Abstract:
An ion source is disclosed comprising a nebuliser and a target. The nebuliser is arranged and adapted to emit, in use, a stream of analyte droplets which are caused to impact upon the target and to ionise analyte to form a plurality of analyte ions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 14/113,151 filed Oct. 21, 2013, which is the National Stage of International Application No. PCT/GB2012/050888, filed 20 Apr. 2012, which claims priority from and the benefit of U.S. Provisional Patent Application Ser. No. 61/614,734 filed Mar. 23, 2012, United Kingdom Patent Application No. 1204937.5 filed on Mar. 21, 2012, U.S. Provisional Patent Application Ser. No. 61/478,725 filed on 25 Apr. 2011 and United Kingdom Patent Application No. 1106694.1 filed on 20 Apr. 2011. The entire contents of these applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE PRESENT INVENTION 
       [0002]    The present invention relates to an ion source for a mass spectrometer and a method of ionising a sample. The preferred embodiment relates to a mass spectrometer and a method of mass spectrometry. 
         [0003]    Atmospheric Pressure Ionization (“API”) ion sources are commonly used to ionize the liquid flow from ELPIC or UPLC chromatography devices prior to analyzing the resulting gas phase ions via a mass spectrometer. Two techniques which are most commonly used comprise Electrospray Ionization (“ESI”) and Atmospheric Pressure Chemical Ionization (“APCI”). ESI is optimal for moderate to high polarity analytes and APCI is optimal for non-polar analytes. API ion sources that combine both of these techniques have been proposed and realized in designs that simultaneously combine ESI and APO ionization using geometries that ensure that the electric fields generated by each technique are shielded and are independent of one another. These so called “multimode” ion sources have the advantage of being able to ionize analyte mixtures containing a wide range of polarities in a single chromatographic run without the need to switch between different ionization techniques. U.S. Pat. No. 7,034,291 discloses a ESI/APCI multimode ionization source comprising an ESI ion source and a downstream corona needle and U.S. Pat. No. 7,411,186 discloses a multimode ESI/APCI ion source. The known multimode ion sources suffer from the problem of being mechanically complex. 
         [0004]    Other universal or multimode ionization sources have been proposed for interfacing, liquid chromatography to mass spectrometry. One such example is a Surface Activated Chemical Ionization (“SACI”) ion source which directs a vapour stream from a heated nebuliser probe towards a broad area charged target plate which is situated close to the ion inlet aperture of the mass spectrometer and 15-20 mm away from the end of the nebuliser. The spray point of the SACI ion source is within the heated nebuliser probe so that the typical distance between the spray point of the SACI ion source and the target plate is 70 mm. This geometry with a relatively large distance between the sprayer and the target produces a divergent spray with a dispersed reflected flow at the target which generally results in lower sensitivities when compared to optimized ESI and APCI sources. U.S. Pat. No. 7,368,728 discloses a known Surface Activated Chemical Ionisation ion source. 
         [0005]    It is also known to place a small target in the form of a bead at close proximity to the nebulised spray point in impactor nebulisers which are used in atomic absorption spectroscopy. An impactor nebuliser is, for example, disclosed in Anal. Chem. 1982, 54, 1411-1419. The known impactor nebuliser is not used to ionise a sample. 
         [0006]    It is desired to provide an improved ion source for a mass spectrometer. 
       SUMMARY OF THE INVENTION 
       [0007]    According to an aspect of the present invention there is provided an ion source comprising: 
         [0008]    one or more nebulisers and one or more targets; 
         [0009]    wherein 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 to ionise the droplets to form a plurality of ions. 
         [0010]    The droplets preferably comprise analyte droplets and the plurality of ions preferably comprise analyte ions. 
         [0011]    However, according to another embodiment the droplets may comprise reagent droplets and the plurality of ions may comprise reagent ions. 
         [0012]    According to the preferred embodiment reagent ions which are created may react, interact with or transfer charge to neutral analyte molecules and cause the analyte molecules to become ionised. Reagent ions may also be used to enhance the formation of analyte ions. 
         [0013]    According to an embodiment one or more tubes may be arranged and adapted to supply one or more analyte or other gases to a region adjacent the one or more targets. 
         [0014]    The reagent ions are preferably arranged so as to ionise the analyte gas to form a plurality of analyte ions. 
         [0015]    An analyte liquid may be supplied to the one or more targets and may be ionised to form a plurality of analyte ions and/or a reagent liquid may be supplied to the one or more targets and may be ionised to form reagent ions which transfer charge to neutral analyte atoms or molecules to form analyte ions and/or which enhance the formation of analyte ions. 
         [0016]    The one or more targets preferably comprise one or more apertures and wherein the analyte liquid and/or reagent liquid is supplied directly to the one or more targets and emerges from the one or more apertures. 
         [0017]    According to an embodiment the one or more targets may be coated with one or more liquid, solid or gelatinous analytes and wherein the one or more analytes are ionised to form a plurality of analyte ions. 
         [0018]    The one or more targets may be formed from one or more analytes and the one or more analytes may be ionised to form a plurality of analyte ions. 
         [0019]    According to the preferred embodiment the ion source comprises an Atmospheric Pressure Ionisation (“API”) ion source. 
         [0020]    The one or more nebulisers are preferably arranged and adapted such that the majority of the mass or matter emitted by the one or more nebulisers is in the form of droplets not vapour. 
         [0021]    Preferably, at least 50%, 55%, 60%, 65%, 70%, 75%, 10%, 85%, 90% or 95% of the mass or matter emitted by the one or more nebulisers is in the form of droplets. 
         [0022]    The one or more nebulisers are preferably arranged and adapted to emit a stream of droplets wherein the Sauter mean diameter (“SMD”, d32) of the droplets is in a range: (i) &lt;5 μm; (ii) 5-10 μm; (iii) 10-15 μm; (iv) 15-20 μm; (v) 20-25 μm; or (vi) &gt;25 μm. 
         [0023]    The stream of droplets emitted from the one or more nebulisers preferably forms a stream of secondary droplets after impacting the one or more targets. 
         [0024]    The stream of droplets and/or the stream of secondary droplets preferably traverse a flow region with a Reynolds number (Re) in the range: (i) &lt;2000; (ii) 2000-2500; (iii) 2500-3000; (iv) 3000-3500; (v) 3500-4000; or (vi) &gt;4000. 
         [0025]    According to the preferred embodiment substantially at the point of the droplets impacting the one or more targets the droplets have a Weber number (We) selected from the group consisting of (i) &lt;50; (ii) 50-100; (iii) 100-150; (iv) 150-200; (v) 200-250; (vi) 250-300; (vii) 300-350; (viii) 350-400; (ix) 400-450; (x) 450-500; (xi) 500-550; (xii) 550-600, (xiii) 600-650; (xiv) 650-700; (xv) 700-750; (xvi) 750-800; (xvii) 800-850; (xviii) 850-900; (xix) 900-950; (xx) 950-1000; and (xxi) &gt;1000. 
         [0026]    According to the preferred embodiment substantially at the point of the droplets impacting the one or more targets the droplets have a Stokes number (S k ) in the range: (i) 1-5; (ii) 5-10; (iii) 10-15; (iv) 15-20; (v) 20-25; (vi) 25-30; (vii) 30-35; (viii) 35-40; (ix) 40-45; (x) 45-50; and (xi) &gt;50. 
         [0027]    The mean axial impact velocity of the droplets upon the one or more targets is preferably selected from the group consisting oE (i) &lt;20 nals; (ii) 20-30 m/s; (iii) 30-40 m/s; (iv) 40-50 mls; (v) 50-60 m/s; (vi) 60-70 m/s; (vii) 70-80 m/s; (viii) 80-90 m/s (ix) 90-100 m/s; (x) 100-110 m/s; (xi) 110-120 m/s; (xii) 120-130 mls; (xiii) 130-140 m/s; (xiv) 140-150 m/s; and (xv) &gt;150 m/s. 
         [0028]    The one or more targets are preferably arranged &lt;20 mm, &lt;19 mm, &lt;18 mm, &lt;17 mm, &lt;16 mm, &lt;15 mm, &lt;14 mm, &lt;13 mm, &lt;12 mm, &lt;11 mm, &lt;10 mm, &lt;9 min, &lt;8 mm, &lt;7 mm, &lt;6 mm, &lt;5 mm, &lt;4 mm, &lt;3 mm or &lt;2 mm from the exit of the one or more nebulisers. 
         [0029]    The one or more nebulisers are preferably arranged and adapted to nebulise one or more eluents emitted by one or more devices over a period of time. 
         [0030]    The one or more devices preferably comprise one or more liquid chromatography separation devices. 
         [0031]    The one or more nebulisers are preferably arranged and adapted to nebulise one or more clients, wherein the one or more eluents have a liquid flow rate selected from the group consisting of &lt;1 μl/min; (ii) 1-10 μL/min; (iii) 10-50 μL/rain; (iv) 50-100 μL/min; (v) 100-200 μL/min; (vi) 200-300 μL/min; (vii) 300-400 (viii) 400-500 μL/min; (ix) 500-600 μL/min; (x) 600-700 μL/min; (xi) 700-800 μL/min; (xii) 800-900 (xiii) 900-1000 μL/min; (xiv) 1000-1500 μL/min; (xv) 1500-2000 μL/min; (xvi) 2000-2500 μL/min; and (xvii) &gt;2500 
         [0032]    The one or more nebulisers may according to a less preferred embodiment comprise one or more rotating disc nebulisers. 
         [0033]    The one or more nebulisers preferably comprise a first capillary tube having an exit which emits, in use, the stream of droplets. 
         [0034]    The first capillary tube is preferably maintained, in use, at a potential: (i) −5 to −4 kV; (ii) −4 to −3 kV; (iii) −3 to −2 kV; (iv) −2 to −1 kV; (v)−1000 to −900 V; (vi) −900 to −800 V; (vii) −800 to −700 V; (viii) −700 to −600 V; (ix) −600 to −500 V; (x)−500 to −400 V; (xi) −400 to −300 V; (xii) −300 to −200 V; (xiii) −200 to −100 V; (xiv) −100 to −90 V; (xv)−90 to −80 V; (xvi) −80 to −70 V; (xvii) −70 to −60 V; (xviii) −60 to −50 V; (xix) −50 to −0.40 V; (xx) −40 to −30 V; (xxi) −30 to −20 V; (xxii) −20 to −10 V; (xxiii) −10 to 0 V; (xxiv) 0-10 V; (xxv) 10-20 V; (xxvi) 20-30 V; (xxvii) 30-40V; (xxviii) 40-50 V; (xxix) 50-60 V; (xxx) 60-70 V; (xxxi) 70-80 V; (xxxii) 80-90 V; (xxxiii) 90-100 V (xxxix) 100-200 V; (xxxv) 200-300 V; (xxxvi) 300-400 V; (xxxvii) 400-500 V; (xxxviii) 500-600 V; (xxxix) 600-700 V; (xi) 700-800 V; (xli) 800-900 V; (xiii) 900-1000 V; (xliii) 1-2 kV; (xliv) 2-3 kV; (xlv) 3-4 kV; and (xlvi) 4-5 kV. 
         [0035]    The first capillary mbe is preferably maintained, in use, at a potential of (i) −5 to −4 kV; (ii) 4 to −3 kV; (iii) −3 to −2 kV; (iv) −2 to −1 kV; (x) −1000 to −900 V; (vi) −900 to −800 V; (vii) −800 to −700 V; (viii) −700 to −600 V; (ix) −600 to −500 V; (x) −500 to −400 V; (xi) −400 to −300 V; (xii) −300 to −200 V; (xiii) −200 to −100 V; (xiv) −100 to −90 V; (xx) 90 to −80 V; (xvi) −80 to −70 V; (xvii) −70 to −60 V; (xviii) −60 to −50 V; (xix) −50 to −40 V; (xx) 40 to −30 V; (xxi) −30 to −20 V; (xxii) −20 to −10 V; (xxiii) −10 to 0 V; (xxiv) 0-10 V; (xxv) 10-20 V; (xxxi) 20-30 V; (xxvii) 30-40V; (xvii) 40-50 V; (xxix) 50-60 V; (xxx) 60-70 V; (xxxi) 70-80 V; (xxxii) 80-90 V; (xxxiii) 90-100 V; (xxxiv) 100-200 V; (xxxv) 200-300 V; (xxxvi) 300-400 V; (xxxvii) 400-500 V; (xxxviii) 500-600 V; (xxxix) 600-700 V; (xi) 700-800 V; (xli) 800-900 V; (xiii) 900-1000 V; (xliii) 1-2 kV; (xliv) 2-3 kV; (xlv) 3-4 kV; and (xlvi) 4-5 kV; relative to the potential of an enclosure surrounding the ion source and/or an ion inlet device which leads to a first vacuum stage of a mass spectrometer and/or the one or more targets. 
         [0036]    According to an embodiment a wire may be located within the volume enclosed by the first capillary tube wherein the wire is arranged and adapted to focus the stream of droplets. 
         [0037]    According to the preferred embodiment: 
         [0038]    (i) the first capillary tube is surrounded by a second capillary tube which is arranged and adapted to provide a stream of gas to the exit of the first capillary tube; or 
         [0039]    (ii) a second capillary tube is arranged and adapted to provide a cross flow stream of gas to the exit of the first capillary tube. 
         [0040]    The second capillary tube preferably surrounds the first capillary tube and/or is either concentric or non-concentric with the first capillary tube. 
         [0041]    The ends of the first and second capillary tubes are preferably either: (i) flush or parallel with each other; or (ii) protruded, recessed or non-parallel relative to each other. 
         [0042]    The exit of the first capillary tube preferably has a diameter D and the spray of droplets is preferably arranged to impact on an impact zone of the one or more targets. 
         [0043]    The impact zone preferably has a maximum dimension of x and wherein the ratio x/D is in the range &lt;2, 2-5, 5-10, 10-15, 15-20, 20-25, 25-30, 30-35, 35-40 or &gt;40. 
         [0044]    The impact zone preferably has an area selected from the group consisting of (i) &lt;0.01 mm 2 ; (ii) 0.01-0.10 mm 2 ; (iii) 0.10-0.20 mm 2 ; (iv) 0.20-0.30 mm 2 ; (v) 0.30-0.40 mm 2 ; (vi) 0.40-0.50 mm 2 ; (vii) 0.50-0.60 mm 2 ; (viii) 0.60-0.70 mm 2 ; (ix) 0.70-0.80 mm 2 ; (x) 0.80-0.90 mm 2 ; (xi) 0.90-1.00 mm 2 ; (xii) 1.00-1.10 mm 2 ; (xiii) 1.10-1.20 mm 2 ; (xiv) 1.20-1.30 mm 2 ; (xv) 1.30-1.40 mm 2 ; (xvi) 1.40-1.50 mm 2 ; (xvii) 1.50-1.60 mm 2 ; (xviii) 1.60-1.70 mm 2 ; (xix) 1.70-1.80 mm 2 ; (xx) 1.80-1.90 mm 2 ; (xxi) 1.90-2.00 mm 2 ; (xxii) 2.00-2.10 mm 2 ; (xxiii) 2.10-2.20 mm 2 ; (xxiv) 2.20-2.30 mm 2 ; (xxv) 2.30-2.40 mm 2 ; (xxi) 2.40-2.50 mm 2 ; (xxvii) 2.50-2.60 mm 2 ; (xxviii) 2.60-2.70 mm 2 ; (xxix) 2.70-2.80 mm 2 ; (xxx) 2.80-2.90 mm 2 ; (xxxi) 2.90-3.00 mm 2 ; (xxxii) 3.00-3.10 mm 2 ; (xxxiii) 3.10-3.20 mm 2 ; (xxxiv) 3.20-3.30 mm 2 ; (xxxv) 3.30-3.40 mm 2 ; (xxxvi) 3.40-3.50 mm 2 ; (xxxvii) 3.50-3.60 mm 2 ; (xxxviii) 3.60-3.70 mm 2 ; (xxxix) 3.70-3.80 mm 2 ; (xi) 3.80-3.90 mm 2 ; and (xli) 3.90-4.00 mm 2 . 
         [0045]    The ion source preferably further comprises one or more heaters which are arranged and adapted to supply one or more heated streams of gas to the exit of the one or more nebulisers. 
         [0046]    According to an embodiment: 
         [0047]    (i) the one or more heaters surround the first capillary tube and are arranged and adapted to supply a heated stream of gas to the exit of the first capillary tube; and/or 
         [0048]    (ii) the one or more heaters comprise one or more infra-red heaters; and/or 
         [0049]    (iii) the one or more heaters comprise one or more combustion heaters. 
         [0050]    The ion source may further comprise one or more heating devices arranged and adapted to directly and/or indirectly heat the one or more targets. 
         [0051]    The one or more heating devices may comprise one or more lasers arranged and adapted to emit one or more laser beams which impinge upon the one or more targets in order to heat the one or more targets. 
         [0052]    According to an embodiment the one or more targets are maintained, in use, at a potential: (i) −5 to −4 kV; (ii) −4 to −3 kV; (iii) −3 to −2 kV; (iv) −2 to −1 kV; (v) −1000 to −900 V; (vi) −900 to −800 V; (vu) −800 to −700 V; (viii) −700 to −600 V; (ix) −600 to −500 V; (x) −500 to −400 V; (xi) −400 to −300 V; (xii) −300 to −200 V; (xiii) −200 to −100 V; (xiv) −100 to −90 V; (xv) −90 to −80 V; (xvi) −80 to −70 V; (xvu) −70 to −60 V; (xviii) −60 to −50 V; (xix) −50 to −40 V; (xx) −40 to −30 V; (xxi) −30 to −20 V; (xxii) −20 to −10 V; (xxiii) −10 to 0 V; (xxiv) 0-10 V; (xxv) 10-20 V; (xxii) 20-30 V; (xxvii) 30-40V; (xxviii) 40-50 V; (xxix) 50-60 V; (xxx) 60-70 V; (xxxi) 70-80 V; (xxxii) 80-90 V; (xxxiii) 90-100 V; (xxxiv) 100-200 V; (xxxv) 200-300 V; (xxxvi) 300-400 V; (xxxvii) 400-500 V; (xxxviii) 500-600 V; (xxxix) 600-700 V; (xi) 700-800 V; (xli) 800-900 V; (xlii) 900-1000 V; (xliii) 1-2 kV; (xliv) 2-3 kV; (xlv) 3-4 kV; and (xlvi) 4-5 kV. 
         [0053]    According to an embodiment the one or more targets are maintained, in use at a potential (i) −5 to −4 kV; (ii) −4 to −3 kV; (iii) −3 to −2 kV; (iv) −2 to −1 kV; (v) −1000 to −900 V; (vi) −900 to −800 V; (vii) −800 to −700 V; (viii) −700 to −600 V; (ix) −600 to −500 V; (x) −500 to −400 V; (xi) −400 to −300 V; (xii) −300 to −200 V; (xiii) −200 to −100 V; (xiv) −100 to −90 V; (xv) −90 to −80 V; (xvi) −80 to −70 V; (xiii) −70 to −60 V; (xviii) −60 to −50 V; (xix) −50 to −40 V; (xx) −40 to −30 V; (xxi) −30−20 V; (xxii) −20 to −10 V; (xxiii) −10 to 0 V (xxiv) 0-10 V; (xxv) 10-20 V; (xxvi) 20-30 V; (xxvii) 30-40V; (xxviii) 40-50 V; (xxix) 50-60 V; (xxx) 60-70 V, (xxxi) 70-80 V; (xxxii) 80-90 V; (xxxiii) 90-100 V; (xxxiv) 100-200 V; (xxxi) 200-300 V; (xxxvi) 300-400 V; (xxxvii) 400-500 V (xxxviii) 500-600 V; (xxxix) 600-700 V; (xi) 700-800 V; (xli) 800-900 V; (xlii) 900-1000 V; (xliii) 1-2 kV; (xliv) 2-3 kV; (xlv) 3-4 kV; and (xlvi) 4-5 kV; relative to the potential of an enclosure surrounding the ion source and/or an ion inlet device which leads to a first vacuum stage of a mass spectrometer and/or the one or more nebulisers. 
         [0054]    According to a preferred embodiment in a mode of operation the one or more targets are maintained at a positive potential and the droplets impacting upon the one or more targets form a plurality of positively charged ions. 
         [0055]    According to another preferred embodiment in a mode of operation the one or more targets are maintained at a negative potential and the droplets impacting upon the one or more targets form a plurality of negatively charged ions. 
         [0056]    The ion source may further comprise a device arranged and adapted to apply a sinusoidal or non-sinusoidal AC or RF voltage to the one or more targets. 
         [0057]    The one or more targets are preferably arranged or otherwise positioned so as to deflect the stream of droplets and/or the plurality of ions towards an ion inlet device of a mass spectrometer. 
         [0058]    The one or more targets are preferably positioned upstream of an ion inlet device of a mass spectrometer so that ions are deflected towards the direction of the ion inlet device. 
         [0059]    The one or more targets may comprise a stainless steel target, a metal, gold, a non-metallic substance, a semiconductor, a metal or other substance with a carbide coating, an insulator or a ceramic. 
         [0060]    The one or more targets may comprise a plurality of target elements so that droplets from the one or more nebulisers cascade upon a plurality of target elements and/or wherein the target is arranged to have multiple impact points so that droplets are ionised by multiple glancing deflections. 
         [0061]    The one or more targets may be shaped or have an aerodynamic profile so that gas flowing past the one or more targets is directed or deflected towards, parallel to, orthogonal to or away from an ion inlet device of a mass spectrometer. 
         [0062]    At least some or a majority of the plurality of ions may be arranged so as to become entrained, in use, in the gas flowing past the one or more targets. 
         [0063]    According to an embodiment in a mode of operation droplets from one or more reference or calibrant nebulisers are directed onto the one or more targets. 
         [0064]    According to an embodiment in a mode of operation droplets from one or more analyte nebulisers are directed onto the one or more targets. 
         [0065]    According to another aspect of the present invention there is provided a mass spectrometer comprising an ion source as described above. 
         [0066]    The mass spectrometer preferably further comprises an ion inlet device which leads to a first vacuum stage of the mass spectrometer. 
         [0067]    The ion inlet device preferably comprises an ion orifice, an ion inlet cone, an ion inlet capillary, an ion inlet heated capillary, an ion tunnel, an ion mobility spectrometer or separator, a differential ion mobility spectrometer, a Field Asymmetric Ion Mobility Spectrometer (“FAIMS”) device or other ion inlet. 
         [0068]    The one or more targets are preferably located at a first distance X 1  in a first direction from the ion inlet device and at a second distance Z 1  in a second direction from the ion inlet device, wherein the second direction is orthogonal to the first direction and wherein: 
         [0069]    (i) X 1  is selected from the group consisting of (i) 0-1 mm; (ii) 1-2 mm, (iii) 2-3 mm; (iv) 3-4 mm; (v) 4-5 mm; (vi) 5-6 mm; (vii) 6-7 mm; (viii) 7-8 mm; (ix) 8-9 mm; (x) 9-10 mm; and (xi) &gt;10 mm; and/or 
         [0070]    (ii) is selected from the group consisting of (1) 0-1 mm; (ii) 1-2 mm; (iii) 2-3 trim; (iv) 3-4 mm; (v) 4-5 mm; (vi) 5-6 mm; (vii) 6-7 mm; (viii) 7-8 mm; (ix) 8-9 mm; (x) 9-10 mm; and (xi) &gt;10 mm. 
         [0071]    The one or more targets are preferably positioned so as to deflect the stream of droplets and/or the plurality of ions towards the ion inlet device. 
         [0072]    The one or more targets are preferably positioned upstream of the ion inlet device. 
         [0073]    The one or more targets preferably comprise either: (i) one or more rods, or (u) one or more pins having a taper cone. 
         [0074]    The stream of droplets is preferably arranged to impact the one or more rods or the taper cone of the one or more pins either: (i) directly on the centerline of the one or more rods or pins or (ii) on the side of the one or more rods or the taper cone of the one or more pins which faces towards or away from the ion inlet orifice. 
         [0075]    The mass spectrometer may further comprise an enclosure enclosing the one or more nebulisers, the one or more targets and the ion inlet device. 
         [0076]    The mass spectrometer may further comprise one or more deflection or pusher electrodes, wherein in use one or more DC voltages or DC voltage pulses are applied to the one or more deflection or pusher electrodes in order to deflect or urge ions towards an ion inlet device of the mass spectrometer. 
         [0077]    According to an aspect of the present invention there is provided a method of ionising a sample comprising: 
         [0078]    causing a stream predominantly of droplets to impact upon one or more targets to ionise the droplets to form a plurality of analyte ions. 
         [0079]    According to an aspect of the present invention there is provided a method of mass spectrometry comprising a method of ionising ions as described above. 
         [0080]    According to an aspect of the present invention there is provided a mass spectrometer comprising: 
         [0081]    an ion source including: 
         [0082]    a target; and 
         [0083]    a nebuliser configured to emit, in use, a stream formed predominantly of droplets which are caused to impact upon the target and to ionise the droplets to form a plurality of ions. 
         [0084]    According to an aspect of the present invention there is provided an ion source comprising: 
         [0085]    a target; and 
         [0086]    a nebuliser configured to emit, in use; a stream formed predominantly of droplets which are caused to impact upon the target and to ionise the droplets to form a plurality of ions. 
         [0087]    According to an aspect of the present invention there is provided a method of mass spectrometry comprising: 
         [0088]    ionising a sample by generating a stream predominantly formed of droplets and ionising the droplets to form a plurality of ions by impacting the droplets upon one or more targets. 
         [0089]    According to an aspect of the present invention there is provided a method of ionising a sample comprising generating a stream predominantly fanned of droplets and ionising the droplets to form a plurality of ions by impacting the droplets upon one or more targets. 
         [0090]    According to an aspect of the present invention there is provided a desolvation device comprising: 
         [0091]    one or more nebulisers and one or more targets; 
         [0092]    wherein 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 cause said droplets to form desolvated gas phase molecules and/or secondary droplets. 
         [0093]    According to an aspect of the present invention there is provided a method of desolvation comprising: 
         [0094]    causing a stream predominantly of droplets to impact upon one or more targets and to cause said droplets to form desolvated gas phase molecules and/or secondary droplets. 
         [0095]    It will be apparent that the present invention extends beyond an ion source or method of ionising a sample to include apparatus and methods for at least partially desolvating or further desolvating a stream of droplets. The resulting gas phase molecules and/or secondary droplets may be subsequently ionised by a separate ion source. 
         [0096]    According to an aspect of the present invention there is provided a mass spectrometer comprising: 
         [0097]    a nebuliser comprising a first capillary tube and having an exit which emits, in use a stream of analyte droplets; and 
         [0098]    a target arranged &lt;10 mm from the exit of the nebuliser; 
         [0099]    characterised in that the mass spectrometer further comprises: 
         [0100]    a liquid chromatography separation device arranged and adapted to emit an eluent over a period of time; and 
         [0101]    an ion source arranged and adapted to ionise the eluent, the ion source comprising the nebuliser and wherein, in use, the stream of analyte droplets is caused to impact upon the target and to ionise the analyte to form a plurality of analyte ions. 
         [0102]    By way of contrast, the target of a SACI ion source is placed downstream of the ion inlet orifice of a mass spectrometer and ions are reflected back towards the ion inlet orifice. 
         [0103]    According to another aspect of the present invention there is provided a method of mass spectrometry comprising: 
         [0104]    providing a nebuliser comprising a first capillary tube and having an exit which emits a stream of analyte droplets; and 
         [0105]    positioning a target &lt;10 mm from the exit of the nebuliser; 
         [0106]    characterised in that the method further comprises: 
         [0107]    providing a liquid chromatography separation device which emits an eluent over a period of time; and 
         [0108]    ionising the eluent by causing the stream of analyte droplets to impact upon the target and to ionise the analyte to form a plurality of analyte ions. 
         [0109]    As discussed above, the spray point of a SACI ion source is within the heated nebuliser probe so that the typical distance between the spray point and a target plate is around 70 mm. By way of contrast, with the preferred impactor ion source the spray point is located at the tip of the inner capillary tube and the distance between the spray point and the target may be &lt;10 mm. 
         [0110]    It will be understood by those skilled in the art that a SACI ion source emits a vapour stream and the impact velocity of the vapour upon the target is relatively low and is approximately 4 nits. By way of contrast, the impactor ion source according to the preferred embodiment does not emit a vapour stream but instead emits a high density droplet stream. Furthermore, the impact velocity of the droplet stream upon the target is relatively high and is approximately 100 m/s. 
         [0111]    It will be apparent therefore that the ion source according to the present invention is quite distinct from known SACI ion sources. 
         [0112]    According to a preferred embodiment a liquid stream is preferably converted into a nebulised spray via a concentric flow of high velocity gas without the aid of a high potential difference at the sprayer or nebuliser tip. A micro target with comparable dimensions or impact zone to the droplet stream is preferably positioned in close proximity (e.g. &lt;5 mm) to the sprayer tip to define an impact zone and to partially deflect the spray towards the ion inlet orifice of the mass spectrometer. The resulting ions and charged droplets are sampled by the first vacuum stage of the mass spectrometer. 
         [0113]    According to the preferred embodiment the target preferably comprises a stainless steel target. However, other embodiments are contemplated wherein the target may comprise other metallic substances (e.g. gold) and non-metallic substances. Embodiments are contemplated, for example, wherein the target comprises a semiconductor, a metal or other substance with a carbide coating, an insulator or a ceramic. 
         [0114]    According to another embodiment the target may comprise a plurality of plates or target elements so that droplets from the nebuliser cascade upon a plurality of target plates or target elements. According to this embodiment there are preferably multiple impact points and droplets are ionised by multiple glancing deflections. 
         [0115]    From an API source perspective, the combination of a close-coupled impactor which also serves as a charged ionization surface provides the basis of a sensitive multimode ionization source. The spray tip and micro target are preferably configured in close proximity with a glancing impact geometry which results in increased spray flux at the target and significantly less beam divergence or reflected dispersion when compared to a known broad area SACI ion source. The preferred embodiment therefore provides a high sensitivity API source. 
         [0116]    The preferred embodiment comprises a multimode ion source which advantageously can ionize high and low polarity analytes at high efficiency without the need to switch hardware or tuning parameters. 
         [0117]    The droplets which impact the one or more targets are preferably uncharged. 
         [0118]    It will be apparent that the ion source and method of ionising ions according to the present invention is particularly advantageous compared with a known SACI ion source. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0119]    Various embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: 
           [0120]      FIG. 1  shows an impactor spray API ion source according to a preferred embodiment of the present invention; 
           [0121]      FIG. 2A  shows a plan view of a target and a first vacuum stage of a mass spectrometer according to a preferred embodiment of the present invention with the nebuliser omitted and  FIG. 2B  shows a side view of the nebuliser or sprayer tip, target and first vacuum stage of a mass spectrometer according to a preferred embodiment of the present invention; 
           [0122]      FIG. 3  shows a conventional APCI ion source with a corona discharge pin; 
           [0123]      FIG. 4  shows the relative intensities of five test analytes measured using a conventional Electrospray on source, a conventional APCI ion source and an impactor ion source according to the preferred embodiment; 
           [0124]      FIG. 5  shows the effect of target potential on ion signal according to a preferred embodiment of the present invention; 
           [0125]      FIG. 6A  shows a mass spectrum obtained from an impactor spray ion source according to a preferred embodiment of the present invention with a target potential of 2.2 kV,  FIG. 6B  shows a mass spectrum obtained from an impactor spray source according to an embodiment of the present invention with a target potential of 0 V and  FIG. 6C  shows a mass spectrum obtained with a conventional Electrospray ion source with an optimized capillary potential of 4 kV; 
           [0126]      FIG. 7  shows a known Surface Activated Chemical Ionisation ion source; 
           [0127]      FIG. 8  shows a comparison of the relative intensities obtained with a conventional SACI ion source and an impactor ion source spray according to a preferred embodiment; 
           [0128]      FIG. 9  shows data obtained from a Phase Doppler Anemometry analysis of the droplets emitted from a preferred nebuliser; and 
           [0129]      FIG. 10  shows a comparison of the radial distribution of the data rate for a pneumatic nebuliser according to an embodiment of the present invention and from a heated nebuliser such as used in a SACI ion source. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0130]      FIG. 1  shows a schematic of the general layout of an impactor spray API ion source according to an embodiment of the present invention. A flow of liquid containing analyte is arranged to enter a nebuliser or sprayer  1  and is delivered to the sprayer tip  2  via a liquid capillary tube  3 . The liquid capillary tube  3  is preferably surrounded by a second capillary  4  which preferably includes a gas inlet  5  to deliver a stream of high velocity gas to the exit of the liquid capillary tube  3 . According to an embodiment the inner diameter of the liquid capillary tube  3  is 130 μm and the outer diameter of the liquid capillary tube  3  is 270 μm. The inner diameter of the second (gas) capillary tube  4  is preferably 330 μm. This arrangement produces a nebulised spray which contains droplets with a typical diameter of 10-20 μm and which have velocities greater than 100 m/s at a close distance from the sprayer tip. 
         [0131]    The resulting droplets are preferably heated by an additional flow of gas that enters a concentric heater  6  via a second gas inlet  7 . The nebuliser or sprayer  1  may be hinged to the right hand side of the ion inlet cone  8  of a mass spectrometer so that it can swing to vary the horizontal distance between the sprayer tip and an ion inlet orifice  9 . The probe may also configured such that the vertical distance between the sprayer tip and the ion inlet orifice  9  can also be varied. A target  10  which preferably has a similar dimension to that of the liquid capillary tube  3  is placed between the sprayer tip and the ion inlet orifice  9 . The target  10  can preferably be manipulated in the x and y directions (in the horizontal plane) via a micro adjuster stage and is preferably held at a potential of 0-5 kV relative to a source enclosure  11  and the ion inlet orifice  9 . The ion inlet cone  8  is surrounded by a metal cone gas housing  12  that is preferably flushed with a low flow of nitrogen gas that enters via a gas inlet  13 . All gasses that enter the source enclosure preferably leave via a source enclosure exhaust  14  or the ion inlet orifice  9  which is pumped by the first vacuum stage  15  of the mass spectrometer. 
         [0132]      FIG. 2A  shows a schematic plan view of an embodiment of the present invention with the nebuliser or sprayer  1  omitted. A target  10  is located adjacent the first vacuum stage  15  of the mass spectrometer. According to an embodiment the target  10  may comprise a 0.8 ram diameter stainless steel pin which preferably incorporates a straight taper section over a distance of 5 mm. The pin is preferably positioned at a horizontal distance X 1  of 5 mm from the ion inlet orifice  9 . The pin  10  is preferably positioned such that the point of impact between the probe axis and the target  10  is on the side of the taper cone that faces the ion inlet orifice  9  as shown in  FIG. 2B . This position results in an optimized glancing angle of incidence shown as an arrowed line  16  in the end view schematic of  FIG. 2B ,  FIG. 2B  also shows the relative vertical positions of the nebuliser or probe  2  and target  10  according to the preferred embodiment i.e. Z=9 min and Z 2 =1.5 mm. The nebuliser or sprayer  2  is preferably maintained at 0 V, the target  10  is preferably held at 2.2 kV, the ion inlet cone is preferably held at 100 V, the cone gas housing is preferably held at 100 V and the heater assembly and source enclosure are preferably held at ground potential. The nitrogen nebuliser gas is preferably pressurized to 7 bar, the nitrogen heater gas flow is preferably pressurized to deliver 1200 L/hr and the nitrogen cone gas flow is preferably pressurized to deliver 150 L/hr. 
         [0133]    A series of tests were conducted to test the relative sensitivities of the preferred impactor spray source, a conventional ESI ion source and a conventional APCI ion source. 
         [0134]    The conventional ESI ion source was constructed by removing the target  10  and applying a potential of 2.5 kV directly to the sprayer tip. All other potentials and gas flows were maintained as above. 
         [0135]    The APCI ion source was constructed by replacing the nebuliser or sprayer  2  with a conventional heated nebuliser probe  17  as shown in  FIG. 3  as used in commercial APCI ion sources and adding a corona discharge pin  18 . The tip of the corona discharge pin  18  was located at a distance X=7 mm and Z=5.5 mm as shown in  FIG. 3 . The APCI ion source probe was operated at 550° C., the heater gas was unheated at a flow rate of 500 L/hr and the corona discharge pin  18  was set at a current of 5 μA. All other settings were as described above. 
         [0136]    A test solution was prepared consisting of 70/30 acetonitrile/water and containing sulphadimethoxine (10 pg/μL), verapamil (10 pg/μL), erythromycin (10 pg/μL), cholesterol (10 ng/μL) and cyclosporin (100 pg/μL). The test solution was infused at a flow rate of 15 μL/min into a carrier liquid flow of 0.6 mL/min of 70/30 acetonitrile/water which was then sampled by the three different API ion sources. 
         [0137]      FIG. 4  shows the relative signal intensities obtained for the five test analytes with a conventional Electrospray ion source, a conventional APCI inn source and an impactor ion source according to the preferred embodiment. For each analyte the signal intensity was monitored for the protonated molecule ([M+H] + ). However, owing to signal saturation with the preferred impactor spray, the cholesterol signal was measured on the carbon-13 isotope of the [M+H] + ) ion. From this figure, it is clear that although the APCI ion source has some advantages over ESI ion sources (e.g. for non-polar analytes such as cholesterol), ESI is generally the more sensitive of these two techniques. It is also clear that the preferred impactor spray source gives rise to significantly greater signal intensities than either the ESI or APCI ion source for all compound types. 
         [0138]    In API ion sources that utilize the SACI ionization technique, a broad area target is maintained at an elevated potential to optimize ion signal.  FIG. 5  shows the effect of varying the target potential on the resulting ion signal for the preferred impactor spray source where the same test mixture was analysed with a target potential of 2.2 kV followed by a target potential of 0 kV. In contrast to SACI, it is apparent that an elevated target potential, although advantageous, is not essential to the ionization process. By contrast, a broad area SACI source would lose &gt;90% of the ion signal under the same experimental conditions (data not shown). 
         [0139]    Although not essential, an elevated target potential is nonetheless advantageous and has the result of improving the qualitative aspects of mass spectral data. To illustrate this,  FIG. 6A  shows a mass spectrum obtained from an impactor ion source according to the preferred embodiment with a target potential of 2.2 kV,  FIG. 6B  shows a mass spectrum obtained from an impactor ion source according to an embodiment with a target potential of 0 V and  FIG. 6C  shows a mass spectrum obtained with a conventional electrospray source with an optimized capillary potential of 4 kV. The mass spectra shown in  FIGS. 6A and 6B  which were obtained using an ion source according to the preferred embodiment are shown to produce more analyte ions than ESI but significantly an elevated target potential also reduces the susceptibility to ion adduct formation ([M+Na] +  and [M+K] + ) such that the protonated molecule ([M+H] + ) is the base peak only for the mass spectrum shown in  FIG. 6A . 
         [0140]    An experiment was conducted to compare the sensitivity of the impactor ion source according to the preferred embodiment with a SACI-type ionization source.  FIG. 7  shows a schematic of the SACI ion source which was used. The SACI ion source was constructed by replacing the impactor pin target  10  with a 0.15 mm thick rectangular tin sheet  19  which measured approximately 30 mm×15 turn. The sheet target  19  was angled at approximately 30° to horizontal and was positioned such that the point of intersection between the nebuliser or probe  2  axis and the target  19  was at X=4 min and Z=4 mm. The SACI ion source was optimised at a nebuliser or sprayer potential of 0 V and a target potential of 1 kV. All other gas flows and voltages were as described for the preferred impactor spray source. 
         [0141]      FIG. 8  compares the relative signal intensities obtained with a SACI ion source and an impactor ion source according to the preferred embodiment. It is observed that the preferred impactor spray ion source is typically between ×5-10 more sensitive than the broad area SACI ion source. 
         [0142]    Further embodiments are contemplated wherein the performance of the preferred impactor ion source may be further improved by positioning a central wire in the bore of the liquid capillary tube  3 . Video photography has shown that the central wire focuses the droplet stream such that the target may be placed at the focal point to further increase the droplet flux density. The position of the focal point is comparable to the sprayer tip/target distance used in the preferred embodiment (1-2 mm). 
         [0143]    As described above a SACI ion source converts a liquid stream into a vapour stream that then impinges on a broad area target. Experiments on SACT (Cristoni et al., J. Mass Spectrom., 2005, 40, 1550) have shown that ionisation occurs as a result of the interaction, of neutral analyte molecules in the gas phase with the proton rich surface of the broad area target. Furthermore, there is a linear relationship between ionisation efficiency and target area within the range 1-4 cm 2 . 
         [0144]    In contrast to SACI, the preferred ion source uses a streamlined target to intercept a high velocity stream of liquid droplets which results in a secondary stream consisting of secondary droplets, gas phase neutrals and ions. 
         [0145]    A pneumatic nebuliser according to an embodiment of the present invention was investigated further. The nebuliser comprised an inner liquid capillary with an internal diameter of 127 μm and an outer diameter of 230 μm. The inner liquid capillary was surrounded by a gas capillary with an internal diameter of 330 μm that was pressurised to 7 bar. 
         [0146]      FIG. 9  shows typical data obtained from a Phase Doppler Anemometry (“PDA”) analysis of the preferred nebuliser for a 1 mL/min liquid flow consisting of 90% water/10% methanol and a nitrogen nebuliser gas. 
         [0147]    The PDA sampling point was scanned radially across the spray (probe axis=0) at an axial distance of 5 mm from the spray point i.e. equivalent to the typical nebuliser/target distance according to the preferred embodiment.  FIG. 9  shows that the nebuliser typically produces liquid droplets with a Sauter mean diameter (d 32 ) in the range 13-20 μm with mean axial velocities in excess of 100 ms −1 . 
         [0148]      FIG. 9  also shows that the very high velocity droplets are well collimated and are typically confined within a radius of 1 mm from the probe axis. 
         [0149]    The upper trace of  FIG. 10  shows the radial distribution of the data rate N/i (number of validated samples per unit time) for the preferred pneumatic nebuliser and experimental conditions as described above. This logrithmic plot demonstrates that the spray is well collimated with greater than two thirds of the total droplet mass being confined to a radius of 1 mm from the probe axis. The lower trace of  FIG. 10  shows the equivalent NIT distribution from a heated nebuliser such as used in a conventional SACI source. The heated nebuliser consists of a pneumatic nebuliser which sprays into a 90 mm long cylindrical tube with a 4 mm diameter bore (tube temperature 600° C.). The N/T data for this nebuliser was obtained at an axial distance of 7 mm from the exit end of the heated tube. It is important to note that the N/Ts for the few detected droplets from the heated nebuliser (d 32  was typically 14 μm, data not shown) are typically three orders of magnitude lower than those obtained from the pneumatic nebuliser according to the preferred embodiment. This is a due to the fact that the overwhelming mass of the liquid is vaporised in the SACI-type heated nebuliser resulting in a stream of vapour that contains a very low number density of surviving droplets. 
         [0150]    Accordingly, a known SACI ion source should be construed as comprising a nebuliser which emits a stream predominantly of vapour and hence a SACT ion source should be understood as not falling within the scope of the present invention. 
         [0151]    Referring to the data presented in  FIGS. 9 and 10  it can be assumed that the physical model of the ion source according to the preferred embodiment is dominated by the impact of high velocity liquid droplets on a target that is indirectly heated by the source heater. Such impact effects give rise to the formation of secondary droplets, where the nature of the droplet breakup is determined by the Weber number W e  which is given by the following: 
         [0000]        W   e   =ρU   2   d/σ   (1)
 
         [0000]    wherein ρ is the droplet density, U is the droplet velocity, d is the droplet diameter and σ is the droplet surface tension. 
         [0152]    If it is assumed that the water droplets are at 40° C., the nitrogen gas environment is at 100° C., d=18 μm and U=50 ms −1  then a value of W e =640 is obtained for the droplets according to the preferred embodiment. It has been shown (in the literature) that the number of reatomised water droplets increases linearly with W e  in the range 50-750 for impact on a heated steel target for temperatures between 260-400° C. At W e =750, a single droplet typically gave rise to 40 secondary droplets. 
         [0153]    It is apparent, therefore, that the impactor target leads to significant droplet breakup to produce a secondary stream that consists of charged droplets, neutrals, ions and clusters. 
         [0154]    The impact efficiency of the system will be largely governed by the Stokes number S k  where: 
         [0000]        S   k   =ρd   2   U/ 18  μa   (2)
 
         [0000]    wherein ρ is the droplet density, d is the droplet diameter, U is the droplet velocity, μ is the gas viscosity and a is the characteristic dimension of the target. 
         [0155]    Impact efficiency increases with increasing S k  and thus favours large droplets with high velocity and a small target diameter. Thus for the preferred impactor spray conditions described above, it may be expected that S k  has a typical value of 30. 
         [0156]    For S k &gt;&gt;1 droplets are highly likely to deviate from the flow streamlines and impact upon the target. In contrast, if the target dimension is increased by an order of magnitude and the velocity is decreased by an order of magnitude (i.e. similar conditions to SACI), then the value of S k  drops to 0.3 at which point the droplets are more likely to follow the gas flow around the target. The impact efficiency is also known to increase with reducing Reynolds numbers which will further favour the streamlined nature of the impactor spray target according to the preferred embodiment. 
         [0157]    The shape of the secondary stream will be governed by the gas flow dynamics and, in particular, the Reynolds number (R e ) which is given by 
         [0000]        R   e   =ρvL/μ   (3)
 
         [0000]    wherein ρ is the gas density, v is the gas velocity, μ is the gas viscosity and L is the significant dimension of the target. 
         [0158]    With a 1 mm diameter impactor target, a gas velocity of 50 ms −1  and nitrogen gas at 100° C. then a value of R e =3000 is obtained. 
         [0159]    Reynolds numbers in the range 2000-3000 generally correspond to the transition region from laminar to turbulent flow. Therefore, it can be expected that the wake from the target contains some turbulence and eddy features. However, severe turbulence that could hinder the sampling of ions or droplets at the ion inlet cone is not expected. 
         [0160]    The preferred ion source can be tuned by swinging the nebuliser to move the impact zone from one side of the rod-like target to the other. This results in changes to the wake which can be visually observed by strong illumination of the secondary droplet stream. Other embodiments are therefore also contemplated wherein similar source optimisation could be achieved with a centralised impact zone and a non-symmetric target cross section e.g. (the profile of) an aircraft wing. 
         [0161]    Although the present invention has been described with reference to preferred embodiments it will be apparent to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the accompanying claims.