Solid phase micro-extraction-high performance liquid chromatography on-line combined detection method of grease antioxidant
Technical Field
The invention belongs to the field of analytical chemistry, and particularly relates to a solid-phase microextraction-high performance liquid chromatography online combined detection method for an oil antioxidant.
Background
If the oil is influenced by factors such as air, illumination, temperature, moisture, impurities, heavy metal ions, microorganisms and the like for a long time in the processing, storage and use processes, oxidative rancidity and deterioration can occur, the rancidity of the oil can not only influence the quality of oil products and products thereof, but also form health hidden troubles such as aging, tumors, cardiovascular diseases and the like for human bodies. In the food industry, it is common to delay or eliminate the oxidative rancidity of fats and oils by adding antioxidants to the fats and oils. Phenolic antioxidants are commonly used antioxidants in fats and oils, and their representative substances mainly include: t-butyl p-hydroxyanisole (BHA), Propyl Gallate (PG), tert-butylhydroquinone (TBHQ), dibutylhydroxytoluene (BHT), and the like. However, it is not negligible that these antioxidants have large toxic and side effects and have adverse effects on human health, such as potential carcinogenicity of BHA, which may inhibit the activity of human respiratory enzymes and cause liver damage. Therefore, it is necessary to detect the antioxidant content in the oil with high sensitivity. Because of the interference of the complex matrix of the actual sample on the analysis and detection, the sample is generally required to be preprocessed before the detection, so as to reduce the interference of impurities, enrich the analysis object and improve the detection sensitivity.
Covalent Organic Frameworks (COFs) are porous organic polymers formed by combining light elements such as C, H, O, N, B and the like through covalent bonds, and show good application potential in sample pretreatment, the action mechanism of the covalent organic frameworks is mainly that the COFs contain rich benzene rings and-C = N functional groups, a strong pi-pi accumulation effect is provided, and meanwhile, the porous structure of the COFs also ensures large adsorption capacity. As phenolic grease antioxidants such as BHA, PG, TBHQ and the like all have pi-pi conjugated systems containing benzene rings in chemical structures, pi-pi stacking interaction can be generated between the substances and COFs, and a new idea is provided for developing selective extraction and enrichment materials of the grease antioxidants. Firstly, preparing a covalent organic framework functionalized solid phase micro-extraction monolithic column; then the monolithic column is used as a solid phase micro-extraction medium, and the synergistic effect of the pi-pi accumulation effect between COFs (chemical organic frameworks) contained in the monolithic column and the phenolic grease antioxidant and the hydrophilic interaction between the urea resin matrix of the monolithic column and the phenolic grease antioxidant is utilized to realize the high-efficiency extraction and enrichment of trace grease antioxidant in the grease sample and eliminate the interference of impurities in the matrix of the actual sample on analysis and detection; and an online combined detection method of the grease antioxidant is established by combining a solid phase microextraction-high performance liquid chromatography online combined system. At present, no report related to on-line enrichment and extraction of the grease antioxidant by using a covalent organic framework functionalized monolithic column exists.
Disclosure of Invention
The invention aims to provide an on-line combined detection method of solid-phase micro-extraction-high performance liquid chromatography of an oil antioxidant. Firstly, preparing a covalent organic framework functionalized solid phase micro-extraction monolithic column; then the monolithic column is used as a solid phase micro-extraction medium; due to the synergistic extraction effect between the integral column and the phenolic grease antioxidant (namely pi-pi accumulation effect of a covalent organic framework and hydrophilic interaction of a urea resin matrix), the efficient extraction enrichment and on-line purification of trace antioxidants in an actual grease sample are realized; and an online combined detection method of the grease antioxidant is established by combining a solid phase microextraction-high performance liquid chromatography online combined system.
In order to achieve the purpose, the invention adopts the following technical scheme:
the construction and operation of the solid phase micro-extraction-high performance liquid chromatography online combined system are as follows:
the invention relates to a solid phase microextraction-high performance liquid chromatography online combined system which is constructed by referring to a related patent (patent number: 2018207623454), and mainly comprises a ten-way valve, a six-way valve, a covalent organic framework functionalized solid phase microextraction monolithic column, a liquid phase chromatography infusion pump-pump A, a 0.5 mL PEEK tube quantitative ring, a liquid phase chromatography infusion pump-pump B, a 0.2 mL PEEK tube quantitative ring, a liquid phase chromatography analysis column and a detector.
The combined system comprises the following specific operation steps:
firstly, a ten-way valve and a six-way valve of an online combined system are both in a LOAD position; the liquid loading is balanced by a pump A to balance the covalent organic framework functionalized solid phase micro-extraction monolithic column, and the flow rate is 0.1 mL/min; the mobile phase passes through the analytical column directly by a pump B to obtain a stable baseline required by chromatographic separation, and the flow rate is 1.0 mL/min; meanwhile, filling a sample solution into a 0.5 mL PEEK tube quantitative ring through a sample injection needle;
secondly, adjusting the six-way valve to an INJECT position, starting on-line solid-phase microextraction, bringing a sample in a 0.5 mL PEEK tube quantitative ring into a covalent organic framework functionalized solid-phase microextraction monolithic column through loading liquid, and adjusting the six-way valve to a LOAD position after a given time;
thirdly, changing the solution conveyed by the pump A from loading liquid to eluent with the flow rate set as 0.1 mL/min, eluting the analysis object enriched on the covalent organic framework functionalized solid phase microextraction monolithic column by using the eluent, and collecting the elution object in a 0.2 mL PEEK tube quantitative ring; when the elution is completed, the ten-way valve is adjusted to the INJECT position, the collected eluent is sent to a liquid chromatographic column by using the mobile phase for separation, and then the detection is carried out by using a detector.
Wherein, the sample solvent and the loading solution comprise: acetonitrile/5 mmol/L pH =6.0 aqueous ammonium formate solution =40/60 by volume fraction ratio; the mobile phase is as follows: acetonitrile/0.05% aqueous formic acid =60/40 by volume fraction; the eluent is as follows: acetonitrile; the analytical column is a C18 liquid chromatographic analytical column; the detector is an ultraviolet detector; the temperature of the column oven is 40 ℃, and the detection wavelength is 201 nm.
The covalent organic framework functionalized solid-phase microextraction monolithic column is prepared by mixing covalent organic framework nano microspheres, urea aqueous solution, formaldehyde aqueous solution and a catalyst, wherein the mass ratio of the covalent organic framework nano microspheres to the urea aqueous solution to the formaldehyde aqueous solution to the catalyst is 2:9:11: 2. The concentration of the urea aqueous solution is 1 g/mL; the mass concentration of formaldehyde in the formaldehyde aqueous solution is 33% -37%; the catalyst is hydrochloric acid aqueous solution, and the concentration of the hydrochloric acid aqueous solution is 0.1 mol/L.
The preparation method of the covalent organic framework functionalized solid phase micro-extraction monolithic column comprises the following steps:
(1) cleaning an empty pipe: washing the empty polytetrafluoroethylene tube with chromatographic pure methanol, and then placing the tube in a drying oven at 60 ℃ for drying for later use; wherein the length of the polytetrafluoroethylene tube is 10 cm, and the inner diameter is 750 mu m;
(2) preparing covalent organic framework nano microspheres; mixing 0.04 mmol of 1,3, 5-tri (4-aminophenyl) benzene and 0.06 mmol of terephthalaldehyde in 5 mL of acetonitrile, carrying out ultrasonic treatment for 1 minute, adding 1 mL of glacial acetic acid of 12 mol/L, carrying out vortex mixing, carrying out violent oscillation for 10 seconds, standing at room temperature for 72 hours, centrifuging, and collecting precipitate; then washing the microspheres with anhydrous tetrahydrofuran and anhydrous ethanol for three times respectively, and drying the microspheres in vacuum at 60 ℃ for 24 hours to obtain the covalent organic framework nano microspheres.
(3) And (3) rapid polycondensation in a tube: uniformly mixing covalent organic framework nano microspheres, formaldehyde aqueous solution, urea aqueous solution and a catalyst in proportion, quickly oscillating for 1 minute, quickly filling the mixture into a clean and dry polytetrafluoroethylene tube, sealing two ends of the polytetrafluoroethylene tube, and soaking the polytetrafluoroethylene tube in a water bath at 65 ℃ to heat the polytetrafluoroethylene tube at constant temperature for 10 minutes;
(4) washing the monolithic column: after the reaction is finished, using water as a mobile phase, washing the monolithic column by using a liquid chromatography pump, and removing residual solvent and unreacted substances in a column bed to obtain the covalent organic framework functionalized solid phase micro-extraction monolithic column; the monolith is equilibrated with a loading liquid for half an hour prior to conducting an on-line solid phase microextraction operation.
The invention has the following remarkable advantages:
1) the traditional enrichment extraction of the phenolic oil antioxidant mainly uses an inverse phase stationary phase, and a large amount of nonpolar substances in the oil cause great interference on the enrichment extraction of an analysis object; the invention takes a covalent organic framework functionalized solid-phase microextraction monolithic column as a solid-phase microextraction medium, and realizes the high-efficiency enrichment and extraction of an analysis object by utilizing the synergistic extraction effect (namely the pi-pi accumulation effect of the covalent organic framework and the hydrophilic interaction of a urea resin matrix) between the monolithic column and a phenolic grease antioxidant; in addition, the hydrophilic action of the monolithic column matrix greatly reduces the adsorption of non-polar impurities and reduces the influence of the actual sample matrix on the enrichment and analysis of an analysis object.
2) The invention carries out analysis and detection based on an online combined system, greatly improves the detection efficiency compared with the conventional detection method, and improves the automation degree of the system.
Drawings
Fig. 1 shows the influence of acetonitrile content in a loading liquid on the enrichment efficiency of an analysis object when a solid phase microextraction-high performance liquid chromatography (hplc) online combined system is constructed by using a covalent organic framework functionalized monolithic column as a solid phase microextraction medium to perform online enrichment detection on Propyl Gallate (PG), tert-butyl hydroquinone (TBHQ) and tert-butyl p-hydroxyanisole (BHA).
FIG. 2 is a chromatogram of an online enrichment detection of trace oil antioxidants added to edible oil by constructing a solid phase microextraction-high performance liquid chromatography online combined system by using a covalent organic framework functionalized monolithic column as a solid phase microextraction medium. a is the analysis chromatogram of a conventional liquid chromatography system; and b is an online coupling system analysis chromatogram.
FIG. 3 is a chromatogram of different edible oil samples on-line enrichment detection by using a covalent organic framework functionalized monolithic column as a solid phase microextraction medium to construct a solid phase microextraction-high performance liquid chromatography on-line combined system. a is an online combined system analysis chromatogram of a certain brand of blend oil; b is an online combined system analysis chromatogram of certain brand sunflower seed oil; c is an online combined system analysis chromatogram of a certain brand of peanut oil.
Detailed Description
In order to make the present invention more comprehensible, the technical solutions of the present invention are further described below with reference to specific embodiments, but the present invention is not limited thereto.
Example 1
The covalent organic framework functionalized solid-phase microextraction monolithic column is prepared by mixing covalent organic framework nano microspheres, urea aqueous solution, formaldehyde aqueous solution and a catalyst, wherein the mass ratio of the covalent organic framework nano microspheres to the urea aqueous solution to the formaldehyde aqueous solution to the catalyst is 2:9:11: 2.
The concentration of the urea aqueous solution is 1 g/mL; the mass concentration of formaldehyde in the formaldehyde aqueous solution is 37%; the catalyst is hydrochloric acid aqueous solution, and the concentration of the hydrochloric acid aqueous solution is 0.1 mol/L.
The preparation method of the covalent organic framework functionalized solid phase micro-extraction monolithic column comprises the following steps:
(1) cleaning an empty pipe: washing the empty polytetrafluoroethylene tube with chromatographic pure methanol, and then placing the tube in a drying oven at 60 ℃ for drying for later use; wherein the length of the polytetrafluoroethylene tube is 10 cm, and the inner diameter is 750 mu m;
(2) preparing covalent organic framework nano microspheres; mixing 0.04 mmol of 1,3, 5-tri (4-aminophenyl) benzene and 0.06 mmol of terephthalaldehyde in 5 mL of acetonitrile, carrying out ultrasonic treatment for 1 minute, adding 1 mL of glacial acetic acid of 12 mol/L, carrying out vortex mixing, carrying out violent oscillation for 10 seconds, standing at room temperature for 72 hours, centrifuging, and collecting precipitate; then washing the microspheres with anhydrous tetrahydrofuran and anhydrous ethanol for three times respectively, and drying the microspheres in vacuum at 60 ℃ for 24 hours to obtain the covalent organic framework nano microspheres.
(3) And (3) rapid polycondensation in a tube: uniformly mixing covalent organic framework nano microspheres, formaldehyde aqueous solution, urea aqueous solution and a catalyst in proportion, quickly oscillating for 1 minute, quickly filling the mixture into a clean and dry polytetrafluoroethylene tube, sealing two ends of the polytetrafluoroethylene tube, and soaking the polytetrafluoroethylene tube in a water bath at 65 ℃ to heat the polytetrafluoroethylene tube at constant temperature for 10 minutes;
(4) washing the monolithic column: after the reaction is finished, using water as a mobile phase, washing the monolithic column by using a liquid chromatography pump, and removing residual solvent and unreacted substances in a column bed to obtain the covalent organic framework functionalized solid phase micro-extraction monolithic column; the monolith is equilibrated with a loading liquid for half an hour prior to conducting an on-line solid phase microextraction operation.
Example 2
The combined system comprises the following specific operation steps:
firstly, a ten-way valve and a six-way valve of an online combined system are both in a LOAD position; the loading liquid passes through a pump A to balance the covalent organic framework functionalized solid phase micro extraction monolithic column (the monolithic column prepared in example 1), and the flow rate is 0.1 mL/min; the mobile phase passes through the analytical column directly by a pump B to obtain a stable baseline required by chromatographic separation, and the flow rate is 1.0 mL/min; meanwhile, filling a sample solution into a 0.5 mL PEEK tube quantitative ring through a sample injection needle;
secondly, adjusting the six-way valve to an INJECT position, starting on-line solid-phase microextraction, bringing a sample in a 0.5 mL PEEK tube quantitative ring into a covalent organic framework functionalized solid-phase microextraction monolithic column through loading liquid, and adjusting the six-way valve to a LOAD position after a given time;
thirdly, changing the solution conveyed by the pump A from loading liquid to eluent with the flow rate set as 0.1 mL/min, eluting the analysis object enriched on the covalent organic framework functionalized solid phase microextraction monolithic column by using the eluent, and collecting the elution object in a 0.2 mL PEEK tube quantitative ring; when the elution is completed, the ten-way valve is adjusted to the INJECT position, the collected eluent is sent to a liquid chromatographic column by using the mobile phase for separation, and then the detection is carried out by using a detector.
According to the technical scheme of the invention, firstly, a covalent organic framework functionalized monolithic column (the monolithic column prepared in example 1) is prepared, and the monolithic column is used as a solid phase microextraction medium, and a solid phase microextraction-high performance liquid chromatography online combined analysis system is constructed by referring to related patents (patent number: 2018207623454). And then, detecting the antioxidant in the grease by means of online enrichment of an online coupling system, and investigating the influence of the acetonitrile content in the loading liquid on the sample enrichment efficiency. The concentration of several antioxidants in the detection sample is 1.0 mg/kg; the loading solution consists of acetonitrile and 5mM ammonium formate aqueous solution with pH =6.0, the sample injection solvent consists of acetonitrile/5 mM ammonium formate aqueous solution with pH =6.0 =40/60 (v/v), the sample injection flow rate is 0.1 mL/min, and the sample injection volume is 500 μ L; the eluent composition is pure acetonitrile, the elution flow rate is 0.1 mL/min, and the elution volume is 200 μ L (the eluent is collected for the first two minutes); the mobile phase for separation was acetonitrile/0.05% aqueous formic acid =60/40 (v/v), the separation flow rate was 1.0 mL/min, the column oven temperature was 40 ℃, and the detection wavelength was 201 nm.
As shown in fig. 1, when the acetonitrile content of the loading solution is increased from 20% (v/v) to 40% (v/v), the enrichment efficiency of the analysis object is continuously improved, which represents the typical hydrophilic interaction between the monolithic urea-formaldehyde resin matrix and the analysis object; as the acetonitrile content of the loading solution is continuously increased from 40% (v/v) to 60% (v/v), the enrichment efficiency of the analysis object is not obviously improved or is reduced, which should be that the pi-pi stacking effect between the analysis object and the covalent organic framework is obviously weakened along with the increase of the acetonitrile content; therefore, we can see that the extraction mode between several antioxidants and the covalent organic framework functionalized monolithic column is the synergistic extraction effect of the hydrophilic effect and the pi-pi stacking effect. Therefore, we chose a loading liquid acetonitrile ratio of 40% (v/v) as the optimum ratio.
Example 3
According to the technical scheme of the embodiment 2, firstly, the covalent organic framework functionalized monolithic column of the embodiment 1 is prepared, and the monolithic column is used as a solid phase microextraction medium, and a solid phase microextraction-high performance liquid chromatography on-line combined analysis system is constructed by referring to a related patent (patent number: 2018207623454). And then, detecting the antioxidant in the grease by virtue of online enrichment of an online coupling system, wherein the operation optimization parameters of the coupling system are as follows: the sample loading fluid was ACN/5 mM pH =6.0 ammonium formate solution =40%/60% (v/v); the eluent is acetonitrile; the sample introduction flow rate is 0.10 mL/min; the elution flow rate is 0.10 mL/min; elution volume 200 μ L (first two minutes of eluate collected); the mobile phase was ACN/0.05% formic acid solution =60%/40% (v/v); the flow rate of the mobile phase is 1.0 mL/min; the temperature of the column oven is 40 ℃; the detection wavelength was set at 201 nm.
FIG. 2 is a chromatogram comparing the on-line system and the conventional liquid chromatography system for detecting several oil antioxidants added at 1.0 mg/kg in edible oil. In fig. 2, the detection peak 1 is Propyl Gallate (PG), the detection peak 2 is tert-butylhydroquinone (TBHQ), and the detection peak 3 is Butylated Hydroxyanisole (BHA). As can be seen from FIG. 2-b, under the conditions of the combined system, the trace amount of the grease antioxidant added into the edible oil is subjected to efficient enrichment extraction and high-sensitivity detection, and the detection limits of several analysis objects are 2.0. mu.g/kg (PG), 0.9. mu.g/kg (TBHQ) and 0.3. mu.g/kg (BHA), respectively. When the same sample is analyzed using a conventional liquid chromatography system, the signal of the detection object is weak and the detection sensitivity is low (FIG. 2-a). FIG. 3 is a chromatogram of on-line enrichment detection of actual samples of different edible oils using the on-line combination system constructed by the present invention. As can be seen, BHA was detected in all 3 edible oil samples, at 140. mu.g/kg (blend oil), 100. mu.g/kg (sunflower oil) and 225. mu.g/kg (peanut oil). This demonstrates the utility and reliability of the method established by the present invention.
The above description is only a preferred embodiment of the present invention, and all equivalent changes and modifications made in accordance with the claims of the present invention should be covered by the present invention.