Patent Publication Number: US-2023164903-A1

Title: Microwave driven plasma ion source

Description:
TECHNICAL FIELD 
     The invention relates to a microwave driven plasma ion source for ionising a sample to be ionised to sample ions. This microwave driven plasma ion source includes a sample intake for inserting the sample from an outside of the microwave driven plasma ion source into an inside of the microwave driven plasma ion source, a microwave generator for generating microwaves for generating a plasma from a plasma gas, and a plasma torch providing a plasma torch orientation direction and having an inside for housing a process of generation of the plasma from the plasma gas and for housing a process of ionising the sample to the sample ions by exposing the sample to the plasma, wherein the plasma torch comprises a torch outlet for letting out the plasma and the sample ions from the inside of the plasma torch essentially in the plasma torch orientation direction to an outside of the plasma torch, the torch outlet having a torch aperture. 
     BACKGROUND ART 
     Microwave driven plasma ion sources for ionising a sample to be ionised to sample ions pertaining to the technical field initially mentioned are known. US 2016/0025656 A1 of Radom Corporation for example describes such a microwave driven plasma ion source including a microwave generator and a plasma torch. This microwave driven plasma ion source further includes a microwave resonator which is supported within a cylindrical radio-frequency shielding. The microwave resonator has the shape of a circular annulus and provides a clearance. The plasma torch is partially arranged in this clearance. In operation, microwaves are generated with the microwave generator and directed through a waveguide to the microwave resonator in the shielding. Furthermore, plasma gas is inserted into the inside of the plasma torch for generating a plasma by inductively coupling the plasma gas, to an electromagnetic field generated by the microwave resonator being exposed to the microwaves generated by the microwave generator. At the same time, the sample is introduced into the inside of the plasma torch and exposed to the plasma for being ionised to the sample ions. 
     The operation parameters of such known microwave driven plasma ion sources can be optimised for maximising the ionisation of the sample in order to obtain a maximum number of sample ions for a given sample. However, the optimal operation parameters can change depending on the use of the microwave driven plasma ion source. In particular, in case an instrumentation which uses the sample ions generated by such a microwave driven plasma ion source is changed, the optimal operation parameters of the respective microwave driven plasma ion source are likely to be changed, too. Thus, such microwave driven plasma ion sources have the disadvantage that their optimisation for a maximised sample ion generation is difficult and complex. This is in particular the case whenever the respective microwave driven plasma ion source is used for generating sample ions that are then used by some other instrumentation. Examples of such other instrumentations are ion mobility analysers and mass analysers. For the same reason, such microwave driven plasma ion sources are difficult to be optimised for maximised sample ion generation whenever they are used as part of an ion mobility spectrometer and/or a mass spectrometer. 
     In the present text, the formulation “and/or” is occasionally used for linking two features. This formulation is to be understood as either one of the two features or both of the features. Thus, “A and/or B” is to be understood as three equivalent options, wherein one option is A, another option is B and yet another option is both A and B. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to create a microwave driven plasma ion source pertaining to the technical field initially mentioned, that enables a simple and stable optimisation of its operation parameters for maximised sample ion generation. 
     The solution of the invention is specified by the features of claim  1 . According to the invention, the microwave driven plasma ion source includes a shielding for shielding off the microwaves from passing from the inside of the microwave driven plasma ion source to the outside of the microwave driven plasma ion source, wherein the shielding comprises a shielding outlet for letting out the plasma and the sample ions from the inside of the microwave driven plasma ion source essentially in the plasma torch orientation direction to the outside of the microwave driven plasma ion source, the shielding outlet having a shielding aperture, wherein the shielding outlet is fluidly coupled to the torch outlet for letting out the plasma and the sample ions from the inside of the plasma torch essentially in the plasma torch orientation direction to the outside of the microwave driven plasma ion source, wherein a size of the shielding aperture is less than 150%, preferably less than 125%, particular preferably less than 110% of a size of the torch aperture, wherein both the size of the shielding aperture and the size of the torch aperture are measured in units of area. 
     According to the invention, the microwave driven plasma ion source includes a sample intake for inserting the sample from an outside of the microwave driven plasma ion source into an inside of the microwave driven plasma ion source. In one example, this sample intake is an intake for inserting a sample from an environment surrounding the microwave driven plasma ion source into the inside of the microwave driven plasma ion source. In another example, the sample intake is a connector for being connected to a sample source for inserting the sample from the sample source via the sample intake into the microwave driven plasma ion source for ionising the sample. 
     Advantageously, the sample is a gas, an aerosol comprising aerosol particles dispersed in a gas or, more broadly, discrete particles entrained in a gas. In case of an aerosol comprising aerosol particles dispersed in a gas, the aerosol particles may be solid or liquid particles. Aerosol particles usually have a size in a range from 10 nm to 10 µm. Aerosol particles smaller than 10 nm have a large surface to size ratio and therefore grow quickly into larger aerosol particles. Aerosol particles larger than 10 µm on the other hand become too heavy to be suspended for a long time and will eventually fall to the ground. For this reason, the typical size range of aerosol particles in a sample being an aerosol is from 50 nm to 2,000 nm or 2 µm, respectively. As compared to the aerosol, the sample being discrete particles entrained in a gas is to be understood broader. Such discrete particles entrained in a gas can be generated right in front of the sample intake of the microwave driven plasma ion source for example by laser ablation of some sample material. In this case, the discrete particles can have sizes that exceed the upper limit of 10 µm of aerosol particles and as well as sizes that fall below the lower limit of 10 nm of aerosol particles. Of course, the discrete particles can have sizes like aerosol particles, too. 
     According to the solution of the invention, the microwave driven plasma ion source includes a microwave generator for generating microwaves for generating a plasma from a plasma gas. Thereby, the microwaves are the electromagnetic waves of radio frequency electromagnetic radiation in the microwave range. This microwave range is advantageously the range of electromagnetic radiation having a frequency in the range from 1 MHz to 10 GHz, particular advantageously from to 30 MHz to 3 GHz, most advantageously from 30 MHz to 300 MHz or from 300 MHz to 3 GHz. 
     The microwaves generated by the microwave generator can be used directly and/or indirectly to generate the plasma from the plasma gas. Thus, in a first example, the plasma gas is exposed to the microwaves generated by the microwave generator for generating the plasma from the plasma gas. In a second example, the microwave driven plasma ion source comprises a microwave resonator, wherein the microwave resonator is exposed to the microwaves generated by the microwave generator such that the microwave resonator generates an electromagnetic field due to a resonant behaviour of the microwave resonator when being exposed to the microwaves, wherein the plasma gas is inductively coupled to the electromagnetic field generated by the microwave resonator for generating the plasma from the plasma gas. In a third example, the first and second examples are combined in that the plasma gas is exposed to microwaves generated by the microwave generator and inductively coupled to the electromagnetic field generated by the microwave resonator for generating the plasma from the plasma gas. 
     In all three examples, the generation of the plasma from the plasma gas is directly or indirectly and thus ultimately driven by the microwaves generated by the microwave generator. Therefore, in all three examples, the microwave generator is a microwave generator for generating microwaves for generating the plasma from the plasma gas. 
     According to the invention, the microwave driven plasma ion source includes a plasma torch providing a plasma torch orientation direction and having an inside for housing a process of generation of the plasma from the plasma gas and for housing a process of ionising the sample to the sample ions by exposing the sample to the plasma. Thereby, the process of generation of the plasma from the plasma gas is run in a first region in the inside of the plasma torch, while the process of ionising the sample to the sample ions by exposing the sample to the plasma is run in a second region in the inside of the plasma torch. Thereby, the first region and the second region can be completely separated regions, partially overlapping regions or fully overlapping regions. In the case where the first region and the second region are fully overlapping regions, in one example, one of the first and second regions is bigger than the other one of the first and second regions and fully covers this other one of the first and second regions. In another example where the first region and the second region are fully overlapping regions, both regions are identical. Independent of how the first region and the second region is shaped and whether they are separate, partially overlapping or fully overlapping, the first region and the second region have in common that they are both located in the inside of the plasma torch and are thus housed in the inside of the plasma torch. Thus, both, the process of generation of the plasma from the plasma gas and the process of ionising the sample to sample ions by exposing the sample to the plasma are housed in the inside of the plasma torch. 
     According to the invention, the microwave driven plasma ion source includes a shielding for shielding off the microwaves from passing from the inside of the microwave driven plasma ion source to the outside of the microwave driven plasma ion source. Thereby, “shielding off” preferably means that less than 5%, particular preferably less than 1%, of a total intensity being a power (energy per time unit) of the electromagnetic radiation being the microwaves generated in the inside of the microwave driven plasma ion source reaches the outside of the microwave driven plasma ion source. Thereby, the intensity of the electromagnetic radiation being the microwaves reaching the outside of the microwave driven plasma ion source is preferably the locally measured intensity integrated over all positions on a surface of a sphere surrounding the microwave driven plasma ion source. 
     According to the invention, the plasma torch comprises a torch outlet for letting out the plasma and the sample ions from the inside of the plasma torch essentially in the plasma torch orientation direction to an outside of the plasma torch, the torch outlet having a torch aperture. Furthermore, according to the invention, the shielding comprises a shielding outlet for letting out the plasma and the sample ions from the inside of the microwave driven plasma ion source essentially in the plasma torch orientation direction to the outside of the microwave driven plasma ion source, the shielding outlet having a shielding aperture. This shielding outlet is fluidly coupled to the torch outlet for letting out the plasma and the sample ions from the inside of the plasma torch essentially in the plasma torch orientation direction to the outside of the microwave driven plasma ion source. Thereby, the plasma torch can be arranged partially inside the microwave driven plasma ion source or fully inside the microwave driven plasma ion source. 
     In one example, the plasma torch comprises a tube surrounding the inside of the plasma torch, wherein one end of the tube is open and forms the torch outlet. In this example, the plasma torch orientation direction is parallel to a length axis of the tube and points from the inside of the tube out of the tube, thereby pointing through the open end of the tube which forms the torch outlet. Thus, the plasma and the sample ions which move with some divergence along the tube move essentially in the plasma torch orientation direction when being let out of the plasma torch through the torch outlet. 
     In this example, in case the tube protrudes through the shielding outlet from the inside of the microwave driven plasma ion source, wherein the torch outlet is outside of the inside of the microwave driven plasma ion source, the plasma torch is arranged partially inside the microwave driven plasma ion source. In case the tube however ends with the torch outlet within the inside of the microwave driven plasma ion source and the rest of the plasma torch is arranged inside the microwave driven plasma ion source, too, the plasma torch is arranged fully inside the microwave driven plasma ion source. In both cases, the shielding outlet is fluidly coupled to the outlet for letting out the plasma and the sample ions from the inside of the plasma torch essentially in the plasma torch orientation direction to the outside of the microwave driven plasma ion source. 
     Independent of this example, the shielding outlet is advantageously arranged in the vicinity of the torch outlet for ensuring direct passage of the plasma and the sample ions from the inside of the plasma torch essentially in the plasma torch orientation direction to the outside of the microwave driven plasma ion source. Preferably, the shielding outlet is arranged at a distance of less than 5 mm, particular preferably less than 2.5 mm from the torch outlet. In a variant, the shielding outlet is arranged flush with the torch outlet. 
     According to the invention, the size of the shielding aperture is less than 150%, preferably less than 125%, particular preferably less than 110% of the size of the torch aperture, wherein both the size of the shielding aperture and the size of the torch aperture are measured in units of area. This means that the shielding aperture has a limited size. Due to this limited size, the microwaves are better shielded off from passing from the inside of the microwave driven plasma ion source to the outside of the microwave driven plasma ion source in an area of the shielding outlet. Consequently, less intensity of the electromagnetic radiation being the microwaves generated in the inside of the microwave driven plasma ion source reaches the outside of the microwave driven plasma ion source in the area of the shielding outlet. Thus, in case a conducting element is arranged in a vicinity of the shielding outlet, this conducting element is exposed to less microwaves resulting in less microwaves being generated by the conducting element due to induction. 
     At the same time, the limited size of the shielding outlet has the effect that in the area of the shielding outlet, the microwaves generated by the conducting element due to induction are better shielded off from passing from the outside of the microwave driven plasma ion source to the inside of the microwave driven plasma ion source. As consequence, a field of microwaves in the inside of the microwave driven plasma ion source is changed less by feedback effects when a conducting element is moved in the vicinity of the shielding outlet. Thus, the field of microwaves in the inside of the microwave driven plasma ion source is more stable and less disturbed by movements of conducting elements in the vicinity of the shielding outlet. Therefore, the operation parameters of the microwave driven plasma ion source can be optimised for maximised sample ion generation and change less when another instrumentation like an ion mobility analyser and/or a mass analyser is moved relative to the microwave driven plasma ion source or when a setup of such another instrumentation is changed. Consequently, the optimisation of the operation parameters of the microwave driven plasma ion source for maximised sample ion generation is simplified and more stable. This advantage is increased with decreased size of the shielding aperture. Therefore, the advantage is increased if the size of the shielding aperture is less than 125% of the size of the torch aperture as compared to when the size of the shielding aperture is less than 150% of the size of the torch aperture. Furthermore, the advantage is increased if the size of the shielding aperture is less than 110% of the size of the torch aperture as compared to when the size of the shielding aperture is less than 125% of the size of the torch aperture. 
     Independent of whether the size of the shielding aperture is less than 150%, less than 125% or less than 110% of the size of the torch aperture, the size of the shielding aperture is advantageously at least 100% of the size of the torch aperture, wherein both the size of the shielding aperture and the size of the torch aperture are measured in units of area. This has the advantage that the shielding aperture is large enough to enable to let the plasma and a maximum number the sample ions out from the inside of the plasma torch to the outside of the microwave driven plasma ion source. 
     Alternatively, the size of the shielding aperture is less than 100% of the size of the torch aperture. 
     Preferably, the plasma torch comprises a plasma gas inlet for inserting the plasma gas from a plasma gas source through the plasma gas inlet into the inside of the plasma torch. Thereby, the plasma gas source may be part of the microwave driven plasma ion source or may be separate from the microwave driven plasma ion source. In the latter case, the microwave driven plasma ion source advantageously comprises a plasma gas source connector fluidly coupled to the plasma gas inlet, the plasma gas source connector being fluidly connectable to the separate plasma gas source for supplying plasma gas from the separate plasma gas source via the plasma gas source connector and the plasma gas inlet into the inside of the plasma torch. 
     Advantageously, the microwave driven plasma ion source includes a plasma gas source fluidly coupleable to the plasma gas inlet. 
     Alternatively, the plasma torch may go without such a plasma gas inlet. 
     Preferably, the plasma torch comprises a sample inlet fluidly coupled to the sample intake for inserting the sample from the sample intake into the inside of the plasma torch. In a variant thereof, the sample inlet and the sample intake may be combined and thus be the same. In this variant, the sample inlet of the plasma torch is at the same time the sample intake of the microwave driven plasma ion source. 
     Alternatively, the plasma torch may go without such a sample inlet. 
     Advantageously, the plasma torch is arranged in the inside, particular advantageously fully in the inside of the microwave driven plasma ion source. This has the advantage that the plasma torch is optimally shielded off from external influences, thus being more stably operatable with given operation parameters. Consequently, an optimisation of the operation parameters for operating the microwave driven plasma ion source for maximised sample ion generation is more stably maintained. 
     Alternatively, the plasma torch is not arranged in the inside of the microwave driven plasma ion source. 
     Preferably, a size of the torch aperture measured in units of area is in a range from 70 mm 2  to 900 mm 2 , particular preferably in a range from 110 mm 2  to 600 mm 2 . This has the advantage that the sample ion generation with the microwave driven plasma ion source can be optimised by balancing the maximisation of the absolute number of sample ions generated by the microwave driven plasma ion source per time unit versus the maximisation of a density of the sample ions generated by the microwave driven plasma ion source in a region of the torch aperture and the shielding aperture, wherein the term density refers to the number of sample ions per spatial unit. 
     Alternatively, the size of the torch aperture is maximally 70 mm 2  or minimally 900 mm 2 . 
     Preferably, the inside of the microwave driven plasma ion source has an outer limit being a closed surface, wherein the shielding defines the closed surface and covers at least 98%, particular preferably at least 99%, most preferably at least 99.5% of the closed surface. Advantageously, at any opening in the shielding, the outer limit of the inside of the microwave driven plasma ion source is the surface connecting the edges of the respective opening to a closed surface having the smallest possible surface area. Advantageously, openings in the shielding which have a maximum diameter being smaller than 20% of a wavelength of the shortest microwaves generatable by the microwave generator, particular advantageously which have a maximum diameter being smaller than 6 mm, are considered as being fully covered by the shielding. Thereby, the maximum diameter is advantageously the longest possible straight line measured across the respective opening from any position on an edge of the respective opening to any other position on the edge of the respective opening. Larger openings in the shielding however are not considered as being covered by the shielding. 
     Alternatively, however, the shielding covers less than 98% of the closed surface being the outer limit of the inside of the microwave driven plasma ion source. 
     Preferably, the shielding is made from metal. In this case, the shielding is a metal shielding. Thereby, it is irrelevant whether the shielding is a metal coating on some shielding support structure or whether the shielding is made from metal plates, mesh or grid, thereby being supported by some shielding support structure or being selfstable due to an integrated support structure or due to forming the support structure by itself. Independent of how the metal shielding is formed, it has the advantage that an efficient shielding for shielding off the microwaves from passing from the inside of the microwave driven plasma ion source to the outside of the microwave driven plasma ion source can be provided. 
     Particular preferably, in a region of the shielding outlet, the shielding is made from tungsten. Since tungsten is the element with the highest melting point, this has the advantage that the shielding withstands the high temperatures when being hit by parts of the plasma and the sample ions passing from the inside of the plasma torch to the outside of the microwave driven plasma ion source. In variant, in the region of the shielding outlet, the shielding is made from another metal than tungsten. 
     In a preferred variant, the entire shielding is made from tungsten. In another preferred variant however, a main part of the shielding is made from another metal than tungsten, while in the region of the shielding outlet, the shielding is made from tungsten. This latter variant has the advantage that the shielding can be constructed less costly. 
     Alternatively, the shielding is made from another material than a metal. 
     Preferably, the microwave driven plasma ion source comprises a cooling liquid circuit for containing a cooling liquid for cooling the shielding, wherein the shielding, in particular in an area of the shielding outlet, is coolable by the cooling liquid. Advantageously, the cooling liquid is part of the microwave driven plasma ion source and contained in the cooling liquid circuit. The microwave driven plasma ion source may however go without the cooling liquid and without the cooling liquid being contained in the cooling liquid circuit. This latter variant may for example be advantageous for transporting the microwave driven plasma ion source. 
     Particular preferably, the cooling liquid circuit is a closed loop circuit. 
     Advantageously, the cooling liquid is water or liquid nitrogen. Nonetheless, the cooling liquid may be some other liquid. 
     Alternatively, the microwave driven plasma ion source may go without such a cooling liquid circuit for containing a cooling liquid for cooling the shielding. 
     Advantageously, the microwave driven plasma ion source includes a microwave resonator for generating an electromagnetic field for inductively coupling the plasma gas to the electromagnetic field for generation of the plasma from the plasma gas, wherein the microwave resonator exhibits a resonant behaviour and generates the electromagnetic field when being exposed to the microwaves generated by the microwave generator. In this case, the plasma is generated by inductively coupling the plasma gas to the electromagnetic field generated by the microwave resonator. In one variant, the microwaves generated by the microwave generator are involved only indirectly because they are used to drive the microwaves resonator. In another variant, however, the plasma gas is exposed to microwaves generated by the microwave generator and inductively coupled to the electromagnetic field generated by the microwave resonator for generating the plasma from the plasma gas. In this latter variant, the microwaves generated by the microwave generator are involved indirectly and directly in generating the plasma from the plasma gas. 
     In the case where the microwave resonator is part of the microwave driven plasma ion source, the shielding for shielding off the microwaves from passing from the inside of the microwave driven plasma ion source to the outside of the microwave driven plasma ion source is preferably for shielding off both the microwaves generated by the microwave generator and the electromagnetic field generated by the microwave resonator. 
     Preferably, the microwave resonator provides a clearance in which a major part of the electromagnetic field generated by the microwave resonator is located when the microwave driven plasma ion source is operated. Thereby, the major part of the electromagnetic field generated by the microwave resonator being located in the clearance of the microwave resonator preferably means that more than half of the total intensity of the electromagnetic field generated by the microwave resonator occurs inside the clearance in the microwave resonator. Advantageously, the plasma torch is arranged at least partially inside the clearance in the microwave resonator. Preferably, the first region in the inside of the plasma torch, where the process of the generation of the plasma from the plasma gas is located, is arranged inside the clearance in the microwave resonator. In a preferred variant, the second region in the inside of the plasma torch, where the process of ionising the sample to sample ions by exposing the sample to the plasma is located, is arranged inside the clearance in the microwave resonator, too. In an alternative variant however, the second region in the inside of the plasma torch, where the process of ionising the sample to sample ions by exposing the sample to the plasma is located, is arranged at least partially or completely outside of the clearance in the microwave resonator. In an alternative to these variants, the first region in the inside of the plasma torch, where the process of generation of the plasma from the plasma gas is located, is arranged at least partially outside or completely outside of the clearance in the microwave resonator. 
     Alternatively, the microwave resonator does not provide such a clearance in which a major part of the electromagnetic field generated by the microwave resonator is located when the microwave driven plasma ion source is operated. 
     In an alternative, the microwave driven plasma ion source goes without a microwave resonator for generating an electromagnetic field for inductively coupling the plasma gas to the electromagnetic field for the generation of the plasma from the plasma gas, wherein the microwave resonator exhibits a resonant behaviour and generates the electromagnetic field when being exposed to the microwaves generated by the microwave generator. 
     In a preferred variant, the microwave driven plasma ion source includes a plasma gas source, wherein the plasma torch comprises a plasma gas inlet for inserting the plasma gas from the plasma gas source through the plasma gas inlet into the inside of the plasma torch, wherein the plasma gas contains a nitrogen content from 80 volume percent to 100 volume percent. This has the advantage that the plasma gas cost-effective and readily available 
     Advantageously, the plasma gas is nitrogen. Thereby, the nitrogen may comprise traces of other gases. Preferably however, in case of the plasma gas being nitrogen, at least 98 volume percent of the plasma gas is nitrogen, thus comprising 2 volume percent or less other gases. 
     In another preferred variant, the microwave driven plasma ion source includes a plasma gas source, wherein the plasma torch comprises a plasma gas inlet for inserting the plasma gas from the plasma gas source through the plasma gas inlet into the inside of the plasma torch, wherein the plasma gas contains an argon content from 0.9 volume percent to 100 volume percent, in particular from 95 volume percent to 100 volume percent. Thereby, the argon may comprise traces of other gases. Preferably however, in case the plasma gas is argon, at least 98 volume percent of the plasma gas is argon, thus comprising 2 volume percent or less other gases. Independent of the amount of any traces of other gases, argon as plasma gas has the advantage that the plasma gas does not chemically react with the sample during ionisation of the sample to sample ions. 
     Alternatively to nitrogen and argon, another gas or another mixture of gasses can be used as plasma gas. Furthermore the microwave driven plasma ion source may go with or without a plasma gas source. In case the microwave driven plasma ion source goes without plasma gas source, the microwave driven plasma ion source is advantageously connectable to a separate plasma gas source. 
     Advantageously, a mass spectrometer for mass analysing a sample includes the microwave driven plasma ion source according to the invention for ionising the sample to sample ions and a mass analyser for mass analysing the sample ions, the mass analyser having an ion inlet for inserting the sample ions from an outside of the mass analyser into an inside of the mass analyser for mass analysing the sample ions, the ion inlet having an aperture. 
     In the present text, “mass analysing the sample” preferably means determining a mass spectrum of the sample. In order to determine a mass spectrum of the sample, the sample is ionised to sample ions, whereafter the mass spectrum of the obtained sample ions is determined. Thus, the formulation “mass analysing the sample” means ionising the sample to sample ions and subsequently mass analysing the sample ions. Thereby, the formulations “mass spectrum of the sample” and “mass spectrum of the sample ions” refer to the same mass spectrum since the mass spectrum ultimately provides information on the distribution of the sample ions with respect to the sample ions’ mass per charge ratio. In the case of obtaining a “mass spectrum of the sample” however, the sample is ionised to sample ions first, whereafter the mass spectrum is determined from the sample ions. 
     Advantageously, the mass analyser is fluidly coupled to the microwave driven plasma ion source for receiving the sample ions which exit the shielding outlet of the shielding of the microwave driven plasma ion source essentially in the plasma torch orientation direction and thereby for receiving the sample ions through the ion inlet for being mass analysed, wherein the ion inlet is arranged at least partially, particular advantageously entirely within a volume starting from the torch outlet and pointing in the plasma torch orientation direction, the volume having a cross section perpendicular to the plasma torch orientation direction corresponding to a projection of the torch aperture onto a plane oriented perpendicular to the plasma torch orientation direction. Thus, the cross section of the volume perpendicular to the plasma torch orientation direction as well as the position of the volume are defined by the torch outlet, while the extension of the volume away from the torch outlet is defined by the plasma torch orientation direction. 
     Independent of an orientation of the ion inlet with respect to the plasma torch orientation direction, the arrangement of the ion inlet at least partially or entirely within the volume starting from the torch outlet and pointing in the plasma torch orientation direction, the volume having a cross section perpendicular to the plasma torch orientation direction corresponding to a projection of the torch aperture onto a plane oriented perpendicular to the plasma torch orientation direction, has the advantage that more of the sample ions which exit the torch outlet and the shielding outlet are subsequently received by the mass analyser through the ion inlet for being mass analysed. Thus, the efficiency of the mass spectrometer is increased. 
     Advantageously, the mass analyser and the microwave driven plasma ion source are moveable relative to each other in a plane having a normal being maximally inclined by an angle of 20° with respect to the plasma torch orientation direction. Thus, the normal of the plane can be parallel to the plasma torch orientation direction or can be tilted relatively to the plasma torch orientation direction with an angle of 20° or less. Therefore, a position of the ion inlet of the mass analyser relative to the shielding outlet of the shielding of the microwave driven plasma ion source can be changed. This has the advantage that a number of ions of the sample ions which exit the torch outlet and the shielding outlet and subsequently pass through the ion inlet for being mass analysed can easily be maximised by moving the mass analyser relatively to the microwave driven plasma ion source. This maximisation enables to increase the efficiency of the mass spectrometer even at different operation parameters of the microwave driven plasma ion source since in the inside of the plasma torch, a position and a shape of the second region where the sample ions are generated by exposing the sample to the plasma as well as a position within the second region where a density of the generated sample ions is the highest varies depending on the operation parameters of microwave driven plasma ion source. Thus, the mass spectrometer enables to optimise not only the efficiency of the generation of the sample ions from the sample but also enables to optimize the efficiency of the transfer of the ions from the microwave driven plasma ion source into the mass analyser for mass analysing the sample ions. 
     Preferably, the movement of the mass analyser relative to the microwave driven plasma ion source is actuated by at least one actuator, in particular at least one motor. Thus, the mass spectrometer preferably comprises at least one actuator, in particular at least one motor, for actuating the movement of the mass analyser relative to the microwave driven plasma ion source. Alternatively, however, the mass spectrometer may go without such an actuator. In this case, the mass analyser may be moveable by hand relative to the microwave driven plasma ion source. 
     In a first preferred variant, the mass analyser and the microwave driven plasma ion source are moveable relative to each other only in the mentioned plane having the normal being maximally inclined by an angle of 20° with respect to the plasma torch orientation direction. In a second preferred variant however, the mass analyser and the microwave driven plasma ion source are moveable relative to each other in other directions than in the mentioned plane having the normal being maximally inclined by an angle of 20° with respect to the plasma torch orientation direction, too. 
     Preferably, the mass analyser and the microwave driven plasma ion source are moveable relative to each other along two axes being oriented perpendicular to each other. 
     In case the mass analyser and the microwave driven plasma ion source are at the same time moveable relative to each other in the plane having the normal being maximally inclined by an angle of 20° with respect to the plasma torch orientation direction, the two axes are preferably arranged within this plane. This has the advantage that the mass analyser and the microwave driven plasma ion source can easily be moved in a controlled manner relative to each other in the plane having the normal being maximally inclined by an angle of 20° with respect to said plasma torch orientation direction. Thus, a well controlled optimisation of the efficiency of the mass spectrometer is enabled. 
     In a variant however, only one of the two axes or none of the two axes is arranged in this plane. 
     Preferably, the mass analyser and the microwave driven plasma ion source are moveable relative to each other along three axes being oriented perpendicular to each other. This has the advantage that the mass analyser and the microwave driven plasma ion source can easily be moved in a controlled manner in all directions relative to each other in order to maximise the efficiency of the mass spectrometer. 
     In a preferred variant however, the mass analyser and the microwave driven plasma ion source are moveable relative to each other only along two axes being oriented perpendicular to each other. Advantageously, these two axes are arranged within in the plane having the normal being maximally inclined by an angle of 20° with respect to the plasma torch orientation direction. 
     In yet another variant, the mass analyser and the microwave driven plasma ion source are moveable relative to each other only along one axis. 
     Independent of whether the mass analyser and the microwave driven plasma ion source are moveable relative to each other in a plane having a normal being maximally inclined by an angle of 20° with respect to said plasma torch orientation direction or not, the mass analyser and the microwave driven plasma ion source are advantageously pivotable relative to each other about one pivot axis which is oriented essentially perpendicular to the plasma torch orientation direction. Thus, the one pivot axis is advantageously tilted with respect to the plasma torch orientation at an angle in a range between 45° and 135°, particular advantageously between 70° and 110°. Most advantageously, the one pivot axis is however tilted with respect to the plasma torch orientation at an angle of 90°. The mass analyser and the microwave driven plasma ion source being pivotable relative to each other about this one pivot axis has the advantage that an orientation of the mass analyser relative to the microwave driven plasma ion source can be optimised in the plane being oriented perpendicular to the pivot axis in order to maximise the number of ions of the sample ions which exit the torch outlet and the shielding outlet and subsequently pass through the ion inlet for being mass analysed. Thus, the mass spectrometer enables to optimise the efficiency of the transfer of the ions from the microwave driven plasma ion source into the mass analyser for mass analysing the sample ions further. 
     In a preferred variant, the mass analyser and the microwave driven plasma ion source are pivotable relative to each other about two pivot axes which are oriented essentially perpendicular to the plasma torch orientation direction. Thus, the pivot axes are advantageously tilted with respect to the plasma torch orientation at an angle in a range between 45° and 135°, particular advantageously between 70° and 110°. Most advantageously, the pivot axes are however tilted with respect to the plasma torch orientation at an angle of 90°. Independent of the precise angle, this has the advantage that an orientation of the mass analyser relative to the microwave driven plasma ion source can be optimised in the planes being oriented perpendicular to the pivot axes in order to maximise the number of ions of the sample ions which exit the torch outlet and the shielding outlet and subsequently pass through the ion inlet for being mass analysed. Thus, the mass spectrometer enables to optimise the efficiency of the transfer of the sample ions from the microwave driven plasma ion source into the mass analyser for mass analysing the sample ions further. 
     Particular advantageous, these two pivot axes are oriented perpendicular to each other. This has the advantage that a systematic optimisation of the efficiency of the transfer from the ions from the microwave driven plasma ion source into the mass analyser for mass analysing the sample ions is enabled. 
     Alternatively to these variants with one or two pivot axes, the mass analyser and the microwave driven plasma ion source can be not pivotable about any axis relative to each other or they can be pivotable relative to each other about more than two pivot axes. 
     Other advantageous embodiments and combinations of features come out from the detailed description below and the entirety of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings used to explain the embodiments show: 
         FIG.  1    a simplified schematic view of a microwave driven plasma ion source according to the invention for ionising a sample to be ionised to sample ions, 
         FIG.  2    a simplified schematic view of another microwave driven plasma ion source according to the invention for ionising a sample to be ionised to sample ions, 
         FIG.  3    a simplified schematic view of a mass spectrometer for mass analysing a sample including a microwave driven plasma ion source according to the invention for ionising the sample to sample ions and a mass analyser for mass analysing the sample ions, 
         FIG.  4    a simplified schematic view of a mass spectrometer for mass analysing a sample similar to the mass spectrometer shown in  FIG.  3   , and 
         FIG.  5    a simplified schematic view of yet another similar mass spectrometer for mass analysing a sample. 
     
    
    
     In the figures, the same components are given the same reference symbols. 
     PREFERRED EMBODIMENTS 
       FIG.  1    shows a simplified schematic view of a microwave driven plasma ion source  1  according to the invention for ionising a sample to be ionised to sample ions. Thereby, the sample is an aerosol comprising aerosol particles dispersed in a gas. Some of these aerosol particles are solid particles, while other ones of the aerosol particles are liquid particles. In a variant however, all aerosol particles are solid particles. In another variant, the aerosol particles are liquid particles. In yet another variant, the sample is a gas. Independent of the type of sample, the microwave driven plasma ion source  1  includes a sample intake  6  for inserting the sample to be ionised from an outside of the microwave driven plasma ion source  1  into an inside  3  of the microwave driven plasma ion source  1 . 
     The microwave driven plasma ion source  1  shown in  FIG.  1    includes a microwave generator  10  for generating microwaves and a microwave resonator  11 , both arranged in an inside  3  of the microwave driven plasma ion source  1 . The microwave generator  10  is an antenna connected to an AC voltage source  30  of the microwave driven plasma ion source  1 . This AC voltage source  30  generates an AC voltage alternating at a frequency in a range from 1 MHz to 10 GHz and supplies this AC voltage to the microwave generator  10 . In a variant, the frequency is in a range from 30 MHz to 3 GHz. In yet another variant, the frequency is in a range from 30 MHz to 300 MHz. In yet another variant, the frequency is in a range from 300 MHz to 3 GHz. In one particular example, the frequency is 250 MHz. In another particular example, the frequency is 350 MHz. 
     The microwave resonator  11  has an annular shape and exhibits a resonant behaviour and generates an electromagnetic field for generation of a plasma  101  from a plasma gas  100  when being exposed to the microwaves generated by the microwave generator  10 . Thus, the microwave generator  10  is for generating microwaves for generating the plasma  101  from the plasma gas  100  indirectly by generating microwaves which excite the microwave resonator  11  to generate the electromagnetic field which inductively couples the plasma gas  100  to the electromagnetic field for generating the plasma  101  from the plasma gas  100 . 
     The microwave driven plasma ion source  1  further includes a plasma torch  20  which extends through an opening in the annular shape of the microwave resonator  11 . This plasma torch  20  comprises a sample inlet  26  fluidly coupled to the sample intake  6  of the microwave driven plasma ion source  1  for inserting the sample from the sample intake  6  into an inside  21  of the plasma torch  20 . Furthermore, the plasma torch  20  comprises a plasma gas inlet  23  for inserting the plasma gas  100  from a plasma gas source  8  of the microwave driven plasma ion source  1  through the plasma gas inlet  23  into the inside  21  of the plasma torch  20 . Thereby, the plasma gas  100  contains a nitrogen content from  80  volume percent to  100  volume percent. In a variant, the plasma gas  100  is nitrogen. In another variant however, the plasma gas  100  is argon. In a further variant, the plasma gas source  8  is not part of the microwave driven plasma ion source  1 . In this case, the microwave driven plasma ion source comprises a plasma gas source connector fluidly coupled to the plasma gas inlet  23 , the plasma gas source connector being fluidly connectable to a separate plasma gas source for supplying the plasma gas from the separate plasma gas source via the plasma gas source connector and the plasma gas inlet  23  into the inside  21  of the plasma torch  20 . 
     The inside  21  of the plasma torch  20  is for housing a process of generation of the plasma  101  from the plasma gas  100  and for housing a process of ionising the sample to the sample ions by exposing the sample to the plasma  1   01 . Thereby, the process of generation of the plasma  101  from the plasma gas  100  is run in a first region  24  in the inside  21  of the plasma torch  20  in an area in the opening of the annular shape of the microwave resonator  11 . In operation of the microwave driven plasma ion source  1 , microwaves generated with the microwave generator  10  excite the microwave resonator to generate an electromagnetic field in the first region  24 . The plasma gas  100  is inductively coupled to the electromagnetic field in the first region  24  for generation of the plasma  101  from the plasma gas  100 . By exposing the sample to the plasma  101  in a second region  25  in the inside of the plasma torch  20 , the process of ionising the sample to the sample ions is run in this second region  25  in the inside of the plasma torch  20 . Thereby, the first region  24  and the second region  25  are overlapping. In another variant, the first region  24  and the second region  25  are identical. 
     The plasma torch  20  further comprises a torch outlet  22  for letting out the plasma  101  and the sample ions from the inside  21  of the plasma torch  20  to an outside of the plasma torch  20 , wherein the torch outlet  22  has a torch aperture. This torch aperture is measured in units of area and is 70 mm 2 . In a variant, the torch aperture is 100 mm 2 . In another variant, the torch aperture is 111 mm 2 . In yet another variant, the torch aperture is 300 mm 2 . In yet another variant, the torch aperture is 500 mm 2 . In yet another variant, the torch aperture is 600 mm 2 . In yet another variant, the torch aperture is 900 mm 2 . 
     The plasma torch  20  has an elongated shape. This shape can be considered as the shape of a tube. In this picture, an open end of the tube forms the torch outlet  22 . Inside this tube, the plasma gas inlet  23  is arranged concentrically with the tube such that the plasma gas  100  is inserted by the plasma gas inlet  23  into the inside  21  of the plasma torch  20  to flow along a longitudinal axis of the tube. In the inside of the plasma torch  20 , the plasma gas  100  is inductively coupled to the electromagnetic field in the first region  24  to generate the plasma  1   01 . The plasma  101  then exits the plasma torch  20  through the torch outlet  22  which is formed by the open end of the tube. 
     Similar to the plasma gas inlet  23 , the sample inlet  26  is arranged in the inside  21  of the plasma torch  20  concentrically with the tube forming the plasma torch  20  such that the sample is inserted by the sample inlet  26  into the inside  21  of the plasma torch  20  to flow along the longitudinal axis of the tube. More precisely, the sample inlet  26  is arranged concentrically in the plasma gas inlet  23  such that the sample is inserted into the inside  21  of the plasma torch  20  to flow through the plasma  101  generated from the plasma gas  100  for being ionised to the sample ions. Since the plasma gas  100 , the plasma  101  and the sample are inserted into the inside  21  of the plasma torch  20  to flow along the longitudinal axis of the tube forming the plasma torch  20 , both the plasma  101  and the sample ions exit the plasma torch  20  through the torch outlet  22  which is the open end of the tube forming the plasma torch  20 . Thus, the flow direction of the plasma gas  100 , the plasma  101  and the sample ions is a plasma torch orientation direction  29  indicated by an arrow in  FIG.  1   . This plasma torch orientation direction  29  runs parallel to the longitudinal axis of the tube forming the plasma torch  20  and points from the inside  21  of the plasma torch  20  through the torch outlet  22  out of the plasma torch  20 . Since the plasma  101  and the sample ions exit the plasma torch  20  in the plasma torch orientation direction  29  in a beam having some divergence, the formulation that the plasma  101  an the sample ions are let out of the torch outlet  22  essentially in the plasma torch orientation direction  29  is applicable. 
     The microwave driven plasma ion source  1  additionally includes a housing  2  surrounding the inside  3  of the microwave driven plasma ion source  1 . This housing  2  is made from tungsten and forms a shielding  4  for shielding off the microwaves from passing from the inside  3  of the microwave driven plasma ion source  1  to an outside of the microwave driven plasma ion source  1 . Thereby, the inside  3  of the microwave driven plasma ion source  1  has an outer limit being a closed surface which is defined by the shielding  4 . However, the shielding  4  comprises a shielding outlet  5  for letting out the plasma  101  and the ions from the inside  3  of the microwave driven plasma ion source  1  to the outside of the microwave driven plasma ion source  1 , wherein the shielding outlet  5  has a shielding aperture. 
     The shielding outlet  5  is fluidly coupled to the torch outlet  22  for letting out the plasma  101  and the sample ions from the inside  21  of the plasma torch  20  essentially in the plasma torch orientation direction  29  to the outside of the microwave driven plasma ion source  1 . Thereby, a size of the shielding aperture is 149% of a size of the torch aperture, wherein both the size of the shielding aperture and the size of the torch aperture are measured in units of area. In a variant, the size of the shielding aperture is 124% of the size of the torch aperture. In yet another variant, the size of the shielding aperture is 109% of the torch aperture. 
     Thus, the shielding aperture is somewhat larger than the torch aperture. In the embodiment shown in  FIG.  1   , the shielding aperture is even larger than an outer diameter of the plasma torch  20 . Thereby, a largest part of the plasma torch  20  is arranged in the inside  3  of the microwave driven plasma ion source  1 . Only a region of the torch outlet  22  of the plasma torch  20  reaches from the inside  3  of the microwave driven plasma ion source  1  through the shielding outlet  5  to the outside of the microwave driven plasma ion source  1 . Thereby, the torch outlet  22  of the plasma torch  20  protrudes from the shielding outlet  5  by 3 mm. 
     Despite the shielding outlet  5  in the shielding  4  of the microwave driven plasma ion source  1 , the shielding  4  covers 98.5% of the closed surface defining the outer limit of the inside  3  of the microwave driven plasma ion source  1 . 
     The microwave driven plasma ion source  1  further comprises a cooling liquid circuit  7  being a closed loop circuit for containing a cooling liquid for cooling the shielding  4 . This cooling liquid circuit  7  comprises a reservoir  9  of cooling liquid and passes amongst others around the shielding outlet  5  for cooling the shielding  4  in an area of the shielding outlet  5 . Thus, the shielding  4  can be cooled efficiently in the area of the shielding outlet  5  where ions of the plasma  101  are most likely to hit the shielding and heat up the shielding  4 . 
     In the embodiment shown in  FIG.  1   , the cooling liquid is water. In a variant however, another cooling liquid than water is used. In one example, the cooling liquid is liquid nitrogen. Thus, in either case, the shielding  4 , in particular the area of the shielding  4  where the shielding outlet  5  is located, is coolable by the respective cooling liquid. 
       FIG.  2    shows a simplified schematic view of another microwave driven plasma ion source  201  according to the invention for ionising a sample to be ionised to sample ions. This microwave driven plasma ion source  201  is in most parts the same as the microwave driven plasma ion source  1  shown in  FIG.  1   . In contrast to the microwave driven plasma ion source  1  shown in  FIG.  1    however, the microwave driven plasma ion source  201  shown in  FIG.  2    comprises a plasma torch  202  which is arranged in the inside  203  of the microwave driven plasma ion source  201 . Nonetheless, the shielding outlet  205  is as well fluidly coupled to the torch outlet  222  for letting out the plasma  101  and the sample ions from the inside  221  of the plasma torch  220  essentially in the plasma torch orientation direction  229  to the outside of the microwave driven plasma ion source  201 . Thereby, there is a gap of 4.5 mm between the torch outlet  222  and the shielding outlet  205 . In a variant, this gap is only 2 mm. In either case, the shielding outlet  205  is arranged in the vicinity of the torch outlet  222 . 
     In the embodiment shown in  FIG.  2   , the size of the shielding aperture is 105% of the size of the torch aperture and the shielding  204  covers 99.1% of the closed surface defined by the shielding  204 . In a variant, the size of the shielding aperture is 100% of the size of the torch aperture while the shielding  204  covers 99.5% of the closed surface defined by the shielding  204 . In another variant, the torch outlet  222  and the shielding outlet  205  are flush and without gap in between. In this latter variant, the size of the shielding aperture is 100% of the size of the torch aperture, too. 
       FIG.  3    shows a simplified schematic view of a mass spectrometer  50  for mass analysing a sample. This mass spectrometer  50  includes a microwave driven plasma ion source according to the invention for ionising the sample to sample ions. In  FIG.  3   , the microwave driven plasma ion source  1  of  FIG.  1    is exemplary shown. Instead, the microwave driven plasma ion source included in the mass spectrometer  50  can also be the microwave driven plasma ion source  201  of  FIG.  2    or any other microwave driven plasma ion source according to the invention. 
     Besides a microwave driven plasma ion source according to the invention, the mass spectrometer  50  includes a mass analyser  70  for mass analysing the sample ions. This mass analyser  70  has an ion inlet  71  for inserting the sample ions from an outside of the mass analyser  70  into an inside  72  of the mass analyser  70  for mass analysing the sample ions. Thereby, a second opening  73  is arranged further inside the mass analyser  70  as compared to the ion inlet  71  such that sample ions having a trajectory with an orientation within a certain range of orientations can pass the ion inlet  71  of the mass analyser  70  first and the second opening  73  second in order to reach into the inside  72  of the mass analyser  70  for being mass analysed. 
     The mass analyser  70  is fluidly coupled to the microwave driven plasma ion source  1  for receiving through the ion inlet  71  the sample ions which exit the shielding outlet  5  of the shielding  4  of the microwave driven plasma ion source  1  essentially in the plasma torch orientation direction  29 . Thereby, the ion inlet  71  is arranged entirely within a volume starting from the torch outlet  22  and pointing in the plasma torch orientation direction  29 , the volume having a cross section perpendicular to the plasma torch orientation direction  29  corresponding to a projection of the torch aperture onto a plane oriented perpendicular to the plasma torch orientation direction  29 . Thus, the cross section of the volume perpendicular to the plasma torch orientation direction  29  as well as the position of the volume are defined by the torch outlet  22 , while the extension of the volume away from the torch outlet  22  is defined by the plasma torch orientation direction  29 . 
     In the inside  21  of the plasma torch  20 , a position and a shape of the second region  25  where the sample ions are generated by exposing the sample to the plasma  101  depends on the operation parameters of the microwave driven plasma ion source  1 . Furthermore, a position within the second region  25  where a density of the generated sample ions is the highest depends on the operation parameters of microwave driven plasma ion source  1 , too. Thus, the position in the inside  21  of the plasma torch  20 , where the most sample ions are generated, changes when the operation parameters of the microwave driven plasma ion source  1  are changed. Consequently, the position in the aperture of the torch outlet  22  where a majority of sample ions emerges from the inside  21  of the plasma torch  20  through the torch outlet  22  essentially in the plasma torch orientation direction  29  changes depending on the operation parameters of the microwave driven plasma ion source  1 . 
     The operation parameters of the microwave driven plasma ion source  1  can be optimised in order to generate a maximum total number of sample ions. Thereby, the optimal operation parameters depend on the type of sample to be ionised to sample ions. In order to optimise additionally the fraction of the total number of the generated sample ions which emerges from the inside of the  21  plasma torch  20  and makes its way through the aperture of the ion inlet  71  into the inside  72  of the mass analyser  70  for being mass analysed, the mass analyser  70  and the microwave driven plasma ion source  1  are moveable relative to each other in a plane having a normal being the plasma torch orientation direction  29 . In the embodiment of the mass spectrometer  50  shown in  FIG.  3   , the mass spectrometer  50  comprises a motor  28  for actuating a movement of the microwave driven plasma ion source  1  along two linear tracks  27 . 1 ,  27 . 2  within the mass spectrometer  50 . These linear tracks  27 . 1 ,  27 . 2  are oriented perpendicular to each other and perpendicular to the plasma torch orientation direction  29 . Thus, the linear tracks  27 . 1 ,  27 . 2  are arranged parallel to the plane having the normal being the plasma torch orientation direction  29 . This allows for changing a position of the ion inlet  71  of the mass analyser  70  relative to the shielding outlet  5  of the shielding  4  of the microwave driven plasma ion source  1 . Therefore, a number of ions of the sample ions which exit the shielding outlet  5  and subsequently pass through the ion inlet  71  for being mass analysed can easily be maximised by moving the mass analyser  70  relatively to the microwave driven plasma ion source  1  to a correct position. This maximisation enables to increase and thus to optimise the efficiency of the mass spectrometer  50  at different operation parameters of the microwave driven plasma ion source  1 . 
     In a variant, the two linear tracks are not oriented perpendicular but at another angle to each other such that the microwave driven plasma ion source  1  can be moved in two different directions being oriented perpendicular to each other in the plane having the normal being the plasma torch orientation direction  29 . 
     In yet another variant, the microwave driven plasma ion source  1  is movable along a first linear track within the mass spectrometer  50 , while the mass analyser  70  is moveable along a second linear track, wherein the first track and the second track are arranged at an angle to each other and parallel to the plane having the normal being the plasma torch orientation direction  29  such that the microwave driven plasma ion source  1  and the mass analyser  70  can be moved in two different directions being oriented perpendicular to each other in the plane having the normal being the plasma torch orientation direction  29 . 
     In yet another variant where the same optimisation can be achieved, the mass analyser  70  is moveable along two linear tracks within the mass spectrometer  50 , the linear tracks being oriented perpendicular or at another angle to each other and being arranged in a plane having a normal being the plasma torch orientation direction  29 . Thereby, it is possible that only the mass analyser  70  is moveable within the mass spectrometer  50  or that both the mass analyser  70  and the microwave driven plasma ion source  1  are moveable within the mass spectrometer  50 . 
     In variation of the before mentioned variants, the plane parallel to which the linear tracks are arranged has a normal being inclined at an angle of 44° to the plasma torch orientation direction  29 . In another variation, the plane parallel to which the linear tracks are arranged has a normal being inclined at an angle of 19° to the plasma torch orientation direction  29 . 
     These variants and variations have in common that the mass analyser  70  and the microwave driven plasma ion source  70  are moveable relative to each other along two axes being oriented perpendicular to each other. 
     In either one of these variants, the mass spectrometer  50  may comprise one motor for actuating the movement of the microwave driven plasma ion source  1  or the mass analyser  70 , respectively, along one track, and another motor for actuating the movement of the microwave driven plasma ion source  1  or the mass analyser  70 , respectively, along the other track. If both the microwave driven plasma ion source  1  and the mass analyser  70  are moveable along two tracks each, then the mass spectrometer  50  may of course comprise four motors for actuating these movements. 
     In either of these variants, mass analyser  70  and the microwave driven plasma ion source  1  are additionally pivotable relative to each other about two pivot axes which are oriented perpendicular to the plasma torch orientation direction  29 . These two axes are structurally incorporated in the two linear tracks  27 . 1 ,  27 . 2 . Thus, the two linear tracks  27 . 1 ,  27 . 2  and their actuation enable a movement of the mass analyser  70  and the microwave driven plasma ion source  1  relatively to each other along the respective linear track  27 . 1 ,  27 . 2  and a pivoting of the mass analyser  70  and the microwave driven plasma ion source  1  relatively to each other about the respective linear track  27 . 1 ,  27 . 2  and thus about the respective axis. Therefore, an orientation of the mass analyser  70  relative to the microwave driven plasma ion source  1  can be optimised in the planes being oriented perpendicular to the pivot axes in order to maximise the number of ions of the sample ions which exit the torch outlet  22  and the shielding outlet  5  essentially in the plasma torch orientation direction  29  and subsequently pass through the ion inlet  71  for being mass analysed. Thus, the mass spectrometer  50  enables to optimise the efficiency of the transfer of the sample ions from the microwave driven plasma ion source  1  into the mass analyser  70  for mass analysing the sample ions further. 
       FIG.  4    shows a simplified schematic view of a similar mass spectrometer  60  for mass analysing a sample similar to the mass spectrometer  50  shown in  FIG.  3   . This mass spectrometer  60  shown in  FIG.  4    as well includes a microwave driven plasma ion source according to the invention for ionising the sample to sample ions. In contrast to the mass spectrometer  50  shown in  FIG.  3   , the mass spectrometer  60  shown in  FIG.  5    however provides a third linear track  27 . 3  along which the microwave driven plasma ion source  1  is moveable. Thus, the microwave driven plasma ion source  1  is moveable along three linear tracks  27 . 1 ,  27 . 2 ,  27 . 3  within the mass spectrometer  50 . These linear tracks  27 . 1 ,  27 . 2 ,  27 . 3  are oriented perpendicular to each other. Two of these linear tracks  27 . 1 ,  27 . 2  are oriented perpendicular to the plasma torch orientation direction  29 , while the third one of these linear tracks  27 . 3  is oriented parallel to the plasma torch orientation direction  29 . Thus, the mass analyser  70  and the microwave driven plasma ion source  1  can easily be moved in a controlled manner in all directions relative to each other in order to maximise the efficiency of the mass spectrometer  60 . 
       FIG.  5    shows a simplified schematic view of yet another similar mass spectrometer  65  for mass analysing a sample. In this embodiment, the mass spectrometer  65  comprises the microwave driven plasma ion source  201  shown in  FIG.  2   . In variants, the mass spectrometer  65  comprises any other microwave driven plasma ion source according to the invention like for example the one shown in  FIG.  1   . 
     The mass spectrometer  65  shown in  FIG.  5    further comprises a mass analyser  170  being a sector mass analyser which uses magnetic field  175  for mass separating the sample ions spatially such that the sample ions reach a position sensitive detector  176  of the mass analyser  170  at positions which depend on the mass to charge ratio of the respective sample ion and thus indicate the mass to charge ratio of the respective sample ion. Thus, a resolution of the mass analyser  170  depends on a divergence of a beam of the sample ions reaching into the inside  172  of the mass analyser  172  for being mass analysed. In particular, the resolution of the mass analyser  170  depends on a divergence of the beam of the sample ions in an ion separation direction in which the sample ions are spatially separated by the magnetic field  175 . 
     The mass analyser  170  comprises an ion inlet  171  and a second opening  173 .The mass analyser  170  and the microwave driven plasma ion source  201  are moveable relative to each other along one axis being oriented perpendicular to the plasma torch orientation direction  229 . Thereby, the axis being is the linear track  27 . 1  along which the microwave driven plasma ion source  201  is moveable for moving the mass analyser  170  relatively to the microwave driven plasma ion source  201 . By moving the mass analyser  170  relatively to the microwave driven plasma ion source  201 , a number of sample ions exiting from the torch outlet and reaching into the inside of the mass analyser  170  for being mass analysed can be maximised. 
     The invention is not limited to the embodiments and variants described above. Other variants are readily available to the person skilled in the art. 
     In summary, it is to be noted that a microwave driven plasma ion source pertaining to the technical field initially mentioned is created, that enables a simple and stable optimisation of its operation parameters for maximised sample ion generation.