Glow Discharge Ion Source

Abstract

A mass spectrometer is disclosed comprising a glow discharge device within the initial vacuum chamber of the mass spectrometer. The glow discharge device may comprise a tubular electrode located within an isolation valve, which is provided in the vacuum chamber. Reagent vapour may be provided through the tubular electrode, which is then subsequently ionised by the glow discharge. The resulting reagent ions may be used for Electron Transfer Dissociation of analyte ions generated by an atmospheric pressure ion source. Other embodiments are contemplated wherein the ions generated by the glow discharge device may be used to reduce the charge state of analyte ions by Proton Transfer Reaction or may act as lock mass or reference ions.

Claims

1. A mass spectrometer comprising: a vacuum chamber; an atmospheric pressure ion source for generating first ions, wherein first ions generated by said atmospheric pressure ion source are transmitted, in use, into said vacuum chamber via a sampling cone or first aperture; and a glow discharge device for generating second ions, wherein second ions generated by said glow discharge device are either generated within said vacuum chamber or are transmitted into said vacuum chamber without being transmitted through said sampling cone or first aperture.

2. A mass spectrometer as claimed in claim 1, wherein said glow discharge device comprises an electrode or pin and wherein said mass spectrometer further comprises a voltage device for supplying or applying a DC and/or RF voltage to said electrode or pin in order to cause or generate a glow discharge.

3. A mass spectrometer as claimed in claim 1, further comprising one or more dispensing devices for dispensing one or more reagents in proximity to said glow discharge device so that said one or more reagents are ionised, in use, by a glow discharge caused or generated by said glow discharge device.

4. A mass spectrometer as claimed in claim 3, wherein said one or more reagents comprise one or more Electron Transfer Dissociation reagents and/or one or more Proton Transfer Reaction reagents and/or one or more lock mass or calibration reagents.

5. A mass spectrometer as claimed in claim 1, wherein said glow discharge device is housed in a housing adjacent said vacuum chamber and wherein second ions generated by said glow discharge device pass from said housing through a second aperture into said vacuum chamber.

6. A mass spectrometer as claimed in claim 1, further comprising either a solid, powdered, partially solid or gel substance, a volatile liquid or a gas which is arranged or supplied in proximity to said glow discharge device so that ions are sputtered, extracted or released from said substance, liquid or gas.

7. A mass spectrometer as claimed in claim 6, wherein said substance comprises caesium iodide.

8. A mass spectrometer as claimed in claim 1, further comprising a supply device for supplying one or more reagents and/or one or more Electron Transfer Dissociation reagents and/or one or more Proton Transfer Reaction reagents and/or one or more lock mass reagents in proximity to said glow discharge device.

9. A mass spectrometer as claimed in claim 1, further comprising an isolation valve arranged in said vacuum chamber and arranged downstream of said sampling cone or first aperture.

10. A mass spectrometer as claimed in claim 9, wherein said glow discharge device is located upstream of said isolation valve, within said isolation valve or downstream of said isolation valve.

11. A mass spectrometer as claimed in claim 9, wherein said isolation valve comprises a first rotatable port wherein when said isolation valve is rotated to a first position then said vacuum chamber downstream of said isolation valve is in fluid communication with said sampling cone or first aperture and when said isolation valve is rotated to a second position then said vacuum chamber downstream of said isolation valve is no longer in fluid communication with said sampling cone or first aperture.

12. A mass spectrometer as claimed in claim 1, wherein said glow discharge device comprises a tube having a sharpened or pointed end.

13. A mass spectrometer as claimed in claim 12, further comprising a supply device for supplying one or more reagents and/or one or more Electron Transfer Dissociation reagents and/or one or more Proton Transfer Reaction reagents and/or one or more lock mass reagents through said tube.

14. A mass spectrometer as claimed in claim 1, wherein said glow discharge device is operated in a continuous or pulsed manner.

15. A mass spectrometer as claimed in claim 1, wherein said glow discharge device is maintained or operated in a mode of operation at a potential selected from the group consisting of: (i) <1 kV; (ii) 900 to 800 V; (iii) 800 to 700 V; (iv) 700 to 600 V; (v) 600 to 500 V; (vi) 500 to 400 V; (vii) 400 to 300 V; (viii) 300 to 200 V; (ix) 200 to 100 V; (x) 100 to 0 V; (xi) 0 to 100 V; (xii) 100 to 200 V; (xiii) 200 to 300 V; (xiv) 300 to 400 V; (xv) 400 to 500 V; (xvi) 500 to 600 V; (xvii) 600 to 700 V; (xviii) 700 to 800 V; (xix) 800 to 900 V; (xx) 900 to 1000 V; and (xxi) >1 kV.

16. A mass spectrometer as claimed in claim 1, wherein said glow discharge device is operated, in use, at a pressure selected from the group consisting of (i) >0.001 mbar; (ii) >0.01 mbar; (iii) >0.1 mbar; (iv) >1 mbar; (v) >10 mbar; (vi) >100 mbar; (vii) <0.001 mbar; (viii) <0.01 mbar; (ix) <0.1 mbar; (x) <1 mbar; (xi) <10 mbar; (xii) <100 mbar; (xiii) 0.001-0.01 mbar; (xiv) 0.01-0.1 mbar; (xiv) 0.1-1 mbar; (xv) 1-10 mbar; (xvi) 10-100 mbar; and (xvii) 0.01-20 mbar.

17. A mass spectrometer as claimed in claim 1, further comprising an Electron Transfer Dissociation fragmentation cell arranged in a second vacuum chamber, wherein said second vacuum chamber is located downstream of said vacuum chamber and wherein, in use, at least some Electron Transfer Dissociation reagent ions generated by said glow discharge device are caused to interact with at least some analyte ions within said Electron Transfer Dissociation fragmentation cell so as to cause at least some of said analyte ions to fragment by Electron Transfer Dissociation.

18. A method of mass spectrometry comprising: providing a vacuum chamber; providing an atmospheric pressure ion source for generating first ions; generating first ions by said atmospheric pressure ion source and transmitting said first ions into said vacuum chamber via a sampling cone or first aperture; providing a glow discharge device for generating second ions; and generating second ions by said glow discharge device, wherein said second ions are generated either within said vacuum chamber or are transmitted into said vacuum chamber without being transmitted through said sampling cone or first aperture.

19. A mass spectrometer comprising: an atmospheric pressure ion source for generating analyte ions; and a glow discharge device for generating reagent ions and/or reference ions, wherein said glow discharge device is located within a vacuum chamber of said mass spectrometer.

20. A mass spectrometer as claimed in claim 19, wherein said mass spectrometer further comprises: (i) an Electron Transfer Dissociation reaction cell and wherein said reagent ions are caused to interact with analyte ions within said reaction cell in order to cause at least some of said analyte ions to fragment by Electron Transfer Dissociation; and/or (ii) a Proton Transfer Reaction reaction cell and wherein said reagent ions are caused to interact with analyte ions within said reaction cell in order to reduce the charge state of at least some of said analyte ions by Proton Transfer Reaction.

21. A method of mass spectrometry comprising: providing an atmospheric pressure ion source for generating analyte ions; and using a glow discharge device for generating reagent ions and/or reference ions, wherein said glow discharge device is located within a vacuum chamber of said mass spectrometer.

22. A method as claimed in claim 21, further comprising: (i) causing said reagent ions to interact with analyte ions within an Electron Transfer Dissociation reaction cell in order to cause at least some of said analyte ions to fragment by Electron Transfer Dissociation; and/or (ii) causing said reagent ions to interact with analyte ions within a Proton Transfer Reaction reaction cell in order to reduce the charge state of at least some of said analyte ions by Proton Transfer Reaction.

23. A mass spectrometer comprising: an atmospheric pressure ion source for generating analyte ions; a vacuum chamber; a nozzle-skimmer interface separating said vacuum chamber from said atmospheric pressure ion source, wherein at least some analyte ions generated by said atmospheric pressure ion source are transmitted, in use, through said nozzle-skimmer interface into said vacuum chamber; an isolation valve located in said vacuum chamber; a glow discharge device for generating reagent ions and/or reference ions, wherein said glow discharge device is located within or downstream of said isolation valve; and a device for supplying reagent to said glow discharge device.

24. A method of mass spectrometry comprising: providing an atmospheric pressure ion source for generating analyte ions; providing a vacuum chamber; providing a nozzle-skimmer interface separating said vacuum chamber from said atmospheric pressure ion source, wherein at least some analyte ions generated by said atmospheric pressure ion source are transmitted through said nozzle-skimmer interface into said vacuum chamber; providing an isolation valve located in said vacuum chamber; generating reagent ions and/or reference ions using a glow discharge device, wherein said glow discharge device is located within or downstream of said isolation valve; and supplying reagent to said glow discharge device.

25. A mass spectrometer comprising: a vacuum chamber; a sub-atmospheric pressure ion source for generating analyte ions; and a glow discharge device for generating reagent or reference ions, wherein said reagent or reference ions generated by said glow discharge device are generated within said vacuum chamber.

26. A method of mass spectrometry comprising: providing a vacuum chamber; generating analyte ions using a sub-atmospheric pressure ion source; and using a glow discharge device to generate reagent or reference ions, wherein said reagent or reference ions generated by said glow discharge device are generated within said vacuum chamber.

27. A mass spectrometer comprising: an atmospheric or sub-atmospheric pressure ion source for generating analyte ions; a vacuum chamber; a nozzle-skimmer interface separating said vacuum chamber from said ion source wherein at least some analyte ions generated by said ion source are transmitted, in use, through said nozzle-skimmer interface into said vacuum chamber; a glow discharge device for generating reagent ions and/or reference ions, wherein said glow discharge device is located within or downstream of said nozzle-skimmer interface and is maintained in use at a sub-atmospheric pressure; and a device for supplying reagent vapour to said glow discharge device.

28. A method of mass spectrometry comprising: providing an atmospheric or sub-atmospheric pressure ion source for generating analyte ions; providing a vacuum chamber; providing a nozzle-skimmer interface separating said vacuum chamber from said ion source wherein at least some analyte ions generated by said ion source are transmitted through said nozzle-skimmer interface into said vacuum chamber; generating reagent ions and/or reference ions using a glow discharge device, wherein said glow discharge device is located within or downstream of said nozzle-skimmer interface and is maintained at a sub-atmospheric pressure; and supplying reagent vapour to said glow discharge device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0175] Various embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

[0176] FIG. 1 shows an embodiment of the present invention wherein a glow discharge ion source for generating reagent ions is provided in a housing adjacent to the first vacuum chamber of a mass spectrometer;

[0177] FIG. 2 shows an embodiment of the present invention wherein two reagents may be supplied to the glow discharge ion source which is provided in a housing adjacent to the first vacuum chamber of a mass spectrometer;

[0178] FIG. 3 shows an embodiment of the present invention wherein a solid reagent is provided within a housing adjacent to the first vacuum chamber of the mass spectrometer and wherein part of the solid reagent is ionised by a glow discharge to provide lock mass ions for calibrating the mass spectrometer;

[0179] FIG. 4 shows an embodiment of the present invention wherein a glow discharge is initiated directly in the first vacuum chamber of a mass spectrometer and wherein a volatile reagent may be fed into the first vacuum chamber downstream of the electrode which is used to generate the glow discharge;

[0180] FIG. 5 shows a tune page from a mass spectrometer and shows mass spectra showing caesium lock mass ions generated by a preferred glow discharge ion source and analyte ions of Leucine Enkephalin generated by an Electrospray ion source;

[0181] FIG. 6 shows a tune page from a mass spectrometer and shows mass spectra showing caesium and diiodomethane lock mass ions generated by a preferred glow discharge ion source and analyte ions of Leucine Enkephalin generated by an Electrospray ion source;

[0182] FIG. 7 shows an embodiment wherein a discharge pin for generating a glow discharge is provided within the body of an isolation valve which is located in the first vacuum chamber of a mass spectrometer;

[0183] FIG. 8 shows an Electron Transfer Dissociation fragmentation spectrum of triply charged substance-P which has been subjected to fragmentation by azobenzene reagent ions generated by a glow discharge source according to an embodiment of the present invention;

[0184] FIG. 9A shows a calibration mass spectrum performed using perfluorotripentylamine (FC70) and caesium ions which were generated by a glow discharge source according to an embodiment of the present invention and FIG. 9B shows an experimental mass spectrum, a reference mass spectrum and a determination of the residual mass errors after calibration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0185] An embodiment of the present invention will now be described with reference to FIG. 1 which shows an atmospheric pressure Electrospray ionisation ion source 7 arranged adjacent to the inlet and sample cone 8 of a mass spectrometer. A glow discharge source comprising an electrode or pin 4 is preferably provided in a housing adjacent to a first vacuum chamber 3 of the mass spectrometer. The electrode or pin 4 is preferably connected to an external high voltage supply. The housing is preferably maintained at a relatively high or intermediate pressure and the application of a relatively high voltage to the electrode or pin 4 preferably causes a glow discharge 1 to occur within the housing.

[0186] A volatile reagent 6 is preferably fed into the housing and is preferably injected into the glow discharge volume 1 which is preferably formed within the housing. The flow of reagent 6 is preferably controlled by a valve or micro-dosing device 5. When a high voltage is applied to the discharge electrode or pin 4 a glow discharge is preferably initiated which preferably ionises the reagent which is fed into the housing so that a plurality of reagent ions are formed within the housing. According to an embodiment a means of enhancing the extraction of reagent ions from the glow discharge volume 1 or otherwise from the housing into the first vacuum chamber 3 via an aperture 2 is preferably provided. For example, an electric field may be maintained between the housing and the first vacuum chamber 1 in order to urge reagent ions from the housing into the first vacuum chamber 1. Additional pumping (not shown) may be provided to the housing in which the glow discharge volume 1 is generated in order to minimise the flow of neutral reagent molecules from the housing via the aperture 2 into the first vacuum chamber 3 and the other ion optic sections of the mass spectrometer. Reagent ions which are generated within the housing and which pass via the aperture 2 into the first vacuum chamber 3 are then preferably onwardly transmitted through an extraction cone 10 into a second vacuum chamber. The second vacuum chamber preferably comprises an ion guide 11 which is preferably arranged to transmit ions through the second vacuum chamber. The reagent ions (preferably reagent anions) are then preferably onwardly transmitted to a collision cell (not shown) located in a further vacuum chamber downstream of the second vacuum chamber.

[0187] According to a preferred embodiment of the present invention the reagent ions which are created within the housing and which are preferably onwardly transmitted to the collision cell are preferably caused to interact with analyte ions (preferably analyte cations). The reagent ions are preferably caused to interact with analyte ions so as to cause the analyte ions to fragment by a process of Electron Transfer Dissociation (ETD) so that a plurality of fragment, daughter or product ions are formed as a result of the Electron Transfer Dissociation process. According to another less preferred embodiment the reagent ions which are generated may be reacted with analyte ions in order to reduce the charge state of the analyte ions by Proton Transfer Reaction without substantially fragmenting the analyte ions.

[0188] According to an alternative or additional embodiment, the glow discharge ion source 1 may act as a source of ions which are preferably used for mass calibration of the mass spectrometer. A particularly preferred aspect of the present invention is that a glow discharge ion source located within the body of the mass spectrometer may be used to generate both reagent ions for use in an Electron Transfer Dissociation and/or Proton Transfer Reaction cell and also calibration or lock mass ions for calibrating the mass spectrometer.

[0189] The glow discharge source is preferably operated in a pulsed manner wherein a high voltage pulse is preferably only applied to the electrode or discharge pin 4 during a lock mass cycle or during a reagent introduction cycle. A lock mass compound is preferably only admitted into the housing during a lock mass cycle. Likewise, a reagent is preferably only admitted into the housing during a reagent introduction cycle.

[0190] Analyte ions are preferably generated by a separate atmospheric pressure ion source such as an Electrospray Ionisation ion source 7 as shown in FIG. 1. The ion source for generating analyte ions is preferably external to the main body of the mass spectrometer.

[0191] A known conventional mass spectrometer comprises two ion sources. The first ion source comprises an Electrospray ion source for generating analyte ions and the second ion source comprises an Atmospheric Pressure Chemical Ionisation ion source for generating reagent ions or calibration ions. The necessity to use two atmospheric pressure ion sources is problematic and makes the ion source geometry relatively complex. The two ion sources can also interfere with each other and can cause problems due to cross-talk. The preferred embodiment is therefore particularly advantageous in that only a single Electrospray ion source needs to be provided. This simplifies the ion source geometry and removes any problem of cross-talk between ion sources.

[0192] A particularly serious problem with known mass spectrometers is that many of the reagents which are ionised to produce reagent ions for use in Electron Transfer Dissociation such as azobenzene are carcinogenic. An important advantage therefore of the preferred embodiment is that reagents which are used to generate reagent ions are not sprayed or otherwise emitted external to the inlet aperture of the mass spectrometer (and hence in the vicinity of a user) but instead are sprayed or otherwise injected internally within a vacuum chamber of the mass spectrometer. As a result, the exposure of a user to potentially carcinogenic reagents is significantly reduced.

[0193] A yet further advantageous feature of the preferred embodiment is that the preferred glow discharge device significantly improves the sensitivity and intensity of generated reagent ions compared with a conventional mass spectrometer.

[0194] It is also contemplated that at least some reagents may be used both as a source of reagent ions for Electron Transfer Dissociation (and/or Proton Transfer Reaction) and also as a source of lock mass or reference ions for calibrating the mass spectrometer. Analyte ions generated by the ion source 7 are preferably drawn through the sample cone 8 of the mass spectrometer into the first vacuum chamber 3 of the mass spectrometer which is preferably pumped by a vacuum pump 9. The first vacuum chamber 3 and the inlet into the mass spectrometer are preferably heated. The analyte ion source and initial stages of the mass spectrometer may comprise a Z-Spray ion source.

[0195] FIG. 2 shows another embodiment of the present invention wherein two different reagents 6a,6b may be introduced into the housing in which the glow discharge volume 1 is generated. According to an embodiment one of the reagents 6a may comprise a reagent which is used to generate reagent ions which are used for ion-ion reactions such as Electron Transfer Dissociation. The other reagent 6b may be used to generate calibrant or lock mass ions. The selection of one species of reagent ions is preferably performed or controlled by use of one or more valves or micro-dosing devices 5a,5b. Alternatively, a mass selective device such as a resolving quadrupole rod set mass filter may be arranged or otherwise provided downstream of the glow discharge source 1 in order to filter out any undesired ions and/or to transmit onwardly only desired ions.

[0196] Further embodiments are contemplated wherein three, four, five, six or more than six different reagents may be selectively introduced into the housing in which the glow discharge 1 is created. A flow of gas may be admitted towards the discharge volume or otherwise more generally into the housing in which the glow discharge 1 is created. The gas may comprise an inert make up gas which is preferably introduced in order to increase the pressure within the discharge chamber or housing relative to the first vacuum stage or chamber 3. Alternatively, the gas may comprise a chemical ionisation (CI) gas which is preferably provided in order to enhance the ionisation of the reagent in the discharge chamber or housing. The gas may flow past or through the reagent as a means of controlling the flow of reagent neutrals into the glow discharge. The addition of gas into the housing in which the glow discharge 1 is generated in combination with suitable or appropriate differential pumping preferably enables the discharge chamber or housing in which the glow discharge 1 is generated to be interfaced to other vacuum regions of the mass spectrometer.

[0197] The first vacuum chamber 3 and the inlet into the mass spectrometer as shown in FIG. 2 may be heated. The analyte ion source and initial stages of the mass spectrometer may comprise a Z-Spray ion source.

[0198] An alternative embodiment for producing lock mass ions is shown in FIG. 3. According to this embodiment a solid highly ionic material 12 such as caesium iodide (Csl) may be placed or located within the housing which is located adjacent to the first vacuum chamber 3 of the mass spectrometer. When a glow discharge 1 is produced within the housing, the glow discharge 1 preferably causes caesium ions to be released from the surface of the caesium iodide block. The caesium ions are preferably sputtered and are preferably extracted from the discharge volume 1 for use as a means of lock mass correction. A reagent introduction system comprising reagent 6, a fluid flow path and a valve 5 may also be provided in order to introduce reagent into the housing. Reagent ions are preferably created by the glow discharge 1 within the housing and may be onwardly transmitted for use as Electron Transfer Dissociation and/or Proton Transfer Reaction reagent ions. Therefore, according to this embodiment both calibrant or lock mass ions and also reagent ions may be generated within the housing and may be onwardly transmitted into the first vacuum stage 3 and subsequent vacuum stages of the mass spectrometer. According to an embodiment a resolving quadrupole rod set mass filter may be provided upstream of an ion-ion reaction cell and downstream of the glow discharge source 1 to ensure that only desired reagent ions (preferably reagent anions) are introduced into the reaction cell. The reagent ions preferably interact with analyte ions (preferably analyte cations) and preferably cause the analyte ions to fragment by means of Electron Transfer Dissociation.

[0199] The first vacuum chamber 3 and the inlet into the mass spectrometer shown in FIG. 3 may be heated. The analyte ion source and initial stages of the mass spectrometer may comprise a Z-Spray ion source.

[0200] FIG. 4 shows a further embodiment wherein a glow discharge is initiated directly within the first vacuum region 3 of the mass spectrometer rather than in a housing adjacent to the first vacuum chamber 3. According to the embodiment shown in FIG. 4 a volatile reagent 6 is preferably fed via a valve 5 directly into the first vacuum chamber 3 of the mass spectrometer. The reagent is preferably introduced into the first vacuum chamber 3 at a location downstream of an electrode or pin 4. The electrode or pin 4 is preferably connected via an electrical connection 13 to a high voltage source. A high voltage is preferably applied to the electrode or pin 4 via the electrical connection 13 and this preferably causes a glow discharge to be created within the first vacuum chamber 3.

[0201] Other embodiments are contemplated wherein the volatile reagent 6 may be replaced with a solid such as caesium iodide (Csl) which is preferably located within the first vacuum chamber 3. The block of caesium iodide is preferably provided within the first vacuum chamber 3 adjacent the region wherein a glow discharge 1 is formed within the first vacuum chamber 3 by the application of a high voltage to the electrode or pin 4.

[0202] The first vacuum chamber 3 and the inlet into the mass spectrometer shown in FIG. 4 may be heated. The analyte ion source and initial stages of the mass spectrometer may comprise a Z-Spray ion source.

[0203] FIG. 5 shows a tune page from a modified Waters Q-Tof Premier mass spectrometer and corresponding mass spectra. The modified mass spectrometer was operated in positive ion mode and was arranged substantially as shown in FIG. 4 except that caesium lock mass ions were generated from a solid block of caesium iodide which was provided within the first vacuum chamber 3 adjacent electrode or pin 4. The glow discharge source 1 was generated using an ESCi power supply operating in constant current mode with approximately 5 A of discharge current at a voltage of approximately +700V. Leucine Enkephalin was used as a test analyte and was ionised by a conventional Electrospray ion source. The corresponding mass spectrum for the test analyte having a mass to charge ratio of 556 is shown in FIG. 5. Caesium lock mass calibration ions having a mass to charge ratio of 133 which were generated by the glow discharge ionising the caesium iodide block were used to calibrate the mass spectrometer. A corresponding mass spectrum showing the caesium lock mass ions is also shown in FIG. 5. Alternating acquisitions of 1 s duration were acquired with an inter-spectrum delay of 0.1 sec.

[0204] FIG. 6 shows a further tune page and corresponding mass spectra which were obtained when Diiodomethane vapour was additionally introduced into the mass spectrometer in a manner substantially as shown in FIG. 4 as an additional calibration compound. A block of caesium iodide was also provided within the first vacuum chamber 3 adjacent the glow discharge 1 so that caesium ions were also released as a source of reference ions. Mass spectra for the analyte ions as generated by an Electrospray ion source and the two lock mass ions as generated by a glow discharge device according to an embodiment of the present invention are shown in FIG. 6.

[0205] FIG. 7 shows a further embodiment of the present invention wherein a discharge pin 14 is incorporated into an isolation valve 15 located within the first vacuum chamber of the mass spectrometer. The isolation valve 15 is located between a skimmer cone 17 and an extraction cone 18 of the mass spectrometer. The discharge pin 14 preferably comprises a stainless steel capillary tube (0.0625 O.D.0.5 mm I.D.) which is preferably arranged to have a sharp or pointed end. According to the embodiment shown in FIG. 7 vapour from a reagent cell 16 is preferably emitted via a reagent vapour outlet tube 19 in close proximity to the sharp or pointed end of the discharge pin 14. The reagent cell may comprise a vial of reagent crystals e.g. azobenzene or fluoranthene. It will be understood that the isolation valve 15 has a cylindrical bore or port which when aligned with the central cylindrical bore of the first vacuum chamber 3 allows ions to pass from the analyte sampling cone 17 towards the extraction cone 18 which leads into the second vacuum chamber. However, when the mass spectrometer is not operational the isolation valve may be rotated by 90 so that the cylindrical bore or port of the isolation valve 15 is no longer in alignment with the cylindrical bore of the first vacuum chamber 3. As a result, the isolation valve 15 acts to seal the vacuum chambers of the mass spectrometer from the atmosphere and therefore assists in maintaining a low pressure within the mass spectrometer when the mass spectrometer is not operational. It will be appreciated that maintaining a low pressure within the mass spectrometer significantly reduces the start-up time when operation of the mass spectrometer is desired to be resumed.

[0206] According to a preferred embodiment of the present invention the reagent cell may 16 be positioned such that reagent vapour flows down and through the tube which preferably forms the discharge pin 14 and emerges from the tube at the sharp or pointed end of the discharge pin 14. According to an embodiment the reagent cell 16 may comprise a vial of reagent crystals e.g. azobenzene or flouranthene may be provided in solid form within the reagent cell 16 which is preferably connected to the tube. A make-up gas (which is preferably inert) such as nitrogen may also be used. The make-up gas may be arranged to flow past the crystals which may be held at room temperature (20 C.). The make-up gas may be supplied at a flow rate of around 20 ml/min.

[0207] According to the preferred embodiment oxygen is preferably substantially prevented from flowing through the discharge region as this can cause a loss of reagent signal. According to the preferred embodiment the source volume may be purged with nitrogen.

[0208] According to an embodiment a voltage of 500 V may be applied to an electrode 20 which is in electrical contact with the discharge pin 14 so that the discharge pin is preferably maintained at a voltage of 500 V in order to generate a glow discharge.

[0209] FIG. 8 shows an ETD fragmentation spectrum which was obtained by interacting triply charged substance-P ions with azobenzene reagent anions. The azobenzene reagent anions were formed by introducing azobenzene reagent vapour through a tubular pin 14 located within the isolation valve of a mass spectrometer in a manner substantially as shown in FIG. 7 and as described above. The end of the tubular pin 14 was pointed and formed the glow discharge device. A high voltage was applied to the electrode or pin 14 in order to induce a glow discharge which resulted in the ionisation of the azobenzene vapour to form azobenzene reagent anions. The azobenzene reagent ions when interacted with triply charged substance-P ions in an Electron Transfer Dissociation cell located in a downstream vacuum chamber caused the triply charged substance-P ions to fragment by Electron Transfer Dissociation.

[0210] FIG. 9A shows a mass spectrum obtained according to an embodiment of the present invention wherein calibration was performed using perflourotripentylamine (FC70) ions and caesium ions. The perfluorotripentylamine (FC70) ions and caesium ions were generated by introducing perfluorotripentylamine vapour through a tubular pin 14 located within the isolation valve of a mass spectrometer in a manner substantially as shown in FIG. 7 and as described above. The end of the tubular pin 14 was pointed and formed the glow discharge device. A high voltage was applied to the electrode or pin 14 in order to induce a glow discharge which resulted in the ionisation of the perfluorotripentylamine vapour to form perfluorotripentylamine reference ions. The caesium reference ions were generated by coating the region around the end of the tubular pin 14 with caesium iodide.

[0211] Other embodiments are contemplated wherein other reagents for calibration/lock mass may be introduced into the glow discharge device including perfluorokerosine (PFK), perfluorotrihexylamine, perfluorotributylamine (FC43), diiodomethane and iodotetrafluoropropane.

[0212] The upper mass spectrum in FIG. 9B shows an nominal mass spectrum obtained from experimental data prior to calibration of the mass spectrometer. The mass or mass to charge ratio values were calculated using an estimated calibration method wherein the mass is proportional to the time of flight squared. Perfluorotripentylamine (FC70) ions were generated by an Electrospray ionisation ion source and were admitted into the mass spectrometer and were subsequently mass analysed.

[0213] The middle mass spectrum shows a reference or theoretical mass spectrum for perfluorotripentylamine and is based upon reference data.

[0214] The mass spectrometer was then calibrated more accurately by applying a higher order (fourth order) time of flight polynomial curve to the experimental data. The RMS of the residual errors of the least squares fitting of the fourth order polynomial curve against the experimental data are shown in the lower figure and was determined to be 0.6 ppm.

[0215] Although according to the preferred embodiment reagent ions are preferably generated either in a housing adjacent to the first vacuum chamber 3 or alternatively directly in the first vacuum chamber 3, according to other less preferred embodiments reagent ions may be generated in a housing adjacent to a second or subsequent vacuum chamber or alternatively may be generated in a second or subsequent vacuum chamber which is preferably arranged downstream of the first vacuum chamber 3. For example, it is contemplated that a glow discharge ion source may be provided in the same vacuum chamber as an Electron Transfer Dissociation reaction cell and/or the same vacuum chamber as a Proton Transfer Reaction reaction cell.

[0216] According to the preferred embodiment of the present invention the glow discharge device comprises a pin or electrode 4,14 to which a DC and/or RF voltage is applied in order to generate a glow discharge 1.

[0217] Although the present invention has been described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made to the particular embodiments discussed above without departing from the scope of the invention as set forth in the accompanying claims.