Preparation of a neutral porous monolith and its evaluation in pressurized capillary electrochromatography with neutral and charged solutes

A neutral, nonpolar monolithic capillary column was evaluated as a hydrophobic stationary phase in pressurized CEC system for neutral, acidic and basic solutes. The monolith was prepared by in situ copolymerization of octadecyl methacrylate and ethylene dimethacrylate in a binary porogenic solvent consisting of cyclohexanol/1,4‐butanediol. EOF in this hydrophobic monolithic column was poor; even the pH value of the mobile phase was high. Because of the absence of fixed charges, the monolithic capillary column was free of electrostatic interactions with charged solutes. Separations of neutral solutes were based on the hydrophobic mechanism with the pressure as the driving force. The acidic and basic solutes were separated under pressurized CEC mode with the pressure and electrophoretic mobility as the driving force. The separation selectivity of charged solutes were based on their differences in electrophoretic mobility and hydrophobic interaction with the stationary phase, and no obvious peak tailing for basic analytes was observed. Effects of the mobile phase compositions on the retention of acidic compounds were also investigated. Under optimized conditions, high plate counts reaching 82 000 plates/m for neutral compounds, 134 000 plates/m for acid compounds and 150 000 plates/m for basic compounds were readily obtained.     

Capillary electrochromatography with monolithic stationary phases. 4. Preparation of neutral stearyl-acrylate monoliths and their evaluation in capillary electrochromatography of neutral and charged small species as well as peptides and proteins

A neutral, nonpolar monolithic capillary column having a relatively strong electroosmotic flow (EOF) yet free of electrostatic interactions with charged solutes was developed for the reversed-phase capillary electrochromatography (RP-CEC) of neutral and charged species including peptides and proteins. The neutral nonpolar monolith is based on the in situ polymerization of pentaerythritol diacrylate monostearate (PEDAS) in a ternary porogenic solvent composed of cyclohexanol, ethylene glycol, and water. PEDAS plays the role of both the cross-linker and the ligand provider, generating a macroporous nonpolar monolith having C17 chains as the chromatographic ligands. Despite the fact that the neutral PEDAS monolith is devoid of fixed charges, the monolithic capillary columns exhibited a relatively strong EOF due to the ability of PEDAS to adsorb sufficient amounts of electrolyte ions from the mobile phase. The adsorbed ions imparted the neutral PEDAS monolith the zeta potential necessary to support the EOF required for mass transport across the monolithic column. The absence of fixed charges on the surface of the neutral PEDAS monolith and in turn the adsorption sites for electrostatic attraction of charged solutes allowed the rapid and efficient separations of proteins and peptides at pH 7.0, with an average plate number of 255,000 and 121,000 plates/m, respectively. To the best of our knowledge, this constitutes the first report on the separation of proteins at neutral pH by RP-CEC using a neutral monolithic column.     

Sulfoalkylbetaine‐based monolithic column with mixed‐mode of hydrophilic interaction and strong anion‐exchange stationary phase for capillary electrochromatography

A novel monolithic stationary phase with mixed mode of hydrophilic and strong anion exchange (SAX) interactions based on in situ copolymerization of pentaerythritol triacrylate (PETA), N,N‐dimethyl‐N‐methacryloxyethyl N‐(3‐sulfopropyl) ammonium betaine (DMMSA) and a selected quaternary amine acrylic monomer was designed as a multifunctional separation column for CEC. Although the zwitterionic functionalities of DMMSA and hydroxy groups of PETA on the surface of the monolithic stationary phase functioned as the hydrophilic interaction (HI) sites, the quaternary amine acrylic monomer was introduced to control the magnitude of the EOF and provide the SAX sites at the same time. Three different quaternary amine acrylic monomers were tested to achieve maximum EOF velocity and highest plate count. The fabrication of the zwitterionic monolith (designated as HI and SAX stationary phase) was carried out when [2‐(acryloyloxy)ethyl]trimethylammonium methylsulfate was used as the quaternary amine acrylic monomer. The separation mechanism of the monolithic column was discussed in detail. For charged analytes, a mixed mode of HI and SAX was observed by studying the influence of mobile phase pH and salt concentration on their retentions on the poly(PETA‐co‐DMMSA‐co‐[2‐(acryloyloxy)ethyl]trimethylammonium methylsulfate) monolithic column. The optimized monolith showed good separation performance for a range of polar analytes including nucleotides, nucleic acid bases and nucleosides, phenols, estrogens and small peptides. The column efficiencies greater than 192 000 theoretical plates/m for estriol and 135 000 theoretical plates/m for charged cytidine were obtained.     

Preparation and evaluation of a sulfoalkylbetaine‐based zwitterionic monolithic column for CEC of polar analytes

A novel polymethacrylate‐based monolithic column with covalently bonded zwitterionic functional groups was prepared by in situ copolymerization of N,N‐dimethyl‐N‐methacryloxyethyl N‐(3‐sulfopropyl) ammonium betaine (SPE), pentaerythritol triacrylate (PETA), and vinylsulfonic acid (VS) in a binary porogenic solvent consisting of cyclohexanol and ethylene glycol. This monolith was developed as a separation column for CEC. While SPE functioned as both an electrostatic interaction stationary phase and the polar ligand provider, VS was employed to generate EOF. PETA, which has much more hydrophilicity due to a hydroxyl sub‐layer, was used to replace ethylene dimethacrylate as a cross‐linker. The monolith provided an adequate EOF when VS level was maintained at 0.6% w/w. Different monolithic stationary phases were easily prepared by adjusting the ratio of PETA/SPE in the polymerization solution as well as the composition of the porogenic solvent. The observed RSD were ≤3.6, ≤4.3 and ≤5.6% for the EOF velocity, retention time, and column efficiency, respectively. The column efficiencies greater than 145 000 theoretical plates/m for thiourea and 132 000 theoretical plates/m for charged cytidine were obtained. The poly(SPE‐co‐PETA‐co‐VS) monolith showed good selectivity for neutral and charged polar analytes. It was found that the separation mechanism of charged polar solutes was attributed to a mixed mode of hydrophilic interaction and electrostatic interaction, as well as electrophoresis. No peak tailing was observed for the separation of basic compounds, such as basic nucleic acid bases and nucleoside on the monolith.     

Naphthyl methacrylate‐based monolithic column for RP‐CEC via hydrophobic and π interactions

A neutral naphthyl methacrylate‐based monolith (NMM) was introduced for RP‐CEC of various aromatic compounds via hydrophobic and π interactions. It was characterized over a wide range of elution conditions to gain insight into its RP retention mechanism toward the various solute probes under investigation. First, the NMM column exhibited cathodal EOF at various mobile phase compositions and pH suggesting that although the NMM column is void of fixed charges, it acquires a negative zeta potential. It is believed that the negative zeta potential is imparted by the adsorption of mobile phase ions to the NMM surface. The NMM column exhibited π–π interactions in addition to hydrophobic interactions due to the aromatic and nonpolar nature of its naphthyl ligands. In all cases, the retention of the various aromatic test solutes including PAHs, benzene derivatives, toluene derivatives, anilines and toluidine, tolunitrile and nitrotoluene positional isomers on the NMM column were compared to their retention on an octadecyl acrylate‐based monolithic column. Not only were the values of the retention factors of the various solutes on the NMM column higher than those obtained on the octadecyl acrylate‐based monolithic column under otherwise the same CEC conditions, but the elution orders were also different on both columns with a superior and unique selectivity exhibited by the NMM column.     

Capillary electrochromatography with monolithic stationary phases: 1. Preparation of sulfonated stearyl acrylate monoliths and their electrochromatographic characterization with neutral and charged solutes

A novel monolithic stationary phase having long alkyl chain ligands (C17) was introduced and evaluated in capillary electrochromatography (CEC) of small neutral and charged species. The monolithic stationary phase was prepared by the in situ copolymerization of pentaerythritol diacrylate monostearate (PEDAS) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) in a ternary porogenic solvent consisting of cyclohexanol/ethylene-glycol/water. While AMPS was meant to support the electroosmotic flow (EOF) necessary for transporting the mobile phase through the monolithic capillary, the PEDAS was introduced to provide the nonpolar sites for chromatographic retention. Monolithic columns at various EOF velocities were readily prepared by conveniently adjusting the amount of AMPS in the polymerization solution as well as the composition of the porogenic solvent. The monolithic stationary phases thus obtained exhibited reversed-phase chromatography behavior toward neutral solutes and yielded a relatively strong EOF. For charged solutes (e.g., dansyl amino acids), nonpolar as well as electrostatic interaction/repulsion with the monoliths were observed in addition to electrophoretic migration. Therefore, for charged solutes, selectivity and migration can be readily manipulated by changing various parameters including the nature of the monolith and the composition of the mobile phase (e.g., pH, ionic strength and organic modifier). Ultrafast separation on the time scale of seconds of 17 different charged and neutral pesticides and metabolites were performed using short capillary columns of 8.5 cm x 100 microm ID.     

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.     

Preparation of organic–inorganic hybrid silica monolith with octyl and sulfonic acid groups for capillary electrochromatograhpy and application in determination of theophylline and caffeine in beverage

An organic–inorganic hybrid silica monolithic column with octyl and sulfonic acid groups has been prepared by sol–gel technique for capillary electrochromatograhpy. The structure of hybrid monolith was optimized by changing the composition of tetraethoxysilane (TEOS), octyltriethoxysilane (C₈-TEOS) and 3-mercaptopropyltrimethoxysilane (MPTMS) in the mixture of precursors. Then, the obtained hybrid monolith was oxidized using hydrogen peroxide (30%, w/w) to yield sulfonic acid groups. The sulfonic acid group, which served as strong cation-exchanger, dominated the charge on the surface of the capillary column and generated stable electroosmotic flow (EOF) in a wide range of pH. The monolithic column was characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy and elemental analysis (EA), and the performance of column was evaluated in detail by separating different kinds of compounds with column efficiency up to 155,000 plates/m for thiourea. In addition, this monolithic column was also applied in the analysis of theophylline (TP) and caffeine (CA) in beverages. The detection limits were 0.39 and 0.48 μg/mL for theophylline and caffeine, respectively. The method reproducibility was tested by evaluating the intra- and inter-day precisions, and relative standard deviations of less than 3.9 and 8.4%, respectively, were obtained. Recoveries of compounds from spiked beverage samples ranged from 87.2 to 105.2%.     

Neutral octadecyl monolith for reversed phase capillary electrochromatography of a wide range of solutes

A neutral octadecyl monolithic (ODM) column for RP capillary electrochromatography (RP-CEC) has been developed. The ODM column was prepared by the in situ polymerization of octadecyl acrylate (ODA) as the monomer and trimethylolpropanetrimethacrylate (TRIM) as the crosslinker, in a ternary porogenic solvent containing cyclohexanol, ethylene glycol, and water. The ODM column exhibited cathodal EOF over a wide range of pH and ACN concentration in the mobile phase despite the fact that it was devoid of any fixed charges. It is believed that the EOF is due to the adsorption of ions from the mobile phase onto the surface of the monolith thus imparting to the neutral ODM column the zeta potential necessary to support the EOF required for mass transport across the monolithic column. Furthermore, the adsorption of mobile phase ions to the neutral monolith modulated solute retention and affected the separation selectivity. The wide applications of the neutral ODM column were demonstrated by its ability to separate a wide range of small and large solutes, both neutral and charged. While the separation of the neutral solutes was based on RP retention mechanism, the charged solutes were separated on the basis of their electrophoretic mobility and hydrophobic interaction with the C18 ligands of the stationary phase. As a typical result, the neutral monolithic column was able to separate peptides quite rapidly with a separation efficiency of nearly 200,000 plates/m, and this efficiency was exploited in tryptic peptide mapping of standard proteins, e. g., lysozyme and cytochrome C, by isocratic elution.     

A novel, neutral hydroxylated octadecyl acrylate monolith with fast electroosmotic flow velocity and its application to the separation of various solutes including peptides and proteins in the absence of electrostatic interactions

A neutral hydroxylated octadecyl monolith (ODM-OH) for reversed-phase capillary electrochromatography has been developed. The ODM-OH was prepared by the in situ polymerization of octadecyl acrylate and pentaerythritol triacrylate (PETA) in a ternary porogenic solvent. Pentaerythritol triacrylate possesses a hydroxyl functional group, which imparts the monolith with a hydrophilic group, thus the acronym ODM-OH. The ODM-OH column exhibited cathodal EOF over a wide range of pH and ACN concentration in the mobile phase despite the fact that it was devoid of any fixed charges. This ODM-OH monolith exhibited stronger EOF than its counterpart the ODM made from the in situ polymerization of octadecyl acrylate and trimethylolpropane trimethacrylate. Similar to ODM, it is believed that the EOF was due to the adsorption of ions from the mobile phase onto the surface of the monolith thus imparting the neutral monolithic column the zeta potential necessary to support the EOF. The higher EOF exhibited by ODM-OH was due to the presence of polar OH groups on its surface, which would favor stronger adsorption of ions from the mobile phase. The wide applications of the neutral ODM-OH column were demonstrated in the separation of a wide range of small and large solutes. As a typical result, the ODM-OH was able to separate proteins quite rapidly yielding 200,000 plates/m.     

A Novel Method for Synthesis of ZSM‐5 Coatings on Monolithic Cordierite Substrate

ZSM‐5 coatings, have been synthesized onto a monolithic cordierite substrate by an environmental friendly and high coating selectivity method—Vapor Phase Transport (VPT). With this method, an aluminosilicate gel coated onto the monolithic cordierite substrate has been transformed into a ZSM‐5 layer under vapors of n‐butylamine and water, n‐Butylamine played a key role in the forming of ZSM‐5 layer on the cordierite substrate. The ZSM‐5/cordierite monolith composites prepared by this method were ion‐exchanged with Cu²⁺ and tested for the selective catalytic reduction of NO by propane. The deNO_x activities of Cu/ZSM‐5 monolith catalysts were not only dependent on the ion‐exchange methods, but also on the ZSM‐5 loading of the monolith catalysts. The best result was obtained over the Cu (B3)/ZSM‐5 monolith catalyst, which had a ZSM‐5 loading of about 13% and was prepared by a pressure exchange procedure. At a temperature of 723 K and a space velocity of 10,000 h^?1 (based on the monolith volume), 85% of NO conversion and 93% of C₃H₃ conversion were achieved over the Cu(B3)/ZSM‐5 monolith catalyst.     

Hydrophilic interaction chromatography: A promising alternative to reversed‐phase liquid chromatography systems for the purification of small protonated bases

The overloaded band profiles of the protonated species of propranolol and amitriptyline were recorded under acidic conditions on four classes of stationary phases including a conventional silica/organic hybrid material in reversed‐phase liquid chromatography mode (BEH‐C₁₈), an electrostatic repulsion reversed‐phase liquid chromatography C₁₈ column (BEH‐C₁₈+), a poly(styrene‐divinylbenzene) monolithic column, and a hydrophilic interaction chromatography stationary phase (underivatized BEH). The same amounts of protonated bases per unit volume of stationary phase were injected in each column (16, 47, and 141 μg/cm³). The performance of the propranolol/amitriptyline purification was assessed on the basis of the asymmetry of the recorded band profiles and on the selectivity factor achieved. The results show that the separation performed under reversed‐phase liquid chromatography like conditions (with BEH‐C₁₈, BEH‐C₁₈+, and polymer monolith materials) provide the largest selectivity factors due to the difference in the hydrophobic character of the two compounds. However, they also provide the most distorted overloaded band profiles due to a too small loading capacity. Remarkably, symmetric band profiles were observed with the hydrophilic interaction chromatography column. The larger loading capacity of the hydrophilic interaction chromatography column is due to the accumulation of the protonated bases into the diffuse water layer formed at the surface of the polar adsorbent. This work encourages purifying ionizable compounds on hydrophilic interaction chromatography columns rather than on reversed‐phase liquid chromatography columns.     

C18‐bound porous silica monolith particles as a low‐cost high‐performance liquid chromatography stationary phase with an excellent chromatographic performance

Ground porous silica monolith particles with an average particle size of 2.34 μm and large pores (363 Å) exhibiting excellent chromatographic performance have been synthesized on a relatively large scale by a sophisticated sol–gel procedure. The particle size distribution was rather broad, and the d(0.1)/d(0.9) ratio was 0.14. The resultant silica monolith particles were chemically modified with chlorodimethyloctadecylsilane and end‐capped with a mixture of hexamethyldisilazane and chlorotrimethylsilane. Very good separation efficiency (185 000/m) and chromatographic resolution were achieved when the C₁₈‐bound phase was evaluated for a test mixture of five benzene derivatives after packing in a stainless‐steel column (1.0 mm × 150 mm). The optimized elution conditions were found to be 70:30 v/v acetonitrile/water with 0.1% trifluoroacetic acid at a flow rate of 25 μL/min. The column was also evaluated for fast analysis at a flow rate of 100 μL/min, and all the five analytes were eluted within 3.5 min with reasonable efficiency (ca. 60 000/m) and resolution. The strategy of using particles with reduced particle size and large pores (363 Å) combined with C₁₈ modification in addition to partial‐monolithic architecture has resulted in a useful stationary phase (C₁₈‐bound silica monolith particles) of low production cost showing excellent chromatographic performance.     

Preparation and evaluation of amphiphilic silica‐based monolithic column having surface‐bound octanoyl‐aminopropyl moieties for capillary electrochromatography

A novel amphiphilic silica‐based monolithic column having surface‐bound octanoyl‐aminopropyl moieties was successfully prepared by a one‐step in situ derivatization process. As expected, the amphiphilic monolithic column exhibited RP chromatographic behavior toward non‐polar solutes (e.g., alkyl benzenes) with high column performance. As the pH of the buffer inside the column increases, the EOF changed from −2.65×10⁻⁸m² V⁻¹s⁻¹ at pH 3.0 to 1.20×10⁻⁸ m² V⁻¹s⁻¹ at pH 8.0 with the reversion of EOF at about pH 6.4. Using acidic mobile phase, five aromatic acids can be efficiently separated in less than 6 min under co‐EOF conditions. For basic compounds, symmetrical peaks were obtained due to the existence of hydrophilic acyl amide group, which can effectively minimize the adsorption of the positively charged basic analyte to the silica‐based surface of the capillary column.     

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.     

Electrochromatographic performance of conventional and polar-embedded C16 silica monolithic stationary phases

A novel monolithic silica column that has a polar‐embedded amide‐secondary amine group linking with C16 functionality for RP‐CEC is described. The amide‐secondary aminealkyloxysilane was synthesized by the reaction of 3‐(2‐aminoethylamino) propyltrimethoxysilane with hexadecanoyl chloride. Then, the silylant agent was bonded to the silica monolith matrix to produce hexadecanamide‐secondary amine bonded silica (HDAIS) monolithic column. The electrochromatographic performance of HDAIS monolithic column for the separation of neutral, basic and polar solutes was studied, which was compared to that using the hexadecyl bonded silica monolithic column. The HDAIS monolithic column displayed reduced hydrophobic retention characteristics in the separation of five hydrophobic n‐alkylbenzenes and four polar phenols when compared to the hexadecyl bonded silica monolithic column. A very much reduced silanol activity of HDAIS monolithic column was observed in the separation of test basic mixture including four aromatic amines, atenolol and metoprolol with 10 mM borate buffer (pH 7.5) containing 30% v/v ACN as the mobile phase. The comparison results indicate good performance for both polar and basic mixtures on HDAIS monolithic column in RP‐CEC, and also show promising results for further applications.     

Preparation and Porous Property of C14-Monolithic Column for Capillary Electrochromatography

Capillary electrochromatography (CEC) has been performed with a series of C₁₄, methlyacrylamide based monolithic columns. These monoliths with different porosities were prepared by in-situ copolymerization in fused-silica capillaries. The porous properties of monoliths were further observed using scanning electron micrographs (SEM) and measured using a mercury porosimeter. The effect of various alcohols as porogens on porous structural properties and chromatographic behaviors were also investigated. The effects of organic additive, pH value and ionic strength in mobile phase on electroosmosis flow (EOF) and separation were further discussed. Meanwhile, the baseline separation of 6 neutral compounds can be well obtained. In addition, the monolithic column demonstrates the high column efficiency and satisfactory reproducibility.Acknowledgements We gratefully acknowledge the support of the National Natural Science Foundation of China.     

Post‐polymerization modification of a new reactive monolith for reversed phase and hydrophilic interaction capillary electrochromatography of neutral,polar, and biologically active compounds

A reactive monolith based on the polymerization of 3‐chloro‐2‐hydroxypropyl methacrylate, (HPMA‐Cl), with a crosslinking agent, ethylene glycol dimethacrylate (EDMA), was synthesized and post‐functionalized with a macromolecular ligand polyethyleneimine. Monolithic columns with controlled permeability and pore structure were prepared by free radical polymerization in the presence of a binary porogenic mixture of isopropanol and decanol. The presence of chloropropyl functionality in the pristine monolith allowed the synthesis of a post‐fuctionalized monolith carrying cationic groups that was used to control the magnitude of electroosmotic flow (EOF) in electrochromatographic separation. In the synthesis of pristine monoliths, the feed concentration of functional monomer (ie, HPMA‐Cl) was changed between 30 and 60 v/v % for obtaining cationic monoliths providing satisfactory electrochromatographic separation. The best electrochromatographic performance was obtained with the polyethyleneimine functionalized monolith prepared by using the pristine monolith obtained by 60% (v/v) monomer concentration. This monolith was used in reversed phase and hydrophilic interaction capillary electrochromatography modes for the separation of alkylbenzenes, polycyclic aromatic hydrocarbons, phenols, and nucleosides, using mobile phases with low acetonitrile (ACN) contents ranging between 20% and 35% (v/v). This ACN range was remarkably lower than the content of ACN used on the hydrophilic polymethacrylate‐based monoliths reported previously (ie, >90%). The plate heights up to 5.3 μm were obtained for the separation of nucleosides with the environmental friendly mobile phases whose ACN contents were also remarkably lower than that of similar polymethacrylate‐based monoliths.     

Preparation and evaluation of rigid porous polyacrylamide-based strong cation-exchange monolithic columns for capillary electrochromatography

A CEC monolithic column with strong cation-exchange (SCX) stationary phase based on hydrophilic monomers was prepared by in situ polymerization of acrylamide, methylenebisacrylamide, and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) in a complete organic binary porogenic solvent consisting of DMSO and dodecanol. The sulfonic groups provided by the monomer AMPS on the surface of the stationary phase generate an EOF from anode to cathode, and serve as an SCX stationary phase at the same time. The monolithic stationary phase exhibited normal-phase chromatographic behavior for neutral analytes. For charged analytes, electrostatic interaction/repulsion with the monolith was observed. The strong SCX monolithic column has been successfully employed in the electrochromatographic separation of basic drugs, peptides, and alkaloids extracted from natural products.     

Use of the Synthesized Titania Monolith to Determine Benzoic Acid and Vanillin in Foodstuffs by HPLC

A simple and reliable HPLC method for the determination of benzoic acid and vanillin in food samples has been developed, in which a pure titania monolithic column synthesized through a template-free sol-gel synthesis route was used as chromatography column. To fully understand the retention mechanism of benzoic acid and vanillin on titania, acetonitrile (ACN) percentage, buffer concentration, and buffer pH of the mobile phase were investigated. The retention mechanism of benzoic acid and vanillin on the titania monolith column belongs to hydrophilic interaction and ligand exchange. When the high %ACN and appropriate acetate existed in eluent, the hydrophilic interaction was the dominant retention mode. Benzoic acid and vanillin in preserved fruit and jelly samples were successfully determined and quantitative analysis was carried out by external standard method with correlation coefficient (R ² ) of 0.9994 for benzoic acid and 0.9989 for vanillin. The relative standard deviations (RSDs) of benzoic acid and vanillin were 0.94% and 1.50%, respectively. The developed titania-based HPLC method is simple, rapid, accurate, and competent for the separation of polar and hydrophilic compounds, and this work has also promoted the application of titania monolith in chromatographic separation.