A direct detection method based on non-polar solvents and ionic liquids

Abstract

The application discloses a direct detection method based on non-polar solvent and ionic liquid, comprising the following steps: (1) establishing a working curve by using an isotope internal standard method: I. preparing a series of samples containing an isotope internal standard; II. assembling an atmospheric pressure sound velocity spray ionization device; III. cutting a sample to be detected and placing the sample on a glass plate sample table, opening a gas source and a mass spectrometer, setting mass spectrometer parameters and detecting; IV. quantifying by using an isotope internal standard method; (2) detecting an actual sample by using the method of step III. The application applies non-polar solvent to surface direct analysis, introduces a double-cationic ionic liquid, and improves the conductivity and ionization efficiency of the liquid.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of detection, in particular to a method for directly detecting perfluorinated compounds based on non-polar solvent desorption and electrostatic binding of dicationic ionic liquid. BACKGROUND

[0002] Mass spectrometry has the characteristics of fast analysis, strong specificity and high sensitivity, and has been favored in the field of analysis and detection. Ion source is an important part of mass spectrometer, and the innovation of ionization technology has expanded the application range of mass spectrometer and promoted the leap development of mass spectrometer. Open ionization technology refers to a new technology for directly ionizing and mass spectrometric detection of samples without or only with simple sample pretreatment and without chromatographic separation. Open ionization technology has the characteristics of convenience and rapidity. Since Professor Cooks of Purdue University in the United States first proposed desorption electrospray ionization in 2004, many kinds of open ionization technologies have been developed. However, for direct analysis of solid sample surface, there are still problems such as serious matrix interference and low desorption efficiency. SUMMARY

[0003] The technical problem to be solved by the present application is to provide a method for directly detecting and analyzing perfluorinated compounds by simultaneously using non-polar solvent desorption and electrostatic binding of dicationic ionic liquid.

[0004] A direct detection method based on non-polar solvent and ionic liquid, comprising the following steps:

[0005] (1) Establishing a working curve by using isotope internal standard method:

[0006] I. Preparation of a series of samples containing isotope internal standard;

[0007] II. Preparation of a core borosilicate glass capillary, 20 microliters of 20 micromole / liter dicationic ionic liquid bis-(3-methyl-1-imidazole) butylene difluoride salt [C4(MIM)2]F2 in n-hexane solution is added, and a normal pressure acoustic velocity spray ionization device is assembled;

[0008] III. The sample to be detected is cut and placed on the glass plate sample table, the gas source and mass spectrometer are turned on, and the mass spectrometry parameters are set for detection; the sample to be detected is a food packaging material;

[0009] IV. Quantification by isotope internal standard method;

[0010] (2) Detecting the actual sample by the method of step III.

[0011] The sound wave spray ionization device is prior art and is not within the protection scope of the present application, the device part of the present application only improves the angle of the sample table, and a new detection method is developed by using the improved device.

[0012] The direct detection method based on the non-polar solvent and the ionic liquid, wherein the method is used for detecting perfluorinated compounds.

[0013] The direct detection method based on the non-polar solvent and the ionic liquid, wherein parameters of the mass spectrometer are set as follows:

[0014] A positive ion detection mode;

[0015] Voltages of a transmission capillary inlet and outlet are 2.0 kilovolts and 2.5 kilovolts respectively;

[0016] A dry gas temperature is 180 degrees Celsius;

[0017] A dry gas flow rate is 4 liters per minute;

[0018] A data acquisition mode is a multiple reaction monitoring mode.

[0019] The direct detection method based on the non-polar solvent and the ionic liquid, wherein secondary mass spectrometry parameters are as follows:

[0020]

[0021] The direct detection method based on the non-polar solvent and the ionic liquid, wherein a preparation method of the glass capillary is as follows: a glass capillary blank is horizontally fixed on a tension arm of a P-1000 type microelectrode drawing instrument, the middle of the capillary is heated by a filament until the glass is melted and broken, two spray capillaries with sharp ends are generated, the inner diameter of the sharp end is 5 micrometers, and the glass capillary is prepared.

[0022] The direct detection method based on the non-polar solvent and the ionic liquid, wherein an incident angle of the glass capillary is 45 degrees, the glass sample table is arranged at an inclination of 30 degrees, a reflection angle between the glass capillary and the glass sample table is 10 degrees, a distance between the sharp end of the glass capillary and a sample inlet of the mass spectrometer is 3 millimeters, and a distance between the sharp end of the glass capillary and the glass plate sample table is 1 millimeter. A distance, at which the sharp end of the glass capillary extends out of a sleeve, is 3 millimeters.

[0023] The direct detection method based on the non-polar solvent and the ionic liquid, wherein nitrogen gas is provided to one end of a three-way vertical head by a nitrogen gas cylinder, and a pressure of the nitrogen gas is set as 6.2 bars.

[0024] The direct detection method based on non-polar solvent and ionic liquid in the application, wherein the sample to be detected is cut into a 1*1 square centimeter square and placed on a glass plate sample stage.

[0025] The direct detection method based on non-polar solvent and ionic liquid in the application, wherein the series sample preparation method containing isotopic internal standards comprises the following steps:

[0026] A blank hamburger wrapping paper is cut into a 1*1 square centimeter square, and a series of mixed standard solutions of perfluorooctanoic acid, perfluorooctane sulfonic acid and isotopic internal standards are prepared with methanol, wherein the concentrations of perfluorooctanoic acid and perfluorooctane sulfonic acid are 1, 2, 5, 10, 20, 50 and 100 nanograms per milliliter respectively, and the concentrations of the two isotopic internal standards are both 20 nanograms per milliliter; 100 microliters of the mixed standard solution is taken with a pipette and uniformly dropped on the cut square hamburger wrapping paper, and naturally dried, thereby obtaining a hamburger wrapping paper sample with a series of standard concentrations of perfluorooctanoic acid and perfluorooctane sulfonic acid and a certain amount of isotopic internal standards.

[0027] The front-end sleeve of the sound wave spray ionization device in the application is a polytetrafluoroethylene sleeve, a coaxial high-speed airflow is generated in the gap between the glass capillary and the polytetrafluoroethylene sleeve, thereby generating a Venturi effect, bringing out the solution in the glass capillary and atomizing it into small droplets. At the same time, the high-speed airflow charges the small droplets due to electrostatic imbalance.

[0028] The charged spray droplets bombard the sample surface, forming a layer of liquid film on the surface and extracting perfluorinated compounds in the sample. At the same time, the gas flow splashes the dicationic ionic liquid and the anion form of perfluorinated compounds to electrostatically combine, which are detected by the mass spectrometer in positive ion mode.

[0029] The direct detection method based on non-polar solvent and ionic liquid in the application is different from the prior art in that:

[0030] Simple open sound wave spray ionization technology can realize ionization of the measured substance through coaxial high flow rate gas of the capillary tip without high voltage and auxiliary heating process, and is the simplest open ionization technology. The solvent-based desorption ionization technology has high requirements for the spray solvent, and in most cases needs water, methanol or mixed solvent with a certain proportion of the two, and it is difficult to realize ionization process for weakly polar or non-polar solvent. However, for the compound with low polarity, according to the similar solubility principle, the extraction ability of the polar solvent for the weakly polar substance is poor. Based on this, we first apply the non-polar solvent to the surface direct analysis, and introduce the double cationic ionic liquid to improve the conductivity of the liquid, so as to improve the ionization efficiency. At the same time, the double cationic ionic liquid as an ionic additive can be combined with the extracted perfluorinated compound through electrostatic interaction. Under the double guarantee of extraction efficiency and ionization efficiency, the detection sensitivity of the method is improved, so as to be used for rapid screening and detection of perfluorinated compounds in samples. The electrostatic combination reaction of the double cationic ionic liquid with perfluorooctanoic acid and perfluorooctane sulfonic acid is as follows:

[0031]

[0032] Electrostatic combination reaction of the double cationic ionic liquid with perfluorooctanoic acid and perfluorooctane sulfonic acid

[0033] The direct detection method based on non-polar solvent and ionic liquid of the application will be further described below in combination with the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 It is the full scan primary mass spectrum of the application for detecting perfluorooctanoic acid and perfluorooctane sulfonic acid in the spiked hamburger packaging paper sample; wherein the embedded part is the secondary mass spectrum and mass spectrum fragmentation diagram of perfluorooctanoic acid and perfluorooctane sulfonic acid;

[0035] Figure 2 It is the influence of different atomization gas pressures on the signal intensity of perfluorooctane sulfonic acid in the application;

[0036] Figure 3 It is the influence of different concentrations of double cationic ionic liquid on the signal intensity of perfluorooctanoic acid and perfluorooctane sulfonic acid in the application;

[0037] Figure 4 It is the contact angle test result of the sample involved in the application;

[0038] Figure 5 It is the influence of different spray solvents on the signal intensity of perfluorooctanoic acid and perfluorooctane sulfonic acid in the application;

[0039] Figure 6Full scan mass spectrum of PFOS in hamburger wrapping paper sample detected by the present application using methanol (a) and n-hexane (b) as spray solvent respectively;

[0040] Figure 7 Secondary mass spectrum of isotopic internal standard of PFOA (a) and PFOS (b) in the present application and working curve of PFOA and PFOS in hamburger wrapping paper sample established by isotopic internal standard method;

[0041] Figure 8 Experimental results of detecting PFOA in a certain popcorn bucket sample in the present application.

[0042] The Chinese translation of all English in the drawings is as follows:

[0043] Relative Abundance: relative abundance; m / z: mass-to-charge ratio; Normalized Intensity: normalized intensity; Intensity: intensity; Concentration: concentration; Pressure: pressure; CA left: left contact angle; CA right: right contact angle; PFOA: perfluorooctanoic acid; PFOS: perfluorooctane sulfonic acid; MeCN: acetonitrile; MeOH: methanol; Me2CO: acetone; IPA: isopropyl alcohol; THF: tetrahydrofuran; EtOAC: ethyl acetate; Hx: n-hexane; Spray Solvent: spray solvent; S / N: signal-to-noise ratio; A / IS: abundance ratio of the analyte to the isotopic internal standard. DETAILED DESCRIPTION

[0044] I. Instruments and devices

[0045] Female threaded straight tee (Shanghai Sivaco Fluid System Technology Co., Ltd. Item No.: SS-200-3TFT), male threaded straight connector (Shanghai Sivaco Fluid System Technology Co., Ltd. Item No.: SS-100-1-2BT); P-1000 microelectrode puller, borosilicate glass capillary (outer diameter 1.5 mm, inner diameter 0.86 mm, length 10 cm) (Sutter Company, USA, Item No.: BF150-86-10); Bruker amaZon ion trap mass spectrometer (Bruker Dalton Corporation, USA); Milli-Q ultrapure water generator (Millipore Corporation, USA); pipette tips used in the experiment are imported (Bio-Rad Corporation, USA).

[0046] Acoustic wave spray ionization device, please refer to the specific structure of the invention patent with application number 201710541576.2, the difference is as follows: the sample stage in the present application is set at an angle of 30 degrees, and the sleeve at the front end of the device is made of a polytetrafluoroethylene sleeve.

[0047] II. Materials and Reagents

[0048] The perfluorooctanoic acid, perfluorooctane sulfonic acid and isotopic internal standard standard materials used in the experiment are commercially available highest purity standard samples, and the perfluorooctanoic acid, perfluorooctane sulfonic acid and isotopic internal standard standard are prepared into 1000 mg / L standard stock solution with methanol, and diluted with methanol according to the need when used; The solvents used are chromatographic pure grade; The dicationic ionic liquid bis-(3-methyl-1-imidazole) butylene difluoride salt is purchased from Shanghai Chengjie Ionic Liquid Co., Ltd., and is prepared and diluted into standard working solution according to the need with different solvents when used.

[0049] III. Detection method

[0050] Example 1

[0051] Establishment of working curve by isotopic internal standard method

[0052] 1. Preparation of series of samples containing isotopic internal standard

[0053] Cut the blank hamburger wrapping paper into 1x1 square centimeter squares, and prepare a series of mixed standard solutions (7) of perfluorooctanoic acid, perfluorooctane sulfonic acid and isotopic internal standard with methanol. The concentrations of perfluorooctanoic acid and perfluorooctane sulfonic acid in the 7 standard solutions are 1, 2, 5, 10, 20, 50, and 100 ng / mL, respectively; The concentrations of the two isotopic internal standards are both 20 ng / mL. Take 100 microliters of the above mixed standard solution with a pipette, and evenly drop it on the above cut square hamburger wrapping paper, and naturally air dry, to obtain a series of hamburger wrapping paper samples containing perfluorooctanoic acid and perfluorooctane sulfonic acid with a certain amount of isotopic internal standard.

[0054] 2. Build device

[0055] A glass capillary was fixed horizontally on the pulling arm of a microelectrode puller (P-1000), and the middle of the capillary was heated by a filament until the glass melted and was pulled apart, resulting in two jetting capillaries with a tip inner diameter of 5 microns; 20 microliters of a 20 micromolar solution of the double-cationic ionic liquid bis-(3-methyl-1-imidazolium) butylene difluoride salt [C4(MIM)2]F2 in n-hexane was added to the capillary using a pipette; the jetting capillary with the pre-added solution was inserted into a 1 / 8 inch stainless steel tee joint and a 1 / 16 adapter and was screwed tightly. The capillary tip was surrounded by a polytetrafluoroethylene sleeve, and the tip extended 3 millimeters out of the sleeve. The incident angle of the simple open sound wave ionization device was adjusted to 45 degrees, the distance between the capillary tip and the inlet of the mass spectrometer was 3 millimeters, the distance between the capillary tip and the glass plate sample stage was 1 millimeter, and the reflection angle between the capillary tip and the glass plate sample stage was 10 degrees; the sample to be tested was placed on the glass plate sample stage; a nitrogen cylinder was used to provide nitrogen to the vertical end of the tee joint, and the pressure of the nitrogen was set to 6.2 bar.

[0056] 3. Mass spectrometric detection

[0057] The parameters of the mass spectrometer were set as follows:

[0058] Positive ion detection mode;

[0059] The voltages at the inlet and outlet of the transfer capillary were 2.0 kilovolts and 2.5 kilovolts, respectively;

[0060] The temperature of the drying gas was 180 degrees Celsius;

[0061] The flow rate of the drying gas was 4 liters per minute;

[0062] Multiple reaction monitoring mode, and the parameters of the secondary mass spectrometric analysis were as follows:

[0063]

[0064] 4. Experimental results

[0065] The primary mass spectra of perfluorooctanoic acid and perfluorooctane sulfonic acid in the spiked hamburger wrapping paper sample are shown in FIG. 1, and the mass spectrometric peaks of perfluorooctanoic acid and perfluorooctane sulfonic acid were detected at m / z 633.1 and 719.1, respectively. Figure 1 Figure 1 The secondary mass spectra and the mass spectrometric fragmentation schematic diagram of perfluorooctanoic acid and perfluorooctane sulfonic acid are shown in FIG. 2.

[0066] 5. Optimization of the pressure of the atomizing gas

[0067] The simple open sound wave ionization technology relies on the coaxial high-speed atomizing gas to charge the small droplets due to electrostatic imbalance, so the pressure of the atomizing gas has a great influence on the ionization efficiency, and therefore the pressure of the atomizing gas was optimized in the experiment.​

[0068] A series of nitrogen pressure (20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120 psi) were set in the experiment, and the signal response intensity of the target perfluorooctane sulfonic acid complex was detected and recorded, and the results are shown in Figure 2 The results show that in the range of 20-90 psi, the signal response intensity increases with the increase of the atomization gas pressure, reaches the maximum at 90 psi, and then decreases with the increase of the atomization gas pressure, so 90 psi is selected as the experimental nitrogen pressure.

[0069] 6. Optimization of the concentration of the dicationic ionic liquid

[0070] The detection mode is changed to positive ion mode due to the electrostatic binding of the dicationic ionic liquid and the anion form of the perfluorinated compound, which significantly improves the detection sensitivity. The concentration of the dicationic ionic liquid has a great influence on the detection results, so the concentration of the dicationic ionic liquid is optimized in this experiment.

[0071] A series of dicationic ionic liquid bis-(3-methyl-1-imidazole) butylene difluoride salt concentrations (5, 10, 15, 20, 25, 30, 35 micromole / liter) were set in the experiment, and the signal intensity of the target was detected and recorded as shown in Figure 3 The experimental results show that in the range of 5-20 micromole / liter, the signal intensity of the target increases with the increase of the concentration of the dicationic ionic liquid, reaches the maximum at 20 micromole / liter, and then there is no obvious change in the signal intensity of the target with the further increase of the concentration of the dicationic ionic liquid, so 20 micromole / liter is selected as the experimental concentration of the dicationic ionic liquid.

[0072] 7. The sample contact angle test involved in the present application

[0073] The physicochemical properties of the sample surface have a great influence on the experimental results. In order to further optimize the experimental conditions, the contact angles of the hamburger packaging paper and the popcorn bucket with water were tested, and the test results are shown in Figure 4 It can be seen from the figure that the contact angles of the hamburger packaging paper (a) and the popcorn bucket (b) are both greater than 90 degrees, i.e. the sample surface shows hydrophobic characteristics.

[0074] 8. Optimization of the spray solvent

[0075] In the simple open spray ionization process, the spray droplets bombard the sample surface to form a liquid film, which extracts the target substance in the sample, so that the subsequent ionization is detected by the mass spectrometer. According to the principle of "like dissolves like", the type of solvent has an important influence on the extraction effect, so the type of spray solvent is optimized in this experiment.

[0076] The experiment set acetonitrile, methanol, acetone, isopropanol, tetrahydrofuran, ethyl acetate and n-hexane as the spray solvent, and other conditions were the same, respectively detected and recorded the comparative target signal intensity as shown in Figure 5 The experimental results show that n-hexane has the best extraction effect on the target due to its strong hydrophobicity, low saturated vapor pressure and other characteristics, and therefore n-hexane is selected as the spray solvent in the experiment.

[0077] In addition, compared with polar solvents such as methanol, n-hexane as a non-polar solvent significantly reduces the matrix interference of surfactants such as alkylphenol polyoxyethylene ether in the simple acoustic wave spray ionization process, and the comparison results are shown in Figure 6

[0078] 9. Solvent surface tension test

[0079] In order to further explain the experimental results of the application, the surface tension of the above different solvents was tested at room temperature 20 degrees Celsius when the concentration of the dicationic ionic liquid was 20 micromole / liter, and the test results are shown in Table 1. The test results show that in general, the smaller the surface tension, the stronger the detection signal intensity.

[0080] Table 1 Surface tension of different solvents when the concentration of the dicationic ionic liquid is 20 micromole / liter

[0081]

[0082] 10. Isotope internal standard method quantification

[0083] The isotope internal standard method was used to establish the working curve of perfluorooctanoic acid and perfluorooctane sulfonic acid, and the secondary mass spectrum of perfluorooctanoic acid (a), perfluorooctane sulfonic acid (b) and its isotope internal standard is shown in Figure 7 The working curve (c) of the method of the application is obtained by plotting the concentration according to the abundance ratio of the test substance to its isotope internal standard, and the results show that the linearity is good in the range of 0.1-100 micrograms / square meter, and the method quantification limit, detection limit and recovery rate are shown in Table 2.

[0084] Table 2 Detection limit, quantification limit and recovery rate of perfluorooctanoic acid and perfluorooctane sulfonic acid in hamburger packaging paper

[0085]

[0086] 11. Actual sample detection

[0087] The perfluorooctanoic acid and perfluorooctane sulfonic acid in 30 pieces of hamburger packaging paper samples and popcorn bucket samples purchased by various means such as fast food store, online shopping, etc. were detected by the method of the application, and the detection results show that perfluorooctanoic acid was detected in a certain popcorn bucket sample, and the quantification results by the isotope internal standard method are as follows Figure 8 ​As shown, according to the established working curve, the perfluorooctanoic acid content is 6.5 micrograms per square meter.

[0088] The above-described embodiments are merely intended to describe the preferred embodiments of the present application, and are not intended to limit the scope of the present application. Various modifications and improvements to the present application made by those skilled in the art, without departing from the design spirit of the present application, shall fall within the scope of protection of the present application.

Claims

1. A direct detection method based on non-polar solvents and ionic liquids, characterized in that: It comprises the following steps:

1. Preparation of series of samples containing isotopic internal standard: Cut the blank hamburger wrapping paper into 1x1 square centimeter squares, and prepare a series of mixed standard solutions of perfluorooctanoic acid, perfluorooctane sulfonic acid and their isotopic internal standards with methanol. The concentrations of perfluorooctanoic acid and perfluorooctane sulfonic acid in the 7 standard solutions are 1, 2, 5, 10, 20, 50, and 100 ng / ml respectively; the concentrations of the two isotopic internal standards are both 20 ng / ml. Take 100 microliters of the above mixed standard solution with a pipette and evenly drop it on the above cut square hamburger wrapping paper, and let it dry naturally. Thus, the hamburger wrapping paper sample with a series of standard concentrations of perfluorooctanoic acid and perfluorooctane sulfonic acid and a certain amount of isotopic internal standard is obtained.

2. Build the device: Fix the glass capillary horizontally on the tension arm of the P-1000 microelectrode drawing instrument, and heat the middle of the capillary with a filament until the glass melts and breaks, producing two sharp spray capillaries with an inner diameter of 5 microns. Add 20 microliters of a 20 micromolar solution of the double-cationic ionic liquid bis-(3-methyl-1-imidazole) butylene difluoride salt [C4(MIM)2]F2 in n-hexane to the capillary with a pipette. Insert the sharp capillary with pre-added spray solution into a 1 / 8 inch stainless steel tee joint and a 1 / 16 adapter and tighten it. The capillary tip is surrounded by a polytetrafluoroethylene sleeve, and the tip extends 3 mm out of the sleeve. Adjust the incident angle of the simple open sound wave spray ionization device to 45 degrees. The distance between the capillary tip and the inlet of the mass spectrometer is 3 mm, and the distance between the capillary tip and the glass plate sample stage is 1 mm. The reflection angle between the capillary tip and the glass plate sample stage is 10 degrees. Place the sample to be tested on the glass plate sample stage. Provide nitrogen gas to the vertical end of the tee joint with a nitrogen cylinder, and set the pressure of the nitrogen gas to 6.2 bar.

3. Mass spectrometric detection Set the mass spectrometer parameters as follows: Positive ion detection mode; The voltages at the inlet and outlet of the transmission capillary are 2.0 kV and 2.5 kV respectively; The drying gas temperature is 180 degrees Celsius; The drying gas flow rate is 4 liters per minute; Multi-reaction monitoring mode, the secondary mass spectrometry parameters are shown in the table above.

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