阅读说明：本技术 一种快速测定水中n-亚硝基二甲胺浓度的方法 (Method for rapidly determining concentration of N-nitrosodimethylamine in water ) 是由 陈忠林 沈琳璐 沈吉敏 王斌远 康晶 于 2020-12-14 设计创作，主要内容包括：一种快速测定水中N-亚硝基二甲胺浓度的方法,本发明涉及高效萃取和检测水中N-亚硝基二甲胺浓度的方法领域。本发明要解决现有饮用水中NDMA测定方法操作复杂且耗时长的技术问题。方法：一、将目标水样进行过滤处理；二、设置仪器；三、萃取；四、切换六通阀改变管路流向,洗脱；五、切换六通阀改变管路流向,分离并检测NDMA,同时活化在线固相萃取柱。本方法可自动实现样品的固相萃取、洗脱分析,无需复杂的手动预处理过程,操作简便快捷、成本适中,克服了传统离线萃取方法存在的问题,可实现快速准确地检测水样中NDMA浓度。本发明用于快速测定水中N-亚硝基二甲胺浓度。(The invention discloses a method for rapidly determining the concentration of N-nitrosodimethylamine in water, and relates to the field of methods for efficiently extracting and detecting the concentration of N-nitrosodimethylamine in water. The invention aims to solve the technical problems of complex operation and long time consumption of the existing NDMA (Newcastle disease Virus-associated antigen) determination method in drinking water. The method comprises the following steps: firstly, filtering a target water sample; secondly, setting an instrument; thirdly, extracting; fourthly, switching the six-way valve to change the flow direction of the pipeline and eluting; and fifthly, switching the six-way valve to change the flow direction of the pipeline, separating and detecting the NDMA, and activating the online solid-phase extraction column. The method can automatically realize solid-phase extraction and elution analysis of the sample, does not need a complex manual pretreatment process, is simple, convenient and quick to operate and moderate in cost, overcomes the problems of the traditional off-line extraction method, and can realize quick and accurate detection of the NDMA concentration in the water sample. The method is used for rapidly determining the concentration of N-nitrosodimethylamine in water.)

1. A method for rapidly determining the concentration of N-nitrosodimethylamine in water is characterized by comprising the following steps:

firstly, filtering a target water sample;

selecting a high performance liquid chromatograph provided with a double-ternary gradient pump, and installing an online solid phase extraction column and an analysis column, wherein the high performance liquid chromatograph is provided with a six-way valve;

thirdly, after the six-way valve is arranged, the extraction mobile phase is pumped in by a 2# pump, the target water sample treated in the first step is injected into the pipeline by the automatic sample injector, and the analysis mobile phase is pumped in by the 1# pump; the extraction mobile phase pushes a target water sample to be extracted through an online solid-phase extraction column, and N-nitrosodimethylamine in the target water sample is enriched in the online solid-phase extraction column;

fourthly, switching the six-way valve to change the flow direction of the pipeline, analyzing the mobile phase to reversely elute the N-nitrosodimethylamine enriched in the online solid-phase extraction column in the third step, and enabling the N-nitrosodimethylamine to enter the analysis column;

fifthly, switching the six-way valve to change the flow direction of the pipeline, analyzing the flow, and successively pushing N-nitrosodimethylamine in the analysis column in the separation step IV to enter a detector for detection; and simultaneously pumping an activated mobile phase into the on-line solid phase extraction column by a No. 2 pump to activate and regenerate the on-line solid phase extraction column, thereby completing the method.

2. The method of claim 1, wherein the packing material of the on-line solid phase extraction column of step two is polar modified octadecylsilane chemically bonded silica; the filling material of the analytical column is high-purity porous silica gel particles.

3. The method of claim 1, wherein the sample volume of the target water sample in step three is 0.5-2.5 mL.

4. The method of claim 1, wherein the analysis mobile phase in step three is acetonitrile aqueous solution with a volume percentage of 5-30% or methanol aqueous solution with a volume percentage of 10%, the extraction mobile phase is ultra-pure water, and the flow velocity of the extraction mobile phase is controlled to be 0.5-3.0 mL/min.

5. The method for rapidly determining the concentration of N-nitrosodimethylamine in water according to claim 1, wherein the operation of the four-step switching six-way valve is performed when the extraction time of the three-step extraction is controlled to be 1.5-3.0 min.

6. The method for rapidly determining the concentration of N-nitrosodimethylamine in water according to claim 1, wherein the elution flow rate in step four is controlled to be 1.0-4.0 mL/min.

7. The method for rapidly determining the concentration of N-nitrosodimethylamine in water according to claim 1, wherein the operation of switching the six-way valve in the fifth step is performed when the elution time of the fourth step is controlled to be 1.5-4.0 min.

8. The method for rapidly determining the concentration of N-nitrosodimethylamine in water according to claim 1, wherein the flow rate of the activated mobile phase in step five is controlled to be 0.5-3.0 mL/min.

9. The method of claim 1, wherein the activating mobile phase in step five is ultrapure water or acetonitrile in water with a volume percentage of 5% to 20%.

10. The method of claim 1, wherein the detection wavelength of step five is 226nm to 232 nm.

Technical Field

The invention relates to the field of methods for efficiently extracting and detecting the concentration of N-nitrosodimethylamine in water.

Background

During the treatment of drinking water using chlorine disinfection, chloramine disinfection, or ozone disinfection processes, a novel nitrogenous disinfection by-product, N-Nitrosodimethylamine (NDMA), is often produced. NDMA is stable in property and strong in toxicity, and is difficult to remove once generated in water, thereby causing potential threat to the safety of drinking water. In response to the frequent detection of NDMA in water, some countries and regions, canada, usa, japan, etc., have developed standards to control their concentrations in drinking water, and the World Health Organization (WHO) also recommends that NDMA concentrations in drinking water not exceed 100 ng/L. In recent years, China pays more attention to the safety problem of drinking water, the requirement on the drinking water is gradually transited from 'qualified water' to 'high-quality water', some developed regions successively release local drinking water quality standards which are strict with the national standards on the basis of the national standards, and NDMA is listed as one of key indexes for improving the water quality. NDMA is listed as an unconventional index and a limit value of 100ng/L is specified in the 'domestic drinking water quality standard' in Shanghai city in China (DB 31/T1091-. In addition, NDMA is also listed as a high-quality drinking water recommendation index in the Key Water quality index control Standard of waterworks in city of province of Jiangsu (DB 32/T3701-2019), and a limit value of 100ng/L is also adopted.

At present, China has not established a standard determination method of NDMA in drinking water. Because NDMA is a small molecular compound and has strong polarity, the detection difficulty is high. The most common analysis method for determining the NDMA concentration in water is to perform traditional off-line solid phase extraction, separate through liquid phase or gas phase, and then enter instruments such as a mass spectrometer for analysis, the method is complex in operation and long in time consumption, large-volume samples (200 plus 1000mL) are required for enrichment, isotope-labeled NDMA standard products (d6-NDMA) are required to be added to each sample to reduce the loss caused by pretreatment, the instruments and consumables can only be used once, and the cost is high. As for the current national conditions of China, the method cannot be popularized and detected in a wider range.

Disclosure of Invention

The invention provides a method for rapidly determining the concentration of N-nitrosodimethylamine in water, aiming at solving the technical problems of complex operation and long time consumption of the existing NDMA determination method in drinking water.

A method for rapidly determining the concentration of N-nitrosodimethylamine in water, which comprises the following steps:

firstly, filtering a target water sample;

selecting a high performance liquid chromatograph provided with a double-ternary gradient pump, and installing an online solid phase extraction column and an analysis column, wherein the high performance liquid chromatograph is provided with a six-way valve;

thirdly, after the six-way valve is arranged, the extraction mobile phase is pumped in by a 2# pump, the target water sample treated in the first step is injected into the pipeline by the automatic sample injector, and the analysis mobile phase is pumped in by the 1# pump; the extraction mobile phase pushes a target water sample to be extracted through an online solid-phase extraction column, and N-nitrosodimethylamine in the target water sample is enriched in the online solid-phase extraction column;

fourthly, switching the six-way valve to change the flow direction of the pipeline, analyzing the mobile phase to reversely elute the N-nitrosodimethylamine enriched in the online solid-phase extraction column in the third step, and enabling the N-nitrosodimethylamine to enter the analysis column;

fifthly, switching the six-way valve to change the flow direction of the pipeline, analyzing the flow, and successively pushing N-nitrosodimethylamine in the analysis column in the separation step IV to enter a detector for detection; and simultaneously pumping an activated mobile phase into the on-line solid phase extraction column by a No. 2 pump to activate and regenerate the on-line solid phase extraction column, thereby completing the method.

Wherein the flow direction of the pipeline in the step three is the same as that of the pipeline in the step five.

And step three, selecting a proper target water sample injection volume, and injecting the sample through a large-volume sample injector, wherein the selected injection volume is 0.5-2.5mL so as to ensure higher recovery rate.

And step three, carrying out solid-phase extraction by adopting a polar embedded C18 online solid-phase extraction column which is high in extraction efficiency and good in target object retention effect, and reasonably purifying and enriching a target water sample with a certain volume by using extraction flow of an online solid-phase extraction pipeline. The selected on-line solid phase extraction column adopts polarity modified octadecylsilane chemically bonded silica as a filler, and the column temperature is between 25 and 30 ℃. The filler in the solid phase extraction column is activated by adopting ultrapure water, so that the carbon chain is fully extended, impurities are removed, and a hydrophilic environment is created for the solid phase extraction column. Ultrapure water is selected as a sample introduction extraction mobile phase, a sample passes through an online solid phase extraction column at a certain flow rate, and the flow rate of the water sample is controlled to be 0.5mL/min-3.0mL/min, so that an analysis sample is retained on a stationary phase, and the purification and separation of analysis substances are realized.

And fourthly, switching the six-way valve of the instrument at a reasonable time point to change the flow direction of the pipeline, reasonably using a mobile phase solvent of the analysis pipeline to reversely elute the N-nitrosodimethylamine, and controlling the time point of the first switching of the six-way valve to be 1.5-3.0min of extraction, so that the sample is kept on the online solid-phase extraction column before the valve is switched and does not flow out along with the mobile phase.

Step four, selecting a proper solvent as a mobile phase during elution to ensure that the target can be completely eluted from the solid phase extraction column. In order to improve the elution efficiency, the NDMA is eluted by acetonitrile and water or methanol and water according to a certain proportion. The elution flow rate is controlled to be 1.0 mL/min-4.0 mL/min.

And fifthly, switching the six-way valve of the instrument for the second time at a reasonable time point to change the flow direction of the pipeline, wherein one part of the pipeline continuously sends the target object to the analytical column for separation from the elution mobile phase and enters a proper detector for detection, and the other pipeline activates and regenerates the on-line solid phase extraction column. And controlling the time point of the second six-way valve switching to be 1.5-4.0 min of elution. The selected analytical column adopts high-purity porous silica gel particles as a filler, adopts an amido-embedded bonding phase, and controls the column temperature to be about 25-35 ℃. NDMA is detected by liquid chromatography, a high performance liquid chromatograph is adopted, and the detection wavelength is 226nm-232 nm.

The invention has the beneficial effects that:

after a target water sample is filtered, an online solid-phase extraction column with high extraction efficiency and good retention effect is selected, a mobile phase solvent of an online solid-phase extraction pipeline is reasonably used for purifying and enriching the target water sample with a certain volume, the first six-way valve of the instrument is switched at a reasonable time point to change the pipeline flow direction, the mobile phase solvent of an analysis pipeline is reasonably used for eluting a target object, the target object is separated by the analysis column and enters a proper detector for detection, the second six-way valve of the instrument is switched at a reasonable time point to change the pipeline flow direction, and the online solid-phase extraction column is activated and regenerated to realize multiple sample injection detection.

Through the method, the rapid solid-phase extraction and elution analysis of the sample can be realized, the complex manual pretreatment process is not needed, the operation is simple, convenient and rapid, the cost is moderate, the consumption of the organic solvent is greatly reduced, in addition, the online solid-phase extraction column used in the treatment process can be repeatedly used for many times through activation and regeneration, the time consumption is short, the efficiency is high, the precision and the accuracy are good, the problems of large solvent consumption, long time consumption and expensive instruments and consumables in the traditional offline extraction method are solved, the blank that the online solid-phase extraction column is used for rapidly extracting and enriching the NDMA in the water environment is filled, and the NDMA concentration in the water sample can be rapidly and accurately detected.

The experimental result shows that the method has the advantages that the recovery rate of the NDMA is 96.62-111.75 percent, the relative standard deviation is less than or equal to 9.50 percent, and the requirement of quantitative analysis on the NDMA is met

The method is used for rapidly determining the concentration of N-nitrosodimethylamine in water.

Drawings

FIG. 1 is a schematic flow diagram of the first step three and the fifth step five of the embodiment;

FIG. 2 is a schematic flow diagram of a four-line flow in one embodiment;

FIG. 3 is a liquid chromatogram for detecting a water sample with NDMA concentration of 20. mu.g/L by using the methods of the first and second examples;

FIG. 4 is a graph of the analysis of the conversion of ozone (1mg/L) in tap water to oxidize NDMA precursors (20 μm) to NDMA in one example.

Detailed Description

The technical solution of the present invention is not limited to the specific embodiments listed below, and includes any combination of the specific embodiments.

The first embodiment is as follows: the embodiment provides a method for rapidly determining the concentration of N-nitrosodimethylamine in water, which comprises the following steps:

firstly, filtering a target water sample;

selecting a high performance liquid chromatograph provided with a double-ternary gradient pump, and installing an online solid phase extraction column and an analysis column, wherein the high performance liquid chromatograph is provided with a six-way valve;

thirdly, after the six-way valve is arranged, the extraction mobile phase is pumped in by a 2# pump, the target water sample treated in the first step is injected into the pipeline by the automatic sample injector, and the analysis mobile phase is pumped in by the 1# pump; the extraction mobile phase pushes a target water sample to be extracted through an online solid-phase extraction column, and N-nitrosodimethylamine in the target water sample is enriched in the online solid-phase extraction column;

fourthly, switching the six-way valve to change the flow direction of the pipeline, analyzing the mobile phase to reversely elute the N-nitrosodimethylamine enriched in the online solid-phase extraction column in the third step, and enabling the N-nitrosodimethylamine to enter the analysis column;

fifthly, switching the six-way valve to change the flow direction of the pipeline, analyzing the flow, and successively pushing N-nitrosodimethylamine in the analysis column in the separation step IV to enter a detector for detection; and simultaneously pumping an activated mobile phase into the on-line solid phase extraction column by a No. 2 pump to activate and regenerate the on-line solid phase extraction column, thereby completing the method.

The second embodiment is as follows: the first difference between the present embodiment and the specific embodiment is: secondly, filling a polar modified octadecylsilane chemically bonded silica into the online solid-phase extraction column; the filling material of the analytical column is high-purity porous silica gel particles. The rest is the same as the first embodiment.

The third concrete implementation mode: the present embodiment differs from the first or second embodiment in that: and step three, the sample volume of the target water sample is 0.5-2.5 mL. The other is the same as in the first or second embodiment.

The fourth concrete implementation mode: the difference between this embodiment mode and one of the first to third embodiment modes is: and step three, the analysis mobile phase is acetonitrile aqueous solution with the volume percentage of 5-30% or methanol aqueous solution with the volume percentage of 10%, the extraction mobile phase is ultrapure water, and the flow speed of the extraction mobile phase is controlled to be 0.5-3.0 mL/min. The others are the same as in one of the first to third embodiments.

The fifth concrete implementation mode: the difference between this embodiment and one of the first to fourth embodiments is: and C, when the extraction time in the third step is controlled to be 1.5-3.0min, the operation of switching the six-way valve in the fourth step is carried out. The other is the same as one of the first to fourth embodiments.

The sixth specific implementation mode: the difference between this embodiment and one of the first to fifth embodiments is: and step four, controlling the elution flow rate to be 1.0-4.0 mL/min. The other is the same as one of the first to fifth embodiments.

The seventh embodiment: the difference between this embodiment and one of the first to sixth embodiments is: and when the elution time of the fourth step is controlled to be 1.5-4.0 min, the operation of switching the six-way valve in the fifth step is carried out. The other is the same as one of the first to sixth embodiments.

The specific implementation mode is eight: the present embodiment differs from one of the first to seventh embodiments in that: and fifthly, controlling the flow rate of the activated mobile phase to be 0.5-3.0 mL/min. The other is the same as one of the first to seventh embodiments.

The specific implementation method nine: the present embodiment differs from the first to eighth embodiments in that: and step five, the activated mobile phase is ultrapure water or acetonitrile water solution with the volume percentage of 5-20%. The rest is the same as the first to eighth embodiments.

The detailed implementation mode is ten: the present embodiment differs from one of the first to ninth embodiments in that: and fifthly, the detection wavelength is 226nm-232 nm. The other is the same as one of the first to ninth embodiments.

The following examples were used to demonstrate the beneficial effects of the present invention:

the first embodiment is as follows:

instrument and reagent

(1) Instrument for measuring the position of a moving object

A Saimeri Ultimate3000 high performance liquid chromatograph, comprising a double ternary gradient pump (equipped with an online degasser), an autosampler (equipped with a 2500 μ L sample injection assembly), a column oven, a Diode Array Detector (DAD); merck Milli-Q ultrapure water instrument.

(2) Preparation of standard solution

Taking 100 mul of NDMA standard substance with the concentration of 1g/L into a 10mL brown polypropylene volumetric flask by adopting a 100 mul microsyringe, and fixing the volume to a scale mark by using methanol to prepare 10mg/L NDMA stock solution, and storing the NDMA stock solution at minus 10 ℃ in a dark place; before sample measurement, 10mg/L NDMA stock solution is taken, diluted by ultrapure water step by step, and the volume is determined to a scale mark to prepare standard solution with required concentration.

Second, the working conditions of the instrument

High performance liquid chromatography conditions

The high performance liquid chromatography is provided with a double-ternary gradient pump, and a pump 1 and a pump 2 can independently operate, so that the functions of on-line solid-phase extraction and elution analysis are realized. In the embodiment, the mobile phase of the No. 2 pump is ultrapure water, and the flow rate is 1.0 mL/min; the No. 1 pump mobile phase is acetonitrile water solution with the volume percentage content of 5 percent, and the flow rate is 2.0 mL/min. The on-line solid phase extraction column was Hypersil GOLD aQ (4.6x 100mm,3 μm) (Seimer Feishell science, USA) at 25 deg.C; the analytical column was a Thermo Acclaim PA II (4.6X 250mm,5 μm) (Saimer Feishel technologies, USA) with a column temperature of 25 ℃; the detection wavelength was 228 nm.

The embodiment relates to a method for rapidly determining the concentration of N-nitrosodimethylamine in water, which comprises the following steps:

firstly, filtering a target water sample by adopting a glass fiber filter membrane, and filtering the target water sample into an 8mL sample introduction brown bottle by using a 0.22 mu m polyether sulfone water phase needle type filter head;

selecting a high performance liquid chromatograph provided with a double-ternary gradient pump, and installing an online solid phase extraction column and an analysis column, wherein the high performance liquid chromatograph is provided with a six-way valve;

setting a six-way valve at the position shown in the figure 1, pumping an extraction mobile phase into the pipeline by a # 2 pump after the six-way valve is set, injecting the target water sample treated in the step one into the pipeline by an automatic sample injector, and pumping an analysis mobile phase into the pipeline by the # 1 pump; the extraction mobile phase pushes a target water sample to be extracted through an online solid-phase extraction column, and N-nitrosodimethylamine in the target water sample is enriched in the online solid-phase extraction column; the extraction mobile phase is ultrapure water, and the analysis mobile phase is acetonitrile water solution with the volume percentage of 5%;

fourthly, when the extraction is carried out for 2min in the third step, a six-way valve is switched to change the flow direction of a pipeline, the six-way valve is positioned at the position shown in the figure 2, the analysis mobile phase carries out reverse elution on the N-nitrosodimethylamine enriched in the on-line solid phase extraction column in the third step, and the N-nitrosodimethylamine enters an analysis column;

and fifthly, when the elution is carried out for 2min in the fourth step, switching a six-way valve to change the flow direction of the pipeline, enabling the six-way valve to be located at the position shown in the figure 1, continuously pushing the N-nitrosodimethylamine in the analysis column in the fourth step by the analysis mobile phase, enabling the N-nitrosodimethylamine to enter a DAD detector for detection and analysis, simultaneously pumping the activated mobile phase into the on-line solid-phase extraction column by a No. 2 pump, activating and regenerating the on-line solid-phase extraction column for 6min, and finishing the method.

Linear response range of the method of the present embodiment

NDMA standards were diluted to 0.1. mu.g/L, 0.2. mu.g/L, 0.5. mu.g/L, 0.8. mu.g/L, 1. mu.g/L, 5. mu.g/L, and 20. mu.g/L, and the measurement was carried out by the method of example one. Drawing a standard curve by taking the concentration of NDMA as an abscissa (x, mu g/L) and the peak area as an ordinate (y, mAU multiplied by min) to obtain a regression equation y of 0.8895x +0.1882 (R)²＝0.9995)。

Analysis of precision, accuracy, recovery of example one

NDMA standard solutions were added to ultrapure water samples and tap water samples, respectively, and quantitative analysis was performed according to the method of example one, with 7 parallel experiments for each addition concentration, and the relative standard deviation and recovery rate were calculated. The recovery and relative standard deviation of NDMA at different spiking concentrations (0.2. mu.g/L, 0.5. mu.g/L, 1. mu.g/L, 5. mu.g/L, 20. mu.g/L) are shown in Table 1.

TABLE 1 NDMA normalized recovery and relative standard deviation

Experimental results show that the method is adopted for analysis, the recovery rate range of the NDMA is 96.62-111.75%, the relative standard deviation is less than or equal to 9.50%, and the requirement of quantitative analysis on the NDMA is met.

Practical application

And (3) taking a laboratory tap water sample for analysis, wherein NDMA in the tap water is not detected within the detection limit of the method. A simulated water distribution experiment was performed using tap water, and the formation of NDMA by oxidation of various NDMA precursors (20 μm) with ozone (1mg/L) (pH 7.0) was measured. FIG. 4 is a graph of the analysis of the conversion of ozone (1mg/L) in tap water to oxidize NDMA precursors (20 μm) to NDMA in one example, whereinRepresents the traditional off-line solid phase extraction-liquid chromatogram-triple quadrupole mass spectrometry (SPE-HPLC-MS/MS) detection method,representing the method of this example, as shown in FIG. 3, the molar conversion rates of precursors with lower molar yields of NDMA, such as dacarbazine and methylene orange, are 0.0041% and 0.0020%, respectively; the NDMA molar conversion rate of the precursor with higher molar yield, such as butyryl hydrazine, unsym-dimethyl hydrazine and the like, can reach 2.47 percent and 4.38 percent. Meanwhile, the experimental result is more consistent with the result measured by the traditional off-line solid phase extraction-liquid chromatography-triple quadrupole mass spectrometry (SPE-HPLC-MS/MS) detection method, which shows that the method can accurately measure the NDMA content in the water.

In the experimental process, a traditional test method is adopted, and about 15 hours are needed for detecting 20 samples from the activation of SPE small columns, the enrichment, elution and nitrogen blowing of the samples, the sample treatment and the liquid chromatography mass spectrometry detection; the method provided by the invention can obtain the detection result in about 4 hours. The method greatly saves the analysis cost and labor force, shortens the analysis time and improves the working efficiency.

Example two:

the difference between this example and the first example is that the mobile phase of the pump # 1 is 10% by volume methanol aqueous solution, and the other conditions are the same as the first example.

The liquid chromatogram for detecting a water sample with NDMA concentration of 20 mu g/L by using the methods of the first embodiment and the second embodiment is shown in figure 3, wherein '-' represents the first embodiment, and '…' represents the second embodiment, and as can be seen from the figure, the characteristic peak of NDMA detected by the method has good peak-out effect and good peak shape, and can be accurately quantified by the peak area or peak height.

Example three:

the difference between this example and the first example is that the flow rate of the mobile phase of the 1# pump is 1.5mL/min, and the other conditions are the same as those in the first example.

Example four:

the difference between the embodiment and the embodiment I is that when the extraction in the step III is controlled for 2.3min, the operation of switching the six-way valve in the step IV is carried out; and (4) controlling the elution in the step four for 1.7min, switching the six-way valve in the step five, and keeping the pumping time of the mobile phase in the step five for 6min, wherein other conditions are the same as those in the first embodiment.

Example five:

the difference between the embodiment and the embodiment I is that when the extraction in the step III is controlled for 2.0min, the operation of switching the six-way valve in the step IV is carried out; and (4) controlling the elution in the step four for 4.0min, switching the six-way valve in the step five, and keeping the pumping time of the flowing phase in the step five for 4.0min, wherein other conditions are the same as those in the first embodiment.

Example six:

the difference between the embodiment and the embodiment I is that when the extraction in the step III is controlled for 2.5min, the operation of switching the six-way valve in the step IV is carried out; and (4) controlling the elution in the step four for 4.0min, switching the six-way valve in the step five, and keeping the pumping time of the flowing phase in the step five for 3.5min, wherein other conditions are the same as those in the first embodiment.

Example seven:

the difference between this embodiment and the first embodiment is that the detection wavelength of the detection analysis of the DAD detector is 230nm, and the other conditions are the same as those in the first embodiment.

Example eight:

this example is different from the first example in that the concentration of NDMA is plotted on the abscissa (x, μ g/L) and the peak height is plotted on the ordinate (y, mAU) to obtain a standard curve, and the peak height is used for quantification.