RU2669390C2 - Method of mass-separation of ions in quadruple mass filter - Google Patents

Abstract

FIELD: physics.SUBSTANCE: invention relates to the quadrupole mass spectrometry technique and can be used for ion analysis mass spectra with high resolution and high transmittance. It is proposed to use a dipole excitation, which is supplied mainly to Y (negative) electrodes with a frequency of 0.5…10 kHz to excite the instability bands along the isolines β=0.005–0.1, where βis determined by the ratio of frequencies β=2ω/Ω.EFFECT: invention made it possible to increase the isotopic sensitivity, as well as the resolution by about two times without significant loss of the transmission coefficient, and to improve the shape of the mass peak of the quadrupole mass filter.2 cl, 6 dwg

Description

The invention relates to techniques for quadrupole mass spectrometry and can be used for mass analysis of ions with high resolution and high transmittance.

Known / 1, 2. 3 / method of separation of ions in a quadrupole mass filter, which is four parallel and symmetrically arranged monopoles (electrodes of cylindrical section). A specific combination of constant and high-frequency voltage is applied to the electrodes in pairs in opposite polarity - (U-V cosΩt) and + (U-V cosΩt), where U is the constant voltage, V cosΩt is the radio frequency component.

In this case, the operation of the quadrupole mass filter is carried out in the stability region on the a , q plane of the parameters of the Mathieu equation (Fig. 1), which describes the movement of ions along x and y of the transverse coordinates of the analyzer by equations / 1-3 /:

Here U and V are the constant voltage and the amplitude of the alternating voltage, m and e are the ion mass and charge, Ω is the angular frequency of the RF voltage, r ₀ is the radius of the inscribed circle between the vertices of the electrodes, ξ ₀ is the initial phase. Adjustment for a given resolution is carried out by changing the slope line a = 2λq to the axis a (Fig. 1). When the scan line approaches the top of the stability diagram, the resolution R = q ₀ / Δq increases with decreasing analyzer transmission. Small angle between the line a = 2λq remediation and y and _y boundary (q) leads to broadening of the mass peaks by small masses. The boundaries of the first stability region are blurred at a finite residence time of ions in the rf field. This additionally increases the broadening of the mass peak from the side of small masses, which leads to a loss of resolution and isotopic sensitivity. The resolution values R and the parameter λ are related by the relation λ = U / V = 0.167843-0.1263 / R [1].

Known / 3-9 / are methods of mass separation of charged particles, namely, that the particles are directed to a quadrupole mass filter, a constant and high-frequency voltage are applied to the mass filter electrodes, the particles are analyzed by mass and recorded at the exit of the quad-polar mass filter characterized in that. in order to increase the resolution and transmittance of the quadrupole mass filter, the amplitudes of the constant and high-frequency voltages are additionally modulated with a periodic signal.

G. Devant / 3 / found that the addition of additional quadrupole RF voltage leads to an improvement in peak shape. M. Κοzο patented the separation regime in the upper stability island formed by two lines of instability / 4, 5 / Later N. Konenkov and M. Sudakov and others first experimentally / 6 / and theoretically / 7 / determined the positions of stability islands on the a , q plane and conditions for their formation / 8, 9 / (Fig. 2).

The idea of the invention is to use an additional dipole alternating voltage to increase the resolution and isotopic sensitivity by “cutting” part of the peak at low resolution by the instability lines along two iso-β _x and iso-β _y lines near the stability boundaries.

The closest is the method / 6 / in which voltage ± (U + V cosΩt), where Ω is the angular frequency and the additional quadrupole voltage of a small value V _q cos (ωt + α) are applied in pairs to 4 parallel electrodes in pairs.

This technical solution defines the ratio of voltages and frequencies to obtain the most optimal parameters of the quadrupole mass filter.

However, it turned out that the operation of the quadrupole mass filter with an additional quadrupole voltage even in the most optimal ratio of parameters can be improved due to the technical solution proposed below.

The technical task of the invention is to increase the isotopic sensitivity, resolution and improve the shape of the mass peak of the quadrupole mass filter.

Offered

1. The method of mass separation of ions in a quadrupole mass filter, which consists in the fact that 4 parallel electrodes are supplied in pairs to opposite pairs of electrodes with voltage ± (U + V cosΩt), where Ω is the angular frequency and additional quadrupole voltage of small magnitude, characterized in that that a resonant voltage of a small value of the form V _y -cos (ω _y t + α _y ) with a frequency ω _y (for parametric excitation of ion vibrations) is applied only to Y electrodes (electrodes with negative potential -U) to excite instability bands along isolines β _y = 0.005-0.1, g e β _y determined frequency ratio β _y = 2ω / Ω, (while the X electrodes with positive potential U resonance excitation voltage is fed to an amplitude equal to zero).

2. The method of mass separation of ions in a quadrupole mass filter, which consists in the fact that 4 parallel electrodes are supplied in pairs to opposite pairs of electrodes with voltage ± (U + V cosΩt), where Ω is the angular frequency and an additional quadrupole voltage of small magnitude, characterized in that that voltage is supplied only to X electrodes (electrodes with positive potential U) to excite instability bands along isolines β _x = 0.95-0.999, (and Y electrodes with negative potential -U supply a resonant excitation voltage with an amplitude equal to th zero).

Dipole resonance excitation of ion vibrations is carried out by applying an additional RF voltage V _y cos (ω _y t + α _y ) to a pair of Y electrodes, as shown in FIG. 3. (Y - electrodes - a pair of electrodes to which a negative constant voltage -U is applied when analyzing positive ions). Similarly, you can apply a dipole voltage V _x cos (ω _x t + α _x ) to X - electrodes (with a constant voltage + U when analyzing positive ions). The resonance frequency coincides with the frequency of the fundamental time harmonic of ion vibrations ω = βΩ / 2.

The equations of motion of ions in the presence of dipole resonant excitation have the form [10]:

where V _x cos (ω _x + α _x ) and V _y cos (ω _y + α _y ) are dipolar voltages with small amplitudes V _x and V _y applied to opposite pairs of electrodes, A1≅0.8 is the amplitude of the dipole spatial harmonic of the field, α _x and α _y - phase stress.

The indicated useful properties of the proposed method of mass separation are illustrated by the following materials obtained on the basis of numerical simulation of the shape of the mass peak (transmission path). The contour T (q) was calculated using an ensemble of 2000 ion trajectories with initial conditions: a random Gaussian distribution of the initial coordinates and velocities with dispersions σx = σy = 0.01r ₀ and σν = σν = 6.4 * 10 ^-3 Ωr ₀ .

In FIG. Figures 4, 5, and 6 show the shape of the mass peak upon excitation of isolines βу = 0.01.0.02 and 0.001 for the indicated values of the excitation parameter qy and λ. The largest peak (black dots) corresponds to the case when excitation is absent (q _y = 0 and q _x = 0). With increasing excitation parameter q _y , that is, the amplitude of the additional voltage V _y (formula 6), the peak width decreases and the amplitude of the peak decreases slightly. In this case, a sharp boundary is observed on the left side of the peak, i.e., the isotopic sensitivity increases. The right side of the peak does not change (there is no excitation along the βx line). At this value, λ = 0.1666 and βy = 0.01 and 0.02, the peak width decreases approximately by half.

With increasing λ, the scan line a = 2λq approaches the top of the stability diagram and crosses the iso-β lines with lower values. Therefore, when programming the dependences λ (M), qy (M), and ω _y (Μ) when scanning over the mass range, it is necessary to reduce the value ω _у = β _y Ω / 2 with increasing mass number M. This is illustrated in FIG. 6 for λ = 0.1675. It is seen that an increase in resolution is possible from R _0.1 = 360 to 820 with an ion sorting time of n = 150 (ion flight time, expressed in the number of RF field periods). R _0.1 is the magnitude of the resolution determined by the 10% peak height level. Thus, working at λ = 0.1675, which corresponds to R _0.1 = 400 (Fig. 1), a good peak shape with a sharp left edge and a resolution of R _{0.1 is} achieved. The improvement in peak shape is characterized by a bell-shaped shape (in the limit, rectangular), which is illustrated by the peak shape in FIG. 6 (curve for q _y = 6 * 10 ^-5 ).

Calculations show that excitation of βx isolines near 0.999–0.980 is not effective; therefore, it is sufficient to remove the low mass tail by dipole excitation of the instability line along iso-β of _the lines. Let us estimate the required values of ω _y and qy for typical values of the mass filter parameters: frequency F of the RF generator F = 1 MHz, fields r ₀ = 5 mm, ion mass 800 Th. Using the formulas,

ω _y = β _y Ω / 2 and

на диапазон значений β_у=0.001-0.02 равными 500 Гц - 10 кГц, соответственно. Параметру q_y=0.0001 соответствует дипольное напряжение

we find the required frequencies

to the range of values β _y = 0.001-0.02 equal to 500 Hz - 10 kHz, respectively. The parameter q _y = 0.0001 corresponds to the dipole voltage

Another useful property of the proposed method for separation of ions by specific charges is the setting for the required resolution when scanning by mass by changing the amplitude of the dipole voltage V _y .

The specified method can be implemented as part of a quadrupole mass spectrometer. The composition of the mass spectrometer:

1. Source of ions

2. Quadrupole mass filter

3. The ion detector.

4. Power supply for supplying constant voltage.

5. The power supply unit of the radio frequency voltage.

6. The power supply unit of dipole voltage with a frequency of 0.5 ... 10 kHz.

7. Vacuum system with pumping and vacuum control.

8. A computer with software.

The application of the proposed technical solution allowed to improve the resolution of the quadrupole mass filter in the process of experimental testing not less than 2 times.

Captions for graphic material.

FIG. 1. The working top of the stability diagram. The lines βy = 0 and βx = 1 correspond to the Y and X boundaries of the stability region. The dashed lines show the contours of the characteristic parameters βу and βх along which instability bands are excited. The straight lines crossing the stability region are the scanning lines a = 2λq and each resolution λ corresponds to the indicated resolution R = q / Δq.

FIG. 2. / 6 /. Islands of stability, initiated by parametric quadrupole resonant excitation of ion vibrations. Island A is a working island of stability, where resolution can be rebuilt by changing the slope of the scan line a = 2λq. The operating frequency f = 2π / Ω = 2.25 MHz, ν = ω / Ω = 9/10, q 'is the dimensionless amplitude of the quadrupole voltage.

FIG. 3. The scheme for supplying the dipole voltage V _y cos (ω _y t + α _у ) through the RF transformer Tr to the Y electrodes of the quadrupole mass filter.

Figure 4. The contours T (q) of the transmission of the mass filter upon excitation of the instability band along the isoline βу = 0.01 at dimensionless amplitudes qy = 0, 0.00005, 0.0001, and 0.0002 for the scan parameter λ = a / 2q = U / V = 0.16659.

FIG. 5. The transmission contours T (q) of the mass filter upon excitation of the instability band along the isoline βу = 0.02 for dimensionless amplitudes qy = 0, 0.0001, and 0.0002 for the scan parameter λ = a / 2q = U / V = 0.1666. The indicated values of R _0.1 correspond to a 10% level of the transmission loop height.

FIG. 6. Transmission contours T (q) of the mass filter upon excitation of the instability band along the isoline βу = 0.001 for dimensionless amplitudes qy = 0 (without dipole excitation), 0.00004, 0.00005, 0.00006, and 0.00007 for the scan parameter λ = a / 2q = U / V = 0.1675. The indicated values of R _0.1 correspond to a 10% level of the transmission loop height.

Literature

1. Dawson, R.N. Quadrupole Mass Spectrometry and Its Applications // American Institute of Physics, Woodbury (NY) - 1995 .-- P. 13-15.

2. March, R.E .; Todd, J.F.J. Quadrupole Ion Trap Mass Spectrometry. Second edition. // A. John Wiley & Sons, Inc. - 2005 .-- P. 81.

3. Devant G. Patent FR 2,620,568. 1989.

4. Kozo M. US Patent 5,227,629. 1993.

5. Kozo, M.J. Development of Quadrupole Mass Spectrometers and Ion Optical Devices. // Mass Spectrom. Soc. Jpn. - 2009. - Vol. 57. No. 1. - p. 23-29.

6. Konenkov, N.V .; Cousins, L. M .; Baranov, V.I .; Sudakov, M.Yu. Quadrupole Mass Filter Operation with Auxiliary Quadrupole Excitation: Theory and Experiment // Int. J. Mass Spectrom. - 2001. - Vol. 208. - P. 17-27.

7. Konenkov N.V .; Sudakov M.U .; Douglas D.J. Matrixs Methods for the Stability Diagrams in Quadrupole MassSpectrometry // Int. J. Mass Spectrom. - 2001. - Vol. 13. - P. 597-613.

8. Konenkov N.V .; Korolkov A.N .; Machmudov M.M. Upper Stability Island of the Quadrupole Mass Filter with Amplitude Modulation of the Applied Voltages. // J. Am. Soc. Mass Spectrom. - 2005. - Vol. 16. - P. 379-387.

9. A.A. Baranov, E.Ya. Chernyak, A.H. Korolkov. Frequency and phase modulation of harmonic power of a quadrupole mass filter. Mass Spectrometry, 2007, 4 (1), p. 31-36.

in einem vierpolfeld. // Zeitschrift fur Physik. - 1959. - P. 1-26.10. Fischer. E. Die dreidimensionale stabilizationierung von

in einem vierpolfeld. // Zeitschrift fur Physik. - 1959. - P. 1-26.

Claims (2)

1. The method of mass separation of ions in a quadrupole mass filter, which consists in the fact that 4 parallel electrodes are supplied in pairs to opposite pairs of electrodes voltage ± (U + VcosΩt), where Ω is the angular frequency and additional quadrupole voltage of small magnitude, characterized in that A dipole voltage of the form V _y cos (ω _y t + α _y ) with a frequency ω _{y is} applied only to Y electrodes to excite instability bands along isolines β _y = 0-0.1, where β _y is determined by the frequency ratio β _y = 2ω _y / Ω. 2. The method of mass separation of ions in a quadrupole mass filter, which consists in the fact that 4 parallel electrodes supply voltage ± (U + VcosΩt) in pairs to opposite pairs of electrodes, where Ω is the angular frequency and additional quadrupole voltage, characterized in that the voltage is applied only on X electrodes with a frequency β _x = 2ω _x / Ω. for the excitation of instability bands along isolines β _x = 0.95-0.999.

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