| Cellulose and cyclodextrin (CD) chiral stationary phases (CSPs) are of great interest in chiral separation field due to their high separation ability. Enantiomer separation can be achieved with high performance liquid chromatography (HPLC) and capillary electrochromatography (CEC) due to the form of diastereomeric molecular complexes with different stabilities between the CSP and two enantiomers. Two enantiomers can be retained or migrated with different velocity under the force created by high pressure pump or electroosmotic flow. In this study, ionic liquid1-allyl-3-methyl-imidazolium chloride (AMIMC1) was used to synthesize two cellulose-based chiral selectors, cellulose tris(3,5-dimethylphenylcarbamate)(CDMPC) and cellulose2,3-bis(3,5-dimethylphenylcarbamate)(CBDMPC), the CDMPC was coated on the surface of (5μm) silica gel, the CBDMPC was bonded on the surface of (5μm) silica gel, and then the CDMPC-coated and-bonded CSPs were prepared, respectively. The resulted chiral materials were slurry packed so as to obtain two kinds of steel chromatographic columns. Furthermore, the α-,β-, and γ-CD were bonded on the surface of (10μm) silica gel, respectively, and three different kinds of cyclodextrin chiral chromatographic columns were also prepared with the identical method by slurry packing. The prepared CDMPC and CBDMPC were characterized by FTIR,1H NMR and elementary analysis. The α-, β-, and γ-CD-bonded CSPs were characterized by FTIR and elementary analysis. The effects of mobile phase composition, flow rate and temperature on the chiral separation of8kinds of typical pesticides were investigated using the above chiral chromatographic columns. In addition, GMA-β-CD was synthesized, isolated, purified, characterized, and used as a functional monomer to prepare a chiral capillary electrochromatography (CEC) organic polymer-based monolithic column. Composition ratio, polymerization time, and temperature were optimized throughout the experiments. The effects of operating voltage, buffer pH, and acetonitrile content on the chiral separation of pesticides were evaluated. The separation mechanisms were discussed to provide an experimental and theoretical basis for developing environmentally-friendly and optically pure chiral pesticides. The results are summarized as follows:1. AMIMC1was used as a Green reaction solvent to synthesize CDMPC. A high recovery rate of the ionic liquid was confirmed by infrared and mass spectra. The effects of different modifiers on chiral separation were compared by normal-phase HPLC using a self-made CDMPC-coated chiral chromatographic column. Isopropanol was a relatively effective modifier for diniconazole, flutriafol, myclobutanil, hexaconazole, and hexythiazox. Ethanol was a relatively effective modifier for paclobutrazol and metalaxyl. Capacity factor (k’), separation factor (a), and resolution (Rs) of the enantiomers of the seven pesticides increased with the reduction in isopropanol content. At a flow rate of1.0ml/min (25℃, n-hexane/isopropanol=98:2(V/V) as mobile phase), the resolutions of diniconazole, flutriafol, paclobutrazol, hexaconazole, metalaxyl and hexythiazox were2.27,1.39,1.00,1.18,1.62and1.51, respectively, and the resolution of myclobutanil (25℃, n-hexane/isopropanol=95:5(V/V) as mobile phase) was1.58. Analysis indicated that enantiomeric separation of metalaxyl, diniconazole, flutriafol, and paclobutrazol were enthalpy-driven over a temperature range of5-25℃, and that the lower temperature of column was more beneficial for the separation of enantiomers. Enantiomeric separations of myclobutanil, hexaconazole, and hexythiazox were neither enthalpy-driven nor entropy-driven, and were not significantly affected by temperature.2. The chiral selector CBDMPC was successfully synthesized with AMIMC1as the green reaction solvent. Ionic liquid was recovered at a high recovery rate. The effect of isopropanol content on chiral separation was investigated by normal-phase HPLC using a self-made CDMPC-bonded chiral chromatographic column. The chiral column could not separate diniconazole and paclobutrazol, but did effectively separate hexaconazole, metalaxyl, myclobutanil, and flutriafol, and weakly separate hexythiazox. The k’, a, and Rs values of the samples increased with the reduction in isopropyl content in the mobile phase. In addition, analysis of flow rate on chiral separation suggested that as flow rate decreased, the elution of enantiomers of hexaconazole, metalaxyl, myclobutanil, flutriafol, and hexythiazox became slower, capacity time between enantiomers and the stationary phase increased, as increased accordingly, and separation was more effective. Although the low flow rate increased resolution, it also resulted in delayed peak time and peak tailing. Furthermore, analysis indicated that separation of metalaxyl, hexaconazole, hexythiazox, flutriafol and myclobutanil were enthalpy-driven over a temperature range of20-40℃, and that the lower temperature of column was more beneficial for the separation of enantiomers. The higher resolution of myclobutanil, hexaconazole, and metalaxyl, and the lower resolution of flutriafol and hexythiazox were detected after the addition of chloroform in the mobile phase; and the lower resolution of all five pesticides was measured after the addition of tetrahydrofuran. In general, the addition of chloroform or tetrahydrofuran during the mobile phase did not significantly reduce or damage column efficiency. It indicated that the chiral stationary phases is enough stable to be used in a variety of commonly mobile phases.3. The effects of acetonitrile content, flow rate, and temperature on metalaxyl separation was investigated by normal-phase HPLC using a self-made CDMPC-bonded chiral chromatographic column. At a flow rate of0.8ml/min (25℃, water/acetonitrile=85:15(V/V) as mobile phase), the resolution of metalaxyl was1.49. Decreased polarity of the mobile phase increased a and Rs, but delayed the peak time and peak tailing of the enantiomers of metalaxyl. The values of k’, a, and Rs increased as flow rate decreased. The separation of metalaxyl was enthalpy-driven over a temperature range of20-45℃. The lower temperature of column was more beneficial for the separation of enantiomers.4. The a-and P-CD-bonded stationary phases were prepared by ether linkage and characterized by FTIR spectra and elemental analysis. Separation effects of the five pesticides by a-and β-CD-bonded chiral columns were compared by reverse-phase HPLC. Factors influencing separation of the four chiral pesticides by the β-CD-bonded chiral column were investigated. Results indicated that the α-CD-bonded chiral column weakly separated hexythiazox, but could not separate tebuconazole, paclobutrazol, diniconazole, or hexaconazole; the β-CD-bonded chiral column separated tebuconazole, paclobutrazol, diniconazole, and hexaconazole to some extent, but could not separate hexythiazox. The β-CD-bonded stationary phase had better separation performance than the a-CD-bonded stationary phase. Decreased acetonitrile content in the mobile phase increased a and Rs, but delayed the peak time and peak tailing of enantiomers. Compared with tebuconazole and paclobutrazol, diniconazole and hexaconazole were more effectively separated by the P-CD-bonded stationary phase. The k’,α, and Rs values of tebuconazole, paclobutrazol, diniconazole, and hexaconazole increased as flow rate decreased. The separation of paclobutrazol diniconazile and hexaconazole were enthalpy-driven over a temperature range of25-45℃, and the separation of tebuconazole was neither enthalpy-driven nor entropy-driven, and was not significantly affected by temperature. 5. The y-CD-bonded stationary phase was prepared by ether linkage and characterized by infrared and elemental analysis. The effects of mobile phase composition, flow rate, and temperature on the chiral separation of paclobutrazol and diniconazole were investigated by reverse-phase HPLC. Results indicated that the y-CD-bonded stationary phase weakly separated paclobutrazol and diniconazole. At a flow rate of1.0ml/min (25℃, water/acetonitrile=90:10(V/V) as mobile phase), the resolutions of paclobutrazol and diniconazole were0.58and0.57, respectively. Both a and Rs increased as acetonitrile content in the mobile phase decreased. The Rs value of both pesticides increased as flow rate decreased. The separation of paclobutrazol and diniconazole were enthalpy-driven over a temperature range of20-40℃. The lower temperature of column was more beneficial for the separation of enantiomers.6. A kind of organic-based monolithic column was successfully prepared by in situ thermal polymerization method, using GMA-β-CD as a functional monomer, EDMA as a cross-linker, AMPS as a electroosmotic flow modifier, AIBN as an initiator and a kind of ternary porogens (DMSO, n-propanol and1,4-butanediol) The obtained capillary column was characterized by SEM and the obtained monolith was evaluated by FT-IR. The prepared monolithic column was used for the chiral separation of three typical pesticides including hexaconazole,imazalil and tebuconazole by CEC. Good separation3was obtained under the optimal conditions with a resolution of1.21,1.51and1.90using a column temperature of25℃, a buffer of50mmol/L ammonium acetate, an injection time of6s, injection pressure of10bar, separation voltage in the range of10-15kV, pH in the range of4.0-5.0and a modifier of acetonitrile in the range of20-50%. |
PDF Full Text Request