Sampler cones for plasma interfaces

Abstract

A sampler cone 4 comprising a hollow truncated cone section with a base 40, an end 41, a longitudinal axis 7 that extends from base to end, and a wall including a main wall section 42 extending at an

Description

Field The invention relates to sampler cones for plasma interfaces, also known as samplers. In particular, the invention relates to samplers for plasma interfaces in analytical instruments, such as mass spectrometers. Typical plasma interfaces in mass spectrometers are atmosphere-to-vacuum interfaces. Background So-called samplers or sampler cones are used in plasma interfaces, for example in mass spectrometers. A portion of a plasma into which an analyte may be introduced passes through a first interface part, comprising the sampler, and a second interface part, comprising a skimmer cone (also known as a skimmer). Each of the cones defines a small openingthat allows a portion of the plasma to pass through while maintaining a pressure difference between the exterior and the interior of the plasma interface. In typical applications, the plasma is at atmospheric pressure while the analytical instrument operates at very low pressures such as ultra-high vacuum. United States patent US 10,998,180 (Thermo Fisher), which is hereby incorporated by reference in this document, discloses a plasma sampling interface for an inductively coupled plasma (ICP) mass spectrometer. The plasma sampling interface comprises a conical sampler and a conical skimmer. Samplers are typically made of metal, for example nickel, aluminium or platinum. This has the disadvantage that elemental background signals are often elevated due to contamination of the metal surface or of the metal material itself. In addition, deposits easily form on the sampler cone. When deposits detach during a measurement, they can disturb the measurement results, resulting in inaccurate or incorrect measurement results. Summary Accordingly, the present invention provides a samplerfor a plasma interface, wherein the sampler comprises a hollow truncated cone section with a base, an end and a longitudinal axis defining an axial direction that extends from the base to the end. The truncated cone section comprises a wall including a main wall section extending at an acute angle relative to the longitudinal axis and having an internal surface that defines a substantially conical interior and an external surface that defines a substantially conical exterior. The wall also includes an end wall section spanning from the internal surface to the external surface and defining an aperture at the end of the truncated cone. The end wall section comprises a bore section that extends from the internal wall section and that extends in the axial direction to be substantially parallel to the longitudinal axis. The end wall section also comprises a rim section that extends from the external wall section and that comprises a substantially flat surface aligned substantially transversely to the longitudinal axis. A funnel section of the end wall section is positioned between the bore section and the rim section that extends in the axial direction at an acute angle relative to the longitudinal axis. The main wall may extend from the base at a constant angle, or may have a varying angle to provide a curved, kinked or stepped cone shape. Although the sampler may have two or more orifices, it is preferred that it has a single orifice. Such a design provides a simple yet effective structure. The single orifice is preferably centrally located in the sampler. The funnel section may comprise only a single section that extends in the axial direction at an acute angle relative to the longitudinal axis. Alternatively, the funnel section may comprise a plurality of sections that each extend in the axial direction at an respective acute angle relative to the longitudinal axis, wherein the acute angle of successive sections of the plurality of sections progressively increases from the section of the plurality of sections adjacent the internal wall section to the section of the plurality of sections adjacent the external wall section. The sampler may have a diameter of between 20 mm and 100 mm, preferably between 25 mm and 75 mm, more preferably between 40 mm and 60 mm, still more preferably about 50 mm. The bore section may extend in the axial direction for a length between 0.05 mm and 2 mm, preferably between 0.1 mm and 1 mm and most preferably about 0.2 mm. A rather short length of 0.2 mm has been found a good compromise between ease of manufacturing and performance. The bore section defines the width (or diameter) of the aperture and the width (or diameter) may be between 0.75 mm and 1.5 mm, preferably between 1.0 mm and 1.25 mm and most preferably about 1.1 mm. The substantially flat surface of the rim section encircles the funnel section and may have a width between 0.25 mm and 1.0 mm, preferably between 0.4 mm and 0.75 mm and most preferably about 0.5 mm. The funnel section may extend in the axial direction for a length between 0.05 mm and 0.5 mm, preferably between 0.2 mm and 0.4 mm and most preferably about 0.3 mm. The funnel section may extend transversely to the longitudinal axis for a length between 0.25 mm and 1.0 mm, preferably between 0.4 mm and 0.75 mm and most preferably about 0.5 mm. The main wall section defines an internal angle of the truncated cone section which may be between 60° and 80°, preferably between 65° and 75° and most preferably about 70°. The sampler may be produced by machining, and may be produced by machining followed by grinding. The orifice may be provided by drilling. There is also provided a plasma interface comprising any of the samplers described above. The plasma interface may further comprise a skimmer. The skimmer may comprise a base section and a cone section protrudingfrom the base section, the cone section having a substantially conical interior and a substantially conical exterior with a top area in which an orifice is provided. The skimmer cone may be made of silicon, preferably silicon with a purity of at least 99.9% purity, more preferably at least 99.9999%. The top area of the skimmer may be substantially flat and may have a diameter of at least 1 mm, preferably at least 2 mm. The substantially flat top area may define a shoulder at the interior of the cone section. The shoulder may have a width between 0.1 mm and 3 mm, preferably between 0.2 mm and 1.5 mm, more preferably between 0.3 mm and 0.5 mm. The skimmer orifice may have a diameter of between 0.5 mm and 2.0 mm, preferably between 0.6 mm and 1.2 mm, more preferably between 0.7 mm and 1.0 mm. The skimmer may have a diameter of between 10 mm and 50 mm, preferably between 20 mm and 25 mm, more preferably between 21 mm and 22 mm. The cone section of the skimmer may define an angle between 30° and 80°, preferably between 45° and 70°, more preferably approximately 60°. The skimmer may be produced by machining, or may be produced by machining followed by grinding. The orifice may be provided by drilling. There is also provided a mass spectrometer comprising any of the samplers described above. The mass spectrometer may further comprise a skimmer and, optionally, a plasma source operable to direct a plasma towards the sampler and skimmer, preferably an inductively coupled plasma source. Brief description of the drawings Fig. 1 schematically shows a plasma interface accordingto the prior art. Fig. 2 shows a cross-sectional view of a sampler accordingto the prior art. Fig. 3 is a detail of the tip of the sampler of Fig. 2. Fig. 4 shows a cross-sectional view of a sampler according to the present invention. Fig. 5 is a detail of the tip of the sampler of Fig. 4. Fig. 6 shows a further cross-sectional view of a sampler according to the present invention. Fig. 7 shows the build-up of matrix deposits on the tip of a sampler according to the prior art. Fig. 8 shows the build-up of matrix deposits on the tip of a sampler according to the present invention. Fig. 9 is a schematic to show ease of cleaning the tip of the sampler of the present invention. Fig. 10 shows a cross-sectional view of a plasma interface according to the present invention. Fig. 11A schematically shows a top view of the skimmer of Fig. 10. Fig. 11B schematically shows a cross-sectional view of the skimmer of Fig. 10. Fig. 12 schematically shows an enlarged version of the cross-sectional view of Fig. 11B. Detailed description of the drawings Fig. 1 shows a plasma source 10 as disclosed in the above-mentioned US Patent No. 10,998,180. The plasma source 10 comprises a plasma torch 1 and a plasma interface 3. The plasma source 1 includes an RF coil 2 which operates to form an inductively coupled plasma (ICP) source 10. The plasma torch 1 is shown to consist of three concentric tubes 11,12 and 13, typically made from quartz. A gas from which the plasma is to be formed (argon in this case, as is typical) is passed between the outer and middle tubes 11 and 12, with an auxiliary gas being supplied between the middle tube 12 and a sample tube 13. A sample to be analyzed can be provided in a carrier gas through the innermost sample tube 13. The plasma torch 1 is placed centrally in the RF coil 2, about 1 -2 cm from the plasma interface 3. The RF coil 2 illustrated in Fig. 1 has three windings 21. A radio frequency (RF) generator (not shown) provides RF power (typically 500 to 1500 W) to the RF coil 2 that generates an intense electromagnetic field near the end of the plasma torch 1 adjacent the plasma interface 3. As argon gas (or another suitable gas) flows through the plasma torch 1, high-voltage sparks are applied to the gas, which causes stripping of electrons from argon atoms and a cascading reaction in which free electrons collide with other argon atoms, stripping still more electrons. The result is a breakdown of the argon atoms into a plasma 80 of ions and electrons. This process is sustained by the continuing transfer of RF energy from the RF coil 2 to the plasma torch 1. The sample is delivered through the innermost tube 13 into the plasma 80. The very high temperature of the plasma, typically 5,000 K to 10,000 K, results in a series of chemical changes, starting with desolvation of the sample (typically provided as an aerosol), followed by gas formation and formation of charged sample ions through the collision of high-energy electrons and argon ions with ground-state atomic species. The arrow A in Fig. 1 indicates the flow of plasma gas 80 that is generated in the plasma torch 1 towards the plasma interface 3. The plasma interface 3 consists of a housing 31 defining an internal chamber 35 which is pumped by a vacuum pump (not shown) via a port 32. Ions from the plasma 80 enter the chamber 35 through a sampler 4 and exit through a skimmer 5. The sampler 4 is typically a conical structure having a small entrance aperture or orifice 41 provided at the apex of the cone (hence, strictly speaking, forming a frustoconical shape to the entrance aperture 41). The entrance aperture 41 has an internal diameter that is typically about 0.8 mm to 1.5 mm (or 1.0 mm to 1.2 mm). The skimmer 5 is typically also a conical structure with an aperture or orifice 51 provided at its apex. The aperture 51 has a diameterthat is typically about 0.4 mm to 0.8 mm (or 0.4 mm to 0.8 mm). An ion guide 90 may be provided downstream of the plasma interface 3 to guide ions exiting the plasma interface 3 towards a mass analyser (not shown), where the mass to charge ratio of the ions may be determined, for example. Samplers 4 according to the prior art are made of metal, for example copper, nickel, aluminium or platinum, or combinations of these or similar metals. Metal may be suitable to withstand the high temperatures involved, but deposits can form on the metal surface. According, the present invention provides a sampler 4 on which deposits are less likely to form on a surface that changes the gas flow. A prior art sampler 4 is shown in Figs. 2 and 3, where Fig. 3 is a detail of the orifice 41 of the sampler 4. The prior art sampler 4 has a substantially round and flat base 40 from which a conical section 42 protrudes. The conical section 42 is stepped, with a relatively flat mid section 43 and a relatively steeper tip section 44. The orifice 41 is formed by drilling straight through the sampler 4 along its longitudinal axis 7, thereby forming a sharp tip 45 to the sampler 4. The wall thickness of the base 40 and mid section 43 is constant, whereas the wall thickness of the tip section 44 tapers to be thinner at the orifice 41. An embodiment of a sampler 4 according to the present invention is shown in Figs. 4 to 6, where Figs. 5 and 6 are details of the orifice 41 of the sampler 4. Similar to the prior art sampler 4, the new sampler 4 has a substantially round and flat base 40 from which a conical section 42 protrudes, and the conical section 42 has a relatively flat mid section 43 and a relatively steeper tip section 44. In this embodiment of the new sampler 4, the wall thickness is the same (or substantially the same) for the base 40, the mid section 43 and the tip section 44. The tip section 44 has, in the embodiment shown, an inside angle 0 of 70° (or approximately 70°), as shown in Fig. 6. When compared to the sharp tip 45 of the prior art sampler 4, the new sampler 4 has a differently-shaped end to the tip section 44 where it forms the orifice 41. The new sampler 4 has a substantially flat top area that forms a rim section 46, from which the aperture 41 extends into the sampler 4. The rim section 46 is flat and lies in a plane aligned transversely (or substantially transversely) to the longitudinal axis 7. A funnel section 47 extends from the rim section inwardly to form an entrance portion of the aperture 41 that narrows as it extends into the sampler 4. Hence, the funnel section 47 extends at an acute angle relative to the longitudinal axis 7. The funnel section 47 is followed by a bore section 48 that defines the narrowest part of the aperture 41. The bore section 48 forms a circular wall that extends in the axial direction to be parallel (or substantially parallel) to the longitudinal axis 7. The bore section 48 ends where it meets the internal surface of the tip section 44 that forms the inside angle 0 of 70° (or approximately 70°). It can also be seen that the sampler cone 4 shown has no sharp corners, as all outside corners have angles of at least 45°. Hence, the sharp tip 45 of the prior art sampler4 is avoided. The present invention alleviates problems with cleaning samplers 4 from sample-matrix related deposits. Conventional cleaning procedures use multiple steps of ultrasonication in de-ionised water (DIW), wiping with cotton swabs, soakingthe tip with dilute nitric acid (2% HNO3), and rinsing. This process is time consuming and the success is dependent on the nature and solubility of the deposits. Due to poor accessibility of the most critical central orifice 41 in prior art samplers 4, insufficient cleaning can occur and remain undetected. The funnel shape of the tip of the sampler 4 according to the present invention has been introduced in order to act as the main deposition area of matrix deposits, moving this area geometrically away from the bore of the aperture 41 defined by the bore section 48, as shown in Figs. 8 and 9. This clogging effect is known as one of the largest problems with conventional samplers 4. With samplers 4 according to the present invention, the overall shape of the tip is maintained to a large extent even with deposits. Fig. 6 shows the sampler tip section 44 of Fig. 5 in more detail. The bore section 48 may extend in the axial direction for a length L between 0.05 mm and 2 mm, preferably between 0.1 mm and 1 mm and most preferably about 0.2 mm. The bore section 48 defines the width WB (or diameter) of the aperture and the width WB (or diameter) may be between 0.75 mm and 1.5 mm, preferably between 1.0 mm and 1.25 mm and most preferably about 1.1 mm. The substantially flat surface of the rim section 46 encircles the funnel section 47. The rim section 46 may extend transversely to the longitudinal axis for a distance Dr between 0.25 mm and 1.0 mm, preferably between 0.4 mm and 0.75 mm and most preferably about 0.5 mm. The funnel section 47 may extend in the axial direction for a length LF between 0.05 mm and 0.5 mm, preferably between 0.2 mm and 0.4 mm and most preferably about 0.3 mm. The funnel section may extend transversely to the longitudinal axis for a length WF between 0.25 mm and 1.0 mm, preferably between 0.4 mm and 0.75 mm and most preferably about 0.5 mm. The funnel section 47 for an outside angle ¢) of between 100° and 135°, preferably between 110° and 125° and most preferably about 118°. Fig. 7 shows matrix deposits that typically form on a prior art sampler 4. The majority of the matrix deposits form close to the orifice 41, as indicated by arrow B, and results in the shape of tip being strongly altered, thus changing the properties of the sampler, as indicated by the arrow C. Fig. 8 shows matrix deposits that typically form on a sampler 4 according to the present invention. Only a small amount of the matrix deposits form at the bore section 48, as indicated by arrow D, whilst the majority forms outside the orifice 41, as indicated by arrow E, thereby preserving overall shape of the tip especially at the orifice 41. An improved distribution pattern of matrix deposits results since most material appears to be deposited rather uniformly around the larger surface area provided by the rim portion 46 and funnel portion 47. It has been found that the tip shape is largely mirrored by the deposition layer, leaving the overall flow pattern of ionized gas through the orifice 41 intact. Since any sample analysis involving high-matrix loads will lead to deposition of solid material on the sampler 4, the present invention is particularly advantageous for applications involving high matrix concentrations of e.g. nickel, precious metals or silicon. Moreover, the tip shape of the sampler 4 according to the present invention has been designed to allow it to be cleaned quickly and easily by mechanical means without damaging the sampler 4 and related deterioration of analytical performance. While conventional samplers 4 need to be treated carefully to avoid damagingthe sharp tip 45, samplers 4 according to the present invention allow intense mechanical cleaning without significant changes to the geometry of the tip. This enables the removal of the sample matrix from the sampler 4 while still maintaining the analytical performance. The funnel shape of the tip facilitates cleaning with commercially available tools such as a rotating brush. Fig. 9 shows schematically cleaning of matrix deposits by a rotating brush from the front side. The brush conforms well to the shape of the orifice 41 ensuring the orifice 41 is cleaned well duringthis procedure. The present invention also benefits analysis of high matrix samples where deposits at the sampler 4 cause analytically unfavourable effects by clogging the aperture 41 which produces signal suppression and signal drifts. These problems can be mitigated using dilution techniques, e.g. using an aqueous or gas dilution, but at the cost of detection capabilities. For full detection power with high sample matrix loads, a compromise between cleaning frequency and signal stability needs to be found. Repetitive cleaning of the sampler 4, especially for hard to remove deposits (e.g. metal deposits), requires frequent re-tuning of the ion coupled plasma mass spectrometer (ICP-MS) and can cause a short life-time of the sampler4. The shape of the sampler 4 has been experimentally optimized to provide good transmission through the sampler 4 resulting in signal levels higher than for conventional cones. A main analytical effect of samplers 4 according to the present invention is an improvement of the long-term drift when analysing high matrix solutions. Further benefits to matrix performance have been found when using samplers 4 according to the present invention, which is thought to arise as a result of: • good long-term behaviour as absolute count rate and as internal standard normalized signal; • uniform signal suppression over a given mass range; • high sensitivity in matrix; • low mass dependence in matrix; and • quick conditioning. The sampler may have a longer lifetime due to the tip geometry, leading to stable tune conditions overtime. Asamplerhavingthe described shape can be cleaned by polishingtechniques: sample deposits can be removed, and the surface can be cleaned, by polishing, without modification of the tip characteristics. Sharp-edged cones are generally not polished at the tip, occasionally just at the inside or in the vicinity of the tip outside, to avoid damage to the relatively sharp and fragile tip geometry, which can cause a loss of performance. Fig. 10 shows a plasma interface 3 comprising a sampler 4 according to the present invention as well as a skimmer 5. The skimmer 5 may be a skimmer as disclosed in international patent application WO 2023 / 117760, the entire contents of which are herewith incorporated by reference in this document. As discussed in WO 2023 / 117760, the skimmer may be made of metal or silicon, for example. An example of a skimmer 5 for use with the present invention is shown in Figs. 11A, 11B and 12, where Fig. 11A shows a top view and Fig. 11B shows a cross-sectional viewalongthe line A-A. Fig. 12 shows an enlarged version of the cross-sectional view along the line A-A. The skimmer 5 of Figs. 11A and 11B has a substantially round and flat base 50 from which a conical section 52 protrudes. The conical section 52 has a substantially flat top area 53, in which an aperture 51 is arranged. The conical section 52 is hollow and has, in the example shown, a smaller wall thickness than the base 50. The conical section 52 has, in the embodiment shown, an inside angle (between the lines S1 and S2 in Fig. 11B) of approximately 60°. As can be seen in Fig. 11B, the skimmer 5 shown has no sharp corners, as all corners have angles of at least 45°. In some examples, these corners can be curved instead of having obtuse angles. In the example shown, only the orifice 51 has angles of approximately 90°, due to the orifice 51 being produced by drilling. In other examples, however, at least some of the angles of the edges of the orifice 51 may also be obtuse. The relatively flat top area 53 at the outside of the skimmer 5 corresponds with a flat area on the inside, resulting in a shoulder 55 in the interior of the skimmer 5. The resulting frustoconical shape further reduces the amount of contaminations in the skimmed plasma, as contaminations will tend to accumulate atthe shoulder55. The top area 53 has a diameter d (see also Fig. 12) of at least 1 mm, preferably at least 2 mm, and typically not more than for example 5 mm. As shown in Fig. 11, plasma 60 passes through the orifice 51 of the skimmer 5. Due to the frustoconical shape of the skimmer 5, any deposits will form mainly in the interior of the skimmer 5, on the shoulder 55 to the side of the orifice 51. At this location, it is very unlikely that deposits will be re-ionized. In addition, the geometry of the skimmer 5 is not altered by the deposits. As a result, ion transmission through the skimmer 5 into the mass spectrometer will remain substantially constant. The skimmer may be made of silicon carbide (SiC), allowing mechanical cleaning. It will be understood by those skilled in the art that the invention is not limited to the embodiments shown and that many additions and modification may be made without departing from the scope of the invention as defined in the appending claims. For example, the dimensions of the sampler 4 maybe chosen in dependence on the particular application, such as on the dimensions of the mass spectrometer interface in which the sampler 4 is used. In the embodiment of Figs. 4 to 7, the base section 40, the mid section 43 and the tip section 44 of the sampler 4 are formed integrally. However, the base section 50 and the conical section 52 (comprisingthe mid section 43 and top section 44) may be constituted by separate parts which are joined after being produced separately. In addition or in the alternative, the mid section 43 and top section 44 may be constituted by separate parts which are joined after being produced separately. As can be seen, the sampler cone 4 shown has no sharp corners, as all corners have angles of at least 45°. In some embodiments, these corners can be curved instead of having obtuse angles although rounded surfaces are generally avoided to facilitate the necessary machining of hard and / or brittle materials via abrasive techniques (and also to allow simple and reliably cleaning with tools, e.g., rotating brushes).

Claims

1. A sampler for a plasma interface, wherein the sampler comprises a hollow truncated cone section with a base, an end and a longitudinal axis defining an axial direction that extends from the base to the end, wherein the truncated cone section comprises a wall including:a main wall section extending at an acute angle relative to the longitudinal axis and having an internal surface that defines a substantially conical interior and an external surface that defines a substantially conical exterior; andan end wall section spanning from the internal surface to the external surface and definingan aperture atthe end of the truncated cone; andwherein the end wall section comprises:a bore section that extends from the internal wall section and that extends in the axial direction to be substantially parallel to the longitudinal axis;a rim section that extends from the external wall section and that comprises a substantially flat surface aligned substantially transversely to the longitudinal axis; anda funnel section positioned between the bore section and the rim section that extends in the axial direction at an acute angle relative to the longitudinal axis.

2. The sampler of claim 1, wherein:the funnel section comprises only a single section that extends in the axial direction at an acute angle relative to the longitudinal axis; orthe funnel section comprises a plurality of sections that each extend in the axial direction at an respective acute angle relative to the longitudinal axis, wherein the acute angle of successive sections of the plurality of sections progressively increases from the section of the plurality of sections adjacent the internal wall section to the section of the plurality of sections adjacent the external wall section.

3. The sampler of either preceding claim, wherein:the bore section extends in the axial direction for a length between 0.05 mm and 2 mm, preferably between 0.1 mm and 1 mm and most preferably about 0.2 mm;and / orthe bore section defines the width of the aperture and the width is between 0.75 mm and 1.5 mm, preferably between 1.0 mm and 1.25 mm and most preferably about 1.1 mm.

4. The sampler of any preceding claim, wherein the substantially flat surface of the rim section encircles the funnel section with a width between 0.25 mm and 1.0 mm, preferably between 0.4 mm and 0.75 mm and most preferably about 0.5 mm.

5. The sampler of any preceding claim, wherein the funnel section extends: in the axial direction for a length between 0.05 mm and 0.5 mm, preferably between 0.2 mm and 0.4 mm and most preferably about 0.3 mm; and / ortransversely to the longitudinal axis for a length between 0.25 mm and 1.0 mm, preferably between 0.4 mm and 0.75 mm and most preferably about 0.5 mm.

6. The sampler of any preceding claim, wherein the main wall section defines an internal angle of the truncated cone section between 60° and 80°, preferably between 65° and 75° and most preferably about 70°.

7. The sampler of any preceding claim, wherein the sampler has a diameterof between 20 mm and 100 mm, preferably between 25 mm and 75 mm, more preferably between 40 mm and 60 mm, still more preferably about 50 mm.

8. The sampler according to any of the preceding claims, which is produced by machining, optionally followed by grinding, and / or the aperture is provided by drilling.

9. A plasma interface comprising a sampler according to any of the preceding claims and a skimmer.

10. The plasma interface of claim 9, wherein the skimmer comprises a base section and a cone section protruding from the base section, the cone section having asubstantially conical interior and a substantially conical exterior with a top area in which an orifice is provided, wherein the top area is substantially flat, and, optionally, the substantially flat top area has a diameter of at least 1 mm, preferably at least 2 mm.

11. The plasma interface according to claim 10, wherein the substantially flat top area defines a shoulder at the interior of the cone section and, optionally, the shoulder has a width between 0.1 mm and 3 mm, preferably between 0.2 mm and 1.5 mm, more preferably between 0.3 mm and 0.5 mm.

12. The plasma interface according to any preceding claim, wherein the orifice has a diameter of between 0.5 mm and 2.0 mm, preferably between 0.6 mm and 1.2 mm, more preferably between 0.7 mm and 1.0 mm.

13. The plasma interface according to any preceding claim, wherein:the skimmer has a diameter of between 10 mm and 50 mm, preferably between 20 mm and 25 mm, more preferably between 21 mm and 22 mm; and / orthe cone section defines an angle between 30° and 80°, preferably between 45° and 70°, more preferably approximately 60°.

14. The plasma interface according to any preceding claim, which is produced by machining and / or the orifice is provided by drilling.

15. A mass spectrometer comprising a plasma interface according to any of claims 9 to 14, and a plasma source operable to direct a plasma towards the sampler and skimmer.

16. The mass spectrometer according to claim 15, wherein the plasma source is an inductively coupled plasma source.15

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