Macrocyclic polyamine-modified poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolith for capillary electrochromatography

1,4,10,13,16-Pentaazatricycloheneicosane-9,17-dione (macrocyclic polyamine)-modified polymer-based monolithic column for CEC was prepared by ring opening reaction of epoxide groups from poly(glycidyl methacrylate-co-ethylene dimethacrylate) (GMA-co-EDMA) monolith with macrocyclic polyamine. Conditions such as reaction time and concentration of macrocyclic polyamine for the modification reaction were optimized to generate substantial EOF and enough chromatographic interactions. Anodic EOF was observed in the pH range of 2.0-8.0 studied due to the protonation of macrcyclic polyamine at the surface of the monolith. Morphology of the monolithic column was examined by SEM and the incorporation of macrocyclic polyamine to the poly(GMA-co-EDMA) monolith was characterized by infrared (IR) spectra. Successful separation of inorganic anions, isomeric benzenediols, and benzoic acid derivatives on the monolithic column was achieved for CEC. In addition to hydrophobic interaction, hydrogen bonding and electrostatic interaction played a significant role in the separation process.     

Adamantyl-functionalized polymer monolith for capillary electrochromatography

An adamantyl (ADM)-functionalized monolithic stationary phase was newly synthesized by a single-step copolymerization of 1-adamantyl-(α-trifluoromethyl) acrylate, ethylene dimethacrylate, and 2-acrylamido-2-methyl-1-propanesulfonic acid in order to prevent the peak tailing of basic solutes in capillary electrochromatography and was compared with butyl methacrylate (BMA)-based one. The ADM structure shields the negatively charged groups on the surface of monolith from basic solutes, resulting in better peak shapes than BMA-based monolithic stationary phase. As the monomers ratio decreased, the monolithic column had lower retention and higher column efficiency which was likely due to lower phase ratio and smaller globule size of monolith, respectively. The ADM-functionalized monolithic columns exhibited a good repeatability and reproducibility of column preparation with relative standard deviation values below 9% in the studied chromatographic parameters.     

Dynamically coated silica monolith with ionic liquids for capillary electrochromatography

An ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) was introduced as dynamic coating of a silica monolithic column for capillary electrochromatography of phenols and nucleoside monophosphates. The run-to-run and column-to-column repeatability of migration time for six phenols were satisfactory on this column with relative standard deviation values less than 0.90 and 4.31%, respectively. Anodic electroosmotic flow (EOF) was observed, which increased with the increase of [BMIM][BF4] concentration within 120 mM and when [BMIM][BF4] concentration was above 120 mM, EOF leveled off due to the saturation of [BMIM][BF4] on the monolith. Efficient separation of phenols and nucleoside monophosphates on this dynamically coated monolithic column was obtained, compared with a dynamically coated fused-silica column and unmodified silica monolithic column. The retention behavior of uncharged phenols is mainly manipulated by hydrophobic interactions due to the presence of butyl groups, and that of nucleoside monophosphates is governed by the electrostatic attraction mechanism based on the interaction between positively charged [BMIM][BF4] moieties and negatively charged phosphate groups. In addition, silica matrix also contributes to the separation resolution.     

Analyses of synthetic antioxidants by capillary electrochromatography using poly(styrene-divinylbenzene-lauryl methacrylate) monolith

In this study, several organic polymer-based monoliths prepared by single step in situ copolymerization of styrene- and methacrylate ester-based monomers (styrene (S), divinylbenzene (DVB) and lauryl methacrylate (LMA)) were developed as stationary phases of capillary electrochromatography (CEC) for the analyses of synthetic antioxidants. These monoliths were characterized by examining the SEM image, IR spectrum, and measuring the pore size, surface area, conversion yield, and thermal decomposition temperature. The polymerization procedure was optimized by varying the reaction temperature, the reaction time, and the LMA-styrene ratio. The LMA-styrene ratio had the most significant influence on the peak symmetry of butylated hydroxyanisole (BHA) and 2, 6-di-tert-butyl-4-methyl phenol (BHT), the latter being greatly affected by excessive peak tailing in the poly(S-DVB) monolith. It showed that the interaction between the poly(S-DVB) monolith and the antioxidant (BHT or BHA) was significantly altered by the insertion of LMA. Compared with the best HPLC and CE methods previously reported, this proposed CEC method provides a comparable separation ability for the five antioxidants analyzed. This study demonstrates that the potentiality of poly(S-DVB-LMA) monolith as stationary phase, especially for CEC system, because of high thermal stability and good column reproducibility.     

Fast preparation of photopolymerized poly(benzyl methacrylate-co-bisphenol A dimethacrylate) monoliths for capillary electrochromatography

A novel porous polymer monolith was prepared in situ in a fused-silica capillary using photoinitiated polymerization. Bisphenol A dimethacrylate (BPADMA) was selected as a crosslinker, copolymerized with benzyl methacrylate (BMA) in the presence of a binary porogenic solvent consisting of cyclohexanol and 1-decanol in ≤10 min. The resulting poly(BMA-co-BPADMA) monoliths exhibited good permeability and mechanical stability. Mixtures of alkylbenzenes, polycyclic aromatic hydrocarbons (PAHs) or phenolic compounds were successfully separated by CEC. A similar monolith was also prepared with ethylene dimethacrylate (EDMA) as the crosslinker instead of BPADMA to compare the separation ability of the resulting monoliths. The results indicated that poly(BMA-co-BPADMA) monoliths have better selectivity for aromatic analytes and greater chromatographic stability in higher aqueous mobile phase.     

Preparation and evaluation of zirconia-coated silica monolith for capillary electrochromatography

Silica monoliths were fabricated inside fused-silica capillaries. Then the monolithic columns were coated with membrane-like zirconia. The zirconia-coated silica monoliths exhibited different EOF behavior comparing with that of bare silica monoliths. The magnitude and direction could be manipulated by changing the running buffers. Due to the amphoteric characteristic of zirconia, the silica monoliths with zirconia surface facilitate the separation of basic compounds. Aromatic amines and alkaloids were separated without obvious peak tailing. The zirconia surface was easily modified with octadecylphosphonic acid for the separation of neutral compounds. Column efficiency as high as 90,000 and 80,000 m⁻¹ was obtained for beberine and naphthalene, respectively. Furthermore, the zirconia coating increased the stability of the monolithic columns. Even after being exposed to severe condition, there was no apparently efficiency decrease for the test samples.     

Postpolymerization modification of a hydroxy monolith precursor. Part I. Epoxy alkane and octadecyl isocyanate modified poly (hydroxyethyl methacrylate‐co‐pentaerythritol triacrylate) monolithic capillary columns for reversed‐phase capillary electrochromatography

A novel precursor monolithic capillary column referred to as “hydroxy monolith” or OHM was prepared by the in situ copolymerization of hydroxyethylmethacrylate (HEMA) with pentaerythritol triacrylate (PETA) yielding the neutral poly(HEMA‐co‐PETA) monolith. The neutral precursor OHM capillary thus obtained was subjected to postpolymerization modifications of the hydroxyl functional groups present on its surface with 1,2‐epoxyalkanes catalyzed by boron trifluoride (BF₃) ultimately providing Epoxy OHM C‐m capillary column at varying alkyl chain lengths where m = 8, 12, 14, and 16 for RP‐CEC. Also, the same precursor OHM was grafted with octadecyl isocyanate yielding Isocyanato OHM C‐18 column to provide an insight into the effect of the nature of the linkage to the surface hydroxyl groups of the OHM precursor. While the epoxide reaction leaves on the surface of the OHM precursor hydroxy‐ether linkages, the isocyanato reaction leaves carbamate linkages on the same surface of the OHM precursor. This study revealed that changing the alkyl chain length resulted in changing the column phase ratio (?) and also the solute distribution constant (K). While increasing the surface alkyl chain length increased steeply the solute hydrophobic selectivity, i.e. methylene group selectivity, the nature of the ligand linkage produced different retention for the same solutes and affected the selectivity of slightly polar solutes. The various monoliths proved very useful for RP‐CEC of different small solutes at varying polarity over a wide range of mobile phase composition.     

One-pot synthesis of N-methylimidazolium-based porous polymer monolith for capillary electrochromatography via free radical copolymerization and quaterisation

One-pot synthesis of porous polymer monolith decorated with N-methylimidazolium in a capillary was described. The polymer matrix was synthesized by in situ copolymerization and quaterization of 3-chloro-2-hydroxylpropyl methacrylate (CHPMA), ethylene dimethacrylate (EDMA), and N-methylimidazole (N-MIz). The influencing factors including amount of cross-linkers, composition of porogenic solvents, and polymerization temperature on the formation of the monolithic column were investigated. The monolithic column exhibited high column efficiency for thiourea, up to 135 000 plates per meter, and phenylmethanol, up to 102 000 plates per meter. Different types of compounds including alkylbenzenes, phenols, and inorganic anions were successfully baseline separated by capillary electrochromatography (CEC). The separation of theses analytes on the column indicated typical reversed-phase and anion-exchange chromatographic retention mechanism.     

Terminal‐vinyl liquid crystal crown ether‐modified,vinyl‐functionalized hybrid silica monolith for capillary electrochromatography

A novel terminal‐vinyl liquid crystal crown ether (2‐[4‐(3‐undeceny‐1‐yloxy)‐phenyl]‐2‐[4′‐(4′‐carboxybenzo‐15‐crown‐5)‐phenyl] propane) (LCCE) was synthesized and used to modify hybrid silica‐based monolithic column possessing vinyl ligands for CEC. The monolithic silica matrix containing vinyl functionalities was prepared by in situ co‐condensation of tetramethoxysilane and vinyl‐trimethoxysilane via sol–gel process and chemically modified with LCCE by free radical polymerization procedure using α,α'‐azobisisobutyronitrile as an initiator. Morphology of the monolithic column was examined by SEM and mercury porosimetry and the successful incorporation of terminal‐vinyl LCCE to the vinyl‐hybrid monolith was characterized by infrared spectra. Polycyclic aromatic hydrocarbons, benzenediols, carbamate pesticides and steroids, were successfully separated on the column. The separations were dominated hydrogen bonding supplied by crown ether and hydrophobic interaction offered by the liquid crystal. The effect of ACN concentration on separation performance was studied and the result indicated that RP retention mechanism played an important role. Reproducibilities of migration times for the six selected polycyclic aromatic hydrocarbons were reasonable, with relative standard deviation less than 3.50% for five consecutive within‐column runs and were 8.38–9.11% for column‐to‐column measurements of three columns.     

Application of poly(methacrylic acid-ethylene glycol dimethacrylate) monolith microextraction coupled with capillary zone electrophoresis to the determination of opiates in human urine

A novel poly(methacrylic acid-ethylene glycol dimethacrylate) (MAA-EGDMA) monolith microextraction method coupled with CZE was proposed for rapidly determining a mixture of opiates comprising heroin, 6-monoacetylmorphine, morphine, codeine, papaverine, and narcotine in human urine. The extraction device contained a regular plastic syringe, the poly(MAA-EGDMA) monolithic capillary tube (530 microm id x 3 cm) and a plastic pinhead, which connected the monolithic capillary tube and the syringe without leakage. In the polymer monolith microextraction, the sample solution was ejected via the monolithic capillary tube by a programmable syringe pump, followed by desorption with an aliquot of appropriate solution, which was collected into a vial for the subsequent analysis by CZE. The best separation was achieved using a buffer composed of 0.1 M disodium hydrogen phosphate (adjusted to pH 4.5 with 1 M hydrochloric acid) and 20% methanol v/v with temperature and voltage of 25 degrees C and 25 kV, respectively. By applying electrokinetic injection with field-enhanced sample stacking, detection limits of 6.6-19.5 ng/mL were achieved. Excellent method of reproducibility was found over a linear range of 80-2000 ng/mL.     

Poly(divinylbenzene-alkyl methacrylate) monolithic stationary phases in capillary electrochromatography

In this study, a series of poly(divinylbenzene-alkyl methacrylate) monolithic stationary phases, which were prepared by single step in situ polymerization of divinylbenzene and various alkyl methacrylates (butyl-, octyl-, or lauryl-methacrylate), were developed as separation columns of benzophenone compounds for capillary electrochromatography (CEC). In addition to the presence of plenty of benzene moieties, the stationary phases contained long and flexible alkyl groups on the surface. With an increase in the molecular length of alkyl methacrylate, the polymeric monolith, which had higher hydrophobicity, effectively reduced the peak tailing of benzophenones, but a weaker retention was observed. The unusual phenomenon was likely due to the π–π interaction between the aromatic compound and the polymeric material. The usage of longer alkyl methacrylate as reaction monomer limited the retention of aromatic compounds on the stationary phase surface, thus the π–π interaction between them was possibly reduced. Consequently, the retention time of aromatic compounds was markedly decreased with an increase in carbon length of alkyl methacrylate that was carried on the polymeric monolith. Compared to previous reports on polystyrene-based columns in which the peak-tailing problem was reduced by decreasing the benzene moieties on the stationary phase, this study demonstrated that the undesirable retention (peak-tailing) could also be improved by the inclusion of long alkyl methacrylate to the polystyrene-based columns.     

Combining poly (methacrylic acid‐co‐ethylene glycol dimethacrylate) monolith microextraction and octadecyl phosphonic acid‐modified zirconia‐coated CEC with field‐enhanced sample injection for analysis of antidepressants in human plasma and urine

A method based on poly (methacrylic acid‐co‐ethylene glycol dimethacrylate) monolith microextraction and octadecylphosphonic acid‐modified zirconia‐coated CEC followed by field‐enhanced sample injection preconcentration technique was proposed for sensitive CE‐UV analysis of six antidepressants (doxepin, clozapine, imipramine, paroxetine, fluoxetine and chlorimipramine) in human plasma and urine. A poly(methacrylic acid‐co‐ethylene glycol dimethacrylate) monolithic capillary column was introduced for the extraction of antidepressants from urine and plasma samples. The hydrophobic main chains and acidic pendant groups of the monolithic column make it a superior material for extraction of basic analytes from aqueous matrix. After extraction, the desorption solvent, which normally provided an excellent medium to ensure direct compatibility for field‐enhanced sample injection in CE, was analyzed by CE directly. By the use of alkylphosphonate‐modified zirconia‐coated CEC for separation of the basic compounds of antidepressants, high separation efficiency and resolution were achieved because that both hydrophobic interaction between analytes and alkylphosphonate‐modified zirconia coat and electrophoretic effect work on the separation of antidepressants. The best separation was achieved using a buffer composed of 0.3 M ammonium acetate (adjusted to pH 4.5 with 1 M acetic acid) and 35% ACN v/v, with a temperature and voltage of 20°C and 20 kV, respectively. By applying both preconcentration procedures, LODs of 11.4–51.5 and 3.7–17.0 μg/L were achieved for the six antidepressants in human plasma and urine, respectively. Excellent method of reproducibility was found over a linear range of 50–5000 μg/L in plasma and urine sample.     

Acrylate ester-based monolithic columns for capillary electrochromatography separation of triacylglycerols in vegetable oils

A simple and reliable method for the evaluation of triacylglycerols (TAGs) in vegetable oils by capillary electrochromatography (CEC) with UV-Vis detection, using octadecyl acrylate (ODA) ester-based monolithic columns, has been developed. The percentages of the porogenic solvents in the polymerization mixture, and the mobile phase composition, were optimized. The optimum monolith was obtained at the following ratios: 40:60% (wt/wt) monomers/porogens, 60:40% (wt/wt) ODA/1,3-butanediol diacrylate and 23:77% (wt/wt) 1,4-butanediol/1-propanol (14 wt% 1,4-butanediol in the polymerization mixture). A satisfactory resolution between TAGs was achieved in less than 12 min with a 65:35 (v/v) acetonitrile/2-propanol mixture containing 5 mM ammonium acetate. The method was applied to the analysis of TAGs of vegetable oil samples. Using linear discriminant analysis of the CEC TAG profiles, the vegetable oils belonging to six different botanical origins (corn, extra virgin olive, hazelnut, peanut, soybean and sunflower) were correctly classified with an excellent resolution among all the categories.     

Poly(ethylene glycol) diacrylate based monolithic capillary columns for the analysis of polar small solutes by capillary electrochromatography

Monolithic stationary phases based on poly(ethylene glycol) diacrylates for capillary electrochromatography were developed. Several poly(ethylene glycol) diacrylates (M_n 250, 575, and 700) were used as single monomers and the resulting columns were carefully compared. Methanol and ethyl ether were selected as porogenic solvents, and in all cases ultraviolet radiation was selected as initiation method to prepare polymeric monoliths. The influence of the monomer chain length and ratio monomer/porogen on the morphological and electrochromatographic properties of the resulting monoliths was investigated. Several families of compounds with different polarity (alkyl benzenes, organophosphorous pesticides, benzoic acid derivatives, and sulfonamides) were selected to evaluate the performance of the fabricated monolithic columns. The best results were obtained for poly(ethylene glycol) diacrylate 700 monoliths affording efficiencies of 144 000 plates/m for retained polar aromatic small molecules and excellent reproducibility in column preparation (RSD values below 2.5%).     

Application of capillary electrochromatography (CEC) for the analysis of phenols in mainstream and sidestream tobacco smoke

Summary This paper describes the application of capillary electrochromatography (CEC) to the rapid quantitative analysis of individual mono- and dihydroxy phenols in tobacco smoke. The method derived provides reproducible and quantitative analysis of real samples, in significantly shorter times than are achieved by current GC and HPLC methods.     

Postpolymerization modification of a hydroxy monolith precursor. Part II. Epoxy biphenyl modified poly (hydroxyethyl methacrylate‐co‐pentaerythritol triacrylate) monolithic capillary columns for reversed‐phase capillary electrochromatography based on π–π and hydrophobic interactions

In this second part of the series of investigations involving the postpolymerization modification of a hydroxy monolith (OHM) capillary column, the surface hydroxyl groups were reacted with epoxy biphenyl thus yielding the so‐called Biphenyl OHM capillary column. The modification involved the epoxy ring opening of the 2‐biphenylyl glycidyl ether catalyzed by BF₃ and its subsequent reaction with the hydroxyl groups on the OHM precursor surface. The Biphenyl OHM capillary column thus obtained exhibited the typical reversed phase behavior by primarily hydrophobic interactions vis‐à‐vis the homologous series of alkyl benzenes and in addition by π–π interactions toward nitroalkane homologous series via their π‐electron rich nitro groups. This dual retention mechanism was very distinctly observed with a set of PAH solutes in the sense that the k values of the PAH solutes were comparable to those obtained on a more non polar stationary phase, namely the Epoxy OHM C‐16 reported in the preceding article. Other aromatic solutes showed the dual retention mechanism on the Biphenyl OHM capillary including phenols, anilines derivatives, and phenoxy acid herbicides. The Biphenyl OHM capillary exhibited good reproducibility from run‐to‐run, day‐to‐day, and column‐to‐column.     

Molecularly bonded chitosan prepared as chiral stationary phases in open-tubular capillary electrochromatography: comparison with chitosan nanoparticles bonded to the polyacrylamide phase

The chiral selector, chitosan (CS), was attached to the silanized capillary via a silane coupling agent, (3-glycidyloxypropyl)trimethoxysilane (GTS), to form the GTS-CS capillary, and results for this capillary were compared with those of a previous study on the copolymerization of CS with methacrylamide (MAA) (forming the MAA-CS capillary). The GTS-CS capillary did not exhibit enantioselectivity for d/l-tryptophan, whereas the GTS-BSA capillary, which was prepared by replacement of CS with bovine serum albumin (BSA), succeeded in the chiral separation with an Rs = 2.4 in Tris buffer (50 mM, pH 8.5). To increase CS attachment, the CS units were crosslinked by succinic acid, and the resulting GTS-CS-s capillary phase improved the resolution to 1.9. Alternatively, the SiH-CS-s capillary was constructed by CS attachment on the silicon hydride phase via stepwise silanization and hydrosilation reactions and crosslinking by succinic acid, but this approach could only achieve a resolution of 1.4 in Tris buffer (50 mM, pH 9.5). Although the GTS-CS-s and SiH-CS-s capillaries were still inferior to the MAA-CS capillary (Rs = 3.8), the enantioselectivities of the three capillaries were all in the range of 1.4-1.6. For the (±)-catechin sample, the plate heights of the GTS-CS-s and SiH-CS-s capillaries conditioned in pH 8.5 Tris buffer with 60% MeOH modifier were 0.9 cm ((−)-catechin) and 6.0 cm ((+)-catechin)) and 2.9 cm (−) and 3.2 cm (+), respectively, and these heights were comparable to the MAA-CS capillary (2.5 cm (−), 6.0 cm (+)) in pH 6.6 phosphate buffer with 80% MeOH. Finally, a racemate of ibuprofen, a weakly acidic anti-inflammatory drug, was successfully baseline resolved by the GTS-CS-s and SiH-CS-s capillaries in the borate buffers, which were 30 mM at pH 7.5 and 10 mM at pH 8.0, respectively.     

Poly(methacrylic acid-ethylene glycol dimethacrylate) monolith in-tube solid-phase microextraction applied to simultaneous analysis of some amphetamine derivatives in urine by capillary zone electrophoresis

A method based on in-tube solid-phase microextraction and capillary zone electrophoresis (CZE) was proposed for simultaneously determining four amphetamines (amphetamine, methamphetamine, 3,4-methylenedioxyamphetamine, and 3,4-methylenedioxymethamphetamine) in urine. A poly(methacrylic acid-ethylene glycol dimethacrylate) monolithic capillary column, which can provide sufficient extraction efficiency, was introduced for the extraction of amphetamines from urine samples. The hydrophobic main chains and acidic pendant groups of the monolithic column make it a superior material for extraction of basic analytes from aqueous matrix. After extraction, the samples were analyzed by CZE. The best separation was achieved using a buffer composed of 0.1 M disodium hydrogen phosphate (adjusted to pH 4.5 with 1 M hydrochloric acid) and 20% methanol v/v, with a temperature and voltage of 25 degrees C and 20 kV, respectively. By applying electrokinetic injection with field-amplified sample stacking, detection limits of 25-34 microg/L were achieved. Excellent method of reproducibility was found over a linear range of 0.1-5 mg/L. Determination of these analytes from abusers' urine sample was also demonstrated.     

Preparation and evaluation of o‐phenanthroline immobilized on a hybrid silica monolith modified with ionic liquids for reversed‐phase pressurized capillary electrochromatography

A novel o‐phenanthroline‐immobilized ionic‐liquid‐modified hybrid monolith for capillary electrochromatography was synthesized based on chloropropyl‐silica, which was prepared by the in situ polymerization of tetramethoxysilane and 3‐chloropropyltrimethoxysilane via a sol–gel process. The morphology of the hybrid monolith was characterized by scanning electron microscopy, and relatively stable anodic electroosmotic flow was observed under a broad pH ranged from pH 3.0 to 9.0. The separation mechanism was investigated by separating four neutral molecules (toluene, dimethylformamide, formamide, and thiourea). The obtained hybrid monolith possessed an obviously reversed‐phase retention mechanism, but when the acetonitrile content in the mobile phase was >90% v/v, a weak hydrophilic mechanism was observed on the resultant o‐phenanthroline‐modified chloropropyl‐silica hybrid monolith. The reproducibility of the column was also investigated by measuring relative standard deviations of the migration time for four neutral molecules. Relative standard deviations of run to run (n = 3), day to day (n = 3), and column to column (n = 3) were in the range of 0.4–0.7, 0.9–2.1, and 1.4–3.3%, respectively. Basic separations of various polar analytes including phenols and aromatic amines were successfully achieved.     

Enantioseparations on amylose tris(5-chloro-2-methylphenylcarbamate) in nano-liquid chromatography and capillary electrochromatography

Amylose tris(5-chloro-2-methylphenylcarbamate) was coated onto native and aminopropylsilanized silica in order to prepare chiral stationary phases (CSP) for enantioseparations using nano-liquid chromatography (nano-LC) and capillary electrochromatography (CEC). The effect of the nature of silica, the particle size and pore diameter, the chiral selector loading onto silica, the mobile phase composition and pH, as well as separation variables such as a linear flow rate of the mobile phase, applied voltage in CEC, etc. on the separation of enantiomers was studied. It was found that CSPs based on amylose tris(5-chloro-2-methylphenylcarbamate) can be used for preparation of stable capillary columns for enantioseparations by nano-LC and CEC in combination with polar organic and aqueous–organic mobile phases. Higher peak efficiency was observed in CEC than in nano-LC.