JP3707082B2 - Quadrupole mass spectrometer - Google Patents

Description

[0001]
[Industrial application fields]
The present invention separates target ions by passing only ions having a predetermined mass (actually, the value m / z obtained by dividing the mass number m by the charge z) or scans a predetermined mass range. The present invention relates to a quadrupole mass spectrometer that performs analysis.
[0002]
[Prior art]
The quadrupole mass filter (QMF) used in the quadrupole mass spectrometer has a pair of electrode rods in the X-axis direction and the Y-axis direction orthogonal to the ion traveling direction (this is the Z-axis). It is arranged parallel to the Z axis, and by applying a voltage in which a DC voltage and an AC voltage are superimposed between them, only target ions having a mass corresponding to the voltage are passed in the Z axis direction. Of ions with a mass of In order for the quadrupole mass filter to function correctly and with high sensitivity as such a filter, the incident range of ions and the direction of motion at a predetermined position on the incident side of the quadrupole mass filter are determined based on the quadrupole mass filter. It must be compatible with the acceptance of the filter.
[0003]
The acceptance indicates the particle acceptance characteristics of each quadrupole mass filter, and includes the spatial range of incident ions necessary to keep ions emitted from the exit of the quadrupole mass filter within a predetermined target range, and It is expressed in the range of movement direction. The acceptance is represented by ranges in the XX ′ plane and the YY ′ plane as shown in FIG. 8. For example, in the X-axis direction, the X coordinate of the incident ions falls within the range at a predetermined position on the incident side. In addition, when the incident direction (X ′) is within the range, the ions emitted from the exit of the quadrupole mass filter are within the target range. The same applies to the Y-axis direction. Regarding the shape of acceptance, the following can generally be said. For example, as shown in FIG. 7A, when a large number of ions are emitted from a certain point F, the coordinates X1 (or Y1) of the ion passage position in the XY plane with Z = 0. As the value of becomes larger, the inclination X′1 (or Y′1) of the ion movement direction also increases in proportion thereto. Accordingly, the acceptance in this case is a straight line rising to the right as shown by the line Dv in FIGS. On the other hand, as shown in FIG. 7B, when a large number of ions fly so as to be focused on a certain point, the gradient X′1 (or Y′1) increases as the coordinate X1 (or Y1) increases. ) Increases in the negative direction. In this case, the acceptance is a straight line descending to the right as shown by a line Cv in FIGS.
[0004]
In the case where ions are incident on a quadrupole mass filter, in order to adapt the position and direction of motion of the incident ions to the acceptance of the quadrupole mass filter, the incident ions are conventionally placed between the ion source and the quadrupole mass filter. An optical system was provided. By matching the ion emission characteristics of this incident ion optical system (referred to as emittance) with the acceptance of the quadrupole mass filter, ions can be detected efficiently in a quadrupole mass spectrometer and highly sensitive analysis can be performed. Can be performed.
[0005]
An incident ion optical system in a conventional quadrupole mass spectrometer generally focuses ions by applying a DC voltage around the ion passage, and the applied voltage is fixed. The shape and electric field of the incident ion optical system were rotationally symmetric with respect to the ion incident axis.
[0006]
[Problems to be solved by the invention]
Even if the focusing characteristic of the incident ion optical system is constant, the emittance changes when the energy of the incident ions changes. For example, in an inductively coupled plasma mass spectrometer (ICP / MS), the kinetic energy of ions is given by plasma expansion, so the ion velocity is almost the same regardless of the mass. As shown in FIG. The energy increases with time. Therefore, if the applied voltage of the incident ion optical system is kept constant, the emittance appears more strongly in the second and fourth quadrants as the incident ion energy increases, as shown in FIGS. It becomes a thing with strong convergence. Therefore, when changing the target ion passing through the quadrupole mass filter side or performing mass scanning, the incident energy changes accordingly, so the match between the emittance of the incident ion optical system and the acceptance of the quadrupole mass filter is disrupted, There arises a problem that the sensitivity varies depending on the mass of ions.
[0007]
Further, as apparent from FIG. 11, at the same incident energy, the emittance in the X direction and the emittance in the Y direction have substantially the same shape. This reflects the fact that the incident ion optical system is axially symmetric. However, the acceptance of the quadrupole mass filter is not actually axially symmetric, and as shown in FIG. 10, the X and Y directions have different shapes. Therefore, in a conventional axially symmetric incident ion optical system, sufficient matching between emittance and acceptance cannot be obtained, and sufficient efficiency cannot be obtained.
[0008]
The present invention has been made to solve such problems, and the object of the present invention is to always match the emittance of the incident ion optical system and the acceptance of the quadrupole mass filter well. It is to provide a quadrupole mass spectrometer.
[0010]
[Means for Solving the Problems]
The quadrupole mass spectrometer according to the present invention made to solve the above problems has a Z axis as a central axis, and a pair of electrode rods are arranged in the X axis direction and the Y axis direction perpendicular to the Z axis. Quadrupole mass spectrometry with a quadrupole mass filter that consists of quadrupoles and allows only ions of a predetermined mass to pass in the Z-axis direction by applying a voltage superimposed with a DC voltage and an AC voltage between the electrode rods. In the device
An incident ion optical system that provides different convergence or divergence in the X-axis direction and the Y-axis direction with respect to the incident ions, the incident ion optical system including a quadrupole and a Z-axis that are rotationally symmetric with respect to the Z-axis A combination of an aperture plate having an opening around the aperture, and using the quadrupole and the aperture plate, restricts the spatial passage range of ions and the inclination of the movement direction with respect to the X-axis direction and the Y-axis direction. Thus, the emittance shape of the incident ion optical system is made to coincide with the acceptance shape of the quadrupole mass filter in both the X-axis direction and the Y-axis direction.
[0012]
[Action]
In the quadrupole mass spectrometer of the present invention, the incident ion optical system is not axially symmetric as in the prior art, but is set to give different convergence or divergence in the X-axis direction and the Y-axis direction. The emittance of the incident ion optical system can always be matched with the acceptance of the quadrupole mass filter having a different shape in each axial direction. In addition, since the voltage applied to the quadrupole mass filter changes according to the mass of the target ion to be passed through the quadrupole mass filter, use information on the change in the applied voltage of the quadrupole mass filter. Thus, the convergence or divergence in the X-axis direction and the Y-axis direction may be changed according to the mass of incident ions.
[0013]
【Example】
First, an embodiment of a quadrupole mass spectrometer to which the present invention is applied will be described with reference to FIGS. As shown in FIG. 1, the quadrupole mass spectrometer of the present embodiment focuses ions emitted from the ion source 11 with an Einzel lens 12 that is an incident ion optical system, and a quadrupole mass filter (QMF) 13. To enter. A DC / AC superimposed voltage is applied to the quadrupole mass filter 13 from a quadrupole voltage application circuit (AC / DC) 18, whereby only ions having a target mass pass through the quadrupole. The target ions that have passed through the quadrupole mass filter 13 are deflected by the deflector 14 and detected by the ion detector 15.
[0014]
In the conventional quadrupole mass spectrometer, the DC voltage applied to the lens 12 is fixed. However, in the apparatus of this embodiment, as shown in FIG. 1, the lens voltage application circuit (DC) 17 is variable DC. A voltage can be applied to the lens 12, and the value of the applied voltage can be controlled by a control signal from the control unit 19.
[0015]
In general, as shown in FIG. 2, the Einzel lens 12 is composed of three electrode plates 12a, 12b, and 12c having a circular hole 21 in the center. A lens voltage application circuit (DC) is connected to each of the electrode plates 12a, 12b, and 12c. In contrast, a DC voltage is applied independently. The emittance by the lens 12 changes depending on the change of the applied voltage to any of the electrode plates 12a, 12b, and 12c, but the emittance (that is, the degree of convergence) changes by mainly changing the applied voltage to the intermediate electrode plate 12b. Control becomes easier.
[0016]
In the quadrupole mass spectrometer of the present embodiment, the control unit 19 controls both the quadrupole voltage application circuit 18 and the lens voltage application circuit 17. As described above, the quadrupole voltage changes according to the mass of the target ion to be passed through the quadrupole mass filter 13. Therefore, the control unit 19 also gives a control signal corresponding to the mass of the passing target ions to the lens voltage application circuit 17 to change the voltage applied to the lens 12. As a result, even if the mass of the target ion changes and its energy changes, the value of the voltage applied to the lens 12 changes at the same time, so that the emittance of the incident ions always matches the acceptance of the quadrupole mass filter 13. .
[0017]
An embodiment of the present invention will be described with reference to FIGS. In the above embodiment, the incident optical system is axisymmetric with respect to the Z axis as shown in FIG. 2, but the acceptance of the quadrupole mass filter 13 is the X axis direction and the Y axis direction as shown in FIG. The shape is different. Therefore, in the quadrupole mass spectrometer according to the present embodiment, the space inside the nose electrode 22 provided between the incident ion optical system 12 and the quadrupole mass filter 13 in the conventional quadrupole mass spectrometer is the second. A two-incidence ion optical system 24 is added, thereby providing a difference in emittance between the X-axis direction and the Y-axis direction.
[0018]
As shown in FIG. 4, the second incident ion optical system 24 includes a quadrupole 25 and a double aperture 26. The quadrupole 25 is the same as the quadrupole mass filter 13 except that four electrode rods 25a to 25d are arranged rotationally symmetrically with respect to the Z axis. Unlike the quadrupole mass filter 13, only a DC voltage is applied. To do. That is, a + V DC voltage is applied to the pair of electrode rods 25b and 25d in the X-axis direction, and a -V DC voltage having the same polarity and the opposite polarity to the pair of electrode rods 25a and 25c in the Y-axis direction. Apply. Here, V is a positive or negative voltage, and is set according to the characteristics of the first incident ion optical system 12 and the acceptance of the quadrupole mass filter 13 as described later. The voltage + V0 shown in FIG. 4 will be described later.
[0019]
The double aperture 26 includes two aperture plates 26a and 26b having the same openings 26c and 26d. The openings 26c and 26d are symmetric with respect to both the X-axis and the Y-axis, spatially restrict the ion passage range with respect to the X-axis direction and the Y-axis direction, and depending on the distance from the quadrupole 25, The inclinations X ′ and Y ′ of the ion movement direction are also limited. Although it is possible to limit the passage range and the inclination of the movement direction with only one aperture plate, the ion beam source generally has a spatial extension (this means that the emittance is a straight line as shown in FIG. 10). Therefore, by using two aperture plates, the passage range and the direction of movement are more reliably restricted.
[0020]
In the quadrupole mass spectrometer of the present embodiment, the first incident ion optical system 12 and the second incident ion optical system 24 are set as follows. First, as shown in FIG. 5, the virtual ion source (of FIG. 7A or 7B) obtained from the acceptance of the quadrupole mass filter 13 in the X-axis direction is used as the focal point of the first incident ion optical system 12. It is set to be an intermediate point FC between the position FX of the focusing point F) and the position FY of the virtual ion source obtained from the acceptance in the Y-axis direction. Next, a pair of electrodes arranged in the X-axis direction of the quadrupole 25 so that the focal point FC moves to the position of the virtual ion source FX in the X-axis direction of the quadrupole mass filter 13 by a distance dX. The DC voltage V applied to the bars 25b and 25d is set. In the case of FIG. 5, in order to move the focusing point far away, the quadrupole 25 needs to be set so that ions focused by the first incident ion optical system 12 are slightly diverged in the X-axis direction. Therefore, for example, when handling positive ions, a negative voltage V is applied to the pair of electrode rods 25b and 25d in the X-axis direction. As described above, since a voltage −V in the reverse direction is applied to the pair of electrode rods 25a and 25c in the Y-axis direction, ions passing through the quadrupole 25 are focused in the Y-axis direction, and the focusing point is Move forward by a distance dY to coincide with FY.
[0021]
Thereby, in the XX ′ plane and the YY ′ plane, the emittance inclination coincided with the acceptance inclination of the quadrupole mass filter. Next, as described above, the ion passing range and the movement direction are limited by the two aperture plates 26a and 26b, so that the maximum emittance range in the XX ′ plane and the YY ′ plane is the maximum acceptance. Match the range. Accordingly, the emittance shape of the incident ion optical system including the first incident ion optical system 12 and the second incident ion optical system 24 is the acceptance shape of the quadrupole mass filter 13 in both the X-axis direction and the Y-axis direction. To come to match.
[0022]
The nose electrode 22 is conventionally disposed at the entrance of the quadrupole mass filter 13 in the quadrupole mass spectrometer, and is formed at the edge of the quadrupole mass filter 13 by applying a bias voltage. It is provided to shorten the time for ions to stay in the electric field. In this embodiment, the same effect is obtained by applying the bias voltage V0 to the entire second incident ion optical system 24 (quadrupole 25 and aperture 26).
[0023]
In the above embodiment, the quadrupole 25 as shown in FIG. 4 is used in order to independently control the focusing property of the ions in the X-axis direction and the Y-axis direction. Two pairs of flat plate electrodes 31, 32, 33, and 34 as shown in FIG. The voltage application method in this case is the same as in the above embodiment. A crossed lens as shown in FIG. 6B can also be used. In the crossed lens, three or more aperture lenses 41, 42, and 43 having openings symmetrical with respect to two axes in the X-axis direction and the Y-axis direction are arranged by rotating each other by 90 °. In this case, the aperture lens 41 , 42 and 43 independently control the focusing property of the ions in the X-axis direction and the Y-axis direction, and regulate the passing range and the moving direction thereof so that the emittance is adapted to the acceptance of the quadrupole mass filter.
[0025]
【The invention's effect】
In the quadrupole mass spectrometer of the present invention, since the emittance of the incident ion optical system can be controlled separately in the X-axis direction and the Y-axis direction, the acceptance of the quadrupole mass filter having different shapes in both directions is achieved. It can always be in a combined state, and high sensitivity mass spectrometry can always be performed.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a quadrupole mass spectrometer according to a first embodiment.
FIG. 2 is a perspective view of an incident ion optical system according to a first embodiment.
FIG. 3 is a configuration diagram of a main part of a quadrupole mass spectrometer according to a second embodiment.
FIG. 4 is a perspective view of a second incident ion optical system of the quadrupole mass spectrometer of the second embodiment.
FIG. 5 is an explanatory diagram showing a relationship between a first incident ion optical system and a second incident ion optical system.
FIG. 6 is a perspective view showing another example of the second incident ion optical system.
FIG. 7 is a graph showing the relationship between the coordinate on the XY plane and the direction of motion when ions diverge from one point (a) and when they converge at one point (b).
FIG. 8 is a graph showing acceptance or emittance on the XX ′ plane and the YY ′ plane in the case of one-point divergence (Dv) and one-point convergence (Cv).
FIG. 9 is a graph showing the relationship between the mass and energy of incident ions in ICP / MS.
FIG. 10 is a graph showing the acceptance of a quadrupole mass filter.
FIG. 11 is a graph showing that emittance of a conventional incident ion optical system changes due to energy change.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Ion source 12 ... Incident ion optical system 13 ... Quadrupole mass filter 14 ... Deflector 15 ... Ion detector 17 ... Lens voltage application circuit 18 ... Quadrupole voltage application circuit 19 ... Control part 21 ... Nose electrode 24 ... No. Two-incidence ion optical system 25 ... quadrupole 26 ... double aperture

Claims (1)

It consists of a quadrupole with the Z axis as the central axis and a pair of electrode bars arranged in the X axis direction and Y axis direction orthogonal to the Z axis, and a voltage on which a DC voltage and an AC voltage are superimposed is applied between the electrode bars. In a quadrupole mass spectrometer equipped with a quadrupole mass filter that allows only ions of a predetermined mass to pass in the Z-axis direction,
An incident ion optical system that provides different convergence or divergence in the X-axis direction and the Y-axis direction with respect to the incident ions, the incident ion optical system including a quadrupole and a Z-axis that are rotationally symmetric with respect to the Z-axis A combination of an aperture plate having an opening around the aperture, and using the quadrupole and the aperture plate, restricts the spatial passage range of ions and the inclination of the movement direction with respect to the X-axis direction and the Y-axis direction. By doing so, the emittance shape of the incident ion optical system is made to coincide with the acceptance shape of the quadrupole mass filter in both the X-axis direction and the Y-axis direction. .

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