WO2025036729A1

WO2025036729A1 - Mass spectrometer comprising a vacuum system and a method of operation

Info

Publication number: WO2025036729A1
Application number: PCT/EP2024/071892
Authority: WO
Inventors: Hans-Juergen Schlueter
Original assignee: Thermo Fisher Scientific Bremen GmbH
Current assignee: Thermo Fisher Scientific Bremen GmbH
Priority date: 2023-08-17
Filing date: 2024-08-01
Publication date: 2025-02-20
Anticipated expiration: 2026-02-17
Also published as: GB202312561D0; GB2632683A

Abstract

The present disclosure relates to a mass spectrometer (10) comprising a vacuum system (14) and a method of operating a vacuum system (14). The method of operating the vacuum system (14) of the mass spectrometer (10) is for preventing vapours from diffusing from a vacuum pump (41) of the mass spectrometer (10) into an analyser (12) of the mass spectrometer (10). The mass spectrometer (10) comprises the analyser (12), the vacuum pump (41) and a vacuum conduit (45) extending between the vacuum pump (41) and the analyser (12). The method comprises operating the mass spectrometer (10) in a standby mode wherein a sample is not present in the analyser (12) for analysis; and inducing a standby fluid flow in the vacuum conduit (45) away from the analyser (12) towards the vacuum pump (41) while the mass spectrometer (10) is in the standby mode.

Description

MASS SPECTROMETER COMPRISING A VACUUM SYSTEM AND A METHOD OF OPERATION

Technical Field

The present disclosure relates to a mass spectrometer comprising a vacuum system and a method of operating a vacuum system.

Background

Mass spectrometers are used to analyse chemical samples. In general, mass spectrometers analyse a sample by converting a portion of the sample into ions (negatively charged particles or, more commonly, positively charged particles due to removal of electrons), and then passing those ions through a mass separation device, such as an analyser. The mass separation device uses any of a magnetic field, a combination of magnetic and electric sector fields, an RF filter such as a quadrupole, a DC trap such as an orbitrap, an RF trap, an time of flight analyser, or a combination of any of the above. The mass spectrometer comprises the analyser in which the ions of the sample are analysed. By measuring metrics, such as a mass to charge ratio of particles within the sample, a mass spectrum of the sample can be analysed. This can be useful for identifying a chemical make-up of the sample.

One type of mass spectrometer is an inductively coupled plasma-mass spectrometer (ICP- MS). In an ICP-MS, a plasma torch is used to ionise the particles in the sample. The plasma torch uses an inert gas, such as argon, which is raised to a very high temperature by means of an alternating magnetic field. The sample is then introduced into the plasma torch such that the sample is atomized, and a portion of the sample is ionised due to the high temperatures. These ions are passed into the analyser for analysis. In an ICP-MS, the sample is introduced into the analyser via an interface, which is described in more detail below.

The mass separation properties of the mass spectrometer rely on the fact that the path of the ions is determined only by starting properties (e.g., kinetic energy, angle) of the ions, electric and magnetic field properties on the pathway of the ions and the ion mass to charge ratio. Any collisions, e.g., with neutral particles, can decrease the performance of the analyser. In other words, a high mean free path for the particles is required within the analyser. This may be ensured by keeping the analyser under a vacuum in which other particles are not likely to obstruct the flight path of the analysed ions. To create the vacuum in the analyser, a vacuum system comprising at least one vacuum pump and a vacuum conduit extending between the analyser and the vacuum pump may be used. The vacuum pump draws gas and other molecules from the analyser along the vacuum conduit and then subsequently exhausts the gas to the atmosphere. This ensures that the analyser operates under a vacuum, and so a high free mean path of particles is present.

The level of vacuum required for the analyser is generally known as a high vacuum. The high vacuum is one in which there is a very low level of gas particles in the analyser. This is often created by using a vacuum system with two vacuum pumps, a first vacuum pump and a second vacuum pump.

The first vacuum pump is preferably a fore vacuum pump, arranged to create a rough vacuum within the analyser and vacuum system. This pump may be a rotary vane pump which is effective at pumping large volumes of fluid efficiently. In addition, rotary vane pumps are low complexity pumps and therefore generally available at a relatively low cost. However, the rotary vane pump is not able to efficiently create a high vacuum of the level required for the mass spectrometer.

Therefore, the second vacuum pump is preferably a high vacuum pump, able to produce a high vacuum. The second vacuum pump is often a turbo molecular pump which is able to create a high vacuum. However, a turbo molecular pump is configured to only operate in a molecular gas range, which occurs when the rough vacuum is present (and not at pressures near atmospheric pressure). Modern turbo molecular pumps may also be able to operate in a transition range, which has a slightly higher gas pressure than the molecular range. The molecular range is defined by a feature that a mean free path of gas particles is at least as long as the distance required to be travelled by a gas particle between two surfaces present. Because of this, at an outlet of a turbo molecular pump that can partially operate in the transition region, the pressure must be below, e.g., 10 mbar. Therefore, a turbo molecular pump may be combined with the first pump to ensure that a rough vacuum is present and therefore ensure that the turbo molecular pump operates in the correct pressure regime. The vacuum system is generally arranged with the vacuum conduit extending from the analyser to the fore vacuum pump and the high vacuum pump located on the vacuum conduit between the analyser and the fore vacuum pump.

Although a high vacuum is present in the analyser, the sample may be in a sample chamber at atmospheric pressure prior to entering the mass spectrometer. This is desirable because it can avoid the need to evacuate the sample chamber prior to analysis.

Therefore, the sample has to pass from the sample chamber at atmospheric pressure into the analyser held at a high vacuum. To accomplish this, the sample is passed through the interface between the sample chamber and the analyser.

In order to allow a sufficient number of ions to enter the analyser, it is not possible for the interface to simply comprise a very small opening to allow the ions to be introduced directly into the analyser from the sample chamber. Instead, the interface comprises two openings which are in line. Most of the sample, in gas form, entering the first opening is pumped out, and only a small fraction of this gas, together with a fraction of the ions, enters the analyser. Therefore, a very high gas flow is pumped out of the interface when a sample is analysed. There is a second vacuum conduit extending from the interface to the vacuum pump, specifically the fore vacuum pump, to evacuate the interface during operation.

In addition to needing to be under a high vacuum to analyse a sample, residual gas particles within the mass spectrometer may also impact the lifetime and performance of the mass spectrometer.

To address this issue, there is room for improvement to the current vacuum systems of mass spectrometers.

Summary

It has been noted by the inventors of the present invention that one source of residual gas particles in the analyser is vapour, particularly hydrocarbon vapour, diffusing into the analyser. Impurities containing carbon, such as hydrocarbons, may particularly impact the lifetime and performance of the mass spectrometer. Carbon containing impurities can accelerate the aging of detectors used in mass spectrometers, such as secondary electron multipliers. Another negative factor of hydrocarbons and similar impurities is that they may build up surface layers of limited conductivity, leading to reduced efficiency of charging and a distortion of the electric fields. Additionally, hydrocarbons may lead to spatially different electron work function, also leading to a distortion of the electric fields.

This vapour can originate from the vacuum pump, and specifically the fore vacuum pump which may require high levels of oil lubrication to function effectively. Vapours from the vacuum pump are able to diffuse along the vacuum system and into the analyser.

The inventors have noted that while a sample is being analysed, particularly in an ICP-MS, a large flow of excess argon gas, from the plasma torch, travels along the conduit system towards the vacuum pump. Therefore, when the sample is being analysed, it should be impossible for vapour to diffuse from the vacuum pump into the analyser, since this would require diffusion upstream against a large flow of argon gas.

In the light of this, the inventors have determined that hydrocarbon vapour can only diffuse from the vacuum pump into the analyser when a sample is not being analysed. This is when the mass spectrometer is in a standby mode. An object of the present invention is to provide systems and methods for preventing vapour from diffusing from the vacuum pump, through the vacuum system, to the analyser in standby mode.

Potential solutions include the use of oil-free fore vacuum pumps, or oil traps in the vacuum system. However, oil-free fore vacuum pumps can be large and expensive as compared to rotary vane pumps, and oil traps may be large, costly, cause vacuum restriction; and need maintenance. Therefore other, more efficient, solutions are needed.

There is therefore provided a method of operating the vacuum system of a mass spectrometer, for preventing vapours from diffusing from a vacuum pump of the mass spectrometer into an analyser of the mass spectrometer. The mass spectrometer comprises the analyser, the vacuum pump, and a vacuum conduit extending between the vacuum pump and the analyser. The method comprises operating the mass spectrometer in a standby mode wherein a sample is not present in the analyser for analysis, and inducing a standby fluid flow in the vacuum conduit away from the analyser towards the vacuum pump while the mass spectrometer is in the standby mode.

The standby fluid flow in the vacuum conduit away from the analyser towards the vacuum pump advantageously can prevent vapour from diffusing from the vacuum pump, through the vacuum system, to the analyser in standby mode. The vapour can be unable to diffuse upstream against the standby fluid flow and can therefore be prevented from entering the analyser. This can improve the lifetime and performance of the mass spectrometer.

Preferably, the mass spectrometer is operable in at least two modes including an analysing mode in which a sample is analysed, and a flow of an analysing fluid is thereby induced to travel in the vacuum conduit towards the vacuum pump; and the standby mode. The two modes can allow the mass spectrometer to analyse the sample when operated, and be in standby when not operated, to reduce power consumption and yet preserve the vacuum within the mass spectrometer. Preferably, the flow of analysing fluid in the analysing mode has a larger magnitude than the standby fluid flow in the standby mode. This can enable the standby mode to have a low energy consumption as compared with the analysing mode, while keeping the standby fluid flow large enough for preventing vapour from diffusing from the vacuum pump, through the vacuum system, to the analyser.

Optionally, the standby fluid flow is constantly present while the mass spectrometer is in the standby mode. Advantageously, this can ensure that the vapour will be constantly prevented from diffusing from the vacuum pump, through the vacuum system, to the analyser.

Preferably, the mass spectrometer further comprises a fluid container and a flow conduit extending between the vacuum conduit and the fluid container, wherein in the standby mode the standby fluid flow in the vacuum conduit is induced by fluid flowing from the fluid container, through the flow conduit into the vacuum conduit. Optionally, the fluid container may be any source of fluid. Alternatively, the mass spectrometer may not comprise a fluid container, in which case the flow conduit extends from the vacuum conduit.

Advantageously, these features can provide a standby fluid flow in a passive manner without requiring additional steps or electrical power (except for the increased power needed by the vacuum pump to account for the additional flow). Preferably, the flow conduit comprises a section with reduced diameter as compared with the vacuum conduit. Advantageously, this can regulate the size of the standby fluid flow so that the vacuum pump is still able to create an effective vacuum, while still benefitting from the standby fluid flow for preventing vapour from diffusing into the analyser.

Optionally, the fluid container comprises air at atmospheric conditions. In addition, and/or alternatively, the flow conduit may be open to the atmosphere at one end. Advantageously, this can provide a readily available source of fluid for the standby fluid flow. In such an embodiment, the flow conduit preferably comprises a section with dimensions of between 8 mm and 30 cm length, and/or between 100 pm and 250 pm inner diameter. These dimensions advantageously can provide a desired level of standby fluid flow, when using atmospheric air, for preventing vapour from diffusing into the analyser.

Alternatively, the fluid container contains analysing fluid. Advantageously, this can make use of a readily available source of fluid for the standby fluid flow since the mass spectrometer already has a fluid container containing analysing fluid for use in the analysing mode. This can therefore reduce the number of modifications which need to be made to existing mass spectrometers. Preferably, the analysing fluid comprises argon; and the flow conduit comprises a section with dimensions of between 7 cm and 120 cm length, and/or between 75 pm and 150 pm inner diameter. These dimensions advantageously can provide a desired level of standby fluid flow, when using argon, preferably at 5 bar pressure, for preventing vapour from diffusing into the analyser.

Preferably, the vacuum pump is a first vacuum pump, and the apparatus further comprises a second vacuum pump located on the vacuum conduit between the first vacuum pump and the analyser, and a backing valve located on the vacuum conduit between the second vacuum pump and the first vacuum pump. Advantageously this can allow two different vacuum pumps to be used to produce a high vacuum for the mass spectrometer, and the backing valve can enable the use of methods of the present disclosure.

Optionally, the flow conduit extends from the vacuum conduit at one of the following locations: from the second vacuum pump; between the backing valve and the second vacuum pump; or between the first vacuum pump and the backing valve. The locations all advantageously can provide the standby fluid flow. The location of between the first vacuum pump and the backing valve is particularly beneficial since it can enable the mass spectrometer to be run in a test mode.

Preferably the method further comprises closing the vacuum conduit at a location between the vacuum pump and the analyser; increasing the fluid pressure in the vacuum conduit between the location and the analyser; and re-opening the vacuum conduit at the location to thereby induce the standby fluid flow towards the vacuum pump. These steps advantageously can produce the standby fluid flow without significant structural modification to the mass spectrometer. This method also advantageously does not introduce an additional flow of gas into the vacuum system which is then evacuated out (thereby increasing the work of the vacuum pump). Optionally, closing the vacuum conduit comprises closing the backing valve, and re-opening the vacuum conduit comprises opening the backing valve. This advantageously can make use of existing components with the mass spectrometer to perform the method, and therefore reduces the need for significant structural modification to the mass spectrometer. Preferably, the method comprises determining the fluid pressure in the vacuum conduit between the location and the analyser; and if the determined fluid pressure is above a threshold, re-opening the vacuum conduit at the location to thereby induce the standby fluid flow towards the vacuum pump. Beneficially, this can allow the method to ensure that the pressure difference across the closure location is sufficiently large to induce a standby fluid flow of a sufficient size. Optionally, the fluid pressure is determined by monitoring the electrical current supplied to the second vacuum pump to operate the second vacuum pump. This advantageously can make use of existing components within the mass spectrometer to perform the method, and therefore reduces the need for additional sensors within the mass spectrometer.

Preferably, the vacuum conduit comprises at least one section with reduced diameter relative to an adjacent section of the vacuum conduit, the section with reduced diameter for preventing vapour from diffusing from the vacuum pump into the analyser. This advantageously can increase the flow speed of the standby fluid flow (by reducing the available cross-sectional area) and so can aid the prevention of vapour diffusing upstream. Optionally, the at least one section with reduced diameter is sized according to the following formula:

wherein j is the standby fluid flow in the vacuum conduit away from the analyser

towards the vacuum pump; I (mm) is the length of the section with reduced diameter; and d mm² is the diameter of the section with reduced diameter. This advantageously can result in the standby fluid flow having a sufficient flow speed, while keeping the loss of total pressure across the section with reduced diameter at an acceptably low level.

There is further provided a mass spectrometer comprising an analyser and a vacuum system. The vacuum system comprises a vacuum pump and a conduit system comprising at least one conduit extending from the vacuum pump to the analyser. The conduit system comprises at least one section with reduced diameter relative to an adjacent section of the conduit system, the section with reduced diameter for preventing vapour from diffusing from the vacuum pump into the analyser.

The section with reduced diameter advantageously can prevent vapour from diffusing from the vacuum pump, through the vacuum system, to the analyser in standby mode. This can improve the lifetime and performance of the mass spectrometer.

Optionally, the conduit system comprises a vacuum conduit extending from the vacuum pump to the analyser and a flow conduit extending between the vacuum conduit and a fluid container. The flow conduit comprises the at least one section with reduced diameter. The flow conduit is for preventing vapour from diffusing from the vacuum pump into the analyser by inducing a standby fluid flow in the vacuum conduit away from the analyser towards the vacuum pump. The standby fluid flow is induced by fluid flowing from the fluid container, through the flow conduit, into the vacuum conduit. Optionally, the mass spectrometer may not comprise a fluid container, in which case the flow conduit extends from the vacuum conduit. Advantageously, these features can provide a standby fluid flow in a passive manner without requiring additional steps or electrical power (except for the increased power needed by the vacuum pump to account for the additional flow). Beneficially, the section with reduced diameter can regulate the size of the standby fluid flow so that the vacuum pump is still able to create an effective vacuum, while still benefitting from the standby fluid flow for preventing vapour from diffusing into the analyser.

Preferably, the at least one section with reduced diameter includes a section at one or more of the following locations between the backing valve and the second vacuum pump; or between the first vacuum pump and the backing valve. The section with reduced diameter at any of these locations advantageously can increase the flow speed of the standby fluid flow (by reducing the available cross-sectional area) and so can aid the prevention of vapour diffusing upstream.

Brief Description of the Drawings

For a better understanding of the invention, and to show how the same may be put into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

Figure 1 is a schematic diagram showing an embodiment of a mass spectrometer which can be used with the methods of the present disclosure; Figure 2 is a schematic diagram showing an embodiment of a mass spectrometer in accordance with the present disclosure;

Figure 3 is a schematic diagram showing a modification of the mass spectrometer of Figure 2;

Figure 4 is a schematic diagram showing a second modification of the mass spectrometer of Figure 2;

Figure 5 is a schematic diagram showing a third modification of the mass spectrometer of Figure 2;

Figure 6 is a schematic diagram showing a further embodiment of a mass spectrometer in accordance with the present disclosure;

Figure 7 is a schematic diagram showing a modification of the mass spectrometer of Figure 6;

Figure 8 is a schematic diagram showing a further embodiment of a mass spectrometer, combining features of the mass spectrometers of Figures 4 and 7;

Figure 9 is a schematic diagram showing a further embodiment of a mass spectrometer, combining features of the mass spectrometers of Figures 5 and 7; and Figure 10 is a flow chart showing a method of operating a mass spectrometer in accordance with the present disclosure.

Detailed Description

Figure 1 shows a mass spectrometer 10. The mass spectrometer 10 comprises an analyser 12 and a vacuum system 14. The mass spectrometer 10 is arranged to analyse a sample in the analyser 12, and the vacuum system 14 is arranged to keep the analyser 12 under vacuum, so that it can function correctly.

The mass spectrometer 10 illustrated may be an ICP-MS in which the sample is ionised using inductively coupled plasma. In such a case, the mass spectrometer 10 further comprises an interface 18 through which the sample is passed into the analyser 12. The ions are accelerated through the interface 18 into the analyser 12 for analysis. During analysis, a large amount of gas is pumped out of the interface 18.

The analyser 12 may include an RF filter, such as a quadrupole mass analyser, a magnetic sector field analyser, a double focussing analyser with both electric and magnetic sector fields, a time of flight analyser, an RF trap, a DC trap (e.g. an orbitrap), or a combination of any of the above. The analyser 12 is arranged to analyse the ions in the sample to determine the mass spectrum of the sample. The vacuum system 14 comprises a vacuum pump 41 and a conduit system 43. The conduit system 43 comprises a vacuum conduit 45 extending from the vacuum pump 41 to the analyser 12.

The conduit system 43 may further comprise a secondary conduit 47 extending from the vacuum pump 41 , and/or the vacuum conduit 45, to the interface 18. The secondary conduit 47 evacuates gas from the interface 18 which does not enter the analyser 12. During operation, this is a majority of the gas passing through the interface 18.

As illustrated in Figure 1 , the vacuum pump 41 may be a first vacuum pump 41 , and the vacuum system 14 may further comprise a second vacuum pump 49. The first vacuum pump 41 may be a fore vacuum pump configured to create a rough vacuum within the vacuum system 14. One type of pump used for the first vacuum pump 41 may be a rotary vane pump which is efficient for creating the rough vacuum. However, there are different types of pumps that may be used for the fore pump.

The second vacuum pump 49 may be a high vacuum pump which is arranged to provide a high vacuum within the analyser 12. A typical pump used for the second vacuum pump 49 is a turbo-molecular pump which is able to create the high vacuum. As illustrated, the second vacuum pump 49 is located along the vacuum conduit 45 extending between the first vacuum pump 41 and the analyser 12. The second vacuum pump 49 can be said to split the vacuum conduit 45 into an upstream conduit section 51 , and a downstream conduit section 53. In other words, the upstream conduit section 51 is between the second vacuum pump 49 and the analyser 12, and the downstream conduit section 53 is between the second vacuum pump 49 and the first vacuum pump 41 . The upstream conduit section 51 may be part of the analyser 12. Generally, an aim is to maximize the conductance (fluid conductance) of the upstream conduit section 51 .

The second vacuum pump 49 can evacuate gas particles from the upstream conduit section 51 , and therefore the analyser 12, and exhaust the gas particles into the downstream conduit section 53. The gas particles in the downstream conduit section 53 subsequently flow to the first vacuum pump 41 where they can be exhausted into the atmosphere. Additionally, the vacuum system 14 may further comprise a valve 61 . The valve 61 may be referred to as a backing valve 61 and may be located on the conduit 45 between the first vacuum pump 41 and the second vacuum pump 49. In other words, the backing valve 61 may be located on the downstream conduit section 53. The backing valve 61 may be present for a start-up procedure associated with the mass spectrometer 10.

As illustrated, a secondary valve 63 may also be present within the vacuum system 14. The secondary valve 63 is be located on the secondary conduit 47 in between the vacuum pump 41 and the interface 18. The secondary valve 63 can enable a standby mode of the analyser 18.

In the standby mode, there is no plasma at an opening of the interface 18, but rather a fluid, such as air, at atmospheric pressure may be present. The gas density of the plasma, used in operation, is generally lower (by more than one order of magnitude) than atmospheric pressure. Therefore, in the standby mode, if the secondary valve 63 was not present, the amount of gas to be pumped through the secondary conduit 47 may be so large that, using a typical fore vacuum pump 41 , the pressure in conduit system 43 would be too high for the second vacuum pump 49 to operate. Therefore, the secondary valve 63 can be closed in the standby mode so that fluid does not flow down the secondary conduit 47 from the interface 18.

As shown in in the illustrated embodiment, a third valve 65 may be present between the analyser 12 and interface 18. The third valve 65 may be relevant for the start-up procedure associated with the mass spectrometer 10. Although not shown in all the figures, the third valve 65 may be present in any embodiment.

The mass spectrometer 10 is operable in at least two modes. These include an analysing mode and the standby mode. The start-up procedure referenced above is the process of changing from the standby mode to the analysing mode. In the analysing mode, a sample is analysed within the analyser 12. In the process of analysis, an analysing fluid flow, comprising an analysing fluid, is induced to travel in the conduit system 43 towards the vacuum pump 41 . The analysing fluid may be formed of or comprise an inert gas, such as argon gas. The analysing fluid is used to ionise the sample and the analysing fluid enters the interface 18 along with the sample. The analysing fluid flow leaves the interface 18 and/or analyser 12 into the conduit system 43 and into the vacuum pump 41 where it is exhausted to the atmosphere. A majority of, and generally a large majority of, the analysing fluid leaves the interface 18 via the secondary conduit 47. A smaller portion of the analysing fluid enters the analyser 12 together with the sample ions, and is pumped down the upstream conduit section 51 , and the vacuum conduit 45 in general, to the vacuum pump 41.

In the standby mode, a sample is not present in the analyser 12 (nor the interface 18) for analysis. Therefore, in the standby mode the analysing fluid flow is not induced to travel in the conduit system 43 towards the vacuum pump 41 . In addition, the secondary valve 63, as well as the third valve 65 can be closed in the standby mode.

The mass spectrometer 10 may comprise a controller for operating the mass spectrometer 10 and performing the methods of the present disclosure. The controller is arranged to operate the mass spectrometer 10 in the analysing mode and standby mode. The controller may comprise a computer and a computer readable memory loaded with a computer readable code.

Impurities within the analyser 12 can affect parts of the mass spectrometer 10, such as SEM detectors. Therefore, it is desirable to reduce the number of unwanted particles and impurities within the analyser 12. One source of potential impurities in the analyser 12 is vapour, particularly hydrocarbon vapour, diffusing into the analyser 12. A source of the vapour may be the vacuum pump 41 , especially if the vacuum pump 41 is a rotary vane pump which may require high levels of oil lubrication to function more effectively. Oil also aids the rotary vane pump to function as it can ensure a vacuum seal between a vane and a pump body of the rotary vane pump.

In addition, the vacuum pump 41 is often held at a high temperature, when compared with the rest of the vacuum system 14, in order to operate more effectively, such as 80°C. This results in the oil vapor pressure at or near the vacuum pump 41 to be higher than the vapor pressure throughout the rest of the conduit system 43 which is at room temperature. This can increase diffusion from the vacuum pump 41 into the vacuum conduit 45. As a result, vapours from the vacuum pump 41 can diffuse through the vacuum system 14, along the vacuum conduit 45, and into the analyser 12.

When a sample is being analysed in the mass spectrometer 10, especially if the mass spectrometer 10 is an ICP-MS, a large volume of analysing fluid flow travels along the vacuum conduit 45, and an even larger volume of analysing fluid flow travels along the secondary conduit 47, towards the vacuum pump 41 . Therefore, at this time, vapour is not able to diffuse from the vacuum pump 41 into the analyser 12, since this would require diffusion upstream against the large analysing fluid flow. As a result, hydrocarbon vapour can only diffuse from the vacuum pump 41 into the analyser 12 when a sample is not being analysed, i.e., when the mass spectrometer 10 is in standby mode.

On a general level, the described mass spectrometer 10 and methods herein below are arranged for preventing vapour from diffusing from the vacuum pump 41 through the vacuum system 14 to the analyser 12 in the standby mode.

One aspect of the present disclosure includes inducing a standby fluid flow in the conduit system 43, and specifically the vacuum conduit 45, away from the analyser 12 and towards the vacuum pump 41 while the mass spectrometer 10 is in the standby mode. The standby fluid flow towards the vacuum pump 41 prevents vapour from diffusing from the vacuum pump 41 upstream towards the analyser 12 because the flow speed downstream towards the pump 41 is sufficient for preventing upstream diffusion. In this embodiment, a standby fluid flow will be present in the vacuum conduit 45 towards the analyser 12 during standby mode, when there is no analysing fluid flow. The standby fluid may be formed of or comprise air or the inert gas, such as argon gas.

The standby fluid flow may be of a lower magnitude as compared with the analysing fluid flow. The analysing fluid flow in the analysing mode along the secondary conduit 47 may be around 32 ml/sec STP, and the flow through the vacuum conduit 45 may be around 0.05 ml/sec STP. The standby fluid flow may be of the order of 10 ml/min to 100 ml/min and may be around 50 ml/min.

A further aspect of the present disclosure includes features of the conduit system 43 arranged for preventing vapour from diffusing from the vacuum pump 41 upstream towards the analyser 12, regardless of the standby fluid flow.

Although the mass spectrometer 10 illustrated in Figure 1 may be used with the methods of the present disclosure, as described below, the mass spectrometer 10 may be further improved as shown in Figures 2 to 8. On the most general level, the modification of the mass spectrometer 10 is the inclusion of a section with reduced diameter 101 within the conduit system 43. The section with reduced diameter 101 has a smaller internal diameter as compared with other sections of the conduit system 43. The section with reduced diameter 101 is arranged for preventing vapour from diffusing from the vacuum pump 41 into the analyser 12. The section with reduced diameter 101 may also have a minimum length.

Figure 2 illustrates an improved mass spectrometer 10 in accordance with the present disclosure.

As shown in Figure 2, the conduit system 43 further comprises a flow conduit 103 extending from the vacuum conduit 45. The flow conduit 103 extends from the vacuum conduit 45 and optionally to a fluid container. In this embodiment, the flow conduit 103 is open to the atmosphere, and therefore the fluid container contains air at atmospheric conditions. The container need not be a physical container with solid walls, and can rather refer to an area containing fluid, such as the space adjacent to the flow conduit 103.

The flow conduit 103 prevents vapour from diffusing from the vacuum pump 41 into the analyser 12 by inducing the standby fluid flow in the vacuum conduit 45 away from the analyser 12 and towards the vacuum pump 41 . The standby fluid flow is induced due to fluid flowing through the flow conduit 103 and into the vacuum conduit 45. . The standby fluid flow towards the vacuum pump 41 prevents vapour from diffusing from the vacuum pump 41 upstream towards the analyser 12 because the flow speed of the standby fluid flow downstream towards the pump 41 is sufficiently large for preventing upstream diffusion.

As illustrated, the flow conduit 103 comprises the at least one section with reduced diameter 101 . The at least one section with reduced diameter 101 may also be referred to as, and/or have the characteristics of, a capillary.

The section with reduced diameter 101 , in this embodiment, is necessary to ensure that the correct magnitude of flow is induced through the vacuum conduit 45 towards the vacuum pump 41 . If no section with reduced diameter 101 was present, or the section with reduced diameter 101 has too large a diameter, then the standby fluid flow into the vacuum system 14 would be too high for the vacuum pump 41 to effectively create a vacuum in the vacuum system 14 and the analyser 12. However, if the section with reduced diameter 101 has too small a diameter, then the standby fluid flow would not be large enough for preventing upstream diffusion of vapours from the vacuum pump 41 towards the analyser 12. Therefore, in the present embodiment, the section with reduced diameter 101 has dimensions of 4 cm length and 150 pm in their diameter. In general, the section with reduced diameter 101 may be sized according to the following formula: d⁴ — = 1.3 x 10^-5 mm^A3 wherein d is the diameter, and I is the length of the section with reduced diameter 101. Therefore, the dimensions may be between 8 mm and 30 cm length, and between 100 pm and 250 pm diameter, and preferably between 2 cm and 12 cm length, and between 130 pm and 200 pm diameter or between 2 cm and 8 cm length, and between 130 pm and 180 pm diameter. When considering the properties of the flow induced by having the flow conduit 103 open to the atmosphere, the aforementioned sizes of the section with reduced diameter 101 provides the correct level of standby fluid flow within the vacuum conduit 45 for preventing vapour from diffusing into the analyser 12.

In the embodiment of Figure 2, the flow conduit 103 connects to the second vacuum pump 49, and/or connects to the vacuum conduit 45 at the second vacuum pump 49. Specifically, the flow conduit 103 may connect to the vacuum pump 49 between a turbo stage and a Holweck stage of the vacuum pump 49. It may be less beneficial for the flow conduit 103 to connect to the vacuum conduit 45 at a location between the second vacuum pump 49 and the analyser 12 (in the upstream conduit section 51 ), because the standby fluid flow may not function correctly in the molecular flow regime, which is present upstream of the vacuum pump 49. In addition, doing so would increase the decrease the degree of vacuum in the analyser 12.

As illustrated in Figures 3 and 4, the flow conduit 103 may connect to the vacuum conduit 45 at other locations. For example, Figure 3 shows that the flow conduit 103 may connect to the vacuum conduit 45 at a location between the second vacuum pump 49 and the backing valve 61 , and/or the first vacuum pump 41 . In this embodiment, the flow conduit 103 connects to the vacuum conduit 45 in the downstream conduit section 53.

Figure 4 shows that the flow conduit 103 may connect to the vacuum conduit 45 at a location between the backing valve 61 and the first vacuum pump 41 . Any of these locations will induce a standby fluid flow in the vacuum conduit 45 away from the analyser 12 and towards the vacuum pump 41 . Therefore, any of these locations are suitable in the context of the present disclosure. The embodiment of Figure 4 in which the flow conduit 103 connects to the vacuum conduit 45 in between the valve 61 and the vacuum pump 41 , is the preferred embodiment. This embodiment is preferred because during the start-up procedure the valve 61 is closed for a certain time. During this time, it is advantageous if the pressure directly downstream of the second vacuum pump 49 and upstream of the valve 61 does not increase by a large amount. If the flow conduit 103 is connected to the vacuum conduit 45 upstream of the valve 61 (as shown in Figures 2 and 3) then pressure directly downstream of the second vacuum pump 49 may increase when the valve 61 is closed, due to the standby fluid flow. Therefore, the embodiment of Figure 4 is preferable since it does not cause the pressure directly downstream of the second vacuum pump 49 to increase by a large amount when the valve 61 is closed.

A further advantage of the embodiment of Figure 4 is that it allows a test mode to be operated. The mass spectrometer 10 may be operable in the test mode to test for leaks in the vacuum system 14. The test mode includes closing the valve 61 and measuring the increase in an electric current used to operate the second vacuum pump 49 against time. If the flow conduit 103 is connected to the vacuum conduit 45 upstream of the valve 61 (as shown in Figures 2 and 3), this test is not possible, since a deliberate leak, in the form of flow conduit 103, is present upstream of the valve 61 , and the electric current used to operate the second vacuum pump 49 will not be a reliable indicator of unwanted leaks. Therefore, the embodiment of Figure 4 is preferable.

The fluid container does not have to contain air at atmospheric conditions, and the flow conduit 103 does not need to be open to the atmosphere. Figure 5 shows an embodiment in which the fluid container comprises a tank of gas 105. A preferred gas is argon gas. The mass spectrometer 10 often requires an inert gas such as argon to function (especially for an ICP-MS). Therefore, existing mass spectrometers come with a ready supply of argon gas. As a result, it is particularly efficient to connect the flow conduit 103 to a fluid container comprising argon 105. This may be the same fluid container comprising argon 105 which is used in the mass spectrometer 10 to ionise and subsequently analyse the sample.

Given the typical conditions of argon gas used in mass spectrometer systems, ideal dimensions for the section with reduced diameter 101 on the flow conduit 103 is a length of 23 cm and/or inner diameter of 100 pm. In general, when the flow conduit 103 is connected to a fluid container comprising argon 105 containing, for example, argon at five bar pressure, the section with reduced diameter 101 may be sized according to the following formula:

wherein d is the diameter, and I is the length of the section with reduced diameter 101. Therefore, the dimensions may be between 7 cm and 120 cm length, and between 75 pm and 150 pm diameter, and preferably between 7 cm and 66 cm length, and between 75 pm and 130 pm diameter.

Figure 5 illustrates the flow conduit 103 leading from the fluid container comprising argon 105 to the vacuum conduit 45 as connecting to the vacuum conduit 45 in between the second vacuum pump 51 and the backing valve 61 (in other words, in the downstream conduit section 53). However, any of the connection locations on the vacuum conduit 45 described in relation to Figures 2, 3 and 4 would be suitable for connecting the flow conduit 103 in this embodiment as well.

At the opening of the flow conduit 103 to atmosphere (Figure 2, 3 or 4) or to the fluid container comprising argon 105 (Figure 5), there may be an additional valve (not shown) for sealing the flow conduit 103. The additional valve could be closed in the test mode.

The section with reduced diameter 101 need not be, or need not only be, located on flow conduit 103 or present in connection with the presence of flow conduit 103. In Figure 6, an embodiment is shown in which there is no flow conduit 103. As illustrated, the section with reduced diameter 101 is located on the vacuum conduit 45.

In this case, the section with reduced diameter 101 does not have the sole function of regulating flow from the fluid container into the vacuum conduit 45. Rather, the section with reduced diameter 101 prevents vapour from diffusing from the vacuum pump 41 into the analyser 12 in its own right. The section with reduced diameter 101 causes a restriction in gas flow which can restrict the diffusion upstream from the vacuum pump 41 towards the analyser 12. The section with reduced diameter 101 can cause in increase in a flow speed of the standby fluid flow by virtue of the reduced diameter. Since the flow rate will be determined by the fluid conditions, decreasing the diameter of the conduit will increase the flow speed to allow the correct flow rate. Therefore, there will be a greater flow speed, in the section with reduced diameter 101 , against which vapour will have to diffuse to travel into the analyser 12, and so diffusion upstream will be restricted. The flow speed may be between 1 m/s and 300 m/s and is preferably around 10 m/s.

Although Figure 6 shows the section with reduced diameter 101 on the vacuum conduit 45 between the second vacuum pump 49 and the backing valve 61 (on the downstream conduit section 53) other locations are equally viable for the same effect. Figure 7 illustrates an embodiment in which the at least one section with reduced diameter 101 is located on the vacuum conduit 45 between the backing valve 61 and the second vacuum pump 49 and an additional section with reduced diameter 101 ’ is located in between the backing valve 61 and the first vacuum pump 41 . Figure 8 illustrates an embodiment in which the section with reduced diameter 101 is located on the vacuum conduit 45 solely between the backing valve 61 and the vacuum pump 41 .

The section with reduced diameter 101 when present on the vacuum conduit 45 is dimensioned with the following considerations. The section with reduced diameter 101 should have dimensions such that the flow speed through the section with reduced diameter 101 is large enough to the extent that upstream diffusion of vapour from the vacuum pump 41 towards the analyser 12 is sufficiently restricted. However, the section with reduced diameter 101 should have dimensions which do not create a significant pressure drop across the restriction. If the section with reduced diameter 101 has dimensions which are, for example, too narrow, and/or too long, then there will be a significant drop in total pressure across the section with reduced diameter 101 . This will result in the vacuum pump 41 having to use more energy and work harder to provide the desired vacuum. Therefore, the section with reduced diameter 101 on the vacuum conduit 45 is sized according to the following formula:

j

is the standby fluid flow in the vacuum conduit 45 away from the analyser 12 towards the vacuum pump 41 . 1 (mm) is the length of the section with reduced diameter 101 . d (mm is the diameter of the section with reduced diameter 101 .

If the standby fluid flow is, for example, between 10 ml/min and 100 ml/min, the section with reduced diameter 101 may have an internal diameter of between 5 mm and 10 mm or between 3 mm and 30 mm and a length of between 5 mm to 100 mm. The typical dimension of a full-diameter conduit in the conduit system is around 30 mm inner diameter and around 10 cm length. The above dimensions of the section with reduced diameter 101 would lead to a negligible pressure drop across the restriction but would aid the prevention of vapour diffusing from the vacuum pump 41 to the analyser 12.

As shown in Figures 8 and 9, the various embodiments described above can be combined within the vacuum system 14.

As shown in Figure 8, the conduit system 43 may comprise a flow conduit 103 open to the atmosphere and joining the vacuum conduit 45 between the valve 61 and the vacuum pump 41 . In the illustrated embodiment, the conduit system 43 further comprises a first section with reduced diameter 101 in between the vacuum pump 41 and the location at which the flow conduit 103 connects to the vacuum conduit 45. There is an additional section with reduced diameter 101 ’ located on the flow conduit 103 in the manner discussed above in relation to Figure 2.

In the embodiment of Figure 9, the sections with reduced diameter 101 and 10T within the vacuum conduit 45 upstream and downstream respectively of the backing valve 61 (like the arrangement shown in Figure 7) are combined with the flow conduit 103 and the fluid container comprising argon 105 described in relation to Figure 5.

In this embodiment, there are three sections with reduced diameter 101 present. A first section with reduced diameter 101 is present between the backing valve 61 and the second vacuum pump 49. A second section of reduced diameter 10T is present between the backing valve 61 and the first vacuum pump 41 . Both of these sections with reduced diameter 101 , and 101 ’ are located on the vacuum conduit 45. A third section with reduced diameter 101 ” is located on the flow conduit 103 between the fluid container comprising argon 105 and the vacuum conduit 45 for the reasons explained above in relation to Figures 2 to 5.

In the embodiments shown in Figures 8 and 9, the sections with reduced diameter on the vacuum conduit 45 (such as the first and second sections of reduced diameter 101 , 10T in Figure 9) are present to restrict diffusion of vapours from the vacuum pump 41 towards the analyser 12 as described above in relation to Figure 6. The section of reduced diameter 101 ” on the flow conduit 103 is present to regulate the flow from the flow conduit 103 into the vacuum conduit 45 towards the vacuum pump 41 , as described above in relation to the embodiments of Figures 2 to 5. A method of operating the vacuum system 14 of the mass spectrometer 10 is now described. On a general level, the method involves operating the mass spectrometer 10 in the standby mode and inducing the standby fluid flow in the vacuum conduit 45 away from the analyser 12 towards the vacuum pump 41 while the mass spectrometer 10 is in the standby mode. This can be accomplished in a plurality of ways, as described below.

A first way is by including the flow conduit 103 as described in relation to Figures 2 to 5, 8 and 9. As previously described, the standby fluid flow in the vacuum conduit 45 is induced by fluid flowing from the fluid container (optionally the fluid container comprising argon 105) through the flow conduit 103 and into the vacuum conduit 105. In this embodiment, the standby fluid flow is constantly present while the mass spectrometer 10 is in the standby mode.

A second way of inducing the standby fluid flow is now described in relation to Figure 10. Figure 10 is a flowchart showing a method for inducing the standby fluid flow and can be performed with the mass spectrometer 10 shown in Figure 1 , as well as the mass spectrometer 10 of Figures 2 to 9. The method of Figure 10 is especially useful when used with the mass spectrometer 10 shown in Figures 1 , 6 and 7 where no flow conduit 103 is present but may be used in the other embodiments as well. For example, the method of Figure 10 may also be implemented using the mass spectrometer 10 of Figure 8. The method may be performed by the controller.

At step 201 the mass spectrometer 10 is operated in the standby mode.

At step 203, while the mass spectrometer 10 is in the standby mode, the vacuum conduit 45 is closed at a location between the vacuum pump 41 and the analyser 12. The vacuum conduit 45 may be closed by closing the backing valve 61 .

At step 205 the fluid pressure in the vacuum conduit 45 between the location where the vacuum conduit 45 is closed and the analyser 12 is increased. The fluid pressure at this location may be increased by the second vacuum pump 49 which is drawing fluid from the upstream conduit section 51 and exhausting it to the downstream conduit section 53. This causes a build-up in fluid pressure in the downstream fluid section 53 whilst the backing valve 61 , or the vacuum conduit 45 in general, is closed. Optionally, at step 207 the fluid pressure in the vacuum conduit 45 between the location of closure and the analyser 12 is determined. If the vacuum conduit 45 is closed using the backing valve 61 , then the pressure is determined in the vacuum conduit 45 between the backing valve 61 and the second vacuum pump 49, i.e., in the downstream conduit section 53.

The fluid pressure in the vacuum conduit 45 may be simply determined by including a pressure sensor in the vacuum conduit 45. Alternatively, and more preferably, the fluid pressure may be determined by monitoring the electrical current supplied to the second vacuum pump 49 in order to operate the second vacuum pump 49. As pressure in the downstream conduit section 53 is increased, the second vacuum pump 49 will have to work harder to continue operating and pumping fluid from the upstream conduit section 51 to the downstream conduit section 53 (due to the larger pressure gradient against which fluid is being pumped). The pressure in the upstream conduit section 51 should remain roughly constant. Therefore, the work being performed by the second vacuum pump 492, which can be measured by the electrical current supplied to the second vacuum pump 49, is a convenient way to measure the pressure within the downstream conduit section 53.

Alternatively, the fluid pressure in the vacuum conduit 45 between the location of closure and the analyser 12 may not be determined. Instead, a time delay may be included after the vacuum conduit 45 is closed, to give time for the fluid pressure in the vacuum conduit 45 to increase. The length of the time delay may be calibrated to ensure that the pressure increase is sufficient to induce a standby fluid flow which is suitable for preventing upstream diffusion of vapour from the vacuum pump 41 towards the analyser 12.

At step 209, the vacuum conduit 45 is reopened at the location of closure. This thereby induces the standby fluid flow towards the vacuum pump 41 . This occurs due to the increased pressure in the vacuum conduit 45 upstream of the location of closure. When the vacuum conduit 45 was closed using the backing valve 61 , then the vacuum conduit 45 is reopened by opening the backing valve 61 . If the fluid pressure in the vacuum conduit 45 between the location of closure and the analyser 12 is determined, then the vacuum conduit 45 is reopened at the location of closure dependent on the determined fluid pressure being above a threshold value. The threshold pressure is selected such that the difference in pressure between upstream and downstream of the location of closure is sufficient to induce a standby fluid flow which is suitable for preventing upstream diffusion of vapour from the vacuum pump 41 towards the analyser 12. It should be ensured that the vacuum pump 41 is operating when the vacuum conduit 45 is reopened such that the standby fluid flow is exhausted to the atmosphere.

Whilst the backing valve 61 is closed (or in general the vacuum conduit 45 is closed at the location of closure) hydrocarbon vapour cannot flow upstream from the vacuum pump 41 towards the analyser 12 because it is blocked by the closure in the vacuum conduit 45. Therefore, it is not relevant that there is no standby fluid flow at these times. In the method of Figure 10, the standby fluid flow is only present intermittently. However, the standby fluid flow is present whenever necessary, since these are the times when the vacuum conduit 45, and the backing valve 61 in particular, are open, thereby enabling vapour to diffuse upstream towards the analyser 12.

Claims

CLAIMS:

1 . A method of operating the vacuum system of a mass spectrometer, for preventing vapours from diffusing from a vacuum pump of the mass spectrometer into an analyser of the mass spectrometer, the mass spectrometer comprising: the analyser; the vacuum pump; and a vacuum conduit extending between the vacuum pump and the analyser, wherein the method comprises: operating the mass spectrometer in a standby mode wherein a sample is not present in the analyser for analysis; and inducing a standby fluid flow in the vacuum conduit away from the analyser towards the vacuum pump while the mass spectrometer is in the standby mode.

2. The method of claim 1 , wherein the mass spectrometer is operable in at least two modes including: an analysing mode in which a sample is analysed, and a flow of an analysing fluid is thereby induced to travel in the vacuum conduit towards the vacuum pump; and the standby mode.

3. The method of claim 2, wherein the flow of analysing fluid in the analysing mode has a larger magnitude than the standby fluid flow in the standby mode.

4. The method of any preceding claim, wherein the standby fluid flow is constantly present while the mass spectrometer is in the standby mode.

5. The method of any preceding claim, wherein the mass spectrometer further comprises a fluid container and a flow conduit extending between the vacuum conduit and the fluid container, wherein in the standby mode the standby fluid flow in the vacuum conduit is induced by fluid flowing from the fluid container, through the flow conduit into the vacuum conduit.

6. The method of claim 5, wherein the flow conduit comprises a section with reduced diameter as compared with the vacuum conduit.

7. The method of claim 5 or 6, wherein the fluid container comprises air at atmospheric conditions.

8. The method of claim 7, wherein the flow conduit comprises a section with dimensions of between 8 mm and 30 cm length, and/or between 100 pm and 250 pm inner diameter.

9. The method of claim 5 as dependent on claim 2, wherein the fluid container contains analysing fluid.

10. The method of claim 9, wherein: the analysing fluid comprises argon; and the flow conduit comprises a section with dimensions of between 7 cm and 120 cm length, and/or between 75 pm and 150 pm inner diameter.

11 . The method of any preceding claim, wherein the vacuum pump is a first vacuum pump, and the apparatus further comprises: a second vacuum pump located on the vacuum conduit between the first vacuum pump and the analyser, and a backing valve located on the vacuum conduit between the second vacuum pump and the first vacuum pump.

12. The method of claim 11 as dependent on any of claims 5 to 10, wherein the flow conduit extends from the vacuum conduit at one of the following locations: from the second vacuum pump; between the backing valve and the second vacuum pump; or between the first vacuum pump and the backing valve.

13. The method of any preceding claim, wherein the method further comprises: closing the vacuum conduit at a location between the vacuum pump and the analyser; increasing the fluid pressure in the vacuum conduit between the location and the analyser; and re-opening the vacuum conduit at the location to thereby induce the standby fluid flow towards the vacuum pump.

14. The method of claim 13 as dependent on claim 11 , wherein closing the vacuum conduit comprises closing the backing valve, and re-opening the vacuum conduit comprises opening the backing valve.

15. The method of claim 13 or 14, wherein the method comprises: determining the fluid pressure in the vacuum conduit between the location and the analyser; and if the determined fluid pressure is above a threshold, re-opening the vacuum conduit at the location to thereby induce the standby fluid flow towards the vacuum pump.

16. The method of claim 15, as dependent on claim 14, wherein the fluid pressure is determined by monitoring the electrical current supplied to the second vacuum pump to operate the second vacuum pump.

17. The method of any preceding claim, wherein the vacuum conduit comprises at least one section with reduced diameter relative to an adjacent section of the vacuum conduit, the section with reduced diameter for preventing vapour from diffusing from the vacuum pump into the analyser.

18. The method of claim 17, wherein the at least one section with reduced diameter is sized according to the following formula:

wherein: j

is the standby fluid flow in the vacuum conduit away from the analyser towards the vacuum pump;

I (mm) is the length of the section with reduced diameter; and d mm² is the diameter of the section with reduced diameter.

19. A mass spectrometer comprising: an analyser; and a vacuum system, wherein the vacuum system comprises: a vacuum pump; and a conduit system comprising at least one conduit extending from the vacuum pump to the analyser, wherein the conduit system comprises at least one section with reduced diameter relative to an adjacent section of the conduit system, the section with reduced diameter for preventing vapour from diffusing from the vacuum pump into the analyser.

20. The mass spectrometer of claim 19, wherein the conduit system comprises: a vacuum conduit extending from the vacuum pump to the analyser; and a flow conduit extending between the vacuum conduit and a fluid container, wherein: the flow conduit comprises the at least one section with reduced diameter; the flow conduit is for preventing vapour from diffusing from the vacuum pump into the analyser by inducing a standby fluid flow in the vacuum conduit away from the analyser towards the vacuum pump; and the standby fluid flow is induced by fluid flowing from the fluid container, through the flow conduit, into the vacuum conduit.

21 . The mass spectrometer of claim 20, wherein the fluid container comprises air at atmospheric conditions.

22. The mass spectrometer of claim 21 , wherein the at least one section with reduced diameter has dimensions of between 8 mm and 30 cm length, and/or between 100 pm and 250 pm inner diameter.

23. The mass spectrometer of claim 20, wherein the fluid container comprises argon.

24. The mass spectrometer of claim 23, wherein the at least one section with reduced diameter has dimensions of between 7 cm and 120 cm length, and/or between 75 pm and 150 pm inner diameter.

25. The mass spectrometer of any of claims 19 to 24, wherein the vacuum pump is a first vacuum pump, and the vacuum system further comprises: a second vacuum pump located on the conduit system between the first vacuum pump and the analyser, and a backing valve located on the conduit system between the second vacuum pump and the first vacuum pump.

26. The mass spectrometer of claim 25 as dependent on any of claims 20 to 24, wherein the flow conduit extends from the vacuum conduit at one or more of the following locations: from the second vacuum pump; between the backing valve and the second vacuum pump; or between the first vacuum pump and the backing valve.

27. The mass spectrometer of claim 25 or 26, wherein the at least one section with reduced diameter includes a section at one or more of the following locations: between the backing valve and the second vacuum pump; or between the first vacuum pump and the backing valve.