RU2634614C1 - Method of mass-analysis with resonant excitation of ions and device for its implementation - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method of mass-analysis is the impact on the oscillations of ions in a quadrupole HF field by a homogeneous exciting field directed along one of the asymptotes of the quadrupole field. The method is realized in a mass analyzer with planar discrete electrodes, where a superposition of the quadrupole RF and a homogeneous excitation field along one axis is created. The use of the planar discrete electrodes allows to improve the quality of the homogeneous field and to decrease the amplitudes of the higher harmonics of ion oscillations along the excitation axis.

EFFECT: improving the quality of the field and the operating mode of quadrupole mass spectrometers with the resonant ion output, the method and device improve the shape of the mass peaks and 2-3 times increase the resolving power of linear ion traps with the resonant excitation.

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Description

The invention relates to the field of mass spectrometric analysis of a substance and can be used to improve the analytical and commercial characteristics of linear ion traps with resonant ion excitation. The technical task of the invention is to improve the design of the ion-optical system (IOS), improve the quality of the exciting field and increase the resolution of quadrupole mass analyzers with resonant ion output. Known devices of this type are linear ion traps with hyperbolic or cylindrical electrodes with dipole excitation of ions [2]. Commercial instruments with analyzers of this type are widely used for routine analyzes with a resolution of up to several thousand. Their analytical capabilities are limited by the geometry and design features of the IOS of quadrupole analyzers with hyperbolic or round electrodes [1]. The proposed method and device, implemented using IOS with planar discrete electrodes [3], can improve the design, analytical and commercial parameters of mass spectrometers of this class.

A new method of mass analysis with resonant excitation of ions consists in acting on charged particles oscillating with limited amplitudes along the XYZ axes in a superposition of the high-frequency and static electric field-delaying X-axis quadrupole in the XOY plane in an analyzer with a geometric parameter r _o that excites along the axis X by an alternating electric field, characterized in that the exciting field is uniform and directed along one of the asymptotes of the quadrupole field.

In known quadrupole analyzers (Fig. 1, a), the distribution of the RF potential is described by the expression

where U _RF = V⋅cos (ωt), V and ω are the amplitude and frequency of the RF supply voltage, r ₀ is the geometric parameter of the quadrupole analyzer. Oscillations of ions along the X and Y axes in the RF field (1) are described by differential equations [3]

where q = 2eV / (r ₀ ² ω ² m) is the Mathieu parameter, m is the mass of ions. For q <0.3, the solution of differential equations (2) are described by harmonic functions

- секулярная частота колебания ионов.where x ₀ , y ₀ , ν _0x , ν _0y are the initial coordinates and ion velocities along the X and Y axes,

- secular frequency of ion vibrations.

During dipole excitation in known quadrupole analyzers, an exciting alternating electric field with a frequency of Ω _{in is} applied to the RF field (1), which can only be considered homogeneous near the X axis (Fig. 1, b). If the frequencies Ω are equal _in ≈Ω _s under the action of the exciting field, the amplitude of ion vibrations along the X axis increases and they are output through the slots in the electrodes located on the X axis to the recording device. Inhomogeneity of the exciting field and deviations near the gaps of the quadrupole rf field in existing mass analyzers with resonant ion extraction limit their resolution and sensitivity. In addition, the ion vibrations along the X axis along with the secular components (3) contain components with frequencies nω ± Ω, where n = 1, 2, 3 ..., whose amplitudes increase with increasing parameter q. The presence of these components of oscillations of large-level ions distorts the mass scale and reduces the resolution of the analyzers.

The proposed method involves the resonant excitation of ions in a linear ion trap by an alternating electric field uniform along the X axis, formed by a pair of planar discrete electrodes with exciting potentials U ₁ = -U ₂ (Fig. 1, d). The RF potential distribution in the IOS in FIG. 1, in the coordinate system x ', y' are described by the expression

, x_а, y_а - размеры ИОС с планарными дискретными электродами по осям X' и Y'. Электрические поля в квадрупольной ИОС Фиг. 1, а и в ИОС с планарными дискретными электродами Фиг. 1, в совпадают при повороте системы координат X'OY' относительно системы XOY на угол π/4.Where

, x _a , y _a - the sizes of the IOS with planar discrete electrodes along the axes X 'and Y'. Electric fields in quadrupole IOS FIG. 1a and in the IOS with planar discrete electrodes of FIG. 1c coincide when the X'OY 'coordinate system is rotated relative to the XOY system by an angle π / 4.

The rotation of the coordinate system does not affect the secular ion vibrations described by expressions (3), but changes the ratio of the amplitudes of the higher harmonics to frequencies nω ± Ω - the amplitudes increase along one axis and decrease along the other. In the case of resonant ion extraction, higher harmonics distort the shape of the mass peaks and reduce the resolution of the analyzer. To achieve high resolution, the level of higher harmonics of ion vibrations must be reduced. This is realized in planar IOS (Fig. 1, c, d), where the level of higher harmonics along the X 'axis is an order of magnitude lower than along the X axis in the quadrupole IOS (Fig. 1, a, b).

Thus, IOS with planar discrete electrodes solves two important problems for linear ion traps with resonant excitation:

- increases the uniformity of the exciting field;

- reduces the level of higher harmonics of ion vibrations along the excitation axis.

This allows a 2-3-fold increase in the resolution of mass analyzers of ions of this class.

Mass analysis method with resonant excitation of ions and a device for its implementation

FIG. 1 a, b - quadrupole ion-optical system; a - circuit RF power and distribution of the RF field; b - power scheme and distribution of the exciting field; c, d - ion-optical system with planar discrete electrodes 1, 2; c is the distribution of the rf field, g is the distribution of the exciting field.

Literature

1. D.J. Douglas, N.V. Konenkov. Mass selectivity of dipolar resonant excitation in linear quadrupole ion trap // Rapid Communications in Mass Spectrometry. 2014. V. 28. P. 430-438.

2. Collings B.A., Stott W.R., Londry F.A. Resonant excitation in low-pressure linear ion trap // J. Am. Soc. Mass Spectrom. 2003. Vol. 14. P. 522-534.

3. Mammoths E.B. A method of forming a two-dimensional linear high-frequency electric field and a device for its implementation. RF patent No. 2497226. 2012 year

Mass analysis method with resonant excitation of ions and a device for its implementation

FIG. 1 a, b - quadrupole ion-optical system; a - circuit RF power and distribution of the RF field; b - power scheme and distribution of the exciting field; c, d - ion-optical system with planar discrete electrodes 1, 2; c is the distribution of the rf field, g is the distribution of the exciting field.

Claims (2)

1. The method of mass analysis with resonant excitation of ions, which consists in acting on charged particles oscillating with limited amplitudes in a superposition of a quadrupole in the XOY plane high-frequency and static analyzer-delaying electric fields along the Z axis with a geometric parameter r ₀ of the alternating electric excitation along the X axis field, characterized in that the exciting field is uniform and directed along one of the asymptotes of the quadrupole field. 2. Device for mass analysis with resonant excitation of ions, containing an ion-optical system for forming in the XOY plane a superposition of high-frequency quadrupole and alternating exciting electric fields, characterized in that the ion-optical system consists of 2 in the planes x = ± x _a , discrete along the Y axis with a step Δy with dimensions 2y _a >> 2x _a , 2z _a >> 2x _a along the X and Z axes, electrodes with antiphase superpositions of discrete-linear and discrete-uniform distributions of high-frequency and variable exciting potentials on them, about generating in the working region of the ion-optical system | x | <(x _a -Δy), | y | <(y _a -x _a ), | z | <(z _a -x _a ) is a superposition of the high-frequency and homogeneous quadrupole in the CW plane along the X axis of the alternating exciting electric fields, and in the middle of the electrodes, parallel to the Z axis, slits are cut with a width of Δx <Δy and a length of 2Δz> [2 (z _a -x _a )] for resonant output of ions from the analyzer to the recording device.

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