Femtosecond laser assisted chemical ionization method

Abstract

The invention relates to a femtosecond laser-assisted chemical ionization method for mass spectrometry of organic molecules. According to the method, high-concentration primary reagent ions are provided for subsequent chemical ionization by femtosecond laser ionization; high-concentration charges carried by primary reagent ions are transferred to a low-concentration to-be-analyzed sample by chemical ionization, the ionization volume of femtosecond laser ionization is equivalently enlarged, and meanwhile, the soft ionization characteristic of two ionization methods is maintained, so that the high-concentration to-be-analyzed sample can be analyzed under the condition of an open or closed air chamber. And carrying out ultrahigh-sensitivity mass spectrometry on the to-be-analyzed sample. The method is combined with a desorption technology or a liquid sample gas distribution device, and can be expanded to analysis of gas, solid and liquid samples; the device is compatible with wide air pressure conditions and can be applied to closed air chamber or open detection environment; the universality and soft ionization characteristics of femtosecond laser ionization and chemical ionization are reserved, and an ionization method with universality and ultrahigh sensitivity is provided for mass spectrometric detection of organic molecules.

Description

Technical Field

[0001] The present invention relates to the field of mass spectrometry analysis of organic molecules, and in particular to a femtosecond laser-assisted chemical ionization method for mass spectrometry analysis of organic molecules. Background Art

[0002] Ultra-high sensitivity analysis of organic molecules is of great significance in many fields including chemistry, biology, medicine and environmental science. Mass spectrometry is widely regarded as the most sensitive analytical method. With the continuous development of mass analyzers, ion sources have become a bottleneck restricting the further development of mass spectrometry technology. Among the many ionization methods currently available, inductively coupled ionization can provide the highest sensitivity, thus achieving ppq (parts-per-quadrillion, 10 -15 )-level elemental mass spectrometry analysis, but this ionization method will lose the molecular information of the analyzed sample, which greatly limits its application in the field of organic molecule detection. In comparison, ion sources based on chemical ionization or femtosecond laser ionization can achieve ppt (parts-per-trillion, 10 -12 )-level sensitivity detection while retaining the characteristics of soft ionization, that is, no or only very little molecular fragmentation is caused when the molecules are ionized, thereby retaining the overall structural information of the organic molecules.

[0003] In chemical ionization, electron ionization is usually used to produce primary reagent ions, and then the ionized products of the analyte are produced through a series of reactions between the primary ions, reagent molecules and analyte molecules. However, in order to obtain a high concentration of primary reagents, the required electron kinetic energy is much greater than the first ionization energy of the organic molecules. The electron ionization of high-energy electrons will cause the fragmentation of the primary reagent ions, thereby reducing the chemical reactivity of the reagent ions, and ultimately limiting the ionization efficiency of the analyte. In order to alleviate the above dilemma, most ion sources based on electron ionization operate in a high vacuum environment, thereby reducing the high pressure that must be used to penetrate the background gas. However, this working condition reduces the collisions between molecules or ions required for subsequent chemical ionization, making it difficult to continue to improve the ionization efficiency. Ultraviolet photoionization is also an optional ionization method for producing primary reagent ions in chemical ionization. However, in order to achieve efficient single-photon ionization, the single photon of the light source used in ultraviolet photoionization must be greater than the first ionization energy of the molecule to be ionized. Without losing universality, vacuum ultraviolet wavelengths need to be used. This excludes the application of chemical ionization ion sources based on vacuum ultraviolet photoionization under open conditions and puts additional requirements on the design of the gas chamber.

[0004] Femtosecond laser ionization is another common soft ionization method. Due to its strong field ionization nature, it can achieve ionization efficiency close to 1 for a wide range of molecules. However, strong field ionization has a threshold effect. The pulses output by common commercial femtosecond lasers must be focused to achieve considerable ionization efficiency near the focus. Therefore, the actual ionization volume of femtosecond laser ionization is limited to less than the cubic millimeter level, which greatly weakens its advantage of high ionization efficiency, resulting in the sensitivity of mass spectrometry based on femtosecond laser ionization being clamped at the ppt level. Summary of the invention

[0005] In view of the shortcomings of the above-mentioned existing ionization methods, the present invention combines femtosecond laser ionization and chemical ionization, complements the advantages of the two, and proposes a novel femtosecond laser-assisted chemical ionization method.

[0006] The present invention utilizes femtosecond laser ionization to provide high-concentration primary reagent ions for subsequent chemical ionization; utilizes chemical ionization to transfer the high-concentration charge carried by the primary reagent ions to the low-concentration sample to be analyzed, equivalently expanding the ionization volume of the femtosecond laser ionization while maintaining the soft ionization characteristics of the two ionization methods involved, thereby achieving the purpose of ultra-high sensitivity mass spectrometry analysis of the sample to be analyzed under open or closed gas chamber conditions.

[0007] To achieve the above object, the method of the present invention comprises the following steps:

[0008] 1) Use a focused femtosecond laser to irradiate the reagent molecules, emitting multiphoton ionization or tunnel ionization near the focus to generate a high-concentration primary reagent ion plume.

[0009] In step 1), the strong field ionization caused by the femtosecond laser can be described by multiphoton ionization and tunnel ionization in the strong field ionization theory based on the Keldysh ionization theory, especially the molecular PPT theory for predicting the molecular ionization rate.

[0010] Furthermore, the pulse repetition frequency of the femtosecond laser can be 10 Hz to 100 MHz, the pulse width can be 5 fs to 500 fs, the central wavelength can be 200 nm to 3 μm, and the pulse energy can be 100 nJ to 10 mJ; the beam diameter of the femtosecond laser irradiation can be 1 μm to 1 mm, and the peak light intensity of the irradiation can be 10 11 ~10 15 W / cm 2 It is of high order of magnitude and can achieve considerable ionization efficiency for most organic molecules without selectivity, which is universal.

[0011] Preferably, the central wavelength of the femtosecond laser is 600nm or above, so that its single photon energy is small, reducing the additional kinetic energy transferred to the molecules during the femtosecond laser ionization process and reducing the fragmented ion products; the pulse width of the femtosecond laser pulse is below 50fs, so that the action time of light and molecules, that is, the ionization process is shorter than the time for common organic molecules to undergo vibrational dissociation after obtaining photon energy. The beneficial effect is that the generation of fragments during the ionization process is further reduced, which is conducive to enhancing the soft ionization characteristics of femtosecond laser ionization, thereby maintaining the chemical reaction activity of the primary reagent ions.

[0012] Further preferably, the peak light intensity of femtosecond laser irradiation is 10 13 ~10 14 W / cm 2 The magnitude enables effective ionization of most organic molecules while avoiding the generation of high-valence ions resulting in Coulomb explosion due to excessive light intensity. The beneficial effect is that high-efficiency ionization is achieved while maintaining the integrity of the primary reagent ions.

[0013] In step 1), the ionization process may occur under high pressure, normal pressure and vacuum, or in an atmosphere assisted by a background gas, wherein the background gas pressure may be 1 mbar to 10 bar.

[0014] Furthermore, the background gas can be helium, argon, nitrogen, air or other gases, and its selection conditions include: the background gas molecules are not easy to react with the reagent molecules and ions, so as to reduce the consumption of the reagent molecules and ions by the background gas molecules; the ionization threshold of the background gas molecules is significantly higher than that of the reagent molecules, so as to avoid the excessive plasma density in the femtosecond laser ionization volume, and when the space charge saturation effect occurs, the higher concentration of background gas ions limits the production of primary reagent ions; the relative molecular mass is small, so as to efficiently collisionally cool the molecules and ions participating in the reaction, and ensure the soft ionization characteristics of chemical ionization.

[0015] Preferably, helium is used as the auxiliary background gas. The first ionization energy of helium is 25.6eV, which is much higher than the first ionization energy of common organic molecules of 6 to 12eV. At the same time, the relative molecular mass of helium is very small, and it can effectively transfer kinetic energy in the collision with reagents and analyte molecules and ions, and play a role in collision cooling of particles involved in chemical ionization. Its beneficial effect is to avoid the interference of auxiliary gas on the ionization process; at the same time, the excess kinetic energy is transferred through collision cooling, further improving the soft ionization characteristics.

[0016] Furthermore, the ionization process in step 1) can occur under open conditions or in a gas chamber. For example, a separate reagent molecule ionization zone can be designed to adjust the gas pressure in the femtosecond laser ionization zone and the chemical ionization zone respectively. The beneficial effect is that the ionization of the analyte molecules while obtaining the primary reagent ions can be avoided, ensuring that all analyte ionization products are produced from chemical ionization, avoiding the interference of femtosecond laser ionization on chemical ionization, and reducing the consumption of reagent gas.

[0017] Furthermore, under open conditions, when the analyte can react chemically with water molecules or their ionization products, the reagent can be water molecules in the atmospheric environment. The beneficial effect is that the ionization method of the present invention can be applied to in-situ atmospheric component detection; when combined with desorption technology compatible with open conditions, such as femtosecond laser desorption technology, it can be applied to in-situ mass spectrometry imaging analysis and other in-situ, online mass spectrometry analysis technologies.

[0018] In step 1), the concentration of the reagent molecules is much greater than the concentration of the analyte molecules. Preferably, the concentration of the reagent molecules is 10 times the concentration of the analyte molecules. 2 ~10 6 times. In the electron ionization process used in traditional chemical ionization, high-density gas molecules have a great barrier to electrons. Femtosecond laser ionization is insensitive to the density of the ionized gas and can obtain a primary reagent ion concentration far exceeding that produced by electron ionization. Its beneficial effect is to ensure that there are enough primary reagent ions to transfer charge to the analyte molecules in the subsequent chemical ionization process, thereby increasing the final ion yield.

[0019] 2) The primary reagent ion plume generated in step 1) diffuses, and the volume after diffusion is much larger than the ionization volume of the femtosecond laser. Preferably, the volume of the primary reagent ion plume after diffusion is more than 10,000 times the ionization volume of the femtosecond laser.

[0020] In step 2), the diffusion process can occur completely passively, that is, caused by the difference in ion concentration inside and outside the femtosecond laser ionization volume. The diffusion process can also be actively promoted and controlled by designing an accelerating electric field, an ion lens, or setting a purge gas or a gas pressure gradient. The beneficial effect is that the direction and speed of ion diffusion can be optimized, providing favorable primary conditions for chemical ionization.

[0021] 3) During and after the diffusion process, primary reagent ions, reagent molecules, and analyte molecules collide with each other, resulting in one or more chemical reactions, with the final result being that the gaseous analyte molecules are ionized through chemical ionization.

[0022] In step 3), the analyte molecules may originate from a gas sample, gaseous molecules desorbed from a solid sample, or gaseous molecules evaporated from a liquid sample.

[0023] Furthermore, when the analyte molecules come from a mixture, if the mixture contains a component that can undergo a charge transfer chemical reaction with the analyte, the component can be regarded as a reagent gas, thereby performing femtosecond laser-assisted chemical ionization and achieving direct mass spectrometry analysis of the mixture.

[0024] Furthermore, when the analyte molecules originate from desorption products, the ionization method of the present invention can be used as a post-ionization technique in desorption ionization mass spectrometry.

[0025] In step 3), the reaction area of ​​chemical ionization can be an open port, a gas chamber, a quadrupole or a capillary, and its function is to mix the reagent molecules, the primary reagent ions and the analyte molecules so that they fully collide and chemically react.

[0026] Furthermore, to ensure sufficient reaction, the length of the chemical ionization reaction region is more than 5 mm. Background gas such as helium and nitrogen can be added to promote collisions between molecules and ions.

[0027] In step 3), the chemical ionization includes proton transfer reaction, charge exchange reaction and electrophilic addition reaction. The energy transfer in the chemical reaction process involved is low, so the charge transfer process from the primary reagent ion to the analyte molecule is more gentle, causing less fragmentation of the analyte molecule. The chemical ionization products of the analyte molecule include protonated ions, free radical ions, molecular ions, adduct ions and a small amount of fragment ions.

[0028] 4) Analyzing the generated ionized products of the analyte by a mass analyzer, which may be a cascade of one or more of a time-of-flight, quadrupole, ion trap, orbital trap, and Fourier transform ion cyclotron resonance mass analyzer.

[0029] In summary, based on the above steps, when applied to mass spectrometry analysis of organic molecules, the femtosecond laser-assisted chemical ionization method of the present invention can achieve the following beneficial effects compared with the existing ionization method:

[0030] (1) Both ionization methods involved in the ionization method of the present invention have soft ionization characteristics, which can retain the structural information of organic molecules and obtain a mass spectrum that is easy to interpret.

[0031] (2) Both ionization methods of the present invention are applicable to vacuum, normal pressure and high pressure conditions; and femtosecond laser ionization, benefiting from its strong field ionization nature, can achieve non-selective and high-efficiency ionization of almost all organic molecules. At the same time, the selection of appropriate reagent gas for the analyte gives femtosecond laser-assisted chemical ionization a universal characteristic, which greatly expands the scope of application and application field of the ionization method of the present invention, including but not limited to: in-situ biomass spectrometry imaging, in-situ, rapid atmospheric component detection, direct detection of mixture components, etc.

[0032] (3) Compared with the single femtosecond laser ionization method, the present invention introduces chemical ionization, whereby reagent molecules with a concentration far higher than that of analyte molecules obtain more charge from the femtosecond ionization process, and then transfer the charge to the analyte through diffusion and chemical reaction, thereby equivalently expanding the ionization volume of femtosecond laser ionization.

[0033] (4) Compared with the conventional chemical ionization which uses electron ionization or ultraviolet photoionization to generate primary reagent ions, the present invention uses femtosecond laser ionization as the ion source to generate primary reagent ions, which has higher ionization efficiency and better soft ionization characteristics, thereby retaining the chemical reactivity of the reagent ions; at the same time, it removes the restrictions on vacuum background pressure or special gas chamber structure in conventional chemical ionization. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments of the present invention. Obviously, the following drawings are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0035] Figure 1 A schematic diagram of femtosecond laser-assisted chemical ionization under closed chamber conditions provided in the first embodiment of the present invention.

[0036] Figure 2 These are the femtosecond laser-assisted chemical ionization mass spectra and single femtosecond laser ionization mass spectra under closed gas chamber conditions.

[0037] Figure 3 A schematic diagram of femtosecond laser-assisted chemical ionization under open conditions provided for the second embodiment of the present invention.

[0038] Figure symbols: 1- femtosecond laser; 2- objective lens; 3- femtosecond laser ionization region; 4- chemical ionization reaction region; 5- mass spectrometer; 6- gas chamber; 7- gas inlet; 8- barometer; 9- purge gas; 10- gas nozzle; 11- stray light collector. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. The present invention can also be implemented or applied through other different specific implementation methods. The details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0040] At the same time, it should be understood that the protection scope of the present invention is not limited to the specific embodiments described below; it should also be understood that the terms used in the embodiments of the present invention are for describing specific embodiments rather than for limiting the protection scope of the present invention.

[0041] Example 1: Femtosecond laser-assisted chemical ionization method under open conditions

[0042] Figure 1 It is a schematic diagram of femtosecond laser-assisted chemical ionization under closed chamber conditions provided by the present invention, using a closed chamber and auxiliary gas to adjust the gas pressure and control the reagent concentration.

[0043] The ionization process is as follows:

[0044] 1) Gaseous analytes, high-concentration reagent gas and high-pressure auxiliary gas enter the gas chamber 6 through the gas inlet 7, and the pressure in the gas chamber is monitored by the gas pressure gauge 8; the femtosecond laser 1 is focused into the gas chamber with a light-transmitting window through the objective lens 2, and near the femtosecond laser ionization area (focus) 3, the light intensity exceeds the strong field ionization threshold of the reagent molecules, and femtosecond laser ionization occurs, generating high-concentration primary reagent ions; femtosecond laser ionization also directly generates analyte ions at the same time, but their concentration is much lower than that of the primary reagent ions.

[0045] 2) Driven by the pressure difference between the high-pressure gas chamber and the mass spectrometer vacuum chamber, the primary reagent ions at the focus diffuse and enter the chemical ionization reaction area (capillary) 4 with other gases in the gas chamber.

[0046] 3) Under the promotion of high-pressure auxiliary gas and the constraint of capillary, primary reagent ions, reagent molecules and analyte molecules collide and chemical ionization occurs; the capillary is part of the interface of mass spectrometer 5 and is also used as a chemical ionization reaction area in this embodiment.

[0047] 4) The ion products of the analyte enter the mass spectrometer for mass spectrometry analysis. -9 ) acetone as analyte, 1000ppm (parts-per-million, 10 -6) cyclohexene as the reagent, helium as the auxiliary gas, and a comparison experiment between femtosecond laser-assisted chemical ionization and single femtosecond laser ionization was carried out under the conditions of 1.21 bar pressure and 300 K temperature. The parameters of the femtosecond laser are pulse half-width 45fs, central wavelength 800nm, single pulse energy 0.85μJ, focal spot radius of about 6.4μm after focusing by 4x objective lens, and peak light intensity of about 1.1x10 14 W / cm 2 The generated analyte ionization products were analyzed by a time-of-flight mass spectrometer. The femtosecond laser-assisted chemical ionization mass spectra were obtained with and without the addition of reagent gas ( Figure 2 (a)) and individual femtosecond laser ionization mass spectra ( Figure 2 (b)). Figure 2 The soft ionization characteristics of the two methods involved in the present invention can be clearly seen in the figure; from the comparison of Figures (a) and (b), it can be seen that the signal of the analyte ionization product of femtosecond laser assisted chemical ionization is greatly improved compared with that of single femtosecond laser ionization, which is about two orders of magnitude. Combined with the detection limit of hundreds of ppq level of the reported femtosecond laser ionization method, the femtosecond laser assisted chemical ionization method provided by the present invention can achieve sub-ppq level detection sensitivity for gas-phase organic molecules.

[0048] Example 2: Femtosecond laser-assisted chemical ionization method under open conditions

[0049] Figure 3 This is a schematic diagram of femtosecond laser-assisted chemical ionization under open conditions provided by the present invention. This method can be combined with other technologies under open conditions to achieve rapid mass spectrometry detection of atmospheric components or biological tissues in situ. The ionization process is as follows:

[0050] 1) The femtosecond laser is focused to the capillary mouth through the objective lens, and the reagent molecules are ionized near the focus to generate primary reagent ions. The stray light collector 11 is used to collect the divergent laser; the reagent is selected from water molecules in the atmosphere or as one of the components of the purge gas 9, and is blown to the mass spectrometer by the gas nozzle 10.

[0051] 2) Driven by the purge gas and the pressure difference between the inside and outside of the mass spectrometer, the primary reagent ions at the focus diffuse and enter the capillary with other gases in the gas chamber.

[0052] 3) Primary reagent ions, reagent molecules, analyte molecules and other component molecules in the atmosphere collide, resulting in chemical ionization.

[0053] 4) The ion products of the analyte enter the mass spectrometer for mass spectrometry analysis.

[0054] The above descriptions are only some embodiments of the method of the present invention, and do not limit the present invention in any form. Without departing from the scope of the technical solution of the present invention, technicians in this field can make some changes or modifications to the technical contents disclosed above to obtain equivalent embodiments with equivalent changes. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A femtosecond laser-assisted chemical ionization method, characterized in that: The method comprises the following steps: 1) The focused femtosecond laser irradiates the reagent molecules, causing multiphoton ionization and tunnel ionization near the focus, generating a high-concentration primary reagent ion plume; 2) the primary reagent ion plume generated in step 1) diffuses; 3) During and after the diffusion of the primary reagent ion plume, the primary reagent ions, reagent molecules, and analyte molecules collide with each other, and one or more chemical reactions occur, so that the gaseous analyte molecules are ionized through chemical ionization; 4) Analyzing the ionized product of the analyte produced in step 3) by a mass analyzer.

2. The femtosecond laser-assisted chemical ionization method according to claim 1, characterized in that: In step 1), the femtosecond laser has a pulse repetition frequency of 10 Hz to 100 MHz, a pulse width of 5 fs to 500 fs, a central wavelength of 200 nm to 3 μm, and a pulse energy of 10 nJ to 10 mJ; The diameter of the femtosecond laser beam is 1 μm to 1 mm, and the peak intensity is 10 11 ~10 15 W / cm 2 Magnitude.

3. The femtosecond laser-assisted chemical ionization method according to claim 1, characterized in that: The ionization process described in step 1) can occur under open conditions or in a gas chamber with a background pressure of 1 mbar to 10 bar.

4. The femtosecond laser-assisted chemical ionization method according to claim 1, characterized in that: The concentration of the reagent molecule in step 1) is 10 times the concentration of the analyte molecule. 2 ~10 6 times.

5. The femtosecond laser-assisted chemical ionization method according to claim 1, characterized in that: The volume of the primary reagent ion plume after diffusion in step 2) is more than 10,000 times the ionization volume of the femtosecond laser.

6. The femtosecond laser-assisted chemical ionization method according to claim 1, characterized in that: The analyte molecules described in step 3) are derived from gas samples, gaseous molecules desorbed from solid samples, or gaseous molecules evaporated from liquid samples.

7. The femtosecond laser-assisted chemical ionization method according to claim 1, characterized in that: The reaction area of ​​chemical ionization in step 3) is selected from an open area, a gas chamber, a quadrupole or a capillary; The length of the chemical ionization reaction area is not less than 5 mm.

8. The femtosecond laser-assisted chemical ionization method according to claim 1, characterized in that: The chemical reaction in step 3) includes a proton transfer reaction, a charge exchange reaction and an electrophilic addition reaction. The low energy transfer characteristic of the above chemical reactions is utilized to achieve soft ionization of the analyte molecules to obtain an ionized product of the analyte; The ionization products generated by the chemical ionization include: protonated ions, free radical ions, molecular ions, adduct ions and fragment ions of the analyte.

9. The femtosecond laser-assisted chemical ionization method according to claim 1, characterized in that: The mass analyzer described in step 4) is a cascade of one or more of a time-of-flight, a quadrupole, an ion trap, an orbital trap and a Fourier transform ion cyclotron resonance mass analyzer.

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