Impactor Spray Ion Source

Abstract

There is provided an ion source comprising one or more nebulisers and one or more targets, wherein said one or more nebulisers are arranged and adapted to emit, in use, a stream predominantly of droplets which are caused to impact upon said one or more targets and to ionise said droplets to form a plurality of ions, wherein said one or more targets further comprise one or more structures configured to disturb gas flowing along a surface of said one or more targets.

Claims

1. An ion source comprising: one or more nebulisers and one or more targets, wherein said one or more nebulisers are arranged and adapted to emit, in use, a stream predominantly of droplets which are caused to impact upon said one or more targets so as to ionise said droplets to form a plurality of ions; and wherein said one or more targets further comprise: one or more structures configured to disturb gas flowing along a surface of said one or more targets.

2. An ion source as claimed in claim 1, wherein said one or more structures comprises one or more vortex generating structures.

3. An ion source as claimed in claim 1, wherein said one or more structures are configured to promote surface flow vortices that encourage gas flow to remain attached to said surface.

4. An ion source as claimed in claim 1, wherein said one or more structures comprise an aerodynamic shape or profile configured to promote surface flow vortices that encourage gas flow to remain attached to said surface.

5. An ion source as claimed in claim 1, wherein said one or more structures are positioned downstream of a stagnation point or line, and/or upstream of a separation point or line.

6. An ion source as claimed in claim 1, wherein said one or more structures comprises a protuberance extending from a surface of said one or more targets and/or a notch or cavity extending into a surface of said one or more targets.

7. An ion source as claimed in claim 1, wherein said one or more structures comprise one or more strakes or fins having a longitudinal axis that is parallel, off-parallel or perpendicular to the general direction of gas flowing over or around the target.

8. An ion source as claimed in claim 1, wherein said one or more structures comprises at least one of: (i) a single structure or a plurality of structures; (ii) a single row or multiple rows of structures; (iii) a cubic, cuboid, cylindrical, or polyhedral structure; (iv) structures having an irregular spacing between structures; and (v) a continuous micro-patterned surface that is imprinted, etched or micro-machined into a surface of said one or more targets.

9. An ion source as claimed in claim 1, wherein said one or more structures is positioned or aligned within a predominant direction of gas flowing past said one or more targets.

10. (canceled)

11. An ion source as claimed in claim 1, wherein said one or more targets comprises a cylindrical tube or rod, and a or the predominant direction of gas flowing past said one or more targets is around a portion of the circumference of said cylindrical tube or rod.

12. An ion source as claimed in claim 1, wherein said one or more targets comprises a planar surface in the form of a plate, and a or the predominant direction of gas flowing past said one or more targets is across or along said planar surface.

13. An ion source as claimed in claim 1, wherein a height or depth of said one or more structures is equivalent to, or comparable to a boundary layer thickness of said gas flowing past said one or more targets.

14-15. (canceled)

16. A mass spectrometer comprising an ion source as claimed in claim 1.

17. A method of ionising a sample comprising: providing one or more nebulisers and one or more targets, wherein said one or more targets comprises one or more structures configured to disturb gas flowing along a surface of said one or more targets; causing said one or more nebulisers to emit a stream predominantly of droplets which are caused to impact upon said one or more targets so as to ionise said droplets to form a plurality of ions; and disturbing gas flowing along a surface of said one or more targets using said one or more structures.

18. (canceled)

19. An ion source comprising: one or more nebulisers and one or more targets, wherein said one or more nebulisers are arranged and adapted to emit, in use, a stream predominantly of droplets which are caused to impact upon said one or more targets; and wherein said one or more targets further comprise: one or more structures configured to disturb gas flowing along a surface of said one or more targets, wherein said one or more structures are configured to promote surface flow vortices that encourage gas flow to remain attached to said surface.

20. An ion source as claimed in claim 19, wherein said one or more targets comprise a curved surface onto with said stream predominantly of droplets is caused to impact upon.

21. An ion source as claimed in claim 19, wherein said one or more structures comprises a notch or cavity extending into a surface of said one or more targets.

22. An ion source as claimed in claim 19, wherein said one or more structures comprises a continuous micro-patterned surface that is imprinted, etched or micro-machined into a surface of said one or more targets.

23. An ion source as claimed in claim 19, wherein a height or depth of said one or more structures is less than 500 μm.

24. An ion source as claimed in claim 19, wherein said ion source is an Electrospray ionisation (“ESI”) ion source.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0086] Various embodiments of the present disclosure will now be described, by way of example only, and with reference to the accompanying drawings in which:

[0087] FIG. 1 shows a conventional impactor spray ion source;

[0088] FIG. 2 shows a schematic of the stagnation zone for gas flowing past a cylinder;

[0089] FIG. 3 shows counter-rotating vortices in gas flowing past a cylinder, from Kestin and Wood (1970);

[0090] FIG. 4 shows a microvorticity relationship graph from Kestin and Wood (1970);

[0091] FIG. 5 shows a Scanning Electron Microscope (“SEM”) image of a cylindrical impactor spray target;

[0092] FIG. 6 shows an impactor spray ion source comprising a target incorporating a surface groove;

[0093] FIG. 7 shows a graph illustrating a relationship between groove position and signal intensity; and

[0094] FIG. 8 shows an embodiment of the present disclosure.

DETAILED DESCRIPTION

[0095] Developments in relation to an Impactor Spray ion source, and specifically the gas flow and vortex flow behaviour will now be described.

[0096] When a flow of gas approaches a solid object, a point may be reached where the flow becomes attached to the surface and the local surface velocity may become zero. This may be known as the stagnation point 11 and is shown schematically for an Impactor Spray geometry in FIG. 2.

[0097] The stagnation region 13 may be bounded by the stagnation point 11 where the flow optionally becomes attached to the surface, and the separation point 12 where the flow optionally separates from the surface. Although FIG. 2 shows the gas streamline displaced to the right hand side of the rod axis, it is understood that a centralized gas flow from the Impactor Spray nebulizer may result in two symmetrical streamlines on either side of the target 5.

[0098] The vortex phenomena that occur in the stagnation region 13 have been modelled (“On the Stability of Two-Dimensional Stagnation Flow” by J. Kestin and R. T. Wood, Fluid Mech. (1970), vol. 44, Part 3, pp. 461-479, referred to herein as “Kestin and Wood (1970)”) for a cylindrical geometry in cross flow. Such vortex phenomena may be encountered in an Impactor Spray ion source. The theory characterizes the well-established observations that cylinders in cross flow may exhibit a linear series of counter-rotating surface vortices whose axes of rotation are aligned with the gas flow streamlines.

[0099] FIG. 3 shows an illustration of a counter-rotating pair of surface vortices. The distance spanned by a counter-rotating pair may be known as the disturbance wavelength, λ, which may be found to be directly proportional to the cylinder diameter, D, and may be inversely proportional to the square root of the Reynolds number, R.sub.e;

λ=constDR.sub.e.sup.−0.5  (i)

and

R.sub.e=ρvD/μ  (ii)

wherein ρ is the gas density, v is the free-stream gas velocity (away from the surface) and μ is the gas viscosity. A plot of λ/D versus R.sub.e.sup.−0.5 for various turbulence intensities (Tu) is shown in FIG. 4.

[0100] FIG. 5 shows a Scanning Electron Microscope (“SEM”) image of an Impactor Spray target (for example a 1.6 mm diameter, stainless steel Impactor Spray target) which was used as described above for the analysis of analytes contained in protein-precipitated human plasma. The granular, circular “halo” is due to the deposition of involatile components of the plasma and is outside of the area of interest for the present discussion.

[0101] The SEM image was taken in the same direction as the impinging droplet stream and nebulizer gas jet. The cross (+) in FIG. 5 may represent an approximation of the impact point of the centre of the incoming gas jet. A close examination of the circled region of the image reveals a linear series of striation marks which are aligned with the direction of the flow streamlines. These striation marks may be evidence of the existence of counter-rotating surface vortices as described.

[0102] Referring to FIG. 1, the distance y.sub.1 between the nebulizer tip and the target is typically 3 mm. At such close distances, the gas velocity may be supersonic, where, at for example Mach 1, we can estimate R.sub.e to be approximately 30,000 for nitrogen gas at a temperature of 100° C. If we transpose this value onto the plot shown in FIG. 4, we obtain a disturbance wavelength value of λ=37 μm for D=1.6 mm and assuming Tu=4%. This compares to an experimentally determined λ=23 μm from FIG. 5, assuming that three striation marks represent the outer extent and centre of one counter-rotating vortex pair.

[0103] Thus, there appears to be some correlation between the observed experimental data and the theory of vorticity for cylinders in cross-flow.

[0104] It follows from equations (i) and (ii) that a greatest concentration of surface vortices may result from the use of dense gases which have low viscosities, i.e. those that give rise to high Reynolds numbers, R.sub.e. If we compare the available data for carbon dioxide and butane (at 400 K), we would increase R.sub.e by factors of 1.77 and 4.6, respectively, over those obtained with nitrogen as the nebulising gas. Thus, if vorticity is an important factor for Impactor Spray sources, this may advocate the use of high density, low viscosity nebulizer gases.

[0105] We can define a ratio of density, R.sub.ρ, between a chosen gas (X) and nitrogen (N.sub.2) as:

R.sub.ρ=ρ(X)/ρ(N.sub.2)  (ii)

and a ratio of viscosity, R.sub.μ, where:

R.sub.μ=μ(X)/μ(N.sub.2)  (iv)

It follows from equations (i) and (ii) that increased microvorticity will result from the use of nebulising gases that fulfil the condition:

R.sub.ρ/R.sub.μ>1  (v)

[0106] These surface vortices may play an important role in the shearing of liquid droplets which could enhance the so-called “ion spray” and “sonic spray” mechanisms that yield gas phase ions and charged droplets in API sources. Furthermore, these cross flow surface channels may guide surface liquid towards the separation point where secondary droplets or ions may be ejected following a period of double layer formation within the surface liquid filaments (or rolling droplets).

[0107] Referring to FIG. 5, if we assume that the cross (+) represents the approximate location of the flow stagnation point (or line) and the end of the striation marks represent the flow separation point (or line), we can determine from a simple geometric projection that the Impactor Spray target stagnation zone may subtend a radial angle of approximately 46 degrees.

[0108] For a 1.6 mm diameter rod target, as typically used in Impactor Spray sources, this may equate to a stagnation zone that is typically 0.65 mm long. Since the surface vorticity is associated with the stagnation zone, one might assume that any gross interference with this region would have detrimental effects on the performance of the Impactor Spray source.

[0109] An experimental geometry is shown schematically in FIG. 6, in which a surface groove 14, with an equivalent width to the stagnation length (0.65 mm), is cut longitudinally into a 1.6 mm diameter stainless steel rod target 50. It has been shown that by rotating the position of the groove 14 with respect to the stagnation region (upper right hand quadrant), significant sensitivity decreases may be observed when the groove overlaps the stagnation region.

[0110] FIG. 7 shows the effect of target groove position on the relative signal intensity for an Impactor Spray/Mass Spectrometry analysis of busiprone and reserpine which were infused into the source at a concentration of 0.125 pg/μL and a flow rate of 0.8 mL/min. In the illustrated embodiment, the highest sensitivity is observed when the groove is positioned well away from the stagnation zone (upper right hand quadrant). The lowest sensitivity is observed when the groove completely overlaps the upper quadrant, where presumably, the stagnation region is overwhelmed by turbulence such that the clear definition between a stagnation zone and free-stream flow no longer exists. The two additional reference points for busiprone and reserpine were obtained from a different target which contained no groove, but had a 1.6 mm diameter.

[0111] This experiment does not necessarily distinguish between the relative importance of vorticity or the spray steering (Coanda) effect of the gas flow which directs ions and charged droplets towards the ion inlet cone. However, it may be reasonable to suggest that by increasing the length of the existing stagnation region on the standard rod target, it may be possible to increase the sensitivity of an Impactor Spray ion source.

[0112] It is known from aircraft wing design that flow at a surface is more likely to become detached under conditions of low turbulence. Thus in order to increase the length of the stagnation region and hence reduce the chances of stalling under high angles of attack, aircraft wings incorporate vortex generators which are attached along the length of the wing in a position that is downstream but close to the stagnation line. These are typically triangular, rectangular or square features that are most effective when their height is equivalent to the thickness of the boundary layer at their point of attachment to the wing. A vortex generator can also take the form of an elongated strake or fin that is aligned in the direction of the flow streamlines.

[0113] If we assume a planar surface geometry, the thickness of the boundary layer (δ) is given by:

δ=4.91×R.sub.e.sup.−0.5  (vi) for laminar flow, or

δ=0.38×R.sub.e.sup.−0.2  (vii) for turbulent flow,

wherein x is the distance from the stagnation point and R.sub.e is the Reynolds number for the free stream flow.

[0114] For typical Impactor Spray operating conditions, the close positioning of the target surface to the nebulizer tip is such that the free stream gas velocity is supersonic and at Mach 1 we would expect R.sub.e to be of the order 30,000. In this case, equations (i) and (ii) would yield boundary layer thicknesses of δ=6 μm and 10 μm, respectively, for x=0.2 mm which is approximately one third of the distance from the start to the end of the stagnation region. This represents a lower limit for the height of a vortex generating structure, or structures, in the case of a 1.6 mm diameter target rod. Historical hot-wire measurements have also shown that surface vortex disturbances can extend to as far as fifty boundary layer thicknesses so it may be expected that the useful height range of a vortex generating structure may be 1-50 times the boundary layer thickness (δ).

[0115] An embodiment of the present disclosure will now be described.

[0116] FIG. 8 shows a schematic example of a cylindrical rod target 50 in accordance with an embodiment. Target 50 may have surface structures 15, or microstructures, that may serve the purpose of creating surface flow vortices. The surface flow vortices may encourage the flow to remain attached to the target surface.

[0117] The size of the structures is exaggerated in FIG. 8 (which is schematic) and may be 10-100 μm in size. The target may be 1.6 mm in diameter. The microstructures may be located downstream from a stagnation line 16 and may be located upstream from a separation line (17). The size or height of the microstructures may be comparable or equivalent to the thickness of the boundary layer of gas flowing around the target. This can create the most effectiveness when attempting to generate vortices using the microstructures.

[0118] Although the microstructures are shown on the upper right hand quadrant of the target in FIG. 8, an additional set of microstructures may be placed symmetrically on the upper left hand quadrant. The incoming nebulizer droplet stream 18 may be symmetrical, i.e. directed to the Top Dead Centre (“TDC”) of the target.

[0119] In an embodiment, the cylindrical rod target 5 could instead be a plate target, optionally comprising a planar surface in the form of a plate. The plate target may comprise one or more structures or microstructures on its surface.

[0120] The structures or microstructures in any of the aspects or embodiments disclosed herein may not be limited to those shown in FIG. 8, and could comprise or further comprise at least one of:

[0121] (i) a single structure or a plurality of structures;

[0122] (ii) a single row or multiple rows of structures, for example between the stagnation and separation lines;

[0123] (iii) any shape of structure, for example cubes, rectangular cubes, cylinders, or pyramids;

[0124] (iv) structures wherein there is an irregular spacing between structures; and

[0125] (v) a continuous micro-patterned surface that is imprinted, etched or micro-machined into the target.

[0126] The structures or microstructures could comprise or further comprise one or more strakes or fins. The strakes or fins may have a longitudinal axis that is parallel, off-parallel or perpendicular to the general direction of gas flowing over or around the target. The strakes or fins may act to alter the direction of gas flowing past the surface and/or promote surface flow vortices to optionally encourage gas flow to remain attached to said surface. The strakes or fins may achieve this by having an aerodynamic shape or profile.

[0127] The disclosed aspects and embodiments optionally increase the sensitivity of existing Impactor Spray ion sources and optionally provide a wider range of target types and geometries.

[0128] Although the present disclosure 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 disclosure as set forth in the accompanying claims.