TY - JOUR
AU - Wang, Zhi
AB - Abstract Porous organic polymers have gained great research interest in the field of adsorption. A benzoxazine porous organic polymer (BoxPOP) constructed from p-phenylenediamine, 1,3,5-trihydroxybenzene and paraformaldehyde was fabricated and explored as an adsorbent for solid-phase extraction (SPE) of four chlorophenols from water and honey samples. Under the optimized SPE conditions, the response linearity for the analysis of the SPE extract of the chlorophenols by high performance liquid chromatography-diode array detector was observed in the range of 0.2–40.0 ng mL−1 for water samples and 5.0–400.0 ng g−1 for honey samples. The method detection limits of the analytes were 0.06–0.08 ng mL−1 for water samples and 1.5–2.0 ng g−1 for honey samples. The recoveries of the analytes from fortified water and honey samples ranged from 84.8 to 119.0% with the relative standard deviations below 8.4%. The results indicate that the prepared BoxPOP is an effective adsorbent for the chlorophenols. The established method provides an alternative approach for the determination of chlorophenols in real samples. Introduction Chlorophenols (CPs) are organochlorides of phenols that comprise one or more chlorine atoms linked with covalent bonds. CPs can be divided into mono-chlorinated phenols (MCPs), di-chlorinated phenols (DCPs), tri-chlorinated phenols (TCPs), tetra-chlorinated phenols (TTCPs) and penta-chlorinated phenol (PCP) (1). They are widely used as disinfectants in agriculture and industries because of their antimicrobiological properties (2). They can be formed from degradation of the bactericide triclosan and some pesticides (3, 4). Among the 19 CP congeners, 2-CP, 4-CP and 2,4-DCP are the most significant by-products formed in water chlorination (5). CPs are highly toxic with mutagenic, estrogenic and carcinogenic effects and their transformation and migration to foods has attracted extensive attention in recent years (6). For the above reason, the European Union has set the maximum allowable concentration of CPs in drinking water at 0.5 μg L−1 (7). Since CPs usually exist at trace levels in real samples, the analytes enrichment by sample pretreatment techniques is often necessary and critical before instrumental analysis. So far, many sample extraction techniques, such as partitioned liquid-phase microextraction (PLPME) (8), solid-phase microextraction (SPME) (9), magnetic solid-phase extraction (MSPE) (10) and solid-phase extraction (SPE) (11), have been reported for the extraction of CPs. Among them, SPE is still the most commonly used technique because relative to conventional liquid–liquid extraction, it is more effective and requires less organic solvent. In SPE, a suitable adsorbent is the key for the effective extraction of certain analytes and therefore the development of new SPE adsorbents has become the current research focus. In the past few decades, porous materials have gained great research interest in the field of adsorption (12). According to the elemental composition and bonding mode, porous materials mainly include three forms: inorganic porous materials (e.g., zeolites, porous carbons and mesoporous silica), inorganic–organic hybrid porous materials (e.g., metal–organic frameworks (MOFs)) and organic porous materials (e.g., porous organic polymers (POPs), organic porous cages and supramolecular organic frameworks) (13, 14). Among them, POPs consist of light elements (such as H, C, O, B, N, etc.) connected by stable covalent bonds. Because of their good pore structures, low skeleton density and easy functionalization, POPs have intrigued an increased research interest. They have shown promising applications in the fields of energy storage (15–18), catalysis (19–21), gas adsorption (22–25) and extraction (26). Some of the POPs, including covalent-organic frameworks (COFs), amorphous hypercross-linked polymers (HCPs) and porous aromatic frameworks (PAFs), have been employed as the adsorbents for the enrichment of different pollutant residues. For example, COF-SCU1 (SCU for Sichuan University) was coated on SPME fiber and applied to extract some volatile benzene homologs from indoor air samples prior to their determination by gas chromatography–mass spectrometry (27). Wu et al. (28) employed triphenylamine-based hypercrosslinked organic polymer (named as PPTPA) as the SPE adsorbent for the extraction of phenylurea herbicides from water, milk and tomato juice samples followed by high performance liquid chromatographic (HPLC) analysis. A porous aromatic framework of type PAF-6 was synthesized by Wang et al. (9) and explored as the SPME fiber coating for the enrichment of polycyclic aromatic hydrocarbons (PAHs), phthalate plasticizers and n-alkanes. In this work, a heteroatom-containing POP, benzoxazine porous organic polymer (BoxPOP), was synthesized through the polycondensation of p-phenylenediamine, 1,3,5-trihydroxybenzene and paraformaldehyde (29). Since the heteroatoms can provide stronger adsorbate-adsorbent interactions (30), the heteroatom-containing BoxPOP is expected to be an efficient SPE adsorbent for the extraction and enrichment of some target analytes. The BoxPOP was explored for the first time as the SPE adsorbent for the enrichment of four CPs (2-CP, 4-CP, 2,3-DCP and 2,4-DCP) from water and honey samples. The experimental conditions for the SPE were optimized. Finally, a BoxPOP based SPE in combination with HPLC detection was developed for the determination of the four CPs in water and honey samples. The method showed good performance for the analysis of the CPs in real water and honey samples. Materials and Methods Chemicals and materials The CP standards (2-CP, 99%; 4-CP, 99%; 2,3-DCP, 98%; 2,4-DCP, 99.5%) were acquired from Aladdin Reagent (Shanghai, China). 1,4-Dioxane (≥99.0), 1,3,5-trihydroxybenzene (≥99.0), p-phenylenediamine (≥97.0) and paraformaldehyde (analytical grade) were obtained from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). The HPLC-grade acetonitrile, methanol and acetone were supplied by Merda Technology Inc. (USA). All of the reagents above were used as received. The water used in the experiments was prepared by the 1820D ultra-pure water purification system (Chongqing, China). The mixture stock solution for the four CPs was prepared in methanol at the concentration of 40.0 μg mL−1. A series of lower concentrations of the CPs mixture stock solutions (20.0, 10.0, 5.0, 2.0, 1.0, 0.5, 0.1 and 0.05 μg mL−1) were prepared by diluting an appropriate higher concentration solution in a 10-mL volumetric flask with methanol. All of the above mixture solutions were stored in the fridge at 4°C. The lake water samples were taken at three different points from the lake at the west campus of Hebei Agriculture University and then mixed together. The river water samples were collected from three different parts of Qingshui River (Baoding, China) and then pooled together. Glass containers were used for the collection of the waters and all of the collected samples were stored in the fridge at 4°C. Honey samples were purchased from local supermarket (Baoding, China). Instruments The details about the instrumentation in this work are given in the Electronic Supporting Material (ESM). Synthesis of BoxPOP The BoxPOP was synthesized according to the literature method with some modifications (29). Briefly, 1.08 g (0.01 mol) p-phenylenediamine was dispersed in 20 mL 1,4-dioxane and then the mixture was dropped slowly into a solution of 20 mL 1,4-dioxane containing 1.2 g (0.04 mol) paraformaldehyde with the pH being adjusted in the range of 9.5–10.5 with sodium hydroxide. After the mixture was sonicated for 5 min and stirred at 5–10°C for 90 min, 5.35 g (0.033 mol) 1,3,5-trihydroxybenzene in 20 mL 1,4-dioxane was added. Then, the mixture was kept at 80°C under stirring for another 6 h. After being cooled down to room temperature, the precipitate was isolated by filtration and washed by Soxhlet extraction with 1,4-dioxane for 24 h, methanol for 8 h and alkaline water for 24 h. Finally, the desired BoxPOP was obtained by washing the precipitate with water several times and then dried in the vacuum at 60°C for 24 h (Figure 1). Figure 1 Open in new tabDownload slide Procedures for the preparation of the BoxPOP. Figure 1 Open in new tabDownload slide Procedures for the preparation of the BoxPOP. Figure 2 Open in new tabDownload slide The SEM images at scale bar of 5 μm at ×10,000 magnification (a) and 1 μm at ×50,000 magnification (b), the nitrogen sorption isotherms at 77 K (c) and the pore size distribution (d), the FT-IR spectrum (e) and the XRD pattern (f) of the BoxPOP. Figure 2 Open in new tabDownload slide The SEM images at scale bar of 5 μm at ×10,000 magnification (a) and 1 μm at ×50,000 magnification (b), the nitrogen sorption isotherms at 77 K (c) and the pore size distribution (d), the FT-IR spectrum (e) and the XRD pattern (f) of the BoxPOP. Figure 3 Open in new tabDownload slide Effect of sample solution pH (a), sample loading rate (b), sample volume (c), the eluent type (d) and eluent volume (e, f) on the extraction. Figure 3 Open in new tabDownload slide Effect of sample solution pH (a), sample loading rate (b), sample volume (c), the eluent type (d) and eluent volume (e, f) on the extraction. Table I The analytical data for the determination of CPs by current method Samplea . Analyte . Linear range . r . RSD (%) (n = 5) . LOD . LOQ . Water 2-CP 0.20–40.0 0.9993 2.8 0.06 0.20 4-CP 0.20–40.0 0.9970 2.2 0.06 0.20 2,3-DCP 0.50–40.0 0.9993 2.3 0.08 0.26 2,4-DCP 0.50–40.0 0.9980 4.9 0.08 0.26 Honey 2-CP 10.0–400.0 0.9901 3.8 2.0 6.6 4-CP 5.0–400.0 0.9912 5.4 1.5 5.0 2,3-DCP 5.0–400.0 0.9971 1.9 1.5 5.0 2,4-DCP 5.0–400.0 0.9940 5.1 1.5 5.0 Samplea . Analyte . Linear range . r . RSD (%) (n = 5) . LOD . LOQ . Water 2-CP 0.20–40.0 0.9993 2.8 0.06 0.20 4-CP 0.20–40.0 0.9970 2.2 0.06 0.20 2,3-DCP 0.50–40.0 0.9993 2.3 0.08 0.26 2,4-DCP 0.50–40.0 0.9980 4.9 0.08 0.26 Honey 2-CP 10.0–400.0 0.9901 3.8 2.0 6.6 4-CP 5.0–400.0 0.9912 5.4 1.5 5.0 2,3-DCP 5.0–400.0 0.9971 1.9 1.5 5.0 2,4-DCP 5.0–400.0 0.9940 5.1 1.5 5.0 aThe unit for the linear range, LOD and LOQ is ng mL−1 for water sample and ng g−1 for honey sample. Open in new tab Table I The analytical data for the determination of CPs by current method Samplea . Analyte . Linear range . r . RSD (%) (n = 5) . LOD . LOQ . Water 2-CP 0.20–40.0 0.9993 2.8 0.06 0.20 4-CP 0.20–40.0 0.9970 2.2 0.06 0.20 2,3-DCP 0.50–40.0 0.9993 2.3 0.08 0.26 2,4-DCP 0.50–40.0 0.9980 4.9 0.08 0.26 Honey 2-CP 10.0–400.0 0.9901 3.8 2.0 6.6 4-CP 5.0–400.0 0.9912 5.4 1.5 5.0 2,3-DCP 5.0–400.0 0.9971 1.9 1.5 5.0 2,4-DCP 5.0–400.0 0.9940 5.1 1.5 5.0 Samplea . Analyte . Linear range . r . RSD (%) (n = 5) . LOD . LOQ . Water 2-CP 0.20–40.0 0.9993 2.8 0.06 0.20 4-CP 0.20–40.0 0.9970 2.2 0.06 0.20 2,3-DCP 0.50–40.0 0.9993 2.3 0.08 0.26 2,4-DCP 0.50–40.0 0.9980 4.9 0.08 0.26 Honey 2-CP 10.0–400.0 0.9901 3.8 2.0 6.6 4-CP 5.0–400.0 0.9912 5.4 1.5 5.0 2,3-DCP 5.0–400.0 0.9971 1.9 1.5 5.0 2,4-DCP 5.0–400.0 0.9940 5.1 1.5 5.0 aThe unit for the linear range, LOD and LOQ is ng mL−1 for water sample and ng g−1 for honey sample. Open in new tab Sample preparation Water samples (lake water and river water) were filtered through a 0.22 μm membrane to remove undissolved substances. A total of 100 mL of the water filtrate was used for the following SPE. For honey samples, 10 g of honey sample was diluted with water to 100 mL. After it was filtered through a 0.22 μm membrane to get rid of the suspended particulates, 80 mL of the resulting honey sample solution was used for the following SPE. SPE process For the preparation of the SPE extraction cartridge, a sieve plate was first placed on the bottom of an empty 3-mL SPE cartridge, and then 40 mg of the BoxPOP adsorbent was added, followed by placing another sieve plate on the top. Then, the packed cartridge was washed with 5 mL acetonitrile and 5 mL water prior to sample loading. After 100 mL of water sample (or 80 mL of diluted honey sample) was passed through the cartridge at the loading rate of 5 mL min−1, the analytes adsorbed on the SPE adsorbent were eluted with 0.3 mL alkaline acetonitrile (1.0 mol L−1 NaOH solution: acetonitrile = 1: 99 (v/v)). After the eluate was neutralized by adding 3 μL of 1 mol L−1 hydrochloric acid and then filtered through a 0.22 μm syringe filter, 20 μL of the eluate was injected into the HPLC for analysis. Prior to the next SPE, 2 mL acetonitrile and 2 mL water were used to wash the cartridge. Results Characterization of the BoxPOP The morphology of the BoxPOP was observed by SEM. Figure 2 (a and b) reveals that the BoxPOP is spherical particles with the sizes around 1 μm. The porosity of the BoxPOP was characterized by nitrogen sorption. The isotherm curves shown in Figure 2c exhibit the characteristics of both type II and type IV isotherm curves with a sharp growth at low relative pressures (P/P0 < 0.02), reflecting an abundant microporous structure. The hysteresis in the curve indicates that the BoxPOP also has mesopores. The Brunauer–Emmett–Teller (BET) specific surface area and the total pore volume for the BoxPOP were measured to be 106.5 m2 g−1 and 0.57 cm3 g−1 (P/P0 = 0.99), respectively. The pore size distribution of the BoxPOP is shown in Figure 2d. The total adsorption average pore width and Barrett–Joyner–Halenda (BJH) median pore width were 5.9 and 2.3 nm, respectively. The FT-IR spectrum for the BoxPOP was investigated to evaluate the formation of the benzoxazine linkage. As shown in Figure 2e, the aromatic ring vibrations are observed at 1511 and 1614 cm−1, and the aromatic ether vibrations appear at 1263 and 1115 cm−1, which can be attributed to the C-O-C asymmetric and symmetric stretching vibrations in the benzoxazine ring, respectively. The characteristic peak of phenyl affiliated to oxazine rings is at 940 cm−1. The peaks for the residual phenol groups can be observed in the region of 3500–3200 cm−1for the polymer networks (31, 32). These above results suggest that the BoxPOP was successfully synthesized. Table II The analytical results for the determination of CPs in real samples Analytes . Spiked (ng mL−1) . River water . Lake water . Spiked (ng g−1) . Honey sample 1 . Honey sample 2 . Found (ng mL−1) . MRa (%) . RSD (%) . Found (ng mL−1) . MR (%) . RSD (%) . Found (ng g−1) . MR (%) . RSD (%) . Found (ng g−1) . MR (%) . RSD (%) . 2-CP 0 <LOQ <LOQ 0 ndb ndb 1.0 0.99 99.0 6.7 1.03 103.0 5.7 10.0 11.5 115.0 8.2 11.8 118.0 1.0 5.0 4.66 93.2 1.6 5.02 100.4 2.7 50.0 48.9 97.8 5.6 46.6 93.2 6.6 10.0 9.58 95.8 1.6 9.76 97.6 1.9 100.0 94.4 94.4 3.8 99.5 99.5 5.4 4-CP 0 nd nd 0 <LOQ nd 1.0 0.96 96.0 6.2 1.0 100.0 6.7 10.0 10.2 102.0 8.0 12.0 120.0 8.3 5.0 4.67 93.4 4.0 4.71 94.2 3.2 50.0 51.0 102.0 4.5 42.4 84.8 2.3 10.0 9.96 99.6 1.8 9.92 99.2 2.9 100.0 98.0 98.0 1.2 106.2 106.2 4.5 2,3-CP 0 nd nd 0 nd nd 1.0 0.96 96.0 5.1 1.02 102.0 5.6 10.0 9.9 99.0 4.4 11.4 114.0 7.8 5.0 4.90 98.0 3.4 5.09 101.8 3.8 50.0 51.0 102.0 6.0 51.2 102.4 6.5 10.0 10.26 102.6 4.7 10.03 100.3 4.0 100.0 96.1 96.1 1.9 99.1 99.1 3.7 2,4-CP 0 nd nd 0 nd nd 1.0 1.04 104.0 5.0 0.99 99.0 3.1 10.0 9.5 95.0 8.4 11.9 119.0 2.8 5.0 4.89 97.8 3.4 4.81 96.2 4.8 50.0 52.7 105.4 7.5 48.4 96.8 7.2 10.0 9.67 96.7 2.1 9.91 99.1 3.8 100.0 97.4 97.4 5.1 100.1 100.1 3.4 Analytes . Spiked (ng mL−1) . River water . Lake water . Spiked (ng g−1) . Honey sample 1 . Honey sample 2 . Found (ng mL−1) . MRa (%) . RSD (%) . Found (ng mL−1) . MR (%) . RSD (%) . Found (ng g−1) . MR (%) . RSD (%) . Found (ng g−1) . MR (%) . RSD (%) . 2-CP 0 <LOQ <LOQ 0 ndb ndb 1.0 0.99 99.0 6.7 1.03 103.0 5.7 10.0 11.5 115.0 8.2 11.8 118.0 1.0 5.0 4.66 93.2 1.6 5.02 100.4 2.7 50.0 48.9 97.8 5.6 46.6 93.2 6.6 10.0 9.58 95.8 1.6 9.76 97.6 1.9 100.0 94.4 94.4 3.8 99.5 99.5 5.4 4-CP 0 nd nd 0 <LOQ nd 1.0 0.96 96.0 6.2 1.0 100.0 6.7 10.0 10.2 102.0 8.0 12.0 120.0 8.3 5.0 4.67 93.4 4.0 4.71 94.2 3.2 50.0 51.0 102.0 4.5 42.4 84.8 2.3 10.0 9.96 99.6 1.8 9.92 99.2 2.9 100.0 98.0 98.0 1.2 106.2 106.2 4.5 2,3-CP 0 nd nd 0 nd nd 1.0 0.96 96.0 5.1 1.02 102.0 5.6 10.0 9.9 99.0 4.4 11.4 114.0 7.8 5.0 4.90 98.0 3.4 5.09 101.8 3.8 50.0 51.0 102.0 6.0 51.2 102.4 6.5 10.0 10.26 102.6 4.7 10.03 100.3 4.0 100.0 96.1 96.1 1.9 99.1 99.1 3.7 2,4-CP 0 nd nd 0 nd nd 1.0 1.04 104.0 5.0 0.99 99.0 3.1 10.0 9.5 95.0 8.4 11.9 119.0 2.8 5.0 4.89 97.8 3.4 4.81 96.2 4.8 50.0 52.7 105.4 7.5 48.4 96.8 7.2 10.0 9.67 96.7 2.1 9.91 99.1 3.8 100.0 97.4 97.4 5.1 100.1 100.1 3.4 aMR, recovery of the analyte for the method bnd, not detected Open in new tab Table II The analytical results for the determination of CPs in real samples Analytes . Spiked (ng mL−1) . River water . Lake water . Spiked (ng g−1) . Honey sample 1 . Honey sample 2 . Found (ng mL−1) . MRa (%) . RSD (%) . Found (ng mL−1) . MR (%) . RSD (%) . Found (ng g−1) . MR (%) . RSD (%) . Found (ng g−1) . MR (%) . RSD (%) . 2-CP 0 <LOQ <LOQ 0 ndb ndb 1.0 0.99 99.0 6.7 1.03 103.0 5.7 10.0 11.5 115.0 8.2 11.8 118.0 1.0 5.0 4.66 93.2 1.6 5.02 100.4 2.7 50.0 48.9 97.8 5.6 46.6 93.2 6.6 10.0 9.58 95.8 1.6 9.76 97.6 1.9 100.0 94.4 94.4 3.8 99.5 99.5 5.4 4-CP 0 nd nd 0 <LOQ nd 1.0 0.96 96.0 6.2 1.0 100.0 6.7 10.0 10.2 102.0 8.0 12.0 120.0 8.3 5.0 4.67 93.4 4.0 4.71 94.2 3.2 50.0 51.0 102.0 4.5 42.4 84.8 2.3 10.0 9.96 99.6 1.8 9.92 99.2 2.9 100.0 98.0 98.0 1.2 106.2 106.2 4.5 2,3-CP 0 nd nd 0 nd nd 1.0 0.96 96.0 5.1 1.02 102.0 5.6 10.0 9.9 99.0 4.4 11.4 114.0 7.8 5.0 4.90 98.0 3.4 5.09 101.8 3.8 50.0 51.0 102.0 6.0 51.2 102.4 6.5 10.0 10.26 102.6 4.7 10.03 100.3 4.0 100.0 96.1 96.1 1.9 99.1 99.1 3.7 2,4-CP 0 nd nd 0 nd nd 1.0 1.04 104.0 5.0 0.99 99.0 3.1 10.0 9.5 95.0 8.4 11.9 119.0 2.8 5.0 4.89 97.8 3.4 4.81 96.2 4.8 50.0 52.7 105.4 7.5 48.4 96.8 7.2 10.0 9.67 96.7 2.1 9.91 99.1 3.8 100.0 97.4 97.4 5.1 100.1 100.1 3.4 Analytes . Spiked (ng mL−1) . River water . Lake water . Spiked (ng g−1) . Honey sample 1 . Honey sample 2 . Found (ng mL−1) . MRa (%) . RSD (%) . Found (ng mL−1) . MR (%) . RSD (%) . Found (ng g−1) . MR (%) . RSD (%) . Found (ng g−1) . MR (%) . RSD (%) . 2-CP 0 <LOQ <LOQ 0 ndb ndb 1.0 0.99 99.0 6.7 1.03 103.0 5.7 10.0 11.5 115.0 8.2 11.8 118.0 1.0 5.0 4.66 93.2 1.6 5.02 100.4 2.7 50.0 48.9 97.8 5.6 46.6 93.2 6.6 10.0 9.58 95.8 1.6 9.76 97.6 1.9 100.0 94.4 94.4 3.8 99.5 99.5 5.4 4-CP 0 nd nd 0 <LOQ nd 1.0 0.96 96.0 6.2 1.0 100.0 6.7 10.0 10.2 102.0 8.0 12.0 120.0 8.3 5.0 4.67 93.4 4.0 4.71 94.2 3.2 50.0 51.0 102.0 4.5 42.4 84.8 2.3 10.0 9.96 99.6 1.8 9.92 99.2 2.9 100.0 98.0 98.0 1.2 106.2 106.2 4.5 2,3-CP 0 nd nd 0 nd nd 1.0 0.96 96.0 5.1 1.02 102.0 5.6 10.0 9.9 99.0 4.4 11.4 114.0 7.8 5.0 4.90 98.0 3.4 5.09 101.8 3.8 50.0 51.0 102.0 6.0 51.2 102.4 6.5 10.0 10.26 102.6 4.7 10.03 100.3 4.0 100.0 96.1 96.1 1.9 99.1 99.1 3.7 2,4-CP 0 nd nd 0 nd nd 1.0 1.04 104.0 5.0 0.99 99.0 3.1 10.0 9.5 95.0 8.4 11.9 119.0 2.8 5.0 4.89 97.8 3.4 4.81 96.2 4.8 50.0 52.7 105.4 7.5 48.4 96.8 7.2 10.0 9.67 96.7 2.1 9.91 99.1 3.8 100.0 97.4 97.4 5.1 100.1 100.1 3.4 aMR, recovery of the analyte for the method bnd, not detected Open in new tab The crystalline structure of the BoxPOP was investigated by XRD. The XRD pattern in Figure 2f shows that there is a broad peak between 20o and 40o, indicating an amorphous nature. SPE conditions To get the best extraction efficiency for the analytes, the main experimental parameters that affect the extraction were investigated, including the sample solution pH, sample loading rate, sample volume and the type and volume of eluent. Three parallel experiments were performed for each condition and the average value of the results was used for the evaluation. All the optimized experiments were carried out with aliquots of 100 mL water solution containing each of the CPs at 50.0 ng mL−1. According to the optimization results shown in Figure 3, the following conditions were chosen for the experiments: sample solution pH: no adjustment; the sample loading rate: 5 mL min−1; sample volume: 100 mL of water or 80.0 mL of honey solution; eluent: 0.3 mL of alkaline acetonitrile. Evaluation of the method The linear range for the determination of the analytes was evaluated by linear regression equation and correlation coefficients (r). To assess the sensitivity of the developed method, both the limits of detection (LODs) for the target CPs at a signal-to-noise ratio of 3 (S/N = 3) and the limits of quantification (LOQs) at S/N = 10 were measured. The repeatability for the determination of the CPs with the current method was evaluated by the relative standard deviations (RSDs) obtained by five replicate determinations with both spiked water (1.0 ng mL−1) and spiked honey samples (100.0 ng g−1). In order to cancel out the matrix effect, the matrix-matched calibrations were established by spiking the CPs into the CPs-free water and honey samples. The water and honey calibration samples were prepared at different concentration levels (0.05, 0.5, 1.0, 2.0, 5.0, 10.0, 20.0 and 40.0 ng mL−1 for water samples and 0.5, 1.0, 5.0, 10.0, 20.0, 50.0, 100.0 and 400.0 ng g−1 for honey samples). The relevant results are listed in Table I. For water samples, good linearity was obtained in the range of 0.20–40.0 ng mL−1 for both 2-CP and 4-CP, and 0.50–40.0 ng mL−1 for both 2,3-DCP and 2,4-DCP with r ≥ 0.9970, and the LODs were measured to be in the range of 0.06–0.08 ng mL−1 and the LOQs were from 0.20 to 0.26 ng mL−1. For honey samples, the response (peak area) was found to be linear in the range of 10.0–400.0 ng g−1 for 2-CP and in the range of 5.0–400.0 ng g−1 for the other three analytes with r ≥ 0.9901, and the LODs were in the range of 1.5–2.0 ng g−1 and the LOQs were in the range of 5.0–6.6 ng g−1, respectively. The resulting RSDs were <5.4%. The higher LOD and LOQ values for the honey samples relative to water samples are due to the sample dilution for the honey sample which was necessary to reduce its viscosity, matrix effect and higher baseline noise. The linear regression equations for the analytes for both water and honey samples are given in Supplementary Table S1. Discussion Optimization of SPE conditions Influence of the sample solution pH The existing forms of the CPs can be affected by the solution pH change because they are weak acidic organic compounds with their pKa values of 8.49, 9.18, 7.44 and 7.89 for 2-CP, 4-CP, 2,3-DCP and 2,4-DCP, respectively. Under alkaline conditions, they can be ionized into strong hydrophilic phenonium ions, which is unfavorable for the extraction. On the other hand, when the CPs exist in molecular forms, the interactions between the CPs and the adsorbent will be strong, which is favorable for the extraction. Therefore, the pH of the sample solution should be optimized to ensure an effective adsorption of the adsorbent for the target analytes. Figure 3a shows that when sample pH is changed from 2 to 8, the peak areas remain nearly unchanged, but when the sample pH was further increased from 8 to 11, they decline sharply. On the basis of the above results and in consideration that the sample solution pH involved in this study was <7, there was no need to adjust the pH of the sample solutions. Influence of loading rate In SPE, the sample loading rate not only influences the whole analysis time but also has an effect on the extraction recoveries of the analytes. In this study, the effect of sample loading rate was investigated in the range of 1–5 mL min−1. The results in Figure 3b show that the peak areas of the CPs had no obvious changes when the loading rate was increased from 1 to 5 mL min−1. To save the analysis time, the sample loading rate was selected at 5 mL min−1. Influence of sample volume When the SPE cartridge was packed with 40.0 mg of the BoxPOP, different volumes of the sample spiked with the analytes at 50 ng mL−1 were tested. For water samples (Figure 3c), the extraction recoveries had no distinct differences when the volume of the sample was changed from 20 to 100 mL; when the sample volume was >100 mL, the recoveries for the four analytes decreased. For honey sample, when the analytes concentration in the diluted honey solution was 50 ng mL−1, the extraction recoveries declined when the sample volume exceeded 80 mL. Therefore, 100 mL of water and 80.0 mL of honey sample solutions were chosen for the experiments. Figure 4 Open in new tabDownload slide The typical chromatograms of water sample (a), the water sample spiked with the CPs at each concentration of 5.0 ng mL−1 (b), honey sample (c), and the honey sample spiked with the CPs at each concentration of 50.0 ng g−1 (d). Peak identification: 1. 2-CP; 2. 4-CP; 3. 2,3-DCP; 4. 2,4-DCP. Detection wavelength: 280 nm. The mobile phase was the mixture of acetonitrile-water at 40:60 (v/v) for (a) and (b), and at 38:62 (v/v) for (c) and (d). Figure 4 Open in new tabDownload slide The typical chromatograms of water sample (a), the water sample spiked with the CPs at each concentration of 5.0 ng mL−1 (b), honey sample (c), and the honey sample spiked with the CPs at each concentration of 50.0 ng g−1 (d). Peak identification: 1. 2-CP; 2. 4-CP; 3. 2,3-DCP; 4. 2,4-DCP. Detection wavelength: 280 nm. The mobile phase was the mixture of acetonitrile-water at 40:60 (v/v) for (a) and (b), and at 38:62 (v/v) for (c) and (d). Table III Comparison with other reported methods for the determination of CPs in real sample analysis Method . Adsorbent . LODs . Linearity . MR (%) . Sample . Ref. . HFSLM-HPLC-UV dihexyl ether 0.3–0.4 μg mL−1 1.0–200.0 ng mL−1 71.6–120.0 Water [33] MSPE-HPLC-UV Zn/Co-MPC 0.1 ng mL−1 0.2 ng mL−1 0.5–100 ng mL−1 1.0–100 ng mL−1 86.7–114.0 80.8–110.3 Tap water honey tea [34] DLLME-HPLC-UV [C4MIM][NTf2] 1.6–6.4 ng g−1 20.0–400.0 ng g−1 91.6–114.3 Honey [6] HF-LPME-HPLC-UV [C8MIM][PF6] 0.5–1.0 ng mL−1 5.0–400.0 ng mL−1 70.0–95.7 Water [35] HS-SPME-GC–MS COF 0.3–0.7 ng g−1 0.8–1.8 ng g−1 1.0–250.0 ng g−1 3.0–300.0 ng g−1 70.2–113.0 Honey canned-yellow-peach [36] SPE-HPLC-UV BoxPOP 0.06–0.08 ng mL−1 1.5–2.0 ng g−1 0.20–40.0 ng mL−1 5.0–400.0 ng g−1 93.2–104.0 84.8–120.0 Water honey This work Method . Adsorbent . LODs . Linearity . MR (%) . Sample . Ref. . HFSLM-HPLC-UV dihexyl ether 0.3–0.4 μg mL−1 1.0–200.0 ng mL−1 71.6–120.0 Water [33] MSPE-HPLC-UV Zn/Co-MPC 0.1 ng mL−1 0.2 ng mL−1 0.5–100 ng mL−1 1.0–100 ng mL−1 86.7–114.0 80.8–110.3 Tap water honey tea [34] DLLME-HPLC-UV [C4MIM][NTf2] 1.6–6.4 ng g−1 20.0–400.0 ng g−1 91.6–114.3 Honey [6] HF-LPME-HPLC-UV [C8MIM][PF6] 0.5–1.0 ng mL−1 5.0–400.0 ng mL−1 70.0–95.7 Water [35] HS-SPME-GC–MS COF 0.3–0.7 ng g−1 0.8–1.8 ng g−1 1.0–250.0 ng g−1 3.0–300.0 ng g−1 70.2–113.0 Honey canned-yellow-peach [36] SPE-HPLC-UV BoxPOP 0.06–0.08 ng mL−1 1.5–2.0 ng g−1 0.20–40.0 ng mL−1 5.0–400.0 ng g−1 93.2–104.0 84.8–120.0 Water honey This work HFSLM-HPLC-UV, hollow fiber based supported liquid membrane detection-high performance liquid chromatography-ultraviolet; MSPE, magnetic solid-phase extraction; Zn/Co-MPC, Zn/Co-magnetic porous carbon; DLLME, dispersive liquid–liquid microextraction; [C4MIM][NTf2], IL(1-butyl-3-methylimidazoliumbis (trifluoromethylsulfonyl) imide); HF-LPME, hollow fiber-liquid phase microextraction; [C8MIM][PF6], IL (1-octyl-3-methylimidazolium hexafluorophosphate); HS-SPME-GC-MS, headspace solid-phase microextraction gas chromatography–mass spectrometry; COF, TpBD, a covalent organic framework that consists of 1,3,5-triformylphloroglucinol (Tp) and benzidine (BD); SPE, solid-phase extraction Open in new tab Table III Comparison with other reported methods for the determination of CPs in real sample analysis Method . Adsorbent . LODs . Linearity . MR (%) . Sample . Ref. . HFSLM-HPLC-UV dihexyl ether 0.3–0.4 μg mL−1 1.0–200.0 ng mL−1 71.6–120.0 Water [33] MSPE-HPLC-UV Zn/Co-MPC 0.1 ng mL−1 0.2 ng mL−1 0.5–100 ng mL−1 1.0–100 ng mL−1 86.7–114.0 80.8–110.3 Tap water honey tea [34] DLLME-HPLC-UV [C4MIM][NTf2] 1.6–6.4 ng g−1 20.0–400.0 ng g−1 91.6–114.3 Honey [6] HF-LPME-HPLC-UV [C8MIM][PF6] 0.5–1.0 ng mL−1 5.0–400.0 ng mL−1 70.0–95.7 Water [35] HS-SPME-GC–MS COF 0.3–0.7 ng g−1 0.8–1.8 ng g−1 1.0–250.0 ng g−1 3.0–300.0 ng g−1 70.2–113.0 Honey canned-yellow-peach [36] SPE-HPLC-UV BoxPOP 0.06–0.08 ng mL−1 1.5–2.0 ng g−1 0.20–40.0 ng mL−1 5.0–400.0 ng g−1 93.2–104.0 84.8–120.0 Water honey This work Method . Adsorbent . LODs . Linearity . MR (%) . Sample . Ref. . HFSLM-HPLC-UV dihexyl ether 0.3–0.4 μg mL−1 1.0–200.0 ng mL−1 71.6–120.0 Water [33] MSPE-HPLC-UV Zn/Co-MPC 0.1 ng mL−1 0.2 ng mL−1 0.5–100 ng mL−1 1.0–100 ng mL−1 86.7–114.0 80.8–110.3 Tap water honey tea [34] DLLME-HPLC-UV [C4MIM][NTf2] 1.6–6.4 ng g−1 20.0–400.0 ng g−1 91.6–114.3 Honey [6] HF-LPME-HPLC-UV [C8MIM][PF6] 0.5–1.0 ng mL−1 5.0–400.0 ng mL−1 70.0–95.7 Water [35] HS-SPME-GC–MS COF 0.3–0.7 ng g−1 0.8–1.8 ng g−1 1.0–250.0 ng g−1 3.0–300.0 ng g−1 70.2–113.0 Honey canned-yellow-peach [36] SPE-HPLC-UV BoxPOP 0.06–0.08 ng mL−1 1.5–2.0 ng g−1 0.20–40.0 ng mL−1 5.0–400.0 ng g−1 93.2–104.0 84.8–120.0 Water honey This work HFSLM-HPLC-UV, hollow fiber based supported liquid membrane detection-high performance liquid chromatography-ultraviolet; MSPE, magnetic solid-phase extraction; Zn/Co-MPC, Zn/Co-magnetic porous carbon; DLLME, dispersive liquid–liquid microextraction; [C4MIM][NTf2], IL(1-butyl-3-methylimidazoliumbis (trifluoromethylsulfonyl) imide); HF-LPME, hollow fiber-liquid phase microextraction; [C8MIM][PF6], IL (1-octyl-3-methylimidazolium hexafluorophosphate); HS-SPME-GC-MS, headspace solid-phase microextraction gas chromatography–mass spectrometry; COF, TpBD, a covalent organic framework that consists of 1,3,5-triformylphloroglucinol (Tp) and benzidine (BD); SPE, solid-phase extraction Open in new tab Influence of elution conditions In order to achieve a complete elution of the analytes from the adsorbent, an optimization of the elution conditions is necessary. The CPs are weak acidic organic compounds and an alkaline condition should be favorable for their elution from the adsorbent. Therefore, in this study, the three commonly used organic solvents methanol, acetonitrile, acetone and their respective alkaline solutions (containing 1% 1.0 mol L−1 NaOH solution) were tested as the possible eluents for the CPs. The results in Figure 3d indicate that the alkaline acetonitrile was the best eluent among them. Also, different percentages of 1.0 mol L−1NaOH solution in acetonitrile (from 1 to 5%) were tested for the elution. As a result, no significant changes for the extraction recoveries of the analytes were observed in the investigated range, meaning that the percentage of the 1.0 mol L−1 NaOH solution in acetonitrile at 1% was enough. Therefore, the alkaline acetonitrile (containing 1% 1.0 mol L−1NaOH solution) was chosen as the eluent. Then, effect of the eluent volume was studied in the range of 0.3–2.1 mL. Figure 3e indicates that when the eluent volume was larger than 0.9 mL, the extraction recoveries of the analytes reached the highest and became stable. However, according to Figure 3f, the detection sensitivity for the analytes was decreased with increased eluent volume from 0.3 to 2.1 mL due to the dilution effect. Therefore 0.3 mL of the alkaline acetonitrile was finally chosen for the elution in the subsequent experiments. Prior to the next SPE, the cartridge was washed with 2 mL acetonitrile and 2 mL water, respectively. After such washing, no carryover from the cartridge was observed. Adsorption studies To evaluate the adsorption performance for CPs by the BoxPOP, a comparison study was made with the commonly used and commercially available SPE sorbents, multiwalled carbon nanotubes (MWCNTs) and C18. In the study, 40 mg of each of the adsorbents was packed into 3-mL SPE cartridge and 100 mL water solution spiked with 50 ng mL −1 of the CPs was loaded into the cartridges. A total of 0.9 mL alkaline acetonitrile was used to elute the CPs. The results in Supplementary Figure S1 show that the BoxPOP has a better extraction recovery than either MWCNTs or C18, illustrating that the BoxPOP is a good adsorbent for the CPs. Application of the method To verify the usefulness of the method, the method was applied to determine the CPs in real samples (river water, lake water and honey samples). As a result, 2-CP was detected out in two water samples, but the concentration was lower than its LOQ in both cases. 4-CP was found in honey sample 1 but at a concentration below its LOQ. The accuracy and precision of the method were assessed by analyzing the water samples spiked with the CPs at 1.0, 5.0, 10.0 ng mL−1 and the honey samples spiked with the CPs at 10.0, 50.0 and 100.0 ng g−1, respectively. The relevant data are listed in Table II. The method recoveries (MR) for the analytes were in the range of 93.2–104.0% for water samples and 84.8–119.0% for honey samples, with the RSDs below 8.4%. The typical chromatograms for the spiked real samples are shown in Figure 4. Comparison with other methods The current method was compared with other reported methods for the analysis of CPs, and the related data are listed in Table III (6, 33–36). Some important parameters including the extraction methods involved, the adsorbent materials used, LODs, linear range, MR, and sample matrices are compared. As can be seen from Table III, the LODs of the current method are lower than those obtained by HFSLM-HPLC-UV (33), MSPE-HPLC-UV (34), HF-LPME-HPLC-UV (35) and DLLME-HPLC-UV (6), but higher than those by HS-SPME-GC–MS (36). Conclusion In this work, a BoxPOP was investigated as the SPE adsorbent to enrich four CPs from water and honey samples for the first time. The BoxPOP exhibited high extraction efficiency for the target analytes. A BoxPOP based SPE method coupled with HPLC was established for the determination of some CPs in water and honey samples. The method showed low LODs, wide linear ranges, acceptable MR and repeatabilities for the determination of the CPs in real samples. However, the adsorption mechanism between the adsorbent and the analytes needs to be further studied for a better understanding of the interactions between them. Besides, further studies need to be done for the introduction of other functional groups into the BoxPOP to further improve its adsorption capability and selectivity toward certain analytes. Acknowledgments This work was supported by the National Natural Science Foundation of China (31571925, 31671930), the Natural Science Foundation of Hebei Province (B2017204025, B2020204001), the Scientific and Technological Research Programs for Hebei Provincial Universities (ZD2016085, ZD2020196) and the Natural Science Foundation of Hebei Agricultural University (LG201806). Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Ethical Approval This article does not contain any studies with human or animal subjects. 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TI - Benzoxazine Porous Organic Polymer as an Efficient Solid-Phase Extraction Adsorbent for the Enrichment of Chlorophenols from Water and Honey Samples
JF - Journal of Chromatographic Science
DO - 10.1093/chromsci/bmaa106
DA - 2020-12-24
UR - https://www.deepdyve.com/lp/oxford-university-press/benzoxazine-porous-organic-polymer-as-an-efficient-solid-phase-l9Gsqc7IoU
SP - 1
EP - 1
VL - Advance Article
IS - 
DP - DeepDyve
ER -