Background signal processing circuit and method for a quadrupole mass spectrometer

Abstract

The application provides a background signal processing circuit and method for a quadrupole mass spectrometer, which comprises a pre-signal processing circuit and a data acquisition circuit, the pre-signal processing circuit comprises an amplification circuit, a fast discrimination circuit, a pulse shaping circuit and a differential output circuit connected in series, the amplification circuit is connected with an electron multiplier, and weak current pulse signals output by the electron multiplier are input to the amplification circuit; the data acquisition circuit comprises an FPGA core circuit and a pulse receiving circuit, a network port communication circuit, a quadrupole power voltage control circuit and an SPI interface circuit connected with the FPGA core circuit respectively, the differential output circuit is connected with the pulse receiving circuit, and the SPI interface circuit is connected with the fast discrimination circuit. By improving the hardware circuit structure, the application can automatically filter various noise signals in front of pulse counting, ensure the accuracy of pulse counting, and solve many problems caused by background signal interference in mass spectrum.

Description

Technical Field

[0001] This invention relates to the field of quadrupole mass spectrometry, and in particular to a background signal processing circuit and method for quadrupole mass spectrometry. Background Technology

[0002] Quadrupole mass spectrometers are among the most widely used scientific instruments in the field of elemental analysis. They consist of the following parts: a sample introduction system, an ion source, an ion transport system, a mass analyzer, and a detector. After the sample passes through the sample introduction system, it is ionized by the ion source to form an ion stream. This stream enters the ion transport system under vacuum, where, under the influence of an electromagnetic field, positively charged ions are focused and enter the mass analyzer. According to the Mathieu equation, by changing the ratio of DC to RF voltage on the quadrupole mass analyzer, ions with different mass-to-nucleus ratios are separated and enter the detector at a specific angle. The ions are converted into electrons, which are then amplified stepwise to form an electron stream, outputting a weak current pulse signal from the electrodes. After amplification and noise reduction by the pre-signal processing module, the signal enters the data acquisition module to complete pulse counting. The data is then processed by software to obtain the concentration of trace elements in the sample, thus achieving quantitative elemental analysis.

[0003] Currently, the most commonly used detector in mass spectrometers is the electron multiplier. Based on the electrode arrangement, they can be classified as: discontinuous electrode electron multipliers, continuous electrode electron multipliers, switching electrode electron multipliers, and dual-mode electron multipliers. Among these, the dual-mode (analog mode and pulse mode) electron multiplier is the most widely used in quadrupole mass spectrometers, characterized by high sensitivity, fast response, wide dynamic range, and high signal-to-noise ratio (SNR). SNR is often used as a primary indicator for evaluating detector performance. Electron multiplier noise consists of dark current noise, scattering noise, and electrode output noise, with dark current noise being the dominant factor. The sample background noise, the electron multiplier's dark current noise, and the output noise of the pre-amplifier circuit all directly affect the pulse count value and, as the background signal of the mass spectrometer, influence key performance indicators such as the sensitivity and detection limit of the quadrupole mass spectrometer. Existing background signal processing circuits cannot simultaneously remove various types of noise and suffer from the inconvenience and lack of flexibility in manually adjusting the background signal. Summary of the Invention

[0004] The purpose of this invention is to provide a background signal processing circuit and method for a quadrupole mass spectrometer. By setting up a pre-signal processing circuit and a data acquisition circuit, and improving the hardware circuit, it is possible to automatically filter out various noise signals at the pulse counting front end, ensure the accuracy of pulse counting, and solve many problems caused by background signal interference in the mass spectrometer spectrum.

[0005] To achieve the above objectives, the present invention provides the following solution:

[0006] A background signal processing circuit for a quadrupole mass spectrometer includes: a pre-signal processing circuit and a data acquisition circuit. The pre-signal processing circuit includes an amplification circuit, a fast discrimination circuit, a pulse shaping circuit, and a differential output circuit connected in series. The amplification circuit is connected to an electron multiplier, and the weak current pulse signal output by the electron multiplier is input to the amplification circuit.

[0007] The data acquisition circuit includes an FPGA core circuit and a pulse receiving circuit, a network communication circuit, a four-stage power supply voltage control circuit, and an SPI interface circuit, all connected to the FPGA core circuit. The differential output circuit is connected to the pulse receiving circuit, and the SPI interface circuit is connected to the fast discrimination circuit.

[0008] Furthermore, the fast discrimination circuit includes a high-precision DAC chip, an operational amplifier chip, and a fast discriminator connected in sequence, and the SPI interface circuit is connected to the high-precision DAC chip.

[0009] Furthermore, the network port communication circuit is connected to the PC.

[0010] The present invention also provides a background signal processing method for a quadrupole mass spectrometer, applied to the aforementioned background signal processing circuit for a quadrupole mass spectrometer, comprising the following steps:

[0011] Based on the output signal characteristics of the electron multiplier and the design experience of the amplifier circuit, the initial value of the threshold voltage of the fast discrimination circuit is set.

[0012] The FPGA core circuit sets the quadrupole DC voltage and RF voltage for the target mass number;

[0013] Set the integration time, configure the high-precision DAC chip, and initialize the threshold voltage. Continuously record the pulse count N within the set integration time, and compare the average value N0 with 0. When N0 > the first set value, set the threshold step voltage b to 5mV; when the first set value > N0 > the second set value, set the threshold step voltage b to 2mV; when the second set value > N0 > the third set value, set the threshold step voltage b to 1mV; when N0 < the third set value, set the threshold step voltage b to 0.5mV; when N0 = 0, the current threshold voltage is recorded in the FPGA core circuit, and background signal processing automatically ends. The order of priority is: first set value > second set value > third set value.

[0014] Preferably, the initial value of the threshold voltage is -10mV.

[0015] Preferably, the integration time is 50ms and the set number of times is 5.

[0016] Preferably, the first setting value is 1000, the second setting value is 100, and the third setting value is 50.

[0017] Furthermore, the pulse count value consists of pulses generated by the target element ions and noise pulses, wherein the noise includes dark current noise, sample body noise, and circuit output noise.

[0018] According to specific embodiments provided by the present invention, the following technical effects are disclosed: The background signal processing circuit and method for a quadrupole mass spectrometer provided by the present invention includes a pre-signal processing circuit and a data acquisition circuit. The pre-signal processing circuit includes an amplification circuit, a fast discrimination circuit, a pulse shaping circuit, and a differential output circuit connected in series. The amplification circuit is connected to an electron multiplier. The data acquisition circuit includes an FPGA core circuit and a pulse receiving circuit, a network communication circuit, a quadrupole power supply voltage control circuit, and an SPI interface circuit, all connected to the FPGA core circuit. The differential output circuit is connected to the pulse receiving circuit, and the SPI interface circuit is connected to the fast discrimination circuit. By improving the hardware circuit, automatic discrimination and recording of pulse signals are achieved, thus completing the automatic processing of background signals. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of the background signal processing circuit for a quadrupole mass spectrometer according to the present invention;

[0021] Figure 2 This is a schematic flowchart of the background signal processing method for a quadrupole mass spectrometer according to the present invention;

[0022] Figure 3 This is a mass spectrum of elements in a standard solution obtained in an embodiment of the present invention. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] The purpose of this invention is to provide a background signal processing circuit and method for a quadrupole mass spectrometer. By automatically setting the pre-processing circuit and data acquisition circuit through signal feedback, it can automatically filter out various noise signals at the pulse counting front end, ensure the accuracy of pulse counting, and solve many problems caused by background signal interference in the mass spectrometer spectrum.

[0025] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0026] like Figure 1 As shown, the background signal processing circuit for a quadrupole mass spectrometer provided by the present invention includes: a pre-signal processing circuit and a data acquisition circuit. The pre-signal processing circuit includes an amplification circuit, a fast discrimination circuit, a pulse shaping circuit and a differential output circuit connected in series. The amplification circuit is connected to an electron multiplier, and the weak current pulse signal output by the electron multiplier is input to the amplification circuit.

[0027] The data acquisition circuit includes an FPGA core circuit and a pulse receiving circuit, a network communication circuit, a quadrupole power supply voltage control circuit, and an SPI interface circuit, all connected to the FPGA core circuit. The differential output circuit is connected to the pulse receiving circuit, and the SPI interface circuit is connected to the fast discrimination circuit. The network communication circuit is connected to a PC. The FPGA core circuit includes an FPGA chip, an SRAM chip, a Flash chip, and an active crystal oscillator. The FPGA chip is connected to other circuits via I / O pins and controls the output of other circuits through programming. The pulse receiving circuit includes a differential transmission receiver chip and a monostable multivibrator chip. The quadrupole power supply voltage control circuit includes a high-precision dual-channel DAC chip and an IV converter chip, which are connected to the FPGA chip via a parallel port.

[0028] The fast discrimination circuit includes a high-precision DAC chip, an operational amplifier chip, and a fast discriminator connected in sequence, and the SPI interface circuit is connected to the high-precision DAC chip.

[0029] After the sample to be tested passes through the quadrupole mass spectrometer's sample introduction system, it is ionized to form an ion stream, which enters the quadrupole mass analyzer through the lens system. The DC and RF voltages on the quadrupole are set according to the element's mass number, and the ions of the target element are selected to enter the electron multiplier and output a weak current pulse signal.

[0030] The current pulse signal output by the electron multiplier is amplified after IV conversion. The pulse signal is then filtered by a high-precision DAC chip using a preset threshold voltage via the FPGA core circuit. After filtering, the pulse signal enters the pulse shaping circuit, where the pulse width is adjusted to 15ns, and pulse edge jitter is filtered out, facilitating more accurate pulse signal recording. The shaped pulse signal is then output by a differential circuit with strong anti-interference capabilities and transmitted to the data acquisition circuit. After passing through the pulse receiving circuit, it is acquired and recorded by the FPGA core circuit.

[0031] The weak current pulse signal output by the electron multiplier is amplified by IV conversion into a positive pulse voltage signal containing a DC component. At the same time, the pulse height generated by the target element ions is greater than the pulse height generated by the noise. By setting the threshold of the high-bandwidth fast discrimination circuit, the effective pulse signal is identified and the noise signal is subtracted. The pulse width of the effective pulse signal is adjusted to 15ns by the pulse shaping circuit. A differential signal output circuit with strong anti-interference capability is selected to complete the signal transmission with the data acquisition circuit.

[0032] The FPGA core circuit controls the radio frequency voltage of the quadrupole to screen ions of the target element. Simultaneously, it receives pulse signals from the pre-processing circuit, records the pulse count within the integration time, and uploads it to the PC via the network port. The SPI interface circuit enables communication between the FPGA and the high-precision DAC (digital-to-analog converter) in the high-bandwidth, high-speed discrimination circuit.

[0033] The FPGA core circuit can automatically filter out background signals by setting the quadrupole RF voltage of the reference quality number, counting the pulse count value within the integration time and comparing it with 0, and setting the threshold voltage of the fast discriminator output by the DAC via the operational amplifier in real time.

[0034] As described in the background section on the principle of quadrupole mass spectrometers, a mass spectrum is plotted by pulse count values ​​corresponding to the mass numbers of elements after PA correction. The pulse count values ​​consist of pulses generated by the target element ions and noise pulses (dark current noise, sample noise, and circuit output noise). Rapidly identifying the pulses generated by the target element ions is crucial for background signal processing and for the clear presentation of the target element's spectral peaks in the mass spectrum. This invention designs a pre-signal processing circuit and a data acquisition circuit, using hardware to automatically identify and record pulse signals, thus completing the automatic processing of the background signal.

[0035] like Figure 2 As shown, the present invention also provides a background signal processing method for a quadrupole mass spectrometer, applied to the aforementioned background signal processing circuit for a quadrupole mass spectrometer, comprising the following steps:

[0036] Based on the output signal characteristics of the electron multiplier and the design experience of the amplifier circuit, the initial value of the threshold voltage of the fast discrimination circuit is set to -10mV.

[0037] The FPGA core circuit sets the quadrupole DC voltage and RF voltage for the target mass number;

[0038] Set the integration time to 50ms, configure a high-precision DAC chip, and initialize the threshold voltage. Record the pulse count N for five consecutive integration intervals, and compare the average value N0 with 0. When N0 > 1000, set the threshold step voltage b to 5mV; when 1000 > N0 > 100, set the threshold step voltage b to 2mV; when 100 > N0 > 50, set the threshold step voltage b to 1mV; when N0 < 50, set the threshold step voltage b to 0.5mV; when N0 = 0, the current threshold voltage is recorded in the FPGA core circuit, and background signal processing ends automatically.

[0039] The pulse count value is composed of pulses generated by the target element ions and noise pulses, including dark current noise, sample body noise, circuit output noise, etc.

[0040] When N0=0, the current threshold voltage is recorded in the FPGA and remains unchanged. This completes the threshold voltage setting. Selecting the target mass number as a reference for background signal filtering allows for the simultaneous filtering of various background signals, including sample background noise, multiplier dark current noise, and preamplifier output noise.

[0041] In a specific embodiment of this invention, a mixed standard solution containing 1 μg / L of Be(9), Co(59), Y(89), In(115), and Bi(209) elements was selected for testing, and the background signal processing method of this invention was described in detail. The specific steps are as follows:

[0042] A: Selecting the target mass number of the background signal. According to the principle of the quadrupole mass spectrometer and the characteristics of chemical elements, it can be seen that when the target mass number m is set to 220, there are no elemental peaks in the mass spectrum. The pulse count value includes the count of various background signals such as sample background noise, multiplier dark current noise, and preamplifier output noise. Therefore, it is the most suitable as a reference background signal.

[0043] B: FPGA settings include the quadrupole DC voltage and RF voltage when the mass number m=220, the integration time is set to 50ms, a high-precision DAC chip is configured, and the threshold voltage is set to -10mV. The pulse count N is recorded five times within the integration time. The average value N0 is compared with 0. When N0>1000, the threshold step voltage b is set to 5mV; when 1000>N0>100, the threshold step voltage b is set to 2mV; when 100>N0>50, the threshold step voltage b is set to 1mV; when N0<50, the threshold step voltage b is set to 0.5mV; when N0=0, the current threshold voltage is recorded in the FPGA, and background signal processing ends automatically.

[0044] C: In peak sweep mode, the quadrupole DC voltage and RF voltage of elements Be(9), Co(59), Y(89), In(115), and Bi(209) were set, and the elemental mass spectra were obtained as shown in the attached figure. Figure 3 As shown.

[0045] From the appendix of the embodiments Figure 3 As can be seen, the automatic background signal processing method of the present invention has significant effects, clear target element spectral peaks, effectively frees up manpower, and solves the complexity of background signal processing from the perspective of hardware circuits.

[0046] This invention provides a background signal processing method for quadrupole mass spectrometers, applied to the processing of multiplier output signals in quadrupole mass spectrometers. It can automatically and effectively resolve background signal interference caused by the combination of various noises (sample background noise, electron multiplier dark current noise, circuit output noise, etc.) in a single, one-time operation, achieving clear presentation of target element spectral peaks. This invention avoids the inconvenience and lack of flexibility of manually adjusting the background signal, providing stable, reliable, and accurate pulse counting.

[0047] In summary, the background signal processing circuit and method for a quadrupole mass spectrometer provided by this invention includes a pre-signal processing circuit and a data acquisition circuit. The pre-signal processing circuit comprises an amplification circuit, a fast discrimination circuit, a pulse shaping circuit, and a differential output circuit connected in series. The amplification circuit is connected to an electron multiplier. The data acquisition circuit includes an FPGA core circuit and a pulse receiving circuit, a network communication circuit, a quadrupole power supply voltage control circuit, and an SPI interface circuit, all connected to the FPGA core circuit. The differential output circuit is connected to the pulse receiving circuit, and the SPI interface circuit is connected to the fast discrimination circuit. By improving the hardware circuit, automatic discrimination and recording of pulse signals are achieved, thus completing the automatic processing of the background signal.

[0048] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A background signal processing circuit for a quadrupole mass spectrometer, characterized by, The application relates to a background signal processing circuit for a quadrupole mass spectrometer. The background signal processing circuit comprises a front signal processing circuit and a data acquisition circuit, the front signal processing circuit comprises, in sequence, an amplification circuit, a fast discrimination circuit, a pulse shaping circuit and a differential output circuit, the amplification circuit is connected with an electron multiplier, and a weak current pulse signal output by the electron multiplier is input to the amplification circuit. The data acquisition circuit comprises an FPGA core circuit and a pulse receiving circuit, a network port communication circuit, a four-stage rod power voltage control circuit and an SPI interface circuit connected with the FPGA core circuit respectively, the differential output circuit is connected with the pulse receiving circuit, and the SPI interface circuit is connected with the fast discrimination circuit. The fast discrimination circuit comprises, in sequence, a high-precision DAC chip, an operational amplifier chip and a fast discriminator, the SPI interface circuit is connected with the high-precision DAC chip. The application further discloses a background signal processing method applied to the background signal processing circuit for the quadrupole mass spectrometer. According to the signal characteristics output by the electron multiplier and the design experience of the amplification circuit, the initial value of the threshold voltage of the fast discrimination circuit is set. The FPGA core circuit sets the quadrupole direct current voltage and the radio frequency voltage of a target mass number. The integral time is set, the high-precision DAC chip is set so that the threshold voltage is the initial value, the pulse count value N in the integral time is recorded continuously for a set number of times, the average value N0 is compared with 0, when N0 is greater than a first set value, the threshold step voltage b is set to 5mv, when the first set value is greater than N0 and N0 is greater than a second set value, the threshold step voltage b is set to 2mv, when the second set value is greater than N0 and N0 is greater than a third set value, the threshold step voltage b is set to 1mv, when N0 is less than the third set value, the threshold step voltage b is set to 0.5mv, when N0 is 0, the current threshold voltage is recorded in the FPGA core circuit, and the background signal processing is automatically completed; wherein the first set value is greater than the second set value, and the second set value is greater than the third set value. The pulse count value is composed of the pulses generated by target element ions and noise, and the noise comprises dark current noise, sample body noise and circuit output noise.

2. Background signal processing circuit for a quadrupole mass spectrometer according to claim 1, characterized in that, The network port communication circuit is connected with a PC end.

3. The background signal processing circuit for a quadrupole mass spectrometer of claim 1, wherein, The initial value of the threshold voltage is -10mv.

4. The background signal processing circuit for a quadrupole mass spectrometer of claim 1, wherein, The integral time is 50ms, and the set number of times is 5.

5. The background signal processing circuit for a quadrupole mass spectrometer of claim 1, wherein, The first set value is 1000, the second set value is 100, and the third set value is 50.

Images