A method for real-time monitoring of electrochemical reaction based on potential difference in paper spray source and application

Abstract

The application belongs to the technical field of electrochemical analysis, and particularly relates to a real-time monitoring method for electrochemical reaction based on potential difference in paper spray source and application, which takes paper spray ionization source as a carrier, integrates an electrochemical reaction cell in the ionization source, adjusts the circuit voltage division through series connection of an adjustable resistor, accurately controls the potential difference in the system, and drives the measured substance to generate electrochemical reaction. The intermediates and products generated in the reaction are synchronously ionized by electrospray ionization at the tip of the paper substrate, and directly enter a mass spectrometer to realize rapid detection. The method realizes spatial coupling of the reaction process and mass spectrometric detection, greatly shortens the transmission time of intermediates from the ionization source to the mass spectrometer, and the time resolution can reach microseconds or even nanoseconds, so that short-lived reaction intermediates (such as dopamine quinone radical, N,N-dimethyl aniline radical, nitrogen bin and other short-lived species) with a half-life of milliseconds, microseconds or even nanoseconds can be effectively captured, so that the electrochemical reaction mechanism can be comprehensively and deeply studied.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical analysis technology, specifically relating to a method and application for real-time monitoring of electrochemical reactions based on the potential difference within a paper spray source. Background Technology

[0002] Electrochemical mass spectrometry (EC-MS) has become an indispensable tool in the study of electrochemical reaction mechanisms due to its ability to directly correlate electrical signals with material structure information, especially in the capture and identification of intermediates. However, traditional EC-MS techniques often employ external coupling or integrated separation systems between the electrochemical flow cell and the mass spectrometer, resulting in a long transport distance of reactants from the electrode to the mass spectrometer detector and a significant time delay, making it difficult to effectively capture short-lived intermediates with microsecond to millisecond lifespans.

[0003] To overcome this limitation, researchers have focused on source integration technology, which involves conducting electrochemical reactions directly within the ionization source of the mass spectrometer. The core of this technology is to integrate electrodes directly into the ionization source of the mass spectrometer to minimize the transmission distance and time of electrochemical intermediates to the mass spectrometer inlet, thereby improving the detection capability of transient intermediates. Currently, this technology has spawned a variety of advanced EC-MS coupling methods. Typical integration strategies are mainly divided into three categories: (1) Integration of dual-electrode or triple-electrode systems: Taking double-barrel nanoelectrospray ionization (Double-barrel nanoESI) as an example, electrochemical reactions are independently controlled by adding additional reaction electrodes. The reaction efficiency is high and oxidation and reduction processes can be monitored simultaneously. However, the device integration is complex, requiring precision processing and alignment of multiple components, and is prone to stability problems such as blockage and contamination. (2) Integrated wireless bipolar electrode: Taking wireless bipolar nanoelectrospray ionization (Wireless bipolar nanoESI) as an example, this type of device utilizes bipolar electrochemistry, which can simultaneously initiate electrochemical and ESI processes without the need for an additional power source. It has a simple structure and is easy to array design. However, it is highly dependent on a high electric field, limited by micro-nano scale, and the reaction potential cannot be independently controlled. Furthermore, the fabrication of micro-nano structure electrodes is difficult and easily damaged. (3) Utilizing the inherent electrochemical characteristics of electrospray ionization sources: Taking nanoelectrospray ionization (NanoESI) as an example, it directly uses the electrodes (spray needles) of the ionization source itself as the electrochemical reaction site. There is no need to modify the ionization source. The structure is simple and the cost is low. However, due to the limitation of the electrolysis current by the spray current and the limitation of the spray polarity by the electrode reaction anode and cathode processes, there are limitations such as low electrolysis efficiency, insufficient sensitivity of intermediate detection, and a single detection mode.

[0004] Overall, despite significant progress in in-source integration technology, existing methods still face challenges such as complex system integration, limited stability, insufficient versatility, limited applicability, or high operational barriers. Therefore, developing a novel in-source electrochemical-mass spectrometry method that is structurally simple, low-cost, easy to prepare and operate, and widely applicable is of great importance for advancing research on electrochemical reaction mechanisms, especially for real-time in-situ analysis of complex electrocatalytic systems. Summary of the Invention

[0005] To overcome the shortcomings of existing electrochemical mass spectrometry techniques in in-situ analysis and capture of transient intermediates, this invention proposes a real-time monitoring method and application of electrochemical reactions based on the potential difference within a paper spray source. This method integrates the electrochemical reaction cell and the paper spray emitter into the pipette tip, enabling simultaneous on-site electrochemical reaction and mass spectrometry detection, thereby effectively capturing transient intermediates.

[0006] This invention innovatively integrates an electrochemical cell within a paper spray ionization source, using a paper spray ionization source as the carrier. By selectively connecting resistors in series at the front or rear of the ionization source, a controllable polarization potential (ΔV interface) is established at the interface between the paper spray emitter tip and the internal solution using circuit voltage division. Due to different connection methods between the high-voltage electrode and the paper tip, the paper tip can exhibit anodic (oxidation) or cathodic (reduction) characteristics. The direction and amplitude of this interface polarization potential directly drive the analyte to undergo corresponding electro-oxidation or electro-reduction reactions at the paper tip. Simultaneously, relying on the electrospray emitter performance of the ionization source itself, the intermediates and products generated by the reaction are directly sprayed into the mass spectrometry detection system under the action of an electric field, achieving precise online capture of short-lived intermediates of redox reactions. It has significant advantages such as simple operation, high analytical sensitivity, and long-term continuous monitoring. Crucially, compared to traditional dual-electrode, triple-electrode, and bipolar electrode EC-MS devices, this device can efficiently complete in-situ electrochemical reactions and instantaneous ionization directly using the ionization source's own electrodes, without the need for additional electrodes, significantly simplifying the system architecture. Furthermore, by using anodic corrosion of metal electrodes to in-situ precipitate metal ions to explore the electrocatalytic reaction mechanism, the addition of external electrocatalytic materials can be eliminated, fundamentally avoiding the introduction of impurities and system contamination. This integrated paper spray ionization source-in-source electrochemical coupling detection technology significantly shortens the mass transport path from electrochemical reaction to mass spectrometry detection, improving the time resolution to the microsecond or even nanosecond level. It can efficiently capture short-lived reaction intermediates, providing strong technical support for in-situ real-time monitoring of electrochemical reaction processes and in-depth analysis of reaction mechanisms.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A method for real-time monitoring of electrochemical reactions based on the potential difference within a paper spray source includes the following steps: Step 1, Preparation of paper spray ionization source: Cut 3-5 mm from the tip of a disposable pipette tip, and take a pre-cut triangular paper matrix with a base width of 3-5 mm and a height of 7-10 mm. Roll the triangular paper matrix into a cylindrical shape and embed it into the cut tip of the pipette tip as an electrospray emitter, and make the tip of the triangular paper matrix protrude 1-3 mm from the outside of the pipette tip to obtain the paper spray ionization source; Step 2, conductive pretreatment of the front end of the paper spray ionization source: Cut a rectangular copper conductive tape with a side length of 8~12mm, take the above ionization source, stick one side of the rectangular copper conductive tape along the front end of the ionization source and keep the edge of the tape flush with the cut surface of the suction head, perform conductive pretreatment, and ensure that the tape is completely attached to the tube wall to obtain the pretreated paper spray ionization source. Step 3, Paper Spray Ionization Source - In-situ Electrochemical Reaction - Rapid Mass Spectrometry Analysis: Place the pretreated paper spray ionization source containing the analyte solution directly opposite the mass spectrometer inlet, maintaining a distance of 8-12 mm between the paper matrix tip and the mass spectrometer inlet. Depending on the target electrochemical reaction type (oxidation or reduction), selectively connect the grounded metal wire with a series adjustable resistor to the positive / negative output terminals of the high-voltage DC power supply to the front or rear end of the ionization source, respectively. Adjust the voltage distribution of the circuit through the adjustable resistor to form a controllable interfacial polarization potential on the surface of the paper matrix tip. The polarity and magnitude of this interfacial polarization potential determine the type of electrochemical reaction (oxidation or reduction) occurring at the paper matrix tip, thereby driving the analyte compound to undergo an electrochemical reaction at the paper matrix tip. The reaction products and intermediates are electrosprayed and ionized along with the solution at the paper matrix tip, and the generated ions are captured and detected by the mass spectrometer, ultimately obtaining the corresponding mass spectrometry information.

[0008] Based on the above technical solution, in step 1, the length of the disposable pipette tip after cutting is 4.6 cm, and the outer diameter of the tip is 1.2 mm; the base width of the triangular paper matrix is ​​4 mm, and the height is 7 mm; the tip of the paper matrix protrudes 1.2 mm from the outer end of the pipette tip; the paper matrix is ​​ordinary cellulose quantitative filter paper; the volume of the pipette tip is 200 μL, and the length of the pipette tip after cutting is 4.6 cm; the paper matrix is ​​ordinary cellulose quantitative filter paper.

[0009] Based on the above technical solution, the rectangular copper conductive tape in step 2 has a side length of 10mm, the spray voltage in positive ion mode is 3.5~4.0kV, and the spray voltage in negative ion mode is -2.0~-4.0kV.

[0010] Based on the above technical solution, the electrochemical-mass spectrometry device within the paper spray ionization source described in step 3 includes four operating modes, such as... Figure 1As shown, a. positive ion mode tip electro-oxidation reaction, b. positive ion mode tip electro-reduction reaction, c. negative ion mode tip electro-oxidation reaction, d. negative ion mode tip electro-reduction reaction. The real-time monitoring method for electrochemical reactions used in all subsequent embodiments is based on these four modes for in-situ analysis and transient intermediate capture.

[0011] Based on the above technical solution, the concentration of the solution containing the test reactant in step 3 is 18~20 mg / L. - 1 An acetonitrile solution of N,N-dimethylaniline containing 18-20 mg / L -1 A methanolic solution of 6-nitroquinoline containing 35-38 mM ammonia at a concentration of 18-20 mg / L -1 A methanol solution of uric acid containing 10-12 mM ammonium acetate at a concentration of 18-20 mg / L. -1 An acetonitrile solution of 4-nitrobenzoic acid containing 0.3-0.4% formic acid at a concentration of 2-3 mg / L. -1 A methanol solution of 4-aminobiphenyl containing 5-6 mg / L -1 Phethiazide and 10-12 mg L -1 A methanol solution of N,N'-dimethylaniline containing 0.1-0.2% acetic acid at a concentration of 18-20 mg / L. -1 An acetonitrile solution of dopamine; the adjustable resistor has a resistance of 100~1000MΩ.

[0012] Based on the above technical solution, when monitoring the tip electro-oxidation reaction in positive ion ionization mode (e.g.) Figure 1 As shown in a), a solution containing the analyte is introduced into a paper spray ionization source. A grounded metal wire (cathode) with a series adjustable resistor is inserted into the solution at the rear end of the ionization source. The front end of the ionization source is connected to the conductive pretreatment section using a metal electrode and a DC voltage (anode) is applied. When monitoring the tip electroreduction reaction in positive ionization mode (e.g.) Figure 1 As shown in b), the solution containing the test reactant is introduced into the paper spray ionization source. The conductive pretreatment part at the front end of the ionization source is connected to a grounded metal wire (cathode) with an adjustable resistor in series. At the rear end of the ionization source, a metal electrode is inserted into the internal solution and a DC voltage (anode) is applied.

[0013] Based on the above technical solution, in positive ion mode, when monitoring the tip electro-oxidation reaction, the applied DC voltage is 3.5~4.0kV and the adjustable resistance is 300~600MΩ; in positive ion mode, when monitoring the tip electro-reduction reaction, the applied DC voltage is 3.5~3.6kV and the adjustable resistance is 300~400MΩ.

[0014] Based on the above technical solution, when conducting advanced electro-oxidation reaction monitoring in negative ion ionization mode (e.g.) Figure 1 As shown in c), a solution containing the analyte is introduced into a paper spray ionization source. The conductive pretreatment section at the front end of the ionization source is connected to a grounded metal wire (anode) with a series adjustable resistor. At the rear end of the ionization source, a metal electrode is inserted into the internal solution and a DC voltage is applied (cathode). When monitoring the tip electroreduction reaction in negative ion ionization mode (e.g., Figure 1 As shown in d), the solution containing the reactant to be tested is introduced into the paper spray ionization source. A grounded metal wire (anode) with an adjustable resistor in series is inserted into the solution at the rear end of the ionization source. The front end of the ionization source is connected to the conductive pretreatment section using a metal electrode and a DC voltage is applied (cathode).

[0015] Based on the above technical solution, in negative ion mode, when monitoring the tip electro-oxidation reaction, the applied DC voltage is -2.0~-2.1kV and the adjustable resistance is 700~800MΩ; in negative ion mode, when monitoring the tip electro-reduction reaction, the applied DC voltage is -3.9~-4.0kV and the adjustable resistance is 700~800MΩ.

[0016] Based on the above technical solution, when the solution containing the test reactant in step 3 has a concentration of 18~20 mg / L... -1 Electro-oxidation of N,N-dimethylaniline in acetonitrile solution was carried out in positive ion mode with an applied DC voltage of 3.5~3.6kV and an adjustable resistance of 200~300MΩ. This method was used to study the oxidation mechanism of N,N'-dimethylaniline. When the concentration of the test reactant solution in step 3 is 18~20 mg / L -1 In a methanol solution of 6-nitroquinoline, an electroreduction reaction was carried out in positive ion mode with an applied DC voltage of 3.5~3.6kV and an adjustable resistance of 200~300MΩ, which was used to study the reduction mechanism of 6-nitroquinoline. When the solution containing the test reactant in step 3 contains 35-38 mM ammonia water with a concentration of 18-20 mg / L... -1 The uric acid in methanol solution was electro-oxidized in negative ion mode by applying a DC voltage of -2.0~-2.1kV and an adjustable resistance of 700~800MΩ, for the purpose of studying the mechanism of uric acid oxidation reaction. When the solution containing the test reactant in step 3 contains 10-12 mM ammonium acetate at a concentration of 18-20 mg / L... -1 In an acetonitrile solution of 4-nitrobenzoic acid, an electroreduction reaction was carried out in negative ion mode, with a DC voltage of -3.9 to -4.0 kV applied and an adjustable resistance of 700 to 800 MΩ, for the study of the reduction reaction mechanism of 4-nitrobenzoic acid; When the solution containing the test reactant in step 3 contains 0.3-0.4% formic acid at a concentration of 2-3 mg / L... -1 In a methanol solution of 4-aminobiphenyl, an electro-oxidation reaction is carried out in positive ion mode, with an applied DC voltage of 3.9~4.0kV and an adjustable resistance of 600~700MΩ, for the study of the oxidation reaction mechanism of 4-aminobiphenyl; When the solution containing the test reactant in step 3 contains 5~6 mg / L -1 Phethiazide and 10-12 mg L -1 In a methanol solution of N,N'-dimethylaniline, an electro-oxidation reaction was carried out in positive ion mode with an applied DC voltage of 3.5~3.6kV and an adjustable resistance of 200~300MΩ. This reaction was used to study the electro-coupling reaction mechanism of N,N'-dimethylaniline and phenothiazine. When the solution containing the test reactant in step 3 contains 0.1-0.2% acetic acid at a concentration of 18-20 mg / L... -1 The electro-oxidation reaction of dopamine in acetonitrile solution under positive ion mode was carried out with a DC power supply of 3.9~4.0kV and an adjustable series resistor of 300~400MΩ, which was used to study the mechanism of dopamine electrocatalytic oxidation reaction.

[0017] Based on the above technical solution, using a multimeter, the changes in the potential difference ΔV and current I within the paper spray ionization source under positive ion mode were measured, as a function of applied high voltage and resistance value. Figure 2 As shown, a and b represent the trends of ΔV and I when no resistor is connected at 3.5kV; c and d represent the trends of ΔV and I when the voltage is changed while the 300MΩ resistor is fixed; e and f represent the trends of ΔV and I when the resistance is changed while the 3.5kV voltage is fixed; the trends of negative ion mode are the same as those of positive ion mode.

[0018] To address the aforementioned technical problems, this invention provides a real-time monitoring method for electrochemical reactions, enabling in-situ analysis and transient intermediate capture in electrochemical redox reactions or electrocatalytic reactions. This includes studies on the oxidation mechanisms of N,N'-dimethylaniline, the reduction mechanisms of 6-nitroquinoline, the oxidation mechanisms of uric acid, the reduction mechanisms of 4-nitrobenzoic acid, the oxidation mechanisms of 4-aminobiphenyl, the coupling reaction mechanism of N,N'-dimethylaniline with phenothiazine, and the electrocatalytic oxidation mechanism of dopamine.

[0019] Beneficial effects (1) The present invention provides a method and application for real-time monitoring of electrochemical reactions based on the potential difference in a paper spray source. The electrochemical cell is integrated inside the ionization source using the paper spray ionization source as a carrier. By utilizing the inherent electrochemical characteristics of the ionization source itself, a circuit is formed by connecting a resistor in series at one end of the ionization source to achieve precise internal voltage division. The generated potential difference (ΔV) is used to drive the electrochemical reaction to proceed efficiently at the paper tip, which minimizes the transmission path of electrochemical intermediates to the mass spectrometer injection port and effectively avoids the loss of short-lived intermediates during transmission. The simultaneous in-situ reaction and real-time detection are successfully achieved.

[0020] (2) This invention provides a real-time monitoring method and application of electrochemical reactions based on the potential difference within a paper spray source. By controlling the connection position and resistance parameters of the series resistor, the electrochemical properties of the spray tip and the potential difference (ΔV) inside the ionization source are precisely controlled, thereby achieving directional regulation of the electrochemical reaction process. Based on this design, this invention can cover four monitoring modes: oxidation and reduction under positive and negative ion modes. It is particularly suitable for the real-time capture and analysis of active intermediates with half-lives of only milliseconds (ms), microseconds (μs), or even nanoseconds (ns), such as dopaquinone radicals, N,N-dimethylaniline radicals, and nitrobenes, thereby enabling in-depth research on electrochemical reaction mechanisms, such as the coupling reaction between N,N-dimethylaniline and phenothiazines.

[0021] (3) The present invention provides a real-time monitoring method and application of electrochemical reaction based on the potential difference in the paper spray source, which can generate free metal ions through anodic corrosion to carry out electrocatalytic reaction, avoiding the introduction of impurity ions to interfere with the reaction, such as Fe(III) electrocatalytic dopamine oxidation.

[0022] (4) The present invention provides a method and application for real-time monitoring of electrochemical reactions based on the potential difference in a paper spray source. This method has the advantages of simple device, convenient operation, no delay time, high ion transport efficiency, low cost and wide applicability. It does not require the complex external voltage and debugging required by traditional electrochemistry, nor does it require the use of additional reaction electrodes. By utilizing the inherent characteristics of the ionization source itself, the coupling of in-situ reaction and real-time detection can be realized, which significantly simplifies the system architecture and provides a powerful in-situ, real-time analysis tool for the study of reaction mechanisms in the fields of chemical synthesis, drug metabolism, biological metabolism and electrocatalysis. Attached Figure Description

[0023] Figure 1 The diagrams show four modes of the electrochemical-mass spectrometry device in the paper spray ionization source in Examples 1-5 (a. positive ion mode tip electro-oxidation reaction, b. positive ion mode tip electro-reduction reaction, c. negative ion mode tip electro-oxidation reaction, d. negative ion mode tip electro-reduction reaction). Figure 2The following are the trends of ΔV and I in positive ion mode of the electrochemical-mass spectrometry device in the paper spray ionization source in Examples 1-7, as a high voltage is applied and the resistance value changes (a. ΔV without resistance, b. I without resistance, c. ΔV under voltage change with a fixed 300MΩ resistance, d. I under voltage change with a fixed 300MΩ resistance, e. ΔV under resistance change with a fixed 3.5kV voltage, f. I under resistance change with a fixed 3.5kV voltage). Figure 3 The electrochemical-mass spectrometry method in the paper spray ionization source of the present invention was used in Example 1 to study the oxidation reaction mechanism of N,N'-dimethylaniline in positive ion mode (a. Schematic diagram of electro-oxidation process in positive ion mode, b. without resistor, c. with 300MΩ resistor). Figure 4 In Example 2, the electrochemical-mass spectrometry method of the paper spray ionization source of the present invention was used to study the reduction mechanism of 6-nitroquinoline in positive ion mode (a. Schematic diagram of electroreduction process in positive ion mode, b. without resistor, c. with 300MΩ resistor). Figure 5 Example 3 illustrates the use of the paper spray ionization source internal electrochemical-mass spectrometry method of the present invention for the study of the uric acid oxidation reaction mechanism in negative ion mode (a. Schematic diagram of the electro-oxidation process in negative ion mode, b. without resistor, c. with 800MΩ resistor). Figure 6 Example 4 illustrates the use of the paper spray ionization source internal electrochemical-mass spectrometry method of the present invention for the study of the reduction reaction mechanism of 4-nitrobenzoic acid in negative ion mode (a. Schematic diagram of the electroreduction process in negative ion mode, b. without resistor, c. with 800MΩ resistor). Figure 7 Example 5 illustrates the use of the paper spray ionization source internal electrochemical-mass spectrometry method of the present invention for the study of the generation mechanism of 4-aminobiphenyl nitride ions (a. Schematic diagram of the electro-oxidation process in positive ion mode, b. without resistor, c. with 600MΩ resistor). Figure 8 In Example 6, the electrochemical-mass spectrometry method within the paper spray ionization source of the present invention was used to study the coupling reaction mechanism of N,N'-dimethylaniline and phenothiazine (a. Schematic diagram of electrochemical coupling reaction in positive ion mode, b. Test results of DMA system with 300MΩ resistor, c. Test results of PTA system with 300MΩ resistor, d. Test results of DMA system and PTA mixed system with 300MΩ resistor). Figure 9Example 7 illustrates the use of the paper spray ionization source internal electrochemical-mass spectrometry method of the present invention for the study of the electrocatalytic oxidation reaction mechanism of dopamine (a. Schematic diagram of electrocatalytic oxidation process in positive ion mode, b. without resistor, c. with 300MΩ resistor). Detailed Implementation

[0024] To make the objectives and technical solutions of this invention clearer, the following embodiments are provided for further explanation. However, the scope of protection of this invention is not limited to these embodiments; the embodiments are merely for illustrative purposes. Those skilled in the art should understand that any changes or equivalent substitutions that do not depart from the concept of this invention are included within the scope of protection of this invention.

[0025] Unless otherwise specified, all reagents and raw materials used in this invention are obtained through purchase.

[0026] Example 1: A method and application for real-time monitoring of electrochemical reactions based on the potential difference within a paper spray source includes the following steps: Cutting off 4mm from the tip of a disposable pipette tip, then taking a pre-cut triangular paper matrix with a base width of 4mm and a height of 7mm, and embedding it inside the pipette tip as an electrospray emitter, ensuring that the tip of the paper matrix protrudes by approximately 1.2mm; attaching a 10mm rectangular copper conductive tape around the tip flush with the front end for conductive pretreatment, thus obtaining a pretreated paper spray ionization source; subsequently filling the pipette tip with a concentration of 20mg L... -1 An acetonitrile solution of N,N-dimethylaniline (DMA) was prepared, and the pipette tip was horizontally aligned with the mass spectrometer inlet, maintaining a distance of 10 mm. The tip tip was connected to the conductive pretreatment section via a metal electrode, and a 3.5 kV DC voltage was applied. A metal guidewire was inserted into the rear end of the tip, connected in series with a 300 MΩ resistor, and then grounded. Electrochemical-mass spectrometry analysis was then performed directly (device example: ...). Figure 1 a, the tip electro-oxidation reaction in positive ion mode), the results are as follows Figure 3 As shown. In positive ion mode, without a resistor connected (e.g.) Figure 3 As shown in b), only the base peak (m / z 122) attributed to the reactant DMA was observed, indicating that no significant electrochemical oxidation reaction occurred. Conversely, after connecting a 300 MΩ resistor in series, new peaks belonging to N,N'-dimethylaniline radical, N,N,N',N'-tetramethylbenzidine radical, and N,N,N',N'-tetramethylbenzidine (TMB) appeared at m / z 121, 240, and 241, respectively. Figure 3 As shown in c, this indicates that the electrochemical oxidation of DMA dimers to form TMB via the formation of DMA radical intermediates. In particular, the half-life of the DMA (m / z 121) radical is on the order of μs, demonstrating excellent transient intermediate capture capabilities, making this method suitable for detecting ultrafast redox kinetics.

[0027] Example 2: A method and application for real-time monitoring of electrochemical reactions based on the potential difference within a paper spray source includes the following steps: the paper spray ionization source is the same as in Example 1, and then the suction head is filled with a concentration of 20 mg / L. -1 A methanol solution of 6-nitroquinoline (6-NQ) was prepared, and the pipette tip was horizontally aligned with the mass spectrometer inlet, maintaining a distance of 10 mm. The tip tip was connected to the conductive pretreatment section via a metal guidewire, and grounded after being connected in series with a 300 MΩ resistor. The rear end of the tip was inserted into a metal electrode and a 3.5 kV DC voltage was applied for direct electrochemical-mass spectrometry analysis (device as follows). Figure 1 b, positive ion mode tip electroreduction reaction), the results are as follows Figure 4 As shown. Figure 4 The diagram shows that the reduction of 6-nitroquinoline to 6-aminoquinoline (6-AQ) occurs through the sequential formation of two main intermediates: 6-nitrosoquinoline (6-NSQ) and 6-hydroxyaminoquinoline (6-HAQ). Figure 4 As shown in b, when no resistor is connected, the protonated 6-NQ at a mass-to-charge ratio of m / z 175 is the main peak; however, after connecting a 300 MΩ resistor in series, the peak intensities of the product 6-AQ (m / z 145) and the intermediates 6-NSQ (m / z 159) and 6-HAQ (m / z 161) increase significantly, especially 6-HAQ at a mass-to-charge ratio of m / z 161. These results indicate that this real-time monitoring method for electrochemical reactions within a paper spray source can regulate the voltage difference polarity between the paper matrix tip and the metal electrode by changing the connection position of the series resistor relative to the electrospray emitter, thereby achieving precise control of the electrode polarity (anodine / cathode) at the paper tip. This provides a crucial control method for the real-time monitoring of different electrochemical reactions (oxidation / reduction).

[0028] Example 3: A method and application for real-time monitoring of electrochemical reactions based on the potential difference within a paper spray source includes the following steps: the paper spray ionization source is prepared as in Example 1, and then the suction head is filled with 40 mM ammonia water (NH3·H2O) at a concentration of 20 mg / L. -1 A methanol solution of uric acid (UA) is prepared, and the pipette tip is then horizontally aligned with the mass spectrometer inlet, maintaining a distance of 10 mm. The tip tip is connected to the conductive pretreatment section via a metal guidewire, and grounded after being connected in series with an 800 MΩ resistor. The rear end of the tip is inserted into a metal electrode and a -2.0 kV DC voltage is applied for direct electrochemical-mass spectrometry analysis (device as follows). Figure 1 c, the tip electro-oxidation reaction in negative ion mode). The results are as follows: Figure 5As shown in b, without a resistor and no electrochemical reaction occurring, only the characteristic peak of uric acid (UA) deprotonation at a mass-to-charge ratio of m / z 167 exists. After connecting an 800 MΩ series resistor, the signal of the diimine intermediate (DI) at m / z 165 is significantly enhanced (e.g., ...). Figure 5 (As shown in c). This intermediate has a half-life of only 23 ms in a solution at pH 8, and the signal enhancement confirms that it originates from the electro-oxidation process of uric acid. This result indicates that real-time monitoring of electro-oxidation reactions can be achieved in negative ion mode, providing a new detection method for substances that are not suitable for analysis in positive ion mode.

[0029] Example 4: A method and application for real-time monitoring of electrochemical reactions based on the potential difference within a paper spray source includes the following steps: the paper spray ionization source is prepared as in Example 1, and then the suction head is filled with 10 mM ammonium acetate (NH4AC) at a concentration of 20 mg / L. - 1 An acetonitrile solution of 4-nitrobenzoic acid (4-NBA) was prepared, and the pipette tip was then horizontally aligned with the mass spectrometer inlet, maintaining a distance of 10 mm. The tip was connected to the conductive pretreatment section via a metal electrode, and a -4.0 kV DC voltage was applied. A metal guidewire was inserted into the rear end of the tip, connected in series with an 800 MΩ resistor, and then grounded. Electrochemical-mass spectrometry analysis was then performed directly (device as shown). Figure 1 d, tip electroreduction reaction in negative ion mode), the results are as follows Figure 6 As shown. Without a series resistor, no electroreduction reaction occurs in the system; only the base peak signal of 4-NBA (m / z 166) can be observed. Figure 6 b); After connecting an 800MΩ series resistor, 4-NBA was reduced to the target product 4-aminobenzoic acid (4-ABA, m / z 136), and the signals of the intermediates 4-nitrosobenzoic acid (4-NSoBA, m / z 150) and 4-hydroxyaminobenzoic acid (4-HABA, m / z 152) could be observed simultaneously. Figure 6 c). Combining the electroreduction results of 6-NQ in the positive ion mode and 4-NBA in the negative ion mode, it can be inferred that the electroreduction reaction mechanism of nitro aromatic compounds is as follows: through the sequential generation of nitroso compounds and hydroxylamine intermediates, they are finally reduced to the corresponding aromatic amine products.

[0030] Example 5: A method and application for real-time monitoring of electrochemical reactions based on the potential difference within a paper spray source includes the following steps: the paper spray ionization source is prepared as in Example 1, and then the suction head is filled with 0.3% formic acid (FA) at a concentration of 2 mg / L. -1A methanol solution of 4-aminobiphenyl (4-ABP) was prepared, and the pipette tip was horizontally aligned with the mass spectrometer inlet, maintaining a distance of 10 mm. The tip tip was connected to the conductive pretreatment section via a metal electrode, and a 4.0 kV DC voltage was applied. A metal guidewire was inserted into the rear end of the tip, connected in series with a 600 MΩ resistor, and then grounded. Electrochemical-mass spectrometry analysis was then performed directly (device example: [instrument name]). Figure 1 a, the tip electro-oxidation reaction in positive ion mode), the results are as follows Figure 7 As shown. Without a series resistor, no electrochemical reaction occurs in the system; only the base peak signal of 4-ABP (m / z 170) can be observed. Figure 7 b); After connecting a 600MΩ series resistor, it can be clearly observed that it belongs to the 4-ABP radical cation (m / z 169, Figure 7 c) and the nitride ion with a half-life of approximately 370 ns (m / z 168, Figure 7 c) The characteristic peak signal confirms that this method can effectively capture nanosecond-level transient intermediates, significantly outperforming the timeliness of traditional EC-MS coupling techniques. Thanks to the rapid response characteristics of this coupling interface, a low-delay coupling scheme between electrochemistry and mass spectrometry has been successfully realized, enabling convenient real-time detection of short-lived intermediates generated during the electro-oxidation of 4-aminobiphenyl.

[0031] Example 6: A method and application for real-time monitoring of electrochemical reactions based on the potential difference within a paper spray source includes the following steps: the paper spray ionization source is prepared as in Example 1, and then the suction head is filled with a solution containing 6 mg L... -1 Phethatid (PTA) and 10 mg L -1 A methanol solution of N,N'-dimethylaniline (DMA) was prepared, and the pipette tip was horizontally aligned with the mass spectrometer inlet, maintaining a distance of 10 mm. The tip was connected to the conductive pretreatment section via a metal electrode, and a 3.5 kV DC voltage was applied. A metal guidewire was inserted into the rear of the tip, connected in series with a 300 MΩ resistor, and then grounded. Electrochemical-mass spectrometry analysis was then performed directly (device example: [instrument name missing]). Figure 1 a, the tip electro-oxidation reaction in positive ion mode), the results are as follows Figure 8 As shown. When separate experiments were conducted on DMA and PTA with a series resistor connected, the mass spectrometry results showed that the DMA system ( Figure 8 In b), a protonated DMA base peak (m / z 122), a DMA radical (m / z 121), and its self-coupling product N,N,N',N'-tetramethylbenzidine (m / z 241) can be observed; the PTA system ( Figure 8c) The protonated PTA base peak (m / z 200), PTA radical cation (m / z 199), and PTA nitrogen radical (m / z 198) were detected. When a mixed solution of both was placed in the same ionization source, the protonated coupling product (m / z 319) and coupling product radical (m / z 318) were clearly detected, as shown in the results below. Figure 8 As shown in d. The above phenomenon confirms that, with the help of the electric field driving effect in the paper spray ionization source, the cross-coupling reaction between DMA and PTA can be effectively realized, and a new research method has been established for the real-time investigation of the electro-oxidative CH / NH cross-coupling reaction.

[0032] Example 7: A method and application for real-time monitoring of electrochemical reactions based on the potential difference within a paper spray source includes the following steps: the paper spray ionization source is prepared as in Example 1, and then the suction head is filled with 0.1% acetic acid (HAc) at a concentration of 20 mg / L. -1 A dopamine (DA) solution in acetonitrile is prepared. The pipette tip is then horizontally aligned with the mass spectrometer inlet, maintaining a distance of 10 mm. The tip tip is connected to the conductive pretreatment section via a metal guidewire, and grounded after being connected in series with a 300 MΩ resistor. The rear end of the pipette tip is inserted into a metal iron electrode to apply a 4.0 kV DC voltage, allowing for direct electrochemical-mass spectrometry analysis (device example: [instrument name would be inserted here]). Figure 1 b, substrate electro-oxidation reaction in positive ion mode), the results are as follows Figure 9 As shown. Without a series resistor, no electrochemical reaction occurs in the system; only the base peak signal of dopamine (DA) (m / z 154) can be observed. Figure 9 b); After connecting a 300MΩ series resistor, the metal anode corrodes and first generates Fe(II) in situ, which is then further oxidized to Fe(III). The resulting Fe(III) can catalyze the oxidation of dopamine to generate dopaquinone (DAQ, m / z 152), and the signal of this product is successfully captured. Figure 9 c). This result indicates that the electron transfer process between Fe(III) and dopamine dominates the Fe(III)-mediated dopamine oxidation reaction pathway. Elucidating this mechanism is of significant reference value for studying the toxic effects of dopamine under metal-mediated oxidation.

[0033] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for real-time monitoring of electrochemical reactions based on the potential difference within a paper spray source, characterized in that, Includes the following steps: Step 1, Preparation of paper spray ionization source: Cut 3-5 mm from the tip of a disposable pipette tip, and take a pre-cut triangular paper matrix with a base width of 3-5 mm and a height of 7-10 mm. Roll the triangular paper matrix into a cylindrical shape and embed it into the cut tip of the pipette tip as an electrospray emitter, and make the tip of the triangular paper matrix protrude 1-3 mm from the outside of the pipette tip to obtain the paper spray ionization source; Step 2, conductive pretreatment of the front end of the paper spray ionization source: Cut a rectangular copper conductive tape with a side length of 8~12mm, take the above ionization source, stick one side of the rectangular copper conductive tape along the front end of the ionization source and keep the edge of the tape flush with the cut surface of the suction head, perform conductive pretreatment, and ensure that the tape is completely attached to the tube wall to obtain the pretreated paper spray ionization source. Step 3, Paper Spray Ionization Source - In-situ Electrochemical Reaction - Rapid Mass Spectrometry Analysis: Place the pretreated paper spray ionization source containing the analyte solution directly in front of the mass spectrometer inlet, maintaining a distance of 8-12 mm between the paper matrix tip and the mass spectrometer inlet. By selectively connecting a resistor in series at the front or rear of the ionization source, a controllable polarization potential is established at the interface between the paper spray emitter tip and the internal solution using the circuit voltage divider. This allows the paper tip to exhibit anodic or cathodic characteristics, driving the analyte to undergo corresponding electro-oxidation or electro-reduction reactions at the paper tip. The reaction products and intermediates undergo electro-spray ionization along with the solution through the paper matrix tip, and the generated ions are captured and detected by the mass spectrometer, ultimately obtaining the corresponding mass spectrometry information.

2. The method for real-time monitoring of electrochemical reactions according to claim 1, characterized in that, In step 1, the length of the disposable pipette tip after cutting is 4.5~4.7cm, and the outer diameter of the tip is 1.2~1.5mm; the base width of the triangular paper matrix is ​​3~5mm, and the height is 7~10mm; the tip of the paper matrix protrudes 1~3mm from the outside of the pipette tip; the paper matrix is ​​ordinary cellulose quantitative filter paper.

3. The method for real-time monitoring of electrochemical reactions according to claim 1, characterized in that, The rectangular copper conductive tape mentioned in step 2 has a side length of 8~12mm, a spray voltage of 3.5~4.0kV in positive ion mode, and a spray voltage of -2.0~-4.0kV in negative ion mode.

4. The method for real-time monitoring of electrochemical reactions according to claim 1, characterized in that, In step 3, the concentrations of the test reactant solutions are 18-20 mg / L. -1 An acetonitrile solution of N,N-dimethylaniline containing 18-20 mg / L -1 A methanolic solution of 6-nitroquinoline containing 35-38 mM ammonia at a concentration of 18-20 mg / L -1 A methanol solution of uric acid containing 10-12 mM ammonium acetate at a concentration of 18-20 mg / L. -1 An acetonitrile solution of 4-nitrobenzoic acid containing 0.3-0.4% formic acid at a concentration of 2-3 mg / L. -1 A methanol solution of 4-aminobiphenyl containing 5-6 mg / L -1 Phethiazide and 10-12 mg L -1 A methanol solution of N,N'-dimethylaniline containing 0.1-0.2% acetic acid at a concentration of 18-20 mg / L. -1 An acetonitrile solution of dopamine; the adjustable resistor has a resistance of 100~1000MΩ.

5. The method for real-time monitoring of electrochemical reactions according to claim 1, characterized in that, When monitoring tip electro-oxidation reactions in positive ion ionization mode, a solution containing the analyte is introduced into a paper spray ionization source. A grounded metal wire with a series adjustable resistor is inserted into the solution at the rear end of the ionization source, and a metal electrode is connected to the conductive pretreatment section at the front end of the ionization source and a DC voltage is applied. When monitoring tip electro-reduction reactions in positive ion ionization mode, a solution containing the analyte is introduced into a paper spray ionization source. A grounded metal wire with a series adjustable resistor is connected to the conductive pretreatment section at the front end of the ionization source, and a metal electrode is inserted into the internal solution at the rear end of the ionization source and a DC voltage is applied.

6. The method for real-time monitoring of electrochemical reactions according to claim 5, characterized in that, In positive ion mode, when monitoring the tip electro-oxidation reaction, the applied DC voltage is 3.5~4.0kV and the adjustable resistance is 300~600MΩ; in positive ion mode, when monitoring the tip electro-reduction reaction, the applied DC voltage is 3.5~3.6kV and the adjustable resistance is 300~400MΩ.

7. The method for real-time monitoring of electrochemical reactions according to claim 1, characterized in that, When monitoring the tip electro-oxidation reaction in negative ion ionization mode, a solution containing the analyte is introduced into the paper spray ionization source. The conductive pretreatment section at the front end of the ionization source is connected to a grounded metal wire with an adjustable resistor in series. At the rear end of the ionization source, a metal electrode is inserted into the internal solution and a DC voltage is applied. When monitoring the tip electro-reduction reaction in negative ion ionization mode, a solution containing the analyte is introduced into the paper spray ionization source. A grounded metal wire with an adjustable resistor in series is inserted into the solution at the rear end of the ionization source. At the front end of the ionization source, a metal electrode is connected to the conductive pretreatment section and a DC voltage is applied.

8. The method for real-time monitoring of electrochemical reactions according to claim 7, characterized in that, In negative ion mode, when monitoring the tip electro-oxidation reaction, the applied DC voltage is -2.0~-2.5kV and the adjustable resistance is 700~800MΩ; in negative ion mode, when monitoring the tip electro-reduction reaction, the applied DC voltage is -3.8~-4.0kV and the adjustable resistance is 700~800MΩ.

9. The real-time monitoring method for electrochemical reactions according to any one of claims 1 to 8 is used for in-situ analysis of electrochemical redox reactions or electrocatalytic reactions and for the capture of transient intermediates.

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