RU2740604C1 - Method for mass analysis of ions in quadrupole fields with excitation of oscillations on boundaries of stability - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to mass analysis of matter in quadrupole HF fields and is aimed at improving analytical and commercial characteristics of mass spectrometers with resonance output of ions. Method of analyzing ions in quadrupole fields with oscillation excitation at the stability boundary, involving action on charged particles oscillating in a superposition of a quadrupole in the X0Y plane and analyzer with geometrical parameters r₀ exciting on axis Ζ fields exciting field by X axis. Exciting field consisting of sum of quadrupole quasistatic and homogeneous static fields is applied to quadrupole high-frequency field, wherein excitation of oscillations, consisting in unlimited growth of coordinate x of ions, occurs as a result of slow increase in constant component of quadrupole field, when the parameter a(m) of ions with mass m at time t(m) reaches the boundary value a₀(m) and ions in turn in order of decreasing mass are output from the analyzer for registration.

EFFECT: high resolution and sensitivity of quadrupole mass analyzers, improved high-frequency power and sweep devices.

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Description

The invention relates to the field of mass analysis of matter in quadrupole high-frequency (HF) fields and is aimed at improving the analytical and commercial characteristics of mass spectrometers with resonant excitation of ion oscillations. The technical problem of the present invention is to improve the resolution and sensitivity of quadrupole mass analyzers, improve their RF power supply and mass sweep devices. The known devices of this type are linear ion traps with hyperbolic [1] or planar discrete [2] electrodes. The resolving power of mass analyzers of this type is limited by the complex form of excited oscillations of ions containing many higher harmonics, and the rate of increase of oscillations during excitation is low [3].

The proposed method makes it possible to improve the shape and parameters of the excitation function and thereby increase the resolution of this type of quadrupole mass analyzers, as well as improve the system of its high-frequency RF power supply and mass sweep.

In known quadrupole mass analyzers with resonant excitation of ion oscillations, a uniform (or nearly uniform) exciting harmonic field is superimposed on the quadrupole HF field. In this case, the equation of ion motion along the excitation axis has the form

- параметр Матье, V и ω - ампли туда и частота ВЧ питающего напряжения, U_в и Ω_в - амплитуда и частота возбуждающего напряжения. Решение уравнения (1), описывающее колебания ионов, состоит из множества гармонических составляющих с частотами nω ± Ω_c, где n=0, 1, 2, …, Ω_c=βω/2 - секулярное частота, β - параметр устойчивости, зависящий от параметра Матье q. В процессе развертки масс при изменении амплитуды ВЧ напряжения по линейному закону V(t) = υt, υ - скорость развертки, секулярная частота колебаний ионов

также линейно нарастает. С приближением частоты Ω_c к частоте возбуждающего поле амплитуда x_m колебаний ионов но оси X возрастает. Процесс нарастания описывается функцией возбуждения [3]. Когда амплитуда x_m достигает размера r₀, ионы выводятся из анализатора на регистрацию. Метод ограничен большим временем нарастания функции возбуждения

где T - время разве ртки, ΔΩ_c - диапазон сканирования секулярной частоты, а также ее сложным, колебательным характером. При этом шкала масс анализатора оказывается нерегулярной и многозначной, ч то снижает разрешающую способность.where e and m are the charge and mass of both ions, r ₀ is the geometric parameter of the quadrupole analyzer,

is the Mathieu parameter, V and ω are the amplitude and frequency of the HF supply voltage, U _in and Ω _in are the amplitude and frequency of the exciting voltage. The solution of equation (1), which describes the oscillations of ions, consists of a set of harmonic components with frequencies nω ± Ω _c , where n = 0, 1, 2, ..., Ω _c = βω / 2 is the secular frequency, β is the stability parameter depending on the Mathieu parameter q. In the process of sweeping the masses with a change in the amplitude of the RF voltage according to the linear law V (t) = υt, υ is the sweep rate, the secular frequency of ion oscillations

also increases linearly. As the frequency Ω _c approaches the frequency of the exciting field, the amplitude x _m of ion oscillations along the X axis increases. The growth process is described by the excitation function [3]. When the amplitude x _m reaches the size r ₀ , the ions are removed from the analyzer for registration. The method is limited by a long rise time of the excitation function

where T is the sweep time, ΔΩ _s is the scanning range of the secular frequency, as well as its complex, oscillatory nature. In this case, the mass scale of the analyzer turns out to be irregular and multi-valued, which reduces the resolution.

The proposed method of mass analysis of ions consists in the action on charged particles that oscillate in a superposition of a quadrupole in the plane 0Υ and blocking along the-axis of the analyzer fields with a geometric parameter r ₀ , exciting the field along the X axis, differs in that an exciting field is superimposed on the quadrupole HF field. field consisting of the sum of quasi-static quadrupole and homogeneous static fields, and the excitation of oscillations, consisting in an unlimited increase in the x coordinate of ions, occurs during a slow increase in the constant component of the quadrupole field, when the parameter a (m) of ions with mass m at time t (m ) reaches the boundary value a ₀ (m) and the ions alternately in the order of decreasing mass are taken out of the analyzer for registration.

In this case, the excitation of ion oscillations occurs at the boundary a ₀ (q) of the Mathieu stability diagram (Fig. 1) under the action of a superposition of quasi-static quadrupole and homogeneous static fields

where U ₀ (t) is the constant component of the supply voltage of the quadrupole analyzer slowly varying during the mass sweep, U _in is the static excitation voltage. In this case, the equation of ion motion along the excitation axis x in this case in the first approximation using the effective potential distribution can be written in the form

- эффективный потенциал квадрупольного поля.Where

is the effective potential of the quadrupole field.

The solution to equation (3) in the stability region consists of three independent components

The first two components in (4) depend on the initial coordinates х ₀ and velocities υ _{0 of} ions, and the last one on the exciting field

During the mass sweep, the constant component of the supply voltage changes linearly

и секулярной частотыwhere υ _ρ = U _{0 max} / T is the sweep speed, U _{0 max} is the maximum value of the constant component, Τ is the sweep time. This will cause the Mathieu parameter to change

and secular frequency

In this case, the amplitude of the first two components in (4) will vary according to the law

and the amplitude of the third component according to the law

As we approach the stability boundary a ₀ (q), the secular frequency is Ω _c → 0 and ion oscillations are excited. The vibration components determined by the initial parameters change linearly upon excitation, and the component caused by the exciting field according to the quadratic law

The ion trajectory in the course of mass sweep and oscillation excitation is shown in Fig. 2. In this case, the resulting coordinate x (t) will be determined to a greater extent by the exciting field than by the parameters x ₀ and υ ₀ . As a result, the influence of the spread of the initial coordinates and ion velocities on the ion extraction time t (m) decreases, and the analyzer's resolution increases.

The proposed method of excitation of ion oscillations also simplifies the power supply system and the sweep of the mass analyzer, since during the sweep of the masses, its parameters, amplitude and frequency, remain constant. This helps to increase the amplitude and increase the stability of the RF voltage parameters. The mass sweep scheme is simplified, since the sweep is carried out by changing the constant component of the supply voltage.

Literature

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

2. E.V. Mamontov. Method for mass analysis with resonant excitation of ions and device for its implementation. Invention patent RU 2634614, 02/11/2017. Application No. 2016149722 dated 12.16.2016.

3. Mamontov V.V., Sudakov M.Yu., Dyatlov R.N. Free and forced oscillations of charged particles in inertial-nonstationary rapidly oscillating quadrupole electric fields. Radio engineering and electronics. 2020, volume 65, no. 2, p. 197-202.

Claims (1)

A method for analyzing ions in quadrupole fields with excitation of oscillations at the stability boundary, which consists in acting on charged particles oscillating in a superposition of a quadrupole in the X0Y plane and blocking the fields along the axis of an analyzer with geometric parameters r ₀ , exciting a field along the X axis, which differs in that that on the quadrupole high-frequency field, an exciting field is superimposed, consisting of the sum of the quasi-static quadrupole and homogeneous static fields, and the excitation of oscillations, consisting in an unlimited increase in the x coordinate of ions, occurs as a result of a slow increase in the constant component of the quadrupole field when the parameter a (m) of ions with mass m at the moment of time t (m) reaches the boundary value a ₀ (m) and the ions, in order of decreasing mass, are taken out of the analyzer for registration.

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