A method for mass axis correction for a quadrupole mass spectrometer

Abstract

The present application belongs to the technical field of mass spectrometer, and particularly relates to a mass axis correction method for a quadrupole mass spectrometer. The method obtains the relationship between the mass number and the AC / DC voltage through the ion motion equation in the quadrupole, obtains the relationship between the RF voltage DAC code value and the AC / DC voltage through experiments, and further obtains the relationship between the RF voltage DAC code value and the mass number. Meanwhile, according to the mass axis and resolution correction requirements, the mass axis and resolution coefficient corresponding to multiple mass numbers meeting the requirements are obtained, and finally the mass axis and resolution coefficient in the full mass range are obtained by using polynomial fitting. The present application realizes the consistency of the mass axis and resolution in the full mass range by using the polynomial fitting method, improves the sensitivity at the low mass number end, and further improves the mass axis accuracy and test performance of the instrument.

Description

Technical Field

[0001] This invention relates to the field of mass spectrometry, and in particular to a method for mass axis correction in a quadrupole mass spectrometer. Background Technology

[0002] Mass spectrometry is one of the most widely used and promising scientific instruments today. It can rapidly and simultaneously detect almost all elements on the periodic table, making it the most powerful and recognized method for elemental analysis. A mass spectrometer mainly consists of the following parts: a sample introduction system, an ion source, an interface section, an ion focusing system, a mass analyzer, and a detector. There are many types of mass spectrometers, but the quadrupole mass spectrometer is currently the most mature and widely used.

[0003] The basic principle of a quadrupole mass spectrometer is as follows: the sample to be analyzed is ionized by an ion source to form an ion stream, which enters the vacuum system through an interface. In the ion focusing system, neutral ions and photons are intercepted, while positive ions pass through normally and are focused into the mass analyzer. The quadrupole mass analyzer system separates the ions according to their mass-to-charge ratio (m / z) and finally introduces them into the detector. The detector converts the ions into electron pulses, which are then collected and counted by the data acquisition module. The count value is related to the concentration of the analyte ions in the sample. By comparing the count with a standard of known concentration, quantitative analysis of trace elements in unknown samples can be achieved.

[0004] A quadrupole mass analyzer typically consists of four cylindrical or hyperboloid electrode rods. The four rods are paired, and each pair is applied with an AC voltage (V) and a DC voltage (±U) of radio frequency (RF). The applied RF voltage amplitudes can be the same, but their phases must differ by 180°. The ions to be analyzed are introduced axially into one end of the quadrupole mass analyzer, their velocity determined by their energy and mass. The applied RF voltage deflects all ions into an oscillating path through the rods. If the RF and DC voltages are appropriately selected, only ions with a given m / z will achieve a stable path through the rods and exit from the other end. Other ions will be over-deflected, collide with the rods, and be neutralized and lost.

[0005] The motion of ions in a quadrupole mass analyzer can be described by the solution of the Mathieu equation, which has two parameters, a and q. The solution corresponds to three stability regions: the first, second, and third stability regions. These stability regions are essentially functions of q from a to a certain value (a ~ U (m / z)). In quadrupole mass analyzers, the most commonly used region is the first stability region near the origin, which is roughly triangular in shape. Figure 7 As shown, as long as the absolute values ​​of a and q are both within the triangle, the ion has a stable trajectory and can smoothly pass through the quadrupole field to reach the ion detection system.

[0006] Mass axis correction involves adjusting the mass calibration scale to match the mass scale of a known element. Simultaneously, it allows adjustment of the resolution (defined in GB / T 34826-2017, Methods for Determining the Performance of Quadrupole Inductively Coupled Plasma Mass Spectrometers), specifically the peak width at 10% of the mass peak height of a given element. To provide m / z separation, traditional literature often employs a constant a / q value, applying a scan line with "U / V = constant." At specific U and V values, each given point on the scan line corresponds to a specific m / z value. When U and V values ​​change, the a and q values ​​also change, but the ratio remains constant, effectively shifting from one m / z value along the scan line to another. Figure 8 As shown.

[0007] However, in the above correction method, since U / V is a constant value, the peak width for different mass numbers increases with the increase of the mass number, meaning that the resolution differs for each mass number. Therefore, this method cannot meet the requirement of consistent resolution across the entire mass range.

[0008] In existing technologies, to ensure consistent resolution across the entire mass range, a common approach is to select appropriate RF and DC values ​​for each mass number and perform a linear fit using the least squares method to achieve a constant peak width across the entire mass range. However, for the low mass number range, the weights allocated during the fitting process are relatively low, resulting in a significant difference between the fitted RF and DC voltages and the optimal values. Consequently, the sensitivity at the low mass number range is lower than the actual value. Summary of the Invention

[0009] The purpose of this invention is to provide a mass axis calibration method for a quadrupole mass spectrometer. This method uses polynomial fitting to perform automatic mass axis calibration across the entire mass range, achieving consistency between the mass axis and resolution across the entire mass range, and improving sensitivity at the low mass number end, thereby enhancing the instrument's mass axis accuracy and testing performance.

[0010] To achieve the above objectives, the present invention provides the following solution:

[0011] A method for mass axis calibration of a quadrupole mass spectrometer, the method comprising the following steps:

[0012] S1. Prepare standard solution 1 and standard solution 2;

[0013] S2. Based on the equation of motion of ions in a quadrupole, calculate the theoretical relationship between the mass number m and the RF voltage DAC code value;

[0014] S2. Based on the equation of motion of ions in a quadrupole, calculate the theoretical relationship between the mass number m and the DC voltage DAC code value;

[0015] S4. Set the quality number m, as well as the corresponding RF voltage DAC code value and DC voltage DAC code value;

[0016] S5. Set the quality scan range;

[0017] S6. Set the mass axis calibration coefficient;

[0018] S7, Set the resolution calibration factor;

[0019] S8. Use a quadrupole mass spectrometer to measure standard solution 1. Perform spectral peak scanning according to the mass scanning range described in S5. Adjust the mass axis calibration coefficient and the resolution calibration coefficient according to the peak center position and resolution, so that the mass axis is within ±0.1 amu of the calibration value and the resolution is <0.8 amu.

[0020] S9. Set the RF voltage DAC code value and DC voltage DAC code value corresponding to different mass numbers in descending order of mass number, and adjust the mass axis calibration coefficient and the mass axis calibration coefficient for each mass number according to the steps described in S5-S8 to obtain the mass axis calibration coefficient and the mass axis calibration coefficient corresponding to each mass number.

[0021] The S10 quadrupole mass spectrometer software uses a polynomial fitting method to obtain the mass axis calibration coefficient and resolution calibration coefficient corresponding to each mass number across the entire mass range, based on the mass axis calibration coefficient and resolution calibration coefficient corresponding to each mass number.

[0022] S11. The calibration solution 2 is tested using the quadrupole mass spectrometer software to obtain the spectral peaks of each element in the calibration solution 2.

[0023] Furthermore, the quadrupole mass spectrometer is an inductively coupled plasma mass spectrometer (ICP-MS), a liquid chromatography-mass spectrometer (LC-MS), or a gas chromatography-mass spectrometer (GC-MS).

[0024] Furthermore, the standard solution 1 is a 1 ng / ml standard solution prepared by mixing 100 ug / ml multi-element standard solution with 18.25 MΩ·cm high-purity water and high-purity nitric acid solution.

[0025] Furthermore, the standard solution 1 is a multi-element standard solution containing Be, Co, Y, In, Ce, Bi, and U elements.

[0026] Furthermore, the standard solution 2 is a 1 ng / ml standard solution prepared by mixing 100 ug / ml multi-element standard solution with 18.25 MΩ·cm high-purity water and high-purity nitric acid solution.

[0027] Furthermore, the standard solution 2 contains Li and Be elements.

[0028] Furthermore, the mass axis calibration method for the quadrupole mass spectrometer is integrated into the mass spectrometer's application software, and the application software for the mass spectrometer performs automatic calibration of the mass axis.

[0029] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects: The present invention uses polynomial fitting to perform automatic mass axis correction across the entire mass range, achieving consistency between the mass axis and resolution across the entire mass range, improving the sensitivity at the low mass number end, and thus improving the mass axis accuracy and testing performance of the instrument. Compared with the single linear fitting correction method in the prior art, the mass axis obtained by the present invention is closer to the theoretical mass axis (±0.1 amu), resulting in higher signal strength and higher overall detection accuracy. Attached Figure Description

[0030] 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.

[0031] Figure 1 The spectrum of the 7Li element corrected for mass axis using a single linear fitting method according to the present invention;

[0032] Figure 2 The spectrum of 7Li element after mass axis correction using the method of this invention;

[0033] Figure 3 A comparison diagram of the mass axis correction of the 7Li element of the present invention using the first linear fitting method and the method of the present invention;

[0034] Figure 4 The spectrum of the 9Be element of this invention after mass axis correction using a single linear fitting method;

[0035] Figure 5 The spectrum of the 9Be element after mass axis correction using the method of this invention;

[0036] Figure 6 A comparison diagram of the mass axis correction of the 9Be element of this invention using a linear fitting method and the method of this invention;

[0037] Figure 7 This is an image of the I-th stable region near the origin in the prior art;

[0038] Figure 8 An image where the m / z value moves along the scan line in the prior art;

[0039] Figure 9 This is a flowchart of the mass axis calibration method for a quadrupole mass spectrometer according to the present invention. Detailed Implementation

[0040] 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.

[0041] The purpose of this invention is to provide a mass axis calibration method for a quadrupole mass spectrometer. The calibration coefficient can be obtained by using calibration elements to achieve consistency between the mass axis and resolution across the entire mass range, and improve the sensitivity at the low mass number end, thereby improving the mass axis accuracy and testing performance of the instrument.

[0042] 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. Example 1:

[0043] like Figure 9 As shown, the mass axis correction method for a quadrupole mass spectrometer of the present invention can obtain correction coefficients through correction elements, and specifically includes the following steps:

[0044] S1. Prepare standard solution 1 and standard solution 2;

[0045] S2. Based on the equation of motion of ions in a quadrupole, calculate the theoretical relationship between the mass number m and the radio frequency voltage DAC code value (RF code);

[0046] S2. Based on the equation of motion of ions in a quadrupole, calculate the theoretical relationship between the mass number m and the DC voltage DAC code value (DC code);

[0047] S4. Set the RF code and DC code corresponding to the mass number m when it is 89amu;

[0048] S5. Set the quality scan range to 85-95 amu (5 amu on each side of the mass to be measured).

[0049] S6. Set the mass axis calibration coefficients kRF=1 and bRF=0;

[0050] S7. Set the resolution calibration coefficients kDC=0.6 and bDC=0;

[0051] S8. Using a quadrupole mass spectrometer, measure the standard solution 1. Perform peak scanning according to the mass range in A5. Adjust the mass axis calibration coefficient kRF and resolution calibration coefficient kDC according to the peak center position (mass axis) and resolution, so that the mass axis m is within ±0.1 amu of the calibration value and the resolution is 0.7 to 0.8 amu.

[0052] S9. Sequentially set the RF code and DC code corresponding to the mass numbers 9, 59, 89, 115, 140, 209, and 238, and adjust the kRF and kDC coefficients for each mass number according to steps A5-A8 to obtain the kRF and kDC corresponding to each mass number.

[0053] The S10 quadrupole mass spectrometer software uses a polynomial fitting method to obtain the kRF_m and kDC_m corresponding to the mass number m in the full mass range (1-250 amu) based on the seven mass numbers 9, 59, 89, 115, 140, 209 and 238 (where the subscript m represents 9, 59, 89, 115, 140, 209 and 238).

[0054] S11. In the quadrupole mass spectrometer software system, the calibration solution 2 is tested using the correction coefficient to obtain the spectral peaks of each element in the calibration solution 2.

[0055] In this embodiment, the quadrupole mass spectrometer can be an inductively coupled plasma mass spectrometer (ICP-MS), a liquid chromatography-mass spectrometer (LC-MS), or a gas chromatography-mass spectrometer (GC-MS).

[0056] In this embodiment, standard solution 1 is a 1 ng / ml standard solution prepared by mixing 100 ug / ml multi-element standard solution with 18.25 MΩ·cm high-purity water and high-purity nitric acid solution. The multi-element standard solution contains Be, Co, Y, In, Ce, Bi and U.

[0057] In this embodiment, standard solution 2 is a 1 ng / ml standard solution prepared by mixing 100 ug / ml multi-element standard solution with 18.25 MΩ·cm high-purity water and high-purity nitric acid solution. This multi-element standard solution contains Li and Be.

[0058] In this embodiment, the mass axis calibration method for a quadrupole mass spectrometer of the present invention can be integrated into the application software of the mass spectrometer, and the mass axis calibration can be automatically completed when the mass spectrometer is working.

[0059] Example 2:

[0060] (1) In this embodiment, the quadrupole mass spectrometer used is an ICP-MS (inductively coupled plasma mass spectrometer).

[0061] (2) The standard solution 1 described in S1 was tested. According to theoretical calculations, the RF codes and DC codes of the seven elements 9Be, 59Co, 89Y, 115In, 140Ce, 209Bi and 238U were set respectively, as well as kRF_m and kDC_m, and the spectra of each element were obtained.

[0062] (3) For the test spectra in (2), the software automatically adjusts the kRF_m and kDC_m of each spectrum to obtain 7 sets of kRF_m and kDC_m that meet the conditions.

[0063] (4) Apply the least squares method to the 7 groups of kRF_m and kDC_m in (3) to perform a linear fit and obtain the fitting curve. From this, the RF code and DC code of any mass number element can be obtained.

[0064] (5) Apply a quadratic polynomial to the 7 groups of kRF_m and kDC_m in (3) to obtain the fitting curve, and from this, the RF code and DC code of any mass number element can be obtained.

[0065] (6) Taking 7Li and 9Be as examples, the standard solutions described in S1 were tested to obtain the spectra of 7Li and 9Be under two fitting methods.

[0066] Figure 1 The spectrum of 7Li element after mass axis correction using a linear fitting method; Figure 2 The spectrum of 7Li after mass axis correction using the method of this invention; Figure 3 A comparison of the mass axis correction for 7Li element using a single linear fitting method and the method of the present invention; Figure 4 The spectrum of the 9Be element after mass axis correction using a linear fitting method; Figure 5 The spectrum of 9Be element after mass axis correction using the method of this invention; Figure 6 A comparison chart showing the mass axis correction for the 9Be element using a linear fitting method and the method of this invention.

[0067] from Figure 1-6 The comparison shows that after correction using the method of the present invention, the mass axis is closer to the theoretical mass axis (±0.1 amu) than the single linear fitting correction method, and the resulting signal strength is also higher than that of the single fitting. This embodiment significantly improves the accuracy of the instrument's mass axis and the test performance.

[0068] The above descriptions only illustrate some embodiments of the present invention and are quite specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

[0069] Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of this invention should be within the protection scope of the appended claims. In the above description, numerous specific details have been set forth to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that these specific details are not necessary to practice the invention. In other instances, to avoid obscuring the invention, well-known techniques, such as specific construction details, operating conditions, and other technical conditions, have not been specifically described.

[0070] 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 method for mass axis calibration in a quadrupole mass spectrometer, characterized in that, Includes the following steps: S1. Prepare standard solution 1 and standard solution 2; S2. Based on the equation of motion of ions in a quadrupole, calculate the theoretical relationship between the mass number m and the RF voltage DAC code value; S3. Based on the equation of motion of ions in a quadrupole, calculate the theoretical relationship between the mass number m and the DC voltage DAC code value; S4. Set the quality number m, as well as the corresponding RF voltage DAC code value and DC voltage DAC code value; S5. Set the quality scan range; S6. Set the mass axis calibration coefficient; S7, Set the resolution calibration factor; S8. Use a quadrupole mass spectrometer to measure standard solution 1. Perform spectral peak scanning according to the mass scanning range described in S5. Adjust the mass axis calibration coefficient and the resolution calibration coefficient according to the peak center position and resolution, so that the mass axis is within ±0.1 amu of the calibration value and the resolution is <0.8 amu. S9. Sequentially set the RF voltage DAC code value and DC voltage DAC code value corresponding to multiple different mass numbers, and adjust the mass axis calibration coefficient and the resolution calibration coefficient for each mass number according to the steps described in S5-S8 to obtain the mass axis calibration coefficient and the resolution calibration coefficient corresponding to each mass number. The S10 quadrupole mass spectrometer software uses a quadratic polynomial fitting method to obtain the mass axis calibration coefficients and resolution calibration coefficients corresponding to the mass numbers across the entire mass range, based on the mass axis calibration coefficients and resolution calibration coefficients corresponding to each mass number. S11. The standard solution 2 is tested using the quadrupole mass spectrometer software to obtain the spectral peaks of each element in the standard solution 2.

2. The mass axis calibration method for a quadrupole mass spectrometer according to claim 1, characterized in that, The quadrupole mass spectrometer is an inductively coupled plasma mass spectrometer (ICP-MS), a liquid chromatography-mass spectrometer (LC-MS), or a gas chromatography-mass spectrometer (GC-MS).

3. The mass axis calibration method for a quadrupole mass spectrometer according to claim 1, characterized in that, In step S1, the standard solution 1 is a 1 ng / ml standard solution prepared by mixing 100 ug / ml of the first multi-element standard solution with 18.25 MΩ·cm of high-purity water and high-purity nitric acid solution.

4. The mass axis calibration method for a quadrupole mass spectrometer according to claim 3, characterized in that, The standard solution 1 is a first multi-element standard solution, which contains Be, Co, Y, In, Ce, Bi and U elements.

5. The mass axis calibration method for a quadrupole mass spectrometer according to claim 1, characterized in that, The standard solution 2 is a 1 ng / ml standard solution prepared by mixing 100 ug / ml of the second multi-element standard solution with 18.25 MΩ·cm of high-purity water and high-purity nitric acid solution.

6. The mass axis calibration method for a quadrupole mass spectrometer according to claim 5, characterized in that, The second multi-element standard solution contains Li and Be elements.

7. The mass axis calibration method for a quadrupole mass spectrometer according to claim 1, characterized in that, The mass axis calibration method for a quadrupole mass spectrometer is integrated into the mass spectrometer's application software, which is used for automatic mass axis calibration.

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