Quadrupole mass spectrometer

Abstract

A mass spectrometer according to an embodiment of the present invention comprises a voltage generation unit (53) that generates an RF voltage to be applied to an electrode forming a quadrupole mass filter. The voltage generation unit includes: an RF signal generation unit (530) that generates a sine wave signal using a DDS; a first filter (531) that removes high-frequency noise superposed on the sine wave signal; a waveform conversion unit (5320) that receives a sine wave signal that is an output of the first filter as an input, and generates a rectangular wave signal having a frequency of 1 / N (where N is an integer of at least one) of the frequency of the sine wave signal; and a second filter (5321) that converts the rectangular wave signal generated by the waveform conversion unit into a sine wave signal, by removing high-frequency components from the rectangular wave signal.

Description

quadrupole mass spectrometer

[0001] The present invention relates to a quadrupole mass spectrometer, and more particularly to an electrical system technology for driving a quadrupole mass filter in a quadrupole mass spectrometer.

[0002] A quadrupole mass filter used in a quadrupole mass spectrometer typically includes four rod electrodes, each of which is applied with a voltage that is the sum of an RF (radio frequency) voltage and a DC voltage. Of the various ions introduced into the quadrupole mass filter, only ions with a predetermined mass-to-charge ratio (m / z) determined by the amplitude of the RF voltage and the DC voltage, or ions with m / z values ​​within a predetermined m / z range, pass through the quadrupole mass filter stably, while other ions diverge along the way. In this way, the quadrupole mass filter selects ions according to their m / z. Therefore, if the RF and DC voltages applied to each rod electrode are unstable, the performance of the quadrupole mass filter, specifically the m / z accuracy and sensitivity of the passing ions, will be reduced. Therefore, both the RF and DC voltages must be highly accurate, stable, and low noise.

[0003] The RF voltage is typically a large-amplitude AC voltage with a frequency of about 1 MHz and a maximum amplitude of about 3000 V peak. To obtain such a large-amplitude RF voltage, a typical quadrupole drive circuit employs a method in which an RF signal of a predetermined frequency, generated by an RF signal source and amplified by an amplifier circuit or the like, is input to a series LC resonant circuit consisting of an inductance L of a coil and a capacitance C of a capacitor between multiple rod electrodes, and the LC resonant circuit operates to generate a large amplitude. Conventionally, oscillator circuits using quartz crystal resonators have been widely used as RF signal sources. However, in recent years, devices using a direct digital synthesizer (DDS) have become known, such as the mass spectrometer described in Patent Document 1.

[0004] As described in Non-Patent Documents 1 and 2, a DDS is a device that can generate signal waveforms such as sine waves, sawtooth waves, triangular waves, and rectangular waves having any frequency from a reference clock signal, and is characterized by its ability to easily and instantly switch frequencies and phases. However, due to the principle of operation of a DDS, unnecessary high-frequency noise called spurious is superimposed on the output RF signal. For this reason, it is recommended to provide a low-pass filter (LPF) circuit downstream of the DDS to pass the frequency components of the target RF signal while removing high-frequency noise components.

[0005] WO 2022 / 158430

[0006] Art Pini, "The Basics of Direct Digital Synthesizers (DDS): How to Select and Use Them," [Online] [Retrieved February 22, 2024], DigKey, Internet <URL: https: / / www.digikey.jp / ja / articles / the-basics-of-direct-digital-synthesizers-ddss> Erbe D Reyta and two others, "Achieve Accurate RF Testing with a DDS-Based Signal Generator Using a Raspberry Pi," [Online] [Retrieved February 22, 2024], Analog Dialogue, Internet <URL: https: / / www.analog.com / media / jp / analog-dialogue / volume-57 / number-2 / achieve-accurate-rf-testing-with-a-raspberry-pi-based-dds-signal-generator_jp.pdf>

[0007] By appropriately defining the characteristics (such as cutoff frequency and in-band flatness) of the LPF circuit placed downstream of the DDS, high-frequency noise superimposed on the RF signal (sine wave signal) output from the DDS can be removed. However, the RF signal output from the DDS may contain not only high-frequency noise but also voltage noise in a frequency band significantly lower than the frequency of the RF signal, which is observed as fluctuations or undulations in the RF signal envelope. Such low-frequency noise cannot be removed by the LPF circuit, and may ultimately reduce the accuracy and stability of the RF voltage applied to the rod electrodes, resulting in a deterioration in the performance of the quadrupole mass filter.

[0008] The low-frequency noise described above is affected by various factors, including the stability and noise characteristics of the analog power supply voltage supplied to the DDS device, the specifications and performance of the DDS device itself, the performance of peripheral components used in addition to the device, and the device's mounting state, including the circuit board pattern and wiring layout. Therefore, to suppress its generation, it is necessary to take a variety of comprehensive measures, such as selecting an appropriate device, which makes device selection and circuit design difficult and increases costs. For these reasons, there is a demand for a method that can stably remove low-frequency noise superimposed on signals output from a DDS at low cost.

[0009] The present invention has been made to solve these problems, and its main object is to provide a quadrupole mass spectrometer that is capable of performing analysis with high accuracy, resolution, sensitivity, etc., even when low-frequency noise caused by the influence of the power supply voltage, etc., is present in the output of the DDS used as an RF signal source.

[0010] One aspect of the quadrupole mass spectrometer according to the present invention has a voltage generating unit that generates an RF voltage to be applied to electrodes that constitute a quadrupole mass filter, and the voltage generating unit comprises: an RF signal generating unit that generates a sine wave signal using a direct digital synthesizer (DDS); a first filter that removes high-frequency noise superimposed on the sine wave signal; a waveform converting unit that receives the sine wave signal that is the output of the first filter as input and generates a square wave signal having a frequency that is 1 / N (N is an integer greater than or equal to 1) of the frequency of the sine wave signal; and a second filter that converts the square wave signal generated by the waveform converting unit into a sine wave signal by removing high-frequency components from the square wave signal.

[0011] Another aspect of the quadrupole mass spectrometer according to the present invention has a voltage generating unit that generates an RF voltage to be applied to electrodes that constitute a quadrupole mass filter, the voltage generating unit comprising: an RF signal generating unit that generates a sine wave signal using a direct digital synthesizer (DDS); a first filter that removes high frequency noise superimposed on the sine wave signal; a square wave generating unit that binarizes the sine wave signal that is the output of the first filter to generate a square wave signal; a waveform converting unit that receives the square wave signal generated by the square wave generating unit as input and shapes the square wave signal using a logical operation circuit or divides the square wave waveform by 1 / N (where N is an integer of 2 or more) to generate a new square wave signal; and a second filter that converts the square wave signal output from the waveform converting unit into a sine wave signal by removing high frequency components from the square wave signal.

[0012] According to the above-described aspect of the quadrupole mass spectrometer of the present invention, even if the output of the DDS contains low-frequency noise due to the influence of the power supply voltage or the like that is not removed by the first filter, the waveform converter and the second filter can substantially remove such noise, thereby obtaining a high-purity sine wave signal. By generating RF voltages to be applied to each electrode of the quadrupole mass filter based on this sine wave signal, the quadrupole mass filter can be driven in a near-ideal state, enabling analysis with high accuracy, resolution, sensitivity, etc.

[0013] 1. A block diagram of the main parts of a quadrupole mass spectrometer according to one embodiment of the present invention. 2. A schematic block diagram of an RF voltage generating unit in the mass spectrometer of this embodiment. 3. A schematic block diagram of an RF voltage generating unit according to one modified example. 4. A schematic block diagram of an RF voltage generating unit according to another modified example. 5. A schematic block diagram of an RF voltage generating unit according to another modified example. 6. A schematic block diagram of an RF voltage generating unit according to another modified example. 7. A diagram showing waveforms of main parts in a conventional RF voltage generating unit when DDS voltage noise is relatively small. 8. A diagram showing waveforms of main parts in a conventional RF voltage generating unit when DDS voltage noise is relatively large. 9. A diagram showing the shape of the mass peaks in FIG. 7. 10. A diagram showing the shape of the mass peaks in FIG. 11. 11. A diagram showing waveforms of main parts in an RF voltage generating unit using a comparator built into a DDS device when DDS voltage noise is relatively large. 16 is a diagram showing waveforms of essential parts when the DDS voltage noise is relatively large in an RF voltage generating unit that uses a comparator built into the DDS device. A diagram showing ripple waveforms of the analog power supply voltage and digital power supply voltage of a DDS device. A diagram showing the shapes of mass peaks in the cases of FIGS. 13 and 14. A diagram showing waveforms of essential parts when the DDS voltage noise is relatively large in an RF voltage generating unit that uses a comparator built into the DDS device and is provided with a noise removal unit. A diagram showing the shape of the mass peak in the case of FIG. 17.

[0014] An embodiment of a quadrupole mass spectrometer according to the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a block diagram of the essential parts of the quadrupole mass spectrometer of this embodiment;

[0015] In FIG. 1, an ion source 1 ionizes component molecules contained in a sample. Ions generated by the ion source 1 are transported by an ion transport optical system 2 and introduced into a quadrupole mass filter 3. The quadrupole mass filter 3 includes four rod electrodes 31, 32, 33, and 34 that surround a central axis (ion optical axis) C and extend in the direction of the central axis C. A predetermined voltage is applied to each of the four rod electrodes 31 to 34 from a power supply unit 5, and only ions having an m / z value corresponding to that voltage selectively pass through the quadrupole mass filter 3. A detector 4 detects the ions that have passed through and outputs a detection signal corresponding to the amount of ions.

[0016] The power supply unit 5 includes a voltage control unit 51, a DC (direct current) voltage generator 52, an RF voltage generator 53, and an RF / DC voltage adder 54. Under the control of the voltage control unit 51, the DC voltage generator 52 and the RF voltage generator 53 generate a DC voltage (±U) having a voltage value corresponding to the m / z of the ions to be passed through the quadrupole mass filter 3 and an RF voltage (±V cosωt) having an amplitude corresponding to the m / z. The RF / DC voltage adder 54 adds the DC voltage and the RF voltage and applies the result to the four rod electrodes 31 to 34. The voltage control unit 51 is mainly composed of, for example, a microcomputer, and controls the DC voltage generator 52 and the RF voltage generator 53 to obtain predetermined RF and DC voltages based on instructions from a control unit 6 that controls the entire apparatus.

[0017] In general, of the four rod electrodes 31 to 34 constituting the quadrupole mass filter 3, an RF voltage of the same phase and amplitude and a DC voltage of the same polarity and value are applied to two rod electrodes facing each other across the ion optical axis C, for example, rod electrodes 31 and 33, and an RF voltage of the same amplitude and phase but opposite to each other and a DC voltage of the same voltage and opposite to each other are applied to two adjacent rod electrodes around the ion optical axis C, for example, rod electrodes 31 and 32. This is exactly the same as in a conventional, general quadrupole mass spectrometer.

[0018] Fig. 2 is a block diagram showing a schematic configuration of the RF voltage generating unit 53 in Fig. 1. This RF voltage generating unit 53 includes a DDS 530, a first LPF 531, a noise removing unit 532, a variable gain amplifier (VGA) 533, an error amplifier 534, and an amplifier (AMP) 535.

[0019] The DDS 530 can be a commonly available DDS device (IC) described in, for example, Non-Patent Documents 1 and 2. The DDS 530 includes blocks such as a phase accumulator 5300 that generates digital sawtooth voltage data of a frequency specified by a voltage control unit 51 based on an externally supplied reference clock CLK, a sine wave conversion unit 5301 that includes a conversion table that converts the sawtooth voltage data into sine wave data, and a digital-to-analog converter (DAC) 5302 that converts the sine wave data into an analog signal, and outputs a sine wave signal of a specified frequency. While DDS devices are available from various manufacturers, an example that can be used is the AD9838 from Analog Devices, Inc.

[0020] The first LPF 531 is, for example, a seventh-order Butterworth low-pass filter having a predetermined cutoff frequency. The cutoff frequency is set to a frequency higher than the fundamental frequency of the sine wave signal input to the first LPF 531 and capable of sufficiently removing high-frequency signal components superimposed on the sine wave signal. Of course, filter characteristics other than Butterworth can be used, and the order of the filter can also be selected appropriately. The first LPF 531 is generally required to remove high-frequency noise (spurious) superimposed on the output voltage of the DDS 530, and its use is recommended, for example, in Non-Patent Document 1.

[0021] The noise removal unit 532 includes a square wave conversion unit 5320 and a second LPF 5321. The square wave conversion unit 5320 converts an input sine wave signal (or a square wave signal, as described later) into a square wave signal. A comparator is generally used to binarize a sine wave signal, but in this case, a logic operation device (IC) such as a CMOS may be used instead of a comparator.

[0022] For example, in an inverter circuit, the simplest logic operation circuit, the output inverts from high to low when the input signal rises and exceeds a predetermined threshold, and conversely, the output inverts from low to high when the input signal falls and falls below the predetermined threshold. Therefore, although the polarity is reversed, a sine wave signal can be binarized. However, in such logic operation circuits, the rising and falling thresholds are not necessarily the same, and the threshold is not 1 / 2 the amplitude of the sine wave signal. Therefore, it is difficult to achieve a 50% duty ratio for the binarized square wave signal using a simple inverter circuit or buffer circuit. Therefore, when using a logic operation device as the square wave conversion unit 5320, it is recommended to use an N-divider circuit where N is 2 or greater. Specifically, a counter circuit or a flip-flop circuit can be used. Furthermore, to prevent chattering when high-frequency noise is superimposed on the input signal, it is also effective to use a logic operation device with a Schmitt trigger input characteristic.

[0023] The second LPF 5321 converts the square wave signal into a sine wave signal by sufficiently blunting the rising and falling edges of the square wave signal with a duty ratio of 50%. The combination of the square wave converter 5320 and the second LPF 5321 can remove (reduce) low-frequency voltage noise superimposed on the input sine wave signal (or square wave signal).

[0024] VGA 533 is an amplifier whose gain changes in response to an external command (here, the output voltage of error amplifier 534). Here, the difference (error) between the monitor value of the voltage actually applied to rod electrodes 31 to 34 and the target value of the voltage to be applied to rod electrodes 31 to 34 is fed back to VGA 533 via error amplifier 534, and the gain of VGA 533 is adjusted accordingly so that the voltage applied to rod electrodes 31 to 34 approaches the target value. The next-stage amplifier 535 is a power amplifier with a fixed gain.

[0025] In a conventionally considered configuration of an RF voltage generator using a DDS, the dashed line 539 shown in FIG. 2 is connected, and the output of the first LPF 531 is directly connected to the input of the VGA 533 (i.e., the noise elimination unit 532 is omitted). Because the DDS 530 is largely comprised of digital circuits operating at high frequencies, high-frequency spurious signals are generated, as described above. These spurious signals can be removed by the first LPF 531 to a degree that leaves virtually no effect. Meanwhile, while studying the configuration of an RF voltage generator using a DDS 530, the inventors discovered that noise with a frequency significantly lower than that of the sine wave signal is superimposed on the sine wave signal output from the DDS 530, and this significantly affects the performance of the quadrupole mass filter. It is believed that this low-frequency noise is caused by fluctuations in the power supply voltage of the digital system, which are subject to relatively large fluctuations when the digital circuits built into the DDS 530 and the control microcomputer upstream of the DDS 530 operate, affecting the output stage of the DDS.

[0026] FIG. 7 shows waveforms obtained when the constants of the external components of the DDS 530 are adjusted by trial and error to minimize the low-frequency noise. FIG. 8 shows waveforms obtained when standard constants (recommended by the device manufacturer) are used without such trial and error adjustment. (a) shows the observed waveforms of the outputs of the DDS, VGA, and AMP. (b) shows the observed waveforms over a longer period of time with the horizontal axis of the VGA and AMP outputs scaled down. (c) shows a partially enlarged waveform of (b). In both cases, noise corresponding to envelope fluctuations at a frequency much lower than the sine wave signal is observed, but the noise is greater in FIG. 8 than in FIG. 7. Specifically, according to experiments conducted by the inventors, the voltage noise level is 12 mVp-p in FIG. 7 compared to 20 mVp-p in FIG. 8, an increase of approximately 60 to 70%.

[0027] Figure 9 shows the shapes of mass peaks for multiple m / z values ​​in the case of Figure 7 . Figure 10 shows the shapes of mass peaks for multiple m / z values ​​in the case of Figure 8 . Comparing Figures 9 and 10 , the latter shows an overall decrease in intensity of about 20% compared to the former. Furthermore, while the intensity decreases, the peak widths remain the same or slightly wider, indicating a decrease in mass resolution. Thus, the increase in noise observed on the sinusoidal signal waveform shown in Figure 8 clearly contributes to a decrease in performance as a mass filter. As shown in Figures 7 and 9 , adjusting the constants of the external components of the DDS 530 may reduce the noise and prevent the performance degradation of the mass filter. However, this approach is not robust enough to withstand differences in devices, variations in the constants of various components, or changes in the surrounding environment. Therefore, a robust solution that can prevent the performance degradation of the mass filter even in the presence of low-frequency power supply voltage noise is desirable.

[0028] In contrast, in the RF voltage generation unit 53 of this embodiment, as shown in FIG. 2, a noise removal unit 532 is provided between the first LPF 531 and the VGA 533. That is, the sine wave signal from which high-frequency noise (spurious) has been removed by the first LPF 531 is input to the square wave conversion unit 5320, where it is converted into a square wave signal with a frequency that is, for example, 1 / N of the frequency of the sine wave signal. The value of N can be determined appropriately, but can be, for example, 10. To configure a 10-frequency divider circuit using a logical operation device, for example, a 74HC390 can be used. In this case, since the frequency of the final sine wave signal is 1 / 10 of the frequency of the sine wave signal generated by the DDS 530, it is necessary for the DDS 530 to generate a sine wave signal with a frequency that is 10 times the frequency of the sine wave voltage (±Vcosωt) applied to the rod electrodes 31-34. This is true regardless of the value of N.

[0029] Low-frequency power supply voltage noise superimposed on the sine wave signal input to the noise elimination unit 532 is substantially removed during square wave conversion in the square wave conversion unit 5320. High-frequency components such as rising and falling edges contained in this square wave signal from which the power supply voltage noise has been removed are removed by the second LPF 5321, and the signal is converted into a sine wave signal of the same frequency. If the duty ratio of the square wave signal is 50%, a sine wave signal with a good waveform shape, free of distortion and superimposed low-frequency power supply voltage noise, can be obtained at the output stage of the second LPF 5321. This sine wave signal is input to the VGA 533, amplified with a gain corresponding to the output voltage from the error amplifier 534, and further amplified with a predetermined gain by the next-stage amplifier 535. The amplified sine wave signal is applied to the rod electrodes 31 to 34 through the LC resonant circuit.

[0030] Fig. 11 shows waveforms at various parts when the power supply voltage noise is large and the noise removal unit 532 is used as described above. Fig. 12 shows the shapes of mass peaks for multiple m / z values ​​in this case. In this case, even if the power supply voltage noise in the output from the DDS 530 is large, voltage fluctuations in the output of the amplifier 535 are suppressed, and it can be seen that, reflecting this, the intensity of the mass peaks is approximately the same as in the case shown in Fig. 9 when the power supply voltage noise is small.

[0031] When a logical operation IC is used as the square wave conversion unit 5320, although this is a digital circuit, it is desirable to separate the power supply voltage supply line from the digital power supply voltage supply line of the preceding DDS 530. This is because, to reliably remove power supply voltage noise in the square wave conversion unit 5320, it is important that the power supply voltage in this circuit is stable (that is, that the power supply voltage does not fluctuate due to the operation of the preceding digital circuit).

[0032] 7, 8, and 11 (and similar figures described below) are examples of observed waveforms when a specific device is used as the DDS 530, but the level of power supply voltage noise at the output of the DDS 530 varies considerably depending on the characteristics of the DDS device itself and the circuit pattern around the device. This is because the output voltage of the DDS 530 is generated by current-to-voltage conversion of the output current of the DAC 5302 built into the DDS 530 using an external resistor, and the stability of the output current of the DAC 5302 is affected by various factors, such as the DAC signal specifications of the DDS device, the specifications of the DDS device, the internal reference voltage of the DDS device, the stability and noise of the analog power supply voltage of the DDS device, and the characteristics of the external resistor used to adjust the full scale of the DAC output current. Therefore, it should be noted that the noise characteristics at the output voltage of the DDS 530 will vary depending on the type of DDS device used and the type of circuit pattern, including the upstream microcontroller, etc.

[0033] In other words, although there are cases where the noise elimination unit 532 is not necessary by selecting an appropriate DDS device and designing an appropriate circuit pattern, such special consideration becomes unnecessary by providing the noise elimination unit 532. In other words, the range of selectable DDS devices is expanded, and the degree of freedom in designing the circuit pattern also increases, which can bring about the benefit of cost reduction.

[0034] Next, a modified example of the RF voltage generator described above will be described. FIG. 3 is a schematic block diagram of the RF voltage generator 53X of this modified example. Components that are the same as or equivalent to those in FIG. 2 are assigned the same reference numerals. Some DDS devices output a sine wave signal and have a built-in comparator circuit. For example, the AD9838 from Analog Devices, Inc. described above, as shown in FIG. 3, has a built-in comparator 5303 and is equipped with an input terminal and an output terminal for the comparator 5303. By inputting the sine wave signal that is the output of the first LPF 531 to the input terminal of the comparator 5303, a rectangular wave signal can be obtained by binarizing the sine wave signal in the comparator 5303.

[0035] Therefore, the inventors initially intended to use a comparator 5303 instead of the square-wave conversion unit 5320 in the configuration shown in FIG. 2 , and devised a modified configuration in which the components are connected as shown by the dashed line 539 in FIG. 3 . FIGS. 13 and 14 show waveforms of the various components in this configuration. As is clear from FIG. 14( c), relatively large envelope fluctuations are observed in the sine wave signal. FIG. 15 shows the observed ripple waveforms of the analog power supply voltage and digital power supply voltage of the DDS 530. Because the digital power supply voltage line is shared with the power supply voltage line of other digital circuits such as microcontrollers, noise due to the operation of other digital circuits is also superimposed on the digital power supply voltage of the DDS. Although comparators are generally not digital circuits, the power supply voltage of the comparator inside the DDS device used is supplied from the digital power supply voltage line of the DDS. Therefore, it is assumed that the ripple waveform of the digital power supply voltage as described above also appears in the comparator output.

[0036] Figure 16 shows the mass peak shapes for multiple m / z values. It can be seen that the mass peak shapes are noticeably worsened and the intensities are significantly reduced, reflecting the large fluctuations in the envelope of the sinusoidal signal.

[0037] Therefore, in one modification, as shown in FIG. 3 , even when a square wave signal is extracted from the DDS 530 using a comparator 5303 built into the DDS 530, the square wave signal is first input to a square wave converter 5320, which then shapes the square wave signal. Specifically, a square wave signal with a duty ratio of 50% and minimal fluctuation in high-level and low-level voltages is obtained. The output of the square wave converter 5320 is input to a second LPF 5321, which converts the square wave signal back into a sine wave signal. The square wave converter 5320 can be the same as that used in the RF voltage generator 53 of the above embodiment, such as a 1 / N divider using a logical operation circuit.

[0038] FIG. 17 shows the waveforms of the various parts in this case, and FIG. 18 shows the shape of the mass peaks at this time. From FIG. 17, it can be seen that the voltage fluctuations have been greatly improved. Furthermore, from FIG. 18, it can be confirmed that the mass peak shape has improved and that the intensity is as high as in FIG. 9. In this way, the noise removal unit 532 is also useful when outputting a square wave signal from the DDS 530 using the comparator 5303 built into the DDS 530.

[0039] 2, 3, etc., a digital circuit section including the DDS 530 and an analog circuit section mainly consisting of the VGA 533 and subsequent circuits are mixed together, and various situations can be considered in circuit design, such as when it is desired to divide the board into multiple sections because they cannot all fit on a single board, or when it is desired to separate the digital circuit section and the analog circuit section. In the latter case, the digital circuit section and the analog circuit section may be placed on separate boards, or the two may be completely separated within a single board.

[0040] In such a case, the RF voltage generating units 53 and 53X can have the configurations shown in Figures 4 to 6. In Figures 4 to 6, reference numeral 53A denotes a digital circuit unit, reference numeral 53B denotes an analog circuit unit, and reference numeral 53C denotes wiring between the circuit units that connects them. Figures 4 and 5 show configuration examples corresponding to the circuit shown in Figure 2, and Figure 6 shows a configuration example corresponding to the circuit shown in Figure 3.

[0041] 4, a buffer amplifier 536 is provided at the output of the first LPF 531, and a sine wave signal amplified by the buffer amplifier 536 is sent to the analog circuit unit 53B via the inter-circuit wiring 53C. In the analog circuit unit 53B, the sine wave signal is converted into a square wave signal by the square wave converter 5320, and then converted back into a sine wave signal by the second LPF 5321. When the sine wave signal is transmitted via the inter-circuit wiring 53C, noise due to external interference or the like may be superimposed on the sine wave signal or distortion may occur. However, when the square wave converter 5320 receives the sine wave signal and converts it into a square wave signal, the noise and distortion are removed, and a sine wave signal with a good waveform can be obtained at the output stage of the second LPF 5321.

[0042] 5, a square wave converter 537 is provided at the output of the first LPF 531, and the square wave signal binarized by the square wave converter 537 is sent to the analog circuit 53B via the inter-circuit wiring 53C. In the analog circuit 53B, the square wave signal is shaped by the square wave converter 5320 and converted back into a sine wave signal by the second LPF 5321. Therefore, in this configuration, the signal passing through the inter-circuit wiring 53C is a square wave signal, i.e., a digital signal. Even in this case, there is a possibility that noise due to external interference may be superimposed on the square wave signal or cause distortion. However, when the square wave converter 5320 receives the square wave signal and shapes it, the noise and distortion are removed, and a sine wave signal with a good waveform is obtained at the output stage of the second LPF 5321.

[0043] 6, a square wave signal output by the comparator 5303 built into the DDS 530 is sent to the analog circuit unit 53B via the inter-circuit wiring 53C. In the analog circuit unit 53B, the square wave signal is shaped by the square wave converter 5320 and converted back into a sine wave signal by the second LPF 5321. Therefore, even in this configuration, the signal passing through the inter-circuit wiring 53C is a square wave signal, i.e., a digital signal. In this case, there is also a possibility that noise due to external interference may be superimposed on the square wave signal or distortion may occur. However, when the square wave converter 5320 receives this noise and distortion and shapes the square wave signal, the noise and distortion are removed, and a sine wave signal with a good waveform can be obtained at the output stage of the second LPF 5321.

[0044] By adopting the configurations shown in Figures 4 to 6 above, not only can RF voltages based on sine wave signals with good waveform shapes be applied to the rod electrodes 31 to 34, but there are also advantages in that the degree of freedom in designing the substrate and housing is increased, design is made easier, and this leads to cost reduction.

[0045] As already mentioned, it goes without saying that the type of device of the DDS 530 used in the above-mentioned RF voltage generating units 53 and 53X and the specific configuration (devices used) of the rectangular wave converting unit 5320 can be selected appropriately within the range that satisfies the required functions.

[0046] 2 to 6 are merely schematic block diagrams, and it will be clear to those skilled in the art that when implementing these, either a single-ended input configuration or a differential input configuration can be selected. As is well known, a differential input configuration requires more complex wiring and more circuit elements, resulting in higher costs, but it has advantages such as the ability to remove common-mode noise.

[0047] Furthermore, although the embodiment of the apparatus shown in FIG. 1 is an application of the present invention to a single-type quadrupole mass spectrometer, it goes without saying that the present invention can be applied to other types of mass spectrometers equipped with a quadrupole mass filter, such as a triple quadrupole mass spectrometer or a quadrupole time-of-flight mass spectrometer.

[0048] Furthermore, the above-described embodiment and modified examples are merely examples of the present invention, and it goes without saying that any appropriate modifications, alterations, additions, etc. made within the spirit of the present invention will also be encompassed within the scope of the claims of the present application.

[0049] Various Aspects It will be appreciated by those skilled in the art that the exemplary embodiments described above are examples of the following aspects.

[0050] (Item 1) One aspect of the quadrupole mass spectrometer according to the present invention has a voltage generating unit that generates an RF voltage to be applied to electrodes that constitute a quadrupole mass filter, and the voltage generating unit comprises: an RF signal generating unit that generates a sine wave signal using a direct digital synthesizer; a first filter that removes high frequency noise superimposed on the sine wave signal; a waveform converting unit that receives the sine wave signal that is the output of the first filter as input and generates a square wave signal having a frequency that is 1 / N (where N is an integer greater than or equal to 1) of the frequency of the sine wave signal; and a second filter that converts the square wave signal generated by the waveform converting unit into a sine wave signal by removing high frequency components from the square wave signal.

[0051] (Item 4) Another aspect of the quadrupole mass spectrometer according to the present invention has a voltage generating unit that generates an RF voltage to be applied to electrodes that constitute a quadrupole mass filter, the voltage generating unit comprising: an RF signal generating unit that generates a sine wave signal using a direct digital synthesizer; a first filter that removes high frequency noise superimposed on the sine wave signal; a square wave generating unit that binarizes the sine wave signal that is the output of the first filter to generate a square wave signal; a waveform converting unit that receives the square wave signal generated by the square wave generating unit as input and shapes the square wave signal using a logical operation circuit or divides the square wave waveform by 1 / N (where N is an integer greater than or equal to 2) to generate a new square wave signal; and a second filter that converts the square wave signal output from the waveform converting unit into a sine wave signal by removing high frequency components from the square wave signal.

[0052] According to the quadrupole mass spectrometers described in paragraphs 1 and 4, even if the output of the RF signal generator included in the DDS device contains low-frequency noise due to the influence of the power supply voltage or the like that is not removed by the first filter, the waveform converter and second filter can substantially remove such noise, thereby obtaining a high-purity sine wave signal. By generating RF voltages to be applied to each electrode of the quadrupole mass filter based on this high-purity sine wave signal, the quadrupole mass filter can be driven in a near-ideal state, enabling analysis with high precision, resolution, sensitivity, etc.

[0053] (Items 2 and 5) In the quadrupole mass spectrometer described in items 1 or 4, the waveform converter may be an N-divider circuit, where N is 2 or more, configured using a logical operation device.

[0054] According to the quadrupole mass spectrometer described in paragraphs 2 and 5, a rectangular wave signal with a duty ratio of approximately 50% can be output from the waveform converter, and a sine wave signal with a good waveform shape without distortion can be output from the second filter, thereby making it possible to make the waveform shape of the RF voltage applied to the rod electrodes closer to ideal, and further improving the analytical precision, resolution, and sensitivity.

[0055] (Item 6) In the quadrupole mass spectrometer described in Item 4, the RF signal generator and the square wave generator may be built into the same direct digital synthesizer device. The square wave generator here is, for example, the comparator in the above embodiment.

[0056] (Clause 3, 7) The quadrupole mass spectrometer described in clause 1 or 4 may further include a variable gain amplifier that amplifies the sine wave signal output from the second filter, and whose gain is adjusted according to the difference between the monitor value and the target value of the voltage applied to the electrode.

[0057] In the quadrupole mass spectrometer described in paragraphs 3 and 7, the gain can be appropriately adjusted by the variable gain amplifier, and an appropriate voltage corresponding to a target value can be applied to the electrodes constituting the quadrupole mass filter, thereby enabling the quadrupole mass filter to effectively select ions having a desired mass-to-charge ratio.

[0058] 1...Ion source 2...Ion transport optical system 3...Quadrupole mass filter 31 to 34...Rod electrodes 4...Detector 5...Power supply unit 51...Voltage control unit 52...DC voltage generation unit 53, 53X...RF voltage generation unit 54...RF / DC voltage addition unit 530...Direct digital synthesizer (DDS) 5300...Phase accumulator 5301...Sine wave conversion unit 5302...Digital-to-analog converter (DAC) 5303...Comparator 531...First low-pass filter (LPF) 532...Noise removal unit 5320...Square wave conversion unit 5321...Second low-pass filter (LPF) 533...Variable gain amplifier (VGA) 534...Error amplifier 535...Amplifier 536...Buffer amplifier 537...Square wave conversion unit 53A...Digital circuit unit 53B...Analog circuit unit 53C...Wiring between circuit parts

Claims

1. A quadrupole mass spectrometer having a voltage generation unit that generates an RF voltage to be applied to electrodes that constitute a quadrupole mass filter, the voltage generation unit comprising: an RF signal generation unit that generates a sine wave signal using a direct digital synthesizer; a first filter that removes high frequency noise superimposed on the sine wave signal; a waveform conversion unit that receives the sine wave signal output from the first filter as input and generates a square wave signal with a frequency that is 1 / N (N is an integer greater than or equal to 1) of the frequency of the sine wave signal; and a second filter that converts the square wave signal generated by the waveform conversion unit into a sine wave signal by removing the high frequency components of the square wave signal.

2. The quadrupole mass spectrometer according to claim 1, wherein the waveform converter is an N-divider circuit, where N is 2 or more, configured using a logical operation device.

3. The quadrupole mass spectrometer of claim 1, further comprising a variable gain amplifier that amplifies the sine wave signal output from the second filter, the gain of which is adjusted according to the difference between the monitor value and the target value of the voltage applied to the electrode.

4. A quadrupole mass spectrometer having a voltage generation unit that generates an RF voltage to be applied to electrodes that constitute a quadrupole mass filter, the voltage generation unit comprising: an RF signal generation unit that generates a sine wave signal using a direct digital synthesizer; a first filter that removes high-frequency noise superimposed on the sine wave signal; a square wave generation unit that binarizes the sine wave signal that is the output of the first filter to generate a square wave signal; a waveform conversion unit that receives the square wave signal generated by the square wave generation unit as input and shapes the square wave signal using a logical operation circuit or divides the square wave waveform by 1 / N (where N is an integer of 2 or more) to generate a new square wave signal; and a second filter that converts the square wave signal output from the waveform conversion unit into a sine wave signal by removing high-frequency components.

5. The quadrupole mass spectrometer according to claim 4, wherein the waveform converter is an N-divider circuit, where N is 2 or more, configured by a logical operation device.

6. The quadrupole mass spectrometer according to claim 4, wherein the RF signal generating section and the square wave generating section are built into the same direct digital synthesizer device.

7. A quadrupole mass spectrometer as described in claim 4, further comprising a variable gain amplifier that amplifies the sine wave signal output from the second filter, the gain of which is adjusted according to the difference between the monitor value and the target value of the voltage applied to the electrode.

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