RU2693570C1 - Multi-electrode harmonized ion trap of kingdon with merged inner electrodes - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to mass-spectrometry. Ion trap contains at least two external electrodes elongated along the longitudinal axis of the trap, and two pairs of internal electrodes elongated along the longitudinal axis of the trap and arranged so that each electrode from one pair touches the electrode of the other pair at least part of its surface.

EFFECT: increased compactness of the mass spectrometer realized on the basis of the possibility of capture and retention of ions created inside the trap, which will allow to avoid use of the additional radio-frequency trap used in the traditional Orbitrap for ion input; configuration of internal electrodes of the disclosed ion trap provides reduced sensitivity to errors in making electrodes.

1 cl, 8 dwg

Description

Technical field

The invention relates to the field of mass spectrometry, in particular to the design of a modified ion trap of Kingdon, including a system of electrodes, forming a field that retains ions. The invention can find application in many areas of technology.

The level of technology

Modern mass spectrometry is a sensitive, fast and informative analytical method that has the widest application in many areas of technology. The use of mass spectrometry to identify biological objects requires high resolution and accuracy of mass measurement. For a long time, these requirements were satisfied by mass spectrometry of ion cyclotron resonance (ICR) with Fourier transform, but a significant drawback of this method is the need to use cryo-magnets with high (7 Tesla and higher) magnetic fields, which leads to high operating costs and large size, as well as the weight of the device itself. Another type of mass spectrometers approaching in resolution and accuracy to ICR-based instruments are Orbitrap instruments using the orbital ion trap principle — the principle of ion retention inside an ion trap using electrostatic fields, first proposed by Kingdon (hereinafter referred to as Kingdon, KH: it has been greatly improved in the work of Knight (Knight, RD: Storage); ions from laser produced plasmas. Appl. Phys. Lett. 38, 221–223 (1981)), which he built a mass spectrometer based on such a trap, and subsequently the work of Makarov, who created such a mass spectrometer, named by Orbitrap (Eliuk, S., Makarov, A .: Evolution of Orbitrap mass spectrometry instrumentation. Annu. Rev. Anal. Chem 8, 61–80 (2015)). Orbital ion trap Orbitrap uses a symmetric static electric field between the external and internal electrodes of a special form. Ions entering the field with a pulse sufficient for trapping, introduced from the outside perpendicular to the trap's axis not in the center of the trap, begin to move along stable cyclic trajectories around the central electrode and simultaneously oscillate along the axis of the central electrode. Although the radial and angular frequencies also depend on the m / z ion, the harmonic oscillation of ions along the z axis does not depend on these quantities. Ions are detected by induced current on external electrodes. Due to the fact that axial oscillation does not depend on the ion energy and the fact that the electric field is established with high accuracy and stability, high resolution can be achieved and the mass can be determined with high accuracy by the measured frequency of axial oscillations. The orbital trap is also characterized by a higher ion capacity.

Orbitrap devices on the market today use a single internal electrode with a complex hyperbolic surface, in combination with two external electrodes for holding ions inside a trap and measuring current induced by ions (Eliuk, S., Makarov, A. Annu. Rev. Anal. Chem. 8, 61–80 (2015)). The possibility of the existence of orbital ion trap structures with several internal electrodes was previously predicted by Golikov and (Golikov, YK, et al., Integrable electrostatic ion traps. Appl. Phys. (Russian). 5, 50–57 (2006)), however specific versions that provide certain practical advantages were not considered in these works. The work of Golikov shows that to create a trap, it is possible to use any number of internal electrodes, elongated parallel to the axis of the external electrode, which do not prevent the creation of a quadratic potential in the space between the internal and external electrodes. Despite the commercial success of Orbitrap devices, they are also not without drawbacks. In particular, Orbitrap cannot capture the ions created inside the trap, therefore the Orbitrap trap can only be used in combination with another trap (for example, the C-trap in Thermo Scientific mass spectrometers based on Orbitrap). The use of an additional radio frequency trap with a complex system of deflectors for introducing ions into the Orbitrap trap significantly increases the device’s energy consumption and also makes it more difficult to implement as a portable device or as a device for space applications (size plays a key role in creating spacecraft instruments) . Thus, today there is still a need for inexpensive high-resolution portable mass spectrometers on the market, and this invention has a number of properties necessary to solve the problem.

Summary of Invention

The present invention is the creation of a modified ion trap of Kingdon, which is the basis of an inexpensive high-resolution portable mass spectrometer. This problem is solved by using a specific configuration of the internal electrodes of the ion trap, which provides the ability to capture and hold the ions created inside the trap, as well as providing reduced sensitivity to errors in the manufacture of electrodes. This problem is solved by creating an ion trap in the mass spectrometer used for the accumulation and detection of ions, containing: (a) at least two external electrodes elongated along the longitudinal axis of the trap; (b) two pairs of internal electrodes elongated along the longitudinal axis of the trap and positioned in such a way that each electrode from one pair contacts the other electrode with at least a part of its surface, while the internal surface of the external electrodes and the external surface of the internal electrodes correspond to equipotential surfaces electric potential c (x; y; z), existing between external and internal electrodes, and calculated by the formula (1):

where x and y are the directions of the axes perpendicular to each other and the axes of the trap z; the values of a, b, c and d are constants, and the values –a, a, –b, b determine the coordinates of the centers of the four internal electrodes at z = 0; at the same time, the linear dimensions of the electrodes are limited by the absolute value of the potential applied to them, not exceeding 1.5 kV.

In preferred embodiments of the invention, this trap is characterized by the fact that it accumulates ions created inside the trap in the space between the external and internal electrodes.

When implementing the invention, the following technical result is achieved: the proposed ion trap provides an increase in the compactness of the mass spectrometer implemented on its basis due to the possibility of trapping and holding the ions created inside the trap, which will prevent the use of an additional radio frequency trap used in the traditional Orbitrepe for ion injection. Also, the configuration of the internal electrodes of the proposed ion trap provides reduced sensitivity to errors in the manufacture of electrodes.

Brief description of drawings

Fig. 1. An example of a harmonic Kingdon trap with four internal electrodes. In this example, the values a = 0.9, b = 0.9, c = 0.2, d = 0.5 for formula (1) are used, the bias potential of the external electrodes is U_high = 7, and the bias potential of the internal electrodes is U_low = 1.

Fig. 2. An example of ionic orbital rotation around all internal electrodes (right) and around two internal electrodes (left) in the four-electrode orbital harmonized trap of Kingdon; ions also oscillate along the z axis due to the quadratic potential in z. The figure shows the projections of ion trajectories onto the trap cross section at z = 0. When ions are introduced outside, it is necessary to instantaneously increase the bias potential on the internal electrodes with a pulse in order to reduce the kinetic energy of the ions to the required value for orbital capture — otherwise the ions will collide with the external electrodes. Ions are introduced into a trap not in the center (z = 0), but at some distance from the center along the z axis, which initiates oscillations of the cloud along the z axis. The point of entry of ions along the z axis determines the amplitude of oscillations.

Fig. 3. Three-dimensional schematic representation of a four-electrode trap with two coalesced internal electrodes.

Fig. 4. The difference between the distribution of the actual electric potential along the z axis (for x = 0, y = 0) and its theoretical value for an ideal trap for different variants of its truncation (see the captions on the graph). The bias potential on the internal electrodes is 4 kV, and the external electrode is grounded. "Extension" are an additional 3-millimeter 2D-sections that are added on both sides of the trap at the truncation points. The shape of the electrodes in these 2D sections repeats the shape of the truncated electrodes.

Fig. 5. Sections x-y (a), x-z (d) and y-z (c) (not to scale) show the electric potential differences of an ideal trap and traps with electrodes with a non-ideally smooth surface (modeled using a grid of 75 μm). The external electrode is grounded, the potential bias on the internal electrodes is -4 kV.

Fig. 6. The initial positions of the ions in the average cross section (z = 0) of a four-electrode harmonized trap with coalesced electrodes (left), and their positions after 5 ms of flight (right). Ions have masses ranging from 50 Da to 2000 Da.

Fig. 7. Amplitude versus time for the frequency sweep signal (top) and the position of ions in the z-Ez phase plane at the end of the frequency sweep excitation (bottom). The coordinates and velocities obtained from the previous modeling of the accumulation of ions were used as initial conditions for the ions (see Fig. 6). Ions oscillate with an amplitude of 5-6 mm, forming rather compact groups depending on their mass-to-charge ratio, which is a prerequisite for successful registration of induced currents.

Fig. 8. (A) Current induced on the halves of the external electrode as a function of time. (B) The mass spectrum of trapped ions in the range from 50 to 2000 Da after the Fourier transform. For some peaks, resolution is indicated.

Detailed disclosure of the invention

For a better understanding of the present invention, here are some terms used in the present description of the invention. In the description of this invention, the terms "includes" and "including" are interpreted to mean "includes, among other things." These terms are not intended to be interpreted as “consists only of”. Unless otherwise defined, technical and scientific terms in this application have standard meanings commonly accepted in scientific and technical literature.

The traditional mass analyzer based on the Kingdon ion trap is a high-precision mechanical device that includes electrodes with hyperbolic surfaces as part of an ion trap. The fabrication of the hyperbolic surfaces of the electrodes requires high accuracy, which leads to a rise in the cost of the device or, potentially, to a decrease in the resolution characteristics when imperfectly making the shape of the electrodes. Also, the Orbitrap traditional orbital ion trap uses only one internal electrode to trap ions inside the ion trap using electrostatic fields. The use of an ion trap with a different configuration of internal electrodes will allow the functionality of a mass spectrometer based on it to be expanded, namely, to make it portable by eliminating the additional radio frequency trap.

In the present invention, the authors used simulation and simulation experiments to study a set of four-electrode harmonic Kingdon traps (with quadratic potential dependence on the coordinate along the axis of the trap) to determine their sensitivity to manufacturing errors and electrode assembly, as well as to the occurrence of the edge effect caused by the trap truncation. The difference between the electrostatic field of the orbital ion trap and the ideal one is the cause of the dephasing of ions in ion clouds, which limits the resolution of mass spectrometers using traps of this type. In Kingdon’s multielectrode harmonic traps, as in Orbitrap, the field distribution is quadratically in one of the directions (usually along the axis of the electrodes, i.e. along the z axis). The movement of ions in this direction is a harmonic oscillation, in which the oscillation frequency depends solely on the ratio of the mass to the charge of the ions; therefore, by measuring the frequency of oscillations, you can determine the mass of ions. The distribution of the electric potential for Kingdon’s four-electrode harmonic trap (Fig. 1), that is, φ (x, y, z), can be obtained from solving the Laplace equation, using the square dependence of the potential on one of the coordinates as a constraint; the result is an expression (formula 1):

(1)

(one)

where x and y are directions perpendicular to each other and trap axes (z), and a, b, c and d are constants (pairs-a, a and-b, b are coordinates of the centers of four internal electrodes at z = 0). In such ion traps, in addition to the orbital rotation of the ions around the internal electrodes, orbital rotations around subsets of the internal internal electrodes are also possible, as shown in Fig. 2 and oscillations with “eight-shaped” trajectories between the electrodes.

After calculating the potential and shape of the electrodes (the shape of the electrodes should follow the equipotential surfaces), the solution can be normalized so that the potential of the external electrode is zero and the bias potential of the internal electrode is -1. The choice of equipotential surfaces for positioning the internal and external electrodes was made on the basis of two requirements: firstly, to limit the electric field on the surface of these electrodes in order to avoid the autoemission of electrons, and secondly, to create space d To capture enough ions to trap for a high dynamic range. Consequently, the actual solution of the equation is a normalized solution multiplied by the required voltage of the internal rods (electrodes), which in this example was 4 kV. In Fig. Two ion trajectories were estimated using the SIMAX program (www.mssoft.pro), which used the calculated electrical potentials from the SIMION program (http://simion.com) as input data and accurately tracked particles in these fields. In Fig. 2, on the left, an ion with a mass of 500 Da begins to move from the point xo = 20 mm, yo = 20 mm and zo = 0 mm with an energy of 2000 eV along the y axis; similarly, in Fig. 2, on the right, the same ion begins to move from the point x0 = 13.5 mm, y = 13.5 mm and zo = 0 mm, with an initial energy of 500 eV along the y axis and 500 eV along the x axis. Note that the oscillations along the z axis are harmonic and strictly isochronous, with a frequency that depends only on the ratio of the mass to the ion charges.

The fields of the four-electrode harmonic traps of Kingdon can also be obtained as a superposition of the fields of two two-electrode traps. Internal pairs of electrodes of four-electrode traps can be relative to each other at any angle around the z axis. At small angles between the planes in which there are pairs of electrodes of two-electrode traps, the electrodes from different two-electrode traps will merge, resulting in a four-electrode trap reduced to a two-electrode, having internal electrodes of a more complex shape than conventional electrodes of classical two-electrode traps. Further, a study was conducted of the many four-electrode harmonic traps of Kingdon to determine their sensitivity to the errors of manufacturing and assembly of electrodes, as well as to the occurrence of the edge effect caused by the truncation of the trap. Such studies have practical value, since the nonideality of the electric field (i.e., the difference of its values from the quadratic term and (x, y) is the dependence of the coefficient that multiplies the quadratic term in the polynomial equation describing the dependence on z of the electric potential in the volume in which the movement of the ion occurs) leads to a rapid dephasing of harmonic oscillations along the z axis, which leads to a loss of resolution. The study of the effect of electrode roughness was done as follows. In the SIMION program, a uniform grid was used to represent the field surface, and the electrodes are represented with the same voltage across the entire electrode. With curved electrodes, the grid points do not fall exactly on the edges of the electrodes. This can be considered as an electrode with an uneven, distorted shape. The authors created such potential arrays with a grid spacing of 0.075 mm for both the classical two-electrode trap with two rods, and for the configuration of the four-electrode trap with merged electrodes (reduced to a two-electrode trap). These arrays were used further in the simulation with current measurement of induced ions. According to the simulation results, the dephasing of ionic clouds due to the mechanical roughness of electrodes of 75 μm occurs within 10 ms for a classical two-electrode trap and for more than 20 ms for a more complex geometry with coalesced electrodes (reduced four-electrode trap). An ion trap with fused electrodes has an additional degree of freedom to change the shape of the rods (the angle between the lines connecting the electrode pairs) to make the geometry less sensitive to mechanical distortion of the electrodes. This trap configuration can be further further optimized if necessary. The simulation results show that traps with two internal confluent electrodes have the lowest sensitivity to similar distortions in the geometry of the electrodes. The parameters used in the optimal solution are a = 0.1, b = 0.73, c = 2 and d = 0; the shape of the external electrode (which is determined by the shape of the equipotential surface for the selected potential) was given by the condition U_high = 2.0, and the potential of the internal electrode is U_low = -2.8. The trap has a linear dimension along the z axis of 66 mm, while all the electrodes were extended by 3 mm at both ends using cylinders with diameters that coincide with the diameters of the electrodes at the truncation point to correct field distortion due to edge effects. The three-dimensional view of such a trap is shown in Fig. 3. In this construction, the authors tried to minimize the influence of the penetration of external fields into the trap, which occurs after the truncation of the electrodes (finite dimensions instead of theoretically infinite).

In Fig. 4 shows the difference in the distribution of the electric potential along the z axis from the ideal (that is, calculated analytically using formula (1)) for different values of the z coordinate at which the electrodes were truncated. In all cases, adding cylindrical tips reduces the difference; the lowest of the graphs in Fig. 4 (corresponding to a truncation of 33 mm from the center of the trap with added tips of 3 mm) shows almost complete suppression of field distortion near the center of the trap. Although the graphics in Fig. 4 shows the behavior of the potential deviation from the ideal along the z axis; these curves are not fully informative from the point of view of estimating nonlinear distortions. The deviations calculated in Fig. 4 contain a constant that does not affect the movement of ions at all; the quadratic term, which only slightly changes the quadratic potential (and, therefore, only the absolute value of the oscillation frequency); and higher order terms that contain information about non-linear deviations (amplitude frequency dependence of oscillation frequency). To calculate the latter, the authors calculated the amplitude of the quadrupole component for the unperturbed field. For this, the potential along the z axis, that is, φ (z), was represented as

(2)

(2)

 where U_rods is the bias potential on the internal electrodes with a grounded external electrode, zo = 33 mm, i.e. the distance from the center of the trap to the ends (truncation points) of the electrodes, and A2 is the amplitude of the quadrupole field. The equation of motion of ions with mass m and charge e in such a potential has the form

(3)

(3)

that describes harmonic oscillations with an angular frequency Ω equal to

(4)

(four)

The simulation of this system showed that for singly charged ions with a mass of 500 Da, the frequency of axial oscillations is f = 186.690 Hz. From this value, the authors calculated the amplitude of the quadrupole field as

(5)

(five)

the value of A2 is close to unity, and this served as the basis for comparison with the amplitudes of nonlinear distortions, which were calculated further.

To calculate the amplitudes of nonlinear distortion, it is convenient to use polynomial regression. In this case it is necessary to take into account two circumstances. First, the interpolation must be calculated in the region in which the ions move during capture (calculations showed that this is an area near the center of the trap, limited by dimensions - 6 mm <z <6 mm). Secondly, higher-order polynomials should not be used for interpolation, since the amplitudes of higher-order polynomials increase rapidly. In this study, the authors used polynomials up to the sixth order inclusive. Thus, the interpolation polynomial for the potential distribution along the z axis has the form

(6)

(6)

On a line starting at x = 0, y = 0, the interpolation polynomial contains only even powers of z due to the symmetry of the distribution with respect to the center of the trap. Calculations using the MathCad 13 program (https://www.ptc.com/en/products/mathcad/) showed the amplitudes given in Table 1. The magnitudes of the amplitudes are small, but a change in the amplitudes of distortions of high orders shows that the best trap among the many analyzed is trap No. 3. The numbering given here for the trap geometry in the first column of the table corresponds to the numbering in Fig. 4, namely, the 1st and 2nd rows of the table - traps, truncated at a distance of 31 and 33 mm from the center of the trap and without adding extensions. Rows 3 and 4 of the table are the same traps as 1 and 2, but with the addition of additional tips on both sides of the truncation. The fifth line corresponds to a trap, truncated at a distance of 33 mm from the center of the trap, and having an error in the manufacture of electrode surfaces of 0.075 mm.

The authors also investigated the effect of inaccuracies in the manufacture of electrodes on the field inside the trap using the SIMION program. The surface of each electrode must correspond to an equipotential surface; Considering that in SIMION the electrode is represented on a single grid, the nodes of which do not fall on the curved surface of each electrode, therefore, this can be interpreted as the unevenness of the electrode surface. Fig. 5 shows the difference between the exact potential value and the value found by the SIMION program using the best grid size for displaying electrodes (grid spacing 75 μm); The amplitudes of nonlinear distortions for this case are given in the fifth row of Table 1 — this result not only reflects the effect of inaccuracy in the production of electrodes, but also the edge effects due to the truncation of the electrodes. In this case, the nonlinear amplitudes are rather large compared to the other examples considered; However, taking into account that electrodes can be manufactured with high accuracy (no worse than 5 microns) using precision machining, these values will be an order of magnitude smaller and should not significantly affect the analytical characteristics of the device.

For the final determination of all the parameters of the trap design shown in Fig. 5, the authors conducted a more detailed study of the deviation of the axial potential distribution from the ideal distribution due to the truncation of the ends of the trap electrodes. In this case, the difference in the distribution of potentials along the z axis was calculated for an ideal trap and for a trap truncated along lines with constant (x, y) values; these functions were then subjected to polynomial interpolation (that is, formula (6)). The study showed that at distances from the trap axis in the x-direction more than 20 mm, the difference between the distributions in the ideal trap and the truncated trap is negligible. This is understandable, since edge effects in this area are also insignificant. For other lines, the results are shown in table 2.

The calculations in Table 2 show quite significant changes in the U2 values, which means a change in the frequency of the axial oscillations of the ions. However, this is true under the condition that the movement of ions along the z-direction will occur at a constant value of x and y; in fact, as calculations show, when an ion moves along the z axis, oscillations along the x and y directions also occur. In this case, the values of U2 and the values of other non-linear amplitudes Un correlate with each other. As a result, the dependence of the frequency of oscillations of ions along the z-direction weakly depends on the initial position of the ion. This was proved by calculating the oscillation frequencies of ions with a mass of 50 Da, starting the movement from a position z0 = 6 mm and different positions (x, y) with zero initial energy, which leads to harmonic oscillation of ions along the z-direction in the range of -6 mm to 6 mm. Frequencies were calculated using Fourier analysis of the signal generated by these oscillations. The authors did not find any differences in the frequency of ion oscillations, taking into account the error of 1 Hz.

The following examples of the work of the trap are given in order to disclose the characteristics of the present invention and should not be construed as in any way limiting the scope of the invention.

Investigation of the properties of a four-electrode harmonized orbital ion trap with merged internal electrodes.

A practical multielectrode harmonized orbital ion trap has finite-sized electrodes with surfaces that coincide, as far as possible, with the equipotential surfaces of an ideal electric field, obtained from formula (1); Obviously, the shape and size of these electrodes is determined by the choice of the potential value that is applied to the electrodes. When choosing the geometry of the electrodes, they should be located so as not to interfere with the movement of trapped ions, that is, they should not be in areas of stable ionic movement. In the case of a four-electrode harmonized orbital ion trap, when these conditions are met, the electrodes can merge and, as already mentioned, instead of four internal electrodes, form two electrodes with a more complex geometric shape (Fig. 3). The advantage of this design is that the truncated ends of the external electrodes almost completely cover the inner region of the trap, and prevent the penetration of fields from external sources, which leads to minimal distortion of the field inside the trap. Also, the internal electrodes are thick enough to be fixed inside the trap.

It is obvious that such a degenerate four-electrode trap can work as an orbital ion trap. However, more importantly, such a trap with coalesced internal electrodes has an unusual property: it can hold ions near its center (z = 0) that were created directly inside the trap and not introduced from outside; such ions are able to accumulate inside the trap on oscillatory trajectories (Fig. 6). This feature is not obvious and very important, since it eliminates the need to use a complex system of baffles to introduce ions from an external source (additional trap); instead, ions can be created directly inside the trap by electron impact or by photoionization (as described, for example, in Mass Spectrometry Principles and Applications Third Edition, Edmond de Hoffmann Université Catholique de Louvain, John Wiley & Sons Ltd) . After creation, the ions begin sustained oscillations between the inner electrodes in the direction of the coordinate perpendicular to the z axis.

In order to measure the signal from ions in a trap of the type being studied, it is necessary to excite oscillations of ions along the z axis. Ions will not be excited automatically, as it happens in Orbitrap traps, in which ions are introduced from the outside through the z axis with z ≠ 0, where the potential is not minimal, and the ions begin to interact with the field and oscillate along the z axis immediately after their introduction. A similar mode of introducing ions can also be used in the four-electrode trap under consideration with coalesced electrodes. However, in a preferred embodiment, ions are formed in the center of the trap with zero potential energy; therefore, it is necessary to apply a resonant radio frequency field to excite their harmonic oscillations along the z axis. Measurement of mass spectra using four-electrode

traps with two merged electrodes can work as follows:

(i) First, ions are created by electron impact or photoionization in the space between the electrodes (z = 0) and begin to produce stable oscillations in the effective potential well in the z = 0 plane;

(ii) Then, when a sufficient amount of ions accumulate, their movement along the z axis is excited by a signal with a frequency scan and a phase scan (for example, as described in US Pat. No. 4,761,545) covering the entire frequency range of axial oscillations of trapped ions. To generate a dipole field, each of the internal electrodes can be divided into two parts.

(iii) An external electrode is used to measure induced currents, followed by a Fourier transform to obtain a mass spectrum. It can also be divided into two parts, isolated by RF voltage, and having the same electrostatic potential (0 V in this example). One pair (external) will be used to measure the signal induced by ions, and the other for the excitation of ions.

Example. Simulation of a four-electrode trap with two coalescent electrodes as a mass spectrometer.

When simulating the first stage of work (accumulation of ions in the trap), the ions used in this simulation had a mass-to-charge ratio in the range from 50 to 2000 Da with a uniform random distribution of initial positions in the range of 0 mm <x <25 mm and 0 mm <y <2 mm; In addition, the ions had a thermal distribution of initial energies with an average value of 0.025 eV. In simulations, it is predicted that about 13.5% of the ions are trapped in the trap and fluctuate steadily in the x-y plane. The simulation of the second stage of operation of this four-electrode trap (excitation of axial oscillations of ions) is shown in Fig. 7. Excitation was achieved using a broadband frequency sweep signal, which was fed to half of the internal rods (the internal electrodes were cut in half). The frequency sweep signal is a polyharmonic signal containing the frequencies of trapped ions. In this simulation experiment, the frequency sweep signal was created using a special application that first calculates the signal as a harmonic signal with variable frequency and amplitude; the application then performs a Fourier transform of the signal and removes all components of the spectrum that do not correspond to any of the trapped ions. In the simulation experiment, only the frequency range from 60 to 600 kHz remains. Further, the application performs the inverse Fourier transform of the received signal and saves its temporary implementation as a text file. The frequency sweep signal thus obtained does not contain unnecessary harmonics. In this simulation experiment, the resulting frequency sweep signal had a duration of 13 ms and an actual maximum amplitude of 20 V (Fig. 7a). As can be seen from Fig. 7a, with an increase in the frequency of the signal, its amplitude decreases; This is necessary for more uniform excitation of ions of different masses to the same amplitude of oscillations along the z axis. The distribution of ions based on their mass-to-charge ratio is shown in Fig. 7b.

At the end of the frequency sweep signal, the ions begin to perform harmonic oscillations along the z axis, causing a signal on another pair of external trap electrodes (the third stage of the four-electrode trap with two coalesced electrodes). In Fig. 8 shows the time dependence of this signal during the first second. It is seen that the signal of the induced current decreases slightly with time; this is probably due to errors in the integration of the equations of motion of ions — neither collisions with a gas, nor field distortions were taken into account in this simulation.

In Fig. 8 shows the mass spectrum of trapped ions obtained by the Fourier transform of the induced current as a function of time. For a mass-to-charge ratio of 50 Da, the resolution is 609,000; this is a very large value, which was obtained without any apodization and other methods of processing the Fourier spectrum used by specialists. Using existing spectral processing techniques, the resolution can potentially be doubled. As already mentioned, the exact analytical field of the trap calculated by a special application and stored in the PA format of SIMION program files was used in this simulation. Thus, the effects of truncation of the trap electrodes along the z axis, as well as the potential irregularity and inaccuracy of the shape of the electrodes, were not taken into account in their production. Given these factors, the resolution of a real trap will not be as high, but with the development of precision production technologies, these restrictions may become insignificant.

Thus, based on the analysis of the results of studies of various designs of four-electrode orbital harmonic traps, the present invention proposes the use of a new type of trap with two coalesced internal electrodes, which has the ability to capture and accumulate the ions formed inside the trap. It has been demonstrated that a mass spectrometer based on this type of trap can have high resolution. Also, a similar configuration of the internal electrodes of the proposed orbital ion trap provides reduced sensitivity to errors in the manufacture of the exact shape of the electrodes. The creation of electrodes of this form is a rather complicated process, however, this became possible with the development of layered synthesis technology (3D printing). The implementation of the synthesis of the electrodes of the proposed trap will require high resolution manufacturing processes using materials (metals) with high electrical conductivity, resistance to high temperatures and ultrahigh vacuum.

Although the invention has been described with reference to the disclosed embodiments, it should be obvious to those skilled in the art that the specific experiments described in detail are given only for the purpose of illustrating the present invention and should not be construed as limiting the scope of the invention in any way. It should be clear that it is possible to implement various modifications without departing from the gist of the present invention.

Claims (9)

1. Ion trap in the composition of the mass spectrometer used for accumulation and detection of ions, containing: (a) at least two external electrodes extended along the longitudinal axis of the trap; (b) two pairs of internal electrodes, elongated along the longitudinal axis of the trap and positioned in such a way that each electrode from one pair contacts the electrode of the other pair by at least a part of its surface, the inner surface of the outer electrodes and the outer surface of the inner electrodes correspond to the equipotential surfaces of the electric potential φ (x; y; z) existing between the outer and inner electrodes and calculated by the formula

where x and y are the directions of the axes perpendicular to each other and the axes of the trap z; the values of a, b, c and d are constants, and the values –a, a, –b, b determine the coordinates of the centers of the four internal electrodes at z = 0; and at the same time, the linear dimensions of the electrodes are limited by the absolute value of the potential applied to them, not exceeding 1.5 kV. 2. The trap according to claim. 1, characterized in that it accumulates the ions created inside the trap in the space between the external and internal electrodes.

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