Fabrication of an ionic liquid-based macroporous polymer monolithic column via atom transfer radical polymerization for the separation of small molecules
Graphical abstract
Introduction
Over the past two decades, compared to the silica-based columns, there has been a strong interest in polymer monolithic columns[1], [2] due to their advantages of time-saving preparation, enhanced permeability, reduced mass transfer resistance, and better pH tolerance [3], [4], [5]. Polymer monoliths have been successfully used for the separation of large biomolecules, such as proteins, oligonucleotides, and peptides [6], [7], [8], [9], [10], [11].
In contrast, it is unfavorable for the chromatographic performance of polymer monolithic columns for the separation of small molecules as a result of low mesopore volume and inhomogeneity of the structure [12], [13]. Several methods have been proposed to solve this problem, such as applying a single cross-linker [14], using various living-control radical polymerization [15], [16], [17], adding nanostructures [18], [19], and introducing click chemistry reaction [9], [20], [21], which mainly focused on the improvement of the performance of capillary monoliths. To some extent, all these methods led to varying degrees of improvement of column efficiency. The highest efficiency of obtained monoliths exceeded 100,000 plates/m for capillary liquid chromatography (CLC). However, there are still many difficulties to overcome. Many of click chemistry reactions are easily to tolerate conditions that will be damaging for processes currently used for the preparation of monoliths with some reactive chemistries [5]. As for nanostructure substance, because of its poor dispersion in the polymer matrix, it is suitable to be attached on the pore surface of the well-defined monolith in case of being buried and inaccessible, which is more time-consuming and complex operation compared to one-pot polymerization [5], [22]. In addition, in terms of capillary monolith column, it also brings difficulties to the process of analysis for the too low capacity and poor reproducibility. For routine monolithic column (50×4.6 mm i.d.), there is not any prominent progress in recent time. The attempts mainly concentrated on the variation of monomers, cross-linkers, and porogen solvent mixture, the adjustment of the polymerization conditions (including temperature, time, and polymerization technique) [23], but these methods have not improved the applicability of the polymer monoliths to the desired degree. In order to develop a uniform and stable system with simple optimization of polymerization conditions, especially focus on improving the homogeneity of the structure and enhancing column efficiency, the green solvent ionic liquid (IL) as co monomer was proposed in the present work.
Ionic liquids (ILs) are molten salts containing relatively asymmetric organic cations and inorganic or organic anions, whose physical and chemical properties are convenient to be changed by controlling the cations and anions [24]. Furthermore, contributed to the properties of high thermal stability, low volatility, good adjustability, high electrolytic conductivity, and miscibility [25], IL has been widely applied in chromatography [26], [27], [28] and aroused considerable scientific interest in the fabrication of monolithic column. IL-based monolithic columns [29], [30] have been successfully utilized to separate a variety of analytes especially complex biological samples by CEC., These success mainly benefited from the property of good electrolytic conductivity of IL to generate a stable reversed EOF in a wide pH range for the CEC mode. However, it didn't help improve the morphology and enhance the separation of small molecules in HPLC. Our group [31], [32] employed ILs as function monomer for the preparation of polymer monolithic columns via in situ free radical polymerization. The obtained monolithic columns, as the station phase for reversed-phase liquid chromatography (RPLC), exhibited good separation performance for small molecules. It demonstrated that the addition of IL made the structure more uniform with larger surface area. Furthermore, the conditions of polymerization could be optimized to get better mechanical and thermal stability, greater performance improvement and higher repeatability.
The combination of the unique advantages of porous polymer monoliths, and the specific features of IL might present a promising improvement in the performance of monoliths for HPLC. Herein, IL (1-allyl-3-methylimidazolium chloride, AMIM+Cl−) and a high concentration of crosslinking monomer triallyl isocyanurate (TAIC) build up a co-monomers system, with ethylene dimethacrylate (EDMA) as single cross-linker. The atom transfer radical polymerization (ATRP) technique was introduced to prepare the monolith. After the optimization of porogen system and polymerization conditions, a series process of characterization were carried out respectively including scanning electron microscopy, infrared spectrometer, nitrogen adsorption/desorption measurement, mercury intrusion porosimetry and thermal gravimetric analysis. Besides, the separation ability of these columns was estimated comprehensively by separating a series of basic and acidic small molecules, isomers and homologues by HPLC.
Section snippets
Materials and methods
Triallyl isocyanurate (TAIC) was purchased from Shanghai Huangguan Chemical company (Shanghai, China). Ethylene dimethacrylate (EDMA) was obtained from Maya-Reagent (Zhejiang, China). Polyethylene glycol 200 (PEG-200), 1,4-butanediol, N, N- dimethylformamide (DMF), CCl4, and FeCl2 were supplied by Tianjin Guangfu Fine Institute of Chemistry (Tianjin, China). Meanwhile, 2,2-Azobisisobutyronitrile(AIBN), HPLC-grade methanol (MeOH), acetonitrile (ACN), and KBr were produced by Kermel Chemical
Optimization of the monolithic column
Results listed in Table 1 showed the ratio of the three porogens dramatically affected the permeability of the monoliths. With the increasing content of PEG200, the permeability significantly decreased from 9.68 to 3.22 (×10−14 m2, according to columns A – C), calculated according to the Darcy's Law [33] of permeability.
B0=FηL/ΔPπr2 where F was the flow rate of the mobile phase (m3 s−1), η was the dynamic viscosity of the mobile phase (Pa s), L was the length of column (m), ΔP was the back
Conclusions
A facile monolithic column was fabricated by IL-co-TAIC-co-EDMA system via ATRP technique. Contributed to the porous and uniform construction, the resulted monoliths exhibited good thermal and chemical stabilities and better chromatographic efficiency than traditional polymer monoliths prepared via in situ free radical polymerization. The combination of ILs and living-control radical polymerization technique benefited in enhancing the chromatographic performance of small molecules separation.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 21175031, No. 21505030), Natural Science Foundation of Hebei Province (No. B2015201024, B2013201082), National Science Foundation of Hebei University (No. 2013-247), and the Post-graduate's Innovation Fund Project of Hebei University (No. X2015076).
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