Ambient ionisation source unit

Abstract

An ambient ionisation source unit (10) is provided comprising: a housing (12) containing a first device (14) for generating analyte material from a surface of a sample to be analysed and a sampling inlet (16) for receiving analyte material liberated from the surface of the sample. The position(s) of the first device and/or sampling inlet is (are) fixed relative to the housing. Thus, the first device and the sampling inlet are integrated into a single unit that can be installed onto the front-end of an ion analysis instrument with minimal or reduced user interaction.

Claims

1. An ambient ionisation source unit comprising: a housing containing a first device for generating analyte material from a surface of a sample to be analysed and a sampling inlet for receiving analyte material liberated from the surface of the sample, wherein the first device comprises a sprayer device comprising a spray capillary for generating a pneumatic spray of solvent droplets; wherein the position of the first device is fixed relative to the housing; and wherein the sampling inlet is adjustable relative to the housing between two or more discrete positions; wherein the ambient ionisation source unit is connected via transfer tubing to an ion analysis instrument so that analyte material generated using the first device is collected by the sampling inlet and transferred via the transfer tubing towards an inlet of the ion analysis instrument, wherein the transfer tubing comprises one or more flexible regions for accommodating movement of the ambient ionisation source unit relative to the ion analysis instrument.

2. The source unit of claim 1, wherein the first device comprises an ambient ionisation probe.

3. The source unit of claim 1, wherein the first device comprises a desorption electrospray ionisation (“DESI”) or DESI-derived sprayer device.

4. The source unit of claim 1, wherein the first device comprises a nozzle or shield having an aperture, wherein the spray capillary is arranged to direct the spray of solvent droplets through the aperture.

5. The source unit of claim 4, wherein the nozzle or shield is grounded or wherein a voltage is applied to the nozzle or shield to electrostatically charge or direct the solvent droplets as the spray of solvent droplets passes through the aperture.

6. The source unit of claim 1, wherein the first device and sampling inlet are recessed into the housing so that the first device and sampling inlet do not protrude or extend beyond the housing.

7. The source unit of claim 1, wherein the first device and/or sampling inlet protrude through or extend beyond a surface of the housing.

8. The source unit of claim 1, wherein the source unit is a handheld source unit.

9. The source unit of claim 1, wherein the source unit defines, in use, a local sampling volume, and optionally wherein the local sampling volume is provided with a gas such as nitrogen.

10. The source unit of claim 1, wherein a voltage is applied to the sampling inlet.

11. The source unit of claim 1, wherein the housing comprises one or more connectors for allowing connections to be made to one or more of: (i) an electrical power supply; (ii) a supply of solvent gas; (iii) a supply of nebulising gas; and (iv) transfer tubing for transferring analyte material collected by the sampling inlet towards an inlet of an ion analysis instrument.

12. The source unit of claim 1, wherein the transfer tubing comprises a heated portion or is heated.

13. An ion analysis system comprising: an ion analysis instrument such as a mass and/or ion mobility spectrometer; an ambient ionisation source unit as claimed in claim 1; wherein the transport tubing transports analyte material from the sampling inlet of the ambient ion source unit to an inlet of the mass spectrometer so that the analyte material can be analysed by the mass spectrometer.

14. An apparatus for producing ions from a sample comprising: a first device configured to direct a spray of droplets or a laser beam onto a sample; and an inlet configured to collect the analyte from the sample; wherein the first device and the inlet are integrated into a single sampling head or probe; wherein the position of the first device is fixed relative to the housing; and wherein the sampling inlet is adjustable relative to the housing between two or more discrete positions; wherein the ambient ionisation source unit is connected via transfer tubing to an ion analysis instrument so that analyte material generated using the first device is collected by the sampling inlet and transferred via the transfer tubing towards an inlet of the ion analysis instrument, wherein the transfer tubing comprises one or more flexible regions for accommodating movement of the ambient ionisation source unit relative to the ion analysis instrument.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various embodiments will now be described, by way of example only, and with reference to the accompanying drawings in which:

(2) FIG. 1 shows schematically an example of a source unit in accordance with various embodiments;

(3) FIG. 2 shows schematically an example of a mass spectrometry system comprising a source unit of the type shown in FIG. 1;

(4) FIG. 3 shows schematically another example of a mass spectrometry system in accordance with various embodiments;

(5) FIG. 4 shows schematically the reduction in geometric degrees of freedom that may be provided in accordance with various embodiments;

(6) FIG. 5 illustrates an example of the optimal physical parameters of the ambient ionisation source unit in accordance with various embodiments;

(7) FIGS. 6 & 7 show examples of sprayer devices that may be used in accordance with various embodiments described herein;

(8) FIG. 8 shows an example of base peak intensities for lipid species obtained from a tissue section illustrating the effect of spray capillary voltage;

(9) FIG. 9 illustrates the effects of heating a portion of the inlet path on the robustness and signal intensity;

(10) FIG. 10 illustrates exemplary data that may be obtained using a source unit according to various embodiments described herein;

(11) FIG. 11 shows the effect of the length of transfer tubing on signal intensity in both positive and negative ion modes;

(12) FIG. 12 show two possible designs of source units in accordance with various embodiments;

(13) FIG. 13 shows a prototype of a handheld sampling probe in accordance with various embodiments; and

(14) FIG. 14 shows schematically an example of a source unit incorporating a laser probe.

DETAILED DESCRIPTION

(15) Various examples of an ambient ionisation source unit will now be described.

(16) FIG. 1 shows an example of an ambient ionisation source unit 10 in accordance with an embodiment of the present disclosure. The ambient ionisation source unit 10 comprises a first device that is configured to generate analyte material from a sample and a sampling capillary 16 integrated into a single housing 12. The first device comprises a sampling probe 14 which may generally comprise any suitable and desired ambient ionisation probe. For example, in embodiments, the sampling probe may comprise a laser ablation or plasma desorption probe. However, in FIG. 1, the sampling probe 14 is in the form of a desorption electrospray ionisation (‘DESI’) sprayer device that acts to direct a spray of solvent droplets onto the surface of a sample that is to be analysed. The source unit 10 may be connected to an analytical instrument via one or more flexible tubes e.g. which may comprise a liquid (solvent) supply tube 20, a gas supply tube 21, and a transfer tube 22 for transporting analyte material towards the inlet of the analytical instrument.

(17) FIG. 2 shows an example of an ion analysis system wherein an ambient ionisation source unit 10 of the type shown in FIG. 1 is connected to the front end of an analytical instrument such as a mass spectrometer 30. As shown, the source unit 10 is positioned above a sample 40 and the sampling probe 14 is used to direct a spray of droplets onto the surface of the sample 40. The solvent droplets act to desorb analyte material from the surface of the sample. The analyte material that is liberated (desorbed) from the sample 40 by the sampling probe 14 is then collected by a sampling inlet of a sampling capillary 16 and transferred towards an atmospheric pressure inlet 130 of an ion analysis instrument 30 such as a mass and/or ion mobility spectrometer via suitable transfer tubing 22.

(18) Optionally, as shown in FIG. 2, an organic solvent such as isopropanol is added to the analyte material liberated from the surface of the sample prior to the atmospheric pressure inlet 130 of the instrument 30. This may be done by a suitable solvent dosing device 150. However, the addition of an organic solvent is not essential.

(19) FIG. 3 shows another example of an ambient ionisation source unit 10 in accordance with an embodiment of the present disclosure. As shown, a connector 200 is provided on the housing 12 that allows for suitable transfer tubing 22 to be connected to the housing so that the ambient ionisation source unit 10 may be readily installed onto the front-end of the ion analysis instrument 130. Various other connectors 18 are also provided for allowing the housing 12 to be connected to suitable supplies of solvent and nebulising gas, and also for connecting the housing to a voltage source.

(20) The positions of the sampling probe 14 and sampling capillary 16 are both fixed within the housing in a pre-determined (e.g. optimal) geometry. Thus, the only geometrical degree of freedom available to the user is the height of the ambient ionisation source unit 10 above the sample surface. In FIG. 3 the height of the ambient ionisation source unit 10 above the sample surface is controlled by an adjustable vertical stage 24. Thus, as shown, the transfer tubing 22 comprises a flexible region 22A that allows the transfer tubing 22 to flex to accommodate the vertical movement of the ambient ionisation source unit 10. The flexible region 22A is then connected via a suitable connector 22B to a further (heated) region 22C leading to the inlet of the ion analysis instrument. However, various other arrangements are of course possible. For instance, in embodiments, substantially the entire length of transfer tubing 22 may be flexible. The transfer tubing may be formed from Tygon® or other suitable materials.

(21) FIG. 4 illustrates the reduction in geometric degrees of freedom that is offered by the fixed geometry ambient ionisation source unit compared to a conventional DESI source. In a conventional DESI sources, the user would have to manually set and optimise the positions, angles and rotations of both the sprayer and capillary relative to the sample surface. This can be a very time consuming and difficult task. Furthermore, this may lead to a lack of reproducibility between experiments, e.g. performed in different laboratories. It is believed that this has presented a significant barrier towards greater uptake of DESI techniques despite the potential advantages offered thereby. By contrast, for a fixed geometry ambient ionisation source unit the only remaining geometric degree of freedom is the height of the probe above the surface.

(22) FIG. 5 shows one example of an optimal geometry (determined from repeated experiments on adjustable DESI systems) wherein the sprayer device 14 is positioned at an angle of about 75 degrees to the horizontal (i.e. to the surface of the sample when the source unit is held parallel to the sample) whereas the sampling capillary 16 is angled at about 10 degrees to the horizontal. The spacing between the sprayer device 14 and the sampling inlet 16 is about 5 mm. However, it will be appreciated that other geometries may suitably be used depending on the application and the user's requirements. For example, when the source unit is used as a handheld analysis probe e.g. for point-of-contact applications, the sampling capillary 16 may be angled closer to the horizontal, e.g. at less than 10 degrees to the horizontal.

(23) FIG. 6 shows further details of a DESI sprayer device 14 that may be used according to various embodiments described herein. In general, a DESI sprayer device comprises a spray capillary 50 for generating a pneumatic spray of solvent droplets. Solvent is introduced into the spray capillary 50 which is then nebulised at the exit of the capillary by a nebulising gas flow (not shown) provided around the capillary 50. The spray of solvent droplets 56 that is generated can thus be directed onto the sample surface in order to liberate analyte material according to known desorption ionisation processes.

(24) Thus, in order to generate the solvent spray 56, a liquid solvent is fed into the spray capillary alongside a high velocity nebulizing gas flow so that the nebulizing gas acts to nebulize the solvent exiting the spray capillary. A voltage may be applied to the DESI sprayer, or to the flow of liquid solvent, in order to charge the solvent droplets. The charged solvent may thus be pneumatically driven by the gas flow from the spray capillary onto the sample surface. The DESI sprayer thus directs a spray of charged solvent droplets onto the sample surface. Although an electrospray-type sprayer has been described, it will be appreciated that various suitable devices that are capable of generating a stream of solvent droplets carried by a jet of nebulizing gas may be used to form the spray of (charged) solvent droplets. For instance, although FIGS. 6 and 7 illustrate a DESI-MS interface, various similar solvent-driven ionisation interfaces have been developed and are known that operate according to similar physical principles to DESI and to which the techniques of the present invention may also be extended. For instance, by way of one example, Desorption ElectroFlow focussing ionisation (“DEFFI”) sources may also suitably by used to generate the analyte ions. Particularly, it is also contemplated that the solvent may not be charged in the sprayer device, as described above, but rather that the droplets of solvent may subsequently be activated or charged after their deposition onto the sample. For example, a voltage may be applied to the tissue section substrate to provide the charges.

(25) In any case, the solvent droplets (whether charged or not) impact on and interest with the surface of the sample in order to generate analyte ions. There are understood to be two main kinds of ionisation mechanism for DESI analyses, which may depend e.g. on the nature of the sample and the operating conditions of the DESI sprayer.

(26) The first main ionisation mechanism is via a desorption process wherein the solvent droplets hit the surface of the sample and then spread out over a larger diameter and act to dissolve the analyte material with the dissolved analyte material then being released from the surface generating analyte ions as the solvent is evaporated. For example, the droplets may form a thin film of solvent on the surface of the sample that desorbs the analyte molecules, and the desorbed analyte may then be released as secondary droplets by vaporisation or due to the impact of further solvent droplets on the sample. This may result in similar spectra to conventional electrospray ionisation (“ESI”) techniques wherein primarily multiply charged ions are observed. It is believed that this mechanism leads to more multiply charged ions because multiple charges in the solvent droplets may easily be transferred to the desorbed analyte molecules. This mechanism may also be referred to as the “droplet pick-up” ionisation mechanism. This ionisation mechanism may be particularly suited for the ionisation and analysis of larger molecules such as peptides and proteins.

(27) The second main ionisation mechanism is via direct charge transfer, either between a solvent ion and an analyte molecule on the surface of the sample; or between gas phase ions and analyte molecules on the surface or in the gas phase. This mechanism may be similar to what is observed in easy ambient sonic spray ionisation (“EASI”) techniques, and typically generates only singly charged ions. This mechanism is normally observed for relatively smaller or lower molecular weight species compared to the desorption mechanism described above.

(28) It will be understood that these techniques, including DESI, are generally “ambient” ionisation techniques. That is, the sample may be maintained and analysed under ambient or atmospheric conditions. Ambient ionisation ion sources such as DESI sources may further be characterised by their ability to generate analyte ions from a native or unmodified sample. For example, this is in contrast to other types of ionisation ion sources such as Matrix Assisted Laser Desorption Ionisation (“MALDI”) ion sources that require a matrix or reagent to be added to prepare the sample prior to ionisation. It will be apparent that the requirement to add a matrix or a reagent to a sample impairs the ability to provide a rapid simple analysis of target material. Ambient ionisation techniques such as DESI are therefore particularly advantageous since firstly they do not require the addition of a matrix or a reagent and since secondly they enable a rapid simple analysis of target material to be performed. Ambient ionisation techniques such as DESI do not generally require any prior sample preparation or offline sample pre-treatment or separation. As a result, the various ambient ionisation techniques enable tissue samples to be analysed without necessitating the time and expense of adding a matrix or reagent to the tissue sample or other target material.

(29) In other words, ambient ionisation techniques such as DESI may allow for a substantially direct analysis of the sample, i.e. without requiring any specific offline sample preparation or separation steps to be performed prior to the analysis. It will be appreciated that in the context of ambient ionisation the meaning of “direct” analysis is a well understood term of the art referring to in situ analyses performed directly from the surface of a sample. Direct analyses may thus avoid the need for any time-consuming sample separation or off line preparation steps e.g. using a matrix. Particularly, ambient ionisation techniques such as DESI may allow for samples to be directly analysed essentially in their native form. Naturally, this does not preclude any other steps that do not significantly alter the sample such as steps of washing the sample or mounting the sample. Furthermore, it is also contemplated that the sample may be treated with an enzyme such as protease in order to instigate digestion of the tissue, as explained further below, with the digested tissue then being analysed directly.

(30) As shown in FIG. 6, the spray capillary 50 is located behind a nose cone (or shield) 52 with the spray capillary 50 located in-line with an aperture 54 provided in the nose cone 52 such that the solvent spray 56 is directed from the spray capillary 50 through the apertures 54 onto the sample surface. The nose cone 52 may thus act to protect the spray capillary 50, which may be relatively fragile (e.g. comprising fused silica). The aperture may also provide some focussing of the solvent spray 56. The nose cone 52 may be grounded as shown in FIG. 4A. As shown in FIG. 6, the spray capillary tip is positioned between about 0.1 and 2 mm behind the aperture. In some examples, a 200 micron aperture is used in combination with a 360 micron (OD) and 75 micron (ID) fused silica spray capillary. However, it is envisaged that a range of different combinations may suitably be used.

(31) FIG. 7 shows an alternative arrangement wherein the nose cone 52 is also connected to a high voltage (HV) source. Although shown in FIG. 7 as comprising separate high voltage (HV) sources for the nose cone 52 and the spray capillary 50, in general, these voltages may both be applied from a single (external) voltage source, e.g. via a suitable connector provided on the housing, with suitable internal wiring or circuitry being provided within the housing to provide the desired voltage to each of the different components. The nose cone 52 may thus be maintained at a certain voltage e.g. to provide additional electrostatic charging or focussing of the spray droplets (e.g. for DESI operation). In other embodiments, a voltage may only be applied to the nose cone 52 (and not the spray capillary 50), so that the solvent droplets are charged only as the pass through the aperture.

(32) In some cases, the spray droplets may not be charged at all.

(33) According to various embodiments described herein, the geometric parameters of the sampling unit may be substantially optimised and then fixed to minimise the required user interaction. The source unit may thus be controlled by carefully controlling the (other, non-geometric) ionisation or instrument parameters. For instance, where the source unit comprises a DESI probe, as described above, the ionisation may be controlled by adjusting e.g. the nebulising gas pressure, solvent flow, and so on. It will be appreciated that these parameters may be controlled directly from the instrument, or control software, so that, again, the requirement for the user to spend significant time optimising the set-up is avoided.

(34) For example, at least for some tissue imaging experiments, the following operating ranges and optimal parameter values have been determined (although naturally other parameters may be suitably used e.g. depending on the application and the details of the instrument being used):

(35) TABLE-US-00001 Parameter Operating Range Optimum Gas pressure 1 to 10 bar 4 bar Solvent flow 0.05 to 10 μL/min 2 μL/min Solvent voltage 0 to 5 kV 2.5 kV Capillary temperature 0 to 600° C. 550° C.

(36) Other potential suitable operating parameters for DESI sources are described in United Kingdom Patent Application No. 1708835.2 filed on 2 Jun. 2017, which is incorporated herein by reference.

(37) For instance, FIG. 8 shows the effect of varying the spray voltage on base peak intensities of lipid species from tissue section with a remote acquisition of 2.5 metres transfer tubing. As shown, there is a clear optimal voltage at about 2.5 kV where a stable spray through the aperture is set up.

(38) FIG. 9 illustrates the effects of heating the transfer tubing. As shown, heating the final portion of the transfer tubing (i.e. that leading into the inlet of the mass spectrometer) may increase robustness and signal intensity by helping to control the evaporation of droplets prior to their arrival at the inlet/source of the ion analysis instrument. As shown in FIG. 9, when the transfer tubing 22 comprises a heated portion 22C, the signal intensity may be increased by an order of magnitude compared the same system without heating.

(39) In both cases (whether or not heating is applied), the fixed geometry probe allows for significant improvement in signal intensity compared to conventional DESI. For instance, FIG. 10 shows an example of tissue imaging results that may be obtained with the configuration described above. As shown, the signal intensity is high and the spatial resolution is comparable to what could be achieved with conventional DESI. Thus, the use of the combined ambient ionisation source probe described herein may help to remove user involvement in obtaining high quality data from an ambient ionisation sampling experiment.

(40) The length of the transfer tubing can easily be varied. FIG. 11 illustrates the effects of varying the length of the transfer tubing for both positive and negative ion modes of operation. (In general, the ambient ionisation ion system may be operated in negative ion mode or positive ion mode. However, the Applicants have found that better classification accuracy can generally be achieved using negative ionisation mode. Thus, according to various embodiments generating analyte ions from the sample using ambient ionisation comprises using ambient ionisation in negative ionisation mode.) As shown, after an initial drop in intensity from adjacent acquisition (˜2 centimetres) to remote acquisition (˜60 centimetres), there is no further significant loss of signal intensity up to a transfer length of 2.5 metres. Such a system may thus allow the sampling device (i.e. source unit) to be decoupled from the analyser, increasing the flexibility of use as many of the physical constraints are removed.

(41) The housing may generally take any suitable and desired form. For instance, although illustrated in the figures above as comprising a substantially cuboid form, it will be appreciated that the form of the housing may take any suitable and desired form. The sampling probe and capillary may be fully contained within the housing or may protrude through a lower surface. Both options are shown in FIG. 12.

(42) FIG. 12 shows two possible designs for a source unit. In the first (top) design, the sampling probe 94 and sampling inlet 96 protrude through a surface 92 of the housing 90. This may help allow the sampling probe 94 and inlet 96 to be brought very close to a sample.

(43) In the second (bottom) design, the sampling probe 104 and capillary 106 are fully contained within the housing 100. Thus, as shown, the sampling probe 104 and capillary 106 are recessed into the body of source unit. In this case, the combination can still generally be brought close enough (e.g. ˜1 millimetre above) to a sample for optimal sampling but there are now no protruding components, which may otherwise be problematic.

(44) Because of the lack (or protection of) fragile parts such as the DESI emitter and the lack of any need for manual optimisation, the source units described herein may be particularly suitable for integration into automated surface or tissue sampling systems. For instance, the source unit may be integrated into an automated imaging system.

(45) For example, for the system shown in FIG. 3 the position (height) of the ambient ionisation source unit 10 above the sample may be automatically controlled using the vertical stage 24 (e.g. in combination with a horizontal stage for moving the sample 30 underneath the ambient ionisation source unit 10) in order to automatically probe or image a sample. One or more sensors may be provided that are configured to determine the presence (or absence) and/or location of a sample (or the presence (or absence) and/or location of a product of which the sample is part) to be analysed. The one or more sensors may comprise, for example, one or more (e.g. mechanical) sensors configured to determine the presence of a sample (product) when its weight or another force caused by the sample (product) is detected. It would also or instead be possible for the one or more sensors to utilise, for example, image recognition techniques, etc.

(46) However, various other arrangements are of course possible. For instance, the source unit may be provided at the end of a relatively long transfer tubing (e.g. greater than 2 metres) so that the source unit can be used as a handheld analysis probe that can be manually brought into close contact with a sample by the user. FIG. 13 shows an example of a prototype handheld sampling unit incorporating a source unit of the type described herein. In FIG. 13 the transfer tubing connecting the sampling probe to the mass spectrometer inlet comprises 2.5 metres of Tygon tubing. However, it will be appreciated that the length and material of the transfer tubing may be adjusted as desired, e.g. depending on the application. FIG. 13 also shows the various connections to the sampling probe.

(47) Although the examples described above relate to particularly to DESI systems, it will be appreciated that the features described herein may in general relate to various types of (ambient) ionisation sources. For instance, various DESI-derived techniques have been developed and the techniques presented herein may be applied equally to these.

(48) In other examples, the sampling probe may comprise a laser ablation or plasma desorption probe. For example, FIG. 14 shows an example of another ambient ionisation source unit 10 in accordance with an embodiment wherein the sampling probe 14 comprises a laser probe. As shown, a fibre optic laser guide 140 is provided externally to the housing. The sampling probe 14 thus acts to direct the laser beam onto the surface of the sample in order to ablate analyte material from the surface thereof. The ablated analyte material can thus be collected by the sampling inlet 16 and transported by flexible transfer tube 22 towards the inlet of an analytical instrument such as a mass spectrometer.

(49) In general, the sampling probe may alternatively, or additionally, comprise any of: (i) a rapid evaporative ionisation mass spectrometry (“REIMS”) ion source; (ii) a desorption electrospray ionisation (“DESI”) ion source; (iii) a laser desorption ionisation (“LDI”) ion source; (iv) a thermal desorption ion source; (v) a laser diode thermal desorption (“LDTD”) ion source; (vi) a desorption electro-flow focusing (“DEFFI”) ion source; (vii) a dielectric barrier discharge (“DBD”) plasma ion source; (viii) an Atmospheric Solids Analysis Probe (“ASAP”) ion source; (ix) an ultrasonic assisted spray ionisation ion source; (x) an easy ambient sonic-spray ionisation (“EASI”) ion source; (xi) a desorption atmospheric pressure photoionisation (“DAPPI”) ion source; (xii) a paperspray (“PS”) ion source; (xiii) a jet desorption ionisation (“JeDI”) ion source; (xiv) a touch spray (“TS”) ion source; (xv) a nano-DESI ion source; (xvi) a laser ablation electrospray (“LAESI”) ion source; (xvii) a direct analysis in real time (“DART”) ion source; (xviii) a probe electrospray ionisation (“PESI”) ion source; (xix) a solid-probe assisted electrospray ionisation (“SPA-ESI”) ion source; (xx) a cavitron ultrasonic surgical aspirator (“CUSA”) device; (xxi) a focussed or unfocussed ultrasonic ablation device; (xxii) a microwave resonance device; or (xxiii) a pulsed plasma RF dissection device.

(50) Although the present invention has been described with reference to preferred 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.