A method for detecting phenolic antioxidant additives in insulating oil

Abstract

The application discloses a detection method of phenolic antioxidant additives in insulating oil, and belongs to the technical field of insulating oil detection. The application adopts a nuclear magnetic resonance hydrogen spectrum internal standard method for quantitative detection, directly determines the mixed oil sample, an internal standard substance and a deuterated reagent, and calculates the additive content according to the integral area ratio of characteristic quantitative peaks. The chemical shifts of the characteristic quantitative peaks of the internal standard substance and the characteristic quantitative peaks of the phenolic antioxidant additives in the insulating oil are located in the medium field intensity area of the spectrum, and the chemical shifts are not overlapped. The application overcomes technical difficulties such as complicated pretreatment, interference of complex oil matrix, use of toxic reagents and strong dependence on standard products of traditional methods. The detection method has the advantages of small sample consumption, fast analysis speed, green environmental protection, simple operation and the like, and the detection limit can reach 0.004%, and can realize rapid, accurate and reliable quantitative analysis of the phenolic antioxidant additives in the insulating oil.

Description

Technical Field

[0001] This application belongs to the field of power oil testing technology, and particularly relates to a method for detecting phenolic antioxidant additives in insulating oil. Background Technology

[0002] Insulating oil is widely used in power equipment such as transformers, high-voltage switches, power capacitors, and cables. It serves both as an insulating medium and a cooling agent. During long-term operation, insulating oil inevitably undergoes oxidation, generating peroxides and a series of oxidation products such as alcohols, aldehydes, ketones, and acids. These oxidation products can severely affect the insulation performance and operational reliability of power equipment, posing a potential threat to the overall safety of the power grid.

[0003] To inhibit this oxidation process and improve the oxidation stability of oils, phenolic antioxidants are commonly added to insulating oils in the industry, with T501 (2,6-di-tert-butyl-p-cresol) being the most widely used. Its mechanism of action involves capturing active free radicals in the oxidation chain reaction, thereby effectively preventing further deterioration of the oil. According to the national standard GB / T 7595-2008 "Quality of Transformer Oil in Operation," the T501 content in new and recycled oils should not be less than 0.3%-0.5%, and in operating oils, it should not be less than 0.15%. If it is lower than this, it needs to be added. Therefore, in the operation and maintenance of power equipment, it is usually necessary to periodically test the T501 content in insulating oil, which is an important means of assessing the oil's antioxidant capacity and predicting deterioration trends. Currently, methods for detecting T501 in insulating oil include spectrophotometry, liquid chromatography, and infrared spectroscopy.

[0004] However, existing detection methods have the following drawbacks: First, they are susceptible to interference from complex matrices, raising questions about accuracy. Insulating oil itself is a complex mixture of hydrocarbons, and various oxidation products are continuously generated and accumulated during operation. These matrix components can severely interfere with traditional detection methods. For example, spectrophotometry is easily affected by the deepening of oil color and the presence of coexisting chromophores; chromatography may lead to peak overlap or inaccurate quantification due to the similarity of chromatographic properties between oxidation products and the target analyte. This matrix effect often prevents the detection results from accurately reflecting the actual content of T501, affecting the accurate assessment of the oil's condition. Second, they are cumbersome to operate and highly dependent on standards. Existing methods typically rely on standard curves for quantification, requiring the prior acquisition of high-purity T501 standards. More importantly, to establish an accurate standard curve, specially purified blank oil, i.e., base oil from which all original additives have been removed, must be used. This purification process is extremely complex and time-consuming, and the availability and consistency of blank oil are difficult to guarantee, directly affecting the operability of the method and the reliability of the results. Third, it involves toxic and harmful reagents, resulting in poor safety and environmental protection: In the sample pretreatment or analysis process, existing methods generally require the use of toxic and harmful organic reagents such as concentrated sulfuric acid, carbon tetrachloride, and methanol.

[0005] Therefore, there is an urgent need to develop a new detection method that can overcome the above-mentioned defects in order to meet the pressing need for accurate, efficient and green detection of additives in complex operating insulating oils. Summary of the Invention

[0006] This application discloses a method for detecting phenolic antioxidant additives in insulating oil, aiming to solve the technical problems of existing detection methods, such as complex pretreatment, dependence on blank oil and toxic reagents, cumbersome operation steps, and interference from the oil matrix.

[0007] This application applies the internal standard method of proton nuclear magnetic resonance spectroscopy to the direct quantification of phenolic antioxidants in complex insulating oil systems, enabling direct, rapid, accurate, and environmentally friendly detection of phenolic antioxidant additives in insulating oils. By carefully selecting internal standards with specific structures, the quantitative proton nuclear signals of both the internal standard and the target additive are located in the region of high field strength and stable baseline in the spectrum, and both are sharp single peaks. This fundamentally avoids interference from the complex oil matrix and achieves excellent signal-to-noise ratio and integral accuracy. Based on this, by optimizing parameters such as the amount of test sample and the number of scans, high-sensitivity and high-precision quantification of trace additives is achieved without the need for pretreatment.

[0008] To achieve the above objectives, the technical solution of this application is: The first aspect of this application provides a method for detecting phenolic antioxidant additives in insulating oil, which uses the internal standard method of nuclear magnetic resonance hydrogen spectroscopy for quantitative detection; The detection method includes: After mixing the oil sample to be tested, the internal standard, and the deuterated reagent, the 1H NMR spectrum was determined to obtain the 1H NMR spectrum. The characteristic quantitative peaks of the internal standard and the characteristic quantitative peaks of phenolic antioxidant additives in the insulating oil are integrated in the 1H NMR spectrum to obtain the integrated area of ​​the characteristic quantitative peaks of the internal standard and the phenolic antioxidant additives in the insulating oil. The content of phenolic antioxidant additives in the insulating oil is determined by combining the mass of the oil sample to be tested and the internal standard, the purity of the internal standard, the molecular weight of the internal standard and the additives, and the number of protons in the quantitative peak. The chemical shifts of the characteristic quantitative peaks of the internal standard and the characteristic quantitative peaks of the phenolic antioxidant additives in the insulating oil are both located in the mid-intensity region of the 1H NMR spectrum, and the chemical shifts do not overlap.

[0009] In conjunction with the first aspect, preferably, the content of phenolic antioxidant additives in the insulating oil is linearly related to the integral area of ​​the quantitative NMR characteristic peak of the phenolic antioxidant additives.

[0010] In conjunction with the first aspect, preferably, the linear relationship is expressed as follows:

[0011] Wherein, Pa: the content of phenolic antioxidant additives in the oil sample to be tested; Ps: Purity of the internal standard; Ia: The integral area of ​​the NMR quantitative peak of phenolic antioxidant additives in the oil sample to be tested; Is: Integral area of ​​the NMR quantitative peak of the internal standard; Na: The number of protons in the quantitative peak of NMR in the oil sample to be tested; Ns: The number of protons in the NMR quantitative peak of the internal standard; Ma: Molecular weight of phenolic antioxidant additives in the oil sample to be tested; Ms: Molecular weight of the internal standard; Wa: Mass of the oil sample to be tested; Ws: The mass of the internal standard.

[0012] Preferably, in conjunction with the first aspect, the internal standard is an aromatic compound that does not contain active hydrogen and whose chemical shift does not overlap with the characteristic hydrogen nucleus chemical shift of phenolic antioxidant additives; The internal standard is at least one of methyl 3,5-dinitrobenzoate, dimethyl terephthalate, 1,3,5-trimethoxybenzene, 3,4,5-trichloropyridine, benzyl benzoate, and pyrazine.

[0013] Preferably, in conjunction with the first aspect, the phenolic antioxidant additive includes at least one selected from 2,6-di-tert-butyl-p-cresol, 2,6-di-tert-butylphenol, and 4,4-methylenebis-(2,6-di-tert-butylphenol); When testing multiple phenolic antioxidant additives, the characteristic hydrogen nucleus peaks of each phenolic antioxidant additive are selected for integration, and then the content of each phenolic antioxidant additive is calculated.

[0014] Preferably, in conjunction with the first aspect, the deuterated reagent is one or more of deuterated chloroform, deuterated tetrahydrofuran, deuterated methanol, deuterated benzene, and deuterated dimethyl sulfoxide.

[0015] In conjunction with the first aspect, preferably, the mass ratio of the oil sample to be tested and the internal standard is (1000-1):(1-5); And / or, the detection amount of the oil sample to be tested is 20-50 mg.

[0016] Preferably, in conjunction with the first aspect, the conditions for the proton nuclear magnetic resonance spectroscopy measurement are: magnetic field strength of 400-600MHz, pulse width of 4-5 μs, spectral width of 19-20 ppm, pre-scan delay of 6-7 μs, and number of scans of 8-256.

[0017] In conjunction with the first aspect, preferably, the conditions for the proton nuclear magnetic resonance spectroscopy measurement are: pulse width of 4.25 μs, spectral width of 19.8352 ppm, pre-scan delay of 6.5 μs, and 64 scans.

[0018] In conjunction with the first aspect, preferably, the detection limit of the phenolic antioxidant additive content is ≤ 0.004%, the quantitation limit is ≤ 0.013%, and the total time for a single detection is ≤ 5 minutes.

[0019] Compared with the prior art, the advantages or beneficial effects of the embodiments of this application include at least the following: The detection method provided in this application uses the internal standard method of nuclear magnetic resonance (NMR) 1H spectroscopy to quantitatively detect the content of phenolic antioxidant additives in insulating oil. On the one hand, this application uses the internal standard method of NMR 1H spectroscopy for quantitative detection, overcoming the problems of matrix interference and dependence on standards. Its quantitative basis is the relative ratio of characteristic peak areas, rather than the absolute response signal. This detection method does not require the use of standards for the additives to be tested to draw a standard curve, nor does it require the use of blank oil, which is difficult to obtain and differs from the oil sample being tested. In principle, it eliminates the systematic errors and operational bottlenecks caused by standards and blank oil in traditional methods, and can still ensure quantitative accuracy in complex operating insulating oil systems.

[0020] On the other hand, the detection method used in this application has simple pretreatment and high analytical efficiency. Sample pretreatment only requires mixing a small amount of the oil sample to be tested with an internal standard and a deuterated reagent. There is no need for complex pretreatment operations using hazardous reagents such as concentrated sulfuric acid, nor for extraction, derivatization, or other operations. The entire sample preparation process can be completed within a few minutes. Combined with nuclear magnetic resonance spectroscopy scanning, the total detection time for a single sample can be controlled within 5 minutes, which is much shorter than traditional methods. It is particularly suitable for rapid screening of large batches of samples.

[0021] Thirdly, the detection method adopted in this application requires a small amount of oil sample, making it environmentally friendly. The optimal amount of oil sample required is only about 30 mg, which can meet the needs of trace analysis scenarios where sampling of low-oil electrical equipment is difficult. The entire analytical process only requires the use of solid internal standards in milligrams and a small amount of deuterated reagents, completely avoiding the use of large amounts of toxic and harmful solvents such as concentrated sulfuric acid, carbon tetrachloride, and methanol in traditional methods. This reduces chemical hazards and the generation of hazardous waste liquids from the source, which is in line with the development trend of green analytical chemistry.

[0022] Fourthly, the detection method used in this application is highly versatile and yields accurate and reliable results. By selecting appropriate internal standards, it can detect a variety of phenolic antioxidants and achieve simultaneous quantification of multiple components. Its relative standard deviation is less than 5%, and the limit of quantitation is ≤0.013%, fully meeting the national standard requirements for the monitoring sensitivity of operating oil (≥0.15%), ensuring accurate and reliable detection results.

[0023] Fifthly, the detection method adopted in this application is simple to operate and easy to standardize. The entire detection process mainly consists of weighing, mixing, injection and data processing. The operation steps are highly standardized, have little dependence on personnel experience, are easy to promote across laboratories, and improve the consistency and comparability of test results. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic flowchart of the method for detecting antioxidants in the oil sample to be tested, as described in an embodiment of this application. Figure 2 This is a fitting curve diagram of T501 in the oil sample to be tested in the embodiments of this application; Figure 3 This is a repeatability analysis chart of the T501 content measurement in oil samples according to an embodiment of this application; Figure 4 The above is the 1H NMR spectrum of a blank oil sample used in the infrared method of the prior art of this application for the determination of T501 content; Figure 5 The above is a hydrogen NMR spectrum of transformer oil 1 from an embodiment of this application, showing the T501 content. Figure 6 The above is a hydrogen NMR spectrum of transformer oil 2 from an embodiment of this application, showing the T501 content. Figure 7 This is the 1H NMR spectrum of the transformer oil 3 in this embodiment of the application, which shows the content of T501. Detailed Implementation

[0026] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0027] In the following description of this embodiment, the term "and / or" is used to describe the association relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, B existing alone, and A and B existing simultaneously. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0028] In the following description of this embodiment, the term "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0029] Those skilled in the art should understand that, in the following description of the embodiments of this application, the sequence of numbers does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0030] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms "a" and "the" as used in the embodiments of this application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0031] It should be noted that all raw materials and / or reagents in the embodiments of this application were purchased on the market or prepared according to conventional methods known to those skilled in the art.

[0032] In a first aspect, embodiments of this application provide a method for detecting phenolic antioxidant additives in insulating oil, using the internal standard method of nuclear magnetic resonance hydrogen spectroscopy for quantitative detection; The detection method includes: After mixing the oil sample to be tested, the internal standard, and the deuterated reagent, the 1H NMR spectrum was determined to obtain the 1H NMR spectrum. The characteristic quantitative peaks of the internal standard and the characteristic quantitative peaks of phenolic antioxidant additives in the insulating oil are integrated in the 1H NMR spectrum to obtain the integrated area of ​​the characteristic quantitative peaks of the internal standard and the phenolic antioxidant additives in the insulating oil. The content of phenolic antioxidant additives in the insulating oil is determined by combining the mass of the oil sample to be tested and the internal standard, the purity of the internal standard, the molecular weight of the internal standard and the additives, and the number of protons in the quantitative peak. The chemical shifts of the characteristic quantitative peaks of the internal standard and the characteristic quantitative peaks of the phenolic antioxidant additives in the insulating oil are both located in the mid-intensity region of the 1H NMR spectrum, and the chemical shifts do not overlap.

[0033] In this application, the key to achieving high sensitivity, low detection limit, and accurate quantification lies in the following aspects: Firstly, the optimized design of the internal standard and signal peaks: the quantitative hydrogen nuclear signals of the internal standard and phenolic antioxidant additives are selected, and their chemical shifts are all located in the mid-intensity region of the 1H NMR spectrum. This region has the most stable baseline, minimizing interference from other component signals and baseline drift in the complex insulating oil matrix. Simultaneously, the selected quantitative peaks are all sharp, symmetrical, isolated single peaks, avoiding signal overlap. This not only makes the peak area integration more accurate but also significantly improves the signal-to-noise ratio of the effective signal, achieving the structural basis for high sensitivity. Secondly, targeted optimization: based on the above signal characteristics, through systematic optimization of key parameters, the signal-to-noise ratio is improved to the level required for ultra-low concentration quantification while ensuring rapid analysis. This optimization is achieved without relying on complex sample pretreatment, demonstrating the high efficiency of the method itself.

[0034] It should be noted that the mid-intensity region in a 1H NMR spectrum is defined based on the chemical shift range of hydrogen nuclear resonance absorption. The industry-standard definition is that the chemical shift range of 2.0–6.0 ppm constitutes the mid-intensity region. Insulating oil is primarily composed of hydrocarbon compounds (alkanes, cycloalkanes, etc.), and its hydrogen nuclear resonance signals are almost entirely concentrated in the 0–3.0 ppm high-intensity region. In contrast, trace impurities in the oil (such as aromatics and carbonyl compounds) mostly exhibit hydrogen nuclear resonance signals in the low-intensity region above 6.0 ppm. The mid-intensity region represents a signal blank area for the insulating oil matrix. Selecting a quantitative hydrogen nuclear signal within this range can completely avoid the superposition and interference of NMR signals from the matrix and impurities, significantly improving the accuracy of quantitative integration.

[0035] In this embodiment, the internal standard is preferably at least one of methyl 3,5-dinitrobenzoate, dimethyl terephthalate, 1,3,5-trimethoxybenzene, 3,4,5-trichloropyridine, benzyl benzoate, and pyrazine. More preferably, it is methyl 3,5-dinitrobenzoate, whose methyl hydrogen signal is a sharp single peak, and its chemical shift (approximately 4.0 ppm) can be effectively separated from the phenolic hydroxyl proton signal of T501 (approximately 5.0 ppm) and the proton signals of most alkanes in the oil matrix (0.5-2.5 ppm), and the signal is stable, making it an ideal internal standard choice. The internal standard is a compound that does not contain active hydrogen and whose chemical shift does not overlap with the characteristic hydrogen nucleus chemical shifts of phenolic antioxidant additives. These internal standards are chemically stable, and their hydrogen nucleus chemical shifts hardly change with experimental conditions during testing, ensuring the accuracy of quantitative analysis; furthermore, these internal standards are completely miscible with the oil sample to be tested.

[0036] It should be noted that the instrument should first be calibrated: after powering on the instrument, use deuterated chloroform for field locking and sizing, ensuring that the signal rotation sideband is less than 5% and the full width at half maximum (FWHM) of the 1H NMR peak is less than 2 Hz. This indicates normal instrument calibration. Secondly, the 1H NMR testing conditions are as follows: Instrument: Bruker Ascend 600; Frequency: 600 MHz; Pulse width p0 = 4.25 μs; Spectral width SWH = 19.8352 ppm; Acquisition time AQ = 2.75 s; Dwell time = 42 μs; Pre-scan delay DE = 6.50 μs; Deuterated reagent: deuterated chloroform (deuterium content greater than 99.5%, water (HDO) less than 0.2%); NMR sample tube: 5 mm diameter; Insulating oil: homogeneous, free of impurities and particulate precipitates; Internal standard: methyl 3,5-dinitrobenzoate, purity denoted as Ps, molecular weight 226.0226. Finally, the proton NMR spectrum was measured: the sample tube was placed in the probe and the sample was cooled to the probe's set temperature; the sample was automatically scanned and tested according to the set instrument conditions; and the proton NMR spectrum of the sample was obtained.

[0037] In this embodiment, the phenolic antioxidant additive is preferably at least one of 2,6-di-tert-butyl-p-cresol, 2,6-di-tert-butylphenol, and 4,4-methylenebis-(2,6-di-tert-butylphenol), more preferably 2,6-di-tert-butyl-p-cresol. When detecting multiple phenolic antioxidant additives, the characteristic hydrogen nucleus peaks of each phenolic antioxidant additive are selected for integration, and the content of each phenolic antioxidant additive is calculated using the relationship described in the first aspect. The detection method of this application can achieve simultaneous detection of multiple phenolic antioxidant additives in the oil sample to be tested, and by accurately screening the specific characteristic hydrogen nucleus peaks of each target component, the accuracy and reliability of simultaneous detection of multiple components are effectively guaranteed.

[0038] In this embodiment, the deuterated reagent is preferably one or more of deuterated chloroform, deuterated tetrahydrofuran, deuterated methanol, deuterated benzene, and deuterated dimethyl sulfoxide, more preferably deuterated chloroform. These deuterated reagents can avoid interference from solvent hydrogen nuclei with the sample signal, while ensuring good sample dissolution to obtain a clear and accurate 1H NMR spectrum. Furthermore, they possess good chemical stability, will not react with the sample during the detection process, and guarantee the accuracy of the results.

[0039] In this embodiment, if there are obvious particulate matter in the oil sample before mixing the oil sample, internal standard, and deuterated reagent, a pretreatment step is also included: filtering the oil sample through a filter membrane to remove solid impurities. In actual operation, the oil sample may contain fine particulate matter, such as metal fragments, colloidal precipitates, and dust particles, due to equipment wear, external contamination, or oxidative aging. If these solid impurities directly enter the detection system, they may affect the normal operation of the equipment and increase maintenance costs. Furthermore, the organic or inorganic components in the impurities may weakly interact with the deuterated reagent or generate interfering peaks in the 1H NMR spectrum, causing the characteristic hydrogen nucleus peak signal of the target additive to be masked or the integration accuracy to decrease. Therefore, pretreatment effectively traps any solid particulate matter that may be present in the oil sample, ensuring the homogeneity of the system after mixing with the internal standard and deuterated reagent. This provides a pure sample matrix for NMR detection, reducing the interference of impurities on spectral resolution and quantitative accuracy from the source, and further ensuring the reliability and repeatability of the detection results.

[0040] In this embodiment, the preferred mass ratio of the oil sample to the internal standard is (1000-1):(1-5); the preferred detection amount of the oil sample is 20-50 mg. This ratio range allows the intensity of the hydrogen nuclear resonance signal of the internal standard and the phenolic antioxidant additive in the oil sample to be matched, ensuring the accuracy of the characteristic peak integral calculation, while also adapting to the detection concentration range of various additives. The preferred detection amount of the oil sample is 20-50 mg, more preferably 30 mg. This amount is based on the detection sensitivity of the hydrogen nuclear magnetic resonance spectrum, enabling the target additive to generate an effectively identifiable resonance signal, and also addresses the industry pain point of difficult sampling of low-oil electrical equipment, while simultaneously reducing the amount of auxiliary reagents used, further improving the environmental friendliness of the method.

[0041] In this embodiment, the detection method employed has a detection limit ≤ 0.004%, a quantitation limit ≤ 0.013%, and a single detection time ≤ 5 minutes. It exhibits excellent versatility due to its advantages such as low sample requirement, fast analysis speed, minimal solvent usage, simple operation, and high sensitivity. These characteristics enable it to flexibly meet the analytical needs of various oil products, and it has broad market application prospects in multiple fields such as edible oil quality identification, oil adulteration identification, industrial lubricant condition monitoring, and biodiesel component analysis.

[0042] The technical solution of this application will be further described below with reference to specific embodiments.

[0043] Example 1 This embodiment provides a method for detecting antioxidant T501 in insulating oil, such as... Figure 1 The flowchart shown specifically includes: S101: Weigh the internal standard methyl 3,5-dinitrobenzoate (Ws, about 12 mg) and the oil sample to be tested (Wa, about 6.000 g), and mix them thoroughly in the sample vial; S102: Take a certain amount of mixed sample into an NMR tube, add 500 μL of deuterated chloroform, and seal the NMR tube cap. S103: Place the NMR tube into the pre-calibrated nuclear magnetic resonance spectrometer (such as the Bruker Ascend). TM Nuclear magnetic resonance hydrogen spectra were acquired in a 600 MHz frequency band according to the set number of scans (frequency: 600 MHz; pulse width p0 = 4.25 μs; spectral width SWH = 19.8352 ppm; acquisition time AQ = 2.75 s; dwell time = 42 μs; pre-scan delay DE = 6.50 μs). S104: Analyze the nuclear magnetic resonance (NMR) spectrum. Use the methyl peak (approximately 4.05 ppm) of the internal standard methyl 3,5-dinitrobenzoate and the phenolic hydroxyl peak (approximately 5.07 ppm) of T501 as quantitative peaks. Integrate the peaks to obtain the integrated area Is of the internal standard and the integrated area Ia of the phenolic hydroxyl peak of T501. Substitute these parameters into the formula to calculate the content Pa of T501 in the oil sample.

[0044] The internal standard methyl 3,5-dinitrobenzoate was chosen because its quantitative peak and the quantitative hydrogen nucleus signal of the target additive T501 are both located in the mid-intensity region of the spectrum. Furthermore, the selected quantitative peaks are all sharp, symmetrical, isolated single peaks, avoiding signal overlap. This not only makes the peak area integration more accurate but also significantly improves the signal-to-noise ratio of the effective signal.

[0045] Test Example 1: Optimization of the number of scans and the detection limit and quantitation limit of the method. In order to determine the optimal number of scans, an oil sample with a T501 concentration of 0.30% was prepared. According to the test conditions of Example 1, the signal-to-noise ratio (S / N), scan time and stability of T501 content determination were tested at 8, 16, 32, 64, 128 and 256 scans, respectively. The results are shown in Table 1.

[0046] Table 1 Determining the Optimal Number of Scans

[0047] According to the test results in Table 1, when the number of scans is greater than 32, the signal-to-noise ratio is greater than 150, and the detection results remain relatively stable. When the number of scans is 64, the signal-to-noise ratio of this detection method is 232, which is greater than 150, and the test results tend to be stable. Considering both analysis speed and precision, the optimal number of scans is determined to be 64. At this point, the signal-to-noise ratio is high (>230), the single scan time is about 4 minutes, and the precision is good. Based on this, the limit of detection (LOD) for T501 content in insulating oil is calculated to be 0.004%, and the limit of quantitation (LOQ) is 0.013%. This sensitivity fully meets and is far superior to the national standard monitoring requirement that the T501 content in operating oil should not be less than 0.15%.

[0048] Test Example 2: Optimal Amount of Oil Sample to be Tested. To determine the optimal amount of oil sample to be tested, different sample amounts (2-90 mg) of the same oil sample were taken to prepare NMR samples. Under the test conditions in Example 1, the test results are shown in Table 2.

[0049] Table 2 Sample Usage Analysis

[0050] Table 2 shows that the standard deviation (SD) of data 7-14 was 0.2674, and the relative standard deviation (RSD) was 21.73%, indicating insufficient repeatability and stability. However, the standard deviation (SD) of data 10-14 was 0.02765, and the RSD was 2.85%, demonstrating good repeatability and stability. Furthermore, to balance the signal intensity requirements of the target peak with the operability of the weighing process, the optimal amount of oil sample added was 30 mg.

[0051] Test Example 3: A blank oil with a T501 content of 0.0% was selected for infrared spectroscopy. The oil sample was tested under the optimized conditions described above. The sample size was 30 mg, and the number of scans was 64. The 1H NMR spectrum is shown below. Figure 4 As shown, the parameters are: Solvent (deuterated reagent), Temperature (test temperature), Number of Scans (number of scans), Relaxation Delay (relaxation time), Spectrometer Frequency (NMR frequency), and Nucleus (proton). Analysis of the 1H NMR spectrum yielded an internal standard quantitative peak integral area of ​​Is = 3.0 and a phenolic hydroxyl integral area of ​​T501 of Ia = 0.186. Substituting these parameters into the formula, the content of T501 in the oil sample was calculated to be Pa = 0.0355%. The calculation process is as follows:

[0052] Pa: The content of antioxidant T501 in the oil sample to be tested; Ps: Purity of the internal standard, such as 98% for methyl 3,5-dinitrobenzoate used; Ia: The integral area of ​​the T501 NMR quantitative peak of the antioxidant in the oil sample to be tested. If the phenolic hydroxyl hydrogen is used as the quantitative peak, it is the integral area of ​​the chemical shift at 5.0 ppm, Ia = 0.186. Is: The integrated area of ​​the internal standard NMR quantitative peak, with the methyl group of methyl 3,5-dinitrobenzoate as the quantitative peak, is the integrated area at a chemical shift of 4.0 ppm, Is = 3; Na: The number of protons in this signal of the oil sample to be tested. The number of protons is taken as the phenolic hydroxyl hydrogen of the antioxidant T501, and Na = 1. Ns: The number of protons in the signal from the internal standard; the methyl group of methyl 3,5-dinitrobenzoate is the quantitative peak, Ns = 3; Ma: The molecular weight of the oil sample to be tested; the molecular weight of antioxidant T501 is 220.1827. Ms: The molecular weight of the internal standard; the molecular weight of methyl 3,5-dinitrobenzoate is 226.0226. Wa: The actual mass of the oil sample to be tested, Wa = 6000.3 mg; Ws: The weighed mass of the internal standard, Ws = 12.0 mg.

[0053] The above tests demonstrate that traditional infrared testing methods may fail to detect trace residues due to insufficient sensitivity or matrix interference, and also reveal the potential background interference in the so-called "blank oil" used in existing technologies. In contrast, the detection method of this application eliminates the reliance on blank oil, avoiding systematic errors caused by background interference from blank oil at the detection principle level, and achieving accurate and interference-free detection of trace residues.

[0054] Example 2 Method linear range: A series of T501 standard oil samples with different concentration gradients were prepared and mixed with the test oil samples at concentrations of 0.1001%, 0.2004%, 0.3009%, 0.4016%, 0.5025%, and 0.6048%, respectively. Nuclear magnetic resonance (NMR) 1H spectrometry was performed under the optimal conditions described in Example 1. The T501 integral area Ia was calculated according to the formula after detection, as shown in Table 3.

[0055] Table 3 Linearity and Quantitative Analysis Performance

[0056] Based on the actual weighed concentration c and the integral area Ia of T501 in Table 3, a linear regression analysis was performed. With the actual weighed concentration (c) of T501 as the abscissa and the integral area Ia of T501 as the ordinate, the linear fitting equation was obtained as c = 491.8774 Ia, R0. 2 =0.9997, R is the linear correlation coefficient, such as Figure 2 As shown, the method exhibits good linearity within the range of 0.1%-0.6%, fully meeting the requirements for determining the content of phenolic oxidizing additives in insulating oil. This also fully demonstrates that the detection method described in this application possesses excellent detection accuracy and reliability.

[0057] Example 3 Repeated analysis of transported oil samples: The same transported oil sample was selected and analyzed repeatedly 8 times, numbered 21-28. The detection and analysis were carried out according to the steps of Example 1, and the results are shown in Table 4.

[0058] Table 4. Repeated analysis using oil transport samples

[0059] Based on the test results in Table 4, a plot was created with the T501 integral area Ia as the ordinate and the number of test repetitions as the abscissa. Figure 3 The T501 content obtained from multiple repeated tests was statistically analyzed, and the average T501 content was calculated to be 0.2645%, with a standard deviation SD of 0.00661 and a relative standard deviation RSD of 2.50%. The results demonstrate that the analytical detection method of this application has good repeatability.

[0060] Example 4 The detection method of this application was used to test three different transformer oil samples in operation. The NMR spectra are shown below. Figure 5-7 , Figure 5 The above is a hydrogen NMR spectrum of transformer oil 1 from an embodiment of this application, showing the T501 content. Figure 6 The above is a hydrogen NMR spectrum of transformer oil 2 from an embodiment of this application, showing the T501 content. Figure 7 The image shows the 1H NMR spectrum of transformer oil 3 in this embodiment of the application for testing the T501 content. Analysis of the 1H NMR spectrum yielded the integral area Ia of the phenolic hydroxyl group of T501. Substituting this area into the formula, the T501 content (Pa) in the operating transformer oil sample was calculated. The calculation results are shown in Table 5.

[0061] Table 5. Test data of T501 content in transported oil samples.

[0062] As shown in Table 5, all sample testing proceeded smoothly, with stable baselines in the 1H NMR spectra, good separation between the target peak and the internal standard peak, and accurate integration, fully verifying the reliability of the detection method proposed in this application. This method not only requires only a small amount of oil sample for analysis and detection, significantly reducing the required sample size, but also enables rapid detection and efficient, accurate quantification of the T501 additive content, providing convenient and reliable technical support for the quality monitoring of transformer oil in operation.

[0063] In summary, the detection method for phenolic antioxidant additives in insulating oil provided in this application achieves direct, rapid, and high-precision quantification of target additives in complex insulating oil matrices by carefully selecting internal standards and employing the 1H NMR internal standard method. This method completely eliminates the reliance on blank oil, high-purity standards, and toxic reagents, and possesses outstanding advantages such as strong anti-interference ability, high sensitivity, simple operation, and environmental friendliness, providing a powerful and innovative tool for monitoring the oil condition of power equipment.

[0064] The various embodiments in this specification are described in a progressive manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0065] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit this application. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of this application.

Claims

1. A method for detecting phenolic antioxidant additives in insulating oil, characterized in that, Quantitative detection was performed using the internal standard method of proton nuclear magnetic resonance spectroscopy. The detection method includes: After mixing the oil sample to be tested, the internal standard, and the deuterated reagent, the 1H NMR spectrum was determined to obtain the 1H NMR spectrum. The characteristic quantitative peaks of the internal standard and the characteristic quantitative peaks of phenolic antioxidant additives in the insulating oil are integrated in the 1H NMR spectrum to obtain the integrated area of ​​the characteristic quantitative peaks of the internal standard and the phenolic antioxidant additives in the insulating oil. The content of phenolic antioxidant additives in the insulating oil is determined by combining the mass of the oil sample to be tested and the internal standard, the purity of the internal standard, the molecular weight of the internal standard and the additives, and the number of protons in the quantitative peak. The chemical shifts of the characteristic quantitative peaks of the internal standard and the characteristic quantitative peaks of the phenolic antioxidant additives in the insulating oil are both located in the mid-intensity region of the 1H NMR spectrum, and the chemical shifts do not overlap.

2. The method for detecting phenolic antioxidant additives in insulating oil according to claim 1, characterized in that, The content of phenolic antioxidant additives in the insulating oil is linearly related to the integral area of ​​the quantitative NMR characteristic peak of the phenolic antioxidant additives.

3. The method for detecting phenolic antioxidant additives in insulating oil according to claim 2, characterized in that, The linear relationship is expressed as follows: ; Wherein, Pa: the content of phenolic antioxidant additives in the oil sample to be tested; Ps: Purity of the internal standard; Ia: The integral area of ​​the NMR quantitative peak of phenolic antioxidant additives in the oil sample to be tested; Is: Integral area of ​​the NMR quantitative peak of the internal standard; Na: The number of protons in the quantitative peak of NMR in the oil sample to be tested; Ns: The number of protons in the NMR quantitative peak of the internal standard; Ma: Molecular weight of phenolic antioxidant additives in the oil sample to be tested; Ms: Molecular weight of the internal standard; Wa: Mass of the oil sample to be tested; Ws: The mass of the internal standard.

4. The method for detecting phenolic antioxidant additives in insulating oil according to claim 1, characterized in that, The internal standard is an aromatic compound that does not contain active hydrogen and whose chemical shift does not overlap with the characteristic hydrogen nucleus chemical shift of phenolic antioxidant additives. The internal standard is at least one of methyl 3,5-dinitrobenzoate, dimethyl terephthalate, 1,3,5-trimethoxybenzene, 3,4,5-trichloropyridine, benzyl benzoate, and pyrazine.

5. The method for detecting phenolic antioxidant additives in insulating oil according to claim 1, characterized in that, The phenolic antioxidant additive includes at least one of 2,6-di-tert-butyl-p-cresol, 2,6-di-tert-butylphenol, and 4,4-methylenebis-(2,6-di-tert-butylphenol); When testing multiple phenolic antioxidant additives, the characteristic hydrogen nucleus peaks of each phenolic antioxidant additive are selected for integration, and then the content of each phenolic antioxidant additive is calculated.

6. The method for detecting phenolic antioxidant additives in insulating oil according to claim 1, characterized in that, The deuterated reagent is one or more of deuterated chloroform, deuterated tetrahydrofuran, deuterated methanol, deuterated benzene, and deuterated dimethyl sulfoxide.

7. The method for detecting phenolic antioxidant additives in insulating oil according to claim 1, characterized in that, The mass ratio of the oil sample to be tested to the internal standard is (1000-1):(1-5); And / or, the detection amount of the oil sample to be tested is 20-50 mg.

8. The method for detecting phenolic antioxidant additives in insulating oil according to claim 1, characterized in that, The conditions for the proton nuclear magnetic resonance (NMR) spectroscopy measurement are as follows: magnetic field strength of 400-600 MHz, pulse width of 4-5 μs, spectral width of 19-20 ppm, pre-scan delay of 6-7 μs, and number of scans of 8-256.

9. The method for detecting phenolic antioxidant additives in insulating oil according to claim 8, characterized in that, The conditions for the proton nuclear magnetic resonance (NMR) spectroscopy measurement were as follows: pulse width of 4.25 μs, spectral width of 19.8352 ppm, pre-scan delay of 6.5 μs, and 64 scans.

10. The method for detecting phenolic antioxidant additives in insulating oil according to claim 1, characterized in that, The detection limit for the phenolic antioxidant additive content is ≤ 0.004%, the quantitation limit is ≤ 0.013%, and the total time for a single test is ≤ 5 minutes.

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