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Rigid porous polymeric monoliths are robust, highly efficient, versatile stationary phases. They offer simple preparation and convenient modification provided by a whole range of synthesis factors, e.g., starting monomers, cross-linkers, initiators, porogens, polymerization techniques, and temperature. The main aim of this study was to synthesize polymeric monoliths and determine the correlation between polymerization parameters and the porosity and thermal stability of the obtained materials. Polymeric monoliths were synthesized directly in HPLC columns using N-vinyl-2-pyrrolidone (NVP) and 4-vinylpiridine (4VP) as functional monomers, with trimethylolpropane trimethacrylate (TRIM) serving as the cross-linking monomer. During copolymerization a mixture of cyclohexanol/decane-1-ol was used as the pore-forming diluent. Polymerization was carried out at two different temperatures: 55 and 75 °C. As a result, monoliths with highly developed internal structure were synthesized. The value of their specific surface area was in the range of 92 m2/g to 598 m2/g, depending on the monomer composition and polymerization temperature. Thermal properties of the obtained materials were investigated by means of thermogravimetry (TG). Significant differences in thermal behavior were noticed between monoliths synthesized at 55 and 75 °C. Additionally, the poly(NVP-co-TRIM) monolith was successfully applied in GC analyses.

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Monolithic stationary phases based on poly(ethylene glycol) diacrylates for capillary electrochromatography were developed. Several poly(ethylene glycol) diacrylates (Mn 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%).

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In this study, activated carbon-organic polymer (AC-polymer) extraction columns were prepared via microwave-assisted polymerization. The AC-polymer was applied in polymer monolith microextraction (PMME) of phenolic acids (PAs) in food samples. Structural characterization using Raman and Fourier transform infrared (FTIR) spectroscopy revealed the successful incorporation of AC into polymer monolith. In contrast to neat polymer in which low extraction efficiency (17.0-57.7%) was observed, the AC-polymer (8 mg AC, 4 cm column length) afforded better extraction efficiency for PAs ranging from 75.8 to 99.8% for intra-day with less than 1.4% relative standard deviations (RSDs) and inter-day precision ranging from 74.3 to 100.1% (<2.7% RSDs). The extraction efficiency for column-to-column were found to be in the range of 72.8-100.5% (<2.0% RSDs) (n = 3). Under the optimized conditions, the AC-polymer monolithic column was successfully applied in PMME of PAs in fruit wine and cranberry juice samples at concentrations of <4.8 mg L-1 and 26.54 mg L-1, respectively.

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Porous polymeric monoliths with densities as low as ≈0.060 g cm-3 are prepared in a gel-emulsion template way, of which the stabilizer employed is a newly discovered acidified aramid fiber that is so efficient that 0.05% (w/v, accounts for continuous phase) is enough to gel the system. The porous monoliths as obtained can be dried at ambient conditions, avoiding energy-consuming processes. Importantly, the monoliths show selective adsorption to HCHO, and the corresponding adsorption capacity (M6) is ≈2700 mg g-1 , the best result that is reported until now. More importantly, the monoliths can be reused after drying.

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A polymeric column that contains multiwalled carbon nanotubes-ß-cyclodextrin composite was developed. The composite was wrapped into the poly(butyl methacrylate-ethylene dimethacrylate) monolith column (0.76 mm id and 10 cm in length). The column was then applied for the online solid-phase microextraction of psoralen and isopsoralen from Fructus Psoraleae. Following microextraction, the coumarins were quantified by high-performance liquid chromatography with C18 separation column and UV detection. The effects of sample flow rate, sample volume, and pH value were optimized. The method showed low limits of detection (20 pg/mL, S/N = 3) for both psoralen and isopsoralen. Finally the method was successfully applied to the determination of psoralen and isopsoralen in spiked herb extracts and rat plasma where it gave recoveries that ranged between 93.2 and 102.1%. The empty hydrophobic cavities of ß-cyclodextrin and the hydrophobicity of multiwalled carbon nanotubes provided specific extraction capability for psoralen and isopsoralen.

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A simple approach was developed for rapid preparation of polymeric monolithic columns in UV-transparent fused-silica capillaries via photoinitiated thiol-acrylate polymerization of pentaerythritol diacrylate monostearate (PEDAS) and trimethylolpropane tris(3-mercaptopropionate) (TPTM) within 10min, in which the acrylate homopolymerized and copolymerized with the thiol simultaneously. The morphology, permeability and chromatographic performance of the resulting poly(PEDAS-co-TPTM) monoliths were studied. It could be observed from SEM that the morphology of poly(PEDAS-co-TPTM) monolith was rather different from that of poly(PEDAS) monolith, which was fabricated via photo-induced free radical polymerization using PEDAS as the sole monomer. Compared with poly(PEDAS) monolith, poly(PEDAS-co-TPTM) monolith possessed better permeability when they were fabricated under the same preparation conditions. By adjusting the composition of porogenic solvents, poly(PEDAS-co-TPTM) monolith exhibited lower plate heights (15.7-17.7µm) than poly(PEDAS) monolith (19.1-37.9µm) in µLC. In addition, 66 unique peptides were positively identified on poly(PEDAS-co-TPTM) monolith when tryptic digest of four proteins was separated by µLC-MS/MS, demonstrating its potential in proteome analysis.

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This paper presents a second part of our research devoted to cholesterol-based polymeric monolithic stationary phase. The obtained capillary columns were successfully used for separations of alkylbenzenes, steroid hormones and polycyclic aromatic hydrocarbons during isocratic or gradient elutions. The columns showed excellent thermal stability. Increasing the temperature resulted in decrease of the retention factors and peak widths, but selectivity and efficient separations have been retained in the studied temperature range of 20 to100°C. Additionally, the van't Hoff model presented a non-linear relationship of lnk versus 1/T plots, which is likely the result of liquid crystal properties of cholesterol. The studied capillary monolithic columns showed extremely fast re-equilibration after gradient elution and found to be stable under such conditions as: fast flow rate, high acetonitrile content in the mobile phase (no swelling) and high temperature.

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In this study, an organic polymer monolithic columns, which were prepared via in situ polymerization of alkyl methacrylate-ester (AMA), divinylbenzene (DVB) and vinylbenzyl trimethylammonium chloride (VBTA, charged monomer), were developed as adsorbent for solid-phase microextraction (SPME). Different parameters affecting the extraction efficiency for nine (9) non-steroidal anti-inflammatory drugs (NSAIDs) such as the ratio of the stearyl methacrylate (SMA) to DVB monomer, column length, sample pH, extraction flow rate and desorption solvent were investigated to obtain the optimal SPME condition. Also, the permeability for each poly(AMA-DVB-VBTA) monolithic column was investigated by adding porogenic solvent (poly(ethylene glycol), PEG). Using the optimized condition, a series of AMA-based poly(AMA-DVB-VBTA) monolith columns were developed to determine the effect the extraction efficiency of NSAIDs by varying the alkyl chain length of the methacrylate ester (methyl-, butyl-, octyl-, or lauryl-methacrylate; (MMA, BMA, OMA, LMA)). Results showed that decreasing the AMA chain length increases the extraction efficiency of some NSAIDs (i.e. sulindac (sul), naproxen (nap), ketoprofen (ket) and indomethacin (idm)). Among the poly(AMA-DVB-VBTA) monolithic columns, poly(BMA-DVB-VBTA) showed a highly repeatable extraction efficiency for NSAIDs with recoveries ranging from 85.0 to 100.2% with relative standard deviation (RSD) less than 6.8% (n=3). The poly(BMA-DVB-VBTA) can also be reused for at least 50 times without any significant effect in extraction efficiency for NSAIDs. Finally, using the established conditions, the poly(BMA-DVB-VBTA) was used to extract trace-level NSAIDs (100µgL(-1)) in river water with good recoveries ranging from 75.8 to 90.8% (RSD<14.9%).

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A novel, cholesterol-based polymeric monolithic stationary phase for capillary liquid chromatography, was prepared by thermally initiated in-situ polymerization. Cholesteryl methacrylate (CholMA) was used as a functional monomer and trimethylolpropane trimethacrylate (TRIM) was a cross-linker, while azobisisobutyronitrile (AIBN) was an initiator. Isooctane and toluene were chosen as "poor" and "good" solvent, respectively, as constituents of the porogen solvent. Isocratic elutions of alkylbenzenes and separation of the testing mixture of o-terphenyl and triphenylene were conducted for all of the monoliths to assess their hydrophobicity and planar selectivity characteristic for cholesterol-based stationary phases. The synthesized columns demonstrated efficiency exceeding N=10,000 plates and a plate height of ca. H=30 µm. Column preparation was found to be highly reproducible; the relative standard deviation (RSD) values (n=3) for day-to-day and column-to-column were less than 4.08 and 2.02%, respectively, based on retention factor of alkylbenzenes.

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New monolithic reversed-phase liquid chromatography (RPLC) stationary phases based on single multi-acrylate/methacrylate-containing monomers [i.e., 1,12-dodecanediol dimethacrylate (1,12-DoDDMA), trimethylolpropane trimethacrylate (TRIM) and pentaerythritol tetraacrylate (PETA)] were synthesized using organotellurium-mediated living radical polymerization (TERP), which was expected to produce more efficient monolithic columns than conventional free-radical polymerization. The rationale behind selection of porogens, relative concentrations of reagents and polymerization conditions are described. The new monolithic columns were applied to the separation of small molecules (i.e., alkylbenzenes) under isocratic conditions. Chromatographic efficiencies as high as 60,200 plates/m (71,300 plates/m when corrected for extra-column variance) were obtained, showing a general improvement over previous RPLC monoliths.

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Three crosslinking monomers, i.e., 1,6-hexanediol dimethacrylate (HDDMA), cyclohexanediol dimethacrylate (CHDDMA) and 1,4-phenylene diacrylate (PHDA), were used to synthesize highly cross-linked monolithic capillary columns for reversed-phase liquid chromatography (RPLC) of small molecules. Selection of porogen type and concentration was investigated in detail. Isocratic elution of alkylbenzenes at a flow rate of 300nL/min was performed using HDDMA and CHDDMA monolithic columns. Gradient elution of alkylbenzenes using all three monolithic columns showed good separations. Optimized monoliths synthesized from all three crosslinking monomers possessed high permeabilities. Poly(HDDMA) monoliths demonstrated column efficiencies up to 86,000 plates/m. Column preparation of poly(HDDMA) monolithic columns was highly reproducible; the relative standard deviation (RSD) values (n=3) for run-to-run and column-to-column were less than 0.26% and 0.70%, respectively, based on retention times of alkylbenzenes.

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