JP3536029B2 - Ion trap mass spectrometry method and ion trap mass spectrometer - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an ion trap mass spectrometry method and an ion trap mass spectrometer.

[0002]

The basic construction and operation of an ion trap is described in US Pat. No. 2,939,952 by Paul et al.

Regarding a mass spectrometer using an ion trap, JP-A-59-134546 and JP-A-62-37861.
JP-A-7-146283, JP-A-10-294078, and US Pat. No. 5,734,162.

As described in each of the above publications, the ion trap mass spectrometer has a ring electrode and a pair of end cap electrodes, and these electrodes form an ion trap space for trapping ions.

The basic operation of an ion trap mass spectrometer having an electron impact (EI) ion source is to ionize a sample in an ion trap space by applying electrons to the sample and accumulate the ions in the ion trap space. A mass spectrometric step in which the generated ions are discharged from the ion trap space for each mass by a high frequency voltage sweep applied to the electrode and detected by a detector. in this way,
The basic operation of mass spectrometry is to go through each step over time.

[0006]

During the above-mentioned mass analysis step, there should be no new ionization or introduction of ions from the outside in the ion trap space.
If ionization or introduction of ions takes place during the mass analysis in the ion trap space, the ions are ejected out of the ion trap space during the main radio frequency voltage sweep of the mass analysis independent of the mass. This ion is detected by the detector. This becomes random noise that appears on the mass spectrum.

For example, it is assumed that the high frequency applied to the ring electrode is swept and ions having a mass number of 200 or a mass number of 250 are created in the ion trap space at the moment when the ions having a mass number of 300 are being ejected. These mass numbers 20
Ions of 0 or mass number 250 are immediately unstable in the ion trap space due to the quadrupole high frequency electric field in the ion trap space. These ions are immediately discharged from the trap space to the outside, and noise is generated around the mass number 300 of the mass spectrum.

Therefore, in the ion trap mass spectrometer, it is attempted to prevent noise by strictly controlling the ionization stage and the mass spectrometry stage by controlling electrons by an electron gate.

In reality, however, spike-like noise sometimes occurs on the mass spectrum even in the ion trap mass spectrometer using the above electronic gate. Figure 5
A mass spectrum when noise is generated is shown in (B). Here, m3 is a molecular ion in which the sample molecule becomes an ion as it is, and m1 and m2 are fragment ions formed by cleavage of this molecular ion. Originally, the spectrum that should appear is this m as shown in FIG.
It should be only the spectrum of 1 to m3, but in reality,
Many mass peaks other than m1, m2, and m3 appear, and a mass spectrum as shown in FIG. 5B is obtained. Here, noise is indicated by n on the mass peak. Naturally, there is no description of n or m1, m2, etc. on the acquired mass spectrum. As a result, the measurer cannot distinguish between signal and noise. Some of these noises have ionized background components other than the sample components, but these can be identified because they are reproducible. However, in the case of high-sensitivity measurement in which only a very small amount of component is measured, other random noise appears. Since this noise is random noise independent of mass number, it is impossible to assign ions at all, and there is a possibility that highly sensitive quantitative analysis becomes impossible. This noise spoils the high sensitivity of the ion trap mass spectrometer.

An object of the present invention is to solve such a problem and to enable highly sensitive measurement of an ion trap mass spectrometer.

[0011]

There are several possible causes of random noise, but as a result of experiments conducted by the inventor, the following two have been found to be the main causes.

(Cause 1) In the mass analysis stage, ions are introduced into the ion trap space.

As described above, in the mass spectrometry stage, the electron gate is closed (negative voltage is applied) so that the electrons do not enter the ion trap space. However, in order to stabilize emitted electrons in the filament, a current is constantly supplied from the filament power source. for that reason,
A large number of electrons emitted from the filament, electrons reflected by the grid electrode, and the like exist near the tip of the filament. On the other hand, the pressure around the filament is 10-3.
Pa to 10-4 Pa, with many residual gases present. When the residual gas collides with the electrons near the filament, the gas molecules are ionized and become positive ions. The positive ions are accelerated by the negative voltage applied to the electron gate electrode and enter the ion trap space. This ion
Immediately after it is discharged from the ion trap space, it is detected by the detector and becomes random noise.

(Cause 2) Electrons, photons and ions emitted from the filament are directly incident on the detector.

As a detector of the mass spectrometer, there is used a detection system such as a secondary electron multiplier or a photomultiplier which converts ions into electrons and emits light by a scintillator for detection. Further, all the electrons and photons emitted from the filament do not enter the ion trap space, but diffusely reflect on the wall surface or the like in the vacuum container in which the mass spectrometer is housed. Such electrons and photons directly enter the detector and generate noise. Further, the accelerated electrons ionize residual gas molecules in the vacuum container on the way. Even if these ions are directly incident on the detector, they become noise.

The present invention has been made to solve such a problem. Specifically, the electron gate electrode between the filament and the end cap electrode is divided into two parts, one for ionization and one for mass spectrometry. The voltage applied to each is controlled independently. This prevents unwanted ions and electrons from being introduced into the ion trap space during mass spectrometry.

Further, a plurality of cylindrical or plate-like electrodes for blocking electrons, ions and photons are arranged between the filament and the detector. This can prevent ions and electrons scattered in the vacuum container from directly entering the detector.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION (First Embodiment) A first embodiment of the present invention will be described with reference to FIGS. 1, 2, and 4.

First, referring to FIG. 1, a schematic structure of the ion trap mass spectrometer will be described. The ion trap mass spectrometer has one ring electrode 7 having a rotating hyperboloid and a hyperboloid adjacent to the ring electrode 7 in the direction of the axis of rotational symmetry in order to form a space called an ion storage space or an ion trap space 9. Two end cap electrodes 6 and 8 are provided. The space surrounded by these three electrodes is the ion trap space 9. A main high frequency power supply (main RF power supply) 1 is provided between the ring electrode 7 and the two end cap electrodes 6 and 8.
High frequency is applied from 5. As a result, a quadrupole high frequency electric field is generated in the ion trap space 9, and ions having a mass-to-charge ratio (m / z) within a predetermined range can be trapped.

The end cap electrodes 6 and 8 are supplied with a voltage 0 to 10 V from an auxiliary high frequency power source (auxiliary RF power source)
A degree of auxiliary high frequency is applied through the transformer 19. Two
When an auxiliary high frequency is applied between the two end cap electrodes 6 and 8, a dipole electric field is generated in the ion trap space 9. As a result, ions having a specific mass-to-charge ratio (m / z) resonate.

Furthermore, in an ion trap mass spectrometer having an electron impact (EI) ion source, a filament 2, which emits thermoelectrons when heated by an electric current supplied from a filament power source 1, and the periphery of the filament 2. The grid electrode 3 provided on the
An electronic gate power supply 18 for applying a predetermined voltage to the electronic gate electrode 5 and a detector 12 for detecting ions are provided.

The main high frequency power source 15 and the auxiliary high frequency power source 21,
The electronic gate power supply 18 and the like are controlled by the data processing device 14 through the signal lines 22 and 20.

FIG. 2 shows a detailed structure near the electronic gate electrode 5. The electronic gate electrode 5 of the present invention is divided into two. This is shown as a first electron gate electrode 31 and a second electron gate electrode 32. Each of these electrodes is made of a tubular metal. The electronic gate power source 18 is also composed of two components, a first electronic gate power source 33 and a second electronic gate power source 34.

The ion trap mass spectrometer operates by dividing it into several stages (modes) over time. The operation of each stage will be described with reference to FIG. One cycle for obtaining one mass spectrum is about 0.1 to several seconds.

(1) Stage of Ionization (Ion Accumulation) The period corresponding to time t0 to t1 in FIG. 4 is the stage of ionization.

First, the main high frequency power source 15 to the ring electrode 7
The high-frequency voltage applied to is set low so that ions of different masses can be simultaneously trapped in the ion trap space 9.

A voltage of -15 V supplied from the electron acceleration power supply 17 is applied to the grid electrode 3 surrounding the filament 2. The voltage supplied from the first electronic gate power supply 33 is applied to the first electronic gate electrode 31. The first electronic gate power supply 33 is ± 50V to ± 200V
Can be applied to the first electron gate electrode 31 within the range of
Here, a voltage of + 100V is applied. The voltage supplied from the second electronic gate power supply 34 is applied to the second electronic gate electrode 32. The second electronic gate power supply 34 is +1
The voltage can be applied to the second electron gate electrode 32 in the range of 00V to + 300V, but a voltage of + 200V is applied here.

Thermions 4 emitted from the filament 2
Is accelerated by a potential which becomes higher in order of the grid electrode 3, the first electron gate electrode 31, and the second electron gate electrode 32,
It is introduced into the ion trap space 9 from a pore opened at the center of the end cap electrode 6. Where the thermoelectrons are
It collides with the sample gas introduced from the gas chromatograph (GC) 23 through the sample gas guide pipe 16 and ionizes the sample gas molecules. The generated ions form a stable ion trajectory 10 in the ion trap space 9 and are trapped. During the ionization (about 10 μsec to 0.1 sec), the thermoelectrons from the filament 2 are continuously introduced into the ion trap space 9, and the sample is continuously ionized and the ions are accumulated.

In the vicinity of the filament 2, positive ions may be generated due to the interaction between electrons and gas molecules.
When the positive ions are introduced into the ion trap space 9, they are detected as noise, but the generated positive ions are generated by the first electron gate electrode 31 and the filament 2 (the filament 2 is the grid electrode 3). Is almost the same as that of the first electron gate electrode 31). Eventually, the positive ions collide with the grid electrode 3 to lose the charge and disappear, so that they are not introduced into the ion trap space 9.

In addition to positive ions, negative ions may be generated. Negative ions have the same polarity as electrons, so they can interfere. However, the generation probability of negative ions at a pressure of about 10 −3 Pa is 1/103 that of positive ions.
It is as low as about 1/104 and is negligible. As a result, even if the generated negative ions are introduced into the ion trap space 9 together with the electrons, there is no concern that noise will be generated.

(2) Stage of mass spectrometry As shown in FIG. 4, when the time is t1 and the ionization time is over, the process proceeds to the next stage of mass spectrometry. A negative voltage is applied to the first electronic gate electrode 31. Here, −100V is applied. Due to this potential setting, the thermoelectrons 4 emitted from the filament 2 are not accelerated.
Therefore, thermoelectrons cannot pass through the first electron gate electrode 31 and never enter the ion trap space 9. The voltage applied to the second electron gate electrode 32 and the grid electrode 3 is constant from the ionization stage. Here, + 200V is continuously applied to the second electron gate electrode 32, and −15V is continuously applied to the grid electrode 3.

On the other hand, the data processor 14 controls the main high frequency power supply 15 to start sweeping the high frequency voltage applied to the ring electrode 7. As a result, the trapped ions become unstable for each mass of the ions, and are ejected from the pores of the end cap electrode 8 to the outside of the ion trap space 9. The ejected ions 11 are detected by the detector 12. The detected signal is amplified by the DC amplifier 13 and sent to the data processor 14 to give a mass spectrum.

Even in this mass analysis stage, the filament 2 constantly emits thermoelectrons. For this reason,
In the vicinity of the filament 2, positive ions are generated by the interaction between electrons and the surrounding gas. Since a negative voltage is applied to the first electron gate electrode 31 in order to block electrons, the generated positive ions are accelerated in the direction of the first electron gate electrode. However, in the present invention, a positive voltage is applied to the second electron gate electrode 32. That is, the positive ions that have passed through the first electron gate electrode 31 cannot pass through the second electron gate electrode 32 due to the potential difference between the first and second electron gate electrodes. This makes it possible to prevent positive ions from being introduced into the ion trap space 9 even in the mass analysis stage.

(3) Reset When the acquisition of the mass spectrum is completed, the high frequency voltage applied to the ring electrode 7 is once reset to zero. As a result, all the ions having a large mass remaining in the ion trap space 9 are ejected outside the trap or collide with the wall inside the ion trap and lose the charge.

By the operations (1) to (3), one mass spectrum can be obtained (the first scan is completed). By repeating the operations (1) to (3),
Collect multiple mass spectra in succession.

As described above, in the present invention, in the ionization stage, the first and second electron gate electrodes are accelerated so that the electrons are accelerated to the ion trap space 9 and the generated positive ions are removed. The applied voltage is controlled to remove electrons at the first electron gate electrode 31 in the mass analysis stage,
In the second electron gate electrode 32, by controlling the voltage applied to the first and second electron gate electrodes so as to remove positive ions, it is possible to control electrons and ions that become noise. That is, only electrons are introduced into the ion trap space 9 at the ionization stage, and both electrons and positive ions can be blocked at the mass analysis stage. As a result, the generation of random noise during mass spectrometry can be suppressed and eliminated.

In the above-mentioned US Pat. No. 5,734,162, two electron gate electrodes are shown, which is similar to the structure of the present invention. However, US Pat.
In 4,162, two electronic gate electrodes are connected to the same power source, and their functions are considered to be equivalent to one electronic gate electrode. There is no disclosure that the applied voltage to each electronic gate electrode is independently controlled as in the present invention. The removal of random noise is first achieved by independently controlling the applied voltage to the two electron gate electrodes in the ionization step and the mass analysis step, as shown in the present invention.

Further, here, the first and second electron gate electrodes are shown as cylindrical metal electrodes. Alternatively, a disk-shaped metal electrode having a hole through which electrons can pass in the center may be used. Alternatively, a metal mesh or the like may be used. (Second Embodiment) FIG. 3 shows details of the second embodiment of the present invention. This embodiment is intended to reduce noise by preventing electrons, photons, ions, etc., which cause noise generated in the vicinity of the filament 2, from directly entering the detector 12.

The ion trap mass spectrometer is installed in a vacuum container 44 evacuated by a turbo molecular pump 45. Around the filament 2, the first electron gate electrode 31, and the second electron gate electrode 32, electrons and photons accelerated and emitted from the filament 2, secondary electrons generated by collision of electrons with the electrode surface, and reaction with ambient gas There are many ions and the like generated by the action. If even a part of these enters the detector 12, it becomes random noise.

In order to block the charged particles and photons, in this embodiment, the periphery of the filament 2, the first electron gate electrode 31, and the second electron gate electrode 32 is surrounded by the shield electrode 4.
Cover with 1. The shield electrode 41 is set to the ground potential so as not to be charged up even when ions or the like collide.

For the shield electrode 41, a metal plate without holes is effective for shielding charged particles and photons, but this cannot keep the pressure around the filament low. In order to extend the life of the filament 2 and to prevent the electrode near the filament 2 from being contaminated, it is necessary to reduce the pressure around the filament as much as possible. For that purpose, it is necessary to maintain the exhaust conductance at a high level. Therefore, as the shield electrode 41, a porous metal plate or a metal mesh is suitable.

Since it is possible that electrons and the like pass through the shield electrode 41, plate-shaped shield electrodes 42 and 43 are provided in order to capture the passed electrons and the like. The shield electrodes 42 and 43 are installed around the end cap electrodes 6 and 8. This is because the end cap electrodes 6 and 8 operate at almost ground potential, while the ring electrode 7 has 20 kV.
This is because a high-frequency potential close to (peak to peak) is applied, and it is not advisable to bring the shield electrode at the ground potential closer to the ring electrode. The shield electrodes 42 and 43 may be metal plates or meshes. It is also possible to combine two mesh plates to efficiently capture charged particles while maintaining the exhaust conductance. The first and second embodiments can be combined. The structure in this case is as shown in FIG. By controlling the two electron gate electrodes shown in the first embodiment and the action of the shield electrode shown in the second embodiment, it is possible to more reliably prevent unnecessary electrons from entering the detector. It is possible to further reduce the possibility that random noise will occur.

[0043]

As described above, according to the present invention, random noise at the time of mass spectrometry is reduced, and a mass spectrum of a trace amount of component can be obtained with high sensitivity. Moreover, the analysis of the mass spectrum is not disturbed by noise. Furthermore, since noise in the total ion chromatogram (TIC) is also reduced, highly sensitive quantitative analysis of a trace amount of components becomes possible.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of the present invention.

FIG. 2 is a configuration diagram showing a first embodiment of the present invention.

FIG. 3 is a configuration diagram showing a second embodiment of the present invention.

FIG. 4 is an explanatory diagram of the operation of the present invention.

FIG. 5 is a diagram showing a mass spectrum for explaining a measurement result by a conventional device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Filament power supply, 2 ... Filament, 3 ... Grid electrode, 4 ... Electron, 5 ... Electronic gate electrode, 6 ... End cap electrode, 7 ... Ring electrode, 8 ... End cap electrode, 9 ... Ion trap space, 10 ... Trap Generated ions, 11 ... Ions, 12 ... Detector, 13 ... DC amplifier, 14 ... Data processing device, 15 ... Main high frequency power supply, 16
… Sample gas guide pipe, 17… Grid power supply, 18…
Electronic gate power source, 19 ... Transformer, 20 ... Signal line, 21
... auxiliary high frequency power supply, 22 ... signal line, 23 ... gas chromatograph, 31 ... first electronic gate, 32 ... second electronic gate, 33 ... first electronic gate power supply, 34 ... second electronic gate power supply, 41 ... shield electrode, 42 ... Shield electrode, 43 ...
Shield electrode, 44 ... Vacuum container, 45 ... Turbo molecular pump.

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01J 49/00-49/42

Claims (9)

(57) [Claims] 1. A filament for emitting electrons, a ring electrode for forming an ion trap space and a pair of end cap electrodes, a first electron gate electrode provided between the filament and the end cap electrode, and a second electrode. In the mass spectrometric method using an ion trap mass spectrometer having an electron gate electrode and a detector for detecting ions emitted from the ion trap space, a positive voltage is applied to the first and second electron gate electrodes. Then, the electron from the filament is introduced into the ion trap space, an ionization step of ionizing the sample, a negative voltage is applied to the first electron gate electrode, and a positive voltage is applied to the second electron gate electrode, Mass component for continuously detecting and ejecting ions in the ion trap space by sweeping high frequency voltage applied to the ring electrode Mass spectrometry method characterized by comprising the steps, a. 2. The mass spectrometric method according to claim 1, wherein a voltage higher than that of the first electron gate electrode is applied to the second electron gate electrode in the ionization step. 3. The mass spectrometric method according to claim 1, wherein in the mass spectrometric analysis step, a voltage higher in absolute value is applied to the second electronic gate electrode than the first electronic gate electrode. . 4. An ion trap mass spectrometric analysis for obtaining a mass spectrum by ionizing a sample introduced into the ion trap space with an electron emitted from a filament and mass spectrometrically detecting the ion ejected from the ion trap space by a detector. In the meter, the first and second electron gate electrodes are arranged between the filament and the end cap electrode, a positive voltage is applied to the first and second electron gate electrodes during ionization, and the An ion trap mass spectrometer, comprising: an applied voltage control unit for controlling to apply a negative voltage to the first electronic gate electrode and a positive voltage to the second electronic gate electrode. 5. The ion trap mass spectrometer according to claim 4, further comprising a shield electrode that covers the first and second electron gate electrodes. 6. The ion trap mass spectrometer according to claim 5, wherein the shield electrode is maintained at a ground potential. 7. The ion trap mass spectrometer according to claim 5, wherein the shield electrode is made of a porous metal plate or a metal mesh. 8. The ion trap mass spectrometer according to claim 4, wherein a plate-shaped shield electrode is provided around the end cap electrode. 9. The ion trap mass spectrometer according to claim 8, wherein the plate-shaped shield electrode is a metal plate or mesh.