Patent Publication Number: US-2022229027-A1

Title: Detection method based on supercritical fluid chromatography and post-column ionic liquid charge complexation

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International Patent Application No. PCT/CN2021/070260, filed on Jan. 5, 2021, which itself claims priority to and benefit of Chinese Patent Application No. 202010085113.1 filed on Feb. 10, 2020 in the State Intellectual Property Office of P.R. China. The disclosure of each of the above applications is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the technical field of inspection and detection, and in particular relates to a detection method based on supercritical fluid chromatography (SFC) and post-column dicationic ionic liquid (DIL) charge complexation. 
     BACKGROUND OF THE INVENTION 
     Perfluorinated compounds (PFCs) are a class of organic compounds in which hydrogen atoms in hydrocarbons and their derivatives are replaced by fluorine atoms. PFCs are commonly used in household decorative paper, food packaging materials, and surfactants in fire-fighting foams. In textile manufacturing, PFCs are broadly used as finishing agents and surfactants. However, PFCs have a strong bioaccumulation effect. After entering organisms, PFCs will be distributed in serum and liver, resulting in a variety of toxic effects on organisms, such as liver toxicity, cardiovascular toxicity, developmental toxicity, immune system toxicity, endocrine disruption, and potential carcinogenicity. 
     Currently, solid-phase microextraction, ultrasonic extraction, and Soxhlet extraction have been reported for textile sample pretreatment. Nevertheless, these methods are not only time-consuming and use large amounts of organic solvents, but also have a negative impact on the environment and laboratory personnel. As nanostructured liquids, supramolecular solvents (SUPRASs) can improve the extraction efficiency of analytes, reduce extraction time, and consume less solvent. SUPRASs have progressively become a suitable green alternative to conventional organic solvents for sample pretreatment. 
     SUMMARY OF THE INVENTION 
     The technical problem to be solved in the present invention is to provide a detection method using SFC separation, post-column DIL-based charge complexation, and electrospray ionization-mass spectrometry (ESI-MS). 
     Compressed carbon dioxide is the primary mobile phase used in SFC, showing advantageous properties of lower cost, safety benefits, and environmental friendliness characteristics compared to organic solvents. Moreover, SFC is capable of achieving rapid and efficient separation with high reproducibility and reduction in consumption of organic solvents. 
     Negative-ion ESI-MS has been commonly used for the detection of PFCs. However, negative-ion ESI-MS is less sensitive compared to positive-ion ESI-MS. Room temperature ionic liquids are salts with melting points below 100° C. and have been regarded as promising green solvents because of the properties such as thermal stability, nonflammability, low viscosity, good electric conductivity, and tunable immiscibility. DILs are a novel class of ionic liquids consisting of a dication paired with two anions and can be used as post-column ion-pairing reagents for SFC. 
     The invention relates to a detection method based on SFC and post-column DIL charge complexation, which includes the following steps: 
     (1) The SUPRAS was prepared by mixing heptanol, tetrahydrofuran, and water; 
     (2) Sample pretreatment: the SUPRAS was used to extract the sample for subsequent analysis; 
     (3) Analysis of PFCs using supercritical fluid chromatography-mass spectrometry (SFC-MS). 
     According to the detection method based on SFC and post-column DIL charge complexation, the preparation of SUPRAS included the following steps: Aliquots of 3 mL of heptanol, 4 mL of tetrahydrofuran, and 33 mL of water were transferred into a 50-mL glass centrifuge tube, mixed for 3 min on a vortexer, and centrifuged at 3000 r/min for 10 min. The resulting SUPRAS supernatant was collected with a glass syringe and stored at 4° C. 
     According to the detection method based on SFC and post-column DIL charge complexation, the sample pretreatment process included the following steps: Aliquots of 0.50 g of samples were weighed in a 10-mL centrifuge tube, into which 4 mL of the obtained SUPRAS was then added. After vortexing for 3 min, the extract was centrifuged at 3000 r/min for 10 min. Aliquots of 100 μL supernatant portion was collected and diluted 1:1 (v/v) with methanol. The mixture was vortexed and filtered through a 0.22-μm microporous membrane prior to SFC-MS analysis. 
     According to the detection method based on SFC and post-column DIL charge complexation, the SFC-MS method used the following chromatographic conditions: 
     A Torus DIOL chromatographic column (2.1 mm×100 mm, 1.7 μm) was used. The binary mobile phase was composed of pressurized carbon dioxide (A) paired with 0.1% ammonia in methanol (B). The initial conditions were 5% B, and the linear elution gradient was then programmed from 5% B to 20% B within 8.9 min, and held for 0.1 min. At 9.5 min, the gradient was linearly returned to 5% B and maintained for 0.5 min to complete the whole run. The column temperature was set to 40° C. The flow rate was 0.3 mL/min. The automatic back pressure regulator (ABPR) pressure was set to 2000 psi. A sampling volume of 2 μL was injected. The flow rate of make-up solvent was 0.2 mL/min. 
     According to the detection method based on SFC and post-column DIL charge complexation, after SFC separation, a DIL (1,1′-dioctyl-4,4′-bipyridinium dibromide) was dissolved in the make-up solvent of SFC and introduced post-column but before the ESI source. The make-up solvent was a mixture of methanol and water at a ratio of 1:1 (v/v). The mass spectrometric parameters were set as follows: 
     The ESI source under positive ion mode enabled ionization of the analytes with a capillary voltage of 2.30 kV and a nitrogen desolvation gas of 150 L/hr at 350° C. A source temperature of 150° C., collision gas of 0.25 L/hr, and nitrogen cone gas at a flow rate of 150 L/hr were set for the experiments. 
     According to the detection method based on SFC and post-column DIL charge complexation, the PFCs analytes included perfluorododecanoic acid (PFDoDA), perfluorododecanoic acid (PFUnDA), perluorodecanoic acid (PFDA), perfluorononanoic acid (PFNA), perfluorooctanoate (PFOA), perfluoroheptanoic acid (PFHpA), perfluorohexanoic acid (PFHA), perfluoropentanoic acid (PFPA), perfluorooctane sulfonate (PFOS), and perfluorobutane sulfonate (PFBS). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Chemical information for the 10 PFCs analytes 
               
            
           
           
               
               
               
            
               
                   
                 PFCs 
                 Molecular weight 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 PFDoDA 
                 613.9604 
               
               
                   
                 PFUnDA 
                 563.9647 
               
               
                   
                 PFDA 
                 513.9679 
               
               
                   
                 PFNA 
                 463.9711 
               
               
                   
                 PFOA 
                 413.9743 
               
               
                   
                 PFHpA 
                 363.9774 
               
               
                   
                 PFHA 
                 313.9806 
               
               
                   
                 PFPA 
                 263.9838 
               
               
                   
                 PFOS 
                 499.9380 
               
               
                   
                 PFBS 
                 299.9508 
               
               
                   
                   
               
            
           
         
       
     
     The differences between the invention and current techniques are as follows: 
     The invention established an analytical method incorporating SFC separation, post-column DIL-based charge complexation and ESI-MS. The DIL reagent formed positively charged complexes with anionic target analytes during the ESI process, facilitating MS detection of the analytes in the positive ion mode with enhanced detection sensitivity. Moreover, the factors affecting the extraction yield of SUPRAS were optimized by single factor experiment and response surface methodology. 
     The invention incorporating SFC separation, post-column DIL-based charge complexation and ESI-MS is further explained in combination with the following attached figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional objectives, functions, and advantages of the present invention will be set forth in the description of embodiments which follow, with reference to the accompanying drawings in which: 
         FIG. 1A  is a schematic diagram of the effect of alkyl alcohol type on extraction yield. 
         FIG. 1B  is a schematic diagram of the effect of amount of heptanol on extraction yield. 
         FIG. 1C  is a schematic diagram of the effect of amount of tetrahydrofuran on extraction yield. 
         FIG. 1D  is a schematic diagram of the effect of vortex time on extraction yield. 
         FIG. 2A  is a response surface plot of amounts of tetrahydrofuran and heptanol. 
         FIG. 2B  is a response surface plot of vortex time and amount of heptanol. 
         FIG. 2C  is a response surface plot of amounts of heptanol and SUPRAS. 
         FIG. 2D  is a response surface plot of vortex time and amount of tetrahydrofuran. 
         FIG. 2E  is a response surface plot of amounts of tetrahydrofuran and SUPRAS. 
         FIG. 2F  is a response surface plot of vortex time and amount of SUPRAS. 
         FIG. 3A  is a multiple reaction monitoring (MRM) chromatogram of PFDoDA in the positive ion mode. 
         FIG. 3B  is a MRM chromatogram of PFUnDA in the positive ion mode. 
         FIG. 3C  is a MRM chromatogram of PFOS in the positive ion mode. 
         FIG. 3D  is a MRM chromatogram of PFBS in the positive ion mode. 
         FIG. 3E  is a MRM chromatogram of PFDA in the positive ion mode. 
         FIG. 3F  is a MRM chromatogram of PFNA in the positive ion mode. 
         FIG. 3G  is a MRM chromatogram of PFOA in the positive ion mode. 
         FIG. 3H  is a MRM chromatogram of PFHpA in the positive ion mode. 
         FIG. 3I  is a MRM chromatogram of PFHA in the positive ion mode. 
         FIG. 3J  is a MRM chromatogram of PFPA in the positive ion mode. 
         FIG. 4A  is a total ion current chromatogram for the analysis of the 10 PFCs in the negative ion mode without post-column addition of DIL. 
         FIG. 4B  is a total ion current chromatogram for the analysis of the 10 PFCs in the positive ion mode with post-column addition of DIL. 
         FIG. 4C  is a multiple reaction monitoring chromatogram of PFBS in the negative ion mode without post-column addition of DIL. 
         FIG. 4D  is a multiple reaction monitoring chromatogram of PFOS in the negative ion mode without post-column addition of DIL. 
         FIG. 4E  is a multiple reaction monitoring chromatogram of PFBS in the positive ion mode with post-column addition of DIL. 
         FIG. 4F  is a multiple reaction monitoring chromatogram of PFOS in the positive ion mode with post-column addition of DIL. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     1. Materials and Apparatus 
     1.1. Materials 
     Reference standards of perfluorododecanoic acid (PFDoDA), perfluoroundecanoic acid (PFUnDA), perluorodecanoic acid (PFDA), perfluorononanoic acid (PFNA), perfluorooctanoate (PFOA), perfluoroheptanoic acid (PFHpA), perfluorohexanoic acid (PFHA), perfluoropentanoic acid (PFPA), perfluorooctane sulfonate (PFOS), and perfluorobutane sulfonate (PFBS) were purchased from Sigma-Aldrich (St. Louis, Mo., USA). 1,1′-Dimethyl-4,4-bipyridinium dichloride, 4-aza-1-azoniabicyclo[2.2.2]octane, 1,1′-[1,4-phenylenebis(methylene)]bis(4,4′-bipyridinium) dibromide, 1,1′-diheptyl-4,4′-bipyridinium dibromide, and 1,1′-dioctyl-4,4′-bipyridinium dibromide were obtained from Sigma-Aldrich (St. Louis, Mo., USA). Ultrapure water was produced using a Millipore Milli-Q Integral 5 water purification system (Bedford, Mass., USA). Pentanol, hexanol, octanol, decanol, undecanol, and dodecanol were obtained from J&amp;K Scientific Ltd. (Beijing, China). Heptanol and nonanol were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). 
     1.2. Apparatus 
     ACQUITY UPC 2  system and Xevo TQ-S triple quadrupole mass spectrometer fitted with an ESI source and MassLynx version 4.1 software (Waters, Milford, Mass., USA); AB204-S electronic balance (Mettler Toledo, Columbus, Ohio, USA); Hitachi CR 21N centrifuge (Tokyo, Japan); IKA MS3 vortexer (Staufen, Germany). 
     2. Detection Method 
     2.1. Preparation of SUPRAS 
     Aliquots of 3 mL of heptanol, 4 mL of tetrahydrofuran, and 33 mL of water were transferred into a 50-mL centrifuge tube, mixed for 3 min on a vortexer, and centrifuged at 3000 r/min for 10 min. The resulting SUPRAS supernatant was collected and stored at 4° C. 
     2.2. Sample Pretreatment 
     Aliquots of 0.50 g of samples were weighed in a 10-mL centrifuge tube, into which 4 mL of the obtained SUPRAS was then added. After vortexing for 3 min, the extract was centrifuged at 3000 r/min for 10 min. Aliquots of 100 μL supernatant portion was collected and diluted 1:1 (v/v) with methanol. The mixture was vortexed and filtered through a 0.22-μm microporous membrane prior to SFC-MS analysis. 
     2.3. SFC Conditions 
     A Torus DIOL chromatographic column (2.1 mm×100 mm, 1.7 μm) was used. The binary mobile phase was composed of pressurized carbon dioxide (A) paired with 0.1% ammonia in methanol (B). The initial conditions were 5% B, and the linear elution gradient was then programmed from 5% B to 20% B within 8.9 min, and held for 0.1 min. At 9.5 min, the gradient was linearly returned to 5% B and maintained for 0.5 min to complete the whole run. The column temperature was set to 40° C. The flow rate was 0.3 mL/min. The ABPR pressure was set to 2000 psi. A sampling volume of 2 μL was injected. The flow rate of make-up solvent was 0.2 mL/min. 
     2.4. Mass Spectrometric Conditions 
     After SFC separation, a DIL (1,1′-dioctyl-4,4′-bipyridinium dibromide) was dissolved in the make-up solvent of SFC and introduced post-column but before the ESI source. The make-up solvent was a mixture of methanol and water at a ratio of 1:1 (v/v). The mass spectrometric parameters were set as follows: 
     The ESI source under positive ion mode enabled ionization of the analytes with a capillary voltage of 2.30 kV and a nitrogen desolvation gas of 150 L/hr at 350° C. A source temperature of 150° C., collision gas of 0.25 L/hr, and nitrogen cone gas at a flow rate of 150 L/hr were set for the experiments. 
       FIG. 4A  is a total ion current chromatogram for the analysis of the 10 PFCs in the negative ion mode without post-column addition of DIL.  FIG. 4B  is a total ion current chromatogram for the analysis of the 10 PFCs in the positive ion mode with post-column addition of DIL.  FIG. 4C  is a multiple reaction monitoring chromatogram of PFBS in the negative ion mode without post-column addition of DIL.  FIG. 4D  is a multiple reaction monitoring chromatogram of PFOS in the negative ion mode without post-column addition of DIL.  FIG. 4E  is a multiple reaction monitoring chromatogram of PFBS in the positive ion mode with post-column addition of DIL.  FIG. 4F  is a multiple reaction monitoring chromatogram of PFOS in the positive ion mode with post-column addition of DIL. The multiple reaction monitoring (MRM) chromatograms shown in  FIG. 4C ,  FIG. 4D ,  FIG. 4E  and  FIG. 4F  were obtained for the detection of PFBS and PFOS at 10 ng/mL using 2.5 μM dissolved in methanol/water mixture solution (1:1, v/v) at a flow rate of 1.5 mL/min. Significant enhancement in S/N and signal intensity was achieved in the positive ionization mode compared to the negative ionization mode. Moreover, the peak shape of the analytes at low concentrations in the negative ion mode could easily be distorted due to the unstable ionization. However, the peak shape in the positive ion mode was much better. 
     2.5. Comparative MS Test in the Negative Ion Mode 
     The ESI source under positive ion mode enabled ionization of the analytes with a capillary voltage of 2.30 kV and a nitrogen desolvation gas of 150 L/hr at 350° C. A source temperature of 150° C., collision gas of 0.25 L/hr, and nitrogen cone gas at a flow rate of 150 L/hr were set for the experiments. 
     The precursor and product ions, cone voltage, and collision energy for the analysis of the 10 PFCs are shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 The precursor ion, product ion, cone voltage and 
               
               
                 collision energy for the analysis of the 10 PFCs. 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Precursor 
                 Product 
                 Cone 
                 Collision 
               
               
                 No. 
                 PFCs 
                 ion (m/z) 
                 ion (m/z) 
                 voltage (V) 
                 energy (V) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 1 
                 PFDoDA 
                 613.1 
                 569.0 
                 10 
                 10 
               
               
                 2 
                 PFUnDA 
                 563.1 
                 519.0 
                 2 
                 10 
               
               
                 3 
                 PFDA 
                 513.0 
                 469.0 
                 22 
                 10 
               
               
                 4 
                 PFNA 
                 463.1 
                 419.1 
                 14 
                 10 
               
               
                 5 
                 PFOA 
                 413.0 
                 369.0 
                 21 
                 10 
               
               
                 6 
                 PFHpA 
                 363.1 
                 319.0 
                 10 
                 10 
               
               
                 7 
                 PFHA 
                 313.0 
                 269.0 
                 10 
                 8 
               
               
                 8 
                 PFPA 
                 263.1 
                 219.1 
                 10 
                 8 
               
               
                 9 
                 PFOS 
                 499.0 
                 80.0 
                 26 
                 36 
               
               
                 10 
                 PFBS 
                 299.0 
                 80.0 
                 36 
                 30 
               
               
                   
               
            
           
         
       
     
       FIG. 3A  is a multiple reaction monitoring chromatogram of PFDoDA in the positive ion mode.  FIG. 3B  is a multiple reaction monitoring chromatogram of PFUnDA in the positive ion mode.  FIG. 3C  is a multiple reaction monitoring chromatogram of PFOS in the positive ion mode.  FIG. 3D  is a multiple reaction monitoring chromatogram of PFBS in the positive ion mode.  FIG. 3E  is a multiple reaction monitoring chromatogram of PFDA in the positive ion mode.  FIG. 3F  is a multiple reaction monitoring chromatogram of PFNA in the positive ion mode.  FIG. 3G  is a multiple reaction monitoring chromatogram of PFOA in the positive ion mode.  FIG. 3H  is a multiple reaction monitoring chromatogram of PFHpA in the positive ion mode.  FIG. 3I  is a multiple reaction monitoring chromatogram of PFHA in the positive ion mode.  FIG. 3J  is a multiple reaction monitoring chromatogram of PFPA in the positive ion mode. 
     3. Results and Discussion 
     3.1. Optimization of SUPRAS-Based Extraction with Single Factor Test 
     3.1.1. Optimization of Type and Amount for Alkyl Alcohols 
     A variety of alkyl alcohols with different carbon numbers (each of pentanol, hexanol, heptanol, octanol, nonanol, decanol, undecanol, and dodecanol) were mixed with tetrahydrofuran and water to form SUPRAS for the extraction of PFCs in textiles. As shown in  FIG. 1A , the extraction recovery of PFCs in heptanol/tetrahydrofuran/water system was 91-106%, and the recovery was relatively stable. Thus, heptanol was selected as the best suited among the alkyl alcohols investigated. The amount of heptanol (1-6 mL) was also evaluated. Maintaining fixed volumes of 40 mL of SUPRAS, the extraction yield increased with increasing amount of heptanol from 1 to 3 mL but decreased thereafter ( FIG. 1B ). Thus, 3 mL was selected as an optimum volume for heptanol. 
     3.1.2. Optimization of Amount for Tetrahydrofuran 
     The extraction effect was assessed using a total of 40 mL of SUPRAS containing 2, 4, 6, 8, 10, or 12 mL of tetrahydrofuran (heptanol was kept at 3 mL), among which 4 mL gave the maximum and stable extraction yield. With the increase of tetrahydrofuran, the extraction yield of most compounds first decreased and then basically became stable ( FIG. 1C ). 
     3.1.3. Optimization of Vortex Time 
     The extraction yield of six vortex times (1, 3, 5, 7, 9, and 11 min) were evaluated. It was found that the vortex time of 3 min reached a maximum extraction yield for the 10 PFCs. With the increase of vortex time, the extraction yield tended to be stable. Therefore, 3 min was chosen for further studies.  FIG. 1D  is a schematic diagram of the effect of vortex time on extraction yield. 
     3.1.4. Optimization of SUPRAS Volume 
     After the composition of SUPRAS was determined, the extraction efficiency of different volumes of SUPRAS (2, 3, 4, 5, and 6 mL) was further studied. The experimental results demonstrated that the extraction yield first increased and then gradually stabilized, with 4 mL being the optimal volume of SUPRAS. 
     3.2. Optimization of SUPRAS-Based Extraction by Response Surface Methodology 
     According to the single factor test results, a comprehensive analysis of four critical variables (amounts of heptanol, tetrahydrofuran, SUPRAS, and vortex time) was carried out to examine their influences on extraction efficiency for PFCs. The interaction between the variables was analyzed using the four-factor and three-level response surface methodology. According to multiple regression analyses of the data processing with coded levels (Table 3), analysis of variance (ANOVA) of the fitted quadratic polynomial model identified a p value less than 0.0001, indicating that the regression model was of significance. The calculated lack-of-fit value was 0.2985, exhibiting that the model could sufficiently predict relevant variation while representing the actual relationship between the four variables.  FIG. 2A  is a response surface plot of amounts of tetrahydrofuran and heptanol.  FIG. 2B  is a response surface plot of vortex time and amount of heptanol.  FIG. 2C  is a response surface plot of amounts of heptanol and SUPRAS.  FIG. 2D  is a response surface plot of vortex time and amount of tetrahydrofuran.  FIG. 2E  is a response surface plot of amounts of tetrahydrofuran and SUPRAS.  FIG. 2F  is a response surface plot of vortex time and amount of SUPRAS. The results indicated that interaction between the amounts of heptanol and tetrahydrofuran (AB), interaction between the amount of heptanol and vortex time (AC), and interaction between the amounts of heptanol and SUPRAS (AD) were significantly correlated with the extraction yield of the 10 PFCs. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 ANOVA results for the response surface quadratic model. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Degrees of 
                 Sum of 
                 Mean 
                 F- 
                 P- 
                   
               
               
                 Source 
                 freedom 
                 squares 
                 square 
                 value 
                 value 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Model 
                 14 
                 3266.21 
                 233.30 
                 58.47 
                 &lt;0.0001 
                 significant 
               
               
                 A 
                 1 
                 238.90 
                 238.90 
                 59.87 
                 &lt;0.0001 
               
               
                 B 
                 1 
                 484.57 
                 484.57 
                 121.44 
                 &lt;0.0001 
               
               
                 C 
                 1 
                 224.44 
                 224.44 
                 56.25 
                 &lt;0.0001 
               
               
                 D 
                 1 
                 33.77 
                 33.77 
                 8.46 
                 &lt;0.0001 
               
               
                 AB 
                 1 
                 660.73 
                 660.73 
                 165.59 
                 0.0114 
               
               
                 AC 
                 1 
                 173.14 
                 173.14 
                 43.39 
                 &lt;0.0001 
               
               
                 AD 
                 1 
                 1054.68 
                 1054.68 
                 264.33 
                 &lt;0.0001 
               
               
                 BC 
                 1 
                 42.60 
                 42.60 
                 10.68 
                 &lt;0.0001 
               
               
                 BD 
                 1 
                 47.97 
                 47.97 
                 12.02 
                 0.0056 
               
               
                 CD 
                 1 
                 55.19 
                 55.19 
                 13.83 
                 0.0038 
               
               
                 A2 
                 1 
                 2.41 
                 2.41 
                 0.60 
                 0.0023 
               
               
                 B2 
                 1 
                 46.21 
                 46.21 
                 11.58 
                 0.4499 
               
               
                 C2 
                 1 
                 193.48 
                 193.48 
                 48.49 
                 0.0043 
               
               
                 D2 
                 1 
                 38.30 
                 38.30 
                 9.60 
                 &lt;0.0001 
               
               
                 Residual 
                 14 
                 55.86 
                 3.99 
                   
                 0.0079 
               
               
                 Lack of 
                 10 
                 45.74 
                 4.57 
                 1.81 
                 0.2985 
                 not 
               
               
                 fit 
                   
                   
                   
                   
                   
                 significant 
               
               
                 Pure 
                 4 
                 10.12 
                 2.53 
               
               
                 error 
               
               
                   
               
            
           
         
       
     
     3.3. Model Validation 
     The optimal extraction conditions obtained by response surface methodology were as follows: 4 mL of heptanol, 4 mL of tetrahydrofuran, 3 mL of SUPRAS, and 1 min of vortex time. In order to confirm the accuracy of the established model, the process was repeated three times to extract PFCs under optimal condition. The experimental results under this condition were 106.337%, 108.599%, and 107.840%, with an average value of 107.592%. The relative error between experimental value and predicted value (108.105%) was 0.474%. Thus, the extraction conditions of PFCs optimized by response surface methodology are feasible and reliable. 
     3.4. Optimization of DILs 
     3.4.1. Optimization of Type for DILs 
     The DIL acted as the derivatization reagent for the post-column adducts. The chromatographic effluent was combined post-column with a make-up solvent containing a DIL reagent, leading to the detection of positively charged complexes in the positive ion mode. The DILs of 1,1′-dimethyl-4,4-bipyridinium dichloride, 4-aza-1-azoniabicyclo[2.2.2]octane, 1,1′-[1,4-phenylenebis(methylene)]bis(4,4′-bipyridinium) dibromide, 1,1′-diheptyl-4,4′-bipyridinium dibromide, and 1,1′-dioctyl-4,4′-bipyridinium dibromide were investigated for their ability to pair with PFCs. Among the five DILs investigated, 1,1′-dioctyl-4,4′-bipyridinium dibromide was identified as the most effective, and the total signal intensity and detection sensitivity of the 10 PFCs were the highest. Therefore, 1,1′-dioctyl-4,4′-bipyridinium dibromide was used in the make-up solvent. The dissolving solvent, concentration, and flow rate were optimized. 
     3.4.1. Optimization of Solvent Type and Ratio for DILs 
     Two make-up solvents (acetonitrile and methanol) for dissolving the DIL were evaluated. However, it was found that the baseline was too high. The solvent was then changed to a mixture of methanol/acetonitrile and water with the ratios of 1:1, 2:3, 4:1, 3:2, and 1:4. Based on the comparison in terms of response intensity and S/N value, the best results were achieved using a mixture of methanol and water at a ratio of 1:1 (v/v). 
     3.4.2. Optimization of DIL Concentration 
     The concentration of 1,1′-dioctyl-4,4′-bipyridinium dibromide (20, 15, 10, 5, 4, 3, 2, 1, 0.8, and 0.6 μM) was optimized, among which the most intense signal intensity and S/N value were obtained at a concentration of 2.5 μM. 
     3.4.3. Optimization of DIL Flow Rate 
     Maintaining a fixed concentration of 2.5 μM for 1,1′-dioctyl-4,4′-bipyridinium dibromide and methanol-water 1:1 (v/v) as the make-up solvent, the flow rate of the make-up solvent was then optimized in the range of 1-2.5 mL/min. It was observed that the maximum signal intensity and best peak shape were obtained at a flow rate of 1.5 mL/min. 
     3.5. Analysis of Real Samples 
     The proposed approach was applied to the analysis of real textile samples of different fabrics (e.g., nylon, cotton, polyester and silk). The experimental results revealed that the 10 PFCs were not detected. 
     The foregoing embodiments are merely illustrative of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various variations and modifications made to the technical solutions of the present invention by those skilled in the art without departing from the spirit of the present invention are embraced in the protection scope of the present invention as defined by the appended claims.