Method for improving performance of linear ion trap-time of flight mass spectrometry by digital wave phase modulation

Abstract

The present invention relates to a method for improving the performance of a linear ion trap-time of flight mass spectrometer by digital wave phase modulation. A digital wave radio frequency voltage is generated by a digital wave generating device and applied to the linear ion trap. Ions generated by an ion source enter the linear ion trap and are cooled and stored under the action of the digital wave radio frequency electric field. The digital wave radio frequency is turned off at a certain phase point, and an extraction voltage is applied to extract the ions in the trap axially to a time of flight mass spectrometry detector. By changing the radio frequency phase during ion extraction, the performance of the IS-TOF can be regulated.

Description

Technical Field

[0001] The present invention belongs to mass spectrometry instruments, and particularly relates to a method for improving the performance of a linear ion trap-time of flight mass spectrometer by digital wave phase modulation. Background Art

[0002] In an ion trap, through ion-molecule collisions, an ion cloud is gathered near the center of the ion trap, and its phase space distribution is less than 1 mm. Thus, the ion trap can provide an ion source with smaller spatial divergence for a time-of-flight mass spectrometer. Compared with a 3D ion trap, ions in a linear ion trap are prone to gather in the shape of a cigar-shaped cloud along the Z-axis, and the space charge effect is very small, making the storage capacity of the linear ion trap dozens or even hundreds of times higher than that of a 3D ion trap. Currently, the commonly used ion capture method usually uses a sine wave. To extract ions in the trap more efficiently, the RF voltage should be cut off when injecting the ions into the TOF. In fact, it is very difficult to instantaneously turn off a sine wave, and there will be a residual ringing RF field when it is turned off. The residual RF electric field will bring an unpredictable accelerating electric field, disturbing the phase space distribution of the ion cloud, not only causing a loss in ion extraction efficiency, but also distorting the resolution of the time-of-flight mass spectrometer. Summary of the Invention

[0003] The present invention discloses a method for modulating the performance of an ion storage-time of flight mass spectrometer by phase modulation. In a time-of-flight mass spectrometer with a pre-stage ion trap storage device, by changing the RF phase during ion extraction, the performance of the IS-TOF can be regulated.

[0004] The technical solution adopted by the present invention to achieve the above object is: a method for improving the performance of a linear ion trap-time of flight mass spectrometer by digital wave phase modulation, comprising the following steps:

[0005] Generate a digital wave RF voltage through a digital wave generating device and apply it to the linear ion trap. Ions generated by the ion source enter the linear ion trap, and under the action of the digital wave RF electric field, ion cooling and storage are achieved; at a certain phase point, turn off the digital wave RF and apply an extraction voltage to extract the ions in the trap axially to the time-of-flight mass spectrometer detector.

[0006] Ions generated by the ion source enter the linear ion trap. Lower the voltage of the entrance electrode to a negative voltage to inject the ions into the linear ion trap. At the same time, raise the entrance electrode to a positive voltage to prevent the ions from flowing out, and apply a voltage to the RF electrode to capture the injected ions.

[0007] At a certain phase point, turn off the digital wave RF and apply an extraction voltage to extract the ions in the trap axially to the time-of-flight mass spectrometer detector, comprising the following steps:

[0008] Apply pulsed high voltage with positive pressure and negative pressure to the inlet electrode and the outlet electrode respectively, and the absolute values of the positive pressure and the negative pressure are equal; at the same time, turn off the digital wave radio frequency power supply, and extract the ions in the linear ion trap to the time-of-flight mass spectrometry detector according to the set extraction time.

[0009] The waveform, voltage, phase, duty cycle and frequency of the digital wave can be adjusted arbitrarily.

[0010] The digital wave includes, but is not limited to, square wave and triangular wave.

[0011] The phase of the digital wave realizes modulation at any phase point between 0 and 360°.

[0012] The air pressure in the linear ion trap is regulated within the range of 0.1 - several thousand Pa.

[0013] A system for improving the performance of a linear ion trap-time-of-flight mass spectrometry by digital wave phase modulation, comprising:

[0014] A linear ion trap for injecting and storing ions generated by an ion source;

[0015] A time-of-flight mass spectrometry detector for extracting ions from the ion trap; the inlet corresponds to the outlet of the linear ion trap, and the axis of the time-of-flight mass spectrometry detector coincides with the axis of the linear ion trap;

[0016] A digital wave generating device, connected to the radio frequency electrode of the linear ion trap, for applying a digital wave radio frequency voltage to the linear ion trap.

[0017] The linear ion trap is of a rectangular ion trap, quadrupole or segmented quadrupole structure.

[0018] The ion source includes an in-trap ion source and an out-of-trap ion source.

[0019] The present invention has the following beneficial effects and advantages:

[0020] 1. The present invention applies a digital square wave to an ion storage-multiple reflection time-of-flight mass spectrometry. It can achieve precise control of the radio frequency voltage and effectively avoid the acceleration effect of the ringing electric field during ion extraction.

[0021] 2. By changing the radio frequency phase during ion extraction, that is, controlling the phase space distribution of the ion cloud during extraction, the regulation of the resolution and sensitivity of IS-TOF can be achieved. Description of the Drawings

[0022] Figure 1 Schematic diagram of the linear ion trap-TOF structure;

[0023] 1. Inlet electrode, 2. Outlet electrode, 3. Radio frequency electrode, 4. Digital wave generating device, 5. Time-of-flight mass spectrometry detector;

[0024] Figure 2 Example diagram: Schematic diagram of the axially extracted ion trap-TOF structure;

[0025] Figure 3 Ion trap-TOF working timing diagram;

[0026] Figure 4 In this embodiment, the variation trends of the ion mass spectrometry signal intensities and resolutions of m / z 106, m / z 164, and m / z 258 with the digital wave extraction radio frequency phase;

[0027] Among them, a) Polar coordinate diagram of the ion peak signal intensity with a mass-to-charge ratio of m / z 106; b) Polar coordinate diagram of the resolution with a mass-to-charge ratio of m / z 106 varying with the square wave phase; c) Polar coordinate diagram of the ion peak signal intensity with a mass-to-charge ratio of m / z 164; d) Polar coordinate diagram of the resolution with a mass-to-charge ratio of m / z 164 varying with the square wave phase; e) Polar coordinate diagram of the ion peak signal intensity with a mass-to-charge ratio of m / z 258; f) Polar coordinate diagram of the resolution with a mass-to-charge ratio of m / z 258 varying with the square wave phase. Detailed implementation manners

[0028] The following further elaborates on the present invention in conjunction with the accompanying drawings and embodiments.

[0029] The present invention provides an axially tandem linear ion trap-time of flight mass spectrometer, where the said ion trap and the time of flight mass spectrometer are coaxially placed, and the ion cloud is axially extracted from the ion trap to the time of flight mass spectrometer detector. By changing the radio frequency phase during ion extraction, the performance of the IS-TOF can be regulated.

[0030] A method for improving the performance of a linear ion trap-time of flight mass spectrometer by digital wave phase modulation, including an ion source, an ion trap composed of an inlet electrode, an outlet electrode, and a radio frequency electrode, a digital wave generating device, and a time of flight mass spectrometer. A digital wave radio frequency voltage is generated by the digital wave generating device and applied to the linear ion trap.

[0031] Ions are generated by an in-trap ion source or an out-of-trap ion source and enter the linear ion trap, and the ions are cooled and stored under the action of the digital wave radio frequency electric field. Subsequently, the digital wave radio frequency is turned off at a specific phase point, and a double-pulse voltage is applied to axially extract the ions in the trap to the time of flight mass spectrometer detector at the rear end.

[0032] According to the measurement requirements, the waveform, voltage, phase, duty cycle, and frequency of the digital wave can be adjusted arbitrarily, including but not limited to square waves and triangular waves.

[0033] Linear ions include but are not limited to rectangular ion traps, quadrupoles, and segmented quadrupole structures.

[0034] The storage time of the linear ion trap can be adjusted within the range of 0 - 1000 ms.

[0035] The digital wave phase can achieve modulation of any phase point between 0 and 360°.

[0036] The background gas in the linear ion trap is one or more of helium, argon, krypton, nitrogen, oxygen, or dry air.

[0037] The gas pressure in the linear ion trap can be regulated in the range of 0.1 - several thousand Pa.

[0038] The ion source includes an in-trap ion source and an out-of-trap ion source, and the out-of-trap ion source includes various ion sources at atmospheric pressure and negative pressure.

[0039] The ion extraction method of the ion trap is axial extraction.

[0040] Example 1

[0041] A method for improving the performance of a linear ion trap-time-of-flight mass spectrometer by digital wave phase modulation. It includes a linear ion trap, a digital wave generating device, and a time-of-flight mass spectrometer. A digital wave radio frequency voltage is generated by the digital wave generating device and applied to the linear ion trap.

[0042] Ions are generated by the in-trap ion source or the out-of-trap ion source and enter the linear ion trap, where ion cooling and storage are achieved under the action of the digital wave radio frequency electric field. Then, the digital wave radio frequency is turned off at a specific phase point, and an extraction voltage is applied to extract the ions in the trap axially to the time-of-flight mass spectrometer detector at the rear end.

[0043] As Figure 2 shown, in the example, the linear ion trap is a segmented quadrupole structure, composed of an inlet electrode, segmented electrodes, and an outlet electrode. The segmented electrodes are respectively composed of four groups of parallel electrode arrays; each group of electrode arrays is uniformly spaced by one or more concentric cylindrical electrodes, and the cylindrical diameter is 9.04 mm; the centers of the four groups of electrode arrays are uniformly arranged on a circle with a radius of 8.52 mm; the thickness of the cylindrical electrode is 4 mm, and the interval is 0.5 mm. The inlet electrode hole and the outlet electrode are both circular ring electrodes with an inner diameter of 1.5 mm, an outer diameter of 28 mm, and a thickness of 1 mm. DC voltages are applied to the inlet electrode and the outlet electrode respectively; the concentric cylindrical electrodes on any one of the four groups of electrode arrays of the segmented electrodes are uniformly divided by voltage dividing resistors, and each segmented electrode is connected to an equivalent capacitor. The cylindrical electrodes on each group of segmented electrode arrays are applied with the same radio frequency voltage, and the adjacent segmented electrode array groups are applied with the same radio frequency voltage, and the adjacent cylindrical electrode groups are applied with opposite polarities and the same absolute value of radio frequency voltage. The radio frequency voltage is a digital square wave radio frequency voltage (the amplitude of the radio frequency voltage is 200 V, the frequency is 1.5 MHz, and the duty cycle is 0.5).

[0044] By controlling the voltage parameters of the above electrodes through timing, ion injection, cooling, and extraction can be achieved in the linear ion trap. The timing control is asFigure 3 As shown, the mass spectrometry cycle is controlled by the main trigger frequency. By controlling the main trigger frequency, the storage time of the linear ion trap is regulated. Each trigger cycle consists of three segments: ion injection, ion cooling, and ion extraction. During the ion injection period, ion injection is achieved through a DC electric field. The voltage of the entrance electrode is reduced to -5V, creating a potential gradient with the voltage at the front end of the entrance electrode to enable the injection of ions into the linear ion trap. At the same time, the exit electrode is raised to +3V to prevent ions from flowing out, and the RF power supply is turned on to capture the injected ions. Finally, in the cooling region, both the entrance electrode and the exit electrode are modulated to +10V to achieve axial confinement of the ions. The digital wave RF field confines the ions radially. After a certain period of cooling, the ion cloud gradually converges onto the central axis of the quadrupole in space. During the ion extraction period, double-pulse extraction is employed. Pulse high voltages of +300V and -300V are applied to the entrance electrode and the exit electrode respectively, and at the same time, the digital wave RF power supply is turned off. The extraction time is set to 10 μs to extract the ions in the trap to the time-of-flight mass spectrometry detector at the rear end.

[0045] The ion source is a low-pressure photoionization source, and the detector is a multi-reflection time-of-flight mass spectrometer. With 10 ppbv acetone-benzene-p-xylene-tetrachloroethylene-1,3-hexachlorobutadiene as the sample calibration gas, the linear ion trap storage time set at 2 ms, and the MR-TOF flight at 2 circles, the variation trends of the mass spectrometry signal intensities and resolutions of m / z 106, m / z 164, and m / z 258 ions with the digital wave extraction RF phase were tested. The experimental results are as Figure 4 shown in a) - f) below. It can be seen that the corresponding mass spectrometry peak resolutions and signal intensities show periodic variations with the square wave phase, and the phase point is the optimal extraction phase point when the square wave voltage is 0.

Claims

1. A method for improving the performance of a linear ion trap-time of flight mass spectrometer by digital wave phase modulation, characterized in that Comprising the following steps: A digital wave radio frequency voltage is generated by a digital wave generating device and applied to a linear ion trap. Ions generated by an ion source enter the linear ion trap, and under the action of the digital wave radio frequency electric field, ion cooling and storage are achieved; when the square wave voltage is 0, the digital wave radio frequency is turned off at the phase point, and an extraction voltage is applied to extract the ions in the trap axially to a time-of-flight mass spectrometry detector; Pulsed high voltages with positive and negative pressures are respectively applied to the inlet electrode and the outlet electrode, and the absolute values of the positive and negative pressures are equal; meanwhile, the digital wave radio frequency power supply is turned off, and the ions in the linear ion trap are extracted to the time-of-flight mass spectrometry detector according to the set extraction time.

2. The method for improving the performance of a linear ion trap-time of flight mass spectrometer by digital wave phase modulation according to claim 1, wherein Ions generated by the ion source enter the linear ion trap. The voltage of the inlet electrode is reduced to a negative pressure to inject the ions into the linear ion trap. At the same time, the inlet electrode is raised to a positive pressure to prevent the ions from flowing out, and a voltage is applied to the radio frequency electrode to capture the injected ions.

3. The method for improving the performance of a linear ion trap-time of flight mass spectrometer by digital wave phase modulation according to claim 1, characterized in that: The digital wave is a square wave or a triangular wave.

4. The method for improving the performance of a linear ion trap-time of flight mass spectrometer by digital wave phase modulation according to claim 1, characterized in that: The phase of the digital wave realizes modulation at any phase point between 0° and 360°.

5. The system for improving the performance of a linear ion trap-time of flight mass spectrometer by digital wave phase modulation according to claim 1, wherein Including: A linear ion trap for injecting and storing ions generated by an ion source; A time-of-flight mass spectrometry detector for extracting ions from the ion trap; The inlet corresponds to the outlet of the linear ion trap, and the axis of the time-of-flight mass spectrometry detector coincides with the axis of the linear ion trap; A digital wave generating device, connected to the radio frequency electrode of the linear ion trap, for applying a digital wave radio frequency voltage to the linear ion trap.

6. The system for improving the performance of a linear ion trap-time of flight mass spectrometer by digital wave phase modulation according to claim 5, wherein The linear ion trap is a rectangular ion trap, a quadrupole or a segmented quadrupole structure.

7. The system for improving the performance of a linear ion trap-time of flight mass spectrometer by digital wave phase modulation according to claim 5, characterized in that, The ion source includes an in-trap ion source and an out-of-trap ion source.

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