RU2399985C1 - Method of analysing ions from specific charge in quadrupole time-of-flight mass spectormetres (monopole, tripole and mass filter) - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method of analysing ions from specific charge in quadrupole time-of-flight mass spectrometres (monopole, tripole and mass filter) involves input of analysed ions through an input channel into a mass spectrometre analyser, selective exposure of said ions to a high-frequency quadrupole field, thereby sorting the ions according to specific charge and output of the sorted ions through an output channel to a measurement device. The ions are sorted according to charge through selective phase and selective double spatial focusing of selected ions at the input of the output channel.

EFFECT: increase in resolution of quadrupole mass spectrometres by tens of times, increase in sensitivity by hundreds of times in high resolution mode, shorter length of the electrode system of the mass spectrometer, high speed of ions entering the analyser.

3 cl, 5 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.

A known method for the analysis of ions by specific charges in quadrupole mass spectrometers of the span type (monopole, tripole and mass filter) [1], by which the analyzed ions are introduced through the input channel into the analyzer of the mass spectrometer, selectively affect the ions with a high-frequency quadrupole field, sorting at this ions in specific charges, and output sorted ions through the output channel to the measuring device. In this case, the separation of ions by specific charge is carried out by arranging the working point of the analyzed ion on the general stability diagram near the boundaries of the stability zones along the x and y coordinate axes. If the working point of the ion is located inside the general stability zone, then this ion can be held in the analyzer volume and get into the measuring device. If the working point of the ion in at least one of the coordinate axes is in the unstable zone, then the coordinate of the ion along this axis will continuously increase with time, and the ion will be neutralized at one of the analyzer electrodes without reaching the output channel.

The disadvantage of this method is the fact that in order to obtain a high resolution mass spectrometer, it is necessary to position the ion operating point on the general stability diagram very close to the boundaries of the general stability zone, which leads to a sharp increase in the vibration amplitudes of the ions held in the analyzer volume and, as a result, corresponding decrease in sensitivity. In addition, the circumstances noted lead to an increase in the requirements for the accuracy of the manufacture of analyzer electrodes, the required stability of the supply high-frequency voltage, and the ratio of its amplitude to the constant component.

A known method for the analysis of ions by specific charges in quadrupole mass spectrometers [2, 3], in which the analyzed ions are introduced through the input channel into the analyzer of the mass spectrometer, selectively affect them with a high-frequency quadrupole field, sorting the ions by specific charges, and output sorted ions through the output channel to the measuring device. In the known method, it is possible to partially reduce the stability requirements for the ratio of the amplitude of the RF voltage to the DC component. According to this method, by introducing a corner electrode, only ions with a positive coordinate along one of the axes are left in the analyzer volume. This made it possible to move the working point of the ion on the general stability diagram along one coordinate axis deep into the stable zone, which radically reduced the amplitude of ion vibrations along this coordinate and increased sensitivity. At the same time, the required stability of constancy of the ratio of the amplitude of the RF field to the constant component decreased.

However, although partially using the known method, it was possible to eliminate the disadvantages of the analogue, when implementing the known method, a new disadvantage appeared. In an attempt to increase the degree of sorting by reducing the number of unstable ions by increasing the sorting time (the time of flight of the ions in the analyzer), the bandwidth near the boundary of the stability zone began to sharply decrease, in which the working points of the ions passing to the analyzer output could be located. This worsened the shape of the mass peak, decreasing the relative sensitivity of the device (the fronts of the mass peak sharply increased).

The aim of the invention is to increase the resolution and sensitivity of quadrupole mass spectrometers of the span type by eliminating the above disadvantages.

This goal is achieved by the fact that the analyzed ions are introduced through the input channel into the mass spectrometer analyzer, selectively act on them by a high-frequency quadrupole field, sorting the ions by specific charges, and the sorted ions are output through the output channel to the measuring device. In this case, the ions are sorted by specific charges due to selective phase and selective double spatial focusing of selected ions to the input of the output channel. Selective phase focusing of selected ions is carried out by positioning the working point of the analyzed ions inside the overall stability diagram of the analyzer on one of the quasistability lines (iso-β lines), for which β (stability parameter) is determined by the ratio:

where S is integers (S = 2, 3, ...), and the energy of the ions introduced into the analyzer is chosen such that the flight time of the selected ions between the output of the input channel and the input of the output channel is equal to an integer number of periods of the high-frequency field, a multiple of S. Selective double spatial focusing of the selected ions to the input of the output channel is carried out by combining the working point of the selected ions on the general stability diagram with the intersection point of the quasistability line corresponding to the y and x coordinate axes, while the values of the parameters their lines S _y and S _x are chosen to be multiples of each other and so that S _{y is} greater than or equal to S _x .

This analysis method allows you to drastically increase the sensitivity of the mass spectrometer, bringing up to almost 100% the collection rate of selected ions to the input of the output channel, regardless of the phase of entry of ions into the analyzer, while significantly increasing the resolution by using double focusing (as in x- coordinate, and y-coordinate) of selected ions to the input of the output channel. In addition, the resolution of the proposed method of analysis is increased due to time-of-flight sorting. An additional significant advantage of the proposed method of analysis is the ability to increase the angle of entry of ions into the analyzer and significantly reduce the time of flight of ions to the input of the output channel. The latter not only allows one to reduce the length of the analyzer, but also makes it possible to increase the longitudinal velocity of the ions introduced into the analyzer, which is important because it allows one to reduce the relative dispersion of the introduced ions along the longitudinal velocities and, accordingly, improve the parameters of the instruments. At the same time, an increase in the angle of entry of ions into the analyzer increases the rate of departure of unstable ions from the flow, which allows a high degree of sorting of ions.

Figure 1 shows a General stability diagram for a quadrupole mass filter with pulse power. The RF supply voltage is a “meander” with equal duration of rectangular pulses of different polarity. On the general stability diagram, operating points are noted for which the shape of the mass peaks shown in the following figures was determined by numerical simulation. The values of a ₁ and a ₂ (see figure 1) are determined by the relations:

U ₁ and U ₂ are the amplitudes of pulses of different polarity (V);

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

Y ₀₀ is the radius of the field of the electrode system (m);

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

β is the stability parameter of the solution of the corresponding Hill equation.

Figure 2 and 3 shows the mass peaks obtained by numerical simulation of the operation of the quadrupole mass filter according to the proposed method.

Figure 2 - quadrupole mass filter. The supply signal is a meander, the radius of the field is 6 mm, the length of the electrode system in the radii of the field is 22, the amplitude of the supply voltage is 200 V, the ions are introduced throughout the entire RF field in a parallel flow, the inlet and outlet openings with a diameter of 0.5 mm, the ion energy along z axis 2.78 eV, ion energy in the xy plane 0.1 eV, input angle in the yz plane α = 11 °.

Figure 3 - quadrupole mass filter. The supply signal is a meander without a constant component, the field radius is 6 mm, the length of the electrode system in the field radii is 33, the amplitude of the supply voltage is 200 V, the ions are introduced throughout the entire RF field in a parallel flow, the input and output holes with a diameter of 0.5 mm, energy ions along the z axis 15.6 eV, ion energy in the xy plane 5 eV, input angle in the yz plane α = 30 °.

- отношение выходного ионного тока к входному) от a₂. Здесь Δ - уровень определения разрешающей способности.In Fig.2 and 3 placed tables illustrating the features of the shape of the mass peaks. The mass peak is defined as the dependence of the transmission coefficient (I) in percent of ions through the electrode system (

- the ratio of the output ion current to input) from a ₂ . Here Δ is the level of determination of resolution.

Figure 4 shows the shape of the mass peak (point 3 in figure 1) obtained by numerically simulating the operation of a monopolar quadrupole mass spectrometer by the proposed method (with pulsed power).

Figure 4 - monopole. The supply signal is a meander, the slope of the working line is λ = 1.157725, the field radius is 6 mm, the length of the electrode system in the field radius is 11, the voltage span is 200 V, the ions are introduced throughout the entire RF field in a parallel flow. The inlet and outlet are 0.14 mm in diameter, the inlet is above the corner electrode, the ion energy along the z axis is 4.14 eV, the ion energy in the x-y plane is 0.15 eV, and the input angle in the y-z plane is α = 10.8 °.

There is also a table illustrating the features of the shape of the mass peak and the profile of the monopole electrode system.

Figure 5 shows the trajectories of ions in the xy plane in a tripole quadrupole span mass spectrometer constructed for the point a ₁ = 1.906662 a ₂ = 2.048044 for 12 initial phases uniformly distributed over the period of the rf field (point 4 figure 1). The range of the supply voltage is 200 V, the initial phase in fractions of the RF field period is 0.8, the initial coordinate of the ion along x and y is 0, the ion energy along the x axis is 8.9 eV, and the ion energy along the y axis is 0.575 eV.

. Точки c₀, и d₀ на фиг.2 есть точки пересечения «рабочей прямой» с границами общей зоны стабильности: c₀ - с y-границей (β_y=+1), a d₀ - с x-границей(β_x=-1). Через относительную величину расстояния между точками c₀ и d₀ обычно оценивается максимальное разрешение фильтра масс:

. В нашем случае ρ_m.0≅70. Внутри разрешаемого диапазона значений a₂(a_d0÷a_c0) найдена обозначенная цифрой 1 на фиг.1 точка, соответствующая предлагаемому способу с S_x=S_y=20 (при этом периоды низкочастотных колебаний иона по осям x и y равны: n_x=n_y=40). Другими словами, эта точка является точкой пересечения двух линий квазистабильности с параметром S=20. Из фиг.2 видно, что при реализации предлагаемого способа разрешающая способность, определенная на полувысоте пика, достигает 2000, что почти в 30 раз выше максимальной разрешающей способности (ρ_m.0≅70) «фильтра масс», определенной выше. При этом выбранному значению 2 соответствовала 100% трансмиссия ионов, т.е. проведенное выше сравнение по разрешению проведено практически в режиме 100% трансмиссии.The advantages of the proposed method can be illustrated in figure 2. The working point of the selected ions is indicated in figure 1 by the number 1. In figure 1, a "working line" with the tangent of the angle of inclination is drawn through this point

. The points c ₀ and d ₀ in FIG. 2 are the intersection points of the “working line” with the boundaries of the general stability zone: c ₀ - with the y-border (β _y = + 1), ad ₀ - with the x-border (β _x = -one). Through the relative distance between the points c ₀ and d ₀ , the maximum resolution of the mass filter is usually estimated:

. In our case, ρ _m.0 ≅70. Inside the allowed range of values of a ₂ (a _d0 ÷ a _c0 ), a point indicated by the number 1 in Fig. 1 is found corresponding to the proposed method with S _x = S _y = 20 (in this case, the periods of low-frequency oscillations of the ion along the x and y axes are: n _x = n _y = 40). In other words, this point is the intersection point of two lines of quasistability with parameter S = 20. Figure 2 shows that when implementing the proposed method, the resolution determined at half maximum of the peak reaches 2000, which is almost 30 times higher than the maximum resolution (ρ _m.0 ≅70) of the "mass filter" defined above. In this case, the selected value of 2 corresponded to 100% ion transmission, i.e. the above comparison by resolution was carried out practically in the 100% transmission mode.

A comparison can also be made at close resolution values. To do this, increase λ, which will lead to an increase in the resolution of the mass filter in the known mode. In this case, the working point of the selected ions will shift from the point of intersection of the quasistability lines with an equal value S = S _x = S _y , as a result of which the three-dimensional sorting mode is violated and the transmission decreases.

The described offset mode is illustrated in FIG. With the remaining parameters remaining constant, only the value of λ was increased to λ = 1.243. As a result, the resolved range d ₀ -c ₀ decreased to the level d ₀₂ -c ₀₂ , (corresponds to a resolution of ~ 150) and the sensitivity decreased by almost 100 times (see Fig. 2: the right scale axis refers to the shifted peak). This means that when implementing the proposed three-dimensional focusing mode with approximately equal resolution, it is possible to increase the transmission of the ion flux (respectively, sensitivity) by several hundred times.

The operation of the quadrupole mass spectrometers of the span type in the depth of the general stability zone according to the proposed method is illustrated in FIG. 3. Here, the working point of the analyzed ions (point 2 in FIG. 1) is on the working line with λ = 1 (i.e., the mode of absence of a “constant component” in the RF signal is realized). In this case, the ion working point is located deep in the general stable zone (far from the boundaries of the stability zones) near the central point of the zone with β _x = β _y = 0. The operating point chosen for calculating the mass peak is the intersection point of the quasistability lines corresponding to S _x = S _y = 3 (β _x = β _y = -0.5). According to the proposed method, a 100% transmission mode is implemented at this point with a very significant angle of input of the ion flux of 30 ° (although, at the point β _x = β _y = 0, it is possible to introduce the ion flux into the analyzer at α °45 °.) The stability zone, as shown by the theory of the method, allows you to pass significantly larger ion fluxes through the analyzer than when using the known method. This is explained by the fact that for operating points located deep in the stability zone, the amplitude of ion vibrations in the xy plane is the smallest (for example, at the point β _x = β _y = 0, the largest coordinate in the xy plane at which the ion introduced along the z axis is held field equal to 0.288 relative units (coordinate referred to the radius of the field), while at point 1 (see figure 2) this value is equal to 0.047). The latter means that the flow passed through the analyzer through the analyzer, the flow can be almost 40 times greater than by the known method.

The flight time of selected ions through the analyzer for the point in FIG. 3 is 24 T ₀ , while the low-frequency period of the movement of ions is 6 T ₀ . It is possible to increase the energy of introduced ions by 4 times (up to 62.4 V). In this case, the ions will fly through the analyzer for 12 T ₀ . In this case, the resolution at the level of 0.5 will decrease to 400 ÷ 500, while remaining quite high.

Thus, the proposed method allows using a quadrupole mass filter to analyze ion flows with a high longitudinal velocity.

The geometry of the electrode system of the monopole and “tripole” imposes special conditions on the passage of the analyzed ion flux along the axis of the analyzer. The presence of a corner electrode limits the passage of ions to the output of the analyzer, for which the half-cycle of low-frequency oscillations is less than the transit time along the analyzer. With the known method, this leads to the fact that the working points of the analyzed ions in the overall stability diagram are located in a narrow strip near the boundary of the stability zone along the y coordinate. On the one hand, this band is limited by the y-boundary of the stability zone, and on the other, by the iso-β line, for which the half-period of low-frequency oscillations is equal to the time of flight of the ions through the analyzer. With increasing flight time, the resolution of these devices increases. In this case, the width of the stability band decreases and the sensitivity of the mass spectrometer decreases, since the working point of the analyzed ions approaches the boundary of the stability zone; and the amplitude of ion vibrations increases. This is a disadvantage of the known method.

According to the proposed method, the mass peak of the monopole is formed near the operating point located far from the boundary of the stability zone at the intersection of the lines of quasistability characteristic of this regime. Figure 4 illustrates the shape of a high-resolution mass peak formed near the intersection of the line of quasistability along the x axis, for which the characteristic half-period of low-frequency oscillations is equal to 2 periods of the RF field (iso-β line with β _x = 0) with a line of quasistability along the y axis, for which the half-period of low-frequency oscillations is 20 periods (β _x = 0.987688). In figure 4, the dotted line to the left of the peak shows the coordinate a ₂ corresponding to (at a given value of λ) the boundary of the stability zone. This confirms the possibility of working on the proposed method in the depths of the General stability zone. It should be noted that, as in the case of a quadrupole mass filter, for a monopole, high resolution is realized at 100% transmission of the ion flux.

In a monopoly analyzer, the input channel for introducing ions and the output channel can be located both above and below the z axis. Since the ion flight time in this case should be either equal to or less than a half-period of low-frequency oscillations, in the full focus mode according to the proposed method, it is necessary that the receiving area be a mirror image of the input. For the mode illustrated in FIG. 4, it is assumed that the inlet is above the z axis and the outlet is below. In the "axis input" mode, it can be considered with a certain degree of accuracy that the initial coordinates of the ion introduced into the field are equal to zero. In this case, the conclusion should be carried out “through the axis”, i.e. when x = y _{O O} ≅0 (y _O x and _O - ion coordinates at a start of field).

5, in order to illustrate the proposed method, in one drawing, 12 trajectories of ions in the xy plane are shown, flying into the analyzer in different phases, uniformly distributed over the period of the RF field. The ion working point is located at the intersection of the lines of quasistability: in the x coordinate corresponding to β _x = 0.707107 (half-period of low-frequency oscillations of 4 periods of the RF field), and in the coordinate corresponding to β _y = 0.980785 (half-period of low-frequency oscillations of 16 periods) ( point 4 in figure 1). The trajectories are constructed for a tripole with the input of ions “through the axis”. The phase focusing of the ion flux is clearly visible even with a very complex ion path combined with double focusing along the x and y coordinates. This leads to high resolution and sensitivity of the proposed method.

Thus, numerical modeling shows that the proposed method of analysis allows you to:

- increase the resolution of span quadrupole mass spectrometers dozens of times at 100% transmission;

- with high resolution, increase the sensitivity of span quadrupole mass spectrometers hundreds of times;

- reduce the length of the electrode system of such mass spectrometers;

- increase the speed of ions introduced into the analyzer.

Information sources

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. Von Caan. New mass spectrometer with an electric field // PNI. 1963.V. 34. No. 12. C.1-4.

3. Ivanov V.V., Karnav TB, Dubkov M.V. Span-type hyperboloid mass spectrometers for space research // Promising projects and technologies. 2006. B.1. C.53-57.

Claims (3)

1. The method of analysis of ions by specific charges in span-type quadrupole mass spectrometers (monopole, tripole and mass filter), according to which the analyzed ions are introduced through the input channel into the mass spectrometer analyzer, selectively affect them with a high-frequency quadrupole field, sorting ions by specific charges, and sorted ions are output through an output channel to a measuring device, characterized in that the ions are sorted by specific charges due to selective phase and selective double anstvennoy focusing ions selected input channel to the output. 2. The method according to claim 1, characterized in that the selective phase focusing of the selected ions is carried out by placing the working point of the analyzed ions inside the overall stability diagram of the analyzer on one of the quasistability lines (iso-β-lines) for which β (stability parameter) is determined the ratio

where S is an integer (S = 2, 3, ...), and the energy of the ions introduced into the analyzer is chosen such that the flight time of the selected ions between the output of the input channel and the input of the output channel is an integer number of periods of the high-frequency field, a multiple of S. 3. The method according to claim 1 and claim 2, characterized in that the selective double spatial focusing of the selected ions to the input of the output channel is carried out by combining the working point of the selected ions on a common stability diagram with the intersection point of the quasistability lines corresponding to the y and x coordinate axes, wherein the parameter values of these lines S _y and S _x are chosen to be multiples of each other and so that S _{y is} greater than or equal to S _x .

Images