APPARATUS AND METHOD

Abstract

An ion source (30) for a static gas mass spectrometer is described. The ion source (30) comprises: a source block (310) defining a volume V to receive a sample gas G; an electron source (320) in fluid communication with the source block (310) and configured to provide a flux of electrons E therein for ionising the sample gas G; a set of electrodes (330), including a first electrode (330A), disposed between the electron source (320) and the source block (310); and a controller (not shown) configured to control a voltage applied to the first electrode (330A) to attenuate the flux of the electrons E into the source block (310) during a first time period following receiving of the sample gas G in the source block (310) and to permit the flux of the electrons E into the source block (310) during a second time period following the first time period.

Claims

1. A static gas mass spectrometer comprising an ion source, the ion source comprising: a source block defining a volume to receive a sample gas; an electron source in fluid communication with the source block and configured to provide a flux of electrons therein for ionising the sample gas; a set of electrodes, including a first electrode, disposed between the electron source and the source block; and a controller configured to control a voltage applied to the first electrode to attenuate the flux of the electrons into the source block during a first time period following receiving of the sample gas in the source block and to permit the flux of the electrons into the source block during a second time period following the first time period; wherein the first electrode comprises and/or is a deflector configured to deflect the flux of electrons away from the source block during the first time period.

2. The static gas mass spectrometer according to claim 1, wherein the first electrode comprises and/or is a cathode configured to decelerate the electrons theretowards and/or repel the electrons therefrom.

3. The static gas mass spectrometer according to claim 1, wherein the first electrode comprises and/or is an anode configured to accelerate the electrons theretowards and/or attract the electrons theretowards.

4. The static gas mass spectrometer according to claim 1, wherein the first electrode comprises and/or is a grid configured to interrupt the flux of electrons into the source block.

5. The static gas mass spectrometer according to claim 1, wherein the first electrode is disposed off axis with respect to the flux of electrons into the source block and is arranged to deflect the flux of electrons away from the source block.

6. The static gas mass spectrometer according to claim 1, wherein the controller is configured to control the flux of the electrons provided by the electron source.

7. The static gas mass spectrometer according to claim 1, wherein the electron source comprises and/or is a field emission gun or wherein the electron source comprises and/or is thermionic electron emitter and wherein the controller is configured to control a temperature of the thermionic electron emitter.

8. The static gas mass spectrometer according to claim 1, wherein the controller is configured to control an energy of the electrons provided by the electron source.

9. The static gas mass claim 1, wherein the controller is configured to control the voltage applied to the first electrode to selectively attenuate the flux of the electrons into the source block during the first time period.

10. The static gas mass spectrometer according to claim 9, wherein the controller is configured to control the voltage applied to the first electrode to permit the flux of the electrons into the source block during the first time period.

11. The static gas mass spectrometer claim 1, wherein a ratio of the flux of the electrons into the source block during the first time period to the flux of the electrons into the source block during the second time period is at most 1:100.

12. The static gas mass spectrometer claim 1, wherein the controller is configured to determine the first time period.

13. A static gas mass spectrometer, wherein the flux of electrons emitted by the ion source is constant during the first time period and the second time period.

14. A method of controlling an ion source of a static gas mass spectrometer, the method comprising: receiving, by a volume defined by a source block, a sample gas; providing, by an electron source in fluid communication with the source block, a flux of electrons therein and ionising the sample gas; controlling, by a controller, a voltage applied to a set of electrodes, including a first electrode, disposed between the electron source and the source block, comprising: attenuating, during a first time period following receiving of the sample gas in the source block, the flux of the electrons into the source block by deflecting the flux of electrons away from the source block during the first time period; and permitting, during a second time period following the first time period, the flux of the electrons into the source block.

15. The method according to claim 14, comprising: equilibrating, during the first time period following receiving of the sample gas in the source block, the sample gas in the source block.

16. The method according to claim 14, comprising: determining, by the controller, the first time period.

17. A method of controlling a static gas mass spectrometer, the method comprising: controlling the ion source according to claim 14; and detecting, during the second time period following the first time period, the ions from the sample gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0143] For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:

[0144] FIG. 1 schematically depicts a conventional ion source, in use;

[0145] FIG. 2A shows the intensity of the detected ion beam signal for an isotope of interest, from time-zero or t.sub.0; and FIG. 2B shows the intensity of the detected ion beam signal for the isotope of interest, after discarding data from the first time period;

[0146] FIG. 3A schematically depicts an ion source according to an exemplary embodiment, in use; and FIG. 3B schematically depicts the ion source, in use;

[0147] FIG. 4 shows the intensity of the detected ion beam signal for an isotope of interest, from time-zero or t.sub.0;

[0148] FIG. 5A schematically depicts an electron source for an ion source according to an exemplary embodiment; and FIG. 5B schematically depicts an electron source for an ion source according to an exemplary embodiment;

[0149] FIG. 6 schematically depicts an ion source according to an exemplary embodiment;

[0150] FIG. 7 schematically depicts a method according to an exemplary embodiment; and

[0151] FIG. 8 schematically depicts a method according to an exemplary embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

[0152] FIG. 3A schematically depicts an ion source 30 according to an exemplary embodiment, in use, particularly during the first time period; and FIG. 3B schematically depicts the ion source 30, in use, particularly during the second time period.

[0153] The ion source 30 is for a static gas mass spectrometer. The ion source 30 comprises: [0154] a source block 310 defining a volume V to receive a sample gas G; [0155] an electron source 320 in fluid communication with the source block 310 and configured to provide a flux of electrons E therein for ionising the sample gas G; [0156] a set of electrodes 330, including a first electrode 330A, disposed between the electron source 320 and the source block 310; and [0157] a controller (not shown) configured to control a voltage applied to the first electrode 330A to attenuate the flux of the electrons E into the source block 310 during a first time period following receiving of the sample gas G in the source block 310 and to permit the flux of the electrons E into the source block 310 during a second time period following the first time period.

[0158] That is, in contrast to the conventional ion source 10 as described with respect to FIG. 1, the ion source 30 according to an exemplary embodiment further comprises the set of electrodes 330, including the first electrode 330A, disposed between the electron source 320 and the source block 310; and the controller configured to control the voltage applied to the first electrode 330A to attenuate the flux of the electrons E into the source block 310 during the first time period following receiving of the sample gas G in the source block 310 and to permit the flux of the electrons E into the source block 310 during the second time period following the first time period.

[0159] Typically, a static vacuum mass spectrometer has constant source conditions, so as soon as a sample is allowed into the mass spectrometer, then the extraction of ions and their consequent fractionation occurs.

[0160] The invention temporarily stops ionisation during the equilibration time period, then simultaneously restarts ionisation (a new time-zero is defined) and the data acquisition.

[0161] Hence, no fractionation or consumption of sample occurs during the inlet equilibration time period. Also, the regression of the data set need only be extrapolated back to the point at which the ion extraction was restarted i.e. after equilibration.

[0162] The invention incorporates the use of a grid electrode, for example, between the cathode and source block, whose voltage is controlled with an independent supply. In normal operation, this grid voltage is adjusted to provide the required trap current and ionisation.

[0163] However, to prevent extracted ions and sample fractionation during the equilibration period, the grid can be used to turn off the electron beam, so that no ions are generated within the extraction region C.

[0164] Once the sample has equilibrated, the grid voltage can be restored to the normal operating condition (time-zero) and the data analysis can begin immediately.

[0165] In summary, the grid electrode acts as a tap and is used to stop the fractionation and consumption of the sample during the equilibration process, by preventing the formation of ions in the extraction region of the source. Once equilibration has occurred, then the grid voltage is restored to allow ionisation as before and simultaneously, the data acquisition can start.

[0166] In this example, the ion source 30 is a Nier-type source.

[0167] In this example, the source block 310 is generally as described with respect to the source block 110. Like reference signs denote like features.

[0168] In this example, the source block 310 comprises an electron inlet aperture 311 provided in a wall 312 thereof for the electron flux and an electron outlet aperture 313 provided in an opposed wall 314 thereof. In this example, the source block 310 comprises an ion outlet aperture 315, provided in a wall 316 transverse to the electron inlet aperture 311 and the electron outlet aperture 313. In this example, the source block 310 comprises an ion repeller plate 350. In this example, the ion source 310 comprises a trap 340 for collecting the electron flux exiting the source block 310, via the electron outlet aperture 313 provided in the wall 314 thereof. In this example, the ion source 30 comprises Y focus plates 360 (also known as extraction half plates) for extracting the ions from the volume V of the source block 310, for example via the ion outlet aperture 315 thereof. In this example, the ion source 30 comprises a defining slit 370 (also known as source slit).

[0169] In this example, the electron source 320 comprises a thermionic electron emitter. In this example, the electron source 320 comprises an electron emitter cathode presenting a thermionic electron emitter surface and a heater element electrically isolated from the electron emitter cathode and arranged to be heated by an electrical current therein and to radiate heat to the electron emitter cathode sufficient to liberate electrons thermionically from said electron emitter surface.

[0170] In this example, the electron source 320 is in fluid communication with the source block 310 via an aperture or passageway 311 provided in a wall 312 thereof.

[0171] In this example, the first electrode 330A comprises and/or is a cathode configured to decelerate the electrons theretowards and/or repel the electrons therefrom, for example during the first time period. In this example, the cathode 330A is disposed axially with respect to the flux of electrons into the source block and is arranged to interrupt the flux of electrons into the source block.

[0172] In this example, the first electrode 330A comprises and/or is a grid configured to interrupt the flux of electrons into the source block, as described below.

[0173] In this example, the first electrode 330A comprises and/or is one or more electron extraction grids and the controller is configured to control the voltage applied to the first electrode to attenuate the flux of the electrons into the source block during the first time period following receiving of the sample gas in the source block by applying a negative voltage to the first electrode and to permit the flux of the electrons into the source block during the second time period following the first time period by applying a positive voltage to the first electrode.

[0174] In this example, the controller is configured to control the voltage applied to the first electrode 330A to entirely attenuate (i.e. prevent) the flux of the electrons E into the source block 310 during the first time period

[0175] In this example, the controller is configured to determine the first time period, for example as described below.

[0176] In this example, the first time period is measured by intermittent sampling, for example by selectively attenuating the flux of the electrons into the source block during the first time period so as to permit the flux of the electrons intermittently, for example periodically, into the source block during the first time period.

[0177] FIG. 4 shows the intensity of the detected ion beam signal for an isotope of interest, from time-zero or t.sub.0 (i.e. time=0 seconds in this example, corresponding with the start of the second time period). This example, the extrapolated intercept precision is 0.65% c.f. 0.92% for the conventional ion source of FIG. 2B.

[0178] FIG. 5A schematically depicts an electron source 520A for an ion source according to an exemplary embodiment. The electron source comprises a tungsten wire filament coil 11 having opposite respective wire ends electrically connected to a current input terminal 12 having a first electrical potential, and a current output terminal 13 having a second electrical potential different to the first electrical potential thereby causing an electrical current to flow through the filament coil 11. Sufficient current flows to cause the tungsten filament coil to heat (e.g. incandescently) to a temperature sufficient to cause the surface of the filament coil to emit electrons thermionically from its surface. That is to say, the thermal energy acquired by the electrical heating effect of the electrical current passing though the filament coil is sufficient to imbue electrons in the filament coil to acquire an energy exceeding the surface work function of the filament coil. Although electrons are emitted generally omni-directionally from the filament coil 11, those electrons emitted in a preferred direction (D) are selected for input into a gas-source chamber of a gas-source mass spectrometer with which the filament coil 11 is in communication via an electron input slit 511A formed in a side wall of the source block 510A adjacent which the filament coil 11 is situated. A set of electrodes (not shown), including a first electrode (not shown), is disposed between the electron source 520A and the source block 510A.

[0179] FIG. 5B schematically depicts an electron source for an ion source according to an exemplary embodiment.

[0180] The cathode filament electron source 520B comprises a separated heater element 24 and cathode surface 26. The electron source includes an electron emitter cathode (25, 26) presenting a thermionic electron emitter surface 25 in communication with the source block of the gas-source mass spectrometer for providing electrons thereto. A heater element 24 is electrically isolated from the electron emitter cathode (25, 26) and arranged to be heated by an electrical current therein and to radiate heat to the electron emitter cathode sufficient to liberate electrons thermionically from the electron emitter surface. This provides the source of electrons for use in ionising a gas the gas-source chamber. A benefit of this arrangement is that the emitting surface is exposed to a more uniform acceleration potential resulting in a narrower energy spread of electrons. Consequently, most or all thermionic electrons reside at the same place, or region, within the accelerating electrical potential thereby improving the uniformity of thermionic electrons generated for use in ionising a target gas. An electrical heating current is not passed through the electron emitter surface 26. Instead, an electrical heating current is passed through a separate heating element 24 which becomes heated to sufficient temperature, to radiate heat electromagnetically (e.g. IR radiation) to the electron emitter cathode (25, 26). The cathode absorbs radiated heat energy and emit electrons thermionically in response to that. A flow rate of electrons across the gas chamber, in the electron beam, may exceed 500 A or more. The flow rate of electrons across the gas chamber, in the electron beam, may be between 0.5 mA and 10 mA, e.g. 1 mA or several mA. These electron flow rates may be achievable when the temperature of the electron emitter cathode is less than 2000 C., e.g. about 1000 C. The electron emitter cathode (26, 25) is able to be heated by the heater element 24 to a temperature up to 2000 C. when the electrical power input to the heater element is less than 5 W. Indeed, typically, the electrical power input to the heater element 24 may be between about 0.5 W and about 1 W. The electron emitter cathode (26, 25) is an oxide cathode. In other embodiments an I-cathode (also known as a Ba-dispenser cathode) may be used. It comprises a Ni base part 25 which bears a coating of thermionically emissive material 26 presenting the electron emitter surface. The coating comprises (Ba,Sr,Ca)-carbonate particles or (Ba,Sr)-carbonate particles on a nickel cathode base part. The electron source 20 comprises a Nichrome sleeve 23 which surrounds the heater element 24. The electron emitter surface 26 and base part 25, collectively reside at an end of the sleeve. The base part 25 forms a cap enclosing tat end of the sleeve. The sleeve serves to concentrate heat from the heater element upon the base part 25, which conducts heat to the emitter coating 26. The heater element comprises a tungsten filament 21 coated with an alumina coating. This provides electrical isolation between the heating current within the heater element and the electron emitter cathode ((25, 26). This electron source offers greater electron emission at lower temperatures as compared to the tungsten filament. Typical operation requires 6.3V at 105 mA which is approximately 0.6 W of power. The local temperature on the cathode is then about 1000 C. This produces about 1 mA of electron trap current and a corresponding 5-fold sensitivity increase of the resulting ion beam produced by electron bombardment ionisation of a source gas via the electron beam 6. The lifetime of the cathode filament 20 is estimated to be more than 10 years, which far exceeds the ordinary operating lifetime of the tungsten coil filament 1, if it were to produce a comparable emission current. Benefits of using cathode as a replacement for the tungsten filament 1 include the following: [0181] Higher electron emissions: by a factor of about 5-10 with a comparable lifetime to the existing tungsten filament 1. The tungsten filament coil 1 may produce similar emissions but the lifetime is considerably reduced before replacement is necessary. A filament replacement potentially causes months of down-time. [0182] Lower operating temperatures: This reduces the presence of hydrocarbon volatiles in the vacuum which are ionised and interfere with the isotope species of interest. [0183] The higher levels of emission: This means that the external magnetic field (magnets 14) can be removed. This avoids unwanted effects of this field on the mass analyser. Ion mass discrimination between isotopes is possible, as this tends to be non-linear over a given range of partial pressures of a sample/target material. [0184] No voltage drop across the cathode: This cannot be avoided when using the tungsten filament coil 1. This provides a more homogenous electron energy which will provide greater control on sensitivity. [0185] Mechanical stability: This improves the consistency of the electron source and the ion source which uses it, and avoids step changes in operation during cathode lifetime. [0186] Extended lifetime: The lower operating temperature and conservative design of the cathode 20 results in extended useful life of the cathode coupled with low rates of filament deterioration.

[0187] FIG. 6 schematically depicts an ion source 60 according to an exemplary embodiment. The ion source 60 is generally as described with respect to the ion source 30, description of which is not repeated for brevity and like reference signs indicate like all integers.

[0188] In this example, the first electrode 630A comprises and/or is one or more electron extraction grids and the controller is configured to control the voltage applied to the first electrode to attenuate the flux of the electrons into the source block during the first time period following receiving of the sample gas in the source block by applying a negative voltage to the first electrode and to permit the flux of the electrons into the source block during the second time period following the first time period by applying a positive voltage to the first electrode.

[0189] In this example, the set of electrodes 630 includes a second electrode 630B, an anode, disposed between the first electrode 630A and the source block 610, in tandem with the first electrode 630A. In this example, the controller is configured to apply a variable electrical potential to the second electrode 630B for accelerating electrons emitted from the electron source 620 in a direction towards the source block 610.

[0190] In this example, the set of electrodes 630 includes a third electrode 630C, disposed between the electron source 620 and the source block 610, in tandem with the first electrode 630A and the second electrode 630B. In this example, the third electrode 630C comprises an Einzel lens arranged to focus the electrons from the electron source 620 into the source block 610 via the aperture 611.

[0191] In this example, the controller is configured to control the energy of thermionic electrons for input to the source block 610 by controlling the accelerating voltage(s) applied to the anode 630B or applied to the extraction grid 630A, or both. This controllability is particularly effective and beneficial due to the relatively narrow spread in the distribution of kinetic energy amongst the thermionic electrons emitted from the electron source 610, as compared to the much broader corresponding distribution of kinetic energy amongst the thermionic electrons emitted from a conventional heated tungsten filament.

[0192] FIG. 7 schematically depicts a method according to an exemplary embodiment.

[0193] The method is of controlling an ion source of a static gas mass spectrometer.

[0194] At S701, the method comprises receiving, by a volume defined by a source block, a sample gas.

[0195] At S702, the method comprises providing, by an electron source in fluid communication with the source block, a flux of electrons therein and ionising the sample gas.

[0196] At S703, the method comprises controlling, by a controller, a voltage applied to a set of electrodes, including a first electrode, disposed between the electron source and the source block, comprising: [0197] at S704, attenuating, during a first time period following receiving of the sample gas in the source block, the flux of the electrons into the source block; and [0198] at S705, permitting, during a second time period following the first time period, the flux of the electrons into the source block.

[0199] The method may include any of the steps as described with respect to the third aspect.

[0200] FIG. 8 schematically depicts a method according to an exemplary embodiment.

[0201] The method is of controlling a static gas mass spectrometer.

[0202] At S801, the method comprises the ion source as described with respect to FIG. 7.

[0203] At S802, the method comprises detecting, during the second time period following the first time period, the ions from the sample gas.

[0204] The method may include any of the steps as described with respect to the fourth aspect.

[0205] Although a preferred embodiment has been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims and as described above.

[0206] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0207] All of the features disclosed in this specification (including any accompanying claims and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at most some of such features and/or steps are mutually exclusive.

[0208] Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0209] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.