RU2806730C1 - Four-electrode kingdon trap with equispaced electrodes - Google Patents

Abstract

FIELD: mass spectrometry.

SUBSTANCE: mass spectrometer with electron impact ionization based on a multi-electrode harmonized Kingdon trap. The mass spectrometer is equipped with at least one source of electrons located outside the Kingdon trap, ionizing molecules of the analyte gas inside the trap, located under a negative potential U, sufficient to accelerate the electrons and their penetration into the trap, and configured to ionize the molecules of the analyte gas with electrons, the energy value varies from U in the region near the external electrodes to zero - at the electron turning point, where the potential of the trap field created by the internal electrodes coincides with the potential applied to the electron source.

EFFECT: increase in the reliability of the device and the operating time without replacing the cathode by doubling the working volume of the trap, lowering the coalescence threshold and doubling the independent sources of electrons.

4 cl, 8 dwg

Description

TECHNICAL FIELD

The invention relates to the field of mass spectrometry, and specifically describes a mass spectrometer with electron impact ionization based on a four-electrode Kingdon trap with equidistant electrodes. The proposed mass spectrometer makes it possible to create ions in two directions at once, which doubles the working volume of the trap, reduces the coalescence threshold, allows you to work with two independent sources of electrons, increases the reliability of the device and increases the operating time without replacing the cathode. The invention can find application in many fields of technology.

BACKGROUND OF THE ART

Modern mass spectrometry is a sensitive, fast and informative method for the atomic and molecular analysis of substances, which has wide applications in many fields of science and technology. The use of mass spectrometry to identify molecules requires high resolution and high mass measurement accuracy. For a long time, such requirements were satisfied by ion cyclotron resonance (ICR) mass spectrometry with Fourier transform; however, a significant disadvantage of this method is the need to use superconducting cryo-magnets with high (7 Tesla and above) magnetic induction, which leads to high operating costs and high the size, as well as the weight, of the device itself. Another type of mass spectrometer, approaching the resolution and accuracy of ICR-based instruments, are instruments that use the principle of an orbital ion trap - the retention of ions with high kinetic energy (several kiloelectron volts) inside an ion trap using electrostatic fields, first proposed by Kingdon ( further in this description - the Kingdon trap; Kingdon, K.H.: A method for the neutralization of electron space charge by positive ionization at very low gas pressures. Phys. Rev. 21, 408-418 (1923)) and proposed in the work of Knight (Knight , R.D.: Storage of ions from laser produced plasmas. Appl. Phys. Lett. 38, 221-223 (1981)) as a mass spectrometer, and subsequently in the works of Makarov, who created such a mass spectrometer, which he called Orbitrap ( Orbitrap) (Eliuk, S., Makarov, A.: Evolution of Orbitrap mass spectrometry instrumentation. Annu. Rev. Anal. Chem. 8, 61-80 (2015)). In the Orbitrap orbital ion trap, an axisymmetric static electric field is created by specially shaped electrodes. Ions entering the field with sufficient angular momentum to capture, introduced from the outside perpendicular to the trap axis at some distance from the center of the trap, begin to move along stable cyclic trajectories around the central electrode and simultaneously oscillate along the axis (z axis) of the central electrode in the quadratic potential created by the electrodes traps along the z axis. The frequencies of radial and angular motion, as well as the frequency of vibrations along the z axis, depend on the mass and charge of the ion. Along the z axis, ions perform harmonic oscillations with a frequency inversely proportional to the square root of the ratio of mass to charge of the ion. Due to the fact that the electric potential depends quadratically on the z coordinate, this frequency does not depend on the amplitude of ion vibration. The measured quantity is the potential difference induced by moving ions on the external electrodes. Due to the fact that the frequency of axial oscillation of ions does not depend on their energy and the fact that the electric field is set with high accuracy and stability, high resolution can be achieved and the mass can be determined with high accuracy from the measured frequency of axial oscillations. The orbitrap is also characterized by the ability to simultaneously capture and measure the masses of a relatively large number of ions.

The closest analogue of the proposed invention is the mass spectrometer disclosed in US patent No. US 7989758 B2 (owned by BRUKER DALTONIK GMBH and published on 08/02/2011 IPC B01D59/44; H01J49/00) containing the so-called Cassini trap and also a mass spectrometer based on a multielectrode harmonized Kingdon trap RU 2693570C1 (belonging to the AUTONOMOUS NON-PROFIT EDUCATIONAL ORGANIZATION OF HIGHER EDUCATION "SKOLKOVO INSTITUTE OF SCIENCE AND TECHNOLOGY" and published on 07/03/2019 IPC H01J49/42). However, the devices described in these patents do not provide for the possibility of creating ions inside a trap. The electrode surface geometries proposed in these patents are not optimized for the case where ions are created inside an electrostatic trap - a mass analyzer

The proposed technical solution is aimed at eliminating the shortcomings of the current state of the art and differs from previously known ones in that the proposed solution allows the creation of ions simultaneously in two directions, which doubles the working volume of the trap and reduces the coalescence threshold, allows working with two independent sources of electrons, increases the reliability of the device and increases operating time without cathode replacement.

SUMMARY OF THE INVENTION

The technical problem that the claimed solution is aimed at is the creation of a mass spectrometer with electron impact ionization based on a multielectrode harmonized Kingdon trap.

The technical result consists in doubling the working volume of the trap, lowering the coalescence threshold and doubling the independent sources of electrons, which increases the reliability of the device and increases the operating time without replacing the cathode.

The claimed technical result is achieved by implementing a mass spectrometer with electron impact ionization based on a multielectrode harmonized Kingdon trap, consisting of two external and four internal electrodes, which are located symmetrically, made with the ability to detect a signal induced by ions oscillating in a quadratic potential inside the trap, while a potential is applied to the internal electrodes, creating an electric field outside these electrodes inside the trap, the magnitude of which depends quadratically on the coordinate along the internal electrodes, while the quadratic potential is ensured by the shape of the internal and external electrodes, and the geometry of the surfaces of the internal electrodes coincides with the geometry of the equipotential surfaces corresponding to the voltage -4 kV on the inner electrodes, and the outer electrode has the geometry of an equipotential surface extending at a distance from 20 mm to 90 mm from the center of the trap along the X-axis direction;

in this case, the mass spectrometer is equipped with at least one source of electrons located outside the Kingdon trap, ionizing molecules of the analyte gas inside the trap, located under a negative potential U sufficient to accelerate the electrons and their penetration inside the trap, and configured to ionize gas molecules - analyte by electrons with an energy value varying from U in the region near the external electrodes, to zero - at the electron turning point, where the potential of the trap field created by the internal electrodes coincides with the potential applied to the electron source.

In one embodiment, the electron source may be at least one indirectly heated cathode.

In one embodiment, the electron source may be at least one field emission cathode.

In one embodiment, a single lens may be installed between the electron source and the trap to focus the electrons into the trap inlet.

DESCRIPTION OF DRAWINGS

The implementation of the invention will be described further in accordance with the accompanying drawings, which are presented to explain the essence of the invention and in no way limit the scope of the invention. The following drawings are attached to the application:

Fig. 1 illustrates the potential distribution in the trap.

Fig. 2 illustrates a three-dimensional view of the trap.

Fig. 3, illustrates the potential distribution at a voltage of -4 kV on the rods and a grounded external electrode

Fig. 4 illustrates the potential distribution near the outer electrode, showing the penetration depth of an electron beam with an energy of 300 eV.

Fig. 5, illustrates the trajectories of ions created in the areas -0.1mm<x<0.1mm, 77mm<y<90mm and -0.1mm<y<0.1mm, 77mm<x<90mm

Fig. 6 illustrates the phase portrait of ion motion in the Z direction during ion nucleation and immediately after the exciting pulse.

Fig. 7 illustrates the phase space of ions after 1 ms of flight in the case of ChargeFactor=100.

Fig. 8 illustrates the number of remaining ions in the case of ChargeFactor=100 after 1ms of flight (top) and at the end of the simulation (100ms).

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention sets forth numerous implementation details designed to provide a clear understanding of the present invention. However, it will be apparent to one skilled in the art how the present invention can be used with or without these implementation details. In other cases, well-known methods, procedures and components have not been described in detail so as not to unduly obscure the features of the present invention.

In addition, from the above discussion it will be clear that the invention is not limited to the above implementation. Numerous possible modifications, alterations, variations and substitutions, while retaining the spirit and form of the present invention, will be apparent to those skilled in the art.

For a better understanding of the present invention, below are some terms used in the present description of the invention. As used herein, the terms “includes” and “including” are interpreted to mean “including, but not limited to.” These terms are not intended to be construed as “consisting only of.” Unless otherwise defined, technical and scientific terms in this application have their standard meanings commonly accepted in the scientific and technical literature.

There are several ways to create ions for mass analysis in a harmonized Kingdon trap. It is possible to ionize gaseous substances outside the trap, for example, in a standard Nira ion source, and introduce ions into the trap from this source by capturing them in the field while flying through the trap, as is done in Orbitraps. The advantage of this method is that a standard ionization energy of 70 volts can be used, allowing molecules to be identified using electron impact mass spectral databases and the use of a magnetic field to collimate the ionizing electrons. The disadvantage of the method is the need for a device for accumulating ions before releasing them into the trap, since otherwise the efficiency of selecting ions for mass analysis will be very low.

Another obvious method of ionization is ionization within the trap itself. This can be done in two ways: with grounding of all electrodes of the trap, which allows the electron energy to be recorded, and with operating potentials on the electrodes. In the first case, the duty cycle will be negligibly small, since ions that are trapped only at the moment of ionization (microseconds) can be captured, and in the second case there will be a large spread in ionization energies, since it depends on the place of birth of the ion, which is located at path of ionizing electrons. The electron trajectories do not coincide with the electric field lines inside the trap; the electron energy spread at the moment of ionization is up to 400 electron volts. (The energy of the electrons in the trap ranges from 0 to 400 eV) The mass spectra obtained with such ionization will generally not coincide with the mass spectra obtained with standard ionization energy and will require correction. The ions formed by this method of ionization will be captured by the trap. Simulations of the ionization process show that capture efficiencies range from 10 to 60%. Experiments show that the lifetime of ions in a trap after their formation and capture is more than 1000 milliseconds.

To obtain a signal, it is necessary to excite the movement of ions in the direction in which the potential has a quadratic dependence on the coordinate, namely along the internal electrodes of the trap. This can be done in the case of introducing ions from a storage device by introducing ions not in the center of the trap, as in the Orbitrap trap. Then the ions enter a field with a quadratic potential not at its center, where the potential is minimal, but in a potential region above the potential in the center and begin to perform harmonic oscillations in this quadratic potential. In the case of introducing ions at the center, their movement in a quadratic potential must be excited either by applying an alternating voltage with a frequency coinciding with the resonant frequencies of harmonic oscillations of ions in a quadratic field, to the external electrodes of the trap or to the internal ones, for which the electrodes must be cut along the vertical plane of symmetry. The voltage amplitude is adjusted experimentally to obtain maximum ion signal amplitude and resolution. To selectively excite ions of certain masses, the SWIFT method can be used, as in FT ICR mass spectrometry. In this method, a frequency spectrum is synthesized, the transformation of which produces a time signal of the required frequency and amplitude to excite ions in a certain mass range or ions of certain masses. The programmed SWIFT time signal is fed from the DAC through the amplifier to the excitation electrodes. When externally introducing ions into a Kingdon trap, any type of ion source can be used, which allows the ionization of gaseous, liquid and solid substances. In the case of gas analysis, ions can be created directly in the Kingdon trap itself using electron impact ionization and photoionization.

As the simulation results show, the probability of capturing the ions formed in this way can exceed 50% with the optimal choice of the position of the ionizing beam of electrons and photons. Thus, ions can be accumulated in a trap and analyzed by mass when the required amount is reached. Ionization must be carried out in the plane of symmetry of the trap in order to avoid excitation of the movement of the resulting ions in the longitudinal direction of the trap.

To implement the principle of doubling the working volume of the trap and lowering the coalescence threshold, a mass spectrometer with electron impact ionization was created based on a multielectrode harmonized Kingdon trap, consisting of two external and four internal electrodes, which are located symmetrically.

Trap field and electrode shape.

The trap proposed by Yuri Golikov in the dissertation of Nikitina D.V. was investigated. (Golikov, Y.K., et al., Integrated electrostatic ion traps. Appl. Phys. (Russian). 5, 50-57 (2006)), (Nikitina, D.V.: Ion trap Mass spectrometry in a dynamic mass spectrometry. Thesis for PhD degree, St. Petersburg (2006)).

The formula for calculating the potential is given below:

U(x,y,z)= с*(2*z**2 -x**2 -y**2)/2 +

ln((x+a)**2+(y+b)**2) + ln((x+a)**2+(y+b)**2) +

ln((x-а)**2+(y+b)**2) + ln((x-а)**2+(y-b)**2)

With parameter values a = 1.0, b = 1.0, c = 0.5. The potential is created by a three-dimensional quadrupole field and the fields of four infinite charged filaments.

Figure 1 shows the potential distribution in such a trap. The internal rods were formed by equipotential surfaces corresponding to voltages from 100V to 10kV. The outer electrode is formed by an equipotential surface extending at a distance of 10 mm (acceptable range 5-15 mm) from the center of the trap along the X direction.

Simulation of ion motion taking into account Coulomb interactions.

Figure 4 shows the potential distribution near the external electrode, from which it follows that an electron beam with an energy of 300 eV penetrates into the trap to a depth of 77 mm from its center. In further simulations, ions are created in the region -0.1mm<x<0.1mm, 77mm<y<90mm and -0.1mm<y<0.1mm, 77mm<x<90mm with a uniform random distribution of initial positions and zero energy in all directions. The peculiarity of this trap is that it can create ions in two areas at once.

To avoid Coulomb explosion, ions are created during the first 100 μs of the simulation with a uniform random distribution of appearance times. Immediately after the nucleation of all ions, a voltage pulse is applied and a dipole field is created by the halves of the external electrode in order to excite oscillations of the ions along the Z axis. The pulse duration is 1 μs, the amplitude is 1000 V. Fig. 6. shows a phase portrait of the movement of ions in the Z direction during ion nucleation and immediately after the exciting pulse without taking into account Coulomb interactions.

In the proposed technical solution, the electrodes are separated at the corners of the cube and are located symmetrically. The inner electrodes are formed by equipotential surfaces corresponding to a voltage of 4 kV, and the outer electrode is formed by an equipotential surface extending at a distance from 20 mm to 90 mm from the center of the trap along the X direction.

The dimensions of the trap electrodes are determined by the requirements to achieve the highest possible accuracy in their manufacture. The oscillation frequency of the trap depends on the voltage. In this case, the area of the treated surface should be no more than 200mm x 200mm x 200mm, i.e. no more than the volume of the working area of the most precision milling machines. The larger the size of the working area of the machine, the greater the influence of thermal expansion and the worse the manufacturing accuracy, all other things being equal.

The number of charges in each simulation is controlled by the ChargeFactor parameter.

Two simulations of the movement of CO, N2 and C2H4 ions were carried out with the parameter ChargeFactor=100 and 200 (this is the number of charges on one ion). In the simulation, six groups of ions of 500 each are created - three along the X direction and three along the Y direction. In total, at the beginning of the simulation there are 3000 ions. Taking into account Coulomb interactions, the clouds of ions expand significantly (see Fig. 7) and a significant part of the ions is lost on the rods. So in the case of ChargeFactor=100, 274 ions survive (see Fig. 8). In the case of ChargeFactor=100, this corresponds to 27 thousand charges in the trap.

As the ions move, the ions continue to die and by the end almost all the green ions have disappeared. Therefore, their peak is absent in the spectrum of the induced current (Fig. 9).

In the case of ChargeFactor=200, the clouds are even wider and 165 ions survive. This corresponds to more than 30,000 charges. During the simulation, the ions continue to decrease and after 60 ms only 45 ions remain, which give a single peak in the spectrum (see Fig. 4).

Thus, the proposed technical solution makes it possible to create ions in two directions at once, and also provides the possibility of using from 2 to 4 electron sources. This doubles the working volume of the trap and lowers the coalescence threshold. Indeed, in this trap, coalescence has a developed character at approximately 80,000 charges, whereas for currently used traps it begins already at 25,000 charges. In addition, this design of the trap allows working with two independent sources of electrons, which increases the reliability of the device and increases the operating time without replacing the cathode.

Although the invention has been described with reference to the disclosed embodiments, it will be apparent to those skilled in the art that the specific experiments detailed are provided for purposes of illustrating the present invention only and should not be construed as limiting the scope of the invention in any way. It should be understood that various modifications can be made without departing from the spirit of the present invention.

Claims (5)

1. Mass spectrometer with electron impact ionization based on a multielectrode harmonized Kingdon trap, consisting of two external and four internal electrodes, which are located symmetrically, designed to detect the signal induced by ions oscillating in a quadratic potential inside the trap, while a potential is applied to the internal electrodes , creating an electric field outside these electrodes inside the trap, the magnitude of the potential of which depends quadratically on the coordinate along the internal electrodes, while the quadratic potential is ensured by the shape of the internal and external electrodes, and the geometry of the surfaces of the internal electrodes coincides with the geometry of the equipotential surfaces corresponding to a voltage of -4 kV on the internal electrodes, and the outer electrode has the geometry of an equipotential surface extending at a distance from 20 to 90 mm from the center of the trap along the X-axis direction; wherein the mass spectrometer is equipped with at least one source of electrons located outside the Kingdon trap, ionizing molecules of the analyte gas inside the trap, located under a negative potential U, sufficient to accelerate the electrons and their penetration inside the trap, and configured to ionize molecules of the analyte gas electrons with an energy value varying from U in the region near the external electrodes to zero - at the electron turning point, where the potential of the trap field created by the internal electrodes coincides with the potential applied to the electron source. 2. Mass spectrometer according to claim 1, characterized in that the electron source is at least one indirectly heated cathode. 3. Mass spectrometer according to claim 1, characterized in that the electron source is at least one field emission cathode. 4. The mass spectrometer according to claim 1, characterized in that a single lens is installed between the electron source and the trap to focus electrons into the inlet of the trap.