Abstract
The invention relates to reducing harmonic signals in FT-ICR spectra. Since harmonic signals in quadrupolar 2ω-detection can be more abundant for the same ion motion in the ICR cell as compared to harmonic signals in classical dipolar 1ω-detection, they could hitherto not be reduced to satisfactory levels by any known method, such as gated deflection during ion introduction into, and correcting for an offset electric field axis in the ICR cell. The present disclosure foresees, in addition to other methods carried out for improving the measurement conditions as the case may be, performing the quadrupolar 2ω-detection at least twice, where the phase of the ion excitation radio frequency is turned by 180° in the second measurement. From the sum transient, a Fourier-transformed spectrum is derived. As a result, the broad band spectra of complex substance mixtures like crude oil become cleaner, and misinterpretations of false (harmonic) peaks are minimized.
Claims
1. A method for reducing 1ν-subharmonic signals in measurements of ICR mass spectra by quadrupolar 2ω-detection of transients representing ionic image currents in an ICR cell after excitation of ions, the method comprising the steps: exciting a first bunch of ions using a first excitation wave phase and measuring a first transient by 2ω-detection, exciting a second bunch of ions using a second excitation wave phase differing from the first phase by substantially 180°, and measuring a second transient by 2ω-detection, adding the first and second transients, and transforming a sum of the first and second transients into a frequency spectrum.
2. The method according to claim 1, being applied to a broad band measurement spanning an m/z range of equal to or more than 1000 Dalton.
3. The method according to claim 2, wherein the first and second bunches of ions are derived from a complex substance mixture.
4. The method according to claim 3, wherein the complex substance mixture is derived from one of crude oil, oil distillation residue, and a plant extract.
5. The method according to claim 1, wherein the first and the second bunches of ions comprise substantially equal numbers of ions.
6. The method according to claim 5, wherein the ions of the first and second bunches of ions are generated in an ion source which operates at substantially constant ionic output, and are transferred to the ICR cell using a same transfer procedure.
7. The method according to claim 6, wherein the ion source is fed with substances from a substance separator.
8. The method according to claim 7, wherein the substance separator is one of a chromatograph and an electrophoretic device.
9. The method according to claim 7, wherein the first and second transients are measured immediately subsequently.
10. The method according to claim 1, wherein a first sum-transient is obtained by adding measured transients from several bunches of ions using the first excitation wave phase, and a second sum-transient is obtained in a similar way but using the second excitation wave phase, and the first and second sum-transients are added to obtain the frequency spectrum by Fourier transformation.
11. The method according to claim 10, wherein the first and second sum-transients are obtained by alternately measuring and adding transients from several bunches of ions.
12. The method according to claim 1, wherein also a phase of detection is switched by 180° between a measurement of the first and second transients, and 1ν signals found therein are used to precisely determine an unperturbed cyclotron frequency ν.sub.c.
13. The method according to claim 1, wherein the frequency spectrum is transformed into a mass spectrum.
14. The method according to claim 1, wherein the excitation is dipolar.
15. The method according to claim 1, wherein the ICR cell comprises four quarter cylindrical mantle electrodes and two axial trapping electrodes.
16. The method according to claim 1, wherein the excitation includes irradiating the ICR cell with a pulse radio frequency sweep (“chirp”).
17. A method for measuring ion cyclotron resonance transients that represent ionic image currents in an ICR cell, having 2×n mantle electrodes where n>2 is an integer, after excitation of ions, the method comprising the steps: exciting a first bunch of ions using a first excitation wave phase and measuring a first transient by nω-detection, exciting a second bunch of ions using a second excitation wave phase differing from the first phase by substantially 180°, and measuring a second transient by nω-detection, and adding the first and second transients to form a sum transient.
18. The method according to claim 17, wherein the sum transient is transformed into a frequency or mass spectrum.
19. The method according to claim 18, wherein an intensity of a group of (n−1)ν, (n−3)ν, (n−5)ν, . . . subharmonic signals as well as a group of (n+1)ν, (n+3)ν, (n+5)ν, . . . higher frequency harmonic signals is reduced as compared to that of a main peak nν.sub.+ in the frequency or mass spectrum.
20. The method according to claim 17, wherein both an excitation wave phase as well as a detection phase is switched by substantially 180° between a measurement of the first and second transients.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 shows a simple form of a cylindrical ICR cell (200) with four cylinder mantle electrodes (210) to (212) and two end cap (axial trapping) electrodes (205) and (206).
[0022] FIG. 2 exhibits a schematic diagram of the classical operation of an FT-ICR cell using dipolar excitation and dipolar 1ω-detection.
[0023] FIG. 3a schematically presents the circuit status of the dipolar excitation for a quadrupolar 2ω-detection measurement, and FIG. 3b shows the quadrupolar 2ω-detection.
[0024] FIG. 4 shows, in some detail, a simulated frequency spectrum as obtained from a Fourier transformation of a transient simulating a quadrupolar 2ω-measurement, without using a subharmonic suppression method according to principles of this invention. The simulations were done with a cyclotron orbit of 50% of the ICR cell radius, 10% magnetron orbit, and 10% offset between magnetron center and axis of the ICR cell (illustrated in the insert at the top right). The main signal represents the fundamental peak 2ν.sub.+; next in size is the group of the 1ν-subharmonics, comprising the cyclotron frequency ν.sub.+ and its side band (ν.sub.++ν.sub.−). Furthermore, third, fourth, fifth and sixth harmonics are visible towards larger frequencies as tiny peaks.
[0025] FIG. 5 shows on the left-hand side two excerpts of simulated transients as measured by quadrupolar 2ω-detection, wherein the measurement at the bottom was obtained with an excitation wave phase turned by 180° compared with the excitation wave phase used for the transient at the top. The addition of both transients is shown on the right-hand side.
[0026] FIG. 6a presents the frequency spectrum, which is identical for each of the two transients shown on the left-hand side of FIG. 5, and FIG. 6b shows the frequency spectrum of the summed transient on the right-hand side of FIG. 5. The group of 1ν-subharmonic signals completely disappears (and also the groups of third and fifth harmonic signals).
[0027] FIG. 7 again shows on the left-hand side two excerpts of simulated transients as measured by quadrupolar 2ω-detection, wherein the measurement at the bottom was obtained with both excitation and detection phases turned by 180° compared with the phases of the first measured transient at the top. The addition of both transients is shown on the right-hand side, now showing a kind of beat.
[0028] FIG. 8a presents the frequency spectrum, which is identical for each of the two transients shown on the left-hand side of FIG. 7, and FIG. 8b shows the frequency spectrum of the summed transient on the right-hand side of FIG. 7. Now, the fundamental cyclotron peak 2ν.sub.+ disappears (and also the group of fourth harmonic signals), while the group of 1ν-subharmonics takes full size. This method can be used to precisely measure the side band frequency (ν.sub.++ν.sub.−) which is in fact the unperturbed cyclotron frequency ν.sub.c in the ICR cell. This frequency is not influenced by the electrical (axial) trapping potential of the ICR cell, i.e. by magnetron movement and space charge perturbations.
[0029] FIG. 9 exhibits in the upper panel a measured (not simulated) broadband FT-ICR mass spectrum of sodium trifluoroacetate (NaTFA) that mainly consists of a series of cluster ion peaks, with the strongest peak at m/z 703 Dalton. In the lower panel, FIG. 9 displays a closer view of the group of first subharmonics of this strongest peak, appearing at m/z 1405.6 Dalton, magnified in intensity by a factor of 100, and zoomed-in on the m/z scale. The intensity of this group of peaks, not representing true ionic signals in the spectrum, amounts to about 1 percent of its fundamental peak, potentially giving rise to some misinterpretation of the spectrum. Similar harmonic peaks may be visible for all other mass peaks of the spectrum.
[0030] FIG. 10 demonstrates the effect of the method according to principles of the invention. The intensities of the first harmonics of the spectrum peaks now are reduced by a factor of about ten.
[0031] FIG. 11 shows three measurements of a complex mixture sample SRFA (Suwannee River Fulvic Acids). In the uppermost spectrum, the ICR cell was not correctly trimmed. In the middle spectrum, the cell was almost optimally trimmed and filled using the methods described in U.S. Pat. No. 8,766,174 B1 and U.S. Pat. No. 9,355,830 B2, respectively. The 1ν-subharmonic signals were greatly reduced but did not completely disappear. In the mass spectrum at the bottom, the excitation phase switching according to principles of this invention was used additionally to almost completely eliminate the 1ν-subharmonic signals.
[0032] FIG. 12 shows by way of example an ICR-MS set-up suitable for carrying out the methods according to the invention.
[0033] FIG. 13 shows the three independent ion motions of an ion in an ICR cell. Only the radial motions, the cyclotron motion with frequency ν.sub.+ and the magnetron motion with frequency ν.sub.− are relevant for carrying out the methods according to principles of the invention. It should be mentioned that the orbit diameters of the radial motions are outlined schematically here in such a way as to give the impression that the cyclotron motion is a fast motion (small orbit) and the magnetron motion is a slow (drift) motion (large orbit). In fact, the cyclotron frequency is usually faster by a factor of approximately 10.sup.5, but the cyclotron orbit is usually larger than the magnetron orbit, contrary to what is shown.
DETAILED DESCRIPTION
[0034] While the invention has been shown and described with reference to a number of different embodiments thereof, it will be recognized by those of skill in the art that various changes in form and detail may be made herein without departing from the scope of the invention as defined by the appended claims.
[0035] In a first aspect, the invention aims to suppress the 1ν-subharmonic signals in broad band spectra obtained by quadrupolar 2ω-detection. In spectra of complex mixtures of substances, these signals complicate the interpretation. In broad band spectra measurement, the measurable transients are usually short, only a few seconds, reducing the achievable mass resolution. To enhance the resolution by a factor of two, quadrupolar 2ω-detection can be applied, but the 1ν-subharmonics disturb the spectra.
[0036] The first aspect relates to the 2ω-measurement of the cyclotron frequency, such as conducted with a quadrupolar arrangement of quarter cylindrical excitation and detection electrodes, as shown by way of example in FIG. 1. The principle of quadrupolar 2ω-measurements is illustrated in FIG. 3a and FIG. 3b, exhibiting the switching states for the excitation and detection events. To suppress the 1ν-subharmonics, this quadrupolar 2ω-measurement is performed two times, wherein the phase of the excitation wave is turned by 180° for the second measurement, and the two transients are added together. As a result, the signals of the 1ν-subharmonics are greatly reduced or even eliminated beyond detectability.
[0037] The result can be studied by computer simulations, an example of which is presented in FIG. 4. Here, the cyclotron orbit radius is assumed to amount to 50% of the ICR cell radius, and the magnetron orbit radius to 10% of the ICR cell radius. The center of the magnetron orbit has an offset of about 10% from the axis of the ICR cell. The orbit positions of cyclotron and magnetron are illustrated in the insert at the top right. Under these conditions, a frequency spectrum will be obtained as exhibited in FIG. 4. The designation ν.sub.+ represents the cyclotron frequency, ν.sub.− represents the magnetron frequency. As expected, the most abundant peak appears at 2ν.sub.+, the double cyclotron frequency; but surprisingly the 1ν-subharmonics group (ν.sub.+; ν.sub.++ν.sub.−) has an intensity of about 40% of the main peak. The higher frequency harmonics groups (3ν, 4ν, 5ν and 6ν) are visible but have largely negligible intensities.
[0038] FIGS. 5, 6a, and 6b show the simulation result of the suppression according to principles of this invention. FIG. 5 depicts on the left-hand side two excerpts of simulated transients as measured by 2ω quadrupolar detection, wherein the measurement at the bottom was obtained with an excitation phase turned by 180° compared with the excitation phase used for the transient at the top. The addition of both transients, the basic idea of this invention, is shown on the right-hand side of the figure. FIG. 6b now presents the frequency spectrum of the summed transient on the right-hand side of FIG. 5. As intended by the invention, the 1ν-subharmonics group (ν.sub.+; ν.sub.++ν.sub.−) completely disappears, and also the groups of higher frequency third and fifth harmonics. In contrast, FIG. 6a presents the frequency spectrum of one of the left-hand side transients of FIG. 5 showing the original spectrum with all subharmonics and higher frequency harmonics.
[0039] If both excitation wave phase and detection phase are turned by 180°, the 2ν.sub.+ signal, i.e. the double fundamental frequency, disappears, and the signal of the 1ν-subharmonics group (ν.sub.+; ν.sub.++ν.sub.−) remains, as demonstrated by FIGS. 7, 8a, and 8b. This method can be used to precisely determine frequency or mass values by measuring the side band frequency (ν.sub.++ν.sub.−) which is in fact the unperturbed cyclotron frequency ν.sub.c in the ICR cell. This frequency is not influenced by the electrical (axial) trapping potential of the ICR cell, i.e. by magnetron movement and space charge perturbations.
[0040] Real measurements of the effect of the invention are presented in FIG. 9 and FIG. 10. In both figures, the upper panel shows measured mass spectra of sodium trifluoroacetate (NaTFA), which forms numerous cluster ions. In the bottom panels of the figures, the 1ν-subharmonics group of the main mass peak of 703 Dalton is shown, appearing around m/z 1405.6, enlarged in intensity by a factor of 100, and zoomed-in on the mass scale. In FIG. 9, after application of the shimming and gated deflection methods described in U.S. Pat. No. 8,766,174 B1 and U.S. Pat. No. 9,355,830 B2, respectively, but without application of the method presented herein, the intensities of the 1ν-subharmonics amount to about 1% of the corresponding fundamental 2ν.sub.+ peak. In FIG. 10, applying in addition the principles according to the invention, the signals of the 1ν-subharmonics (ν.sub.+; ν.sub.++ν.sub.−) are reduced in size by a factor of about ten.
[0041] FIG. 11 shows spectra of measurements of a complex mixture sample SRFA (Suwanee River Fulvic Acids) acquired with three different methods. In the uppermost spectrum, the ICR cell was not correctly trimmed by shimming and gated deflection. In the middle spectrum, the cell was optimally trimmed according to the methods described in the documents U.S. Pat. No. 8,766,174 B1 and U.S. Pat. No. 9,355,830 B2 for the optimization of electric fields and reduction of the magnetron orbit in measurement cells of Fourier transform ion cyclotron resonance mass spectrometers. The 1ν-subharmonic signals are greatly reduced but do not completely disappear. In the mass spectrum at the bottom, the excitation (wave) phase switching according to principles of this invention was applied additionally to the aforementioned measures and results in the almost complete elimination of the 1ν-subharmonic signals.
[0042] It should be mentioned here that the method is not restricted to 2ω-detection. In a second aspect, it is possible to apply it to multi electrode nω-detection with n>2. Applying the principles disclosed herein will reduce the (n−1, n−3, n−5, . . . )ν-subharmonics and (n+1, n+3, n+5, . . . )ν-harmonics. But with nω-detection, the very high abundant signals of the (n−2, n−4, n−6, . . . )ν-subharmonics and harmonics are still apparent. Also the low abundance signals of (n+2, n+4, n+6, . . . )ν-harmonics remain.
[0043] The general operation and function of an ion cyclotron resonance mass spectrometer can be briefly described by way of example with reference to FIG. 12. Ions are produced preferably at substantially constant output, for example, by electrospray in a vacuum-external ion source (1). The ion source (1) might receive the liquid to be sprayed from an upstream substance separator (25), such as a liquid chromatograph or an electrophoretic device. The ions can be introduced, together with ambient gas, through a capillary (2) into the first stage (3) of a differential pumping system, which may consist of a series of chambers (3), (5), (7), (9), (11) and (13) and could be pumped by the pumps (4), (6), (8), (10), (12) and (14). Ions in the chambers (3) and (5) can be drawn in by the ion funnels (14) and (15) and transferred into the multipole ion guiding system (16), in which ions can be either guided through or also be stored. Storing allows in particular the repeated gated release of ion bunches having substantially the same ion count. The ions may be subsequently transferred through a quadrupole mass filter (17) and through another multipole ion guide (18) that also allows ion storage, and finally via the main ion transfer system (19) into the ICR cell (200), where they can be captured, trapped and detected.
[0044] The ICR cell (200) may consist of four mantle-shaped enclosing longitudinal electrodes (210) to (212) and of two axial trapping electrodes (205) and (206) with a central hole (20) in each of them, as has been set out with reference to FIG. 1. The ICR cell is preferably located in the homogeneous zone of a strong magnetic field that may be generated by superconducting coils in a helium cryostat (24) and should be kept as constant in time as well as spatially homogeneous as possible. The magnetic field is preferably aligned parallel to the longitudinal mantle electrodes of the ICR cell, as shown.
[0045] The radial motions of an ion in an ICR cell which are relevant for carrying out the methods according to principles of the invention are the cyclotron motion with frequency ν.sub.+ and the magnetron motion with frequency ν.sub.− with reference to FIG. 13. The cyclotron motion is a fast motion perpendicular to the magnetic field lines and the magnetron motion is a slow (drift) motion around the electric trapping field axis, the cyclotron frequency being typically higher by a factor of about 10.sup.5.
[0046] The invention has been described with reference to a number of different embodiments thereof. It will be understood, however, that various aspects or details of the invention may be changed, or various aspects or details of different embodiments may be arbitrarily combined, if practicable, without departing from the scope of the invention. Generally, the foregoing description is for the purpose of illustration only, and not for the purpose of limiting the invention which is defined solely by the appended claims, including any equivalent implementations as the case may be.
