Preparation of Perphenylcarbamoylated β-Cyclodextrin-silica Hybrid

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Preparation of Perphenylcarbamoylated β-Cyclodextrin-silica Hybrid Monolithic Column with “One-Pot” Approach for Enantioseparation by Capillary Liquid Chromatography Zhenbin Zhang,†,‡ Minghuo Wu,† Ren’an Wu,*,† Jing Dong,† Junjie Ou,† and Hanfa Zou*,† †

Key Laboratory of Separation Science for Analytical Chemistry, National Chromatographic R & A Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ‡ Graduate School of Chinese Academy of Sciences, Beijing 100049, China

bS Supporting Information ABSTRACT:

Perphenylcarbamoylated β-cyclodextrin-silica (Ph-β-CD-silica) hybrid monolithic columns for enantioseparation in capillary liquid chromatography (cLC) have been prepared by a “one-pot” approach via the polycondensation of alkoxysilanes and in situ copolymerization of mono (6A-N-allylamino-6A-deoxy)-Ph-β-CD and vinyl group on the precondensed siloxanes. The morphologies of the Ph-β-CD-silica hybrid monolithic columns were characterized by optical microscopy and scanning electron microscopy (SEM), showing the uniform monolithic matrixes tightly bonded onto the capillary wall. The content of Ph-β-CD incorporated in monolithic matrix by the “one-pot” approach was ca. 2.9 times higher than that by postmodification method. The permeability of the Ph-β-CD-silica chiral hybrid monolithic column was 3.63  1014 m2, and the minimum plate height was 12 μm corresponding to 83 300 theoretical plates/meter. Enantioseparations of 13 racemates were achieved by the Ph-β-CD-silica hybrid monolithic column. In this work, since the prepolymerization system mainly consisted of organic solvent (methanol (MeOH), N, N-dimethylformamide (DMF)), the limitation and difficulty of the use of water insoluble organic monomers in the previously reported “one-pot” method was circumvented. Therefore, various β-CD derivatives as well as other hydrophobic monomers could thus be used to prepare organicsilica hybrid monolithic columns with the “one-pot” process.

C

hirality is getting comprehensive attention, especially for biologically active compounds of drugs, agrochemicals, pheromones, food additives, and flavors, which may perform differently in toxicology and pharmacology. Enantioseparation by capillary liquid chromatography (cLC) has been gaining the rising attention due to its distinct merits of high column efficiency, low consumption of sample and solvent, and expensive chiral stationary phases (CSPs).17 In recent years, various chiral selectors such as proteins,8 macrocyclic antibiotics,9 polysaccharide derivatives,10,11 cyclodextrins (CDs) and their derivatives,12 chiral ion exchangers, and chiral ligand exchangers as well as molecularly imprinted polymers (MIPs),13 have been introduced into the capillary format for the purpose of enantioseparations in cLC. Among these CSPs, CD is one of the most important chiral selectors as the truncated cone structure of CD with a cavity offering the remarkable ability in forming inclusion r 2011 American Chemical Society

complexes with a variety of molecules and ions and the chirality of CD14 and its derivatives15 enabling the enantioseparations. However, CD-based chiral columns in cLC are mainly the ones packed with conventional particulate CSPs.16 Instead of the packing process, monolithic capillary columns were synthesized by in situ preparation of polymerization or solgel process. Importantly, the monoliths are tightly anchored on the capillary wall with no need of retaining frits to support the monolithic matrixes. The polymer-based chiral monolithic capillary columns, such as charged low cross-linked polyacrylamide gels with physically17 or covalently18 incorporated β-CD (β-cyclodextrin) moieties have been extensively studied.19 Unfortunately, the high Received: February 22, 2011 Accepted: April 1, 2011 Published: April 01, 2011 3616

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Figure 1. Scheme for (A) the synthesis of mono (6A-N-allylamino-6A-deoxy)-Ph-β-CD and (B) the preparation of the Ph-β-CD-silica hybrid monolithic capillary column.

amount of β-CD bonded on monolith surface could not be expected because of the restriction of mesopores against the high molecular weight chiral selector. Though a solgel process of the use of tetramethoxysilane (TMOS) and an organfunctional silicon alkoxide containing β-CD has been used to prepare the chiral monolithic column in a single step with reduced fabrication time,20 it demonstrated low enantioseparation ability because of the use of native CDs and moreover required more than 36 h for column preparation. Previously, we have developed a simple “one-pot” approach for preparing the organicinorganic hybrid monolithic capillary columns.21 However, the incompatibility between hydrophobic organic monomer and aqueous prepolymerization mixture is the limitation to give a homogeneous solution for the “one-pot” process. In order to conquer this limitation, we developed a new polymerization system which was composed of organic solvent and a little water. A perphenylcarbamoylated β-cyclodextrinsilica (Ph-β-CD-silica) hybrid chiral monolithic column was prepared by the “one-pot” approach in the new polymerization system. The whole process needed less than 24 h only, and the content of Ph-β-CD incorporated in monolithic matrix was ca. 2.9 times higher than that introduced by the postmodification method. Enantioseparation of 13 racemates on the Ph-β-CDsilica hybrid chiral monolithic column were achieved by cLC in either normal-phase mode or reversed-phase mode.

’ EXPERIMENTAL SECTION Materials. Mono-6A-deoxy-6A-(p-tolylsulfonyl) β-CD (Ts-

β-CD) was purchased from CHEMOS GmbH (Germany). γ-Methacryloxypropyltrimethoxysiliane (γ-MAPS), phenyl isocyanate, and 1-allylamine were from Sigma Chemical Co. (St. Louis, MO, USA). Vinyltrimethoxysilane (VTMS) and cetyltrimethylammonium bromide (CTAB) were from Aldrich (Milwaukee, WI). Tetramethoxysilane (TMOS) was from Chemical Factory of Wuhan University (Wuhan, China). 2,2-Azobisisobutyronitrile (AIBN) was from Shanghai Chemical Plant (Shanghai, China) and recrystallized in ethanol before use. Fused-silica capillary with 75 μm i.d. and 375 μm o.d. was from the Reafine Chromatography Ltd. (Hebei, China). HPLC-grade methanol (MeOH) was used for the preparation of mobile phases. All

racemic compounds were purchased from Alfa Aesar China (Tianjin) Co., Ltd. Water used in all experiments was doubly distilled and purified by a Milli-Q system (Millipore Inc., Milford, MA). Other chemical reagents were of analytical grade. Synthesis of Mono (6A-N-Allylamino-6A-deoxy)-perphenylcarbamoylated β-CD (III). The schematic synthesis of mono (6A-N-allylamino-6A-deoxy)-Ph-β-CD was shown in Figure 1A, starting from the readily available material Ts-β-CD (compound I). The synthesis of compound II was according to a previously reported method with a minor modification.2225 A solution of compound I (1.97 g, 1.53 mmol) in allylamine (30 mL, 0.306 mol) was stirred for 6 h at 70 °C. The resulting yellow solution was cooled to room temperature and diluted with MeOH (30 mL). When acetonitrile (100 mL) was added, a colorless solid was precipitated. After filtering and drying in high vacuum, the compound II was obtained (1.50 g, 83.5%) with m/z at 1197.4 by MALDI-TOF MS. The synthesis of compound III was similar to the previously reported method.26 Predried compound II (1.7 mmol) was first dissolved into a flask containing 30 mL of pyridine, and then, 3.0 mL of phenyl isocyanate was added. The mixture was allowed to react at 80 °C for 12 h under nitrogen protection. After that, the cooled solution was poured into a large amount of MeOH, the insoluble fraction was isolated by centrifugation, washed carefully with MeOH, and dried under vacuum to obtain compound III. The product was characterized by IR and MALDI-TOF mass spectrometry with results as follows: IR (KBr): FT-IR (cm1, KBr): 34203161(amide NH), 3113(arom CH), 2998 (CH), 1710 (ester CdO), 1578, 1508, 1462 (arom CdC), 1281, 1036 (COC); MALDI-TOF MS: m/z at 3698.2 [Mþ]. Preparation of Perphenylcarbamoylated β-CD-Silica Hybrid Monolithic Capillary Column. The schematic preparation of Ph-β-CD-silica hybrid monolithic capillary column was illustrated in Figure 1B. At the beginning, a fused-silica capillary was pretreated and rinsed with 1.0 M NaOH for 12 h, water for 30 min, 1.0 M HCl for 12 h, and water for another 30 min, respectively, which was then dried by nitrogen stream at room temperature. Then, the capillary was filled with 50% γ-MAPS solution in MeOH(v/v), sealed with rubbers at both ends, and submerged in a water bath at 45 °C for 12 h. Finally, the capillary 3617

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Table 1. Effects of Synthesis Parameters on the Formation of Perphenylcarbamoylated β-CD-Silica Hybrid Monolitha

a Other preparation conditions: TMOS, 125 μL; DMF, 60 μL; MeOH, 190 μL; H2O, 62 μL; 102 M ammonia solution, 50 μL; β-CD derivative, 26.4 mg. b High resolution images of optical microscopy are illustrated in Supporting Information.

was rinsed with MeOH to flush out the residual reagent and dried by passage of nitrogen. Thus, the inner wall of the capillary was modified by a layer of γ-MAPS, which is available for subsequent attachment of monolith to the wall during the polymerization reaction.27 For the preparation of the Ph-β-CD-silica hybrid monolithic capillary column, a prepolymerizable mixture was prepared by mixing MeOH (190 μL), N,N-dimethylformamide (DMF) (60 μL), H2O (62 μL), TMOS (125 μL), VTMS (150 μL), CTAB (3.2 mg), 102 M ammonia solution (50 μL), mono(6A-N-allylamino-6A-deoxy)-Ph-β-CD, and AIBN (1 mg) at room temperature and sonicating for about 10 min to form a homogeneous solution. After that, the mixture was manually introduced into the pretreated capillary to an appropriate length with a syringe. When both ends of the capillary were sealed with two pieces of rubbers, the capillary was incubated at 40 and 60 °C for 12 h, respectively. The obtained Ph-β-CD-silica hybrid monolithic capillary column was then flushed with MeOH to remove the CTAB and other residuals. Instruments and Methods. cLC experiments were performed on an Agilent 1100 LC system (Hewlett-Packard) equipped with a micropump and a UV detector. The flow rate of the pump was set at 40250 μL/min. For obtaining a flow rate of nanoliters per minute, a T-union connector was used to serve as a splitter, with one end connected to the monolithic column and one end connected to a blank capillary (50 μm i.d.). The actual flow rate in the monolithic column was 80450 nL/min. A 7725i injector equipped with a 20 μL sample loop was equipped before a T-union split with the split ratio of 500:1. A detection window was made by removing the polyimide coating of a fused-silica capillary with a razor blade in the empty section of the capillary at the edge of the hybrid silica continuous bed. The detection wavelength was set at 214 nm. All data obtained were based on three runs. The retention factor (k0 ) was defined as (tr  t0)/t0, where tr and t0 represent the retention times of an analytes and an unretained compound in this work, respectively. The separation factor (R) was defined as k20 /k10 , where k10 and k20 were the retention factor of enantiomer that first and second eluted, respectively.

’ RESULTS AND DISCUSSION Optimization of Synthesis Conditions for the Perphenylcarbamoylated β-CD-Silica Hybrid Monolithic Capillary Column. The major advantage of our procedure is its simplicity for

straightforward preparation of the chiral monolithic column by directly incorporating the Ph-β-CD functionalities into the porous mononlithic matrixes. The physical and chromatographic properties of the monolithic matrixes are easily controlled by changing various factors during the preparation procedure. In the present work, the influences of the ratio of TMOS/VTMS, polycondensation temperature, content of supermolecule template (CTAB), the amount of chiral monomer, the ratio of DMF/ MeOH, and the content of H2O on the column morphology and chromatographic properties were studied in detail. The formation of the Ph-β-CD-silica hybrid monolithic capillary column involves three major reactions: hydrolysis, polycondensation, and copolymerization of the precondensated siloxanes and mono (6A-N-allylamino-6A-deoxy)-Ph-β-CD. The effect of the ratio of TMOS/VTMS in the reaction mixture on the formation of the Ph-β-CD-silica hybrid monolithic capillary column by varying its ratios from 125:100 to 125:175 was investigated. The resulted optical microscope images of obtained columns A1, A2, and A3 corresponding to the ratio of TMOS to VTMS at 125:100, 125:150, and 125:175 were displayed in Table 1, respectively. As seen from Table 1, the lower content of VTMS (as for column A1) in the reaction mixture would result in the transparent monolith inside the capillary; the higher content of VTMS (as for columns A3) would result in the slack monolith. Only the ratio of TMOS to VTMS at 125:150 could result in homogeneous and semitransparent monolithic matrixes within the confine of the capillary. Because the polycondensation is temperature sensitive, the effect of temperature on the morphology and permeability of resultant monolithic columns was thus examined in detail. The optical microscopy images of resulted columns prepared under the copolymerization temperature of 35, 40, and 45 °C, respectively, responding to B1, A2, and B2 were shown in Table 1. 3618

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Analytical Chemistry From Table 1, it was clearly seen that the temperature of 35 °C was not appropriate for the co-condensation of column B1, in which the monolithic matrix was seriously detached from the inner capillary wall due to the incomplete co-condensation of silane monomers. With the increase of the condensation temperature to 40 °C, the obtained monolithic matrixes were homogeneous and fully filled in the capillary. However, further increase of the condensation temperature to 45 °C would result in the transparent monolith with low permeability inside the capillary and bad permeability since it was even hard to pump MeOH through the monolith. These results confirmed that the co-condensation of the silane monomers was temperature dependent and the increase of the temperature would accelerate the co-condensation of silane monomers. The surfactant CTAB acts as supramolecular template in the formation of the solgel monolith, and it can be easily removed by a simple solvent extraction. As has been discussed in previous publications,28 the formation of an organicinorganic hybrid mesostructure is the result of the delicate balance of two competitive processes-organizations of the template and polymerization. Therefore, the contents of CTAB have dramatic effects on the column morphology and efficiency. In our experiment, it was found that, without CTAB or if the content of CTAB was too low in the prepolymerization mixture, it would lead to the difficulty in obtaining a desirable monolithic capillary column. However, when the content of CTAB was too high, it would lead to a substantial deterioration of column efficiency, so an appropriate amount of CTAB should be taken into consideration. The optical microscopy images of resulted columns prepared with 0 mg, 0.8 mg (the concentration of CTAB in the prepolymerization mixture is 1.4 mg/mL), 3.2 mg/mL (5.45 mg/mL), and 9.3 mg (15.8 mg/mL) CTAB, respectively, responding to C1, C2, A2, and C3 were shown in Table 1. As can be seen from Table 1, without CTAB (for column C1), the resulted monolithic matrix in the capillary is transparent and inhomogeneous. When CTAB was added in the polymerization mixture, the obtained monolithic matrix was homogeneous and semitransparent. However, further increase in the content of CTAB (more than 15.8 mg/mL in the polymerization mixture) would lead to the slacked morphology of monolithic matrix. The effects of CTAB contents on the enantioseparation abilities of the resulted chiral capillary monolithic columns were also investigated (Supporting Information Figure 2A). The amount of chiral monomer in the prepolymerization mixture would result in different chromatographic properties of the column, such as the retention, resolution, and efficiency. Here, the influence of the concentration of chiral monomer in the prepolymerization mixture on the enantioseparation abilities of the resulted chiral capillary monolithic columns was investigated in detail (Supporting Information Figure 2B). It can be seen that the retention time and resolution of the enantiomers increased by increasing concentration of chiral monomer from 37.0 mg/mL, 45.0 mg/mL, and 53.3 mg/mL to 60.6 mg/mL in the prepolymerization mixture, and the baseline separation of compound 3 (shown in Figure 4) could be achieved when the concentration of chiral monomer was 45.0 mg/mL. When there was a further increase in the concentration of chiral monomer to 60.6 mg/mL, it was difficult to dissolve in the prepolymerization mixture completely and form a homogeneous solution. The initial purpose of adding DMF in the prepolymerization mixture was to dissolve the chiral monomer; therefore, increasing the volume of DMF in the prepolymerization mixture will allow

TECHNICAL NOTE

Figure 2. SEM photographs of the Ph-β-CD-silica hybrid monolith.

more chiral monomer to dissolve which could obtain a capillary monolithic column with higher enantioseparation ability. Additionally, the presence of DMF in the prepolymerization mixture would act as drying control chemical additive which could avoid the shrinkage of the resulted monolithic columns.29 However, the amount of DMF also has great effect on the morphology and permeability of the resulted capillary monolithic column. Here, we had investigated the effect of the ratio of DMF/MeOH on the preparation of the monolithic capillary column in detail. When the ratio of DMF/MeOH increased from 20:230 to 60:190, more content of the monomer could be dissolved in the prepolymerization mixture without a significant effect on the morphology and permeability of the resulted capillary monolithic column. However, when the ratio of DMF/MeOH increased to 100:150, the silanes could not dissolve in the prepolymerization mixture to form a homogeneous solution and the resulted monolithic matrix in the capillary was transparent and inhomogeneous. Thus, we chose the ratio of DMF/MeOH at 60:190. Due to the hydrophobic property of the TMOS, VTMS, and chiral monomer, the reactants could not mix with each other when only water was utilized as the solvent in the reaction. On the other hand, when DMF and MeOH was substituted for water or the volume of water was too small in the prepolymerization mixture, the hydrolysis of the TMOS and VTMS could not proceed completely; therefore, the subsequent condensation and polycondensation were inhibited. As a result, the reactants were unable to gel into a monolithic matrix. To address the aforementioned problems associated with the single-solvent system, a mixture of water, MeOH, and DMF was used. Detailed studies demonstrated that the most suitable volume of water was 62 μL in a total of 637 μL of a polymerization mixture. In this case, the reactant system underwent transition from a cloudy mixture into a transparent, clear homogeneous phase during a 15 min ultrosonication. After being kept in a water bath at 40 °C for 12 h, the sol solidified into a white gel. Characterization of the Perphenylcarbamoylated β-CDSilica Hybrid Monolithic Capillary Column. After the careful optimization of synthesis conditions, the Ph-β-CD-silica hybrid monolithic capillary column was subsequently prepared and characterized. The scanning electron microscopy (SEM) photographs of Ph-β-CD-silica hybrid monolithic capillary column prepared under the optimized conditions were shown in Figure 2. A uniform Ph-β-CD-silica hybrid monolithic matrix was obtained 3619

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Figure 3. (A) Relationship between flow velocity and the plate height of the Ph-β-CD-silica hybrid monolithic capillary column. Experimental conditions: effective length, 20 cm  75 i.d.; mobile phase, MeOH; test compounds, toluene; detection wavelength, 214 nm. (B) The breakthrough curves of (a) column without chiral monomers; (b) column prepared by postmodification; (c) column prepared by “one-pot” process. Conditions: column, 20 cm  200 μm; mobile phase, MeOH/H2O = 50:50, concentration of 2-naphanol, 1  102 mol/L.

within a 75 μm id capillary, and the resulted chiral monolithic matrix was tightly bonded onto the capillary wall due to the copolymerization among γ-MAPS, mono (6A-N-allylamino-6Adeoxy)-Ph-β-CD, and VTMS. In addition, the residual hydroxyl group on the inner wall surface of the capillary may also take part in the solgel process. The mechanical stability of the Ph-β-CD-silica hybrid monolithic capillary column was examined by connecting the column (effective length, 25 cm) to a cLC pump using acetonitrile (ACN)/H2O = 40:60 as mobile phase with the flow rate ranging from 0.24 to 10.8 μL/min (in split mode). The measured backpressure was linearly increased from 5 to 19 MPa with relation factor of 0.9971 when the flow rate was increased. These results indicated that the obtained chiral monolithic capillary column possessed good mechanical stability under the pressure of 20 MPa. Using Darcy’s Law30 of permeability B0 = FηL/(πr2ΔP), where F is the flow rate of the mobile phase, η is the viscosity of the mobile phase with the value of 0.801 cP,31 L is the effective length of column, r is the inner radius of the column, and ΔP is the pressure drop of the column. The permeability of the monolithic column was calculated as 3.63 1014 m2, which indicated the good permeability of the prepared monolithic column. The column efficiency of the chiral hybrid monolithic capillary column was evaluated in cLC by changing the flow rate from 80 to 450 nL/min. The relationship between the flow velocity and the plate height which was shown in Figure 3A was obtained using toluene as test sample. The minimum plate height was determined as 12 μm corresponding to 83 300 theoretical plates per meter. Because the amount of Ph-β-CD on the surface of the monolithic matrix had a great effect on its enantioseparation ability, the bonding contents of chiral selectors on the surface of the monolithic capillary column with the exact same size (20 cm  200 μm i.d.) prepared by a “one-pot” process and postmodification, respectively, were measured and compared with each other by studying their adsorption capacities of 2-naphthol which could form inclusion complexes with β-CD by a ratio of 1:1. First, the Ph-β-CD-silica hybrid monolithic capillary column and the silica monolithic capillary column prepared without chiral monomer was thoroughly flushed with MeOH/H2O = 50:50, respectively, until a stable baseline was observed at 214 nm.

Second, the tube from the reservoir to the inlet of the monolithic column was filled with a solution of the 2-naphthol in MeOH/H2O = 50:50 (102 mol/L). When the pump (flow rate, 450 nL/min) was started and the signal was simultaneously recorded at 214 nm, the breakthrough curves were obtained and shown in Figure 3B. After that, the silica monolithic capillary column prepared without chiral monomer was filled with mono(6A-N-allylamino-6A-deoxy)-Ph-β-CD solution in DMF at the same concentration as in the preparation of the Ph-β-CD silica hybrid monolithic capillary column through a “one-pot” process. When both ends of the capillary were sealed with two pieces of rubber, the column was put in the water bath at 60 °C for 12 h. Then, the column was thoroughly flushed with DMF and followed by MeOH to remove unreacted monomers. The breakthrough curve was obtained by the same method as described above. The results illustrate the breakthrough time is 38.56 min for the Ph-β-CD-silica hybrid monolithic capillary column prepared through a “one-pot” process and 23.13 min for the column made by the postmodification method. The silica monolithic capillary column was prepared without the chiral selector in which the 2-naphthol could not be retained on and had an breakthrough time of 15 min. When the void volume of the column was considered, the adsorption capacity of the monoliths prepared by a “one-pot” process and postmodification for 2-naphthol were 1.06  104 mmol and 3.66  105 mmol, respectively, which indicates the specific adsorption capacity of the column prepared by the “one-pot” process was approximately 2.9 times higher than the one made by the postmodification method. In order to study the reproducibility of the Ph-β-CD-silica hybrid monolithic capillary column and enantioselectivity of different batches, we have prepared many batches of columns. Column-to-column reproducibility of the monolithic columns was investigated in terms of the retention factor and the enantioselectivity with compound 3 as tested sample. It was observed that the retention factor had a relative standard deviation of 3.19% (n = 3) and the reproducibility of the enantioselectivity for individual analyte was less than 5.66% (n = 3), respectively. These results indicated that the reproducibility of the monolithic columns was quite satisfactory. Enantioseparation of Racemates on the Perphenylcarbamoylated β-CD-Silica Hybrid Monolithic Capillary Column. 3620

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Figure 4. Enantioseparation of chiral compounds on the Ph-β-CD-silica hybrid monolithic column by cLC. Experimental conditions: effective length, 30 cm  75 μm i.d.; mobile phase, (a) MeOH/TEAA (pH = 4.2) = 60:40, MeOH/TEAA (pH = 4.2) = 80:20, Hexane/IPA = 90:10; flow rate, 120 nL/ min; detection wavelength, 214 nm.

Chromatographic separation was first attempted using hexane/ isopropanol (IPA) as mobile phase in the normal-phase mode and MeOH/triethylammonium acetate (TEAA) as mobile phase in the reversed-phase mode. The enantioseparation of 13 various racemates was realized on the Ph-β-CD-silica hybrid monolithic column by cLC, and representative chromatograms were summarized in Figure 4. The separation factor (R)

of compounds 1 and 2 was 1.23 and 1.2, respectively, which was higher than the previously reported value of 1.04 on a conventional packed CD column.25 For compound 9, the R value reached 1.95 which is much higher than the previously reported one of 1.28 on conventional packed CD columns.32 All these results indicated that the higher functionalization contributes to greater selectivity as compared to packed columns. 3621

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’ CONCLUSION The incorporation of chiral monomer of Ph-β-CD derivatives into the silica monolithic matrix was developed by a simple “onepot” approach, with the polycondensation and the polymerization of the organic and inorganic monomers subsequently carried out within the confine of a capillary. The morphologies of the Phβ-CD-silica hybrid monolithic capillary columns were characterized by optical microscopy and scanning electron microscopy, showing a uniform monolithic matrix and tightly bonding onto the capillary wall. The content of Ph-β-CD incorporated into the monolithic matrix prepared by the “one-pot” approach was approximately 2.9 times higher than that by the postmodification method by the breakthrough curves. The permeability of the prepared chiral hybrid monolithic column was calculated as 3.63  1014 m2, and the minimum plate height was determined as 12 μm which corresponds to 83 300 theoretical plates per meter. Enantioseparations of 13 racemates were achieved on the Ph-β-CD-silica hybrid monolithic capillary columns. Importantly, since the prepolymerization system we developed mainly consisted of organic solvent (MeOH, DMF), the limitation and difficulty of the use of water insoluble organic monomers in the “one-pot” method we developed previously was circumvented. Therefore, various β-CD derivatives as well as other hydrophobic monomers could be used directly to prepare organicsilica hybrid monolithic columns with a “one-pot” process. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (H.F. Zou); [email protected] (R.A. Wu).

’ ACKNOWLEDGMENT Financial support from the National Natural Sciences Foundation of China (Nos. 20735004, 20975101, 20875089) and the Creative Research Group Project by NSFC (No.21021004) to Dr. H. Zou and the Hundred Talent Program of Dalian Institute of Chemical Physics of Chinese Academy of Sciences to Dr. R. Wu is greatly acknowledged.

TECHNICAL NOTE

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dx.doi.org/10.1021/ac200414r |Anal. Chem. 2011, 83, 3616–3622