Zhaorui Zeng

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.

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.

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.

A novel calix[4] open‐chain crown ether (p‐tert‐butylcalix[4]arene‐1,3‐bis(allyloxyethy) ether)‐modified, organic–inorganic hybrid silica‐based monolithic column possessing vinyl ligands for CEC is described. The monolithic silica matrix containing a vinyl functionality was synthesized by in situ cocondensation of tetramethoxysilane (TMOS) and vinyltrimethoxysilane (VTMS) via sol–gel process and chemically modified with calix[4] open‐chain crown ether by free radical polymerization procedure using α, α'‐azobisisobutyronitrile (AIBN) as an initiator. Morphology of the monolithic column was examined by SEM and the successful incorporation of calix[4] open‐chain crown ether to the vinyl‐hybrid monolith was characterized by infrared (IR) spectra. Compared with an unmodified vinyl‐hybrid monolithic column, slightly stronger EOF at pH >7.5 was observed for this monolithic column due to the ionization of phenolic hydroxyls on the lower rim of calix[4]arene. VTMS/TMOS ratios in the reaction mixture were varied and 1:4 was found to be optimum to obtain homogeneous monolith with good permeability. The performance of the column was evaluated by nucleotides, β‐blockers, neurotransmitters, and PAHs as test solutes and compared with that of unmodified vinyl‐hybrid monolithic column. Greatly improved column performance was obtained due to the host–guest interaction and intermolecular hydrogen bonding provided by the calix[4] open‐chain crown ether moiety. The column efficiencies for neurotransmitters and nucleotides are up to 120 000 and 110 000 plates/m, respectively. Migration time and theoretical plate number reproducibilities were reasonable with RSDs less than 1.0 and 1.8% each for within column runs and not more than 7.2 and 8.6% each for column‐to‐column measurements, using four nucleotides as test solutes.

This paper describes a novel method that applies pressure‐assisted field‐amplified sample injection with reverse migrating micelles (PA‐FASI‐RMM) for the online concentration of neutral analytes in MEKC with a low‐pH BGE. After injection of a plug of water into the separation capillary, negative voltage and positive pressure were simultaneously applied to initialize PA‐FASI‐RMM injection. The hydrodynamic flow generated by the positive pressure compensated the reverse EOF in the water plug and allowed the water plug to remain in the capillary during FASI with reverse migrating micelles (FASI‐RMM) to obtain a much longer injection time than usual, which improved stacking efficiency greatly. Equations describing this injection mode were introduced and were supported by experimental results. For a 450‐s online PA‐FASI‐RMM injection, three orders of magnitude sample enhancement in terms of peak area could be observed for the steroids and an achievement of detection limits was between 1 and 10 ng/mL.

A novel open‐tubular CEC (OT‐CEC) column was prepared by immobilized 5,11,17,23‐tetra‐tert‐butyl‐25,27‐diethoxy‐26,28‐dihydroxy‐calix[4]arene (Calix[4]) on fused‐silica capillary column with sol‐gel technology. Calix[4] was initially reacted with γ‐glycidoxypropyltrimethoxysilane (KH‐560) to form a new sol‐gel precursor (calix[4]‐KH‐560), and then mixed with another precursor, namely tetraethoxysilane (TEOS). After hydrolysis and condensation, a sol‐gel matrix was obtained, in which calix[4] was incorporated. Successful calix[4]‐modified sol‐gel coating was suggested by infrared (IR) spectra and greatly decreased EOF. In comparison with OT columns prepared by the sol‐gel method with TEOS and KH‐560 only, the calix[4]‐modified sol‐gel column showed greatly improved separation of isomeric toluidines, nitrophenols, picolines and neurotransmitters with structural similarity. Migration time and theoretical plate number reproducibility were satisfactory with RSDs less than 1 and 6% each for within column runs and not more than 3 and 7% each for column‐to‐column measurements, using toluidine and nitrophenol isomers as test solutes on this column. High separation efficiencies (96 000–300 000 plates/m) for basic toluidine isomers were obtained. This column was also successfully employed to combine extraction procedure for the determination of dopamine (DA) and norepinephrine (NE) in Portulaca oleracea L. The recoveries of DA and NE were 93.3 and 94.2%, respectively.

A novel fiber coated with novel sol-gel (5,11,17,23-tetra-tert-butyl-25,27-dihydroxy-26,28-diglycidyloxycalix[4]arene/hydroxy-terminated silicone oil; diglycidyloxy-C[4]/OH-TSO) was prepared for use with headspace solid-phase microextraction (HS-SPME) combined with gas chromatography (GC) and electron capture detection (ECD), which was applied in order to determine nine chlorobenzenes in soil matrices. Due to the improved fiber preparation, which increases the percentage of calixarene in the coating, the new calixarene fiber exhibits very high extraction selectivity and sensitivity to chlorine-substituted compounds. Various parameters affecting the extraction efficiency were optimized in order to maximize the sensitivity during the chlorobenzene analysis. Interferences from different soil matrices with different characteristics were investigated, and the amount extracted was strongly influenced by the matrix. Therefore, a standard addition protocol was performed on the real soil samples. The linear ranges of detection for the chlorobenzenes tested covered three orders of magnitude, and correlation coefficients >0.9976 and relative standard deviations (RSD) <8% were observed. The detection limits were found at sub-ng/g of soil levels, which were about an order of magnitude lower than those given by the commercial poly(dimethylsiloxane) (PDMS) coating for most of the compounds. The recoveries ranged from 64 to 109.6% for each analyte in the real kaleyard soil matrix when different concentration levels were determined over the linear range, which confirmed the reliability and feasibility of the HS-SPME/GC-ECD approach using the fiber coated with diglycidyloxy-C[4]/OH-TSO for the ultratrace analysis of chlorobenzenes in complex matrices.

A simple, sensitive, and accurate method for the determination of propranolol in human urine has been developed based on solid‐phase microextraction (SPME) followed by GC‐flame ionization detection (FID). The sol–gel 5,11,17,23‐tetra‐tert‐butyl‐25,27‐dihydroxy‐26,28‐diglycidyloxycalix[4]arene/hydroxy‐terminated silicone oil (diglycidyloxy‐C[4]/OH‐TSO) fiber was prepared to accommodate to the harsh extraction conditions. It possesses excellent alkali‐proof ability and retains its extraction characteristics intact even after treatment with highly alkaline (4 mol/L) NaOH solution. Direct chemical bonding of the coating to the fiber surface provides it with excellent solvent resistance and the introduction of calixarene enhances its thermal stability. The newly developed sol–gel calixarene coating was effectively used for the extraction of propranolol in human urine. No interference with the determination of propranolol was observed from the urine components. Standard curves were linear in the range 50–5000 μg/L for headspace‐SPME (HS‐SPME) and 25–25 000 μg/L for direct‐SPME (Dir‐SPME) with correlation coefficients better than 0.9999. The detection limit was 0.275 μg/L for HS‐SPME and 0.193 μg/L for Dir‐SPME. The method was validated using standard addition methodology and recovery values were between 91.4 and 117% for both the sampling modes with the RSDs less than 6% at different concentration levels in the linear ranges. The results obtained by both the sampling modes were feasible, and no significant differences between them regarding accuracy, precision, and detection limits were seen.