CN102103974B - A method and device for improving the performance of ion trap collision-induced dissociation - Google Patents

Abstract

The invention relates to a method for improving the Collision Induced Dissociation (CID) performance of an ion trap, wherein the ion trap comprises an end electrode; is characterized in that: in the process of CID of the ion, the voltage signal applied to the terminal electrode contains a plurality of frequency components, wherein the frequency components are within [ (f-1) kHz, (f +1) kHz ], and f (kHz) is the resonance frequency of the ion to be CID, and the voltage signal is a broadband signal containing all frequency components within a certain frequency range, and can also be a frequency sweep signal with the frequency changing with time. The invention also provides a device for improving the collision induced dissociation performance of the ion trap. The invention can eliminate the influence caused by space charge effect or radio frequency binding voltage fluctuation and the like, and improves CID efficiency and repeatability.

Description

A method and device for improving the performance of ion trap collision-induced dissociation

technical field

The invention relates to a method and a device for improving the performance of an ion trap, in particular to a method and a device for improving the performance of an ion trap for collision-induced dissociation.

Background technique

Multistage mass spectrometry is an important function of ion trap mass spectrometers, and plays a pivotal role in the fields of genomics, proteomics, and pharmaceuticals that involve material structure analysis. At the same time, the multi-stage mass spectrometry function is one of the advantages of the ion trap mass spectrometer over other mass spectrometers.

The multi-stage mass spectrometry analysis process is as follows: firstly, the target ion (or parent ion) is isolated, and then the parent ion is fragmented by collision-induced dissociation (CID). The fragment ions obtained after fragmentation are called product ions. Product ions also undergo the same isolation and CID process. By analyzing the fragment ions obtained after ion CID, the structure of the ion can be analyzed and a larger amount of information can be obtained.

At present, the ion trap theory is very mature, here only briefly introduces the theory related to CID. For a three-dimensional ion trap with a radial radius of r ₀ and an axial radius of z ₀ , the stability map parameters a _z and q _z are defined as follows:

${\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; z</span>}}<span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 16</span>}\mathrm{<span\; class="notranslate">\; eU</span>}}{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; z</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; \&Omega;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; q</span>}}_{\mathrm{<span\; class="notranslate">\; z</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 8</span>}\mathrm{<span\; class="notranslate">\; eV</span>}}{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; z</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; \&Omega;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

The binding parameters include the DC voltage U applied on the ion trap electrode, the amplitude V of the radio frequency voltage and the angular frequency Ω of the radio frequency voltage.

The resonant frequency ω (denoted as ω) of ions in the ion trap is directly related to a _z and q _z , and the parameter β is generally introduced to describe the relationship between the ion resonant frequency and the RF voltage frequency Ω, as shown in the following formula:

$\mathrm{<span\; class="notranslate">\; \&omega;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; \&beta;\&Omega;</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

$\mathrm{<span\; class="notranslate">\; \&beta;</span>}<span\; class="notranslate">\; \&ap;</span>\sqrt{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; a</span>}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; q</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

From the above analysis, it can be seen that in a three-dimensional ion trap with a fixed geometric size, ions with different mass-to-charge ratios have different resonance frequencies when the binding parameters are constant.

The basic principle of CID is as follows: when the amplitude and frequency of the RF confinement voltage applied to the ion trap ring electrodes are constant, the ions stored in the ion trap have their own resonance frequencies. Ions resonate when an excitation signal at the same frequency as the ion resonance is applied to the end cap electrodes of the ion trap. During the resonance process, the ions will absorb the energy of the external excitation signal and convert it into kinetic energy; at the same time, a part of the kinetic energy will be converted into internal energy during the collision between the ion and the buffer gas in the ion trap. When the energy increases to overcome the chemical bond energy inside the ion, the ion will dissociate to form daughter ions.

CID efficiency and repeatability are the key factors determining the performance of MSMS analysis.

The CID method currently used by most ion trap mass spectrometers is shown in Figure 2. During the CID process, the RF confinement voltage is kept constant, and a single-frequency sine wave with the same frequency as the ion resonance is applied to dissociate the ions. In fact, the resonant frequency of ions is affected by many factors, such as space charge effects, RF confinement voltage fluctuations, temperature changes, and rounding errors caused by calculations, which may cause the frequency of the external excitation signal to deviate from the resonant frequency of ions. Therefore, using the CID method shown in Figure 2 cannot guarantee the efficiency and repeatability of CID.

To solve the above problems, the Varian mass spectrometer adopts the CID method shown in Figure 3. It achieves the purpose of modulating the ion resonance frequency by modulating the amplitude of the RF confinement voltage. That is to say, the resonant frequency of the ions will vary within a certain range, and the frequency of the externally applied excitation signal only needs to fall within this frequency range. This method improves the efficiency and repeatability of CID to a certain extent, but it has the following disadvantages:

1) As shown in Figure 3, the RF amplitude modulation method actually superimposes the modulation signal on the RF signal. Since the RF circuit is generally an LC resonant circuit, the circuit has a frequency selection function, which limits the frequency of the modulation signal.

2) Modulating the high-voltage radio frequency signal may cause output voltage fluctuations, which also increases the difficulty of circuit implementation.

3) Even if the amplitude modulation range of RF is fixed, the range of ion characteristic frequency changes caused by it is different for ions with different mass-to-charge ratios. That is to say, there is a "quality discrimination" effect in this method.

Contents of the invention

In order to solve the deficiencies of the prior art, the present invention proposes a method for improving CID performance. This method can eliminate the influence of space charge effect or radio frequency confinement voltage fluctuation, etc., and improve the efficiency and repeatability of CID.

In order to realize the above-mentioned purpose of the invention, the present invention adopts the following technical solutions:

A method for improving the performance of collision-induced dissociation of an ion trap, the ion trap including a terminal electrode; characterized by:

In the process of performing CID on ions, the voltage signal applied to the terminal electrode contains two or more frequency components, including frequency components within [(f-1)kHz, (f+1)kHz] , f(kHz) is the resonance frequency of the ion to be CIDed.

Further, the voltage signal is a broadband signal with a frequency range, and the broadband signal includes all frequency components in the frequency range.

Preferably, the voltage signal can be obtained by band-pass filtering a white noise signal.

Further, in the voltage signal, frequency components are discrete.

Further, the voltage signal is a frequency sweep signal whose frequency changes with time, and the start frequency and stop frequency of the sweep signal can be set.

Further, during the CID process, the frequency sweep signal appears twice or more.

Further, the frequency versus time relationship is linear or non-linear.

Further, when the relationship between frequency and time variation is nonlinear, a time weight is set for each frequency component.

Furthermore, the time weight of a frequency component having a high probability of being equal to the resonance frequency of ions is greater than that of other frequency components.

In order to realize the above method, the present invention also proposes such a device for improving the performance of ion trap collision-induced dissociation, the ion trap includes a terminal electrode, and the device includes:

A signal generator, the voltage signal generated by the signal generator is applied to the terminal electrode; the voltage signal includes two or more frequency components, which have a frequency in [(f-1)kHz, (f+1)kHz ], f (kHz) is the resonance frequency of the ion to be CIDed.

Further, the voltage signal is a broadband signal with a frequency range, and the broadband signal includes all frequency components in the frequency range.

Further, in the voltage signal, frequency components are discrete.

Further, the voltage signal is a frequency sweep signal whose frequency changes with time, and the start frequency and stop frequency of the sweep signal can be set.

Further, during the CID process, the frequency sweep signal appears twice or more.

Further, the frequency versus time relationship is linear or non-linear.

Further, when the relationship between frequency and time variation is nonlinear, a time weight is set for each frequency component.

Furthermore, the time weight of a frequency component having a high probability of being equal to the resonance frequency of ions is greater than that of other frequency components.

Compared with prior art, the advantage of the present invention is:

1. During the CID process of ions, a voltage signal with a certain frequency range is applied to the electrode of the ion trap end cover. Even if factors such as space charge effect or confinement voltage drift cause the resonance frequency to change, the frequency range of the voltage signal can also cover this Resonance frequency. This eliminates the influence of ion resonance frequency due to some factors, and improves the repeatability of CID.

2. In the CID process, the ion ejection caused by resonance excitation and the ion dissociation process "compete" with each other. In the traditional way, ions are continuously excited by resonance, and their vibration amplitude may continue to increase to escape the confinement of the ion trap. However, in the present invention, the ions are only periodically excited by resonance, so the ions have sufficient time to collide with the buffer gas and convert the acquired kinetic energy into internal energy. Therefore, this method improves CID efficiency.

3. The external excitation signal is used for frequency modulation, and the range of frequency modulation can be adjusted according to the mass-to-charge ratio of ions to eliminate "mass discrimination".

4. The weight of the frequency component of the voltage signal is adjustable to ensure that the time required for CID is reduced under the condition of improving CID efficiency.

5. The voltage signal is a low-voltage and low-frequency signal. Controlling the voltage signal is easier and more flexible than controlling a high-voltage RF signal.

Description of drawings

Fig. 1 is the structural representation of ion trap mass spectrometer;

Fig. 2 is the timing diagram of traditional CID;

Figure 3 is a timing diagram of the CID used by the Varian mass spectrometer;

Fig. 4 is the timing diagram of the CID adopted by the present invention, wherein the CID voltage signal is a broadband signal;

Fig. 5 is the timing diagram of the CID adopted by the present invention, wherein the CID voltage signal is a frequency sweep signal whose frequency varies with time;

FIG. 6 is a working sequence diagram of the ion trap mass spectrometer in Embodiment 1 of the present invention.

Detailed ways

Example 1:

A device for improving the performance of ion trap collision-induced dissociation, which is applied in an ion trap mass spectrometer.

As shown in FIG. 1 , the ion trap mass spectrometer includes a three-dimensional ion trap, an electron source 106 , an electron gate 104 , an ion detector 105 , and a radio frequency generator 109 . The ion trap includes a ring electrode 101 and terminal electrodes 102 and 103 . When the mass spectrometer is working, a radio frequency confinement voltage is applied to the ring electrode 101 , and the radio frequency confinement voltage is generated by a radio frequency generator 109 .

The device includes a signal generator 107, and the generated voltage signal is converted into a double-terminal signal through a transformer, and applied to the terminal electrodes 102 and 103 respectively. As shown in FIG. 4 , the voltage signal is a broadband signal, and its frequency range is (f-2)kHz-(f+2)kHz, that is, it includes all frequency components within this frequency range. Where f (kHz) is the ion resonance frequency calculated theoretically based on radio frequency confinement voltage and related parameters. The voltage signal is obtained by band-pass filtering white noise.

This embodiment also discloses a method for improving the ion trap collision-induced dissociation performance, which is applied in multi-stage mass spectrometry analysis. As shown in FIG. 6, the multi-stage mass spectrometry analysis includes the following steps:

1. Ionization stage:

A radio-frequency confinement voltage with a certain amplitude and frequency is applied to the electrode 101, and a positive DC voltage is applied to the electrode 104. The electrons emitted by the electron source 106 can enter the ion trap through the electron gate 104, and the neutral sample in the ion trap Ionization into ions, the obtained ions are bound in the ion trap under the action of radio frequency electric field;

2. Ion cooling stage:

The RF confinement voltage applied on the electrode 101 remains unchanged, and a negative DC voltage is applied on the electrode 104 to prevent electrons from entering the ion trap; at this stage, the ions trapped in the ion trap are collided by the buffer gas (He) Gradually lose its kinetic energy, and thus gather in the central area of the ion trap under the confinement of the radio frequency electric field;

3. Quality isolation stage:

The RF confinement voltage amplitude on the electrode 101 is maintained at a fixed value, and the magnitude of the voltage amplitude is set according to the mass-to-charge ratio of the ions to be isolated. Apply a SWIFT signal on the electrodes 102 and 103, the signal contains a frequency component of 50-500kHz, and also contains a frequency missing window, the frequency in this window corresponds to the resonance frequency of the ion to be isolated; after the mass isolation stage, the desensitization in the ion trap Except for the ion of interest (precursor ion), other ions are excited by resonance and pop out of the ion trap or collide with the electrode of the ion trap to annihilate;

4. CID stage:

The RF confinement voltage amplitude on the electrode 101 is set according to the mass-to-charge ratio of the parent ion, so that the q value of the parent ion (q is defined as shown in formula 2) is between 0.2 and 0.45. Simultaneously apply a voltage signal on the end cap electrodes 102 and 103, the voltage signal is a broadband signal, the specific method of generating this signal is: (a) first calculate the resonant frequency of the ion according to the radio frequency confinement voltage and related parameters theory, denoted as f(kHz); (b) Calculate the CID voltage signal, the frequency range of the voltage signal is (f-2)kHz～(f+2)kHz, that is, the voltage signal includes the frequency range (f-2)kHz～( f+2) All frequency components within kHz; (c) Use the auxiliary signal generator 107 to generate the above-mentioned voltage signal, couple it through the transformer 108 and load it on the electrodes 102 and 103 .

The advantages brought by the above-mentioned voltage signal are: (a) It can eliminate the influence of ion resonance frequency shift and improve the repeatability of CID. (b) The ions are intermittently excited, which can fully convert the kinetic energy of external excitation into internal energy, and improve the efficiency of CID.

5. Quality scanning stage:

The radio-frequency confinement voltage applied to the electrode 101 is linearly scanned, and a sine wave excitation signal with a fixed frequency is applied to the electrodes 102 and 103 at the same time. During this process, the ions will emerge from the ion trap sequentially according to their mass-to-charge ratio, and will be detected by the ion detector 105 to obtain the final mass spectrum.

Example 2:

A device for improving the performance of ion trap collision-induced dissociation, the difference from Example 1 is:

1. The frequency components of the voltage signal generated by the signal generator are distributed discretely, within the frequency range (f-2)kHz-(f+2)kHz, there are (f-1.8)kHz, (f-1.6)kHz, ( f-1.4)kHz..., fkHz,...(f+1.4)kHz, (f+1.6)kHz, (f+1.8)kHz and other frequencies.

2. Instead of using white noise filtering to obtain the above voltage signal, other methods are used.

A method for improving ion trap collision-induced dissociation performance, which is different from Embodiment 1 in that:

1. The frequency components of the voltage signal generated by the signal generator are distributed discretely, within the frequency range (f-2)kHz-(f+2)kHz, there are (f-1.8)kHz, (f-1.6)kHz, ( f-1.4) kHz..., f kHz, ... (f+1.4) kHz, (f+1.6) kHz, (f+1.8) kHz and other frequencies.

2. Instead of using white noise filtering to obtain the above voltage signal, other methods are used.

Example 3:

A device for improving the performance of ion trap collision-induced dissociation, the difference from Example 1 is:

The frequency of the voltage signal generated by the signal generator varies linearly with time. As shown in Figure 5, the voltage signal is a frequency sweep signal, the frequency range is (f-2)kHz-(f+2)kHz, the step size is 200Hz, the initial frequency and the speed of frequency sweep are set according to the actual situation . During the CID process, the frequency sweep signal appears repeatedly, and the number of repetitions is related to the frequency sweep speed and the duration of the CID process.

A method for improving ion trap collision-induced dissociation performance, which is different from Embodiment 1 in that:

The frequency of the voltage signal applied to the terminal electrode changes linearly with time, that is, the frequency sweep signal, the frequency range is (f-2)kHz-(f+2)kHz, the step size is 200Hz, the starting frequency and The frequency scanning speed is set according to the actual situation. During the CID process, the frequency sweep signal appears repeatedly, and the number of repetitions is related to the frequency sweep speed and the duration of the CID process.

Example 4:

A device for improving ion trap collision-induced dissociation performance, which is different from Embodiment 3 in that:

The frequency of the voltage signal generated by the signal generator changes nonlinearly with time, the frequency range is (f-2)kHz-(f+2)kHz, and the time weight is set for each frequency component, which has a high probability of being equal to the resonance frequency of the ion The time weight of the frequency components of is greater than that of other frequency components.

A method for improving ion trap collision-induced dissociation performance, which is different from embodiment 3 in that:

The frequency of the voltage signal applied to the terminal electrode changes nonlinearly with time, the frequency range is (f-2)kHz-(f+2)kHz, and the time weight is set for each frequency component, which has the same probability as the resonance frequency of the ion High frequency components have a greater temporal weight than other frequency components.

The weight can be set to be adjustable, thereby reducing the time required for CID while improving CID efficiency.

The above embodiments should not be construed as limiting the protection scope of the present invention. The key of the present invention is that the voltage signal applied to the terminal electrode of the ion trap contains two or more frequency components, wherein there are frequency components in [(f-1)kHz, (f+1)kHz], f (kHz) is the resonance frequency of the ion to be CIDed, and the voltage signal can be a broadband signal containing all frequency components within a certain frequency range, or a frequency sweep signal whose frequency changes with time. Without departing from the spirit of the present invention, any changes made to the present invention shall fall within the protection scope of the present invention.

Claims (3)

1. A method for improving ion trap collision-induced dissociation performance, said ion trap comprising terminal electrodes; characterized in that: In the process of performing CID on ions, the voltage signal applied to the terminal electrode contains two or more frequency components, which have a frequency within [(f-1)kHz, (f+1)kHz] composition, f is the resonance frequency of the ion to be CIDed; The voltage signal is a broadband signal with a frequency range, and the broadband signal includes all frequency components in the frequency range. 2 . The method for improving the performance of ion trap collision-induced dissociation according to claim 1 , wherein the voltage signal can be obtained by band-pass filtering a white noise signal. 3 . 3. A device for improving ion trap collision-induced dissociation performance, said ion trap comprising terminal electrodes, said device comprising: A signal generator, the voltage signal generated by the signal generator is applied to the terminal electrode; the voltage signal includes two or more frequency components, which have a frequency in [(f-1)kHz, (f+1)kHz ], f is the resonance frequency of the ion to be CID; The voltage signal is a broadband signal with a frequency range, and the broadband signal includes all frequency components in the frequency range.

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