RU2447539C2 - Time-of-flight quadrupole mass-spectrometre analyser (mass filter, monopole and tripole type) - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: time-of-flight quadrupole mass-spectrometre analyser (mass filter, monopole, tripole type) relates to mass-spectrometry and can e used in designing high-resolution and high-sensitivity time-of-flight quadrupole mass-spectrometres. The analyser has a device for sorting ions according to specific charge, a device for inputting the analysed ions into the sorting device and a device for outputting the sorted ions from the sorting device. The input and output devices are in form of double-electrode quadrupole cells which, along with a quadrupolar field, can generate a longitudinal (along axis z) uniform electric field, the electrodes of which are electrically insulated from each other, where the working surfaces of the electrodes of the quadrupole cells are defined within the operating volume of the cell by certain ratios.

EFFECT: increased sensitivity and resolution of the analyser.

5 cl, 4 dwg

Description

The invention relates to the field of mass spectrometry and can be used to create quadrupole mass spectrometers of the span type with high resolution and sensitivity.

Known devices for analyzers of quadrupole mass spectrometers of the span type (monopole, tripole and mass filter) [1], which consist of three systems: a system for inputting the analyzed ions into the analyzer, a system for sorting ions by specific charges, and a system for outputting sorted ions from the analyzer to the measuring device . The sorting system contains a set of electrodes that create a quadrupole field in the working volume of the sorting system. Moreover, the accuracy of this field must be maintained at a very high level (for this, the electrodes must be hyperbolic and located in space with high accuracy). The ion input system and the output system in analogues are flat diaphragms with a small hole located on the axis (or near the axis) of the analyzer electrode system having a potential equal to zero. A disadvantage of the known device is the appearance of a distorted quadrupole field in the region of input of ions into the electrode system of the analyzer and in the region of output (the so-called transition fields). A significant uncontrolled change in the parameters of the ion flux introduced into the analyzer takes place in these fields: an increase in the spread of the flux ions along the transverse and longitudinal velocities, phase dependences of the flow parameters, etc. This leads to a significant decrease in the sensitivity of the mass spectrometer (up to blocking the ion current at the exit analyzer) and a significant decrease in the resolution of the device.

A device of analyzers of quadrupole mass spectrometers of the span type (for example, a mass filter) is known [2], in which, to reduce the influence of the transition region on the parameters of the device, the ion input system is made in the form of a flat diaphragm, inserted directly into the electrode system of the mass filter, while potential equal to zero. With this design of the input system, the length of the transition region is slightly reduced, which reduces the spread of the introduced ions at transverse and longitudinal velocities and allows us to slightly improve the parameters of quadrupole mass spectrometers.

However, although partially in the known device it is possible to reduce the influence of the transition region on the passage of the ion flux through the analyzer, it is still not possible to minimize this effect, because the length of the transition region in the prototype decreases, but only to a certain limit, corresponding to an infinitely small distance between the plane of the diaphragm and the electrodes of the analyzer.

The aim of the invention is to increase the resolution and sensitivity of span-type quadrupole mass spectrometers by significantly reducing the effect of transition regions in the mass filter electrode system (monopole and tripole) on instrument parameters.

This goal is achieved by the fact that in the analyzer of a quadrupole mass spectrometer (such as a mass filter, "monopol", "tripole") containing a device for sorting ions by specific charges, made in the form of four hyperbolic electrodes for the mass filter and "tripole" and two hyperbolic electrodes for a monopole, a device for inputting analyzed ions into a sorting device, and a device for outputting sorted ions from a sorting device, an input device and an output device are two-electrode quadrupole cells, sp sobnye along with quadrupole field to create a longitudinal (along the z-axis direction) uniform electric field. The electrodes of the cells are electrically isolated from each other, and the working surfaces of the electrodes of these quadrupole cells are described (within the working volume of the cell) by the relations:

where y ₀ is the "field radius" of the hyperbolic electrodes creating a quadrupole field in the working volume of the sorting device; l is the length of the two-electrode quadrupole cell in the direction of the z axis; K ₀ is the geometric parameter of the electrode system of the quadrupole cell, specified during design. In this case, the electrode (1) is electrically connected to one hyperbolic electrode of the sorting device, and the electrode (2) has a potential equal to zero, and near the point y ~ 0, x ~ 0, z ~ 0, a hole is made in this electrode for input and output of ions .

The proposed input device of the analyzed ions for the mass filter can consist of four two-electrode quadrupole cells with a uniform longitudinal electric field, and all the electrodes of the cells described by relation (2) are electrically connected together, and the electrodes described by relation (1) are electrically connected to one of the four electrodes of the electrode system of the mass filter.

For a tripole, the input device of the analyzed ions can consist of three electrode quadrupole cells. In this case, all three electrodes of the cells described by relation (2) are electrically connected together with the corner electrode of the electrode system of the Tripole analyzer, and the three electrodes described by relation (1) are electrically connected each with one of the three electrodes of the tripole sorting device .

In addition, for the mass filter and “tripole”, all four (for the mass filter) and three (for “tripole”) electrodes described by relation (2) can be made as a single monoblock, and the electrodes described by relation (1), each is mechanically bonded to the corresponding electrode of the “mass filter” and “tripole” sorting devices. Moreover, for the input device of the analyzed ions and for the output device of the sorted ions, the values of the coefficient K ₀ can be different.

The use of two-electrode quadrupole cells with a longitudinal field as elements of input and output systems of sorted ions in quadrupole mass spectrometers of the span type allows one to practically solve the problem of the transition field in such devices. In this case, the sensitivity and resolution of the analyzers become (to within units of%) corresponding to the input and output of ions in an ideal quadrupole field, which is the most important factor (after the accuracy of the rod system of the analyzer), opening up the possibility of obtaining theoretically justified values of sensitivity and resolution in such devices .

Structurally, a two-electrode quadrupole cell with a longitudinal field is part of a quadrupole electrode system in which in the plane perpendicular to the longitudinal axis z, the field is quadrupole, and in the longitudinal (along the z axis) direction, it is constant, independent of the z coordinate.

The potential distribution in such a cell is described by the relation

Так определяется геометрический параметр квадрупольной ячейки.Here U ₍₁₎ is the potential at the electrode described by relation (1) (see the claims). Relations (1) and (2) were obtained after substituting in (3) the values U (x, y, z) = 0 or U ₍₁₎ . In the z = 0 plane, the profile of the cell’s electrode 1 coincides with the profile of the hyperbolic electrode of the mass filter analyzer (either a “monopole” or a “tripole”). With increasing z ≠ 0 (1) describes a hyperbola. The coordinate of the intersection point of this hyperbola (x = 0) with the yy axis ₀₀ increases with increasing z and the hyperbola rises, as it were, along the y axis. For z = l we have

This determines the geometric parameter of the quadrupole cell.

For z = 0, relation (2) describes the profile of the corner (x = y), which coincides with the asymptotes of the quadrupole field. For z> 0 (i.e., as z increases) (2) again describes a hyperbola inscribed in a corner electrode and in the same way as an electrode (1) rising up along the y axis. Thus, the electrodes of the proposed quadrupole cell are hyperbolas, which leads to the fact that in the entire space between the two electrodes the field is quadrupole, coinciding in parameters with the field between the electrodes of the mass filter (monopole or tripole). This is the reason why the quadrupole cell practically does not create a transition field at the input of the mass filter electrode system. The analyzed ion flux is introduced into the analyzer and removed from the mass filter analyzer through small holes made near the point y≅0, x≅0, z≅0 of electrode 2, i.e. at the top of the quadrupole cell.

On figa shows a cross section in the y-z plane, we offer two-electrode quadrupole cells with a longitudinal field. The internal profile of the electrode 1 in FIG. 1a is described by the relation (1) (see the claims), and the internal profile of the electrode 2 in FIG. 1a is described by the ratio (2). The same figure shows spatial images of the proposed quadrupole cell: 1 - electrode, the inner profile of which is described by the relation (1); 2 - electrode, the internal profile of which is described by the relation (2); A - hole for input (output) of charged particles; B is the part of the hyperbolic electrode from which the electrode 1 is joined (connected); C - corner electrode (in the case of a "monopole"), from which electrode 2 is connected (connected).

Figures 2a and 2b show sections in the yz plane of the electrode system of the mass filter analyzer with an input system in the form of a conventional diaphragm pushed into the electrode system of the analyzer (Fig. 2a) (prototype) and an ion source and electrode system of the mass filter analyzer with our proposed two-electrode quadrupole cells with a longitudinal field and an ion source (D) (fig.2b).

On figa shows a General diagram of the stability of the mass filter during pulse power supply (high-frequency pulse voltage, the period consists of two opposite-polarity pulses of rectangular shape).

Figure 3b shows the region of the general stability diagram near the point of intersection of the two boundaries of the stability zones along x and y coordinates. The position of the ion working point (F) is shown, for which comparative calculations of the ion retention efficiency in the mass filter analyzer were carried out with the input system characteristic of the prototype and our input system using a two-electrode quadrupole cell with a longitudinal field.

The values of a ₁ and a ₂ (see figure 3) are determined by the relations

U ₁ and U ₂ are the amplitudes of bipolar pulses (B);

T ₀ - the period of the RF voltage (sec);

y ₀ - "field radius" of the electrode system of the mass filter;

e and m are the charge (C) and the mass (kg) of the ion;

β _0x and β _0y are the stability parameters of the solution of the Hill equation corresponding to two coordinate axes. The working line (E) passes through point (F).

Figure 4 illustrates the phase planes with ellipses of capture of ions in the analyzer field marked on them at a phase equal to 0.75 (Fig. B, c and d), and the input region on the phase plane corresponding to 100% transmission. Figure 4 also shows the region of the location of the phase points of the ions (blackened areas) at the beginning of the ion input process (Figure 4a) and after passing 8.75 periods of the RF field (Figure 4b, 4c and 4d).

The comparison of the ion capture efficiency in the analyzer field when they are introduced into the mass filter analyzer was carried out as follows. During one period of the RF field, 5 · 10 ³ ions with small initial coordinates (5 thousand ions are distributed uniformly in the initial coordinate range ± 0.011 relative units) were “introduced” into the analyzer with a small transverse velocity determined by the energy ≈0.02 eV and by input phases during one period T _{0 in} increments of 10 ^-2 T ₀ . The total "longitudinal" energy is 2.4 eV. The phase points of all ions were thus located at the time of entry into the region of 100% transmission of the mass filter located on the phase plane — the solid black line in Fig. 4a. The trajectories of all ions were calculated on a computer until the ions went beyond the transition region, i.e. fall into a perfect quadrupole field. This required 8.75 periods of the HF field. At a phase 8.75 phase, the phase points of all ions were indicated on the phase plane together with the corresponding capture ellipse, characteristic of an ideal quadrupole field. Since when entering the phase points of the ions were located in the 100% transmission zone, in the ideal case (there is no influence of field distortions in the transition region on the ion paths), the phase points of all ions should be located in the corresponding capture ellipse after 8.75 RF periods. This case is illustrated in fig.4b. In this case, the coefficient K ₀ (see (1), (2) and (3)) is 1. It is seen that after passing the transition region of the mass filter, all ions remained in the capture ellipse, which means that they will not be lost in the future and reach the output device of the analyzer.

On figs illustrates a case characteristic of the prototype (see figa). We see that, after passing through the transition field, the phase points of all ions went beyond the capture ellipse. This means that none of the ions introduced into the field reaches the output device (this is the cutoff mode of the output current and the sensitivity of the device is zero). On fig.4d illustrates the case of using our proposed two-electrode quadrupole cell with a longitudinal field as an input device. Only 8% of the entered number of ions were lost. This is a proof of the advantage of using our proposed two-electrode quadrupole cell with a longitudinal field as an input device for the mass filter. The field of the transition region and ion trajectories were calculated using the SIMION 8.0 program. The parameters of the working point of the ions on the stability diagram: a ₁ = 3.6377349, and ₂ = 2.9254; peak-to-peak voltage 500 V; resolution, determined by the points of intersection of the working line (line E in figa and 3b) with the boundaries of the stability zones - 400; K ₀ = 1 for FIG. 4b, K ₀ = 0 (aperture) for FIG. 4c and K ₀ = 0.9 for FIG. 4d.

Thus, numerical modeling showed that the proposed device of span-type quadrupole mass spectrometers (monopole, tripole, and mass filter) can radically reduce the influence of the transition region of analyzers on instrument parameters, which opens up prospects for a significant increase in their sensitivity and resolution.

Bibliographic data

1. Paul W., Reinchard H.P., von Zahn U. Das elektrische Massenfilter als Massenspectrometer und Isotopentrener // Z. fur Physik. 1958. No. 152. S.143-182.

2. Dubkov M.V. Study of the features of the quadrupole mass filter and the development of analyzers with thin-walled hyperbolic electrodes. Diss ... cand. tech. Science. Ryazan, 1997.223 s.

Claims (5)

1. An analyzer of a quadrupole span mass spectrometer (such as a mass filter, "monopol", "tripole") containing a device for sorting ions by specific charges, made in the form of four hyperbolic electrodes for the mass filter and "tripole" and two hyperbolic electrodes for the monopole, a device for inputting analyzed ions into a sorting device and a device for outputting sorted ions from a sorting device, characterized in that the input device and the output device are made in the form of two-electrode quadrupole cells, along with a quadrupole field, create a longitudinal (along the z axis) uniform electric field, the electrodes of which are electrically isolated from each other, while the working surfaces of the electrodes of the quadrupole cells are described within the cell’s working volume by the relations:

where y ₀ is the “field radius” of the hyperbolic electrodes of the sorting device, l is the length of the two-electrode quadrupole cell with a longitudinal field, K ₀ is the geometric parameter of the electrode system of the quadrupole cell with a longitudinal uniform field, specified during construction, while the electrode described by the relation (1) is electrically connected to one hyperbolic electrode of the sorting device, and the electrode described by relation (2) is grounded and an opening for input and output of ions is made near the point y ~ 0, x ~ 0, z ~ 0. 2. The analyzer of a passing quadrupole mass spectrometer (such as a mass filter) according to claim 1, characterized in that the input device for the analyzed ions contains four two-electrode quadrupole cells with a uniform longitudinal electric field, all cell electrodes described by relation (2) are electrically connected together, and the electrodes described by relation (1) are each electrically connected to one of the four electrodes of the mass filter electrode system. 3. The analyzer of a quadrupole span mass spectrometer (tripol type) according to claim 1, characterized in that the input device for the analyzed ions contains three electrode quadrupole cells with a uniform longitudinal electric field, while all three cell electrodes described by relation (2) are electrically connected together with the corner electrode of the electrode system of the Tripole analyzer, and the three electrodes described by relation (1) are each electrically connected to one of the three electrodes of the Tripoli sorting device. 4. The analyzer of a quadrupole span mass spectrometer (such as a mass filter and a tripole) according to claim 1, 2, or 3, characterized in that all four (for a mass filter) and three (for a tripole) electrodes described relation (2), made in the form of a single monoblock, and the electrodes described by relation (1) are mechanically fastened each with the corresponding electrode of analyzers of sorting devices "mass filter" and "tripole". 5. The analyzer of a flying quadrupole mass spectrometer (such as a mass filter, "monopol", "tripole") according to claim 1, characterized in that the values of the coefficient K ₀ for the input device of the analyzed ions and for the output device of the sorted ions are different.

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