Quadrupole mass filter and quadrupole mass spectrometrometer

Abstract

Four main rod electrodes included in a main electrode section are disposed in a rotationally symmetric manner around an ion optical axis. Among four pre-rod electrodes included in a pre-electrode section disposed in front of the main electrode section, two are in contact with a circle of a radius r.sub.0, whereas the other two are disposed to be in contact with a circle of a radius R.sub.0 larger than r.sub.0, resulting in rotational asymmetry around the ion optical axis. Accordingly, a shape of acceptance on an x-y plane regarding positions of ions in the pre-electrode section becomes elliptical. This allows the shape of the acceptance to become gradually flat as the ions travel along the ion optical axis, reducing a mismatch between emittance of incoming ions and the acceptance on a receiving side, and relieving ion loss during ion introduction.

Claims

1. A quadrupole mass filter comprising: a) a main electrode section including four main rod electrodes disposed to surround a central axis; b) a pre-electrode section including pre-rod electrodes shorter than the main rod electrodes, the pre-rod electrodes being disposed in front of each of the main rod electrodes of the main electrode section along the central axis; c) a first voltage application unit configured to apply, to each of the main rod electrodes, a voltage obtained by adding a direct current voltage and a radio-frequency voltage according to a mass-to-charge ratio of ions that are allowed to pass; and d) a second voltage application unit configured to apply, to each of the pre-rod electrodes, a radio-frequency voltage identical to the radio-frequency voltage in frequency, wherein in the pre-electrode section, first two of the pre-rod electrodes positioned so as to sandwich the central axis and second two of the pre-rod electrodes adjacent to the first two of the pre-rod electrodes around the central axis are disposed at positions where radii of inscribed circles centered on the central axis differ.

2. A quadrupole mass spectrometer comprising the quadrupole mass filter according to claim 1 used as a mass separator.

3. The quadrupole mass filter according to claim 1, wherein each of the pre-rod electrodes of has a same cross-sectional area.

4. The quadrupole mass filter according to claim 3, wherein the main rod electrodes of the main electrode section are disposed a same distance from the central axis.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic configuration diagram of a first embodiment of a quadrupole mass spectrometer using a quadrupole mass filter according to the present invention.

(2) FIG. 2 is a longitudinal sectional view of main rod electrodes and pre-rod electrodes in the mass spectrometer of the first embodiment.

(3) FIG. 3 is a schematic diagram showing shapes of emittance and acceptance in each stage of the quadrupole mass filter in the mass spectrometer of the first embodiment.

(4) FIG. 4 is a diagram showing simulation results of relative ion intensity in the quadrupole mass filter in the mass spectrometer of the first embodiment and a conventional quadrupole mass filter.

(5) FIG. 5 is a longitudinal sectional view of pre-rod electrodes in a mass spectrometer of a second embodiment.

(6) FIG. 6 is a diagram showing simulation results of relative ion intensity in a quadrupole mass filter in the mass spectrometer of the second embodiment and a conventional quadrupole mass filter.

(7) FIG. 7 is a longitudinal sectional view of pre-rod electrodes in a mass spectrometer of a third embodiment.

(8) FIG. 8 is a diagram showing simulation results of relative ion intensity in a quadrupole mass filter in the mass spectrometer of the third embodiment and a conventional quadrupole mass filter.

(9) FIG. 9 is a configuration diagram of a quadruple mass filter and a voltage application unit in a mass spectrometer of a fourth embodiment.

(10) FIG. 10 is a diagram showing simulation results of relative ion intensity in the quadrupole mass filter in the mass spectrometer of the fourth embodiment and a conventional quadrupole mass filter.

(11) FIG. 11(a) is a stable region diagram showing motion conditions of ions passing through the quadrupole mass filter in a configuration in which the pre-rod electrodes are not provided.

(12) FIG. 11(b) is a stable region diagram showing motion conditions of ions passing through the quadrupole mass filter in a configuration in which the pre-rod electrodes are provided.

DESCRIPTION OF EMBODIMENTS

First Embodiment

(13) A first embodiment of a mass spectrometer using a quadrupole mass filter according to the present invention will be described with reference to the accompanying drawings.

(14) FIG. 1 is a schematic configuration diagram of a single type quadrupole mass spectrometer, which is the first embodiment. FIG. 2 is a longitudinal sectional view of a main electrode section and a pre-electrode section in the quadrupole mass spectrometer of the present embodiment.

(15) The quadrupole mass spectrometer of the present embodiment includes an ion source 1, an ion lens 2, a quadrupole mass filter 3, and a detector 4 inside an unillustrated vacuum chamber. The ion source 1 ionizes sample components within a sample gas, for example, by electron ionization. Ions generated by the ion source 1 and pulled out rightward as shown with an outlined arrow in FIG. 1 are converged by the ion lens 2 and introduced into the quadrupole mass filter 3. As will be described later, the quadrupole mass filter 3 includes a main electrode section 31 including four rod electrodes and a pre-electrode section 32 disposed in a preceding stage of the main electrode section 31.

(16) As will be described in detail later, among the ions introduced into a space of a longitudinal direction of the quadrupole mass filter 3 along an ion optical axis C, by an effect of an electric field generated by a radio-frequency voltage and a direct current voltage applied to the rod electrodes of the quadrupole mass filter 3, only ions having a specified mass-to-charge ratio pass near the ion optical axis C while vibrating, whereas other ions are dispersed halfway. The ions that have passed through the quadrupole mass filter 3 reach the detector 4. The detector 4 generates a detection signal according to an amount of reached ions, and sends the detection signal to an unillustrated data processing unit. When the radio-frequency voltage and the direct current voltage to be applied to the rod electrodes of the quadrupole mass filter 3 are each changed while a predetermined relationship is maintained, the mass-to-charge ratio of the ions that can pass through the quadrupole mass filter 3 will change. Therefore, by scanning each of the radio-frequency voltage and the direct current voltage in a predetermined range, it is possible to change the mass-to-charge ratio of the ions that can reach the detector 4 in a predetermined range, and on a basis of the detection signal obtained accordingly, it is possible to create a mass spectrum indicating a relationship between the mass-to-charge ratio and ion intensity.

(17) As shown in FIG. 2 (a), the main electrode section 31 includes four cylindrical main rod electrodes (a, b, c, d) disposed in parallel with the ion optical axis C to surround the ion optical axis C which is also a central axis. Diameters of the main rod electrodes and a radius r.sub.0 of a circle that is centered on the ion optical axis C and inscribed in the main rod electrodes are identical. Meanwhile, as shown in FIG. 2 (b), in a similar manner to the main electrode section 31, the pre-electrode section 32 includes four cylindrical pre-rod electrodes (a, b, c, d) disposed in parallel with the ion optical axis C to surround the ion optical axis C. Although diameters of the pre-rod electrodes are identical, radii of circles that are centered on the ion optical axis C and inscribed in the pre-rod electrodes differ between the pre-rod electrodes 32a and 32c and the pre-rod electrodes 32b and 32d. That is, the radius of the inscribed circle of the two pre-rod electrodes 32b and 32d is the same as the radius r.sub.0 of the inscribed circle of the four rod electrodes constituting the main electrode section 31; however, the radius R.sub.0 of the inscribed circle of the other two pre-rod electrodes 32a and 32c is larger than the radius r.sub.0. Therefore, it can also be considered that the four pre-rod electrodes 32a, 32b, 32c, and 32d are circumscribed on a virtual elliptical pipe centered on the ion optical axis C.

(18) Note that as is conventional, the voltage to be applied to the four pre-rod electrodes 32a, 32b, 32c, and 32d is the same as the radio-frequency voltage to be applied to the main rod electrodes 31a to 31d disposed behind respective pre-rod electrodes 32a to 32d. That is, a radio-frequency voltage V.sub.RF (=V.sub.1 sin t) is applied to the pre-rod electrodes 32b and 32d, whereas a radio-frequency voltage V.sub.RF (=V.sub.1 sin t) having a phase opposite to that of the radio-frequency voltage V.sub.RF and a frequency and amplitude identical to those of the radio-frequency voltage V.sub.RF is applied to the pre-rod electrodes 32a and 32c. In addition to these voltages, although not described in FIG. 2, a common direct current bias voltage is normally applied to all the pre-rod electrodes 32a to 32d.

(19) In the quadrupole mass filter including the four rod electrodes completely rotationally symmetric around the central axis, potential in an x-y plane of a quadrupole electric field generated in the space surrounded by the rod electrodes is generally represented by Equation (3) below.
(x,y,t)={(x.sup.2y.sup.2)/r.sub.0.sup.2}(U.sub.DCV.sub.ACCOSt)(3)
Static potential in Equation (3) is represented by Equation (4).
V.sub.s={(x.sup.2y.sup.2)/r.sub.0.sup.2}U.sub.DC(4)
In addition, the electric field by Equation (3) is represented by Equation (5).

(20) $\left[\mathrm{Formula}1\right]$$\begin{array}{cc}\begin{array}{c}E\left(x,y,t\right)=-\left(x,y,t\right)\\ =\frac{2\left(U_{\mathrm{DC}}-V_{\mathrm{ACCOS}}t\right)}{r_0^2}\left(\begin{array}{c}-x\\ y\\ 0\end{array}\right)\end{array}& \left(5\right)\end{array}$
A dynamic electric field in Equation (5) is represented by Equation (6).

(21) $\left[\mathrm{Formula}2\right]$$\begin{array}{cc}{E}_{\mathrm{AC}}=\frac{2V_{\mathrm{AC}}}{r_0^2}\left(\begin{array}{c}x\\ -y\\ 0\end{array}\right)& \left(6\right)\end{array}$
Pseudo-potential is represented by Equation (7).

(22) $\left[\mathrm{Formula}3\right]$$\begin{array}{cc}\begin{array}{c}{V}^{*}=\frac{e||E_{\mathrm{AC}}{||}^2}{4m{}^2}+V_s\\ =\frac{{\mathrm{eV}}_{\mathrm{AC}}^2}{4m{}^2}\left(x^2+y^2\right)+\left(x^2-y^2\right)\frac{U_{\mathrm{DC}}}{r_0^2}\\ =\frac{m{}^2}{16e}\left\{\left(q^2+2a\right)x^2+\left(q^2-2a\right)y^2\right\}{E}_{\mathrm{in}}\\ .Math.\{\begin{array}{c}{\left(\frac{x}{R_x}\right)}^2+{\left(\frac{y}{R_y}\right)}^{2}1\\ R_x=\frac{4}{}+\sqrt{\frac{{\mathrm{eE}}_{\mathrm{in}}}{m\left(q^2+2a\right)}}\\ R_y=\frac{4}{}+\sqrt{\frac{{\mathrm{eE}}_{\mathrm{in}}}{m\left(q^2-2a\right)}}\end{array}\end{array}& \left(7\right)\end{array}$
The right side of the first line of Equation (7) is based on description of Non Patent Literature 3. The second line of Equation (7) is based on Equation (2) indicating parameters of the stable region. In addition, E.sub.in is energy of incoming ions.

(23) Equation (7) theoretically indicates that the acceptance shape in the x-y plane orthogonal to a z axis (ion optical axis C) is elliptical in the main electrode section including the four main rod electrodes. Meanwhile, a movement state of ions entering the quadrupole mass filter, that is, the emittance shape is almost circular. In the conventional quadrupole mass filter, the emittance shape of ions emitted from the pre-electrode section is also circular. It can be estimated that this difference between the sectional emittance shape of incoming ions and the sectional acceptance shape in the main electrode section is one of the large causes of a decline in ion introduction efficiency. In contrast, in the mass spectrometer of the present embodiment, since the pre-rod electrodes 32a to 32d in the pre-electrode section 32 are disposed to be circumscribed on an elliptical pipe centered on the ion optical axis C as described above, the acceptance shape in the pre-electrode section 32 is not circular but elliptical.

(24) In addition, an outward shifted amount of the pre-rod electrodes 32a and 32c is determined such that ellipticity of the ellipse indicating the acceptance shape in the pre-electrode section 32 is larger than ellipticity of the ellipse indicating the acceptance shape in the main electrode section 31 (that is, close to circular). FIG. 3 shows a relationship among a sectional shape 100 of the emittance of the ions entering the pre-electrode section 32 through the ion lens 2, a sectional shape 101 of the acceptance in the pre-electrode section 32, and a sectional shape 102 of the acceptance in the main electrode section 31. Thus, in the mass spectrometer of the present embodiment, the acceptance shape does not change suddenly as in the conventional mass spectrometer, but as ions travel along the ion optical axis C, the acceptance shape changes gradually, that is, becomes flat. Therefore, the mismatch between the emittance of the incoming ions and the acceptance on a receiving side becomes small, relieving ion loss during ion introduction.

(25) FIG. 4 shows a comparison result of relative ion intensity by simulation calculations between the quadrupole mass filter used in the first embodiment and the conventional quadrupole mass filter. In this simulation, relative intensity of ions passing through the quadrupole mass filter 3 is calculated by calculating a locus of ions having m/z=500 and emitted from a predetermined position on the ion optical axis (z axis) C corresponding to a position of the ion source 1. As is apparent from FIG. 4, the relative intensity of the quadrupole mass filter 3 in the above embodiment is about 1.8 times the relative intensity of the conventional quadrupole mass filter. That is, it can be said that an amount of ions reaching the detector 4 is nearly doubled, and detection sensitivity improves accordingly.

Second Embodiment

(26) FIG. 5 is a longitudinal sectional view of a pre-electrode section 32 in a mass spectrometer that is another embodiment (second embodiment) of the present invention. A configuration other than the pre-electrode section 32 is completely the same as the configuration of the first embodiment. In this mass spectrometer of the second embodiment, all of four pre-rod electrodes 32a to 32d are in contact with a circle having a radius of r.sub.0, but a radius of the pre-rod electrodes 32a and 32c differs from a radius of the pre-rod electrodes 32b and 32d. That is, sectional arc shapes of curved surfaces of the pre-rod electrodes 32a to 32d facing an ion optical axis C differ, and the sectional shapes are rotationally asymmetric around the ion optical axis C. Accordingly, as in the first embodiment, an acceptance shape in the pre-electrode section 32 is not circular but elliptical. Ellipticity of the ellipse can be adjusted with the radius of the pre-rod electrodes 32b and 32d.

(27) FIG. 6 shows a comparison result of relative ion intensity by simulation calculations between the quadrupole mass filter used in the second embodiment and the conventional quadrupole mass filter. As is apparent from FIG. 6, the relative intensity of the quadrupole mass filter 3 in the second embodiment is about 1.3 to 1.4 times the relative intensity of the conventional quadrupole mass filter. This indicates that, as in the first embodiment, detection sensitivity can be improved also in this mass spectrometer of the second embodiment.

Third Embodiment

(28) FIG. 7 is a longitudinal sectional view of a pre-electrode section 32 in a mass spectrometer that is another embodiment (third embodiment) of the present invention. A configuration other than the pre-electrode section 32 is completely the same as the configuration of the first embodiment. In this mass spectrometer of the third embodiment, all of four pre-rod electrodes 32a to 32d are in contact with a circle having a radius of r.sub.0, but a sectional shape of the pre-rod electrodes 32a and 32c is circular, a sectional shape of the pre-rod electrodes 32b and 32d is elliptical. That is, sectional shapes of curved surfaces of the pre-rod electrodes 32a to 32d facing an ion optical axis C differ, and the sectional shapes are rotationally asymmetric around the ion optical axis C. Accordingly, as in the first embodiment, an acceptance shape in the pre-electrode section 32 is not circular but elliptical. Ellipticity of the ellipse can be adjusted with ellipticity of the pre-rod electrodes 32b and 32d.

(29) FIG. 8 shows a comparison result of relative ion intensity by simulation calculations between the quadrupole mass filter used in the third embodiment and the conventional quadrupole mass filter. As is apparent from FIG. 8, the relative intensity of the quadrupole mass filter 3 in the third embodiment is about 1.3 times the relative intensity of the conventional quadrupole mass filter. This indicates that, as in the first embodiment, detection sensitivity can be improved also in this mass spectrometer of the third embodiment.

Fourth Embodiment

(30) FIG. 9 is a configuration diagram of a quadrupole mass filter and a voltage application unit in a mass spectrometer that is another embodiment (fourth embodiment) of the present invention. FIG. 9 illustrates a main electrode section 31 and a pre-electrode section 32 each on an x-y plane orthogonal to an ion optical axis C. In this mass spectrometer of the fourth embodiment, placement and shapes of pre-rod electrodes 32a to 32d are completely the same as conventional placement and shapes, but the configuration of the voltage application unit that applies voltages to the pre-rod electrodes 32a to 32d differs from conventional configuration.

(31) As shown in FIG. 9, predetermined voltages are applied from the voltage application unit including a radio-frequency voltage generation unit 51, a direct current voltage generation unit 52, a bias voltage generation unit 53, and a voltage composition unit 54 to each of a total of eight rod electrodes included in the pre-electrode section 32 and the main electrode section 31.

(32) In more detail, the radio-frequency voltage generation unit 51 generates radio-frequency voltages +V.sub.RF and V.sub.RF having identical amplitude and an opposite phase according to a mass-to-charge ratio of ions to be selected, in response to an instruction from a control unit 50. The direct current voltage generation unit 52 generates direct current voltages +U.sub.DC and U.sub.DC having an identical absolute value of voltage and an opposite polarity according to the mass-to-charge ratio of ions to be selected, in response to an instruction from the control unit 50. In addition, the bias voltage generation unit 53 generates predetermined direct current bias voltages V.sub.B1 and V.sub.B2 so as to produce an appropriate potential difference between these electrodes and electrodes or an ion optical system disposed in a preceding stage or subsequent stage in order to accelerate or decelerate ions. The voltage composition unit 54 includes adders that add voltages and an amplifier that amplifies (or reduces) a voltage. In this voltage composition unit 54, the positive-phase radio-frequency voltage +V.sub.RF and the positive-polarity direct current voltage +U.sub.DC are added, the opposite-phase radio-frequency voltage V.sub.RF and the negative-polarity direct current voltage U.sub.DC are added, and the direct current bias voltage V.sub.B1 is further added to each of the voltages (U.sub.DC+V.sub.RF). Then, resulting voltages are applied to the main rod electrodes 31a to 31d of the main electrode section 31. This is similar to the conventional general quadrupole mass filter.

(33) In the voltage composition unit 54, the positive-phase radio-frequency voltage +V.sub.RF is added to the direct current bias voltage V.sub.B2 and the resulting voltage is applied to the pre-rod electrodes 32b and 32d. In addition, the opposite-phase radio-frequency voltage V.sub.RF is amplified by a factor of by the amplifier and then added to the direct current bias voltage V.sub.B2, and the resulting voltage is applied to the pre-rod electrodes 32a and 32c. That is, the voltage V.sub.RF+V.sub.B2 is applied to the two pre-rod electrodes 32b and 32d sandwiching the ion optical axis C, whereas the voltage V.sub.RF+V.sub.B2 is applied to the other two pre-rod electrodes 32a and 32c. Accordingly, amplitude of the radio-frequency voltages to be applied to the pre-rod electrodes 32a to 32d becomes rotationally asymmetric around the ion optical axis C. Accordingly, as in the first embodiment, an acceptance shape in the pre-electrode section 32 is not circular but elliptical. Ellipticity of the ellipse can be adjusted with an amplification factor of the amplifier.

(34) FIG. 10 shows a comparison result of relative ion intensity by simulation calculations between the quadrupole mass filter used in the fourth embodiment and the conventional quadrupole mass filter. As is apparent from FIG. 10, the relative intensity of the quadrupole mass filter 3 in the fourth embodiment is about 1.5 to 1.6 times the relative intensity of the conventional quadrupole mass filter. This indicates that, as in the first embodiment, detection sensitivity can be improved also in this mass spectrometer of the fourth embodiment.

(35) Note that for easy understanding. FIG. 9 shows a configuration in which the voltage composition unit 54 including the adders and the amplifier generates the voltages to be applied to each of the rod electrodes; however, it is apparent that the circuit configuration for generating similar voltages is not limited to this configuration. A configuration may be used in which, for example, radio-frequency voltage waveforms are generated using digital data, after addition and multiplication are executed in a digital value stage, analog waveforms corresponding to the radio-frequency voltages are generated by performing digital-to-analog conversion, and these waveforms are applied to the rod electrodes via a drive circuit. Of course, other circuit configurations can also be easily considered.

(36) In addition, the first to fourth embodiments are examples in which the quadrupole mass filter characteristic of the present invention is applied to the single type quadrupole mass spectrometer. Of course, this quadrupole mass filter may be applied to the preceding stage quadrupole mass filter and the subsequent stage quadrupole mass filter of the triple quadrupole mass spectrometer, and to the quadrupole mass filter of the Q-TOF mass spectrometer.

(37) Furthermore, the above-described embodiments are only an example of the present invention, and it is natural that the embodiments are included in the claims of this application even if alterations, modifications, and additions are made as appropriate within the spirit of the present invention.

REFERENCE SIGNS LIST

(38) 1 . . . ion source 2 . . . ion lens 3 . . . quadrupole mass filter 31 . . . main electrode section 31a to 31d . . . main rod electrodes 32 . . . pre-electrode section 32a to 32d . . . pre-rod electrodes 4 . . . detector 50 . . . control unit 51 . . . radio-frequency voltage generation unit 52 . . . direct current voltage generation unit 53 . . . bias voltage generation unit 54 . . . voltage composition unit C . . . ion optical axis (central axis)