Method for detecting methyl tert-butyl ether content in soil

Abstract

The application belongs to the technical field of environmental monitoring and analytical chemistry, and discloses a detection method for the content of methyl tert-butyl ether in soil. The method uses surface functionalized mesoporous silica adsorbent for solid phase extraction, combines ultrasonic assisted extraction with gas chromatography-mass spectrometry analysis, the adsorbent has a pore size of 2.5-4.0 nm, and a surface grafted with bifunctional ligands containing tertiary amine groups and short chain alkyl groups, and has high selectivity and adsorption capacity for methyl tert-butyl ether; the mass spectrometry detection adopts double ion confirmation and intensity ratio criterion, effectively eliminating the interference of structural analogues. Through the design of special adsorption materials, the optimization of the pretreatment process and the construction of the mass spectrometry confirmation logic, the application realizes high sensitivity, high selectivity and high reproducibility detection of trace methyl tert-butyl ether, and can be widely applied to industrial pollution site investigation, farmland soil safety evaluation, groundwater pollution tracing and emergency monitoring of environmental incidents.

Description

Technical Field

[0001] This invention belongs to the field of environmental monitoring and analytical chemistry technology, and relates to a method for detecting the content of methyl tert-butyl ether in soil. Background Technology

[0002] Methyl tert-butyl ether (MTBE), an oxygenated organic compound widely used in gasoline additives, is highly water-soluble, has low adsorption capacity, and strong environmental migration ability. In cases of oil spills, tank leaks, or improper disposal, it can easily enter the soil environment, thus posing a potential threat to groundwater systems.

[0003] In existing technologies, the detection of methyl tert-butyl ether (MTBE) in soil often employs headspace gas chromatography (HS-GC) or liquid-liquid extraction combined with gas chromatography-mass spectrometry (GC-MS). While liquid-liquid extraction can achieve a certain enrichment effect, it is complex to operate, requires large amounts of organic solvents, and is easily interfered with by coexisting matrices such as fatty acids, phenols, and aromatic hydrocarbons in the soil, leading to peak tailing, false positives, or quantitative deviations. Headspace methods, on the other hand, are limited by the low partition coefficient of MTBE in the soil solid phase, especially in clay or samples with high organic matter content, where volatilization efficiency decreases significantly, making it difficult to meet the detection limits at the μg / kg level for environmental monitoring.

[0004] Traditional pretreatment techniques require efficient enrichment of target analytes to improve detection sensitivity. However, soil matrices are complex and diverse. If the enrichment process lacks sufficient selectivity, the co-extracted impurities will seriously interfere with subsequent chromatographic separation and mass spectrometry identification, thereby reducing qualitative reliability and quantitative repeatability.

[0005] Existing solid-phase extraction materials mostly employ general-purpose adsorbents such as C18 and PS-DVB. While these adsorbents exhibit good retention capabilities for nonpolar compounds, their adsorption capacity for weakly polar small-molecule ethers like methyl tert-butyl ether (MTBE) is limited, and elution conditions are difficult to optimize, often resulting in large fluctuations in recovery rates and poor batch-to-batch reproducibility. The reason for this is that MTBE molecules lack both strongly polar functional groups for specific recognition and UV or fluorescent chromophores, making them easily masked in complex matrices. Traditional detection strategies struggle to simultaneously achieve the dual goals of high recovery rates and low background interference. Summary of the Invention

[0006] This invention provides a method for detecting methyl tert-butyl ether (MTBE) content in soil, aiming to solve the technical problems of insufficient detection sensitivity, weak qualitative reliability, and poor quantitative repeatability caused by complex soil matrix, weak target polarity, poor adsorption selectivity, and low pretreatment efficiency. To achieve the above-mentioned objective, this invention employs a solid-phase extraction-gas chromatography-mass spectrometry (SPIE) technique. By constructing a dedicated adsorbent material with high affinity and selectivity for MTBE, and combining it with an optimized ultrasonic-assisted extraction and gradient elution program, this invention achieves efficient enrichment of trace MTBE in soil samples, effective removal of matrix interference, and accurate quantitative analysis.

[0007] The detection method of this invention includes the following steps: soil sample collection and pretreatment, ultrasound-assisted solvent extraction, solid-phase extraction purification, gas chromatography-mass spectrometry separation and detection, external standard method quantification, and result output. The adsorbent used in the solid-phase extraction step is a surface-functionalized mesoporous silica material with a pore size range of 2.5-4.0 nm and a specific surface area higher than 600 m². 2 The adsorbent has a surface grafted with a bifunctional ligand containing a tertiary amine group and a short-chain alkyl group, with the structural formula -(CH2)3-N(CH3)-C(CH3)3, which is covalently bonded to the silica framework surface. The static adsorption capacity of this adsorbent for methyl tert-butyl ether is higher than 120 μg / g at 25 °C, and it remains structurally stable in an aqueous environment with pH values ​​of 4-9.

[0008] In the soil sample pretreatment stage, the collected soil samples were air-dried naturally, plant residues and stones were removed, the samples were ground through a 2mm sieve, and vacuum-dried at 40℃ to constant weight. The samples were then sealed and stored for later use. In the ultrasonic-assisted extraction step, 5.00g of the pretreated soil sample was weighed and placed in a 50mL polytetrafluoroethylene centrifuge tube. 20mL of a 7:3 volume ratio acetonitrile-water mixed solvent was added, and the sample was extracted for 30min in an ultrasonic cleaner with a frequency of 40kHz and a power of 200W. Subsequently, the sample was centrifuged at 8000 rpm for 10min, and the supernatant was filtered through a 0.22μm polyvinylidene fluoride membrane to obtain the primary extract.

[0009] The solid-phase extraction purification step uses a 60mg solid-phase extraction column filled with the aforementioned special adsorbent. Before use, the column is activated sequentially with 5mL methanol and 5mL ultrapure water; 10mL of the primary extract is loaded at a flow rate of 1.0mL / min; after loading, it is rinsed with 5mL of 5% methanol aqueous solution to remove polar impurities; then, the target analyte, methyl tert-butyl ether, is eluted with 4mL of n-hexane solution containing 30% acetone at a flow rate of 0.8mL / min; the eluent is concentrated to near dryness by nitrogen blowing, then redissolved with 1.0mL of n-hexane, and filtered through a 0.22μm organic filter membrane to obtain the sample solution to be tested.

[0010] Gas chromatography-mass spectrometry (GC-MS) analysis was performed using a triple quadrupole mass spectrometer equipped with an electron impact ionization source. The GC column was a medium-polarity capillary column with a length of 30 m, an inner diameter of 0.25 mm, and a film thickness of 1.4 μm. The stationary phase was 5% phenyl-95% dimethyl polysiloxane. High-purity helium was used as the carrier gas at a flow rate of 1.0 mL / min. The injection port temperature was set to 250 °C, using splitless injection mode with an injection volume of 1 μL. The temperature program was as follows: initial temperature 40 °C, hold for 3 min, then increase to 200 °C at a rate of 10 °C / min, hold for 5 min. Selected reaction monitoring (SRM) mode was used for mass spectrometry detection. The precursor ion was m / z 88, and the daughter ions were m / z 73 and m / z 57, with collision energies of 12 eV and 15 eV, respectively, and residence times of 100 ms for each.

[0011] External standard method quantification employed a 5-point calibration curve with standard solution concentration gradients of 0.5, 1.0, 5.0, 10.0, and 50.0 μg / L. Linear regression was performed on the concentration using the peak area of ​​methyl tert-butyl ether (MTBE), with a correlation coefficient higher than 0.999. The MTBE content in the sample was calculated using the following formula: C = (A − A0) × V × D / (m × R), where C is the MTBE content in the soil (μg / kg), A is the peak area of ​​the target analyte in the sample, A0 is the peak area of ​​the method blank, V is the final volume (mL), D is the dilution factor, m is the soil sample mass (g), and R is the average recovery correction factor, which was determined through spiked recovery experiments, with typical values ​​ranging from 0.92 to 0.98.

[0012] As a preferred embodiment of the present invention, the preparation method of the special adsorbent includes the following steps: dispersing mesoporous silica microspheres with a particle size of 40-60 μm in anhydrous toluene, adding 3-aminopropyltriethoxysilane, refluxing at 80°C for 12 hours, centrifuging and washing to obtain an amino-functionalized support; reacting the support with tert-butyl isocyanate in anhydrous dichloromethane at room temperature for 24 hours with a reaction molar ratio of 1:1.2, and obtaining the target adsorbent after vacuum drying of the product.

[0013] In another preferred embodiment of the present invention, acetonitrile in the ultrasonic extraction solvent can be replaced with acetone, but the proportion of methanol in the solid phase extraction eluent needs to be adjusted to 8% simultaneously to maintain the matrix removal efficiency.

[0014] This invention effectively distinguishes methyl tert-butyl ether from co-eluent structural analogs such as tert-butanol and isobutene by setting up two daughter ion channels in mass spectrometry and employing a time-resolved acquisition strategy. The m / z 73 fragment ion is formed by the loss of a methyl radical from the methyl tert-butyl ether molecular ion, while the m / z 57 fragment ion corresponds to the tert-butyl cation. The simultaneous appearance of both within the retention time window and their stable intensity ratio of 1.8 ± 0.2 constitute the confirmatory criterion. If only a single ion responds or the intensity ratio deviates from this range, it is determined to be an interfering substance and is not included in the quantitative results.

[0015] Compared with the prior art, the beneficial effects of the present invention are: The technical solution of this invention fundamentally solves the contradiction between enrichment efficiency and selectivity in traditional methods through the synergistic effect of the molecular recognition mechanism of a specialized adsorbent and a multidimensional confirmation strategy based on chromatography-mass spectrometry. The tertiary amine groups on the surface of the adsorbent can form directional adsorption with the ether oxygen atoms of methyl tert-butyl ether through weak hydrogen bonding, while the short-chain tert-butyl groups enhance hydrophobic interactions through van der Waals forces. Both synergistically improve the affinity for the target analyte. Simultaneously, the mesoporous structure restricts the entry of large molecular impurities (such as humic acid and fatty acid polymers), achieving size exclusion at the physical level. This mechanism allows methyl tert-butyl ether to be preferentially captured in complex soil matrices, while most interfering substances are effectively eliminated.

[0016] Furthermore, this invention standardizes the ultrasonic extraction and solid-phase extraction processes, avoiding the problems of emulsification in liquid-liquid extraction and volatilization loss in headspace extraction. Attached Figure Description

[0017] Figure 1 This is a flowchart of the detection method of the present invention. Detailed Implementation

[0018] This invention provides a method for detecting methyl tert-butyl ether (MTBE) content in soil. Its core lies in the synergistic design of specialized adsorption materials and an optimized pretreatment-analysis process, achieving highly selective enrichment, efficient purification, and precise quantification of trace MTBE in complex soil matrices. The technical system constructed by this invention, from sample collection, pretreatment, separation detection to data analysis, all revolve around improving method sensitivity, anti-interference capability, and operational reproducibility, forming a complete, standardized, and applicable analytical scheme suitable for grassroots environmental monitoring institutions.

[0019] The technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples, so as to ensure that those skilled in the art can fully understand and implement the present invention.

[0020] Example 1: Soil type: loam; Special adsorbent pore size: 3.2 nm; Ultrasonic extraction solvent: acetonitrile-water (7:3); Solid phase extraction eluent: 5% methanol aqueous solution; Eluent: 30% acetone-n-hexane; Gas chromatography-mass spectrometry: Selected reaction monitoring mode; Preparation process: Soil pretreatment → Ultrasonic-assisted extraction → Solid-phase extraction purification → Concentration and redissolution → Gas chromatography-mass spectrometry detection → External standard method for quantification.

[0021] Example 2: The soil type is sandy soil, and the rest of the formula and process are the same as in Example 1; Preparation process: Same as in Example 1 (soil type adjusted).

[0022] Example 3: Soil type was clay (organic matter content 4%); the extraction solvent was replaced with acetone; the methanol ratio of the leaching solution was adjusted to 8%; the rest of the formulation and process were the same as in Example 1. Preparation process: Same as in Example 1 (soil type and solvent adjusted).

[0023] Example 4: The special adsorbent has a pore size of 2.5 nm, and the rest of the formulation and process are the same as in Example 1; Preparation process: Same as in Example 1 (adsorbent parameters adjusted).

[0024] Example 5: The special adsorbent has a pore size of 4.0 nm, and the rest of the formulation and process are the same as in Example 1; Preparation process: Same as in Example 1 (adsorbent parameters adjusted).

[0025] Example 6: Ultrasonic extraction time 20 min, other formulations and processes are the same as in Example 1; Preparation process: Same as in Example 1 (extraction parameters adjusted).

[0026] Example 7: Ultrasonic extraction time 40 min, other formulations and processes are the same as in Example 1; Preparation process: Same as in Example 1 (extraction parameters adjusted).

[0027] Example 8: The acetone content in the eluent is 25%, and the remaining formulation and process are the same as in Example 1; Preparation process: Same as in Example 1 (elution parameters adjusted).

[0028] Comparative Example 1: The solid-phase extraction adsorbent was C18; the rest of the formulation and process were the same as in Example 1; Preparation process: Soil pretreatment → ultrasonic extraction → C18 solid phase extraction → detection → quantification.

[0029] Comparative Example 2: Headspace-gas chromatography was used; no ultrasonic extraction or solid-phase extraction was performed; all other detection conditions were the same as in Example 1. Preparation process: Soil pretreatment → headspace sampling → gas chromatography detection → quantification.

[0030] Test method: Sensitivity and accuracy testing: The limit of detection, limit of quantitation, and recovery rate were determined by spiking experiments; the relative standard deviation was calculated to assess precision; and a standard curve was established to verify the linear relationship.

[0031] Anti-interference test: Twelve common interfering substances were added to the soil sample, and the peak shape and quantitative accuracy of the target substances were detected; the matrix interference removal effect was evaluated.

[0032] Stability testing: Repeated intraday and interday testing of the same soil sample was conducted to verify the repeatability of the method; the performance consistency of different batches of adsorbent was tested.

[0033] The test data comparisons are shown in Table 1 and Table 2.

[0034] Table 1 Comparison of Limit of Detection, Limit of Quantitation, and Spiked Recovery Test Project Limit of detection (μg / kg) Limit of quantitation (μg / kg) Spike recovery rate (%) Example 1 0.3 1 95 Example 2 0.28 0.95 97 Example 3 0.32 1.05 92 Example 4 0.35 1.1 93 Example 5 0.29 0.98 96 Example 6 0.33 1.08 91 Example 7 0.27 0.92 98 Example 8 0.31 1.02 94 Comparative Example 1 2.5 8 75 Comparative Example 2 3 10 68 Table 2 Comparison of Relative Standard Deviation, Correlation Coefficient, and Anti-interference Performance Test Project Relative standard deviation (%) Correlation coefficient Anti-interference Example 1 4.1 0.9995 excellent Example 2 3.2 0.9996 excellent Example 3 4.8 0.9993 excellent Example 4 4.3 0.9994 excellent Example 5 3.9 0.9997 excellent Example 6 4.5 0.9992 excellent Example 7 3.8 0.9998 excellent Example 8 4.2 0.9994 excellent Comparative Example 1 8.5 0.995 Difference Comparative Example 2 10.2 0.993 Difference Examples 1-8 showed detection limits ≤0.35μg / kg and recoveries ≥91%, which were far superior to the comparative examples. Comparative example 1 showed poor selectivity of the traditional C18 adsorbent, and comparative example 2 showed low headspace enrichment efficiency. These results demonstrate that the combination of a dedicated adsorbent, ultrasonic extraction, and gas chromatography-mass spectrometry is the key to highly sensitive detection.

[0035] Sandy soil showed the best detection performance (Example 2), while clay required adjustment of the solvent and eluent ratio (Example 3); the adsorbent pore size was suitable for all ranges from 2.5 to 4.0 nm, with the intermediate pore size showing the best overall effect; extending the ultrasonic extraction time slightly improved the recovery rate and sensitivity.

[0036] The example has a low detection limit, meeting the needs of trace detection; high recovery rate, good precision, and accurate quantification; strong anti-interference ability, suitable for complex soil matrices; simple operation, low organic solvent consumption, meeting the requirements of green analysis; and adaptable to different soil types, with a wide range of applications.

[0037] Compared to the traditional C18 adsorbent (Comparative Example 1), the detection limit of the example was reduced by 86% and the recovery rate was increased by 27%; compared to headspace-gas chromatography (Comparative Example 2), the detection limit was reduced by 90% and the recovery rate was increased by 38%, solving the industry problems of insufficient sensitivity and poor anti-interference of traditional methods.

[0038] In summary, the method described in this invention, through the synergistic use of a specialized adsorbent and combined technology, can achieve accurate detection of methyl tert-butyl ether in soil with different parameter combinations, and is suitable for scenarios such as environmental monitoring and pollution investigation.

[0039] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for detecting the content of methyl tert-butyl ether in soil, characterized in that, Includes the following steps: (1) The collected soil samples were air-dried naturally, impurities were removed, ground and sieved, and vacuum dried to constant weight; (2) Add acetonitrile-water mixed solvent to the pretreated soil sample, extract by ultrasonication, centrifuge, and filter the supernatant through a filter membrane to obtain the primary extract; (3) A solid-phase extraction column filled with 60 mg of special adsorbent was used. After the column was activated with methanol and water in sequence, the primary extract was loaded onto the column and then washed with 5% methanol aqueous solution. Finally, the target substance was eluted with n-hexane solution containing 30% acetone. The eluent was concentrated by nitrogen blowing and then redissolved with n-hexane to obtain the sample solution to be tested. (4) A triple quadrupole mass spectrometer coupled with gas chromatography was used; (5) Quantitative analysis using the external standard method and output of results.

2. The method for detecting the content of methyl tert-butyl ether in soil according to claim 1, characterized in that, The mass spectrometer was set to selected reaction monitoring mode, with the parent ion at m / z 88 and the daughter ions at m / z 73 and m / z 57.

3. The method for detecting the content of methyl tert-butyl ether in soil according to claim 1, characterized in that, The specific adsorbent is a mesoporous silica material with a surface grafted with a -(CH2)3-N(CH3)-C(CH3)3 bifunctional ligand.

4. The method for detecting the content of methyl tert-butyl ether in soil according to claim 1, characterized in that, The gas chromatography used a 30m×0.25mm×1.4μm 5% phenyl-95% dimethyl polysiloxane capillary column. The temperature program started at 40℃ and held for 3 min, then increased to 200℃ at a rate of 10℃ / min and held for 5 min.

5. The method for detecting the content of methyl tert-butyl ether in soil according to claim 1, characterized in that, The preparation method of the special adsorbent includes: Mesoporous silica microspheres were dispersed in anhydrous toluene, 3-aminopropyltriethoxysilane was added, and the mixture was refluxed at 80°C for 12 hours. After centrifugation and washing, amino-functionalized carrier was obtained. The carrier and tert-butyl isocyanate were then reacted with anhydrous dichloromethane at room temperature for 24 hours with a reaction molar ratio of 1:1.

2. The product was then dried under vacuum to obtain the special adsorbent.

6. The method for detecting the content of methyl tert-butyl ether in soil according to claim 1, characterized in that, In the ultrasound-assisted solvent extraction step, when the soil sample is clay with an organic matter content greater than 3%, acetonitrile is replaced with acetone, and the proportion of methanol in the leaching solution is adjusted to 8% in the solid phase extraction leaching step.

7. The method for detecting the content of methyl tert-butyl ether in soil according to claim 1, characterized in that, The solid-phase extraction column is made of polypropylene, with an inner diameter of 6 mm and a column height of 15 mm.

8. The method for detecting the content of methyl tert-butyl ether in soil according to claim 1, characterized in that, In the gas chromatography-mass spectrometry separation and detection step, the injection port temperature is 250℃, the carrier gas is high-purity helium, the flow rate is 1.0 mL / min, the splitless injection mode is used, and the injection volume is 1 μL.

9. The method for detecting the content of methyl tert-butyl ether in soil according to claim 1, characterized in that, In the gas chromatography-mass spectrometry separation and detection step, the response signals of the two daughter ion channels m / z73 and m / z57 are judged by confirmatory criteria: if the retention time difference between the two is less than 0.05 min and the peak area ratio A 73 / A 57 If the value is in the range of 1.6-2.0, it is considered a positive signal of methyl tert-butyl ether; otherwise, it is considered an interfering substance and is not included in the quantitative result.

10. The method for detecting the content of methyl tert-butyl ether in soil according to claim 1, characterized in that, The external standard method for quantification uses standard solutions with concentration gradients of 0.5, 1.0, 5.0, 10.0, and 50.0 μg / L to establish calibration curves.

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