METHOD AND APPARATUS

Abstract

An apparatus (100, 300, 700) is described, comprising: a linear ion trap (102) comprising two pairs of pole electrodes and a radiofrequency, RF, electrical potential supply (117) configured to apply respective RF waveforms to the pairs of pole electrodes, thereby forming a RF trapping field component to trap analyte ions (116) radially in a trapping region (115) of the linear ion trap for processing of the analyte ions (116) therein; a charged particle source (101) comprising a pulse valve (103), a conduit (106, 107), having an entrance in fluid communication therewith and an exit, wherein the conduit (106, 107) extends in the direction of the trapping region (115), and a discharge device (108) electrically coupled to an electrical potential supply (109) and disposed between the entrance and the exit of the conduit (106, 107), wherein the pulse valve (103) is configured to release a gas pulse from a gas supply into the entrance of the conduit (106, 107) and wherein the electrical potential supply (109) is configured to apply a high voltage to the discharge device (108) to generate a discharge (110) in the gas pulse in the conduit (106, 107), thereby generating charged particles (114) from the gas and accelerating the generated charged particles in the direction of the trapping region (115). A method is also described.

Claims

1. An apparatus comprising: a linear ion trap comprising two pairs of pole electrodes and a radiofrequency, (RF) electrical potential supply configured to apply respective RF waveforms to the pairs of pole electrodes, thereby forming a RF trapping field component to trap analyte ions radially in a trapping region of the linear ion trap for processing of the analyte ions therein; and a charged particle source comprising a pulse valve, a conduit extending in the direction of the trapping region of the linear ion trap and having an entrance in fluid communication with the pulse valve and an exit, and a discharge device electrically coupled to an electrical potential supply and disposed between the entrance and the exit of the conduit, wherein the pulse valve is configured to release a gas pulse from a gas supply into the entrance of the conduit and wherein the electrical potential supply is configured to apply a high voltage to the discharge device to generate a discharge in the gas pulse in the conduit, thereby generating charged particles from the gas and accelerating the generated charged particles in the direction of the trapping region to activate the analyte ions in the trapping region.

2. The apparatus of claim 1, further comprising a set of electrical conductors arranged between the exit of the conduit and the linear ion trap, the set of electrical conductors being supplied with a first DC signal to guide the charged particles therethrough, in the direction of the trapping region.

3. The apparatus of claim 2, further comprising a divergence electrode disposed between the charged particle source and the set of electrical conductors and configured to control divergence of the charged particles.

4. The apparatus of claim 2, further comprising a deflection electrode disposed between the set of electrical conductors and the linear ion trap, the deflection electrode being supplied with a second DC signal to prevent the charged particles from entering the trapping region.

5. The apparatus of claim 2, wherein the set of electrical conductors comprises an incandescent electron source configured to create reactive neutral particles from the charged particles, which are directed towards the trapping region.

6. The apparatus of claim 5, wherein the set of electrical conductors is configured to provide the charged particles and the reactive neutral particles to activate the analyte ions in the trapping region.

7. The apparatus of claim 6, wherein the deflection electrode is configured to allow the reactive neutral particles (305) to be injected into the linear ion trap to activate the analyte ions in the trapping region.

8. The apparatus of claim 5, wherein the charged particles are neutralized during transport through the incandescent electron source.

9. The apparatus of claim 5, wherein the charged particles interact with the incandescent electron source to capture an electron from the conduction band of the conductor to form the reactive neutral particles.

10. The apparatus of claim 5, wherein the charged particles react with gas phase electrons produced by thermionic emission from the incandescent electron source to form activated or radical neutral particles.

11. The apparatus of claim 5, wherein the kinetic energy of the reactive neutral particles is adjusted by controlling a potential difference between the charged particle source and the electron source.

12. The apparatus of claim 1, wherein the charged particle source is configured to provide the charged particles in the trapping region to activate the analyte ions in the trapping region.

13. The apparatus of claim 1, wherein the charged particles are accelerated to the desired kinetic energy by controlling the voltage applied to the discharge device.

14. The apparatus claim 1, wherein the kinetic energy of the charged particles is adjusted by controlling a potential difference between the charged particle source and the linear ion trap.

15. A method of directing charged particles, and optionally reactive neutral particles, into a trapping region of a linear ion trap, the method comprising: releasing a gas pulse from a gas supply into the entrance of a conduit extending in the direction of the trapping region; and generating a discharge in the gas pulse in the conduit, thereby generating charged particles from the gas and accelerating the generated charged particles in the direction of the trapping region; optionally, converting a fraction of the accelerated charged particles into reactive neutral particles by transporting the charged particles through an incandescent electron source arranged between the discharge and the trapping region; and optionally, directing the remainder fraction of the accelerated charged particles together with the converted reactive neutral particles in the direction of the trapping region.

16. The method of claim 15, further comprising deflecting the remainder fraction of the accelerated charged particles and activating analyte ions stored in the trapping region by the converted reactive neutral particles.

17. An apparatus comprising: a linear ion trap having a trapping region configured to store analyte ions; and a charged particle source comprising: a conduit extending in the direction of the trapping region of the linear ion trap and having an entrance and an exit; a pulse valve disposed in fluid communication with the conduit and configured to release a gas pulse from a gas supply into the entrance of the conduit; and a discharge device electrically coupled to an electrical potential supply and disposed between the entrance and the exit of the conduit, the electrical potential supply being configured to apply a high voltage to the discharge device to generate a discharge in the gas pulse in the conduit, thereby generating charged particles from the gas and accelerating the generated charged particles in the direction of the trapping region of the linear ion trap to activate analyte ions stored in the trapping region.

18. The apparatus of claim 17, further comprising a set of electrical conductors arranged between the exit of the conduit and the linear ion trap, the set of electrical conductors being supplied with a first DC signal to guide the charged particles therethrough in the direction of the trapping region.

19. The apparatus of claim 18, wherein the set of electrical conductors comprises an incandescent electron source configured to create from the charged particles reactive neutral particles which are directed towards the trapping region.

20. The apparatus of claim 18, wherein the set of electrical conductors is configured to provide the charged particles and the reactive neutral particles to activate the analyte ions stored in the trapping region of the linear ion trap.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] 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:

[0059] FIG. 1 schematically depicts an apparatus according to an exemplary embodiment;

[0060] FIG. 2 shows graphs of ion current as a function of time for the apparatus of FIG. 1;

[0061] FIG. 3 schematically depicts an apparatus according to an exemplary embodiment;

[0062] FIG. 4 schematically depicts the apparatus of FIG. 3, in more detail;

[0063] FIG. 5 shows mass spectra for hydrogen atom attachment to heme B ions, obtained using the apparatus of FIG. 3;

[0064] FIG. 6 shows a mass spectrum of the [M+8H].sup.8+ charge state of protonated ubiquitin, obtained using the apparatus of FIG. 3; and

[0065] FIG. 7 schematically depicts an apparatus according to an exemplary embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

[0066] FIG. 1 shows an example schematic diagram of the apparatus 100 highlighting the charged particle source 101 coupled to a linear quadrupole ion trap 102. A pulse valve 103 is connected externally to the vacuum housing 104 and releases pulses of gas through an orifice 105, having a diameter of approximately 150 μm, into a conduit. The conduit comprises a first part 106 and a second part 107 arranged in series for transporting the gas in the direction of the linear ion trap 102. A first electrode 108 is disposed between the first and the second parts of the conduit and supplied with a high voltage signal produced in an external electrical potential generator 109. A dielectric barrier discharge 110 is developed when pressure inside the conduit during the gas pulse is raised above a threshold exceeding the breakdown limit. The high voltage signal applied to the first electrode 108 will also accelerate charged particles in the direction of the trapping region 115.

[0067] Charged particles and electrons generated in the discharge 110 are entrained in the carrier gas flow travelling through the conduit ultimately forming a free jet expansion at the exit end of the second part of the conduit 107. In this example, the inner diameter of the conduit is 2 mm. A second electrode 111 is disposed at the end of the conduit 107 and supplied with a high voltage signal produced in an external electrical potential generator 109. A third electrode 112 is disposed between the charged particle source 101 and the linear ion trap 102 and also supplied with a high voltage signal produced in an external electrical potential generator 109. The second 111 and third 112 electrodes jointly form a focusing lens for controlling the divergence of the charged particle beam 114 in the direction of the trapping region 115. The focusing lens formed between the second 111 and third 112 electrodes can also be used for deflecting electrons entrained in the gas flow at the exit of the conduit 107.

[0068] In this example, an RF electrical potential generator 117 is employed for producing two RF waveforms, each applied to respective pairs of pole electrodes of the linear quadrupole ion trap forming a RF trapping field component a trapping region 115 to trap analyte ions 116 radially. The pulsed beam of charged particles 114 is injected through an aperture on the side pole-electrode of the linear ion trap into the trapping region 115 to activate or dissociate analyte ions 116. An additional focusing electrode 113 may also be disposed near the aperture to optimize the overlap between externally injected charged particles 114 and analyte ions 116. This focussing electrode 113 may additionally and/or alternatively function to deflect the charged particles, as described herein. Similarly, the second 111 and third 112 electrodes may additionally and/or alternatively function individually or together, optionally with the electrode 113, to deflect the charged particles, as described herein. A steering lens may also be disposed between the charged particle source 101 and the linear ion trap 102 to optimize the efficiency of analyte ion activation.

[0069] When a positive voltage bias is applied to the first electrode 108 generating the discharge, a pulse of positively charged particles is measured at the exit of the discharge conduit. In the example shown in FIG. 2 a series of positive ion current measurements 200 is conducted for molecular hydrogen gas using a fast oscilloscope connected to electrodes 113 and 118 respectively. The oscilloscope is triggered by a voltage pulse applied to the gas pulse valve. Oscilloscope traces 201 and 202 are shown for different voltages applied to electrode 111. The transfer time between releasing the gas pulse from the pulse valve and the arrival time of hydrogen ions, H.sup.+, H.sub.2.sup.+ and H.sub.3.sup.+ onto electrodes 113 and 118 are of the order of ˜1 ms.

[0070] The duration of the ion current pulse produced as a result of charged particle formation in the discharge generated in a gas pulse is determined by the time characteristics of the gas pulse profile. The time width of the ion current pulses 201 and 202 may extend from a few hundreds of ps to several tens of milliseconds. In the example shown in FIG. 2, the residence time of the hydrogen gas and hydrogen charged particles in the trapping region is approximately 2.5 ms. A gas pulse with duration greater than 100 ms may have a negative impact on the operation of the vacuum system and the ion stability parameters of the ion trap.

[0071] The peak intensity of the ion current pulse is also determined by the amplitude of the voltage bias applied to the first electrode 108. A lower voltage bias (<1 KV) for a 2 mm internal diameter (i.d.) conduit will generate an ion current pulse with a peak of a few hundreds of nanoampere. A higher voltage bias (>2 KV) for a 2 mm i.d. conduit will generate an ion current pulse with a peak of several tens of microampere. In the example shown in FIG. 2, ion current produced by hydrogen charged particles and measured on electrode 113 is ˜30 μA while losses in transmitting hydrogen charged particles through the trapping region reduce the ion current to ˜3 μA at the opposite end of the linear ion trap on electrode 118.

[0072] The kinetic energy of the charged particles produced in the discharge is of the order of the voltage bias applied to the first electrode 108 integrated between the first and second parts of the discharge conduit 106, 107, as indicated by stopping potential experiments. In one example, the dimensions of the conduit are designed to reduce the amplitude of the voltage signal applied to electrode 108 to ignite the discharge and consequently reduce the kinetic energy of the charged particles produced in the charged particle source. In one example, the second part of the conduit is removed and the second electrode 11 is disposed in the vicinity of the first discharge electrode 108 to provide fine control of the kinetic energy of the charged particles.

[0073] The kinetic energy of the charged particles may also be controlled by replacing the second part of the conduit 107 typically constructed from an insulating material, typically ceramic, with a resistive glass tube. The resistive glass tube will generate a DC gradient within the conduit along the axis of the gas flow and decelerate or accelerate the charged particles to the desired kinetic energy. Experiments have also revealed that the duration of the pulse of charged particles injected through the trapping region can also be controlled by adjusting the DC gradient established across the resistive glass tube.

[0074] In one example shown in FIG. 3, an electron source 301 is integrated into the apparatus 300 and disposed between the charged particle source 101 and the linear ion trap 102. The electron source 301 is arranged downstream from the second 111 and third 112 electrodes for receiving a pulsed beam of charged particles 114 produced in the gas pulse. The electron source 301 is designed for neutralizing the charged particles 114 generated in the discharge 110 and accelerated in the direction of the trapping region 115. The electron source 301 is shaped into a conduit for radial confinement of the gas pulse moving in the direction of the trapping region 115 in the form of a free jet as well as the charged particles 114 entrained therein. In one example also shown in FIG. 4, the electron source is comprised of a series or ribbon or wire filaments 304 stretched through and along the axis of an insulating or a conductive pipe 303, the electron source conduit. The ribbon filaments 304 are connected in series or in parallel and driven to incandescent temperatures by passing several ampere of heating current. Heating currents in excess of 20 A are prohibitive for operating the electron source adjacent to the linear ion trap efficiently due to the excessive heat produced during operation. Thin ribbon filaments 304 or wires are therefore preferred.

[0075] In one example shown in FIG. 4, the conduit of the electron source is a thin wall cylindrical pipe 401 produced from refractory metals, for example tungsten, tantalum or rhenium. Other materials with enhanced thermionic emission properties at elevated pressures are also desirable, for example yttria coated iridium. Thoriated tungsten is may also be a preferred material for the low work function properties and low temperature operation. The cylindrical pipe 401 is supported by two legs 402 attached on either end and carefully aligned with the axis of the pulsed beam of charged particles 114.

[0076] In both configurations described with reference to FIG. 4, thermionic emission of electrons produces a high density electron cloud and a high-temperature low work function surface. Interactions between the charged particles 114 and the electron cloud or the hot surface of the filaments leads to neutralization of charged particles to form reactive neutrals moving in the direction of the trapping region 305. The efficiency of the reaction converting the pulsed beam of charged particles 114 to a pulsed beam of reactive neutrals 305 is also found by experimentation to be temperature dependent. The higher the temperature of the filament is, the greater the conversion efficiency becomes.

[0077] A third electrode 112 is disposed between the second electrode 111 attached to the exit end of the discharge conduit 107 and the electron source 301. The third electrode 112 is supplied with a voltage bias to control the divergence of the pulsed beam of charged particles 114 produced in the discharge 110. Ideally, the voltage bias is adjusted to optimize the angle of incidence of the charged particles onto the hot surface of the filaments 304 or onto the hot surface of the cylindrical refractory metal pipe 401.

[0078] In the present invention, the apparatus 300 can be operated in three different modes. In the first mode of operation, the charged particles 114 formed in the discharge conduit 106, 107 are directed through the electron source 301 and are partially neutralized. The pulsed beam of reactive neutrals 305 formed in the process together with the remaining population of charged particles are injected into the trapping region 115 of the linear ion trap to activate or dissociate analyte ions 116 stored therein.

[0079] In the second mode of operation, an additional electrode, for example electrode 113, disposed downstream of the electron source 301 is supplied with a voltage bias sufficiently high or synchronized with the gas pulse and the corresponding beam of charged particles based on the information provided in Figure to deflect or stop the accelerated charged particles from entering into the trapping region 115 of the linear ion trap. Analyte ions 116 are therefore activated only via interactions with a pulsed beam of reactive neutrals 305 generated by the charged particles 114 neutralized during transport through the electron source 301.

[0080] In the third mode of operation, the heating current for driving the filaments to incandescent temperatures is switched off. The charged particles 114 produced in the discharge 110 are injected into the trapping region 115 for activating or dissociating ions. The voltage signals applied to the second 111 and third 112 electrodes used for controlling the beam divergence of the pulsed beam of charged particles 114 may differ between the different modes of operation. For example, the neutralization process described in the second mode may require a diverging beam entering the source of electrons for optimizing the angle of incidence of the charged particles 114 onto the hot surface, while direct injection of charged particles for optimizing activation of analyte ions 116 may require a converging beam to be transmitted through the electron source 301.

[0081] Biasing the electron source to optimize focusing of charged particles and generation of reactive neutrals is also desirable. This is accomplished by floating the source of heating current to the desired DC level. Additional electrodes installed in the vicinity of the pole-electrodes of the ion trap maybe required to maximize the overlap between the analyte ion stored in the trapping region of the linear ion trap and externally injected charged particles or reactive neutrals.

[0082] An example of generating reactive neutrals using the geometry disclosed in the present invention involves at least one pulse of molecular hydrogen gas released from a gas pulse valve into the discharge conduit. During the high pressure transient of the molecular hydrogen pulse, a discharge is generated by the high voltage applied to the discharge electrode. A series of hydrogen ions are formed, including H.sup.+, H.sub.2.sup.+ and H.sub.3.sup.+ entrained in the flow of gas undergoing free jet expansion. The mass of the ions produced in the discharge have been determined experimentally by disposing a residual gas analyzer (quadrupole mass filter) downstream of the discharge. Experimental results indicated that the relative concentration of atomic hydrogen ions, H.sup.+, is maximized at the lower pressures while the formation of trihydrogen cations, H.sub.3.sup.+, is maximized at the highest pressures. By careful adjustments of the voltages applied to the electrodes of the charged particle and electron sources, a high density pulse of hydrogen neutral radicals, H*, with kinetic energies above thermal is directed into the trapping region of the linear ion trap. A deflection or a stopping potential applied to an electrode disposed between the electron source and linear ion trap may be used to deflect all remaining hydrogen ions that have not been converted into radical neutral species. In this mode of operation, analyte ions will interact only with a pulsed beam of fast neutral radical hydrogen atoms, H*, leading to hydrogen attachment, hydrogen abstraction reactions, the dissociation of double bonds via attachment of hydrogen atom pairs and also to fragmentation of protonated or radical analyte ions. FIG. 5 shows an example of hydrogen atom attachment to heme B ions stored in the trapping region and irradiated by multiple pulses of fast hydrogen radical species. The first mass spectrum 501 shows the isotopic distribution of the heme B ions prior to irradiation while the second mass spectrum 502 highlights the shift in the isotopic distribution observed at the end of the irradiation process due to multiple hydrogen atom attachment. The reduction of double bonds in heme B occurs via attachment of hydrogen atoms in pairs, each accommodated by the unpaired valence electron produced by the opening of the double bond.

[0083] The fragmentation mass spectrum 501 of the [M+8H].sup.8+ charge state of protonated ubiquitin, a small size protein produced by electrospray ionization, using hydrogen charged particles injected in the trapping volume with ˜1 KeV energy is presented in FIG. 6. Complete sequence coverage is obtained 503 with all types of primary fragment ions observed throughout the backbone. A close-up region of the mass spectrum 502 is also shown, highlighting the different types of fragments and the corresponding isotopic distributions assigned manually. Also annotated in the mass spectrum are the precursor ions [M+8H].sup.8+, and a series of activated precursor ions indicating the detachment of electrons to form hydrogen deficient species as well as proton abstraction and charge reduction effects leading to the formation of radical hydrogen-abundant protein ions.

[0084] In yet another example of the present invention shown in FIG. 7, the discharge 110 is ignited between the first electrode 108 and an additional electrode 702 disposed further downstream. In this configuration a second 701 and a third part 702 forming the conduit are required to confine the gas pulse and raise pressure above the breakdown limit. Similarly, a first electrical potential generator 109 is used to generate a voltage applied to the first electrode 108 and a second electrical potential generator 703 is used to generate a second voltage applied to the additional electrode 702. In one example, the first electrical potential generator 109 is floating at the potential produced by the second electrical potential generator 703. The kinetic energy of the charged particles formed in the discharge 110 and injected into the ion trap or into the electron source can then be controlled by adjusting the potential difference between the two electrodes 108 and 702 at different voltage levels. For example, if a potential difference of 500V is required to initiate the discharge, this difference can be generated by two positive voltage signals at any level, by two negative signals at any level or by one positive and one negative voltage signal applied to the two electrodes respectively. Additional electrodes to focus the pulsed beam of charged particles produced in the discharge 110 and disposed further downstream can be utilized to control beam divergence, deflect electrons and also remove charged particles after an electron source for injecting only reactive neutrals into the trapping region with the desired kinetic energy. In another example, the two electrode configuration 108 and 702 driven by two electrical potential generators 109 and 703 as described with reference to FIG. 7 is combined with the source of reactive neutrals 301 disclosed in FIG. 3. Other configurations to control the kinetic energy of the charged particle and reactive neutral species are envisaged, for example using the first electrical potential generator 109 to float the second electrical potential generator 703 shown in FIG. 7.

[0085] 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.

[0086] In summary, the invention provides a hybrid source of charged, and optionally reactive neutral, particles for activation-dissociation of trapped ions.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.