A multipole ion guide and methods of controlling the voltage applied to a multipole ion guide

Abstract

A method of controlling the voltage applied to a multipole ion guide 1 for a mass spectrometer, the multipole ion guide comprising a plurality of rods 10, wherein the method comprises measuring a temp

Description

This invention relates to mass spectrometry, and particularly a multipole ion guide, more particularly to a quadrupole ion guide, and methods of controlling the voltage applied to a multipole ion guide, more particularly a quadrupole ion guide, for use in a mass spectrometer. BACKGROUND Multipole ion guides are used in mass spectrometers for several purposes. One example is a quadrupole mass filter for obtaining a mass spectrum. Direct Current (DC) and Radio Frequency (RF) voltages are applied to the rods (poles) to selectively transmit ions through the quadrupole. At any selected combination of the applied voltages, ions of a corresponding mass-to-charge (m / z) ratio will have a stable trajectory along the ion path and will arrive at a collector of the mass spectrometer of which the ion guide is part. The quadrupole mass filter can therefore be used to filter ions according to their m / z ratio. In use, the RF voltages cause the rods and the dielectric isolators between the rods to heat up and expand. The rods are typically formed from metal. The thermal expansion changes the so-called RO (the radius of the space between the rods) which in turn influences the transmission characteristics of the multipole. The supports (insulators) for the rods are typically made of ceramic. The supports may expand to some extent as temperature increases, partially compensating for the expansion of the rods by increasing the spacing between them. The coefficient of linear expansion of the support and the rod material may be selected to achieve this. However, typically this only partially compensates or even over-compensates the thermal expansion of the rods. In a mass jump (a change in the mass-to-charge ratio (m / z) detected in the mass spectrometer of which the ion guide is part) the RF and DC voltage amplitudes supplied may be changed such that only certain masses or mass ranges are transmitted through the multipole ion guide. In use, the temperature of the supports and the temperature of the rods may be different. This may occur, for example, because in a jump from a lower mass to a higher mass an RF voltage applied to the multipole may generate more heat in the supports (typically formed from ceramic, for example alumina) which in turn heats the rods. In a mass jump from a higher mass to a lower mass, both the rods and the supports are initially at a high temperature but, due to the difference in materials used and the thermal insulation of the rods, the supports cool at a faster rate than the rods. In either case, a change in RO may occur due to the difference in temperatures of the supports and the rods. In a steady state of the multipole ion guide, (e.g. a state in which the ion guide is set to a constant mass over time), RO may vary with temperature due to differences in thermal expansion caused by temperature variations in use. Such temperature variations may occur due to heating from outside the device, for example due the ambient temperature or indirectly due to the plasma source. The rods of the multipole ion guide may be heated up relatively slowly, by contact of the (usually ceramic) rod holder with the vacuum manifold. The ceramic holders may also be heated due to the effects of energy dissipation of the applied RF voltage within the ceramics, which then transfer heat to the rods. JPH05217548 relates to temperature correction means for a quadrupole mass spectrometer. The present disclosure aims to provide an improved multipole ion guide and methods of controlling the voltage applied to a multipole ion guide providing additional compensation for temperature changes to improve the stability of the system. More specifically, the present disclosure aims to provide a method and apparatus having reduced dependence on environmental conditions, reduced startup time and calibration requirements, improved stability during mass jumps and / or improved reliability. SUMMARY The present disclosure provides a method of controlling the voltage applied to a multipole ion guide for a mass spectrometer, the multipole ion guide comprising a plurality of rods, wherein the method comprises: measuring a temperature of at least one rod of the plurality of rods; determining a dimension parameter indicative of a change in a distance between two rods of the plurality of rods based on the measured temperature; and varying a frequency of a voltage applied to the multipole ion guide based on the determined dimension parameter. The method may further comprise measuring a temperature of at least one support of the multipole ion guide; and determining the dimension parameter based on the measured temperatures of both the at least one rod and the at least one support. Measuring a temperature of at least one rod of the plurality of rods may comprise directly measuring the temperature of the rod, wherein directly measuring the temperature of the rod optionally comprises using a non-contact temperature sensor. Variation of the frequency of the voltage applied to the multipole ion guide may be calculated according to the formula: df / f = -dRO / RO wherein df / f is the relative change in frequency of the voltage, and dRO / RO is the relative change in the dimension parameter. The dimension parameter may be a change in the radius of a central space between the rods (dRO). The change in the radius of the central space between the rods (dRO) may defined as: dRO = Rm-Tksupport-dTsupport~ 2.Rrod-Tkrod-dTrod wherein: Tksupport is a coefficient of linear thermal expansion of the support; Tkrod is a coefficient of linear thermal expansion of the rod; dTrod is a difference between the measured temperature of the rod and a previously measured temperature of the rod; and dTsupport is a difference between the measured temperature of the support and a previously measured temperature of the support. The method may further comprise measuring one or more further temperature parameters, the further temperature parameters each being indicative of a temperature of one or more of: a housing configured to house the plurality of rods; an electrical circuit configured to measure or generate one or more voltages applied to the multipole ion guide; an environmental temperature outside of the multipole ion guide; and a local temperature within the multipole ion guide. The method may further comprise using the one or more further temperature parameters in the determination of the dimension parameter. The method may further comprise varying both of: an amplitude of an RF voltage applied to the multipole ion guide; and an amplitude of a DC voltage applied to the multipole ion guide; based on the determined dimension parameter. The method may further comprise using an artificial neural network to determine the dimension parameter based on the measured temperature of the at least one rod of the plurality of rods. The present disclosure also provides a method of stabilising the geometry of a multipole ion guide in use comprising carrying out the method of controlling the voltage applied to a multipole ion guide for a mass spectrometer according to the present disclosure a plurality of times during a single during a single operation of the mass spectrometer. The present disclosure also provides a mass spectrometer comprising a controller configured to carry out the method according to the present disclosure. The mass spectrometer may be an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) or a Gas Chromatography Mass Spectrometer (GC-MS). The present disclosure also provides a multipole ion guide for a mass spectrometer comprising: a plurality of rods; a support configured to support the plurality of rods; a first temperature sensor configured to directly measure a temperature of at least one rod of the plurality of rods; and a second temperature sensor configured to directly measure the temperature of the support. The first temperature sensor and / or the second temperature sensor may each comprise a non-contact temperature sensor directed towards a surface of which the temperature is to be measured. The non-contact temperature sensor may be an infrared temperature sensor. The one or more temperature sensors may further comprise one or more additional temperature sensors, each of the additional temperatures sensors being configured to directly measure the temperature of one of: a housing configured to house the plurality of rods and the support; an electrical circuit configured to measure or generate one or more voltages applied to the multipole ion guide; an environmental temperature outside of the multipole ion guide; and a local temperature within the multipole ion guide. The present disclosure also provides a mass spectrometer comprising a multipole ion guide according to the present disclosure. The mass spectrometer may comprise a controller configured to take repeated temperature measurements from the first temperature sensor and / or the second temperature sensor during operation of the mass spectrometer. The controller may be configured to vary a frequency or amplitude of a voltage applied to the multipole ion guide based on the output of the first and / or second temperature sensors. The mass spectrometer may comprise a Direct Digital Synthesis (DDS) frequency generator configured to generate a waveform of a voltage applied to the multipole ion guide. The mass spectrometer may be an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) or a Gas Chromatography Mass Spectrometer (GC-MS). The present disclosure also provides a method of controlling a voltage applied to a multipole ion guide for a mass spectrometer, the multipole ion guide comprising: a plurality of rods; and a support configured to support the plurality of rods; wherein the method comprises: measuring a temperature of a rod of the plurality of rods; measuring a temperature of the support; and based on the temperature of the rod and the temperature of the support, varying a characteristic of the voltage applied to the multipole ion guide. Varying a characteristic of the voltage applied to the multipole ion guide may comprise varying one or more of: i) a frequency of an RF voltage applied to the multipole ion guide; and ii) the amplitudes of both an RF voltage and a DC voltage applied to the multipole ion guide. Varying a characteristic of the voltage applied to the multipole ion guide may comprise: determining a dimension parameter indicative of a change in a distance between two rods of the plurality of rods based on the temperature of the rod and the temperature of the support; and varying the characteristic of the voltage based on the dimension parameter. The dimension parameter may be a change in the radius of a central space between the rods (dRO) defined as: dRO = Rm-Tksupport-dTsupport ~ 2. Rrod-Tkrod-dTrod wherein: Tksupport is a coefficient of linear thermal expansion of the support; Tkrod is a coefficient of linear thermal expansion of the rod; dTrod is a difference between the measured temperature of the rod and a previously measured temperature of the rod; and dTsupport is a difference between the measured temperature of the support and a previously measured temperature of the support. Variation of the frequency of the RF voltage applied to the multipole ion guide may be calculated according to the formula: df / f= -dRO / RO wherein df / f is the relative change in frequency of the voltage, dRO / RO is the relative change in the dimension parameter. Variation of the amplitudes of both an RF voltage and a DC voltage applied to the multipole ion guide may be calculated according to the formulae: dU / U=2*dR0 / R0 and dV / V=2*dR0 / R0. wherein dll / U is the relative change in the DC voltage, dV / V is the relative change in the RF voltage, and dRO / RO is the relative change in the dimension parameter. The method may further comprise using an artificial neural network to determine the dimension parameter based on the measured temperature of the at least one rod of the plurality of rods. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the disclosure will now be described, by way of example only, with reference to the following non-limiting figures in which: Figure 1 is a perspective view of the rods and supports of a multipole ion guide according to the present disclosure; Figure 2 is a schematic cross-sectional view of a multipole ion guide of the type shown in Figure 1, showing the cross section parallel to the longitudinal direction of the rods such that two rods are visible; Figure 3 is a schematic cross-sectional view of a multipole ion guide of the type shown in Figure 1, showing the cross section perpendicular to the longitudinal direction of the rods; and Figure 4 is a schematic diagram of a mass spectrometer comprising a multipole ion guide according to the present disclosure. DETAILED DESCRIPTION A multipole ion guide 1 according to the present disclosure comprises a plurality of rods 10, for example as shown in Figures 1 and 2. The rods 10 may be arranged around a central axis 11 of the multipole ion guide 1 in a generally symmetrical arrangement. The rods 10 may be elongate in shape and may be parallel to each other and / or to the central axis 11. A central space 12 may be defined between the rods 10, through which ions flow along an ion path in use. The rods 10 may be in pairs, the rods of each pair being arranged on opposite sides of the central space 12 to each other. The plurality of rods 10 are held in position by at least one support 20. The at least one support 20 may be a plurality of insulated supports 20 as shown in the illustrated embodiments. The multipole ion guide 1 comprises at least one temperature sensor. As shown in Figure 2, the multipole ion guide 1 may comprise a first temperature sensor 30a configured to directly measure the temperature of a rod 10 of the plurality of rods, and a second temperature sensor 30b configured to directly measure the temperature of a support 20 of the at least one supports. The one or more temperature sensors may be non-contact temperature sensors, for example infrared sensors. The non-contact temperature sensors may be directed towards a surface of the component of which the temperature is to be measured, as indicated by the dotted lines in Figure 2. The one or more temperature sensors may be arranged on a single printed circuit board (PCB). Figure 3 is a schematic cross-sectional view of a multipole ion guide 1 of the type shown in Figure 2 (with the supports omitted for clarity) taken generally perpendicular to the central axis 11. In the illustrated examples, the multipole ion guide 1 is a quadrupole i.e. it comprises four rods 10 arranged in two pairs. The radius (R0, also written as Ro) of the central space 12 between the rods 10 is defined by the dimensions of the rods 10 and the dimensions of the central space 12 between them. The positions of the rods 10 may be defined by the supports 20, as shown in Figure 2. In use, a Radio Frequency (RF) voltage, V, in combination with a Direct Current (DC) voltage, U, is applied between pairs of rods 10 of the plurality of rods 10. A second aspect of the present disclosure provides a method of controlling a voltage applied to a multipole ion guide 1 for a mass spectrometer, in which a characteristic of a voltage applied to the multipole ion guide 1 is varied to compensate for thermal expansion in one or more components (for example, the plurality of rods 10 and / or the one or more supports 20) of the mass spectrometer. The method comprises measuring a temperature of a rod 10 of a plurality of rods 10 of the multipole ion guide 1 and measuring a temperature of a support 20 configured to support the plurality of rods 10. The method further comprises varying a characteristic of the voltage applied to the multipole ion guide 1 based on the temperature of the rod 10 and the temperature of the support 20. Varying the characteristic of the voltage may comprise varying a frequency of the RF voltage applied to the multipole ion guide 1 and / or varying an amplitude of both the RF voltage and the DC voltage applied to the multipole ion guide 1. Varying the characteristic of the voltage may comprise determining a dimension parameter indicative of a change in a distance between two rods of the plurality of rods 10 based on the temperature of the rod 10 and the temperature of the support 20 and varying the characteristic of the voltage based on the dimension parameter. The dimension parameter may be a change in the radius of the central space between the rods, dRO (also written as dR0. An outer radius Rm of the multipole ion guide 1 may be defined with respect to R0 and the radius of the rods (RrOd) according to the formula: Rm = R0 + 2.Rrod The change in the radius of the central space between the rods, dRO, may therefore be defined as: dRO = Rm-Tksupport-dTsupport~ 2.Rrod-Tkrod-dTrod wherein: Tksupport is a coefficient of linear thermal expansion of the support 20; Tkrod is a coefficient of linear thermal expansion of the rod 10; dTrod is a difference between the measured temperature of the rod 10 and a previously measured temperature of the rod 10; and dTsupport is a difference between the measured temperature of the support 20 and a previously measured temperature of the support 20. The previously measured temperature may be an initial temperature, a steady state temperature of the rod 10, a temperature measured earlier in the process, or a temperature measured at a defined time period t before the current measurement, where t may be, for example, in the range of 100 msec to 1000 seconds. A third aspect of the present disclosure provides a method of controlling the voltage applied to a multipole ion guide 1 for a mass spectrometer. The multipole ion guide 1 comprises a plurality of rods 10. The method comprises measuring a temperature of at least one rod of the plurality of rods 10, determining a dimension parameter indicative of a change in a distance between two rods of the plurality of rods 10 based on the measured temperature, and varying a frequency of a voltage applied to the multipole ion guide 1 based on the determined dimension parameter. The method may therefore comprise changing the frequency of the RF voltage applied to the multipole ion guide 1 in response to temperature changes, to compensate for the thermal expansion of the rods 10. The method may also comprise varying both an amplitude of an RF voltage applied to the multipole ion guide 1 and an amplitude of a DC voltage applied to the multipole ion guide 1 based on the determined dimension parameter. The method may further comprise measuring a temperature of at least one support 20 of the multipole ion guide 1 and determining the dimension parameter based on the measured temperatures of both the at least one rod and the at least one support 20. Measuring a temperature of at least one rod of the plurality of rods 10 may comprise directly measuring the temperature of the rod, using a contact or non-contact temperature sensor, for example temperature sensor 30a as described above. The dimension parameter may be a change in the radius of the central space between the rods, dRO, which may be determined as described above. In any method according to the present disclosure, the variation of the frequency of the voltage applied to the multipole ion guide 1 may be calculated according to the formula: df / f = -dRO / RO or alternatively dU / U=2*dR0 / R0 and simultaneously dV / V=2*dR0 / R0 wherein df / f is the relative change in frequency of the voltage, dll / U the relative change in the DC voltage, dV / V the relative change in the RF voltage, dRO / RO is the relative change in the dimension parameter (a change in the radius of the central space between the rods). These relationships can be derived as follows: Variables a and q may be defined as: in which U is the amplitude of the DC Voltage, V is the amplitude of the RF voltage, f is the frequency of the RF voltage and m the mass. For a stable operation of the multipole ion guide, both a and q must be kept constant. The skilled person will therefore understand that a change in the RO due to temperature effects can be compensated either by: • adjusting the frequency f of the RF voltage; or • changing both U and V at the same time. When both a and q are constant, taking the partial differential: ’ 3 X (hi ., ,, 0 If f is constant, df=O giving: Or, using the definition above and its partial differential for a: r __ .) r r .¾ The same derivation can be performed for q, giving: Therefore, U and V have to be varied simultaneously. If U and V are kept constant, with f as the variable, then, using / / >7 / ) / 5. . / ) / / . -- the following can be derived: .... which resolves to: / / ¾ The same derivation can be applied to parameter q, and therefore only one parameter (f) is required to be varied to compensate for the change of RO on both a and q. The relative (or percentage) change in the frequency is equal to the relative (or percentage) change in RO. In any embodiment of the present disclosure, the one or more temperature sensors may be used to determine a steady state temperature and therefore the radius of the central space between the rods (RO) in the steady state. By taking repeated measurements of the temperature throughout the use of the multipole ion guide 1, the methods and apparatus of the present disclosure may therefore be used to dynamically vary either the RF frequency f and / or the RF and DC voltage amplitudes U and V in response to the temperature dependent dimensional change dRO, which is determined as described above from the measured temperatures. In any embodiment according to the present disclosure, repeated measurements of the temperature of the rod and / or other components (for example, the support) may be taken during an operation of the mass spectrometer. The characteristics of the voltage applied to the multipole ion guide 1 may therefore be repeatedly varied based on the measured temperature(s) according to any of the methods described herein in order to compensate for the dimensional changes caused changes in temperature, for example those resulting from selection of higher ion masses in mass jumps. The methods described herein may therefore be methods of stabilising the geometry of the multipole ion guide 1 in use, optionally comprising carrying out any of the described methods of controlling the voltage applied to the multipole ion guide a plurality of times during a single operation of the mass spectrometer. As shown in Figure 4, a mass spectrometer 40 according to the present disclosure comprises a multipole ion guide 1 as described herein, the multipole ion guide 1 comprising at least one temperature sensor 30. The mass spectrometer may comprise an ion source 42, ion optics 43 and detector 44. The mass spectrometer 40 further comprises a controller 41 configured to take repeated temperature measurements from the at least one temperature sensor during operation of the mass spectrometer 40. The controller 41 may be configured vary a characteristic (for example, a frequency or amplitude) of a voltage applied to the multipole ion guide 1 based on the output of the at least one temperature sensor. The at least one temperature sensor may comprise the first temperature sensor 30a and the second temperature sensor 30b as shown in Figure 2. The variation of the characteristic of the voltage may be carried out according to any of the methods described herein. While embodiments of the present disclosure have been described above and illustrated in the drawings, these are for example only and are non-limiting. It will be appreciated by those skilled in the art that alternatives are possible within the ambit of the disclosure. For example, the multipole ion guide, mass spectrometer and methods of the present disclosure may comprise any combination of the following features. In the illustrated embodiments, the multipole ion guide 1 is shown as a quadrupole ion guide. The skilled person will understand that in any embodiment according to the present disclosure, the multipole ion guide 1 may comprise any number of pairs of rods, for example 6 rods (hexapole) or 8 rods (octopole). The mass spectrometer 40 may be an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) or a Gas Chromatography Mass Spectrometer (GC-MS), for example. In any embodiment of the present disclosure, the multipole ion guide 1 may additionally comprise one or more additional temperature sensors configured to directly measure the temperature of one or more of: • a housing configured to house the plurality of rods and the support; • an electrical circuit configured to measure or generate one or more voltages in the multipole ion guide 1; • an environmental temperature outside of the multipole ion guide 1; and • a local temperature within the multipole ion guide 1, for example this may be a temperature at a point between the multipole rods and the plasma passing through the multipole ion guide. Any method according to the present disclosure may therefore additionally include measuring one or more of the above temperatures using the one or more further temperature parameters in the determination of the dimension parameter. For example, such calculations may take into account any temperature gradients along the rods, differences between internal and surface temperatures, and / or any systematic errors in the temperature measurements. Incorporating such temperature measurements into the determination of the dimension parameter may comprise calibration and / or machine learning processes. In any embodiment, a dimension parameter indicative of a change in a distance between two rods of the plurality of rods may be determined based on a measured temperature by calculation (e.g. by the equations described above) or by other means, for example by use of a look-up table generated from experimental measurements, or through experimentation and use of an artificial neural network. In embodiments comprising measuring a temperature of at least one rod of the plurality of rods and determining a dimension parameter indicative of a change in a distance between two rods of the plurality of rods based on the measured temperature, the determination of the dimension parameter may be carried out based on a single temperature measurement, for example a measurement of the temperature of the rod. For example, this may be done if the temperature of the rods and / or the supports are assumed to be approximately uniform, or if the temperature of the rods and / or the supports are assumed to be constant (i.e. not varying with time). Corrections for non-uniformities may be applied. If a single temperature measurement is used, the dimension parameter may be determined from lookup table generated from experimental measurements. For example, a look-up table may be generated during a calibration process for each instrument. The look up table may provide df or dll and dV values corresponding to the single temperature measurement. Interpolation may be used for measured values between those listed in the table. As used herein, being “configured to directly measure a temperature” of a component refers to measurement of the temperature of a surface or interior of that component, either by physical contact or using a non-contact temperature sensor. In the illustrated examples, the temperature sensors 30a, 30b are infrared sensors. In any embodiment according to the present disclosure the one or more temperature sensors may be any type of suitable temperature sensor. For example, the one or more temperature sensors may be contact-type sensors such as thermocouples or non-contact sensors such as infrared sensors, or infrared cameras used in combination with image processing. In embodiments where the temperature sensor is an infrared sensor, the sensor may comprise simple optics, for example a lens, or may comprise an infrared camera. By using non-contact temperature sensors, for example an infrared sensor or infrared camera, the multipole ion guide 1 may limit any interference effect of the RF voltage with the one or more temperature sensors and may also limit any interference effect the temperature sensor may have on the rods 10. In any embodiment comprising a contactless infrared sensor, the temperature sensor may be place inside or outside of the vacuum chamber. A lens system and / or optical fibre may be provided for the sensor. In any embodiment of the present disclosure, the one or more temperature sensors may comprise fibre optic components and may be arranged outside a vacuum housing of the mass spectrometer 40. The one or more temperature sensors may be infrared temperature sensors. The housing may therefore comprise one or more components which allow the infrared signal to pass through the housing, for example a silica window, plastic optical fibres or lens optics. By measuring the temperature in multiple locations, including at least one location on a rod 10 and at least one location on a support 20, the apparatus and methods according to the present disclosure may provide a more accurate and reliable temperature determination. In any embodiment, the one or more temperature sensors may comprise a plurality of temperature sensors configured to measure the temperature the rod 10 and / or a plurality of temperature sensors configured to measure the temperature of the support 20. In any embodiment of the present disclosure the dimension parameter may be indicative of a change in a distance between any two rods of the plurality of rods 10 arranged opposite each other across the central space 12, for example the dimension parameter may be a radius R0 of the central space 12. In any embodiment of the present disclosure the rods may have a circular or oval cross section. They may be cylindrical or have a hyperbolic form, for example. In some embodiments, the steady state value of R0 may be about 2.3 mm, or about 2.23 mm, although different values of R0 are also possible, for example up to about 6.0 mm. In any method according to the present disclosure, an artificial neural network may be used to calculate the relationship between the measured temperature of at least one rod 10, any additional temperature measurements taken (including, for example, the measured temperature of the support), and the dimension parameter dRO. In order to define this relationship, a measurement program may be performed in which the measured temperatures are recorded during or after one or more of: a mass jump, both upwards and downwards; a jump in plasma power in the mass spectrometer, upwards or downwards; switching on the mass spectrometer or a part of the mass spectrometer; or a change in the environmental temperature around the mass spectrometer or the multipole ion guide 1 (which may be achieved by blocking the ventilation of the mass spectrometer or by locally heating the mass spectrometer). The method may therefore generate a matrix of all changes which may lead to a change in R0. A controller of a mass spectrometer comprising the multipole ion guide 1 may be configured to carry out the measurement program. The determined relationship may be used to determine dRO in any method according to the present disclosure. In any embodiment of the present disclosure the frequency f of the RF voltage may be the angular frequency of the RF voltage. The (controller of the) multipole ion guide 1 and / or the mass spectrometer 40 may use digital electronics instead of, or in addition to, analogue electronics. By varying only one characteristic (such as frequency) of a voltage applied to the multipole ion guide 1, the apparatus and methods of the present disclosure may improve accuracy in such an apparatus in comparison to varying multiple characteristics, such as U and V, for which two adjustments are required and the non-linearity of both of the adjusted factors may have a negative effect. The mass spectrometer 40 may comprise a Direct Digital Synthesis (DDS) frequency generator configured to generate the RF frequency waveform such that the frequency of the RF voltage may be adjusted in small increments. Alternatively, or in addition, phase-locked loop (PLL) synthesis or other methods may be used to generate the RF frequency waveform. In any embodiment, the voltage applied to the multipole ion guide may be a voltage applied to a pair of opposing rods of the multipole ion guide. In a quadrupole ion guide comprising first and second pairs of opposing rods, the voltage may be applied to the first pair of opposing rods, and the opposite voltage may be applied to the second pair of opposing rods. Any controller, multipole ion guide and / or mass spectrometer of the present disclosure may be configured to carry out any method according to the present disclosure. Examples according to the present disclosure are set out in the following numbered clauses: A1. A multipole ion guide for a mass spectrometer comprising: a plurality of rods; a support configured to support the plurality of rods; a first temperature sensor configured to directly measure a temperature of at least one rod of the plurality of rods; and a second temperature sensor configured to directly measure the temperature of the support. A2. The multipole ion guide of clause A1 wherein the first temperature sensor and / or the second temperature sensor each comprise a non-contact temperature sensor directed towards a surface of which the temperature is to be measured. A3. The multipole ion guide of clause A2 wherein the non-contact temperature sensor is an infrared temperature sensor. A4. The multipole ion guide of any preceding clause wherein the one or more temperature sensors further comprise one or more additional temperature sensors, each of the additional temperatures sensors being configured to directly measure the temperature of one of: a housing configured to house the plurality of rods and the support; an electrical circuit configured to measure or generate one or more voltages applied to the multipole ion guide; an environmental temperature outside of the multipole ion guide; and a local temperature within the multipole ion guide. A5. A mass spectrometer comprising a multipole ion guide according to any preceding clause. A6. The mass spectrometer of clause A5 further comprising a controller configured to take repeated temperature measurements from the first temperature sensor and / or the second temperature sensor during operation of the mass spectrometer. A7. The mass spectrometer of clause A6 wherein the controller is configured to vary a frequency or amplitude of a voltage applied to the multipole ion guide based on the output of the first and / or second temperature sensors. A8. The mass spectrometer of any of clauses A5 to A7 further comprising a Direct Digital Synthesis (DDS) frequency generator configured to generate a waveform of a voltage applied to the multipole ion guide. A9. The mass spectrometer of any of clauses A5 to A8, wherein the mass spectrometer is an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) or a Gas Chromatography Mass Spectrometer (GC-MS). A10. A method of controlling a voltage applied to a multipole ion guide for a mass spectrometer, the multipole ion guide comprising: a plurality of rods; and a support configured to support the plurality of rods; wherein the method comprises: measuring a temperature of a rod of the plurality of rods; measuring a temperature of the support; and based on the temperature of the rod and the temperature of the support, varying a characteristic of the voltage applied to the multipole ion guide. A11. The method of clause A10, wherein varying a characteristic of the voltage applied to the multipole ion guide comprises varying one or more of: i) a frequency of an RF voltage applied to the multipole ion guide; and ii) the amplitudes of both an RF voltage and a DC voltage applied to the multipole ion guide. A12. The method of clause A10 or clause A11, wherein varying a characteristic of the voltage applied to the multipole ion guide comprises: determining a dimension parameter indicative of a change in a distance between two rods of the plurality of rods based on the temperature of the rod and the temperature of the support; and varying the characteristic of the voltage based on the dimension parameter. A13: The method of clause A12 wherein the dimension parameter is a change in the radius of a central space between the rods (dRO) defined as: dRO = Rm-Tksupport-dTsupport ~ 2. Rrod-Tkrod-dTrod wherein: Tksupport is a coefficient of linear thermal expansion of the support; Tkrod is a coefficient of linear thermal expansion of the rod; dTrod is a difference between the measured temperature of the rod and a previously measured temperature of the rod; and dTsupport is a difference between the measured temperature of the support and a previously measured temperature of the support. A14. The method of clause A12 or clause A13, as dependent on clause A11, wherein variation of the frequency of the RF voltage applied to the multipole ion guide is calculated according to the formula: df / f = -dRO / RO wherein df / f is the relative change in frequency of the voltage, dRO / RO is the relative change in the dimension parameter. A15. The method of any one of clauses A12 to A14, as dependent on clause A11, wherein variation of the amplitudes of both an RF voltage and a DC voltage applied to the multipole ion guide is calculated according to the formulae: dU / U=2*dR0 / R0 and dV / V=2*dR0 / R0. wherein dll / U is the relative change in the DC voltage, dV / V is the relative change in the RF voltage, and dRO / RO is the relative change in the dimension parameter. A16. The method of any one of clauses A10 to A15 further comprising using an artificial neural network to determine the dimension parameter based on the measured temperature of the at least one rod of the plurality of rods. B1. A method of controlling the voltage applied to a multipole ion guide for a mass spectrometer, the multipole ion guide comprising a plurality of rods, wherein the method comprises: measuring a temperature of at least one rod of the plurality of rods; determining a dimension parameter indicative of a change in a distance between two rods of the plurality of rods based on the measured temperature; and varying a frequency of a voltage applied to the multipole ion guide based on the determined dimension parameter. B2. The method of clause B1 further comprising: measuring a temperature of at least one support of the multipole ion guide; determining the dimension parameter based on the measured temperatures of both the at least one rod and the at least one support. B3. The method of any of clauses B1 or B2 wherein measuring a temperature of at least one rod of the plurality of rods comprises directly measuring the temperature of the rod, wherein directly measuring the temperature of the rod optionally comprises using a noncontact temperature sensor. B4. The method of any of clauses B1 to B3 wherein the variation of the frequency of the voltage applied to the multipole ion guide is calculated according to the formula: df / f = -dRO / RO wherein df / f is the relative change in frequency of the voltage, and dRO / RO is the relative change in the dimension parameter. B5. The method of any of clauses B1 to B4 wherein the dimension parameter is a change in the radius of a central space between the rods (dRO). B6. The method of clause B5, wherein the change in the radius of the central space between the rods (dRO) defined as: dRO = Rm-Tksupport-dTsupport~ 2.Rrod-Tkrod-dTrod wherein: Tksupport is a coefficient of linear thermal expansion of the support; Tkrod is a coefficient of linear thermal expansion of the rod; dTrod is a difference between the measured temperature of the rod and a previously measured temperature of the rod; and dTsupport is a difference between the measured temperature of the support and a previously measured temperature of the support. B7. The method of any of clauses B1 to B6 further comprising measuring one or more further temperature parameters, the further temperature parameters each being indicative of a temperature of one or more of: a housing configured to house the plurality of rods; an electrical circuit configured to measure or generate one or more voltages applied to the multipole ion guide; an environmental temperature outside of the multipole ion guide; and a local temperature within the multipole ion guide. B8. The method of clause B7 further comprising using the one or more further temperature parameters in the determination of the dimension parameter. B9. The method of any of clauses B1 to B8 further comprising varying both of: an amplitude of an RF voltage applied to the multipole ion guide; and an amplitude of a DC voltage applied to the multipole ion guide; based on the determined dimension parameter. B10. The method of any of clauses B1 to B9 further comprising using an artificial neural network to determine the dimension parameter based on the measured temperature of the at least one rod of the plurality of rods. B11. A method of stabilising the geometry of a multipole ion guide in use comprising carrying out the method of controlling the voltage applied to a multipole ion guide for a mass spectrometer according to any of clauses B1 to B10 a plurality of times during a single during a single operation of the mass spectrometer. B12. A mass spectrometer comprising a controller configured to carry out the method according to any of clauses B1 to B11. B13. The mass spectrometer of clause B12, wherein the mass spectrometer is an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) or a Gas Chromatography Mass Spectrometer (GC-MS).

Claims

1. A method of controlling the voltage applied to a multipole ion guide for a mass spectrometer, the multipole ion guide comprising a plurality of rods, wherein the method comprises:measuring a temperature of at least one rod of the plurality of rods;determining a dimension parameter indicative of a change in a distance between two rods of the plurality of rods based on the measured temperature; andvarying a frequency of a voltage applied to the multipole ion guide based on the determined dimension parameter.

2. The method of claim 1 further comprising:measuring a temperature of at least one support of the multipole ion guide; determining the dimension parameter based on the measured temperaturesof both the at least one rod and the at least one support.

3. The method of any preceding claim wherein measuring a temperature of at least one rod of the plurality of rods comprises directly measuring the temperature of the rod, wherein directly measuring the temperature of the rod optionally comprises using a noncontact temperature sensor.

4. The method of any preceding claim wherein the variation of the frequency of the voltage applied to the multipole ion guide is calculated according to the formula:df / f = -dRO / ROwherein df / f is the relative change in frequency of the voltage, and dRO / RO is the relative change in the dimension parameter.

5. The method of any preceding claim wherein the dimension parameter is a change in the radius of a central space between the rods (dRO).

6. The method of claim 5, wherein the change in the radius of the central space between the rods (dRO) is defined as:dRO — Rm-Tksupport-dTsupport~ 2.Rrod-Tkrod-dTrodwherein:Tksupport is a coefficient of linear thermal expansion of the support;Tkrod is a coefficient of linear thermal expansion of the rod;dTrod is a difference between the measured temperature of the rod and a previously measured temperature of the rod; anddTsupport is a difference between the measured temperature of the support and a previously measured temperature of the support.

7. The method of any preceding claim further comprising measuring one or more further temperature parameters, the further temperature parameters each being indicative of a temperature of one or more of:a housing configured to house the plurality of rods;an electrical circuit configured to measure or generate one or more voltages applied to the multipole ion guide;an environmental temperature outside of the multipole ion guide; and a local temperature within the multipole ion guide.

8. The method of claim 7 further comprising using the one or more further temperature parameters in the determination of the dimension parameter.

9. The method of any preceding claim further comprising varying both of: an amplitude of an RF voltage applied to the multipole ion guide; and an amplitude of a DC voltage applied to the multipole ion guide; based on the determined dimension parameter.

10. The method of any preceding claim further comprising using an artificial neural network to determine the dimension parameter based on the measured temperature of the at least one rod of the plurality of rods.

11. A method of stabilising the geometry of a multipole ion guide in use comprising carrying out the method of controlling the voltage applied to a multipole ion guide for a mass spectrometer according to any preceding claim a plurality of times during a single during a single operation of the mass spectrometer.

12. A mass spectrometer comprising a controller configured to carry out the method according to any preceding claim.

13. The mass spectrometer of claim 12, wherein the mass spectrometer is an Inductively 5 Coupled Plasma Mass Spectrometer (ICP-MS) or a Gas Chromatography Mass Spectrometer (GC-MS).

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