Method of calibrating ion signals

Abstract

A method of mass or ion mobility spectrometry is disclosed comprising: providing an ion source for generating analyte ions and reference ions; providing a mass analyzer or ion mobility separator (IMS); providing an ion trap between the ion source and the mass analyzer or IMS; guiding reference ions from the ion source into the ion trap and trapping the reference ions in the ion trap; guiding the analyte ions from the ion source into the mass analyzer or IMS, wherein the analyte ions bypass the ion trap; and releasing reference ions from the ion trap into the mass analyzer or IMS for analysis.

Claims

1. A method of mass or ion mobility spectrometry comprising: providing an ion source for generating analyte ions and an ion source for generating reference ions; providing an analyser; providing an ion trap between the ion source for generating reference ions and the analyser; directing reference ions from the ion source for generating reference ions into the ion trap and trapping the reference ions therein; directing analyte ions from the ion source for generating analyte ions into the analyser without the analyte ions passing into the ion trap, and analysing the analyte ions in the analyser; and releasing reference ions from the ion trap into the analyser and analysing the reference ions, wherein the trapped reference ions are controllably released from the ion trap such that only a portion of the reference ions trapped in the ion trap are released at any given time.

2. The method of claim 1, wherein the reference ions and analyte ions are analysed by the analyser to provide mass to charge ratio measurements or ion mobility measurements, wherein the mass to charge ratio or ion mobility of the reference ions is known or previously determined prior to the analysis of the reference ions in the analyser, and wherein the mass to charge ratio measurements or ion mobility measurements of the analyte ions are adjusted or calibrated based on the difference between the known or previously determined mass to charge ratio or mobility and the measured mass to charge ratio or mobility of the reference ions.

3. The method of claim 1, wherein the reference ions are analysed by the analyser to provide mass to charge ratio measurements or ion mobility measurements, wherein the mass to charge ratio or ion mobility of the reference ions is known or previously determined prior to the analysis of the reference ions in the analyser, and wherein the operation of the mass or ion mobility spectrometer is controlled or adjusted based on the difference between the known or previously determined mass to charge ratio or mobility and the measured mass to charge ratio or mobility of the reference ions so as to maintain a predetermined operational characteristic of the mass or ion mobility spectrometer at a desired level, e.g. to maintain the mass or ion mobility spectrometer at a desired resolution or sensitivity.

4. The method of claim 1, wherein reference ions and analyte ions are guided through the same first ion guide, wherein the reference ions are directed from the ion guide into the trap and trapped therein, and wherein the analyte ions are directed from the ion guide into the analyser for analysis, the analyte ions having bypassed the ion trap.

5. The method of claim 4, wherein the analyte ions and reference ions are guided along an axis through the ion guide, and wherein an electric field is applied to the reference ions whilst they are within the ion guide or at the exit of the ion guide such that the reference ions are diverted off the axis and transmitted downstream into the ion trap and trapped therein; and/or wherein an electric field is applied to the analyte ions whilst they are within the ion guide or at the exit of the ion guide such that the analyte ions are diverted off the axis and transmitted downstream into the analyser whilst bypassing the ion trap.

6. The method of claim 4, wherein a second ion guide is provided between the first ion guide and the analyser for guiding ions to the analyser, wherein analyte ions are transmitted from the first ion guide into the second ion guide and then into the analyser whilst bypassing the ion trap, and wherein reference ions are transmitted from the ion trap into the second ion guide and into the analyser.

7. The method of claim 6, wherein the first and second ion guides have longitudinal axes along which ions travel as they pass through the ion guides, and wherein the longitudinal axes are coaxial and arranged such that ions which exit the first ion guide along its longitudinal axis are directed into the second ion guide.

8. The method of claim 4, wherein a mass analyser and/or ion mobility separator and/or ion filter is provided between the source of analyte ions and the first ion guide for mass analysing analyte ions, for separating analyte ions according to their mass to charge ratios or ion mobilities, or mass selectively transmitting analyte ions; and/or wherein a mass analyser and/or ion mobility separator and/or ion filter is provided between the source of reference ions and the first ion guide for mass analysing reference ions, separating reference ions from other ions according to their mass to charge ratios or ion mobilities, or mass selectively transmitting reference ions.

9. The method of claim 1, wherein the analyte ions are generated by a first ion source and the reference ions are generated by a second, different ion source; wherein analyte ions are guided from the analyte ion source into the analyser by a first ion guide; and wherein reference ions are guided from the reference ion source into the ion trap by a second, different ion guide.

10. The method of claim 9, wherein reference ions are released from said ion trap into said first ion guide and are then guided into the analyser.

11. The method of claim 10, wherein the first ion guide comprises a switching device that operates in a first mode to allow analyte ions to pass from the analyte ion source into the analyser, and that operates in a second mode to prevent analyte ions from passing from the analyte ion source to the analyser and to allow reference ions to pass from the ion trap to the analyser.

12. The method of claim 1, comprising operating a first mode in which analyte ions are analysed in the analyser and reference ions are not, operating a second mode in which reference ions from the ion trap are analysed in the analyser and analyte ions are not, and wherein the method is repeatedly alternated between these first and second modes.

13. The method of claim 1, wherein the analyser discontinuously analyses ions in a plurality of analysis cycles and wherein reference ions are guided into the analyser as a series of ion packets that are synchronised with the analysis cycles such that an ion packet is analysed in at least one cycle or between cycles; and wherein spectral data from the reference ions analysed in separate analysis cycles is combined to produce combined reference ion data or a combined reference ion peak that is used to either: (i) adjust or calibrate the mass or mobility measurements of the analyte ions; or (ii) maintain a predetermined operational characteristic of the mass or ion mobility spectrometer at a desired level, e.g. to maintain the mass or ion mobility spectrometer at a desired resolution or sensitivity.

14. The method of claim 1, wherein the analyte ions do not pass into the ion trap and/or wherein the only ions that enter the ion trap are the reference ions.

15. The method of claim 1, wherein the reference ions are released from the ion trap into the analyser for calibrating the analyser for the analysis of the analyte ions that have bypassed the ion trap.

16. The method of claim 1, wherein the analyte ions are transmitted from the ion source of analyte ions to the analyser without being reacted with other ions or molecules, and/or without being fragmented.

17. The method of claim 1, wherein reference ions are released from the ion trap at a substantially constant charge per second.

18. The method of claim 1, wherein reference ions are discontinuously released from the ion trap in ion packets having substantially the same charge.

19. The method of claim 1, wherein the reference ions are released from the ion trap at a rate such that the reference ions do not saturate the analyser or a detector of said analyser.

20. A mass or ion mobility spectrometer comprising: an ion source for generating analyte ions and an ion source for generating reference ions; an analyser; an ion trap arranged between the ion source for generating reference ions and the analyser; and control means arranged and configured to: direct reference ions from the ion source for generating reference ions into the ion trap and trap the reference ions therein; direct analyte ions from the ion source for generating analyte ions into the analyser without the analyte ions passing into the ion trap, and to analyse the analyte ions in the analyser; and release the reference ions from the ion trap into the analyser so as to analyse the reference ions, wherein the trapped reference ions are controllably released from the ion trap such that only a portion of the reference ions trapped in the ion trap are released at any given time.

21. A method of mass or ion mobility spectrometry comprising: providing an ion source for generating analyte ions and an ion source for generating reference ions; providing an analyser; providing an ion trap between the ion source for generating reference ions and the analyser; directing reference ions from the ion source for generating reference ions into the ion trap and trapping the reference ions therein; directing analyte ions from the ion source for generating analyte ions into the analyser without the analyte ions passing into the ion trap, and analysing the analyte ions in the analyser; and releasing reference ions from the ion trap into the analyser and analysing the reference ions.

22. The method of claim 21, wherein reference ions and analyte ions are guided through the same first ion guide, wherein the reference ions are directed from the ion guide into the trap and trapped therein, and wherein the analyte ions are directed from the ion guide into the analyser for analysis, the analyte ions having bypassed the ion trap.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various embodiments of the present invention will now be described, by way of example only, and with reference to the drawings, in which:

(2) FIG. 1 shows a schematic of a preferred embodiment of the present invention, operating in a mode wherein reference ions are diverted into an ion trap;

(3) FIG. 2 shows the embodiment of FIG. 1, operating in a mode wherein analyte ions bypass the ion trap;

(4) FIG. 3 shows the embodiment of FIG. 1, operating in a mode wherein reference ions from the ion trap are being analysed;

(5) FIG. 4 shows an alternative embodiment of the present invention wherein ions are switched between an ion trap and an ion analyser;

(6) FIG. 5 shows an alternative embodiment that is similar to that of FIG. 4 except that it comprises separate analyte ion reference ion sources; and

(7) FIG. 6 shows a further embodiment of the present invention that is similar to that of FIG. 5, except that additional devices are arranged between one of the ion sources and the ion analyser.

DETAILED DESCRIPTION OF EMBODIMENTS

(8) FIG. 1 shows a preferred embodiment of the present invention comprising an entrance ion guide 2, an ion trap 6 and an exit ion guide 4. The entrance ion guide 2, ion trap 6 and exit ion guide 4 are formed from electrodes and voltages are applied to the electrodes so as to radially confine ions therein. Each of the entrance ion guide 2 and the exit ion guide 4 radially confines ions therein along an axis and the axes of the two ion guides 2,4 are coaxial. The ion trap 6 radially confines ions therein along an axis, that is parallel to and spaced apart from the axes through the entrance and exit ion guides 2,4.

(9) In a first mode of operation reference ions are generated upstream of the entrance ion guide 2 and these ions are received in the entrance ion guide 2. The reference ions are guided through the entrance ion guide 2 and are then radially ejected into the ion trap 6. The reference ions then remain trapped within the ion trap 6 for subsequent use.

(10) FIG. 2 shows a schematic of a second mode of operation, at a point after the reference ions have been trapped in the ion trap 6. In this mode of operation, the reference ions are not supplied to the entrance ion guide 2 and analyte ions are supplied to the entrance ion guide 2 instead. The analyte ions are guided through the entrance ion guide 2 and into the exit ion guide 4. The ions are then guided through the exit ion guide 4 and to a mass analyser or ion mobility separator that is arranged downstream (not shown). It is contemplated that the exit ion guide 4 may form at least a part of the ion mobility separator. This mode of operation enables the analyte ions to bypass the ion trap 6 and to be mass analysed and/or analysed by ion mobility separation.

(11) FIG. 3 shows a schematic or a third mode of operation that may be performed subsequent to the second mode of operation described above in relation to FIG. 2. According to the third mode of operation, the entrance ion guide 2 is operated so as to prevent analyte ions from passing into the exit ion guide 4. The analyte ions may be trapped within the entrance ion guide or may be directed into an analyte ion trap (not shown). At least some of the reference ions within the reference ion trap 6 are then ejected from the ion trap 6 into the exit ion guide 4. These reference ions are then guided downstream to the mass analyser or ion mobility separator and analysed. As the analysed properties of the reference ions are known, the analysis of the reference ions enables the calibration of the mass analyser or ion mobility separator. After the reference ions have been analysed the mode of operation described in relation to FIG. 2 is reverted to and analyte ions are analysed again. If analyte ions were trapped during the period in which the reference ions were analysed, rather than simply being discarded, then these analyte ions may then be analysed. Alternatively, analyte ions that are newly received in the ion guide 2 may be analysed. The method may repeatedly alternate between the modes described in relation to FIGS. 2 and 3 so as to alternately analyse analyte ions and reference ions.

(12) FIG. 4 shows another embodiment comprising a source of reference ions and analyte ions 8, an ion trap 10 and an analyser 12. The source of reference ions and analyte ions 8 may be a single ion source or may comprise a reference ion source and a separate analyte ion source. The analyser 12 may be a mass analyser or an ion mobility separator (IMS). A first ion guide 14 is arranged between the ion source 8 and the analyser 12. A switching mechanism 16 is provided in the first ion guide 14 and a second ion guide 18 extends from the switching mechanism 16 to the ion trap 10. The switching mechanism 16 is configured to divert ions between the analyser 12 and the ion trap 10, as will be described further below. The switching device 16 comprises one or more electrodes for diverting the ions.

(13) In a first mode of operation, reference ions from the ion source 8 pass into the first ion guide 14 and are diverted into the second ion guide 18 by the switching mechanism 16. The reference ions are guided through the second ion guide 18 into the ion trap 10. The reference ions then remain trapped within the ion trap 10 for subsequent use.

(14) In a second mode of operation, at a point after the reference ions have been trapped in the ion trap 10, analyte ions are supplied to the first ion guide 14. The analyte ions are guided through the first ion guide 14 and into the analyser 12. The switching device 16 does not direct the analyte ions into the ion trap 10. This mode of operation enables the analyte ions to bypass the ion trap 10 and be analysed by the analyser 12.

(15) In a third mode of operation that may be performed subsequent to the second mode of operation described above, analyte ions are prevented from passing to the mass analyser 12. This may be performed by the switching device 16 arranging a blocking potential in the first ion guide 14. Analyte ions may be trapped within the entrance end of the first ion guide 14 or may be directed into an analyte ion trap (not shown). At least some of the reference ions within the reference ion trap 10 are then ejected from the ion trap 10 into the second ion guide 18. These reference ions are then guided into the first ion guide 14 and are directed by the switching mechanism 16 to pass into the analyser 12 for analysis. As the analysed properties of the reference ions are known, the analysis of the reference ions enables the calibration of the analyser 12. After the reference ions have been analysed the second mode of operation may be reverted to and the analyte ions may be analysed again. If analyte ions were trapped during the period in which the reference ions were analysed, rather than simply being discarded, then these analyte ions may then be analysed. Alternatively, analyte ions that are newly received in the first ion guide 14 may be analysed. The method may repeatedly alternate between the second and third modes so as to alternately analyse analyte ions and reference ions.

(16) FIG. 5 shows and embodiment that is similar to FIG. 4 and wherein like elements have like reference numbers. However, in the embodiment of FIG. 5 the reference ions and the analyte ions do not both enter the entrance end of the first ion guide 14. Rather, an analyte ion source 22 is arranged at the entrance end of the first ion guide 14 and a separate reference ion source 24 is provided that is interfaced with the ion trap 10 by a third ion guide 26.

(17) In a first mode of operation, reference ions from the reference ion source 24 pass into the third ion guide 26 and are guided through the third ion guide 26 into the ion trap 10. The reference ions then remain trapped within the ion trap 10 for subsequent use.

(18) In a second mode of operation, analyte ions are supplied to the first ion guide 14. The analyte ions are guided through the first ion guide 14 and into the analyser 12. The first and second modes may be operated concurrently or sequentially.

(19) In a third mode of operation that may be performed concurrently or subsequent to the second mode of operation, at least some of the reference ions are ejected from the ion trap 10 into the second ion guide 18. These reference ions are then guided into the first ion guide 14 and are directed by the switching mechanism 16 to pass into the analyser 12 for analysis. As the analysed properties of the reference ions are known, the analysis of the reference ions enables the calibration of the analyser 12. If the second and third modes are performed sequentially, rather than concurrently, then after the reference ions have been analysed the second mode of operation may be reverted to and the analyte ions may be analysed again.

(20) Analyte ions may be prevented from passing to the mass analyser 12 during the third mode. This may be performed by the switching device 16 arranging a blocking potential in the first ion guide 14. Analyte ions may be trapped within the entrance end of the first ion guide 14 or may be directed into an analyte ion trap (not shown).

(21) FIG. 6 shows an embodiment that is similar to FIG. 5 and wherein like elements have like reference numbers. However, in the embodiment of FIG. 6 an ion mobility separator 28 and a quadrupole mass filter 30 are arranged between the source of analyte ions 22 and the first ion guide 14. The ion mobility separator 28 separates the analyte ions according to their ion mobility as they pass through the ion mobility separator 28. The quadrupole 30 may mass selectively transmit analyte ions to the first ion guide 14. The mass to charge ratios of the ions transmitted may vary with time. The analyser 12 in this embodiment is preferably a mass analyser.

(22) It is preferred that the ion trap 10 is filled with reference ions before the start of an experiment. It is also preferred that only some of the reference ions are released from the ion trap 10 during any release cycle such that analyte ion and reference ion analysis cycles can be repeatedly performed without having to refill the ion trap with reference ions.

(23) During the experiment, reference ions can be rapidly delivered to the mass analyser 12 or ion mobility separator 12 and in controlled amounts as required. This allows reference ion spectra to be acquired between analyte ion spectra substantially without discarding analyte ions. This leads to a high duty cycle technique that avoids errors in quantitation due to missing analyte data. By way of example, if a peak comprising 1000 reference ions is required in order to make a statistically accurate reference measurement and a reference measurement is required every 30 seconds, then a trap with a capacity of 10.sup.6 ions would provide enough reference ions for an acquisition period of over 8 hours.

(24) In order to avoid detector saturation, it may be necessary to spread the packet of reference ions released from the ion trap 10 so that reference ions are delivered to the detector 12 over a time period. For example, in an orthogonal acceleration TOF experiment the reference ion packet may need to be spread over multiple pushes of the extraction region, leading to a consequent loss of analyte duty cycle. This problem can be mitigated in the preferred embodiment by arranging for more frequent acquisitions of packets of reference ions containing fewer ions. Multiple reference spectra may then be combined to produce a reference peak containing a sufficient number of reference ions to generate the required statistical precision.

(25) 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.

(26) For example, the source of reference ions 24 may be the same ion source as is subsequently used for generating the analyte ions. Alternatively, a conventional lock mass source may be used for generating the reference ions.

(27) One possible implementation of the ion trap 10 is shown in the figures. Each of the ion trap 10 and ion guides 3,4,14,18,26 may be constructed from a plurality of electrodes that are aligned to form ion guiding paths. A portion of the entrance ion guide is parallel and adjacent to a portion of the ion trap 10. The electrodes of the entrance ion guide 2 and ion trap 10 are configured, and voltages are applied to these electrodes, such that reference ions are radially ejected from the entrance ion guide 2 into the ion trap 10 and are then radially confined within the ion trap 10. Similarly, a portion of the exit ion guide 4 is parallel and adjacent to a portion of the ion trap 10. The electrodes of the exit ion guide 4 and ion trap 10 are configured, and voltages are applied to these electrodes, such that reference ions are radially ejected from the ion trap 10 into the exit ion guide 4 and are then radially confined within the exit ion guide 4. The ion trap 10 may be conjoined with the entrance and exit ion guides 2,4 to perform the above functions by being constructed as described in US 2011/0049357. However, it is also contemplated that other configurations of ion traps and ion guides could be used according to the present invention. It is desirable that it should be possible to extract controlled numbers of ions from the ion trap while the total charge within the trap becomes depleted.

(28) One or more ion species may be used as the reference ions, thus allowing single or multi-point reference correction. During selection of the reference compounds consideration should be given as to the stability of the reference ions within the ion trap, e.g. to avoid unwanted ion-ion reactions or fragmentation.