Mass spectrometers comprising a pre-filter

Abstract

A mass spectrometer comprises an ion source 10 (Figure 1), a mass pre‐filter 20 downstream of the ion source, a collision cell 30 (Figure 1), at least one mass filter downstream of the collision cell,

Description

Field of the invention The present disclosure relates to mass spectrometers. More in particular, the invention relates to mass spectrometers comprising at least one pre-filter to filter ions before they enter a further component of the mass spectrometer, such as a collision cell ora magnetic sector unit. Background of the invention US 2022 / 223401 Al (Schwieters et al.) describes an isotope ratio mass spectrometer comprising an ion source, a static field mass filter, a reaction cell, and a sector field mass analyser for spatially separating ions from the reaction cell according to their mass-charge ratio (m / z ratio). A detector platform is configured for detecting a plurality of different ion species separated by the sector field mass analyser. The static field mass filter acts as a pre-filter to filter ions from the ion source before they enter the reaction cell. The static field mass filter of US 2022 / 223401 Al comprises a first Wien filter that deflects ions away from a longitudinal symmetry axis of the mass spectrometer in accordance with the ions' m / z, and a second Wien filter that deflects ions back towards the longitudinal symmetry axis in accordance with the ions' m / z. An inverting lens 240 is positioned along the longitudinal axis between the Wien filters to invert the direction of deflection of the ions from the first Wien filter. Summary of the invention The present disclosure provides a mass spectrometer comprising an ion source, a pre-filter downstream of the ion source, a collision cell, at least one mass filter downstream of the collision cell, and at least one ion detector downstream of the at least one mass filter. The pre-mass filter can comprise a first magnetic sector unit for separating ions, a second magnetic sector unit for merging separated ions, and a slit arranged between the first magnetic sector unit and the second magnetic sector unit for selectively passing ions. The first magnetic sector unit may be configured for separating ions into parallel, diverging, or converging partial ion beams. The mass pre-filter may comprise an ion lens assembly arranged between the first magnetic sector unit and the second magnetic sector unit, which ion lens assembly may comprise an inversion lens and / or other ion lenses. The ion lens assembly may comprise a first lens or pair of lenses upstream of the slit and a second lens or pair of lenses downstream of the slit, or a single lens or pair of lenses of one side of the slit only. Instead of a single slit, multiple slits and / or diaphragms may be provided. The single or multiple slits and / or diaphragms may be movable. The first pair of lenses may comprise a first zoom lens for converting the parallel ion beams into diverging ions beams and a correction lens for converting the diverging ions beams into parallel ions beams. The second pair of lenses may comprise a first correction lens for converting the parallel ion beams into converging ions beams and a zoom lens for converting the converging ions beams into parallel ions beams. The ion lens assembly may comprise an inversion lens, for example a single inversion lens which is preferably arranged upstream of the slit. Alternatively, the ion lens assembly may comprise a pair of inversion lenses, preferably arranged on either side of the slit. In an embodiment, the ion lens assembly may comprise an inversion lens and a pair of zoom lenses, the zoom lenses being arranged on either side of the inversion lens, the inversion lens preferably being arranged downstream of the slit. The ion source can comprise an inductively coupled plasma (ICP) ion source. The collision cell can include at least one gas inlet for supplying at least one collision gas or reaction gas, so as to facilitate removal of isobaric interferences, such as by inducing charge transfer reactions, and / or mass shift reactions, and / or fragmentation of molecular species, and / or reducing the absolute kinetic energy and / or reducing the energy spread of ions in the ion beam. In some embodiments, the collision cell may be absent. The at least one mass filter downstream of the collision cell preferably comprises a sector field mass analyser. The at least one mass filter may be a double focusing mass analyser arrangement, for example comprising an electrostatic sector downstream of the collision cell followed by a magnetic sector downstream of the electrostatic sector. The at least one ion detector can be a multicollector, for example comprising a plurality of ion detectors for detecting a plurality of different ion species in parallel. The present disclosure further provides a pre-filter for use in a mass spectrometer, the pre-filter comprising a first magnetic sector unit for separating ions, a second magnetic sector unit for merging separated ions, and a slit arranged between the first magnetic sector unit and the second magnetic sector unit for passing selected ions. The present disclosure also provides method of mass spectrometry, comprising: generating a beam of ions from a sample, pre-mass filtering ions from the beam in accordance with their mass to charge ratio, spatially separating selected pre-mass filtered ions in accordance with their mass to charge ratios, and detecting spatially separated ions. The pre-mass filtering comprises using a first magnetic sector unit for separating ions, a second magnetic sector unit for merging separated ions, and a slit arranged between the first magnetic sector unit and the second magnetic sector unit for selectively passing ions. An optional step following the pre-mass filtering and preceding the spatially separating selected ions is passing the mass filtered ions though a collision cell, which contains a collision or reaction gas. Passing the mass filtered ions through the collision cell can remove isobaric mass spectral interferences, through various mechanisms, such as charge transfer reactions, fragmentation of molecular species and / or mass shift reactions in the cell. Such techniques are described in GB2546060 A, the contents of which are incorporated by reference herein in its entirety. Brief description of the drawings Fig. 1 schematically shows, in a perspective view, a mass spectrometer provided with a pre-filter according to the prior art. Fig. 2 schematically shows a first embodiment of a pre-filter according to the present disclosure. Fig. 3 schematically shows a second embodiment of a pre-filter according to the present disclosure. Fig. 4 schematically shows a third embodiment of a pre-filter according to the present disclosure. Fig. 5 schematically shows a fourth embodiment of a pre-filter according to the present disclosure. Fig. 6 schematically shows a fifth embodiment of a pre-filter according to the present disclosure. Fig. 7 schematically shows a sixth embodiment of a pre-filter according to the present disclosure. Fig. 8 schematically shows a seventh embodiment of a pre-filter according to the present disclosure. Fig. 9 schematically shows an eighth embodiment of a pre-filter according to the present disclosure. Fig. 10 schematically shows a first exemplary embodiment of a mass spectrometer provided with a prefilter. Fig. 11 schematically shows a second exemplary embodiment of a mass spectrometer provided with a pre-filter. Description of embodiments This disclosure provides a novel static mass filter and a novel mass spectrometer provided with a static mass filter used as a pre-filter. US 2022 / 223401 Al &&WO 2019 / 180045 Al, which are herewith incorporated by reference in this document, describe a mass spectrometer provided with a static mass filter used as a mass pre-filter. The known static mass filter comprises two Wien filters and an inversion lens. Embodiments of the static mass filters of this disclosure may be provided using conventional components, such as magnetic sector units. In addition, embodiments of the static field mass filters of this disclosure may be provided without an inversion lens. Fig. 1 shows a mass spectrometer as described in US 2022 / 223401 Al &WO 2019 / 180045 Al. The isotope ratio mass spectrometer (IRMS) 100 of Figure 1 includes an ion source 10, a static mass filter (or pre-filter) 20, a collision cell 30, an accelerator 40, an electrostatic sector unit 50, a focusing lens 60, a magnetic sector unit 70, dispersion optics 80 and a detector platform 90. The exemplary ion source 10 includes a triaxial ICP torch 11, an assembly 12 consisting of a sampler cone, one or more skimmer cones, an extraction lens and / or a further skimmer cone, and another ion optical device 13. This ion source arrangement results in a collimated ion beam. Downstream of the ion source 10 is positioned a static field mass filter 20 which will be described in further detail below. The static field mass filter 20 maintains constant electric and magnetic fields, so that transmission of ions through the static field mass filter has a flat response across the selected m / z (mass per elementary charge) range. A quadrupole mass filter, which may be used in this position instead, does not provide such a flat response. In a quadrupole mass filter, the electromagnetic fields change with time according to the applied frequency, which results in a zig zag trajectory of the ions. The prior art static field mass filter 20 of Figure 1 does not introduce a lateral mass discrimination (as would happen in, for example, a magnetic sector analyser) so that the ion beam exiting the static field mass filter 20 can be focused onto the relatively small (ca. 2mm) entrance aperture of a collision cell 30, across the width of the mass window selected for transmission by the static field mass filter 20. Following the collision cell 30, ions are accelerated by an accelerator 40 and focused into the ion optics of a double focusing high resolution multicollector mass spectrometer for simultaneous detection of different isotopes (of a sample and / or a standard). Further, the double focusing high resolution multicollector mass spectrometer includes an electrostatic sector 50 and a magnetostatic sector 70, separated by a focusing lens 60. Downstream of the high resolution multicollector mass spectrometer, the arrangement of Figure 1 contains dispersion optics 80 and finally a detector platform 90 again. A suitable detector platform is described in GB 2541391 A. The static field mass filter 120 Figure 1 is constituted by a double Wien filter. Wien filters employ an arrangement of crossed electrostatic and magnetostatic fields. Ions passing through this arrangement are subject to the magnetic Lorentz force and the electric field strength. In accordance with the present disclosure, however, other types of pre-filters can be used instead of Wien filters. It is noted that in the present document, the terms mass pre-filter, pre-mass filter and pre-filter will be used interchangeably. The pre-filters of the present disclosure are preferably static field mass filters which have static electric and / or magnetic fields, as opposed to multipole (e.g., quadrupole) mass filters, which typically have non-static (i.e., high frequency) electric fields. A first embodiment of a static field mass filter according to the present disclosure, configured for use as a pre-filter in a mass spectrometer, such as an isotope ratio mass spectrometer (I RMS), is schematically shown in Fig. 2. The mass pre-filter 20 comprises a first magnetic sector unit 21, a second magnetic sector unit 22 and a slit or diaphragm 23 arranged between the magnetic sector units 21 &22. The first magnetic sector unit 21 is configured for splitting up the incoming ion beam 1 according to the m / z ratios of the ions. In the embodiment shown, the ion beam is deflected over an angle of 90° by the magnetic field inside the first magnetic sector unit 21 and the resulting partial ions beams 2 are substantially parallel. This can be achieved by the incoming ion beam arranged substantially perpendicular to the entry surface of the first magnetic sector unit 21 and having a 45° angle a on the exit surface. The second magnetic sector unit 22 mirrors the 90° deflection action of the first magnetic sector unit 21 (45° angle a to the ion beam on the entry surface and perpendicular exit surface) and brings the partial ion beams 2 back together, thus forming the outgoing ion beam 3, which is now directed 180° to the incoming ion beam 1. The diaphragm or slit structure 23 allows to selectively pass partial ion beams 2, namely, to select a mass window of ions to pass. The diaphragm or slit structure 23 may be positioned at the plane of symmetry between the magnetic sector units 21 &22 as shown. The diaphragm or slit 23 comprises a plate or other structure provided with an opening. The opening may be round, rectangular, polygonal or have another suitable shape. The width of the opening may be variable to allow the selection of more or less partial ions beams. Additionally, or alternatively, the location of the opening may be variable relative to the partial ion beams 2 so as to be able to select one or more of these partial ion beams. Thus, in the embodiment of Fig. 2, the opening of the slit 23 may be higher or lower than shown. In some embodiments, a structure comprising more than one opening may be provided to pass multiple partial ion beams simultaneously. Instead of a single slit, two, three or more slit structures and / or diaphragms 23 may be used. Conversely, a single structure may comprise two, three or more openings. The mass window of the partial ion beam can be selected by the position and size of the diaphragm or slit structure 23. Thus, Fig. 2 shows a mass pre-filter for use in a mass spectrometer, the mass pre-filter comprising a first magnetic sector unit for separating ions, a second magnetic sector unit for merging separated ions, and a slit arranged between the first magnetic sector unit and the second magnetic sector unit for passing selected ions. In particular, the first magnetic sector unit is configured for separating ions into substantially parallel ion beams. However, in some embodiments the first magnetic sector unit is configured for separating ions into diverging ion beams, as will be discussed later with reference to Fig. 4. In the embodiment of Fig. 2, each magnetic sector unit 21, 22 deflects the (partial) ion beams over an angle of 90°. In some embodiments, other angles can be used, for example 45°, 60° or 80°. In those embodiments, the partial ions beams emerging from the first magnetic sector unit 21 will typically not be parallel. It will be clear that the mass pre-filter of the present disclosure provides a good alternative to the known double Wien filter while using common available components. The magnetic sector units 21 &22 may be commercially available and have a relatively simple structure. As is well known, magnetic sector units use a magnetic field to spatially separate moving ions. Thus, the incoming ion beam 1 is split up into partial ion beams 2, each partial ion beam consisting of ions having a certain mass / charge (m / z) ratio. The spatial separation allows some partial ion beams 2 to be selectively passed using the slit 23, while other partial ion beams are blocked. The mass pre-filter may comprise an ion lens assembly arranged between the first magnetic sector unit and the second magnetic sector unit. An embodiment is shown in Fig. 3, in which the magnetic sectors 21, 22 are like those of Fig. 2, where the ion lens assembly comprises a pair of zoom lenses 24, 25 and a pair of correction lenses 26, 27. The first zoom lens 24 converts the substantially partial beams 2 into diverging beams (apart from the partial beam located on the optical axis) so as to increase the spacing between the partial beams, i.e. the mass dispersion, and thus increase resolution of the mass window selection. The (optional) first correction lens 26 converts the diverging partial beams into substantially parallel partial beams so that the partial beams pass through the slit 23 in parallel (as will be shown later, this is not essential). A second correction lens 27 converts the parallel partial beams 2 into converging partial beams, so that the second zoom lens 25 outputs a set of substantially parallel partial beams again. The ion lenses may be commercially available ion lenses using voltages to generate suitable electric fields. These electric fields are typically static electric fields. Another ion lens assembly is shown in Fig. 4. In this embodiment, the first magnetic sector unit 21 is configured to produces diverging partial ion beams 2. This can be achieved, for example, by using a magnetic sector that provides a rectangular shaped magnetic field. An inversion lens 28 may convert the diverging set of partial ion beams into a converging set of partial ion beams, while the second magnetic sector unit 22 can be arranged to convert the converging set of partial ion beams into a single ion beam. Embodiments can be provided in which the first magnetic sector unit 21 can be arranged to produce converging partial ion beams 2. In such embodiments, the inversion lens may convert a converging set of partial ion beams into a diverging set of partial ion beams, while the second magnetic sector unit 22 can be arranged to convert a diverging set of partial ion beams into a single ion beam. The embodiment of Fig. 5 is similar to the one of Fig. 4 in that the first magnetic sector unit 21 is arranged to produce diverging partial ion beams 2. Due to a first inversion lens 28, however, the partial beams 2 impinge upon the slit structure in parallel. A second inversion lens 29 converts the parallel partial ion beams into converging partial ion beams. As with other embodiments, the converging action of the second inversion lens 29 is only required when off-axis partial ions beams are passed through the slit, as partial ion beams substantially coinciding with the ion optical axis will typically not be affected by the ion lenses. The slit structure 23 can be positioned at the plane of symmetry between the magnetic sectors 21, 22 and the first and second inversion lenses 28, 29 can be positioned symmetrically about the plane of symmetry. In the embodiment of Fig. 6, the angle between the incoming ion beam 1 and the exiting ion beam 3 is not 180°, as in the previous embodiments, but less than 180°. In the example shown, the angle is approximately 80°, but this angle can also be approximately 90°. It can thus be seen that the angle between the incoming ion beam and the exiting ion beam can depend on the geometry of the pre-filter. In particular, the magnetic sector units 21 &22 of Fig. 6 each deflect the ions over less than 90°, for example approximately 45° or approximately 30°. As can be seen, the angle a at which the partial ion beams 2 exit the first magnetic sector unit 21 is less than 45°, in the present example approximately 30° for the center partial beam. Similarly, the angle at which the partial ion beams 2 enter the second magnetic sector unit 22 is the same angle a of less than 45°, in the present example approximately 30° for the center partial beam. A further embodiment is shown in Fig. 7, where the first magnetic sector unit 21 is arranged for producing diverging partial ion beams 2, which are made more divergent by a first zoom lens 24. An inversion lens 28 inverts (that is, converts from diverging into converging) the partial ion beams 2, while a second zoom lens 25 decreases the convergence of the partial ion beams, so that the second magnetic sector unit 22 can bring the filtered partial ion beams together in a single output ion beam. In the embodiment shown in Fig. 7, the slit structure 23 is arranged between the first zoom lens 24 and the inversion lens 28. In alternative embodiments, the slit structure 23 could be arranged between the inversion lens 28 and the second zoom lens 25, for example. A generic embodiment is illustrated in Fig. 8, where the first ion dispersion unit 21 need not be a magnetic sector unit but could be an electric sector unit, for example. Similarly, the second ion dispersion unit 22 need not be a magnetic sector unit but also could be an electric sector unit, for example. As the first ion dispersion unit 21 is arranged for producing a diverging set of partial ion beams, an inversion lens 28 is used. The inversion lens 28 can be positioned at the plane of symmetry, mid-way between the first and second ion dispersion units 21, 22. In the embodiment shown, the inversion lens 28 is arranged downstream of the slit structure 23, but in other embodiments the slit structure may be positioned upstream. In either case, the slit structure will be arranged near, but typically spaced apart from, the inversion lens. A similar embodiment comprising zoom lenses 24 and 25 is schematically shown in Fig. 9. Other embodiments of the pre-filter of the present disclosure can be envisaged, for example embodiments utilizing an ion deflection of up to (or even exceeding) 360°, similar to an omega filter in electron microscopy applications. Such an embodiment can be used to filter a mass window from an ion beam and may comprise a magnetic prism sector with two electrostatic sectors. Between these two electrostatic sectors there is a slit to cut the mass window. The arrangement may be mirrored along the plane which is defined by the slit plate. A first embodiment of a mass spectrometer comprising a static field pre-filter according to the present disclosure is schematically shown in Fig. 10. The mass spectrometer 100 of Fig. 10 is shown to comprise an ion source 10, a pre-filter 20, a collision cell 30, a magnetic sector 70 mass analyser and a detector platform 90. The ion source 10 can be a plasma ion source, such as an inductively coupled plasma (ICP) ion source. In some embodiments, a different type of ion source can be used. The pre-filter 20 can be a static field mass filter as described above. The collision cell 30, which in use contains a suitable gas, can be configured for removing isobaric interferences, such as by charge transfer reactions, fragmentation of molecular species, and / or inducing mass shift reactions, in particular mass shift reactions to higher mass as described in GB 2546060 A. In some embodiments, the magnetic sector 70 may be replaced or complemented by another type of mass analyzer, such as a multipole mass filter, in particular a quadrupole mass analyser, or a time-of-flight mass analyser for example. The detector platform 90 can be a single collector for sequential ion detection or more commonly is a multicollector for simultaneous detection of ions having different masses, e.g., different isotopes, especially useful for accurate isotope ratio measurements. A second embodiment of a mass spectrometer comprising a static field pre-filter according to the present disclosure is schematically shown in Fig. 11. The mass spectrometer 100 of Fig. 11 is shown to comprise an ion source 10, a pre-filter 20, a collision cell 30, an electric sector unit 50, a magnetic sector unit 70 and a detector platform 90. As in Fig. 10, the pre-filter 20 can be a static field mass filter as described above. In some embodiments, the magnetic sector 70 may be replaced or complemented by another type of mass analyzer, such as a multipole mass filter, time of flight analyser etc. The ion source 10, collision cell 30 and detector platform 90 can be the same as in the embodiment shown in Fig. 10. It will be appreciated that other configurations of mass spectrometer may be used than those shown in Figs. 10 and 11, for example the order of the electric sector and magnetic sector may be reversed. It will be understood by those skilled in the art that the disclosure is not limited to the embodiments described above and that many additions and modifications can be made without departing from the scope of the invention as defined in the appending claims.

Claims

1. A mass spectrometer, comprising an ion source, a mass pre-filter downstream of the ion source, a collision cell, at least one mass filter downstream of the collision cell, and at least one ion detector downstream of the at least one mass filter, characterized in that the mass pre-filter comprises a first magnetic sector unit for separating ions, a second magnetic sector unit for merging separated ions, and a slit arranged between the first magnetic sector unit and the second magnetic sector unit for selectively passing ions.

2. The mass spectrometer according to claim 1, wherein the first magnetic sector unit is configured for separating ions into parallel ion beams.

3. The mass spectrometer according to claim 1, wherein the first magnetic sector unit is configured for separating ions into diverging ion beams.

4. The mass spectrometer according to claim 1, wherein the mass pre-filter comprises an ion lens assembly arranged between the first magnetic sector unit and the second magnetic sector unit.

5. The mass spectrometer according to claims 2 and 4, wherein the ion lens assembly comprises a first pair of lenses upstream of the slit and a second pair of lenses downstream of the slit.

6. The mass spectrometer according to claim 5, wherein the first pair of lenses comprises a first zoom lens for converting the parallel ion beams into diverging ions beams and a correction lens for converting the diverging ions beams into parallel ions beams, and wherein the second pair of lenses comprises a first correction lens for converting the parallel ion beams into converging ions beams and a zoom lens for converting the converging ions beams into parallel ions beams.

7. The mass spectrometer according to claims 3 and 4, wherein the ion lens assembly comprises an inversion lens.

8. The mass spectrometer according to claim 7, wherein the ion lens assembly comprises a single inversion lens which is preferably arranged upstream of the slit.

9. The mass spectrometer according to claim 7, wherein the ion lens assembly comprises a pair of inversion lenses, preferably arranged on either side of the slit.

10. The mass spectrometer according to claim 7, wherein the ion lens assembly comprises an inversion lens and a pair of zoom lenses, the zoom lenses being arranged on either side of the inversion lens, the inversion lens preferably being arranged downstream of the slit.

11. The mass spectrometer according to any of the preceding claims, wherein the at least one mass filter comprises a magnetic sector mass filter.

12. The mass spectrometer according to any of the preceding claims, wherein the at least one mass filter comprises an electric sector mass filter.

13. The mass spectrometer according to any of the preceding claims, wherein the at least one mass filter comprises a multipole mass filter.

14. The mass spectrometer according to any of the preceding claims, further comprising an electric sector unit downstream of the collision cell.

15. A mass pre-filter for use in a mass spectrometer, the mass pre-filter comprising a first magnetic sector unit for separating ions, a second magnetic sector unit for merging separated ions, and a slit arranged between the first magnetic sector unit and the second magnetic sector unit for passing selected ions.

16. The mass pre-filter according to claim 15, wherein the first magnetic sector unit is configured for separating ions into parallel ion beams.

17. The mass pre-filter according to claim 15, wherein the first magnetic sector unit is configured for separating ions into diverging ion beams.

18. The mass pre-filter according to claim 15, comprising an ion lens assembly arranged between the first magnetic sector unit and the second magnetic sector unit.

19. The mass pre-filter according to claims 16 and 18, wherein the ion lens assembly comprises a first pair of lenses upstream of the slit and a second pair of lenses downstream of the slit.

20. The mass pre-filter according to claim 19, wherein the first pair of lenses comprises a first zoom lens for converting the parallel ion beams into diverging ions beams and a correction lens for converting the diverging ions beams into parallel ions beams, and wherein the second pair of lenses comprises a first correction lens for converting the parallel ion beams into converging ions beams and a zoom lens for converting the converging ions beams into parallel ions beams.

21. The mass pre-filter according to claims 17 and 18, wherein the ion lens assembly comprises at least one inversion lens.

22. A method of mass spectrometry, comprising:generating a beam of ions from a sample,pre-mass filtering ions from the beam in accordance with their mass to charge ratio, spatially separating selected ions in accordance with their mass to charge ratios, and detecting spatially separated ions,characterized in that the pre-mass filtering comprises using a first magnetic sector unit for separating ions, a second magnetic sector unit for merging separated ions, and a slit arranged between the first magnetic sector unit and the second magnetic sector unit for selectively passing ions.

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