Method for determining anionic surfactants by solid phase extraction-spectrophotometry

Abstract

This application provides a solid-phase extraction-spectrophotometric method for determining anionic surfactants, belonging to the field of environmental monitoring. The method involves filtering a water sample to be tested through a filter membrane and adjusting the pH to 2-3 to obtain a pretreated water sample. The pretreated water sample is then subjected to solid-phase extraction pre-enrichment using a cation exchange resin column, followed by elution with an eluent, and the eluent is collected. The eluent is mixed with a methylene blue solution and placed in a microwave device for microwave heating, causing the anionic surfactant to undergo a complexation reaction with the methylene blue, forming a blue complex. The blue complex is extracted with an organic extractant, and the organic phase is collected. After washing, the absorbance of the organic phase is measured spectrophotometrically, and the content of anionic surfactant in the water sample is calculated. This application significantly improves the speed, sensitivity, and environmental friendliness of anionic surfactant determination through the synergistic effect of solid-phase extraction pre-enrichment and microwave-assisted methylene blue complexation reaction.

Description

Technical Field

[0001] This invention relates to the field of environmental monitoring technology, specifically to a method for determining anionic surfactants using solid-phase extraction-spectrophotometry. Background Technology

[0002] Anionic surfactants are common pollutants in surface water, groundwater, and wastewater, and relevant water quality standards impose strict limits on their discharge concentrations. Currently, the most widely used method for determining anionic surfactants in water is the "Determination of Anionic Surfactants in Water - Methylene Blue Spectrophotometric Method" (GB / T 7494-1987). The principle of this method is as follows: under acidic conditions, the anionic surfactant reacts with methylene blue to form a blue ion-pair complex. After chloroform extraction, the absorbance of the organic phase (chloroform layer) is measured at a wavelength of 652 nm using a spectrophotometer. The absorbance is directly proportional to the concentration of the anionic surfactant. However, this method has the following shortcomings: (1) It is cumbersome to operate, the steps are complicated, the total detection time is up to 2 hours, manual operation is prone to human error, and the repeatability is poor; (2) The amount of toxic solvent used is large, which poses a potential hazard to the health of the analysts and the environment; (3) The extraction efficiency is unstable, the sensitivity is insufficient for low concentration samples (<0.05 mg / L), and the lowest detection concentration is 0.05 mg / L; (4) Interfering substances such as sulfides and organic sulfates are difficult to completely remove, which affects the accuracy.

[0003] To address the aforementioned shortcomings, existing improvement solutions include replacing the extractant, simplifying the number of extractions, and introducing solid-phase extraction pre-enrichment or flow injection methods. However, these improvements are mostly limited to optimizing a single step and fail to systematically solve core problems such as cumbersome operation, large solvent consumption, difficulty in removing interference, and low sensitivity.

[0004] In view of this, it is necessary to design a solid-phase extraction-spectrophotometric method for the determination of anionic surfactants to solve the above problems. Summary of the Invention

[0005] In view of the technical problems existing in the background art, this application provides a method for determining anionic surfactants by solid phase extraction-spectrophotometry, which aims to solve the technical problems of existing detection methods such as cumbersome operation procedures, large solvent consumption, difficulty in removing interference and low sensitivity.

[0006] This application provides a method for determining anionic surfactants using solid-phase extraction-spectrophotometry, comprising the following steps: S1. Filter the water sample to be tested through a filter membrane and adjust the pH value to 2-3 to obtain a pretreated water sample; S2. The pretreated water sample is pre-enriched by solid-phase extraction through a cation exchange resin column, followed by elution with an eluent, and the eluent is collected. S3. The eluent is mixed with the methylene blue solution and placed in a microwave device for microwave heating, so that the anionic surfactant and methylene blue undergo a complexation reaction to form a blue complex. S4. Extract the blue complex from the reaction solution obtained in step S3 using an organic extractant, and collect the organic phase; S5. After the organic phase is washed, the absorbance is measured by spectrophotometry, and the content of anionic surfactant in the water sample to be tested is calculated.

[0007] As a further improvement of this application, in step S2, the cation exchange resin column is a 732 type cation exchange resin column, the resin skeleton is a styrene-divinylbenzene copolymer, and the functional group is a sulfonic acid group.

[0008] As a further improvement to this application, in step S2, the solid-phase extraction pre-enrichment specifically includes the following steps: S21. Wet and activate the cation exchange resin column with methanol; S22. Pass the pretreated water sample through the activated cation exchange resin column at a flow rate of 10~30 mL / min; S23. After the sample loading is completed, wash the cation exchange resin column with deionized water; S24. Elute the target anionic surfactant with an eluent.

[0009] As a further improvement of this application, in step S2, the eluent is a mixed solution of ammonia and methanol, and the volume ratio of ammonia to methanol is 1:(1~3).

[0010] As a further improvement of this application, in step S3, the power of microwave heating is 250~350W, the heating time is 3~10 minutes, and the reaction temperature is controlled below 60℃.

[0011] As a further improvement of this application, in step S3, the concentration of the methylene blue solution is 40~60 mg / L.

[0012] As a further improvement of this application, in step S4, the organic extractant is chloroform or dichloromethane, and the amount used is 5~15mL.

[0013] As a further improvement of this application, in step S5, the wavelength for the spectrophotometric determination is 652 nm.

[0014] As a further improvement to this application, the anionic surfactant includes one or more of sodium linear alkylbenzene sulfonate, sodium alkyl sulfonate, and sodium fatty alcohol sulfate.

[0015] As a further improvement to this application, the detection limit of the method is not higher than 0.03 mg / L.

[0016] The beneficial effects of this application are as follows: This application provides a method for determining anionic surfactants using solid-phase extraction-spectrophotometry. The method involves filtering the water sample to be tested through a filter membrane and adjusting the pH to 2-3 to obtain a pretreated water sample. The pretreated water sample is then subjected to solid-phase extraction pre-enrichment using a cation exchange resin column, followed by elution with an eluent, and the eluent is collected. The eluent is mixed with a methylene blue solution and heated in a microwave device to induce a complexation reaction between the anionic surfactant and the methylene blue, forming a blue complex. The blue complex obtained in step S3 is extracted with an organic extractant, and the organic phase is collected. After washing, the absorbance of the organic phase is measured spectrophotometrically, and the content of anionic surfactant in the water sample is calculated. This application employs solid-phase extraction for pre-enrichment, significantly reducing the detection limit. It utilizes 732 resin with strong acidic sulfonic acid groups (-SO3H), which selectively adsorb anionic surfactants in water samples. It exhibits weak adsorption capacity for non-target anions (such as sulfate and nitrate), effectively eliminating positive interference. Its low adsorption capacity for sulfides avoids negative interference. The application employs a four-step process of activation, loading, washing, and elution, simplifying the operation and replacing the multiple extractions and backwashings of traditional methods, resulting in rapid and efficient operation. It also reduces chloroform usage, minimizing environmental damage and protecting human health.

[0017] Type 732 cation exchange resin is commonly used for water softening in water treatment. It is widely believed in the industry that its exchange kinetics for macromolecular surfactants are slow and inefficient. This application does not utilize the ion exchange softening function of Type 732 cation exchange resin, but rather leverages the synergistic effect of its hydrophobic framework's physical adsorption and ion exchange. At high flow rates, the resin primarily retains the target analyte through surface adsorption, achieving highly efficient enrichment (recovery rate >90%) of trace amounts of anionic surfactants. This application overcomes traditional technical biases in the field, demonstrating the high efficiency of Type 732 cation exchange resin as a SPE (solid-phase extraction) adsorbent in combination with specific eluents (ammonia / methanol).

[0018] This application utilizes the polarization effect of microwaves to force methylene blue cations to collide and combine rapidly with surfactant anions. Microwave heating is uniform and rapid, and complexation can be completed within 5 minutes, while avoiding local overheating and decomposition that may be caused by traditional heating.

[0019] In this application, SPE pre-concentration not only concentrated the target analyte, but more importantly, effectively removed inorganic interfering substances from the water sample, creating clean and safe conditions for the subsequent microwave heating reaction. Meanwhile, microwave technology compensates for the low efficiency of the colorimetric reaction after SPE concentration, significantly shortening the complexation time. The synergistic effect of these two technologies reduces the detection time to 40 minutes and the detection limit to a low 0.022 mg / L.

[0020] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Detailed Implementation

[0021] The embodiments of the technical solution of this application will be described in detail below. The following embodiments are only used to illustrate the technical solution of this application more clearly, and are therefore only examples, and should not be used to limit the scope of protection of this application.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion.

[0023] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0024] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0025] Anionic surfactants are common pollutants in water bodies, and my country's water quality standards impose strict restrictions on their discharge. Currently, the main method for detection is the "Determination of Anionic Surfactants in Water - Methylene Blue Spectrophotometric Method" (GB / T 7494-1987). The principle is that methylene blue reacts with the anionic surfactant under acidic conditions to form a blue complex, which is then extracted with chloroform and quantified by colorimetric analysis. However, this method has significant drawbacks: it is cumbersome, with single-sample testing taking up to 2 hours; the total chloroform consumption exceeds 50 mL, posing a health hazard and subject to strict control; the minimum detectable concentration is only 0.05 mg / L, making it difficult to meet the needs of low-concentration water samples; interfering substances such as sulfides and organic sulfates are difficult to remove; and color development at room temperature requires 30 minutes of shaking, resulting in low efficiency. Existing improvements are mostly limited to optimizing single steps and have failed to systematically address the above problems.

[0026] To address the technical problems of existing detection methods, such as cumbersome operation procedures, large solvent consumption, difficulty in removing interference, and low sensitivity, this application provides a solid-phase extraction-spectrophotometric method for the determination of anionic surfactants. In this method, the target anionic surfactant is selectively adsorbed through a cation exchange resin solid-phase extraction column, and microwave heating is used to accelerate the complexation reaction between methylene blue and the target analyte. Combined with a single chloroform extraction, this method solves many of the pain points of traditional methods while maintaining compatibility with spectrophotometric measurement systems.

[0027] This application provides a method for determining anionic surfactants by solid-phase extraction-spectrophotometry, comprising the following steps: S1. Filter the water sample to be tested through a filter membrane and adjust the pH value to 2-3 to obtain a pretreated water sample; S2. The pretreated water sample was pre-enriched by solid-phase extraction through a cation exchange resin column, followed by elution with eluent, and the eluent was collected. Solid-phase extraction pre-enrichment specifically includes the following steps: S21. Wet and activate the cation exchange resin column with methanol; S22. Pass the pretreated water sample through the activated cation exchange resin column at a flow rate of 10~30mL / min; S23. After the sample loading is completed, wash the cation exchange resin column with deionized water; S24. Elute the target anionic surfactant with an eluent; S3. Mix the eluent with the methylene blue solution and place it in a microwave device for microwave heating, so that the anionic surfactant and methylene blue undergo a complexation reaction to form a blue complex. S4. Extract the blue complex from the reaction solution obtained in step S3 using an organic extractant, and collect the organic phase; S5. After washing, the absorbance of the organic phase is determined by spectrophotometry, and the content of anionic surfactant in the water sample is calculated.

[0028] In the technical solution of this application embodiment, the synergistic effect of the hydrophobic framework and sulfonic acid groups of the cation exchange resin is utilized to selectively enrich anionic surfactants in water samples under acidic conditions through a combination of physical adsorption and ion exchange, while simultaneously removing inorganic interfering substances. Subsequently, in a microwave field, the polarization effect of microwaves forces the methylene blue cations and anionic surfactants to collide and combine rapidly, completing the complexation reaction in a short time to generate a blue ion-pair complex. After a single extraction of this complex with chloroform or dichloromethane, the absorbance is measured at a wavelength of 652 nm, and quantification is performed according to a standard curve. This method achieves rapid, sensitive, and low-toxicity detection through the synergistic effect of solid-phase extraction and microwave assistance.

[0029] Furthermore, in some embodiments, in step S2, the cation exchange resin column is a 732 type cation exchange resin column, the resin skeleton is a styrene-divinylbenzene copolymer, and the functional group is a sulfonic acid group.

[0030] In the technical solution of this application embodiment, the 732 type cation exchange resin utilizes the synergistic effect of physical adsorption and ion exchange of its hydrophobic framework to achieve efficient enrichment of target substances and removal of interfering substances, providing a clean environment for subsequent microwave-assisted colorimetric reactions. Specifically, in solid-phase extraction mode, the adsorption mechanism of the resin for anionic surfactants mainly includes the following two aspects: 1) Hydrophobic adsorption: Strong hydrophobic interactions can be generated between anionic surfactant molecules and the styrene-divinylbenzene framework of the 732 type resin. This interaction allows surfactant molecules to be effectively captured on the surface of the resin framework, rather than relying solely on the electrostatic attraction between sulfonic acid groups and the polar heads of surfactants; 2) Surface adsorption: Due to the high flow rate, anionic surfactant molecules, due to their large molecular weight and strong hydrophobicity, are difficult to diffuse into the deep microporous structure of resin particles in a short time. Instead, they are mainly adsorbed on the outer surface and surface network structure of resin particles. This surface adsorption process has a fast mass transfer rate and can adapt to high flow rate loading conditions, thereby significantly shortening the loading time while ensuring high enrichment efficiency.

[0031] Furthermore, in some embodiments, in step S2, the eluent is a mixed solution of ammonia and methanol, with a volume ratio of ammonia to methanol of 1:(1~3).

[0032] In the technical solution of this application embodiment, the ammonia water in the mixed eluent provides an alkaline environment, which can neutralize the electrostatic attraction between the resin sulfonic acid groups and the anionic surfactant. At the same time, the ammonia molecules compete with the anionic surfactant for the adsorption sites of the resin. Methanol, as an organic solvent, can effectively destroy the van der Waals forces and hydrophobic interactions between the hydrophobic framework of the resin and the hydrophobic tail chain of the surfactant. The synergistic effect of the two enables the enriched anionic surfactant to be efficiently desorbed from the 732 type resin and transferred into the eluent, thereby achieving a high recovery rate elution.

[0033] Furthermore, in some embodiments, in step S3, the microwave heating power is 250~350W, the heating time is 3~10 minutes, and the reaction temperature is controlled below 60°C.

[0034] In the technical solution of this application embodiment, the polarization effect of microwaves is used to make polar molecules (methylene blue cation and anionic surfactant) rotate and collide at high speed, which significantly improves the complexation reaction rate and shortens the traditional room temperature oscillation time of 30 minutes to about 5 minutes. At the same time, the reaction temperature is maintained below 60°C by power control and ice bath cooling, which not only ensures the rapid progress of the colorimetric reaction, but also avoids the decomposition of blue complex or side reactions caused by high temperature, thus ensuring the accuracy and reproducibility of the measurement.

[0035] Furthermore, in some embodiments, in step S3, the concentration of the methylene blue solution is 40~60 mg / L.

[0036] In the technical solution of this application embodiment, a suitable methylene blue concentration range ensures that the blue complex is fully and stably generated, while maintaining a low reagent blank, thereby obtaining good linear response and accuracy; too low a concentration will lead to incomplete complexation reaction and affect the sensitivity of the determination; too high a concentration may increase background absorption due to excessive methylene blue residue in the organic phase, or even cause co-extraction of impurities.

[0037] Furthermore, in some embodiments, in step S4, the organic extractant is chloroform or dichloromethane, and the amount used is 5-15 mL.

[0038] In the technical solution of this application embodiment, chloroform or dichloromethane is used as an organic extractant. Based on its high solubility and selectivity for methylene blue-anionic surfactant ion-pair complexes, efficient separation can be achieved in a single extraction. The appropriate amount of extractant helps to ensure sufficient contact between the extractant and the reaction solution and complete transfer of the complex, which is significantly reduced compared to traditional methods. At the same time, it avoids the problems of difficult phase separation and decreased recovery rate due to insufficient amount. While reducing organic solvent consumption and environmental hazards, it maintains stable extraction efficiency.

[0039] Furthermore, in some embodiments, in step S5, the spectrophotometric measurement wavelength is 652 nm.

[0040] In the technical solution of this application embodiment, spectrophotometric determination at this characteristic wavelength can obtain the highest molar absorptivity and measurement sensitivity. Simultaneously, the background absorption of other colored substances or residual extractants that may coexist in the water sample is weak at this wavelength, effectively reducing spectral interference and ensuring a good linear relationship between absorbance and anionic surfactant concentration, thereby improving the accuracy and precision of quantitative analysis. Specifically, using a 1cm cuvette, the absorbance of the organic phase is measured at a wavelength of 652nm, and the content of anionic surfactant is quantitatively analyzed using a standard curve comparison method; the formula for calculating the content of anionic surfactant is: Where: C - the concentration of anionic surfactant in the water sample, mg / L; C1 - The concentration of anionic surfactant in the eluent, in mg / L, can be determined from the standard curve; C0 - The concentration of anionic surfactant in the blank sample, in mg / L, can be determined from the standard curve; V - Volume at constant volume, mL; V0 - Sampling volume, mL; V1 - Dilution sample volume, mL; V2 - Dilution volume, mL.

[0041] Furthermore, in some embodiments, the anionic surfactant includes one or more of sodium linear alkylbenzene sulfonate, sodium alkyl sulfonate, and sodium fatty alcohol sulfate.

[0042] In the technical solutions of this application, anionic surfactants such as linear alkylbenzene sulfonate sodium, alkyl sulfonate sodium, and fatty alcohol sulfate sodium all have hydrophilic anionic polar heads (such as sulfonic acid groups and sulfate groups) and hydrophobic long-chain alkyl tail chains. Under acidic conditions, their polar heads are negatively charged and can form blue ion-pair complexes with methylene blue cations through electrostatic interactions; simultaneously, their hydrophobic tail chains can undergo hydrophobic adsorption with the styrene-divinylbenzene skeleton of type 732 resin, thereby achieving selective enrichment. This method is applicable to the quantitative analysis of this type of typical anionic surfactant.

[0043] Furthermore, in some embodiments, the detection limit of this method is not higher than 0.03 mg / L.

[0044] In the technical solution of this application embodiment, the target substance in the water sample is concentrated by solid phase extraction pre-enrichment, which significantly improves the detectability of low concentration samples; at the same time, microwave-assisted accelerated complexation reaction ensures sufficient color development, and combined with high-sensitivity detection at the characteristic wavelength of 652 nm, the detection limit of the method is reduced to below 0.03 mg / L, which is far superior to the 0.05 mg / L of the traditional national standard method, meeting the monitoring needs of low concentration environmental water samples such as surface water and groundwater.

[0045] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0046] Example 1 This embodiment provides a method for determining anionic surfactants using solid-phase extraction-spectrophotometry, including the following steps: S1. Take 100 mL of groundwater sample from a monitoring well in a mine, filter it through a 0.45 μm filter membrane to remove suspended solids, and adjust the pH value to 2.5 with dilute sulfuric acid to obtain a pretreated water sample; S2. Prepare a 732 type cation exchange resin column (inner diameter 1cm, column length 10cm). Wet and activate the cation exchange resin column with 5 mL of methanol; The pretreated water sample was passed through the activated cation exchange resin column at a flow rate of 20 mL / min, so that the anionic surfactant was adsorbed and enriched on the resin. After loading the sample, wash the cation exchange resin column with 5 mL of deionized water to remove most of the interfering substances; The target anionic surfactant was eluted with 2 mL of concentrated ammonia / methanol (1:1, v / v) mixture, and the eluent was collected. S3. Transfer the eluent to a microwave reaction tube, add 1 mL of 50 mg / L methylene blue solution, use a microwave extractor, heat at 300 W microwave extraction power for 5 minutes, keep the temperature below 60℃, cool in an ice bath, and the anionic surfactant will undergo a complexation reaction with the methylene blue to form a blue complex. S4. Extract the blue complex from the reaction solution with 10 mL of chloroform, shake for 1 minute, allow to stand for separation, collect the chloroform layer, and use the chloroform layer for determination after washing; S5. Using chloroform as a blank, the absorbance of the chloroform layer was measured at a wavelength of 652 nm using a 1 cm cuvette. The content of anionic surfactant was quantitatively analyzed by standard curve comparison. A standard series of anionic surfactants with concentrations of 0, 0.100 mg / L, 0.250 mg / L, 0.500 mg / L, 1.000 mg / L, 2.500 mg / L, and 4.000 mg / L were prepared. The above measurement procedure was repeated for each standard point. A standard curve was plotted with concentration (c, mg / L) on the x-axis and absorbance (A) on the y-axis. The regression equation was A = 0.229c - 0.003, and the correlation coefficient R0 was [missing value]. 2 =0.9999; The measured value was found to be 0.518 mg / L based on the standard curve, and the blank value was 0.002 mg / L. The content of the anionic surfactant was calculated as C = (0.518 - 0.002). 10 / 100 = 0.052 mg / L.

[0047] Example 2 This embodiment provides a solid-phase extraction-spectrophotometric method for determining anionic surfactants. Compared to Example 1, the only difference is that the water sample is surface water from a river; other experimental parameters and conditions are basically the same as in Example 1 and will not be repeated here. Based on the standard curve, the measured value was 1.217 mg / L, and the blank value was 0.002 mg / L. The calculated content of the anionic surfactant was C = (1.217 - 0.002). 10 / 100 = 0.122 mg / L.

[0048] Example 3 This embodiment provides a solid-phase extraction-spectrophotometric method for determining anionic surfactants. Compared to Example 1, the only difference is that the water sample is obtained by mixing 5.00 mL of laboratory wastewater containing anionic surfactants with 95 mL of deionized water. Other experimental parameters and conditions are basically the same as in Example 1 and will not be repeated here. Based on the standard curve, the measured value was 3.518 mg / L, and the blank value was 0.002 mg / L. The calculated content of the anionic surfactant was C = (3.518 - 0.002). 10 / 5 = 7.032 mg / L.

[0049] Example 4 This embodiment provides a solid-phase extraction-spectrophotometric method for determining anionic surfactants. Compared to Example 1, the only difference is that the standard sample GSB07-1197-2000 (204435) is used for determination, with a standard value of 3.20 ± 0.21 mg / L. The test water sample is obtained by mixing 10.00 mL of the standard sample with 90 mL of deionized water. Other experimental parameters and conditions are basically the same as in Example 1 and will not be repeated here. Based on the standard curve, the measured value is 3.218 mg / L, and the blank value is 0.002 mg / L. The content of the anionic surfactant is calculated as C = (3.218 - 0.002). 10 / 10 = 3.216 mg / L.

[0050] Example 5 This embodiment provides a solid-phase extraction-spectrophotometric method for determining anionic surfactants. Compared to Example 1, the only difference is that the standard sample GSB07-1197-2000 (204430) is used for determination, with a standard value of 1.54 ± 0.12 mg / L. The test water sample is obtained by mixing 10.00 mL of the standard sample with 90 mL of deionized water. Other experimental parameters and conditions are basically the same as in Example 1 and will not be repeated here. Based on the standard curve, the measured value is 1.548 mg / L, and the blank value is 0.002 mg / L. The content of the anionic surfactant is calculated as C = (1.548 - 0.002). 10 / 10 = 1.546 mg / L.

[0051] Example 6 This embodiment provides a solid-phase extraction-spectrophotometric method for determining anionic surfactants. Compared to Example 1, the only difference is that the standard sample GSB07-1197-2000 (204430) is spiked for determination, with a spiked value of 1.54 ± 0.12 mg / L and a spiked amount of 1.50 mg / L. The test water sample is prepared by adding 10.00 mL of the standard sample to 10.00 mL of a standard solution with a concentration of 1.50 mg / L, followed by the addition of 80 mL of deionized water. Other experimental parameters and conditions are basically the same as in Example 1 and will not be repeated here. Based on the standard curve, the measured value is 3.048 mg / L, and the blank value is 0.002 mg / L. The content of the anionic surfactant is calculated as C = (3.048 - 0.002). 10 / 10 = 3.046 mg / L, and the calculated recovery rate is 99.06%.

[0052] Example 7 This embodiment provides a solid-phase extraction-spectrophotometric method for determining anionic surfactants. Compared to Example 1, the only difference is that a standard sample containing 1.000 mg / L of anionic surfactant is used for the determination, and the test water sample is 10.00 mL of this solution. Other experimental parameters and conditions are basically the same as in Example 1 and will not be repeated here. The measured value was found to be 0.995 mg / L according to the standard curve, and the blank value was 0.002 mg / L. The calculated content of the anionic surfactant is C = (0.995 - 0.002). 10 / 10 = 0.993 mg / L.

[0053] Example 8 This embodiment provides a solid-phase extraction-spectrophotometric method for determining anionic surfactants. Compared to Example 1, the only difference is that a standard sample containing 2.000 mg / L of anionic surfactant is used for the determination, and the test water sample is 10.00 mL of this solution. Other experimental parameters and conditions are basically the same as in Example 1 and will not be repeated here. The measured value was found to be 1.985 mg / L according to the standard curve, and the blank value was 0.002 mg / L. The calculated anionic surfactant content C = (1.985 - 0.002) 10 / 10 = 1.983 mg / L.

[0054] The method for determining the limit of detection (LOD) was as follows: Eleven parallel determinations were performed on the same sample. The standard deviation was calculated according to the "Technical Guidelines for the Development of Environmental Monitoring Analytical Methods Standards (HJ 168-2020)". Three times the standard deviation was used as the method LOD, resulting in a LOD of 0.022 mg / L. This result is significantly lower than the LOD of GB / T 7494-1987 (0.05 mg / L).

[0055] Comparative Example 1 Comparative Example 1 provides a solid-phase extraction-spectrophotometric method for determining anionic surfactants. The only difference from Example 1 is that solid-phase extraction pre-concentration was not used; instead, the water sample was directly subjected to a methylene blue colorimetric reaction. Other experimental parameters and conditions are basically the same as in Example 1 and will not be repeated here. A standard curve was established following the same operating steps as in the comparative example, yielding the regression equation A = 0.2168c + 0.001, and the correlation coefficient R0. 2 =0.9985, the measured value is 0.606 mg / L according to the standard curve, and the blank value is 0.006 mg / L. The content of anionic surfactant is calculated as C = (0.606 - 0.006). 10 / 100 = 0.060 mg / L.

[0056] Comparative Example 2 Comparative Example 2 provides a solid-phase extraction-spectrophotometric method for determining anionic surfactants. Compared to Example 1, the only difference is that room temperature shaking for 30 minutes, a traditional method, is used instead of microwave heating. Other experimental parameters and conditions are essentially the same as in Example 1 and will not be repeated here. A standard curve was established following the same operating steps as in the comparative example, yielding the regression equation A = 0.2266c + 0.0025, and the correlation coefficient R0. 2 =0.9994, the measured value is 0.512 mg / L according to the standard curve, and the blank value is 0.004 mg / L. The content of anionic surfactant is calculated as C = (0.512 - 0.004). 10 / 100 = 0.051 mg / L.

[0057] Comparative Example 3 Comparative Example 3 provides a solid-phase extraction-spectrophotometric method for determining anionic surfactants. Compared to Example 1, the only difference is the use of a C18 solid-phase extraction column (1 cm inner diameter, 10 cm length) instead of a 732 type cation exchange resin column. Before use, the C18 column was activated and equilibrated with 5 mL of methanol and 5 mL of deionized water. Other experimental parameters and conditions were essentially the same as in Example 1 and will not be repeated here. A standard curve was established following the same operating steps as in the comparative example. A linear fit was performed with concentration (c) as the abscissa and absorbance (A) as the ordinate, and the correlation coefficient R0 was calculated.2 The value of 0.5173 indicates that the standard curve has no obvious linear relationship, and the C18 column does not effectively retain anionic surfactants under these conditions, making it unsuitable for quantitative analysis.

[0058] Comparative Example 4 Comparative Example 4 provides a method for determining anionic surfactants using solid-phase extraction-spectrophotometry. The same water sample as in Example 1 was used, and the procedure was performed according to the standard method of GB / T 7494-1987: 100 mL of water sample was placed in a separatory funnel. Using phenolphthalein as an indicator, 1 mol / L sodium hydroxide solution was added until the aqueous solution turned pink. Then, 0.5 mol / L sulfuric acid was added dropwise until the pink color just disappeared. 25 mL of methylene blue solution was added, and the mixture was shaken well. Then, 10 mL of chloroform was added, and the mixture was shaken vigorously for 30 seconds. The chloroform layer was placed in a second separatory funnel pre-filled with 50 mL of washing solution. The outlet tube of the first separatory funnel was rinsed with a few drops of chloroform. The extraction was repeated three times. Use 10 ml of chloroform each time; combine all the chloroform into the second separatory funnel, shake vigorously for 30 seconds, and let stand to separate the layers; pass the chloroform layer through glass wool or degreased cotton and put it into a 50 ml volumetric flask, then extract the washing solution twice with chloroform (5 ml each time), and add this chloroform layer into the volumetric flask as well, and add chloroform (3.3) to the mark; using chloroform as a blank, use a 1 cm cuvette to measure the absorbance of the chloroform layer at a wavelength of 652 nm, and quantitatively analyze the content of anionic surfactant by comparing the standard curve; A standard curve was established following the same procedures as the comparative example, yielding the regression equation A = 0.2276c - 0.002 and the correlation coefficient R0. 2 =0.9999, the measured value is 0.520 mg / L according to the standard curve, and the blank value is 0.006 mg / L. The content of anionic surfactant is calculated as C = (0.520 - 0.006). 10 / 100 = 0.051 mg / L.

[0059] Table 1. Test results of examples and comparative examples. As shown in Table 1, the method of this application can accurately determine anionic surfactants in water samples with different matrices, and the detection results are in good agreement with the standard values ​​or spiked values. The detection result of Comparative Example 1 without SPE is 0.060 mg / L, which is higher than 0.052 mg / L after SPE treatment, indicating that the SPE step can effectively remove interfering substances in the water sample and avoid positive interference. The detection time of Comparative Example 2 is significantly extended when room temperature oscillation is used instead of microwave. When Comparative Example 3 uses a C18 column instead of 732 resin, the target analyte is almost not retained. The detection time of the traditional national standard method in Comparative Example 4 is as long as 90 minutes and the solvent consumption is large, indicating that the present invention has significant advantages in terms of speed, sensitivity and environmental friendliness.

[0060] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for determining anionic surfactants using solid-phase extraction-spectrophotometry, characterized in that, Includes the following steps: S1. Filter the water sample to be tested through a filter membrane and adjust the pH value to 2-3 to obtain a pretreated water sample; S2. The pretreated water sample is pre-enriched by solid-phase extraction through a cation exchange resin column, followed by elution with an eluent, and the eluent is collected; the cation exchange resin column is a 732 type cation exchange resin column, the resin skeleton is styrene-divinylbenzene copolymer, and the functional group is sulfonic acid group. S3. The eluent is mixed with the methylene blue solution and placed in a microwave device for microwave heating, so that the anionic surfactant and methylene blue undergo a complexation reaction to form a blue complex. S4. Extract the blue complex from the reaction solution obtained in step S3 using an organic extractant, and collect the organic phase; S5. After the organic phase is washed, the absorbance is measured by spectrophotometry, and the content of anionic surfactant in the water sample to be tested is calculated.

2. The method for determining anionic surfactants by solid-phase extraction-spectrophotometry according to claim 1, characterized in that, In step S2, the solid-phase extraction pre-enrichment specifically includes the following steps: S21. Wet and activate the cation exchange resin column with methanol; S22. Pass the pretreated water sample through the activated cation exchange resin column at a flow rate of 10~30 mL / min; S23. After the sample loading is completed, wash the cation exchange resin column with deionized water; S24. Elute the target anionic surfactant with an eluent.

3. The method for determining anionic surfactants by solid-phase extraction-spectrophotometry according to claim 2, characterized in that, In step S2, the eluent is a mixed solution of ammonia and methanol, with a volume ratio of ammonia to methanol of 1:(1~3).

4. The method for determining anionic surfactants by solid-phase extraction-spectrophotometry according to claim 1, characterized in that, In step S3, the microwave heating power is 250~350W, the heating time is 3~10 minutes, and the reaction temperature is controlled below 60℃.

5. The method for determining anionic surfactants by solid-phase extraction-spectrophotometry according to claim 1, characterized in that, In step S3, the concentration of the methylene blue solution is 40~60 mg / L.

6. The method for determining anionic surfactants by solid-phase extraction-spectrophotometry according to claim 1, characterized in that, In step S4, the organic extractant is chloroform or dichloromethane, and the amount used is 5~15 mL.

7. The method for determining anionic surfactants by solid-phase extraction-spectrophotometry according to claim 1, characterized in that, In step S5, the wavelength for the spectrophotometric measurement is 652 nm.

8. The method for determining anionic surfactants by solid-phase extraction-spectrophotometry according to claim 1, characterized in that, The anionic surfactant includes one or more of linear alkylbenzene sulfonate, alkyl sulfonate, and fatty alcohol sulfate.

9. The method for determining anionic surfactants by solid-phase extraction-spectrophotometry according to claim 1, characterized in that, The detection limit of the method is not higher than 0.03 mg / L.

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