Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/02—Details
  • H01J49/04—Arrangements for introducing or extracting samples to be analysed, e.g. vacuum locks; Arrangements for external adjustment of electron- or ion-optical components
  • H01J49/0431—Arrangements for introducing or extracting samples to be analysed, e.g. vacuum locks; Arrangements for external adjustment of electron- or ion-optical components for liquid samples
  • H01J49/0445—Arrangements for introducing or extracting samples to be analysed, e.g. vacuum locks; Arrangements for external adjustment of electron- or ion-optical components for liquid samples with means for introducing as a spray, a jet or an aerosol
  • H01J49/045—Arrangements for introducing or extracting samples to be analysed, e.g. vacuum locks; Arrangements for external adjustment of electron- or ion-optical components for liquid samples with means for introducing as a spray, a jet or an aerosol with means for using a nebulising gas, i.e. pneumatically assisted
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/02—Details
  • H01J49/10—Ion sources; Ion guns
  • H01J49/14—Ion sources; Ion guns using particle bombardment, e.g. ionisation chambers
  • H01J49/142—Ion sources; Ion guns using particle bombardment, e.g. ionisation chambers using a solid target which is not previously vapourised
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/02—Details
  • H01J49/10—Ion sources; Ion guns
  • H01J49/16—Ion sources; Ion guns using surface ionisation, e.g. field-, thermionic- or photo-emission

Definitions

• the present invention relates generally to mass spectrometry and in particular to mass spectrometers and methods of mass spectrometry.
• Various embodiments relate to an ion source and a method of ionising a sample.
• API Atmospheric Pressure lonisation
• Fig. 1 shows schematically a conventional standard impactor spray source. This comprises a pneumatic nebulizer assembly 1 , a desolvation heater 4, an impactor target 5 and a mass spectrometer inlet assembly comprising cone gas nozzle 6, ion inlet orifice 8 and first vacuum region 9.
• the nebuliser assembly 1 is composed of an inner liquid capillary 2 and an outer gas capillary 3 which delivers a high velocity stream of gas at the nebulizer tip to aid the atomization of the liquid solvent flow.
• the inner liquid capillary 2 may have an internal diameter of 130 ⁇ and an outside diameter of 270 ⁇ .
• the outer gas capillary 3 may have an inside diameter of 330 ⁇ .
• the gas supply for example nitrogen
• the gas supply is pressurized to approximately 7 bar and liquid flow rates of 0.1 to 1 mL/min are commonly used.
• a heated desolvation gas for example nitrogen flows between the nebulizer 1 and the heater 4 at a flow rate of typically
• the high velocity stream of droplets from the nebulizer 1 impact on a 1.6 mm diameter stainless steel, cylindrical rod target 5.
• the surface of the rod target 5 is polished and smooth.
• the illustrated dimensions x ⁇ and y 2 are typically 5 mm, 3 mm and 7 mm, respectively.
• the nebulizer 1 and impactor target 5 are typically held at 0 V and 1 kV, respectively.
• the mass spectrometer inlet is typically close to ground potential (for example 0-100 V).
• a nitrogen curtain gas flow of typically 150 L/hr passes between the cone gas nozzle 6 and the ion inlet cone 10.
• Ions, charged particles or neutrals that are contained within the gas flow wake 7 from the impactor target 5 can enter the mass spectrometer via the ion inlet orifice 8 which forms a boundary between the first vacuum region 9 of the MS and the atmospheric pressure region of the source enclosure.
• the gas flow wake 7 follows the curvature of the target (Coanda effect) and is swung in the direction of the ion inlet orifice 8 which results in a greater ion signal intensity.
• a nebulizer produces a stream of high velocity liquid droplets in a supersonic gas jet that impinges on a metallic rod target that is held at high voltage and is in close proximity to the nebuliser tip.
• WO2013/093517 discloses interfacing capillary electrophoresis to a mass spectrometer via an impactor spray ionisation source.
• WO2014064400 discloses improved reducibility of impact-based ioinisation source for low and high organic mobile phase compositions using a mesh target.
• EP1855306 (“Cristoni”) discloses an ionisation source and method for mass spectrometry.
• WO2004/03401 1 discloses an ionisation source for mass spectrometry analysis.
• an ion source comprising:
• one or more nebulisers and one or more targets wherein the one or more nebulisers are arranged and adapted to emit, in use, a stream predominantly of droplets which are caused to impact upon the one or more targets and or so as to ionise the droplets to form a plurality of ions;
• the one or more targets further comprise:
• one or more structures configured to disturb gas flowing along or across a surface of the one or more targets.
• Modifications to the target surface of an Impactor Spray ion source are proposed, which are designed to encourage additional vortex flow behaviour to enhance the performance of an impactor spray source.
• Conventional Impactor Spray ion sources involve a target that is typically a planar, curved surface and does not comprise a structure that is configured to disturb gas flowing along its surface. It has been recognised that vortex flow patterns at the target surface may play an important role in the nebulisation, desolvation and ionisation processes in Impactor Spray ion sources, and the present disclosure aims to utilise this recognition. It will be appreciated that the above ion source requires the target to comprise the one or more structures configured to disturb gas flowing along or across its surface. This is very distinct from, for example, WO2013/093517 (“Micromass”) in which the surface of the target is completely smooth.
• the stream predominantly of droplets may be caused to impact upon the one or more targets thereby ionising the droplets to form said plurality of ions.
• the one or more structures may comprise one or more vortex generating structures, wherein the vortex generating structures are optionally configured to cause a vortex and/or turbulence in gas flowing past the one or more vortex generating structures.
• the one or more structures may be configured to promote surface flow vortices that encourage gas flow to remain attached to the surface.
• the one or more structures optionally comprise an aerodynamic shape or profile configured to promote surface flow vortices that encourage gas flow to remain attached to the surface.
• the one or more structures may be positioned downstream of a stagnation point or line, and/or upstream of a separation point or line.
• the one or more structures may 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.
• the one or more structures may comprise a protuberance extending from a surface of the one or more targets and/or a notch or cavity extending into a surface of the one or more targets.
• the one or more structures may comprise at least one of:
• the one or more structures may be positioned within a predominant direction of gas flowing past the one or more targets.
• the one or more structures may be aligned with a predominant direction of gas flowing past the one or more targets.
• the one or more targets may comprise a cylindrical tube or rod.
• a or the predominant direction of gas flowing past the one or more targets may be along or around a portion of the surface, circumference, or circumferential surface of the cylindrical tube.
• the one or more targets may comprise a planar surface in the form of a plate, and a or the predominant direction of gas flowing past said one or more targets may be across or along said planar surface.
• a height or depth of the one or more structures may be equivalent to, or comparable to a boundary layer thickness of the gas flowing past the one or more targets.
• a height or depth of the one or more structures may be within +/- 0%, 10%, 15%, 20%, 30%, 40%, 50%, 100%, 200%, 500%, 1000%, 2500% or 5000% of a boundary layer thickness of the gas flowing past the one or more targets.
• a height or depth of the one or more structures, and/or a distance or spacing between adjacent structures may be greater than, equal to, or less than: (i) 1 ⁇ ; (ii) 2 ⁇ ; (iii) 5 ⁇ ; (iv) 10 ⁇ ; (v) 15 ⁇ ; (vi) 20 ⁇ ; (vii) 25 ⁇ ; (viii) 30 ⁇ ; (ix) 35 ⁇ ; (x) 40 ⁇ ; (xi) 45 ⁇ ; (xii) 50 ⁇ ; (xiii) 60 ⁇ ; (ixv) 70 ⁇ ; (xv) 80 ⁇ ; (xvi) 90 ⁇ ; (xvii) 100 ⁇ ; (xviii) 150 ⁇ ; (ixx) 200 ⁇ ; (xx) 300 ⁇ ; (xxi) 400 ⁇ ; or (xxii) 500 ⁇ .
• the ion source may comprise an Atmospheric Pressure lonisation ("API”) ion source.
• API Atmospheric Pressure lonisation
• a mass spectrometer comprising an ion source as described above.
• a method of ionising a sample comprising:
• the one or more targets comprises one or more structures configured to disturb gas flowing along a surface of the one or more targets;
• the one or more nebulisers to emit a stream predominantly of droplets which are caused to impact upon the one or more targets and or so as to ionise the droplets to form a plurality of ions;
• a method of ionising a sample comprising:
• said one or more nebulisers to emit a stream predominantly of droplets which are caused to impact upon said one or more targets and or so as to ionise said droplets to form a plurality of ions;
• R M ⁇ ( ⁇ )/ ⁇ ( ⁇ 2 );
• p(X) is the density of the nebulising gas
• p(N 2 ) is the density of nitrogen
• ⁇ ( ⁇ ) is the viscosity of the nebulising gas
• ⁇ ( ⁇ 2 ) is the viscosity of nitrogen
• Various embodiments involve modifications to an impactor spray source design that encourage additional microvorticity for the purpose of enhancing ionization efficiency.
• Scanning Electron Microscope (“SEM”) images from an impactor spray rod target show strong evidence for the existence of such counter rotating microvortices where the characteristic spacing between vortices bears some resemblance to theory.
• structure may refer to a microstructure, for example having a dimension less than: (i) 1 ⁇ ; (ii) 2 ⁇ ; (iii) 5 ⁇ ; (iv) 10 ⁇ ; (v) 15 ⁇ ; (vi) 20 ⁇ ; (vii) 25 ⁇ ; (viii) 30 ⁇ ; (ix) 35 ⁇ ; (x) 40 ⁇ ; (xi) 45 ⁇ ; (xii) 50 ⁇ ; (xiii) 60 ⁇ ; (ixv) 70 ⁇ ; (xv) 80 ⁇ ; (xvi) 90 ⁇ ; (xvii) 100 ⁇ ; (xviii) 150 ⁇ ; (ixx) 200 ⁇ ; (xx) 300 ⁇ ; (xxi) 400 ⁇ ; or (xxii) 500 ⁇ .
• an ion source selected from the group consisting of: (i) an Electrospray ionisation (“ESI”) ion source; (ii) an Atmospheric Pressure Photo lonisation (“APPI”) ion source; (iii) an Atmospheric Pressure Chemical lonisation (“APCI”) ion source; (iv) a Matrix Assisted Laser Desorption lonisation (“MALDI”) ion source; (v) a Laser Desorption lonisation (“LDI”) ion source; (vi) an Atmospheric Pressure lonisation (“API”) ion source; (vii) a Desorption lonisation on Silicon (“DIOS”) ion source; (viii) an Electron Impact ("El”) ion source; (ix) a Chemical lonisation (“CI”) ion source; (x) a Field lonisation (“Fl”) ion source; (xi) a Field Desorption (“FD”) ion source; (xxi
• Atmospheric Pressure Matrix Assisted Laser Desorption lonisation ion source (xviii) a Thermospray ion source; (xix) an Atmospheric Sampling Glow Discharge lonisation (“ASGDI") ion source; (xx) a Glow Discharge (“GD”) ion source; (xxi) an Impactor ion source; (xxii) a Direct Analysis in Real Time (“DART") ion source; (xxiii) a Laserspray lonisation (“LSI”) ion source; (xxiv) a Sonicspray lonisation (“SSI”) ion source; (xxv) a Matrix Assisted Inlet lonisation (“MAN”) ion source; (xxvi) a Solvent Assisted Inlet lonisation (“SAN”) ion source; (xxvii) a Desorption Electrospray lonisation (“DESI”) ion source; (xxviii) a Laser Ablation Electro
• SID Surface Induced Dissociation
• ETD Electron Transfer Dissociation
• ECD Electron Capture Dissociation
• PID Photo Induced Dissociation
• PID Photo Induced Dissociation
• a Laser Induced Dissociation fragmentation device an infrared radiation induced dissociation device
• an ultraviolet radiation induced dissociation device an ultraviolet radiation induced dissociation device
• a thermal or temperature source fragmentation device an electric field induced fragmentation device
• xv a magnetic field induced fragmentation device
• an ion an ion
• a mass analyser selected from the group consisting of: (i) a quadrupole mass analyser; (ii) a 2D or linear quadrupole mass analyser; (iii) a Paul or 3D quadrupole mass analyser; (iv) a Penning trap mass analyser; (v) an ion trap mass analyser; (vi) a magnetic sector mass analyser; (vii) Ion Cyclotron Resonance (“ICR”) mass analyser; (viii) a Fourier Transform Ion Cyclotron Resonance (“FTICR”) mass analyser; (ix) an electrostatic mass analyser arranged to generate an electrostatic field having a quadro-logarithmic potential distribution; (x) a Fourier Transform electrostatic mass analyser; (
• (I) a device for converting a substantially continuous ion beam into a pulsed ion beam.
• the mass spectrometer may further comprise either:
• a C-trap and a mass analyser comprising an outer barrel-like electrode and a coaxial inner spindle-like electrode that form an electrostatic field with a quadro-logarithmic potential distribution, wherein in a first mode of operation ions are transmitted to the C-trap and are then injected into the mass analyser and wherein in a second mode of operation ions are transmitted to the C-trap and then to a collision cell or Electron Transfer
• Dissociation device wherein at least some ions are fragmented into fragment ions, and wherein the fragment ions are then transmitted to the C-trap before being injected into the mass analyser;
• a stacked ring ion guide comprising a plurality of electrodes each having an aperture through which ions are transmitted in use and wherein the spacing of the electrodes increases along the length of the ion path, and wherein the apertures in the electrodes in an upstream section of the ion guide have a first diameter and wherein the apertures in the electrodes in a downstream section of the ion guide have a second diameter which is smaller than the first diameter, and wherein opposite phases of an AC or RF voltage are applied, in use, to successive electrodes.
• the mass spectrometer further comprises a device arranged and adapted to supply an AC or RF voltage to the electrodes.
• the AC or RF voltage optionally has an amplitude selected from the group consisting of: (i) about ⁇ 50 V peak to peak; (ii) about 50-100 V peak to peak; (iii) about 100-150 V peak to peak; (iv) about 150-200 V peak to peak; (v) about 200-250 V peak to peak; (vi) about 250-300 V peak to peak; (vii) about 300-350 V peak to peak; (viii) about 350-400 V peak to peak; (ix) about 400-450 V peak to peak; (x) about 450-500 V peak to peak; and (xi) > about 500 V peak to peak.
• the AC or RF voltage may have a frequency selected from the group consisting of: (i) ⁇ about 100 kHz; (ii) about 100-200 kHz; (iii) about 200-300 kHz; (iv) about 300-400 kHz; (v) about 400-500 kHz; (vi) about 0.5-1.0 MHz; (vii) about 1.0-1.5 MHz; (viii) about 1.5-2.0 MHz; (ix) about 2.0-2.5 MHz; (x) about 2.5-3.0 MHz; (xi) about 3.0-3.5 MHz; (xii) about 3.5-4.0 MHz; (xiii) about 4.0-4.5 MHz; (xiv) about 4.5-5.0 MHz; (xv) about 5.0-5.5 MHz; (xvi) about 5.5-6.0 MHz; (xvii) about 6.0-6.5 MHz; (xviii) about 6.5-7.0 MHz; (xix) about 7.0-7.5 MHz; (xx) about 7.5-8.0 MHz
• the mass spectrometer may also comprise a chromatography or other separation device upstream of an ion source.
• the chromatography separation device comprises a liquid chromatography or gas chromatography device.
• the separation device may comprise: (i) a Capillary Electrophoresis (“CE”) separation device; (ii) a Capillary Electrochromatography (“CEC”) separation device; (iii) a substantially rigid ceramic-based multilayer microfluidic substrate (“ceramic tile”) separation device; or (iv) a supercritical fluid chromatography separation device.
• the ion guide may be maintained at a pressure selected from the group consisting of: (i) ⁇ about 0.0001 mbar; (ii) about 0.0001-0.001 mbar; (iii) about 0.001-0.01 mbar; (iv) about 0.01-0.1 mbar; (v) about 0.1-1 mbar; (vi) about 1-10 mbar; (vii) about 10-100 mbar; (viii) about 100-1000 mbar; and (ix) > about 1000 mbar.
• analyte ions may be subjected to Electron Transfer Dissociation ("ETD") fragmentation in an Electron Transfer Dissociation fragmentation device.
• ETD Electron Transfer Dissociation
• Analyte ions may be caused to interact with ETD reagent ions within an ion guide or fragmentation device.
• analyte ions are fragmented or are induced to dissociate and form product or fragment ions upon interacting with reagent ions; and/or (b) electrons are transferred from one or more reagent anions or negatively charged ions to one or more multiply charged analyte cations or positively charged ions whereupon at least some of the multiply charged analyte cations or positively charged ions are induced to dissociate and form product or fragment ions; and/or (c) analyte ions are fragmented or are induced to dissociate and form product or fragment ions upon interacting with neutral reagent gas molecules or atoms or a non- ionic reagent gas; and/or (d) electrons are transferred from one or more neutral, non-ionic or uncharged basic gases or vapours to one or more multiply charged analyte cations or positively charged ions whereupon at least some of the multiply charged analyte cations or positively charged ions are induced to
• the multiply charged analyte cations or positively charged ions may comprise peptides, polypeptides, proteins or biomolecules.
• the reagent anions or negatively charged ions are derived from a polyaromatic
• the reagent anions or negatively charged ions are derived from the group consisting of: (i) anthracene; (ii) 9, 10 diphenyl-anthracene; (iii) naphthalene; (iv) fluorine; (v) phenanthrene; (vi) pyrene; (vii) fluoranthene; (viii) chrysene; (ix) triphenylene; (x) perylene; (xi) acridine; (xii) 2,2' dipyridyl; (xiii) 2,2' biquinoline; (xiv) 9-anthracenecarbonitrile; (xv) dibenzothiophene; (xvi) 1 , 10'- phenanthroline; (xvii) 9' anthracenecarbonitrile; and (xviii) anthraquinone; and/or (c)
• the process of Electron Transfer Dissociation fragmentation comprises interacting analyte ions with reagent ions, wherein the reagent ions comprise dicyanobenzene, 4-nitrotoluene or azulene.
• a chromatography detector may be provided wherein the chromatography detector comprises either:
• a destructive chromatography detector optionally selected from the group consisting of (i) a Flame Ionization Detector (FID); (ii) an aerosol-based detector or Nano Quantity Analyte Detector (NQAD); (iii) a Flame Photometric Detector (FPD); (iv) an Atomic- Emission Detector (AED); (v) a Nitrogen Phosphorus Detector (NPD); and (vi) an
• Evaporative Light Scattering Detector or a non-destructive chromatography detector optionally selected from the group consisting of: (i) a fixed or variable wavelength UV detector; (ii) a Thermal Conductivity Detector (TCD); (iii) a fluorescence detector; (iv) an Electron Capture Detector (ECD); (v) a conductivity monitor; (vi) a Photoionization Detector (PID); (vii) a Refractive Index Detector (RID); (viii) a radio flow detector; and (ix) a chiral detector.
• TCD Thermal Conductivity Detector
• ECD Electron Capture Detector
• PID Photoionization Detector
• RID Refractive Index Detector
• radio flow detector and (ix) a chiral detector.
• the mass spectrometer may be operated in various modes of operation including a mass spectrometry ("MS”) mode of operation, a tandem mass spectrometry (“MS/MS”) mode of operation, a mode of operation in which parent or precursor ions are alternatively fragmented or reacted so as to produce fragment or product ions, and not fragmented or reacted or fragmented or reacted to a lesser degree, a Multiple Reaction Monitoring (“MRM”) mode of operation, a Data Dependent Analysis (“DDA”) mode of operation, a Data Independent Analysis (“DIA”) mode of operation, a Quantification mode of operation or an Ion Mobility Spectrometry (“IMS”) mode of operation.
• MRM Multiple Reaction Monitoring
• DDA Data Dependent Analysis
• DIA Data Independent Analysis
• IMS Ion Mobility Spectrometry
• Fig. 1 shows a conventional impactor spray ion source
• Fig. 2 shows a schematic of the stagnation zone for gas flowing past a cylinder
• Fig. 3 shows counter-rotating vortices in gas flowing past a cylinder, from Kestin and Wood (1970);
• Fig. 4 shows a microvorticity relationship graph from Kestin and Wood (1970);
• Fig. 5 shows a Scanning Electron Microscope ("SEM”) image of a cylindrical impactor spray target;
• Fig. 6 shows an impactor spray ion source comprising a target incorporating a surface groove
• Fig. 7 shows a graph illustrating a relationship between groove position and signal intensity
• Fig. 8 shows an embodiment of the present disclosure.
• 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.
• 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.
• 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.
• Fig. 3 shows an illustration of a counter-rotating pair of surface vortices.
• ⁇ /D versus R e "0 5 for various turbulence intensities (Tu) is shown in Fig. 4.
• 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.
• SEM Scanning Electron Microscope
• 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.
• 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.
• the distance between the nebulizer tip and the target is typically 3 mm.
• 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.
• 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).
• FIG. 6 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.
• 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/ ⁇ - and a flow rate of 0.8 mL/min.
• 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.
• 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.
• 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 ( ⁇ ).
• 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.
• the size of the structures is exaggerated in Fig. 8 (which is schematic) and may be 10-100 ⁇ 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 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.
• TDC Top Dead Centre
• 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.
• any shape of structure for example cubes, rectangular cubes, cylinders, or pyramids;
• 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 acheive this by having an aerodynamic shape or profile.

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