Said tip electrode may be in fluid communication with said first fluid channel.

[0058] In a preferred embodiment, said tip electrode may comprise a hollow needle electrode in fluid communication with said first fluid channel.

[0059] Said hollow needle may at least partially surround or enclose an end portion of said first fluid channel.

[0060] In a preferred embodiment, a first pressure adjusting conduit may be adapted to be connected between said fluid reservoir and said second vacuum chamber.

[0061] The first pressure adjusting conduit may comprise a first pressure adjuster, preferably a throttle.

[0062] A second pressure adjusting conduit may connect the fluid reservoir to an external pressure source, or to the ambient environment. This second pressure adjusting conduit may be provided with a second pressure adjuster, preferably a throttle valve.

[0063] This configuration allows supplying the external fluid solution from the fluid reservoir to the tip electrode simply by adjusting the pressure in the fluid reservoir by means of the throttle valve. A separate delivery means such as a pump or piston is not required.

[0064] In a preferred embodiment, the ion source further comprises first voltage connection means adapted to raise said first nozzle to a first voltage and/or second voltage connection means adapted to raise said tip electrode to a second voltage. The first nozzle may serve as a counter electrode for the tip electrode. By adjusting the voltage levels at the first nozzle and the tip electrode, respectively, the electrospray ionization and ion stream from the tip electrode through the nozzle may be suitably adjusted.

[0065] The invention also relates to a method for generating an ion stream comprising the steps of providing a first vacuum chamber and evacuating said first vacuum chamber to a first sub-atmospheric pressure of no higher than 100 mbar, providing a second vacuum chamber in fluid communication with said first vacuum chamber through a first nozzle, said first nozzle having an inlet with a first cross-sectional area and an outlet with a second cross-sectional area, wherein said second cross-sectional area is larger than said first cross-sectional area and a diameter of said inlet is at least 1 mm. The method further comprises the steps of evacuating said second vacuum chamber to a second sub-atmospheric pressure, wherein said second sub-atmospheric pressure is no lower than said first sub-atmospheric pressure and no higher than 500 mbar, and providing an ion emitter unit in said second vacuum chamber, and providing an ion flow from said ion emitter through said first nozzle into said first vacuum chamber.

[0066] In a preferred embodiment, said second vacuum chamber is evacuated only from said first vacuum chamber via said first nozzle.

[0067] Preferably, said first vacuum chamber is evacuated to a pressure of no higher than 20 mbar, preferably of no higher than 10 mbar.

[0068] In a further preferred embodiment, said second vacuum chamber is evacuated via the first nozzle to a pressure of no higher than 250 mbar, preferably of no higher than 150 mbar.

[0069] In a preferred embodiment, the method further comprises the step of supplying a gas stream to said ion emitter from a sheath gas reservoir, in particular from an external reservoir.

[0070] Preferably, said gas stream is the only source of gas to the second vacuum chamber.

[0071] Said gas stream preferably may comprise a stream of any inert gas like air, nitrogen, SF₆ or noble gases or oxygen.

[0072] In a preferred embodiment, said ion emitter unit comprises a tip electrode, and said gas stream is supplied to said ion emitter unit such that it flushes around said tip electrode.

[0073] The method according to the present invention may further comprise the step of supplying a solvent or electrolytic or non-electrolytic fluid from a fluid reservoir to said ion emitter unit to provide a beam of ions.

[0074] The step of supplying said fluid may comprise the step of adjusting a pressure level in said fluid reservoir.

[0075] Said pressure level may be adjusted by establishing a first fluid connection that may serve for pressure equilibration between said fluid reservoir and said second vacuum chamber, and establishing a second fluid connection between said fluid reservoir and an external pressure reservoir. Said external pressure reservoir may be the ambient air environment. Throttle valves may be provided in the first and/or second fluid connections to adjust the pressure level in the fluid reservoir, and hence the flow of the spray needle.

[0076] In a preferred embodiment, the method further comprises the step of raising said first nozzle to a first voltage, and raising said ion emitter unit to a second voltage.

[0077] Said second voltage may differ from said first voltage by at least 0.3 kV, preferably by at least 1 kV.

[0078] The method according to the present invention may employ an ion source with some or all of the features described above.

Detailed Description of Preferred Embodiments

[0079] The details and numerous advantages of the present invention will be best understood from a description of preferred embodiments with reference to the appended drawings, in which:

Fig. 1 is a schematic cross-sectional drawing of an ion source according to an embodiment of the present invention; and

Fig. 2 is a more detailed close-up of an emitter unit and skimmer nozzle of the embodiment shown in Fig. 1.

[0080] Fig. 1 depicts an ion source 10 according to the present invention, which may form part of or serve as a component of an apparatus for electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) or desorption electrospray ionization (DESI). The ion source 10 comprises a first vacuum chamber or outer vacuum chamber 12 and further comprises a second vacuum chamber or inner vacuum chamber 14 separated from said first vacuum chamber 12 by means of a sealing wall 16. A skimmer nozzle 18 formed in an upper section of said sealing wall 16 establishes a fluid connection between said first vacuum chamber 12 and said second vacuum chamber 14. The skimmer nozzle 18 is formed of metal, and first voltage connection means (not shown) are provided to connect said skimmer nozzle 18 to a voltage source (not shown) and to raise said skimmer nozzle 18 to a predefined first potential.

[0081] The skimmer nozzle 18 has a divergent shape, with an inner diameter that increases continuously between an inlet 20 provided at one end of said skimmer nozzle 18 and in fluid communication with said second vacuum chamber 14, and an outlet 22 provided at an opposing end of said skimmer nozzle 18 and in fluid communication with said first vacuum chamber 12. The diameter of said inlet 20 may be 1 mm or more, for instance 4 mm. The diameter of the outlet 22 may amount to at least 1.5 times the inlet diameter, preferably to at least two times the inlet diameter. For instance, the diameter of the outlet 22 may amount to about 12 mm.

[0082] An emitter unit 24 is provided in said second vacuum chamber 14 facing the inlet 20 of the skimmer nozzle 18. The emitter unit 24 comprises a first fluid channel 26 serving as a fluid channel for providing a fluid from a reservoir located externally of said second vacuum chamber and/or said first vacuum chamber. This channel will henceforth be denoted external fluid channel Said external fluid channel 26 is connected at one end to an external fluid reservoir 28 from which a liquid to be sprayed, such as an external fluid 30 is supplied for delivery to the emitter unit 24. At an end opposite to the fluid reservoir 28, the external fluid channel 26 ends in a hollow spray needle 32 for electrospray generation that faces the inlet 20 of the skimmer nozzle 18. The spray needle 32 may be a conventional ESI needle with a tapered tip section and an inner channel diameter of between 5 and 500 µm, preferably of between 20 and 200 µm, for instance 100 µm, and is preferably made of metal. ESI spray needles are well-known in the art, and hence a detailed description will be omitted. Second voltage connection means (not shown) are provided to connect said spray needle 32 to said voltage source (not shown), such that a voltage may be applied between the spray needle 32 and the skimmer nozzle 18. Typical voltages lie in the range of 1 to 3 kV.

[0083] The external fluid reservoir 28 is additionally connected to the second vacuum chamber 14 via a pressure adjusting conduit 44, and may further be connected to an external pressure source (typically at ambient pressure, not shown) to allow for a supply of ambient air to the fluid reservoir 28 via an air supply conduit 46. A pressure adjusting throttle 48 and an air supply throttle valve 50 may be provided in said pressure adjusting conduit 44 and air supply conduit 46, respectively, to adjust the pressure in the external fluid reservoir 28. This allows delivering the external fluid 30 from the external fluid reservoir 28 through an external fluid conduit 52 and the external fluid channel 26 to the spray needle 32 merely by providing an excess pressure in said fluid reservoir 28, without employing any extra delivery means or pump. By establishing a connection to the second vacuum chamber 14 via the pressure adjusting conduit 44 and the pressure adjusting throttle 48, the pressure in the external fluid reservoir 28 may be automatically adjusted in conjunction with the pressure level in the second vacuum chamber 14, which is a further advantage of the configuration shown in Fig. 1.

[0084] The emitter unit 24 further comprises a sheath gas fluid channel 34 in fluid communication with an external sheath gas reservoir (typically at ambient pressure, not shown) via a sheath gas conduit 36. A throttle valve 38 may be provided in the sheath gas conduit 36 to adjust the pressure and flow rate of the sheath gas.

[0085] In the configuration shown in Fig. 1, the sheath gas fluid channel 34 at least partially encloses or surrounds the external fluid channel 26. In other words, the external fluid channel 26 extends through the sheath gas fluid channel 34. The sheath gas fluid channel 34 opens into the second vacuum chamber 14. The opening of the sheath gas fluid channel 34 comprises a second nozzle 40 that directs the flow of the sheath gas towards the inlet 20 of the skimmer nozzle 18. The spray needle 32 extends through said second nozzle 40 and protrudes slightly from the opening of the second nozzle 40. Hence, the sheath gas provided from the sheath gas reservoir (not shown) through said sheath gas conduit 36 and sheath gas fluid channel 34 flushes around the spray needle 32 when leaving the sheath gas fluid channel 34 through the second nozzle 40. As shown in the drawings, the second nozzle 40 may be a de Laval nozzle.

[0086] The spray needle 32 and sheath gas fluid channel 34 share a common central axis z with the skimmer nozzle 18. The axis z may be a common axis of rotational symmetry of the skimmer nozzle 18 and/or spray needle 32 and/or sheath gas fluid channel 34. Hence, the spray needle 32 and the sheath gas fluid channel 34 are centred on the inlet 20 of the skimmer nozzle 18. The end tip of the spray needle 32 is separated and spaced apart from said inlet 20 of said skimmer nozzle 18 by a distance of between 0,5 mm and 20 mm, preferably by a distance of between 4 mm and 10 mm, said distance measured along the common central axis z.

[0087] A metal impingement plate 42 is formed at an inner sidewall of the second vacuum chamber 14 in order to at least partially surround said skimmer nozzle 18. The impingement plate 42 may be electrically connected to the skimmer nozzle 18 and may serve to discharge any stray ions that are not drawn into the inlet 20 of the skimmer nozzle 18. Preferably, the inlet 20 of the skimmer nozzle 18 protrudes from the impingement plate 42 by at least 1 mm, preferably by at least 3 mm towards the spray needle 32. This allows focussing the electric field towards the skimmer nozzle 18 and to avoid the formation of large equipotential surfaces that could detract the ions from the inlet 20 of the skimmer nozzle 18 and towards the sidewalls of the second vacuum chamber 14.

[0088] Fig. 2 shows a more detailed view of the central part of the ion source 10, comprising the emitter unit 24 with the spray needle 32 that faces the inlet 20 of the skimmer nozzle 18.

[0089] Operation of the ion source for electrospray generation will now be described with continued reference to Figs. 1 and 2. The first vacuum chamber 12 will be evacuated to a first sub-atmospheric pressure through a pumping outlet 64 by means of a pump (not shown), for instance to a pressure of no more than 50 mbar and preferably no more than 20 mbar. As an example, the first vacuum chamber 12 may be evacuated to a pressure of about 5 mbar. The first vacuum chamber is in fluid communication with the second vacuum chamber 14 only via the skimmer nozzle 18. Hence, the skimmer nozzle 18 is the only gas drain of the second vacuum chamber 14. In particular, no separate pumping means are provided in the second vacuum chamber 14. By suitably choosing the dimensions of the skimmer nozzle 18 and by adjusting the flow of the sheath gas through the sheath gas fluid channel 34, the second vacuum chamber 14 may be evacuated via the first vacuum chamber 12 and the skimmer nozzle 18 to a second sub-atmospheric pressure of below 500 mbar, preferably of below 150 mbar, and above 20 mbar. For instance, by providing a sheath gas flow of

an inlet diameter of said skimmer nozzle 18 in the range of 4 mm and a pressure of 5 mbar in the first vacuum chamber 12, the pressure in the second vacuum chamber 14 may be adjusted to around 80 mbar.

[0090] The pressure in the external fluid reservoir 28 will be chosen slightly higher, for instance at around 150 mbar, depending on the properties of the fluid 30 and the desired flow rate. The pressure in the externalfluid reservoir 28 may be adjusted by means of the pressure adjusting throttle 48 and the air supply throttle valve 50, but may also be fine-tuned by changing the position and/or height of the external fluid reservoir 28 (typically a syringe without a piston) relative to the emitter unit 24.

[0091] A voltage of between 1 kV and 3 kV, typically of around 1.5 kV, is applied between the skimmer nozzle 18 and the spray needle 32, and a sheath gas such as nitrogen or air is supplied from the sheath gas reservoir (not shown) through the sheath gas conduit 36 and the sheath gas fluid channel 34 such that it flushes around the tip of the spray needle 32. As described with reference to the prior art in the introductory part of the specification, the strong electrical field (in the range of 5 to 10 kV/mm) in the vicinity of the spray needle 32 disperses the liquid 30 supplied from the external fluid reservoir 28 by electrospray into a fine aerosol. Fig. 2 illustrates the flow of the sheath gas 54 and ion beam 56 from the emitter unit 24 towards the first vacuum chamber 12 through the skimmer nozzle 18. The pressure difference between the sheath gas fluid channel 34 and the second vacuum chamber 14 will result in a supersonic expansion of the aerosol and the formation of a first Mach cone between the second nozzle 40 and the skimmer nozzle 18. The geometry of the skimmer nozzle 18 and the distance between the skimmer nozzle 18 and the spray needle 32 may be adjusted to support the formation of a stable and high-flow ion beam 56. The sharp brim of the inlet 20 of the skimmer nozzle 18 allows skimming turbulences 58, which might otherwise interfere with the formation of the first Mach cone. If the skimmer nozzle 18 was positioned too close to the tip of the spray needle 32, for instance closer than 0.5 mm, the turbulences would not be skimmed, and the first Mach cone could not properly form. Moreover, sparkovers between the skimmer nozzle 18 and the spray needle 32 might occur. On the other hand, if the skimmer nozzle 18 was spaced too far away from the tip of the spray needle 32, for instance more than 20 mm, the electrical field strength at the tip of the needle 32 would decrease and the focussing of the electrical field would deteriorate, thereby decreasing the yield of the electrospray generation.

[0092] Forming the outlet of the sheath gas fluid channel 34 as a de Laval nozzle 40 allows achieving particularly high sheath gas velocities, and hence supports an efficient transport of the ions from the second vacuum chamber 14 to the first vacuum chamber 12 through the skimmer nozzle 18.

[0093] The pressure difference between the second vacuum chamber 14 and the first vacuum chamber 12 will result in a further supersonic expansion and the formation of a second Mach cone. Forming the first nozzle as a divergent skimmer nozzle 18 with an inner diameter that increases steadily in the direction of flight of the ions additionally supports the forming of the second Mach cone and minimizes the contact between the inner walls of the skimmer nozzle 18 and the ions, which would lead to a discharging of the ions and hence to a decrease of the yield. In a supersonic expansion the length of a tube is of minor relevance due to the blocking at the entrance of any nozzle or tube. Therefore the divergent form of the skimmer nozzle 18 at a given diameter further minimizes the flow rate of sheath gas and aerosol from the second vacuum chamber 14 into the first vacuum chamber 12. A low flow rate of sheath gas substantially reduces the effort for the pumping means (not shown) connected to the pumping outlet 64.

[0094] After the ion beam 56 has left the skimmer nozzle 18 through the outlet 22, it may optionally pass through one or more radiofrequency funnels 60, 62 for further beam focussing. Mass filters and further pumping stages (not shown) may also be provided downstream of the skimmer nozzle 18 in further vacuum chambers, possibly provided with further pumping outlets 66, depending on the application. These are conventional techniques in the field of electrospray ionization and electrospray ionization mass spectrometry, and hence a detailed description of these techniques will be omitted.

[0095] While the embodiments described previously with reference to Figs. 1 and 2 relate to the generation of an electrospray from a liquid electrolyte solution 30, the invention may also be operated without any electrolyte fluid flow in APCI-mode. This may require slightly higher voltages between the tip electrode 32 and counter electrode 20, for instance in the range of between 1.7 kV and 2 kV, and can be achieved by supplying a carrier gas flow of a carrier gas through the sheath gas fluid channel 34 and the second nozzle 40. It should be noted, that in contrary to normal APCI the pressure in the second vacuum chamber 14 is significantly below ambient pressure, as described above.

[0096] In this mode a corona discharge forms between the tip electrode 32 and the counter electrode 20, ionizing the carrier gas. Unlike conventional corona discharges the ions are transported in a supersonic flow through the skimmer nozzle 18, so they have no opportunity to ionize further neutral gas molecules. The avalanche effect of a conventional corona discharge source is absent. In consequence the corona current increases monotonically with the corona interseption voltage over at least 2 orders of magnitude.

[0097] The present invention allows providing an ion current in the first vacuum chamber in the range of 1.5 µA and above, at a pressure of typically 4 mbar. The yield of the electrospray ionization, measured as the ratio of the ion current in the first vacuum chamber 12 and the current emanating from the spray needle 32, exceeds 85 % at low needle current and exceeds 50 % at high needle current. The high ion current provides up to a 100-fold improvement in throughput over what is conventionally available in the prior art, and allows to significantly reduce the time required for ESI deposition applications.

[0098] The description of the preferred embodiments and the figures merely serve to illustrate the invention and to demonstrate the beneficial effects associated therewith, but should not be understood to imply any limitation. The scope of the invention is to be determined solely from the appended claims.

List of reference signs

[0099] 

10

ion source

12

first vacuum chamber

14

second vacuum chamber

16

sealing wall

18

skimmer nozzle

20

inlet of skimmer nozzle 18 / counter electrode

22

outlet of skimmer nozzle 18

24

emitter unit

26

external fluid channel

28

external fluid reservoir

30

external fluid

32

spray needle / tip electrode

34

sheath gas fluid channel

36

sheath gas conduit

38

sheath gas throttle

40

de Laval nozzle

42

impingement plate

44

pressure adjusting conduit

46

air supply conduit

48

pressure adjusting throttle

50

air supply throttle

52

external fluid conduit

54

sheath gas

56

ion beam

58

turbulences

60, 62

RF funnels

64

pumping outlet of first vacuum chamber 12

66

pumping outlet

Claims

1. An ion source (10) with:

a first vacuum chamber (12);

a second vacuum chamber (14) in fluid communication with said first vacuum chamber (12) through a first nozzle (18), said first nozzle (18) having an inlet (20) with a first cross-sectional area and an outlet (22) with a second cross-sectional area, wherein said second cross-sectional area is larger than said first cross-sectional area and a diameter of said inlet (20) is at least 1 mm; and

an ion emitter unit (24) provided in said second vacuum chamber (14), said ion emitter unit (24) comprising a tip electrode (32), and further comprising a first fluid channel (26) adapted to supply a fluid to said tip electrode (32);

wherein said tip electrode (32) has a spray outlet facing said inlet (20) of said first nozzle (18) and is adapted to spray and direct a fluid in a direction normal to an inlet surface of said first nozzle (18); and
wherein a tip of said tip electrode (32) is spaced 0.5 mm to 20 mm from said inlet (20) of said first nozzle (18).

2. The ion source (10) according to claim 1, wherein said tip of said tip electrode (32) is spaced no more than 15 mm, preferably no more than 10 mm, from said inlet (20) of said first nozzle (18).

3. The ion source (10) according to claim 1 or 2, wherein said first nozzle is a skimmer nozzle (18).

4. The ion source (10) according to any of the preceding claims, wherein said tip electrode (32) is centred on said inlet (20) of said first nozzle (18).

5. The ion source (10) according to any of the preceding claims, wherein an axis (z) of said tip electrode (32) is positioned along said direction normal to said inlet surface of said first nozzle (18), or said axis (z) is inclined with respect to said direction normal to said inlet surface of said first nozzle (18) by an angle of no more than 10°.

6. The ion source (10) according to any of the preceding claims, further comprising evacuation means adapted to evacuate said first vaccum chamber (12) to a first subatmospheric pressure of no more than 100 mbar, preferably no more than 10 mbar.

7. The ion source (10) according to any of the preceding claims, wherein said first nozzle (18) is the only gas drain of said second vacuum chamber (14).

8. The ion source (10) according to any of the preceding claims, wherein an inlet diameter of said first nozzle (18) is at least 2 mm, preferably at least 3 mm.

9. The ion source (10) according to any of the preceding claims, further comprising a metal plate or metal shielding (42) at least partially surrounding said inlet (20) of said first nozzle (18).

10. The ion source (10) according to any of the preceding claims, wherein said ion emitter unit (24) further comprises a second fluid channel (34) that is connectable to a gas reservoir, said second fluid channel (34) at least partially enclosing said tip electrode (32), wherein said second fluid channel (34) has an opening (40) facing said inlet (20) of said first nozzle (18), wherein said opening of said second fluid channel (34) preferably comprises a second nozzle at least partially surrounding or enclosing said tip electrode (32), wherein said second nozzle preferably is a de Laval nozzle (40).

11. The ion source (10) according to any of the preceding claims, wherein said tip electrode comprises a hollow needle electrode (32) in fluid communication with said first fluid channel (26).

12. A method for generating an ion beam, comprising:

providing a first vacuum chamber (12) and evacuating said first vacuum chamber (12) to a first subatmospheric pressure of no higher than 100 mbar;

providing a second vacuum chamber (14) in fluid communication with said first vacuum chamber (12) through a first nozzle (18), said first nozzle (18) having an inlet (20) with a first cross-sectional area and an outlet (22) with a second cross-sectional area, wherein said second cross-sectional area is larger than said first cross-sectional area and a diameter of said inlet (20) is at least 1 mm;

evacuating said second vacuum chamber (14) to a second subatmospheric pressure,

wherein said second subatmospheric pressure is no lower than said first subatmospheric pressure and no higher than 500 mbar; and
providing an ion emitter unit (24) in said second vacuum chamber (14), and providing an ion flow from said ion emitter unit (24) through said first nozzle (18) into said first vacuum chamber (12).

13. The method according to claim 12, wherein said second vacuum chamber (14) is evacuated exclusivly from said first vacuum chamber (12) via said first nozzle (18).

14. The method according to claim 12 or 13, comprising the step of supplying a fluid (30) from a fluid reservoir (28) to said ion emitter unit (34) to provide said ion beam.

15. The method according to any of the claims 12 to 14, employing an ion source (10) according to any of the claims 1 to 11.

Drawing