Method of Generating Electron Transfer Dissociation Reagent Ions

Abstract

A method of mass spectrometry is disclosed wherein ions are subjected to an electron detachment, electron capture or electron transfer process in order to form ions having a different charge state. At least some of the ions having a different charge state are caused to interact with analyte ions to cause at least some of the analyte ions to fragment to form daughter, fragment or product ions.

Claims

1. A method of mass spectrometry comprising: subjecting first ions having a charge state m to an electron detachment, electron capture or electron transfer process in order to form second ions having a charge state n, wherein mn; and causing at least some of said second ions to interact with analyte ions so as to cause at least some of said analyte ions to fragment to form daughter, fragment or product ions.

2. A method as claimed in claim 1, further comprising ionising a reagent compound to form said first ions.

3. A method as claimed in claim 2, wherein the step of ionising said reagent compound comprises subjecting said reagent compound to Electrospray Ionisation (ESI).

4. A method as claimed in claim 1, wherein at least some of said second ions comprise radical ions, reagent ions or radical reagent ions.

5. A method as claimed in claim 1, wherein said second ions comprise Electron Transfer Dissociation (ETD) reagent ions.

6. A method as claimed in claimed in claim 1, wherein said analyte ions fragment to form said daughter, fragment or product ions by Electron Transfer Dissociation (ETD).

7. A method as claimed in claim 1, further comprising storing or trapping said first ions in a first ion trap, first ion trapping region or first reaction cell.

8. A method as claimed in claim 7, further comprising subjecting said first ions to said electron detachment, electron capture or electron transfer process whilst said first ions are stored or trapped in said first ion trap, first ion trapping region or first reaction cell.

9. A method as claimed in claim 1, wherein said electron detachment, electron capture or electron transfer process comprises exposing said first ions to an electron beam.

10. A method as claimed in claim 1, wherein said electron detachment process comprises exposing said first ions to ultra violet radiation, electromagnetic radiation, radical cations, radical anions, ions or metastable atoms.

11. A method as claimed in claim 1, further comprising mass filtering said second ions in order to select certain said second ions to interact with at least some of said analyte ions.

12. A method as claimed in claim 1, further comprising mass filtering at least some of said analyte ions in order to select certain said analyte ions to interact with at least some of said second ions.

13. A method as claimed in claim 1, further comprising storing, trapping or confining at least some of said second ions in a first ion trap, first ion trapping region or first reaction cell and/or in a second ion trap, second ion trapping region or second reaction cell and directing at least some of said analyte ions into said first ion trap, first ion trapping region or first reaction cell and/or into said second ion trap, second ion trapping region or second reaction cell in order to interact with said at least some second ions.

14. A method as claimed in claim 1, further comprising storing, trapping or confining at least some of said analyte ions in a first ion trap, first ion trapping region or first reaction cell and/or in a second ion trap, second ion trapping region or second reaction cell and directing at least some of said second ions into said first ion trap, first ion trapping region or first reaction cell and/or into said second ion trap, second ion trapping region or second reaction cell in order to interact with said at least some analyte ions.

15. A method as claimed in claim 1, further comprising directing at least some of said second ions and at least some of said analyte ions into a first ion trap, first ion trapping region or first reaction cell and/or into a second ion trap, second ion trapping region or second reaction cell in order that said at least some second ions interact with said at least some analyte ions.

16. A method as claimed in claim 1, wherein said analyte ions comprise biomolecular ions, protein ions, peptide ions or metabolite ions.

17. A method as claimed in claim 1, wherein said daughter, fragment or product ions comprise c-type and/or z-type peptide ions.

18. A mass spectrometer comprising: a device arranged and adapted to subject first ions having a charge state m to an electron detachment, electron capture or electron transfer process in order to form second ions having a charge state n, wherein mn; and a device arranged and adapted to cause at least some of said second ions to interact with analyte ions so as to cause at least some of said analyte ions to fragment to form daughter, fragment or product ions.

19. A method of mass spectrometry comprising: subjecting first ions having a charge state m to an electron detachment, electron capture or electron transfer process in order to form second ions having a charge state n, wherein mn or n<m; and causing at least some of said second ions to interact with analyte ions so as to reduce the charge state of said analyte ions.

20. A mass spectrometer comprising: a device arranged and adapted to subject first ions having a charge state m to an electron detachment, electron capture or electron transfer process in order to form second ions having a charge state n, wherein mn or n<m; and a device arranged and adapted to cause at least some of said second ions to interact with analyte ions so as to reduce the charge state of said analyte ions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0114] Various embodiments will now be described, by way of example only, and with reference to the accompanying drawing in which:

[0115] FIG. 1 shows an instrument arrangement which may be used to generate Electron Transfer Dissociation (ETD) reagent ions according to an embodiment.

DETAILED DESCRIPTION

[0116] Electron Transfer Dissociation (ETD) reagent ions for positive ion Electron Transfer Dissociation (ETD) are known which comprise reactive singly charged radical anions formed by techniques such as Chemical Ionisation (CI) or glow discharge from a volatilized sample. However, as will be understood by those skilled in the art, not all radical anions give rise to Electron Transfer Dissociation (ETD). It is known that the best reagents have a favourable Franck-Condon factor (overlap between anionic and neutral states of the reagent) and a relatively low electron affinity.

[0117] The efficiency of Electron Transfer Dissociation (ETD) increases as the target or analyte ion becomes more positively charged and/or as the reagent ions become more negatively charged.

[0118] Methods to produce singly charged Electron Transfer Dissociation (ETD) reagent ions by Electrospray Ionisation (ESI) are known. For example, it is known to subject arenecarboxylic acids which have been ionised by Electrospray Ionisation (ESI) to Collision Induced Dissociation (CID) fragmentation. The resulting fragment or product ions after loss of CO2 may be used as reagent ions. This method circumvents difficulties associated with the formation of high mass to charge ratio reagent anions.

[0119] It is desirable to produce higher charge state Electron Transfer Dissociation (ETD) reagent ions since they may have a relatively larger reaction cross section. It is also desirable to produce candidate radical (reagent) anions or cations for positive or negative Electron Transfer Dissociation (ETD) from many Electrospray Ionisation (ESI) amenable compounds rather than be restricted to a small subset of compounds as at present.

[0120] According to various embodiments reagent ions (second ions) may be generated by electron detachment, electron capture or electron transfer from primary ions (first ions) which may initially be created or generated by Electrospray Ionisation (ESI) or other ionisation processes. The primary ions (first ions) may have a charge state m; the reagent ions (second ions) may have a charge state n; and the charge state m of the primary ions (first ions) is different to the charge state n of the reagent ions (second ions), i.e. mn. The resulting reagent ions (second ions) may then subsequently be reacted (caused to interact) with analyte ions so as to cause analyte ions to fragment, e.g. by Electron Transfer Dissociation (ETD), to produce (Electron Transfer Dissociation (ETD)) daughter, fragment or product ions. The daughter, fragment or product ions may then be analysed, e.g. by mass spectrometry and/or by ion mobility spectrometry.

[0121] According to various embodiments a reagent (compound), e.g. small proteins or large peptides, may be arranged (ionised) so as to form highly charged primary negatively charged ions or anions (first ions) by subjecting the reagent (compound) to Electrospray Ionisation (ESI) in negative Electrospray Ionisation (ESI) mode. Methods of electron detachment are known and may be used to produce abundant relatively stable radical positively charged ions or cations (second ions) of more positive charge than the negatively charged primary ions (first ions).

[0122] Positive radical cations (second ions) for negative Electron Transfer Dissociation (nETD) (i.e. dissociation of negatively charged analyte ions) may be formed by this method. It is known that negative Electron Transfer Dissociation (nETD) may be performed using xenon radical cations.

[0123] Highly charged negatively charged radical ions may be generated and used as Electron Transfer Dissociation (ETD) reagent ions (second ions). For example, ions of the form [M-nH].sup.n where n>2 may be generated from highly charged parent or precursor ions (first ions) such as small proteins.

[0124] The use of highly charged Electron Transfer Dissociation (ETD) reagent ions (second ions) according to various embodiments results in an improvement in the efficiency of Electron Transfer Dissociation (ETD) for multiply charged analyte ions and facilitates efficient Electron Transfer Dissociation (ETD) for singly charged analyte ions.

[0125] Various methods for producing radical cations and/or anions (second ions) from primary ions (first ions) may be utilised according to various embodiments and involve an electron capture, electron detachment or electron transfer process. Examples of some of the electron capture, electron detachment or electron transfer processes which may be utilised according to various embodiments are given below.

[0126] The radical reagent ions (second ions) which may be used for subsequent Electron Transfer Dissociation (ETD) are shown below in bold.

##STR00001##

[0127] Thus according to various embodiments radical reagent ions (second ions) may be formed by (the electron detachment, electron capture or electron transfer process may comprise) Electron Photo Detachment (EPD) wherein for example, ultra violet (UV) radiation having energy hv interacts with an ion (first ion), e.g. [M-nH].sup.n or [M-nH].sup.+n, to form a radical reagent ion (second ion), e.g. [M-nH].sup.(n1)* or [M-nH].sup.+(n+1)* respectively.

[0128] According to other embodiments radical reagent ions (second ions) may be formed by (the electron detachment, electron capture or electron transfer process may comprise) Electron Detachment (Dissociation) (EDD) wherein for example, electrons e interact with an ion (first ion), e.g. [M-nH].sup.n, to form a radical reagent ion (second ion), e.g. [M-nH].sup.(n1)*.

[0129] According to other embodiments radical reagent ions (second ions) may be formed by (the electron detachment, electron capture or electron transfer process may comprise) Electron Capture (Dissociation) (ECD) wherein for example, electrons e interact with an ion (first ion), e.g. [M-nH].sup.+n, to form a radical reagent ion (second ion), e.g. [M-nH].sup.+(n1)*.

[0130] According to other embodiments radical reagent ions (second ions) may be formed by (the electron detachment, electron capture or electron transfer process may comprise) Negative Electron Transfer (Dissociation) (nETD) wherein for example, a radical cation, e.g. A.sup.+*, interacts with an ion (first ion), e.g. [M-nH].sup.n, to form a radical reagent ion (second ion), e.g. [M-nH].sup.(n+1)*.

[0131] According to other embodiments radical reagent ions (second ions) may be formed (the electron detachment, electron capture or electron transfer process may comprise) Electron Transfer (Dissociation) (ETD) wherein for example, a radical anion, e.g. A.sup.*, interacts with an ion (first ion), e.g. [M-nH].sup.+n, to form a radical reagent ion (second ion), e.g. [M-nH].sup.+(n1)*.

[0132] According to other embodiments radical reagent ions (second ions) may be formed by (the electron detachment, electron capture or electron transfer process may comprise) Charge Transfer (Dissociation) (CTD), wherein for example, ions interact with an ion (first ion), e.g. [M-nH].sup.+n, to form a radical reagent ion (second ion), e.g. [M-nH].sup.+(n+1)*.

[0133] According to yet further embodiments radical reagent ions (second ions) may be formed by (the electron detachment, electron capture or electron transfer process may comprise) Metastable Atom (Dissociation) (MAD) wherein for example, metastable atoms interact with an ion (first ion), e.g. [M-nH].sup.+n, to form a radical reagent ion (second ion), e.g. [M-nH].sup.+(n+1)*.

[0134] Thus according to various embodiments the electron detachment, electron capture or electron transfer process may be selected from the group consisting of: (i) Electron Photo Detachment (EPD); (ii) Electron Detachment (Dissociation) (EDD); (iii) Electron Capture (Dissociation) (ECD); (iv) Negative Electron Transfer (Dissociation) (nETD); (v) Electron Transfer (Dissociation) (ETD); (vi) Charge Transfer (Dissociation) (CTD); and (vii) Metastable Atom (Dissociation) (MAD).

[0135] According to various embodiments the electron detachment, electron capture or electron transfer process may comprise exposing the first ions to an electron beam, ultra violet radiation, electromagnetic radiation, radical cations, radical anions, ions or metastable atoms.

[0136] FIG. 1 shows a schematic of a mass spectrometer instrument geometry which may be used according to various embodiments in order to generate reagent ions (second ions) and which also may be used to perform positive and/or negative Electron Transfer Dissociation (ETD) reactions with analyte ions.

[0137] As shown in FIG. 1, a reagent (compound) may first be ionised by Electrospray Ionisation (ESI) (or another ionisation process) in (or via) an ion source 1. As a result of the ionisation process (primary) reagent ions (first ions) are produced and the resulting (primary) reagent ions (first ions) may then be trapped in a first trapping region, ion trap or reaction cell 2. The primary reagent ions (first ions) may then subjected to an electron detachment, electron capture or electron transfer process according to various embodiments.

[0138] For example, according to an embodiment radical (secondary) reagent ions (second ions) may be formed by exposing the primary reagent ions (first ions) to an electron beam or by exposing the primary reagent ions to ultra violet (UV) radiation or by exposing the primary reagent ions (first ions) to radical cations, radical anions, ions or metastable atoms.

[0139] According to various embodiments the electron detachment, electron capture or electron transfer process may be arranged to occur within the ion source 1, ion trap or reaction cell 2 or an ion guide or ion guiding region of the mass spectrometer.

[0140] Thus the mass spectrometer comprises a device (e.g. reaction cell 2) arranged and adapted to subject the first ions having a charge state m to an electron detachment, electron capture or electron transfer process in order to form the second ions having a charge state n, wherein mn.

[0141] Specific secondary reagent ions (second ions) may then be isolated by removing some or all of the secondary reagent ions (second ions) from the first trapping region, ion trap or reaction cell 2 and passing the secondary reagent ions (second ions) through a quadrupole mass filter 3 which may be arranged to mass filter the secondary reagent ions (second ions). The mass filtered secondary reagent ions (second ions) may then be stored in a second trapping region, ion trap or reaction cell 4.

[0142] Analyte ions of opposite polarity to that of the secondary reagent ions (second ions) may then be produced or otherwise generated in the ion source 1 (or a different ion source). The analyte ions (of opposite polarity) may then optionally also be mass filtered by the quadrupole mass filter 3 (or a different quadrupole mass filter) so that only analyte ions having specific mass to charge ratios may then be introduced into the second trapping region, ion trap or reaction cell 4.

[0143] According to various embodiments ion-ion interactions and in particular Electron Transfer Dissociation (ETD) (or other processes) may be arranged to occur within the second trapping region, ion trap or reaction cell 4. The resulting daughter, fragment or product ions may then be sent or otherwise transmitted to a downstream mass analyser 5 for further analysis.

[0144] Thus according to various embodiments the mass spectrometer further comprises a device (e.g. second trapping region, ion trap or reaction cell 4) arranged and adapted to cause at least some of the second ions to interact with the analyte ions so as to cause at least some of the analyte ions to fragment to form daughter, fragment or product ions.

[0145] Other schemes for implementation according to other various further embodiments are also contemplated. For example, according to an embodiment separate ion sources may be used to ionise separately the analyte and the reagent compounds. Other atmospheric ionisation processes may be used to generate primary ions (first ions). For example, according to an embodiment Atmospheric Pressure Chemical Ionisation (APCI) or photo-ionisation may be used to ionise either the analyte and/or the reagent (compound). Secondary reagent ions (second ions) may then be generated from primary reagent ions (first ions) by subsequent electron detachment, electron capture or electron transfer processes.

[0146] Thus according to various embodiments the mass spectrometer may further comprise a device (e.g. ion source 1) arranged and adapted to ionise analyte to form the analyte ions and/or to ionise a reagent compound to form the first ions.

[0147] The same general processes as described above in relation to Electron Transfer Dissociation (ETD) may also be used to create efficient Proton Transfer Reaction (PTR) reagents. For example, further embodiments are contemplated wherein first (reagent) ions having a charge state m may be subjected to an electron detachment, electron capture or electron transfer process in order to form secondary (reagent) ions (second ions) having a charge state n, wherein mn or n<m. The secondary reagent ions (second ions) may then be interacted with analyte ions in order to reduce the charge state of the analyte ions (e.g. by Proton Transfer Reaction) without substantially causing the analyte ions to fragment. Reducing the charge state of the analyte ions by Proton Transfer Reaction is beneficial since it can be hard for the processing system of a mass spectrometer to resolve analyte ions having a relatively high charge state.

[0148] According to these embodiments, the mass spectrometer further comprises a device (e.g. second trapping region, ion trap or reaction cell 4) arranged and adapted to cause at least some of the second ions to interact with analyte ions so as to reduce the charge state of the analyte ions.

[0149] Although the present invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims.