Method for determining smelly substances by using sodium thiosulfate pretreatment in combination with headspace solid-phase microextraction and gas chromatography-mass spectrometry

Abstract

The invention discloses a method for determining odor substances by using sodium thiosulfate pretreatment in combination with headspace solid-phase microextraction and gas chromatography-mass spectrometry, and relates to the technical field of gas chromatography-mass spectrometry detection. According to the method, sodium thiosulfate pretreatment is combined with headspace solid-phase microextraction gas chromatography-mass spectrometry to determine the odor substances, sodium thiosulfate is used for eliminating residual chlorine existing in effluent of a conventional waterworks, the residual chlorine is prevented from interfering with the solid-phase microextraction process and the gas chromatography-mass spectrometry detection process, and therefore the detection accuracy of the instrument on the odor substances is improved. The invention can obtain the method for determining the smelly substances by using sodium thiosulfate pretreatment in combination with headspace solid-phase microextraction and gas chromatography-mass spectrometry.

Description

Technical Field

[0001] The invention relates to the technical field of gas chromatography-mass spectrometry detection, and in particular to a method for determining odor substances by using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry. Background Art

[0002] Solid phase microextraction (SPME) technology is an efficient sample pretreatment adsorption technology. Since it was proposed by Pawlinszyn and his collaborators at the University of Waterloo in Canada in 1989, it has been widely used in many fields such as environment, food, and medicine. This technology is based on the principle of adsorption of trace analytes on the active solid surface. The analyte in the sample is extracted, concentrated, and injected through a quartz fiber coated with a stationary phase. SPME technology does not require a large amount of solvent, is simple and fast to operate, and has the advantages of high sensitivity, high selectivity, and low detection limit. It can not only be used for the analysis of volatile or semi-volatile substances in gas, liquid, solid and other samples, but can also be used in conjunction with a variety of instruments such as gas chromatography (GC) and high performance liquid chromatography (HPLC) to achieve automated operation and online analysis. In addition, SPME technology can also avoid secondary pollution to the environment and improve the working environment of operators.

[0003] Gas Chromatography-Mass Spectrometry (GC-MS) is an advanced analytical method that combines separation and identification functions. This technology uses a gas chromatography column to efficiently separate the components in a complex sample, and then uses a mass spectrometer to measure the mass-to-charge ratio of each separated component to obtain the characteristic mass spectrum of each compound. GC-MS has high separation efficiency, sensitivity and resolution, and can achieve accurate qualitative and quantitative analysis of trace substances. In many scientific research fields such as environmental science, drug metabolism, and food safety, GC-MS has become an indispensable analytical tool.

[0004] At present, my country stipulates that headspace solid phase microextraction gas chromatography mass spectrometry (HS-SPME-GC-MS) is the method for detecting geosmin (GSM) and 2-methylisocanol (2-MIB). Compared with other detectors and extraction methods, this method has significant advantages in sensitivity, accuracy, degree of automation, enrichment recovery rate, etc. In water quality testing, some substances present in conventional processes will compete with odor substances for adsorption sites on the solid phase microextraction column, reducing the enrichment degree of odor substances, thereby causing the test results to deviate from the actual value. Studies have found that the presence of chlorine (Cl2) can affect the detection results of odor substances, and a detection error of 10-70% can be observed, especially when the concentration of odor substances is low and the residual chlorine concentration is high. This effect is more significant. The concentrations of GSM and 2-MIB have a strong odor at the ng / L level. If the absolute difference between the measured value of the odor substance and the actual value is small and the relative error is large, from the perspective of trace analysis, this subtle difference can easily lead to substandard factory water entering the water supply network. Summary of the invention

[0005] The purpose of the present invention is to solve the above technical problems and to provide a method for determining odor substances by using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry.

[0006] A method for determining odor substances using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry is carried out in the following steps:

[0007] Step S1: sample pretreatment;

[0008] Adding sodium chloride to a chromatographic bottle, then adding a water sample containing geosmin and 2-methylisoborneol, and then adding a 2-isobutyl-3-methoxypyrazine solution and a sodium thiosulfate solution, wherein the concentration of the 2-isobutyl-3-methoxypyrazine solution is 10 μg / L, and the concentration of the sodium thiosulfate solution is 0.4-1 g / L; placing the chromatographic bottle in a water bath for water bath heating, placing the chromatographic bottle in a centrifugal shaker for shaking after the heating is completed, and then moving the chromatographic bottle into a headspace solid phase microextraction platform, first extracting at 50-70° C. for 30-60 minutes, and then desorbing at 240-260° C. for 2-5 minutes to obtain a pre-treated sample;

[0009] Step S2: drawing a standard curve;

[0010] A solution containing geosmin and 2-methylisoborneol is prepared as a mixed working liquid, and eight standard mixed solutions with concentrations of geosmin and 2-methylisoborneol of 0 ng / L, 5 ng / L, 10 ng / L, 20 ng / L, 40 ng / L, 60 ng / L, 80 ng / L and 100 ng / L are prepared using the mixed working liquid, and then a 2-isobutyl-3-methoxypyrazine solution and a sodium thiosulfate solution are added to obtain a mixed solution. The concentration of the 2-isobutyl-3-methoxypyrazine solution is 0 ng / L, 5 ng / L, 10 ng / L, 20 ng / L, 40 ng / L, 60 ng / L, 80 ng / L and 100 ng / L are obtained. The concentration of the sodium thiosulfate solution is 10 μg / L, and the concentration of the sodium thiosulfate solution is 0.4-1 g / L; the mixed solution is placed in a water bath for heating, and after heating, it is placed in a centrifugal shaker for shaking, and then moved into the headspace solid phase microextraction platform, first extracted at 50-70°C for 30-60 minutes, and then desorbed at 240-260°C for 2-5 minutes; after desorption, the eight standard mixed solutions are detected by gas chromatography-mass spectrometry, and then a standard curve is drawn based on the detected peak areas and the concentrations of the eight standard mixed solutions;

[0011] Step S3: data processing;

[0012] The pre-treated sample obtained in step S1 is detected by gas chromatography-mass spectrometry to obtain the quantitative ion peak area of ​​geosmin or 2-methylisoborneol and the quantitative ion peak area of ​​2-isobutyl-3-methoxypyrazine, and then the concentration of geosmin or 2-methylisoborneol in the sample is calculated based on the standard curve intercept and the standard curve slope obtained from the standard curve in step S2 and the concentration of the 2-isobutyl-3-methoxypyrazine solution in the sample.

[0013] Beneficial effects of the present invention:

[0014] The present invention uses sodium thiosulfate pretreatment combined with headspace solid phase microextraction gas chromatography-mass spectrometry to determine odor substances, and utilizes sodium thiosulfate to eliminate residual chlorine present in the effluent of conventional water plants, thereby avoiding interference of residual chlorine in the solid phase microextraction process and the gas chromatography-mass spectrometry detection process, thereby improving the accuracy of the instrument in detecting odor substances.

[0015] The invention can obtain a method for determining odor substances by combining sodium thiosulfate pretreatment with headspace solid phase microextraction and gas chromatography-mass spectrometry. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A diagram showing the influence of each process stage on 2-MIB detection;

[0017] Figure 2 A diagram showing the influence of each process section on GSM detection;

[0018] Figure 3 It shows the relative error diagram of residual chlorine in detecting 2-MIB;

[0019] Figure 4 Relative error diagram of residual chlorine in detecting GSM;

[0020] Figure 5 It shows the effect of dechlorination on the detection results of 2-MIB;

[0021] Figure 6 A diagram showing the impact of dechlorination on the results of GSM detection. DETAILED DESCRIPTION

[0022] Specific implementation method 1: This implementation method uses sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry to determine odor substances, and is carried out according to the following steps:

[0023] Step S1: sample pretreatment;

[0024] Adding sodium chloride to a chromatographic bottle, then adding a water sample containing geosmin and 2-methylisoborneol, and then adding a 2-isobutyl-3-methoxypyrazine solution and a sodium thiosulfate solution, wherein the concentration of the 2-isobutyl-3-methoxypyrazine solution is 10 μg / L, and the concentration of the sodium thiosulfate solution is 0.4-1 g / L; placing the chromatographic bottle in a water bath for water bath heating, placing the chromatographic bottle in a centrifugal shaker for shaking after the heating is completed, and then moving the chromatographic bottle into a headspace solid phase microextraction platform, first extracting at 50-70° C. for 30-60 minutes, and then desorbing at 240-260° C. for 2-5 minutes to obtain a pre-treated sample;

[0025] Step S2: drawing a standard curve;

[0026] A solution containing geosmin and 2-methylisoborneol is prepared as a mixed working liquid, and eight standard mixed solutions with concentrations of geosmin and 2-methylisoborneol of 0 ng / L, 5 ng / L, 10 ng / L, 20 ng / L, 40 ng / L, 60 ng / L, 80 ng / L and 100 ng / L are prepared using the mixed working liquid, and then a 2-isobutyl-3-methoxypyrazine solution and a sodium thiosulfate solution are added to obtain a mixed solution. The concentration of the 2-isobutyl-3-methoxypyrazine solution is 0 ng / L, 5 ng / L, 10 ng / L, 20 ng / L, 40 ng / L, 60 ng / L, 80 ng / L and 100 ng / L are obtained. The concentration of the sodium thiosulfate solution is 10 μg / L, and the concentration of the sodium thiosulfate solution is 0.4-1 g / L; the mixed solution is placed in a water bath for heating, and after heating, it is placed in a centrifugal shaker for shaking, and then moved into the headspace solid phase microextraction platform, first extracted at 50-70°C for 30-60 minutes, and then desorbed at 240-260°C for 2-5 minutes; after desorption, the eight standard mixed solutions are detected by gas chromatography-mass spectrometry, and then a standard curve is drawn based on the detected peak areas and the concentrations of the eight standard mixed solutions;

[0027] Step S3: data processing;

[0028] The pre-treated sample obtained in step S1 is detected by gas chromatography-mass spectrometry to obtain the quantitative ion peak area of ​​geosmin or 2-methylisoborneol and the quantitative ion peak area of ​​2-isobutyl-3-methoxypyrazine, and then the concentration of geosmin or 2-methylisoborneol in the sample is calculated based on the standard curve intercept and the standard curve slope obtained from the standard curve in step S2 and the concentration of the 2-isobutyl-3-methoxypyrazine solution in the sample.

[0029] Specific embodiment 2: The difference between this embodiment and specific embodiment 1 is that the mass of sodium chloride described in step S1, the volume of the water sample containing geosmin and 2-methylisoborneol, and the ratio of the volume of 2-isobutyl-3-methoxypyrazine solution to the volume of sodium thiosulfate solution are (4 to 10) g: 10 mL: 10 μL: 100 μL.

[0030] The other steps are the same as those in the first specific implementation.

[0031] Specific implementation method three: The difference between this implementation method and specific implementation method one or two is that in steps S1 and S2, heating is performed in a water bath at 40-70° C. for 3-5 minutes.

[0032] The other steps are the same as those in the first or second embodiment.

[0033] Specific implementation method 4: The difference between this implementation method and specific implementation methods 1 to 3 is that the shaking time in steps S1 and S2 is 3 to 5 minutes.

[0034] The other steps are the same as those in Specific Embodiments 1 to 3.

[0035] Specific embodiment 5: The difference between this embodiment and specific embodiments 1 to 4 is that the chromatographic bottle described in step S1 is baked at 100-120° C. for 0.5 h to 1 h before use.

[0036] The other steps are the same as those in Specific Embodiments 1 to 4.

[0037] Specific embodiment 6: The difference between this embodiment and specific embodiments 1 to 5 is that the extracted fibers in steps S1 and S2 are aged at 230° C. to 270° C. for 1 to 1.5 hours before use.

[0038] The other steps are the same as those in Specific Embodiments 1 to 5.

[0039] Specific embodiment 7: The difference between this embodiment and specific embodiments 1 to 6 is that in steps S2 and S3, the pre-treated samples are first filtered through a 0.45 μm filter membrane before being tested.

[0040] The other steps are the same as those in Specific Embodiments 1 to 6.

[0041] Specific embodiment eight: The difference between this embodiment and specific embodiments one to seven is: the process parameters of the chromatography and mass spectrometry of the gas chromatography-mass spectrometer in step S2 and step S3: the gas chromatography heating program is to first keep warm at 45°C for 3.5 minutes, then heat to 250°C at a heating rate of 8°C / min, and keep warm at 250°C for 5 minutes; the injection mode is splitless injection, and the carrier gas flow rate is 1mL / min; the ion source temperature is 230°C; the mass spectrometer uses an electron ionization source for ionization, and the standard electron energy is 70eV; the scanning mode is selected ion detection.

[0042] The other steps are the same as those in Specific Embodiments 1 to 7.

[0043] Specific embodiment 9: This embodiment differs from specific embodiments 1 to 8 in that the concentration calculation formula of geosmin or 2-methylisoborneol in step S2 and step S3 is:

[0044] ρ i =(A i / A is -a i )×ρ is / b i ;

[0045] Where: i Indicates the concentration of geosmin or 2-methylisoborneol in the sample, in ng / L; A i Indicates the quantitative ion peak area of ​​geosmin or 2-methylisoborneol in the sample, A is Indicates the quantitative ion peak area of ​​2-isobutyl-3-methoxypyrazine in the sample, α i represents the standard curve intercept, ρ is Indicates the concentration of 2-isobutyl-3-methoxypyrazine in the sample, in ng / L; b i represents the slope of the standard curve;

[0046] 1) Detecting the eight standard mixed solutions in step S2 using a gas chromatography-mass spectrometer to obtain the quantitative ion peak areas A of geosmin or 2-methylisoborneol in the eight standard mixed solutions respectively. i and the quantitative ion peak area A of 2-isobutyl-3-methoxypyrazine is , then A i / A is As the y-axis, ρ i / ρ is As the x-axis, the standard curve is drawn, and the intercept α of the standard curve is obtained. i and the slope of the standard curve b i ;

[0047] 2) Detecting the pre-treated sample obtained in step S1 by gas chromatography-mass spectrometry to obtain the quantitative ion peak area A of geosmin or 2-methylisoborneol i and the quantitative ion peak area A of 2-isobutyl-3-methoxypyrazine is , and then the standard curve intercept α obtained from the standard curve in step S2 i and the slope of the standard curve b i , and then according to the concentration of 2-isobutyl-3-methoxypyrazine solution in the sample is , calculate the concentration of geosmin or 2-methylisoborneol in the sample ρ i .

[0048] The other steps are the same as those in Specific Embodiments 1 to 8.

[0049] Specific embodiment ten: This embodiment differs from specific embodiments one to nine in that the volume ratio of the 2-isobutyl-3-methoxypyrazine solution to the sodium thiosulfate solution in step S2 is 1:10.

[0050] The other steps are the same as those in Specific Embodiments 1 to 9.

[0051] The following examples are used to verify the beneficial effects of the present invention:

[0052] The reagents used in the detection method include high-purity nitrogen with a purity of ≥99.999%; pure water; methanol, high-grade purity; sodium chloride, high-grade purity; geosmin and 2-methylisoborneol, with a purity of ≥95%; 2-isobutyl-3-methoxypyrazine solution, with a purity of ≥95%; and sodium thiosulfate, high-grade purity.

[0053] The device used in the detection method includes a full-automatic solid phase microextraction sampling platform, a solid phase microextraction fiber, a chromatographic bottle, a water bath, a centrifugal oscillator, and a gas chromatography-mass spectrometer.

[0054] Example 1: A method for determining odor substances using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry, which is carried out in the following steps:

[0055] Step S1: sample collection;

[0056] Samples are collected in brown glass bottles with polytetrafluoroethylene bottle pads. When sampling, water is added until the bottle is full, and there should be no bubbles in the bottle. After collection, refrigerate and seal the bottle for storage for 24 hours.

[0057] The chromatographic bottle is a 20 mL brown glass bottle with a screw cap covered with a silicone rubber pad wrapped with a polytetrafluoroethylene film, and is baked at 120° C. for 1 hour before use.

[0058] Step S2: sample pretreatment;

[0059] 4 g of sodium chloride was added to a chromatographic bottle, and then 10 mL of a water sample containing geosmin and 2-methylisoborneol was added, and then 10 μL of 2-isobutyl-3-methoxypyrazine solution (concentration of 10 μg / L) and 100 μL of sodium thiosulfate solution (concentration of 0.4 g / L) were added, and the bottle cap was screwed tightly; the chromatographic bottle was placed in a water bath and heated in a water bath at 60° C. for 3 min; after the heating was completed, the chromatographic bottle was placed in a centrifugal shaker for 3 min, and then the chromatographic bottle was moved to a headspace solid phase microextraction platform, and first extracted at 60° C. for 30 min, and the device automatically inserted the extraction fiber into the gas chromatography injection port, and desorbed at 250° C. for 2 min to obtain a pre-treated sample;

[0060] Step S3: drawing a standard curve;

[0061] A solution containing geosmin and 2-methylisoborneol is prepared as a mixed working liquid, and eight standard mixed solutions with the concentrations of geosmin and 2-methylisoborneol being 0 ng / L, 5 ng / L, 10 ng / L, 20 ng / L, 40 ng / L, 60 ng / L, 80 ng / L and 100 ng / L are prepared by using the mixed working liquid, and then a 2-isobutyl-3-methoxypyrazine solution and a sodium thiosulfate solution are added to obtain a mixed solution, wherein the concentration of the 2-isobutyl-3-methoxypyrazine solution is 10 μg / L, and the concentration of the sodium thiosulfate solution is 10 μg / L. The concentration of the sodium sulfate solution is 0.4-1 g / L; the mixed solution is placed in a water bath and heated at 60° C. for 3 min, and after the heating is completed, it is placed in a centrifugal shaker for 3 min, and then moved into a headspace solid phase microextraction platform, first extracted at 50-70° C. for 30-60 min, and then desorbed at 240-260° C. for 2-5 min; after the desorption is completed, the eight standard mixed solutions in step S2 are detected by gas chromatography-mass spectrometry to obtain the quantitative ion peak area A of geosmin or 2-methylisoborneol in the eight standard mixed solutions. i and the quantitative ion peak area A of 2-isobutyl-3-methoxypyrazine is , then A i / A is As the y-axis, ρ i / ρ is As the x-axis, the standard curve is drawn, and the intercept α of the standard curve is obtained. i and the slope of the standard curve b i ;

[0062] First, use the internal standard solution to calibrate the GC / MS. The response value of 2-isobutyl-3-methoxypyrazine should not deviate by more than 30% between each measurement. If the relative standard deviation of the standard curve is less than 10% within the working range, the instrument is considered to be in a stable state.

[0063] In step S2 and step S3, a solid phase microextraction fiber (DVB / CAR / PDMS fiber) is used. Before the extraction fiber is used for the first time, it should be placed in the injection port for aging at an aging temperature of 250° C. for 1 h.

[0064] The process parameters of chromatography and mass spectrometry of the gas chromatography-mass spectrometer in step S2 and step S3 are as follows: the gas chromatography temperature program is first kept at 45°C for 3.5 minutes, then heated to 250°C at a heating rate of 8°C / min, and kept at 250°C for 5 minutes; the injection mode is splitless injection, and the carrier gas flow rate is 1 mL / min; the ion source temperature is 230°C; the mass spectrometer uses an electron ionization source for ionization, and the standard electron energy is 70 eV; the scanning mode is selected ion detection. The parameters are shown in Table 1.

[0065] Table 1

[0066] Components Retention time / min Qualifier ion (m / z) Quantitative ion (m / z) geosmin 17.65 112,125 112 2-Methylisoborneol 13.62 95.107.135 95 2-Isobutyl-methoxypyrazine 13.28 94,124,151 124

[0067] Step S4: data processing;

[0068] The pre-treated sample obtained in step S1 is detected by gas chromatography-mass spectrometry to obtain the quantitative ion peak area A of geosmin or 2-methylisoborneol. i and the quantitative ion peak area A of 2-isobutyl-3-methoxypyrazine is , and then the standard curve intercept α obtained from the standard curve in step S2 i and the slope of the standard curve b i , and then according to the concentration of 2-isobutyl-3-methoxypyrazine solution in the sample is , calculate the concentration of geosmin or 2-methylisoborneol in the sample ρ i .

[0069] The specific calculation formula is as follows:

[0070] ρ i =(A i / A is -a i )×ρ is / b i ;

[0071] Where: i Indicates the concentration of geosmin or 2-methylisoborneol in the sample, in ng / L;

[0072] A i Indicates the quantitative ion peak area of ​​geosmin or 2-methylisoborneol in the sample;

[0073] A is It represents the quantitative ion peak area of ​​2-isobutyl-3-methoxypyrazine in the sample;

[0074] α i represents the intercept of the standard curve;

[0075] ρ is It indicates the concentration of 2-isobutyl-3-methoxypyrazine in the sample, in ng / L;

[0076] b i Represents the slope of the standard curve.

[0077] Before testing the pre-treated samples in step S3 and step S4, they are first filtered through a 0.45 μm filter membrane;

[0078] (1) Qualitative analysis: Retention time of each component: 2-isobutyl-3-methoxypyrazine, 13.28 min; 2-methylisoborneol, 13.62 min; geosmin, 17.65 min.

[0079] (2) Quantitative analysis: A standard curve must be drawn for each batch of samples, and the mass concentration of the samples found on the standard curve is calculated according to the above formula based on the peak area of ​​each component.

[0080] Principle of the present invention:

[0081] When exploring the influence of water quality in each process section of a water plant on the detection results of odorous substances, it was found that pre-ozone, activated carbon filter and disinfection process had a greater influence on the detection results of odorous substances. The degree of influence was: disinfection > pre-ozone > activated carbon. The results are as follows: Figure 1-2 As shown. The disinfection stage has the greatest impact, with relative errors of 18.4% and 12.1% for GSM and 2-MIB test results. The water plant uses sodium hypochlorite for disinfection, and it is speculated that sodium hypochlorite has a greater impact on the detection of odor substances. Related studies have found that residual chlorine will reduce the recovery rate of solid phase microextraction and cause the detection concentration of odor substances to deviate from the true value by 10%-70%.

[0082] The results of the simulated residual chlorine test using sodium hypochlorite are as follows: Figure 3-4 As shown. When the residual chlorine concentration is 0.5-1.5mg / L, 10ng / L, 50ng / L, and 100ng / L of GSM and 2-MIB are added, the relative errors of the two test results caused by residual chlorine are 6.37%-42.10% and 6.82%-31.30%. The errors of most data exceed the 10% error range allowed by the determination method. The smaller the concentration of the spiked odorous substance, the greater the relative error. As the concentration of residual chlorine increases, the influence of residual chlorine on the detection results of odorous substances also increases.

[0083] Since sodium thiosulfate can react with chlorine and sodium hypochlorite in water samples to reduce them to chloride ions, the ion reaction formula is:

[0084]

[0085] According to the free chlorine index of GB5749-2022 and the actual concentration of sodium hypochlorite in the water leaving the factory, a certain amount of sodium thiosulfate is added to the water sample to eliminate the influence of residual chlorine, avoid its oxidation and competition for the adsorption potential of odorous substances on the solid phase microextraction column, thereby increasing the enrichment of odorous substances adsorbed by solid phase microextraction.

[0086] The detection method of the present invention is mainly improved according to "Standard Test Methods for Drinking Water Part 8: Organic Matter Index" GB / T5750-75.1. By utilizing headspace solid phase microextraction gas chromatography-mass spectrometry and combining it with sodium thiosulfate pretreatment, an efficient enrichment-automated detection system for odor substances is established, thereby realizing accurate, efficient and automated detection of typical odor substances in water.

[0087] 1. Quality control:

[0088] Table 2 shows the quantitative ions, qualitative ions, detection limits and determination limits;

[0089] Table 2

[0090]

[0091] 2. Method precision and accuracy:

[0092] This method was used to determine the relative standard deviation and recovery rate of spiked water samples with concentrations of 12 ng / L and 45 ng / L, respectively, and the measurement was repeated 7 times.

[0093] Table 3 shows the precision and accuracy of the method;

[0094] Table 3

[0095]

[0096]

[0097] 3. Experimental part:

[0098] A comparative experiment on sodium thiosulfate pretreatment was carried out. First, the sodium hypochlorite concentration was set to 0.5 and 1.5 mg / L (in terms of Cl2). Then 10 and 100 ng / L of GSM and 8 and 80 ng / L of 2-MIB were added. Two parallel samples were set for each group of experiments. 2 mg / L of sodium thiosulfate solution (NaS2O3) was added to one of the parallel samples to eliminate the influence of residual chlorine. Pure water was added to the other parallel sample for control. Finally, the relative error was measured and calculated.

[0099] The results of testing each sample in this experiment using the method of this embodiment are as follows: Figure 5-6As shown. When there are 0.5 and 1.5 mg / L of residual chlorine, the detection concentration of GSM will be reduced to 73.0%-82.3% of the true value, and can be restored to 85.5%-99.2% after dechlorination; while the detection concentration of 2-MIB will be reduced to 57.4%-84.1% of the true value, and can be restored to 59.5%-91.9% after dechlorination. When the concentration of odorous substances is high, the recovery effect after dechlorination is better than that of low odorous substance concentrations, and GSM is easier to recover to the true value than 2-MIB. This may be because, whether at low or high concentration levels, the amount of sodium thiosulfate required for GSM is always greater than that for 2-MIB, so GSM is less affected by residual chlorine than 2-MIB.

Claims

1. A method for determining odor substances using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry, characterized in that The method proceeds as follows: Step S1: sample pretreatment; Adding sodium chloride to a chromatographic bottle, then adding a water sample containing geosmin and 2-methylisoborneol, and then adding a 2-isobutyl-3-methoxypyrazine solution and a sodium thiosulfate solution, wherein the concentration of the 2-isobutyl-3-methoxypyrazine solution is 10 μg / L, and the concentration of the sodium thiosulfate solution is 0.4-1 g / L; placing the chromatographic bottle in a water bath for water bath heating, placing the chromatographic bottle in a centrifugal shaker for shaking after the heating is completed, and then moving the chromatographic bottle into a headspace solid phase microextraction platform, first extracting at 50-70° C. for 30-60 minutes, and then desorbing at 240-260° C. for 2-5 minutes to obtain a pre-treated sample; Step S2: drawing a standard curve; A solution containing geosmin and 2-methylisoborneol is prepared as a mixed working liquid, and eight standard mixed solutions with concentrations of geosmin and 2-methylisoborneol of 0 ng / L, 5 ng / L, 10 ng / L, 20 ng / L, 40 ng / L, 60 ng / L, 80 ng / L and 100 ng / L are prepared using the mixed working liquid, and then a 2-isobutyl-3-methoxypyrazine solution and a sodium thiosulfate solution are added to obtain a mixed solution. The concentration of the 2-isobutyl-3-methoxypyrazine solution is 0 ng / L, 5 ng / L, 10 ng / L, 20 ng / L, 40 ng / L, 60 ng / L, 80 ng / L and 100 ng / L are obtained. The concentration of the sodium thiosulfate solution is 10 μg / L, and the concentration of the sodium thiosulfate solution is 0.4-1 g / L; the mixed solution is placed in a water bath for heating, and after heating, it is placed in a centrifugal shaker for shaking, and then moved into the headspace solid phase microextraction platform, first extracted at 50-70°C for 30-60 minutes, and then desorbed at 240-260°C for 2-5 minutes; after desorption, the eight standard mixed solutions are detected by gas chromatography-mass spectrometry, and then a standard curve is drawn based on the detected peak areas and the concentrations of the eight standard mixed solutions; Step S3: data processing; The pre-treated sample obtained in step S1 is detected by gas chromatography-mass spectrometry to obtain the quantitative ion peak area of ​​geosmin or 2-methylisoborneol and the quantitative ion peak area of ​​2-isobutyl-3-methoxypyrazine, and then the concentration of geosmin or 2-methylisoborneol in the sample is calculated based on the standard curve intercept and the standard curve slope obtained from the standard curve in step S2 and the concentration of the 2-isobutyl-3-methoxypyrazine solution in the sample.

2. The method for determining odor substances by using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry according to claim 1, characterized in that The mass of sodium chloride described in step S1, the volume of the water sample containing geosmin and 2-methylisoborneol, the volume of the 2-isobutyl-3-methoxypyrazine solution and the volume of the sodium thiosulfate solution are in the ratio of (4 to 10) g: 10 mL: 10 μL: 100 μL.

3. The method for determining odor substances by using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry according to claim 1, characterized in that In both steps S1 and S2, heating is performed in a water bath at 40-70° C. for 3-5 minutes.

4. The method for determining odor substances by using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry according to claim 1, characterized in that The shaking time in steps S1 and S2 is 3 to 5 minutes.

5. The method for determining odor substances by using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry according to claim 1, characterized in that The chromatographic bottle described in step S1 is baked at 100-120° C. for 0.5 h-1 h before use.

6. The method for determining odor substances by using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry according to claim 1, characterized in that The extracted fibers in steps S1 and S2 are aged at 230° C. to 270° C. for 1 to 1.5 hours before use.

7. The method for determining odor substances by using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry according to claim 1, characterized in that In steps S2 and S3, the pre-treated samples are filtered through a 0.45 μm filter membrane before being tested.

8. The method for determining odor substances by using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry according to claim 1, characterized in that The process parameters of chromatography and mass spectrometry of the gas chromatography-mass spectrometer in step S2 and step S3 are as follows: the gas chromatography temperature program is first kept at 45°C for 3.5 min, then heated to 250°C at a heating rate of 8°C / min, and kept at 250°C for 5 min; the injection mode is splitless injection, and the carrier gas flow rate is 1 mL / min; the ion source temperature is 230°C; the mass spectrometer uses an electron ionization source for ionization, and the standard electron energy is 70 eV; the scanning mode is selected ion detection.

9. The method for determining odor substances by using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry according to claim 1, characterized in that The concentration calculation formula of geosmin or 2-methylisoborneol in step S2 and step S3 is: r i =(A i / A is -a i )×ρ is / b i ; Where: i Indicates the concentration of geosmin or 2-methylisoborneol in the sample, in ng / L; A i Indicates the quantitative ion peak area of ​​geosmin or 2-methylisoborneol in the sample, A is Indicates the quantitative ion peak area of ​​2-isobutyl-3-methoxypyrazine in the sample, α i represents the standard curve intercept, ρ is Indicates the concentration of 2-isobutyl-3-methoxypyrazine in the sample, in ng / L; b i represents the slope of the standard curve; The specific calculation method of the concentration of geosmin or 2-methylisoborneol in step S3 is as follows: 1) Detecting the eight standard mixed solutions in step S2 using a gas chromatography-mass spectrometer to obtain the quantitative ion peak areas A of geosmin or 2-methylisoborneol in the eight standard mixed solutions respectively. i and the quantitative ion peak area A of 2-isobutyl-3-methoxypyrazine is , then A i / A is As the y-axis, ρ i / ρ is As the x-axis, the standard curve is drawn, and the intercept α of the standard curve is obtained. i and the slope of the standard curve b i ; 2) Detecting the pre-treated sample obtained in step S1 by gas chromatography-mass spectrometry to obtain the quantitative ion peak area A of geosmin or 2-methylisoborneol i and the quantitative ion peak area A of 2-isobutyl-3-methoxypyrazine is , and then the standard curve intercept α obtained from the standard curve in step S2 i and the slope of the standard curve b i , and then according to the concentration of 2-isobutyl-3-methoxypyrazine solution in the sample is , calculate the concentration of geosmin or 2-methylisoborneol in the sample ρ i .

10. The method for determining odor substances by using sodium thiosulfate pretreatment combined with headspace solid phase microextraction and gas chromatography-mass spectrometry according to claim 1, characterized in that The volume ratio of the 2-isobutyl-3-methoxypyrazine solution described in step S2 to the sodium thiosulfate solution is 1:10.

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