Laser ion source

Abstract

A slide holder for supporting a slide 4 in a laser ion source, the slide holder comprising a supporting member 10 for supporting the slide, wherein the supporting member is arranged such that laser li

Description

FIELD OF THE INVENTION The present invention relates generally to mass and / or ion mobility spectrometers and in particular to ion sources therefor that use a laser to generate ions from an analytical sample. BACKGROUND It is known to irradiate an analytical sample with laser light in order to generate ions therefrom. The ions can then be analysed by a mass and / or ion mobility spectrometer in order to make determinations about the sample. For example, it is known to ionise a sample using a laser in Matrix Assisted Laser Desorption Ionisation (MALDI). In such techniques the sample is often located on a glass slide such as a microscope slide. The slide is then mounted in the ion source of the spectrometer such that the laser beam used to ionise the sample is incident on the sample along an axis that is at a non-orthogonal angle to the surface of the slide. This angled arrangement of the laser beam is advantageous in that it allows for various ion optics to be located above the region where the sample is located on the slide, without the ion optics obstructing the laser beam. However, the angled arrangement of the laser beam can be problematic because if the ion beam passes through the microscopic slide it can be reflected back onto the slide at a location on the sample that is not intended to be illuminated at that time. SUMMARY The present invention provides an ion source comprising: at least one slide supporting member for supporting a slide within the ion source; and a laser for directing laser light onto a first side of the slide when the slide is supported by the at least one slide supporting member; wherein the at least one slide supporting member is arranged within the ion source such that laser light which passes through the slide and out of a second side of the slide continues to propagates away from the slide. As this arrangement allows laser light which passes through the slide to propagate away from the slide, rather than striking a component directly beneath the slide, it assists in preventing or reducing the laser light being reflected back onto the slide, which could potentially undesirably degrade or ionise sample on the slide. Ion sources have been known to incorporate photo-optical devices that may form part of viewing optics for imaging a sample that the ions are generated from and / or for imaging a laser spot used to generate the ions. Such a photo-optical device (e.g. objective, lens, CCD) may be placed very close to the sample in order to suitably image the sample and / or laser spot. In the present invention, the ion source may not comprise any photo-optics for imaging the slide from the second side of the slide. Thus, the ion source may be configured such that, when the slide is supported by at least one slide supporting member, light that has passed through the slide and out of the second side of the slide does not strike a photo-optical device for imaging a sample on the slide. The ion source may not comprise a photo-optical device for imaging a sample on the slide (at all) or the ion source may comprise a photo-optical device located adjacent to the first side of the slide for viewing or imaging a sample on (the first side of) the slide. In embodiments where the ion source is part of a mass and / or mobility spectrometer, the spectrometer as a whole preferably does not comprise any photo-optics for imaging the slide from the second side of the slide. For example, the ion source (or spectrometer) may not comprise a camera or any other type of light detector arranged to receive light which has passed through the slide and out of a second side of the slide. The ion source (or spectrometer) is therefore desirably free from photo-optical devices for imaging the slide (e.g. the sample or laser spot on the slide) from the second side of the slide. The ion source may be configured such that the laser is arranged, or arrangeable within the ion source, such that in use a laser beam from the laser is directed to a major surface of the slide that is supported by the at least one slide supporting member, wherein the laser beam is at an angle to said major surface that is not orthogonal and is not parallel to the major surface. Alternatively, the laser beam may be directed onto the slide along an axis that is orthogonal to the major surface of the slide. The ion source may comprise a wall arranged such that laser light which passes through the slide and out of a second side of the slide strikes the wall. The ion source may be configured for ions to be generated from a sample located on said first side of the slide when the slide is supported by the at least one slide supporting member. In use, laser light directed onto a sample located on the first side of the slide from the laser may pass through the sample before then passing through the slide from the first side to the second side of the slide. The ion source may be configured for ions to, upon being generated from the sample located on said first side of the slide, to travel away from the slide. The ion source may comprise ion optics configured to receive ions generated by the ion source. The ion optics may be arranged in the ion source to be on the first side of the slide, when the slide is supported by the at least one slide supporting member, for receiving ions generated by directing the laser light onto a sample that is arranged on the first side of the slide. The ion source may be configured such that ions generated from a sample located on the first side of the slide are attracted towards the ion optics and away from the slide and / or at least one slide supporting member. The ion source may be configured for ions generated from a sample located on the first side of the slide to travel in a path (e.g. towards ion optics) that does not pass through the slide and / or at least one slide supporting member. The ion optics may be arranged to face the same (first) side of the slide that the laser is directed onto. The ion source may comprise one or more slide supporting member that is configured to only support the slide at one or more locations along the perimeter of the slide. The ion source may be configured such that, when the slide is held by the at least one slide supporting member, a gap is maintained directly below at least a portion of the second side of the slide such that laser light which passes through the slide continues through the gap and propagates away from the slide. For the avoidance of doubt, the gap is maintained such that only gas (e.g. air) is in contact with said at least a portion of the second side of the slide. The at least one slide supporting member may be arranged within the ion source such that, when the slide is supported by the at least one slide supporting member, laser light which passes through the slide is able to continue through the gap in a direction away from the slide for at least 1 mm before striking a part of the ion source. For example, if the laser beam is focussed relatively tightly onto the sample-receiving side of the slide, then the portion of laser light that passes through the slide diverges relatively rapidly. As such, any light that is reflected back will have diverged over a relatively large area and will be of a relatively low intensity by the time it strikes the sample, and so will not be problematic. In such arrangements, the gap below the slide need not be very large, e.g. it may be around 1 mm. On the other hand, if the laser beam is focussed less tightly onto the samplereceiving side of the slide, then the portion of laser light that passes through the slide diverges less rapidly. As such, the gap below the slide may need to be larger in order to allow the laser light that has passed through the slide and been reflected back to diverge a sufficient amount such that it has an intensity that is low enough not to be problematic when to strikes the sample. Accordingly, the ion source may be configured such that laser light that has passed through the slide does not then strike any components for a distance of: >0.2 cm; >0.3 cm; >0.4 cm; >0.5 cm; >0.6 cm; >0.7 cm; >0.8 cm; >0.9 cm; >1 cm; >2 cm; >3 cm; >4 cm; >5 cm; >6 cm; >7 cm; >8 cm; >9 cm; or >10 cm. This can distinguish from arrangements in which a photo-optical device for viewing or imaging a sample on the slide is placed in close proximity to the second side of the slide. The at least one slide supporting member may comprise a supporting material for supporting the second side of the slide over its central area, wherein the supporting material is configured so as to be able to transmit laser light through at least portions thereof and away from the slide. The supporting material may be a solid or semi-solid block of material having no holes passing completely therethrough and that is made from a material that is transparent or translucent to the laser light. The semi-solid block may be, for example, a crystalline solid or (less preferably) a polymer gel. The ion source may comprise a refractive index matching material arranged on the supporting material and that has a different refractive index to that of the supporting material for matching the refractive index of the supporting material with the refractive index of the slide, for reducing the proportion of laser light that would otherwise be reflected at the boundary between the supporting material and slide. The refractive index matching material may have a refractive index that is closer to the refractive index of the slide than the refractive index or air (or a vacuum). The refractive index matching material may have a refractive index that is between that of the supporting material and the slide. Alternatively, the supporting material may have substantially the same refractive index as the slide and a refractive index matching material may not be used. In less preferred embodiments, the supporting material may be a block of material having a refractive index for causing the laser light to be refracted at the boundary between the supporting material and the slide. This may change the angle of the laser beam after it has been transmitted by the slide so that even if the laser light is reflected downstream of the slide it will not be reflected onto the slide (or not to a position on the slide that is, for example, >5 mm or >10 mm from any edge of the slide). The supporting material may have apertures or channels therethrough for allowing laser light to pass through the supporting material. The supporting material may have reflective channels that guide the laser light through the channels and away from the slide. The channels may be arranged so as to be substantially orthogonal to the second side of the slide. This may, for example, prevent any laser light that is reflected from being reflected to a different portion of the slide to that on which it was originally incident. The supporting material may be a mesh, grid or series of spaced apart wires or rods. The supporting material may be formed from a material that is opaque to the laser light. The supporting material may be electrically conductive and may be connected to a voltage supply. This enables the supporting material to be used as an electrode for generating an electric field. The ion source may be configured such that laser light that has passed through the slide, in use, either: (i) is reflected back onto the slide with an intensity that does not degrade or ionise the sample arranged thereon; or (ii) is not reflected back onto the slide; or (iii) is reflected back onto the slide but not to a position that is >5 mm or >10 mm from any edge of the slide. The angle of the laser beam relative to the slide, the size of the gap, and the type of material below the gap that the laser will strike may be selected so as to achieve these features. The present invention may provide an ion source comprising said slide. The slide may be electrically conductive and electrically connected to a voltage supply. This enables the slide to be used as an electrode for generating an electric field. For example, the slide may be a glass slide that is doped with conductive particles (such as metal particles) at a concentration so as to allow the slide to be electrically conductive. Slide supporting members described herein are also new in their own right. Accordingly, the present invention also provides a slide holder for supporting a slide in a laser ion source, the slide holder comprising a supporting material for supporting a first of the major surfaces of the slide over its central area whilst laser light is directed at an opposing, second major surface of the slide, wherein the supporting material is configured so as to be able to transmit laser light through at least portions thereof and away from the second major surface of the slide. The supporting material may be a solid or semi-solid block of material having no holes passing completely therethrough and that is made from a material that is transparent or translucent to the laser light. The semi-solid block may be, for example, a crystalline solid or (less preferably) a polymer gel. The slide holder may comprise a refractive index matching material arranged on the supporting material that has a different refractive index to that of the supporting material for matching the refractive index of the supporting material with the refractive index of the slide, for reducing the proportion of laser light that would otherwise be reflected at the boundary between the supporting material and slide. The refractive index matching material may have a refractive index that is closer to the refractive index of the slide than the refractive index or air (or a vacuum). The refractive index matching material may have a refractive index that is between that of the supporting material and the slide. Alternatively, the supporting material may have substantially the same refractive index as the slide and a refractive index matching material may not be used. In less preferred embodiments, the supporting material may be a block of material having a refractive index for causing the laser light to be refracted at the boundary between the supporting material and the slide. This may change the angle of the laser beam after it has been transmitted by the slide so that even if the laser light is reflected downstream of the slide it will not be reflected onto the slide (or not reflected to a position on the slide that is, say, >5 mm or >10 mm from any edge of the slide). The supporting material may have apertures or channels therethrough for allowing laser light to pass through the supporting material. The supporting material may have reflective channels that guide the laser light through the channels and away from the slide. The channels may be arranged so as to be substantially orthogonal to the second side of the slide. Alternatively, the supporting material may be a mesh, grid or series of spaced apart wires or rods. The supporting material may be formed from a material that is opaque to the laser light. The supporting material may be electrically conductive and may be connected to a voltage supply. The present invention also provides a combination of the slide holder and said slide. The present invention also provides a mass and / or ion mobility spectrometer comprising an ion source as described herein and a mass analyser and / or mobility analyser for analysing ions. The present invention also provides a method of mass and / or ion mobility spectrometry comprising: providing a mass and / or ion mobility spectrometer as described above; generating ions using the ion source; and analysing ions using the mass analyser and / or mobility analyser to analyse ions. The method may comprise irradiating different locations of a single analytical sample with laser light at different respective times so as to generate analyte ions from those different locations; and analysing the analyte ions, or ions derived therefrom. The method may be a method of ion mapping or ion imaging of the sample. BRIEF DESCRIPTION OF THE DRAWINGS Various embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Fig. 1 shows a schematic illustration of a conventional MALDI ion source, in which laser light that is transmitted through the slide is reflected back by the slide holder onto the slide; Fig. 2 shows an embodiment of the present invention in which laser light that is transmitted through the slide is transmitted away from the slide through a gap beneath it; Fig. 3 shows another embodiment of the present invention that is the same as that of Fig. 2, except wherein the gap beneath the slide is filled with a supporting material that is transparent or translucent for the laser beam; and Figs. 4A and 4B show different views of another embodiment of the present invention that is the same as that of Fig. 2, except wherein the gap beneath the slide is filled with a supporting material that has channels extending therethrough for channelling laser light away from the slide. DETAILED DESCRIPTION Figure 1 shows a schematic illustration of a conventional MALDI ion source used to generate ions to be analysed in a mass and / or ion mobility spectrometer. In this ion source arrangement, the analytical sample 2 is mounted on a transparent, glass microscope slide 4. The slide 4 is mounted on a metal slide holder 6. The ion source includes a laser that is mounted relative to the slide holder 6 so as to illuminate the sample 2 on the slide 4 with a laser beam 8 along an axis that is at a non-orthogonal angle to the surface of the slide 4. This arrangement allows for various ion optics to be located above the slide 4 without obstructing the laser beam 8. In use, the laser beam 8 irradiates the sample 2 on the slide 4, ionising material in the sample 2 and causing gas phase ions to be released. This process is well known in the field and will not be described further herein. The ions are then mass and / or ion mobility analysed in the spectrometer according to known techniques. The laser beam 8 may be moved relative to the slide 4 so as to ionise different areas of the sample 2 at different times, thus enabling ions from different areas of the sample 2 to be analysed at different times. This is useful, for example, in ion imaging or ion mapping of the sample 2. However, the inventors have recognised that a portion of the laser beam 8 may penetrate through the sample 2 and the slide 4 and then be reflected by the slide holder 6 back through the slide 4 and onto the sample 2 again. This problem is exacerbated as the slide holder 4 is often relatively highly reflective for the laser light 8, since the slide holder 4 is typically metal. As the laser beam 8 illuminates the sample 2 at an angle to the slide 4, the portion of the laser beam 8 that is reflected back by the slide holder 4 is incident on a different portion of the sample 2 to that which is being illuminated directly from the laser (at that time). As such, the reflected laser beam may heat, denature, degrade or even ionise a region of the sample 2 that the laser is not intended to be illuminating / interrogating. This is undesirable and may be particularly problematic, for example, in MALDI ionisation techniques, wherein the sample comprises an analyte that is mixed with a MALDI matrix for assisting ionisation of the analyte when it is directly illuminated by the laser. The MALDI matrix comprises chromophores that are configured to absorb laser light when directly illuminated by it, but the laser light that is reflected by the slide holder onto the sample may denature such chromophores. This denaturing of the chromophores may reduce the desorption / ionisation efficiency of the sample when the laser beam is subsequently moved so as to directly illuminate that portion of the sample. Also, in techniques such as ion mapping or ion imaging it may not be desirable to irradiate and ionise separate areas of the sample simultaneously. Although the above problem has been described with respect to arrangements in which the laser beam axis is at a non-orthogonal angle to the slide 4, the problem also occurs when the laser beam axis is orthogonal to the slide 4. This is because the laser light that is reflected from the slide holder 6 will be a beam having a larger diameter than the laser beam 8 that is focussed onto the sample receiving side of the slide 4. As such, the reflected light may be incident on portions of the sample 2 that are not intended to be irradiated by the laser. Fig. 2 shows an embodiment of the present invention that is configured to minimize the reflection of the laser beam 8 back onto the sample 2. This embodiment provides one or more slide supporting member 10 that is configured to support the slide 4 only at its perimeter and such that the slide supporting member 10 does not extend below the portion of the slide 4 inward of its perimeter. For example, the slide supporting member 10 may contact only the opposing ends or only the opposing sides of the slide 4. Alternatively, the slide supporting member 10 may have a window therethrough that is sized and configured such that, when the slide 4 is located on the slide supporting member 10, the perimeter of the slide 4 is in contact with the slide supporting member 10 but the portion of the slide inward of its perimeter is coincident with the window through the slide supporting member 10. In such embodiments, the slide supporting member 10 is not in contact with the portion of the slide that is inward of its perimeter. Rather, only a gas is in the space 12 directly beneath said portion of the slide 4. As such, any laser light that passes through the slide 4 may continue and propagate through the gap 12, rather than being reflected back onto the slide 4 by the slide supporting member 10. There may be other components spaced away from the slide 4 beneath it, but such components may have a relatively low reflectivity for the laser beam 8. Additionally, or alternatively, such other components may be spaced beneath the slide 4 by a relatively large distance such that even if they reflect the laser light 8, the laser light is not reflected onto the slide 4, or is not reflected to a location on the slide at which the sample 2 is located (e.g. is not reflected to a location on the slide that is >5 mm or >10 mm from any edge of the slide). The laser beam 8 is focussed to a focal point at the location of the sample 2 that is desired to be ionised, as shown in Fig. 1. It will therefore be appreciated that laser light 8 that passes through the slide 4 will defocus as it propagates. Any components arranged below the slide 4 may be spaced beneath the slide by a relatively large distance such that even if the laser light 8 is reflected by them, the laser light is defocussed to such an extent that when it hits the sample 2 on the slide 4 it does not ionise or degrade the sample significantly. For example, the ion source may be configured such that laser light 8 that has passed through the slide 4 does not then strike any components for a distance of >2 cm; >3 cm; >4 cm; >5 cm; >6 cm; >7 cm; >8 cm; >9 cm; or >10 cm. Fig. 3 shows a schematic of another embodiment of the invention that is the same as that described in relation to Fig. 2, except wherein a non-gaseous supporting material 14 is located in the space 12 that is directly beneath the central portion of the slide 4. The supporting material 14 may be selected to physically support the slide 4, which is particularly useful if the slide 4 is relatively flexible. The supporting material 14 may be selected so as to be transparent or translucent to the laser light 8. The supporting material 14 may be selected so as to have the same or similar refractive index as the slide 4, so as to avoid laser light 8 being reflected at the interface between the slide 4 and the supporting material 14. A coupling fluid or gel (not shown) having a refractive index between that of the slide 4 and the supporting material 14 may be arranged between the slide 4 and supporting material 14 so as to reduce any back reflection of the laser beam 8 onto the slide 4. Less preferably, the refractive index of the supporting material 14 may be chosen so as to be significantly different from that of the slide 4, so as to cause the laser light 8 to be refracted at the boundary between the supporting material 14 and the slide 4. This may change the angle of the laser beam 8 after it has been transmitted by the slide 4 so that if the laser light 8 is reflected downstream of the slide 4 it will not be reflected onto the slide (or to a position on the slide that is, for example, >5 mm or >10 mm from any edge of the slide). Figs. 4A and 4B show another embodiment that is substantially the same as that of Fig. 3, except wherein the supporting material 16 is a honeycomb structure having channels 18 extending in a direction orthogonal to the slide 4. Fig. 4A shows a side view of the arrangement, whereas Fig. 4B shows a plan view of the honeycomb structure 16 and shows the hexagonal channels 18 through the structure 16. As can be seen from Fig. 4A, laser light 8 that passes through the slide 4 will pass into and through the channels 18 in the honeycomb structure 16, rather than being reflected back to the slide 4. The honeycomb structure 16 may be formed from material such that laser light 8 entering any given channel 18 at an upper side of the structure 16 is reflected along that channel 18 and guided to the exit at the lower side of the structure 16. The laser light may then exit the channel 18 in a diffuse manner, e.g. as shown. The honeycomb structure 16 may be a rigid structure such as being formed of metal. It will be appreciated that a honeycomb structure 16 need not be used as the supporting material and that a material having channels 18 of cross-sectional shape other than hexagonal may be used. It is also contemplated that the supporting material beneath the slide 4 may be a planar sheet supporting material having apertures therethrough, e.g. having hexagonal or other shaped apertures, rather than being a 3D material having a significant depth and elongated channels 18. For example, the sheet may be a mesh or grid having hexagonal or other shaped apertures therethrough. Alternatively, a series of parallel or non-parallel wires or rods may be used as the supporting material. It may be desired to apply a voltage to the supporting material beneath the slide 4 in order to generate an electric field, e.g. to help extract analyte ions (generated by the laser beam 8) away from the upper surface of the slide 4. The supporting material may therefore be electrically conductive and connected to a voltage supply. For example, the supporting material may be a metal electrode structure. Although the present invention has been described with reference to various embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims. For example, less preferred embodiments are contemplated in which the supporting material 14 beneath the slide 4 is not a block of rigid material. For example, the supporting material may be a polymer gel. In such embodiments, the supporting material may comprise electrically conductive particles or components such that a voltage can be applied to the supporting material to generate an electric field, e.g. for helping extract analyte ions (generated by the laser beam) away from the upper surface of the slide 4. Embodiments are also contemplated in which the slide supporting member 10 for holding the perimeter of the slide 4 is not present and instead the slide 4 is simply located on the supporting material 14,16 (and any coupling fluid or gel located thereon). Although MALDI techniques have been described herein, it will be appreciated that the present invention is not limited to MALDI but applies to all ionisation techniques that use a laser to ionise the sample. Although embodiments have been described in which the laser beam 8 is directed 5 at a non-orthogonal (and non-parallel) angle to the slide 4, it is contemplated that the laser beam 8 may be directed onto the slide 4 at an angle that is orthogonal to the slide 4. It is also contemplated that the slide 4 need not necessarily be a glass slide, but may be a material other than glass that is transparent or translucent to the laser light. Although embodiments have been described that use a glass microscope slide 4 to 10 support the analytical sample 2, it is envisaged that other types of slides may be used and / or that slides formed from materials other than glass may be used provided that they are transparent or translucent to the laser beam. 16 05 25

Claims

5 1. An ion source comprising:at least one slide supporting member for supporting a slide within the ion source;a laser for directing laser light onto a first side of the slide when the slide is supported by the at least one slide supporting member; andion optics arranged to be on the first side of the slide, when the slide is supported10 by the at least one slide supporting member, for receiving ions generated by directing the laser light onto a sample that is arranged on the first side of the slide;wherein the at least one slide supporting member is arranged within the ion source such that laser light which passes through the slide and out of a second side of the slide continues to propagate away from the slide;15 wherein the ion source does not comprise any photo-optics for imaging the slidefrom the second side of the slide;wherein the at least one slide supporting member comprises a supporting material for supporting the second side of the slide over its central area, and the supporting material is configured so as to be able to transmit laser light through at least portions thereof and20 away from the slide; andwherein the ion source comprises a refractive index matching material arranged on the supporting material and that has a different refractive index to that of the supporting material for matching the refractive index of the supporting material with the refractive index of the slide, for reducing the proportion of laser light that would otherwise be reflected25 at the boundary between the supporting material and the slide.

2. The ion source of claim 1, comprising one or more slide supporting member that is configured to only support the slide at one or more locations along the perimeter of the slide.

303. The ion source of claim 1 or 2, wherein the supporting material is a solid or semisolid block of material having no holes passing completely therethrough and that is made from a material that is transparent or translucent to the laser light35 4. The ion source of any preceding claim, wherein the supporting material iselectrically conductive and is connected to a voltage supply.

5. The ion source of any preceding claim, comprising a wall arranged such that laser light which passes through the slide and out of a second side of the slide strikes the wall.

406. The ion source of any preceding claim, wherein the ion source is configured such that laser light that has passed through the slide does not then strike any components for a distance of: >2 cm; >3 cm; >4 cm; >5 cm; >6 cm; >7 cm; >8 cm; >9 cm; or >10 cm.5 7. The ion source of any preceding claim, comprising said slide.

8. A mass and / or ion mobility spectrometer comprising the ion source of anypreceding claim.10 9. A method of mass and / or ion mobility spectrometry comprising:providing a mass and / or ion mobility spectrometer comprising an ion source as claimed in any one of claims 1-7 and a mass analyser and / or mobility analyser for analysing ions;generating ions using the ion source; and15 analysing ions using the mass analyser and / or mobility analyser to analyse ions.

10. The method of claim 9, comprising irradiating different locations of a single analytical sample with laser light at different respective times so as to generate analyte ions from those different locations; and analysing the analyte ions, or ions derived20 therefrom.

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