Carbon nanotube and carbon nanorod-filled polyacrylonitrile electrospun stationary phase for ultrathin layer chromatography

The application of carbon nanotube or nanorod/polyacrylonitrile (PAN) composite electrospun nanofibrous stationary phase for ultrathin layer chromatography (UTLC) is described herein. Multi-walled carbon nanotubes (MWCNTs) and edge-plane carbon (EPC) nanorods were prepared and electrospun with the PAN polymer solution to form composite nanofibers for use as a UTLC stationary phase. The analysis of laser dyes demonstrated the feasibility of utilizing carbon nanoparticle-filled electrospun nanofibers as a UTLC stationary phase. The contribution of MWCNT or EPC in changing selectivity of the stationary phase was studied by comparing the chromatographic behavior among MWCNT–PAN plates, EPC–PAN plates and pure PAN plates. Carbon nanoparticles in the stationary phase were able to establish strong π–π interactions with aromatic analytes. The separation of five polycyclic aromatic hydrocarbons (PAHs) demonstrated enhanced chromatographic performance of MWCNT-filled stationary phase by displaying substantially improved resolution and separation efficiency. Band broadening of the spots for MWCNT or EPC-filled UTLC stationary phases was also investigated and compared with that for pure PAN stationary phases. A 50% improvement in band dispersion was noted using the MWCNT based composite nanofibrous UTLC plates.     

新型碳纳米管色谱固定相制备的研究进展

碳纳米管（CNTs）作为一种新型的功能材料,具有优异的物理、化学和机械性能,已经在分析化学领域得到了广泛的关注和应用。通过填充法或原位化学气相沉积法,可制备CNTs气相色谱固定相;将CNTs沉积在硅胶微球或有机聚合物基质微球表面,可制备填充式CNTs液相色谱固定相;通过包埋共聚法将CNTs嵌入聚合物整体柱内,可制备毛细管CNTs液相色谱整体柱。本文主要综述了近年来CNTs（单壁碳纳米管和多壁碳纳米管）用于色谱固定相制备的研究现状,包括气相色谱及液相色谱,并对该领域今后的发展进行展望。     

Nanoparticles as stationary and pseudo-station+ary phases in chromatographic and electrochromatographic separations

To improve selectivity, chemical stability, and separation efficiency of chromatography, many past papers reported on nanoparticles (NPs) being used as stationary phases in chromatography. This article covers applications of NPs, including carbon nanotubes, fullerenes, gold NPs, silica NPs, zirconia NPs, and titanium-oxide NPs, as stationary phases in gas chromatography, high-performance liquid chromatography, capillary electrophoresis and capillary electrochromatography.We discuss the advantages and the disadvantages of nanomaterials as stationary phases compared to other materials, including traditional stationary phases. We also discuss future possibilities for developing nanomaterial-based stationary phases.     

Carbon nanotubes: Solid-phase extraction

Since the first report in 1991, carbon nanotubes (CNTs) have shown great possibilities for a wide variety of processes and applications, which include their use as electrodes, sensors (gas, enzymatic, etc.), nanoprobes, electronic materials, field emitters, etc. The combination of structures, dimensions and topologies has provided physical and chemical attractive properties that are unparalleled by most known materials. Their applications have also reached the Analytical Chemistry field in which CNTs are being used as matrices in matrix assisted laser desorption ionization, stationary phases in either gas chromatography, high performance liquid chromatography or capillary electrochromatography, also as pseudostationary phases in capillary electrophoresis, etc. as well as new solid-phase extraction (SPE) materials. Concerning this last application the number of works has considerably increased in the last five years. This review article pretends to focus on the most important features and different applications of SPE using CNTs (including matrix solid-phase dispersion and solid-phase microextraction) covering articles published since their introduction up to now (September 2009).     

基于碳纳米材料的毛细管电色谱固定相研究进展

碳纳米材料由于其具有独特的纳米结构、大的比表面积、较强的热稳定性、良好的导电性以及较好的吸附性能等物理化学性质,因而在分析科学、生命科学、材料科学及环境科学等领域得广泛的应用.结合国内外最新文献,对近5年来碳纳米材料在毛细管电色谱新型固定相的制备研究方面进展进行了评述,包括毛细管电色谱的分类及分离机理、毛细管电色谱柱的制备方法和优缺点,碳纳米材料(石墨烯、碳纳米管、氧化石墨烯、还原氧化石墨烯、富勒烯)的结构性质及制备方法、碳纳米材料在毛细管电色谱柱固定相中的应用及作用机理等,并对其在色谱应用领域的方向进行了展望.     

Tuning retention and selectivity in reversed-phase liquid chromatography by using functionalized multi-walled carbon nanotubes

Aim of this work was to explore the possibility of retention and selectivity tuning in reversed-phase liquid chromatography by means of chemically modified multi-walled carbon nanotubes (MWCNTs). These were synthesized by derivatizing pristine MWCNTs with amino-terminated alkyl chains containing polar embedded groups. A novel hybrid material based on functionalized MWCNTs (MWCNTs-R-NH₂) was prepared, characterized and tested. The idea was to design a mixed-mode separation medium basing its sorption properties on the peculiar characteristics of MWCNTs combined with the chemical interactions provided by the functional chains introduced on the nanotube skeleton. MWCNTs-R-NH₂ were easily grafted to silica microspheres by gamma radiation (using a ⁶⁰Co source) in the presence of polybutadiene as the linking agent. The composite was characterized by scanning electron microscopy (SEM) and Brunauer, Emmett and Teller (BET) analysis in terms of structural morphology, surface area and porosity. The MWCNTs-R-NH₂ sorbent was tested as stationary phase. The reversed-phase behaviour was first proved by analysis of alkylbenzenes, while the key role of CNT derivatization in addressing the selectivity/affinity towards the solutes was evidenced by testing three classes of analytes, viz. barbiturates, steroid hormones and alkaloids. These compounds, with different molecular structure and polarity, were here analysed for the first time on CNT-based LC stationary phases. The behaviour of the novel sorbent was compared in terms of retention capability and resolution with that observed using unmodified MWCNTs, pointing out the mixed-mode characteristics of the MWCNTs-R-NH₂ material. The same test mixtures were analysed also on a conventional mono-modal separation sorbent (C18) to highlight the particular behaviour of the (derivatized)MWCNTs-based stationary phases. The novel material showed better performance in separation of polar compounds, i.e. barbiturates and alkaloids, than the unmodified MWCNTs and than the C18 column. Results showed that MWCNT functionalization is powerful to modulate retention/selectivity in reversed-phase liquid chromatography.     

Additional studies on shape selectivity by using the carotenoid test to classify C18 bonded silica

Numerous chromatographic tests are applied to study the C18 bonded phases, either to classify these phases, or to better determine their properties or their chromatographic behaviours. Because the carotenoid test is developed in supercritical fluid chromatography (SFC), many correlations with high performance liquid chromatography (HPLC) results have been necessary to ensure the trueness of the classification reached in these analytical conditions. Consequently, the analytical conditions of the carotenoid test were chosen to fit with the TbN/BaP values from the NIST 869a test, which describe the shape selectivity of the alkyl bonded phases. Additional studies performed in this paper by using well standardized silica (J'Sphere, YMC Pack ODS A, Wakosil II), which varied from their surface coverage (J'Sphere), their pore shape (YMC), or their bonding type and carbon content (Wakosil II). The use of these C18 stationary phases allows to reach more accurate conclusion on the comparison of the shape selectivity values provided either by the cis/trans β-carotene selectivity or by the TRI/oTER or TbN/BaP ones. The extension of the studies to many other C18 stationary phases allows clarifying the relationships between the carotenoid test and the tests based on the use of PAH, as well as the usage limit of TRI/oTER and TBN/BaP selectivity in regards of the bonding density of the stationary phases for both monomeric and polymeric phases. By checking other aromatic compounds, another selectivity (anthracene/oTER) display greater correlation with the carotenoid test, that suggest an improvement of the relevance of this new compound couple for the steric selectivity study.     

A new type of triptycene-based stationary phases with alkylated benzimidazolium cations for gas chromatographic separations

The new type of triptycene-based stationary phases with alkylated benzimidazolium cations shows high selectivity towards aromatic isomers.     

Molecularly imprinted polymers: towards highly selective stationary phases in liquid chromatography and capillary electrophoresis

Molecularly imprinted polymers (MIPs) are synthetic polymers with a predetermined selectivity for a given analyte, or group of structurally related compounds, that make them ideal materials to be used in separation processes. In this sense, it is not surprising that the first applications of MIPs were as tailor-made chiral stationary phases in liquid chromatography. However, peak broadening and tailing, especially of the more retained enantiomers, were observed. Accordingly, this paper gives an overview of the attempts carried out during the recent years to improve the chromatographic performance of MIPs in liquid chromatography and capillary electrophoresis as well as the more recent applications. We conclude that MIPs are very promising materials to be used as selective stationary phases in chromatography although further developments are necessary in order to fully exploit their potential.     

阳离子取代基对离子液体气相色谱固定相保留性能的影响

以双-三氟甲烷磺酰亚胺离子（［NTf₂］^-）、三氟甲基磺酸根离子（［TFO］^-）和六氟磷酸根离子（［PF₆］^-）为阴离子,合成了含有丙基、丁基、壬基、乙苯基或氰甲基等阳离子取代基的聚合乙烯基咪唑基离子液体固定相,制备了13种离子液体石英毛细管色谱柱。考察了离子液体固定相结构与色谱柱McReynolds常数、热稳定性和测试化合物保留行为之间的构效关系,研究了阳离子取代基对固定相保留性能的影响规律;同时考察了测试化合物保留指数随温度变化的规律。结果表明,所合成的离子液体固定相具有较强的极性,其保留性能不仅与阳离子取代基的性质密切相关,而且受取代基结构和阴离子影响明显;在考察的温度范围内,测试化合物的保留指数变化规律与传统色谱固定相完全相同。     

Supercritical ammonia as mobile phase in capillary chromatography

Instrumentation was assembled that allows the use of supercritical ammonia as mobile phase in capillary supercritical fluid chromatography. Several modifications of the typical chromatographic system were necessary, especially with respect to injection and detection. In addition, the stabilities of various polysiloxane stationary phases were studied. The chromatography of polarizable and polar basic materials was demonstrated using a nonpolar polysiloxane stationary phase.     

Investigation of polar stationary phases for the separation of sympathomimetic drugs with nano-liquid chromatography in hydrophilic interaction liquid chromatography mode

In this study, the retention and selectivity of a mixture of basic polar drugs were investigated in hydrophilic interaction chromatographic conditions (HILIC) using nano-liquid chromatography (nano-LC). Six sympathomimetic drugs including ephedrine, norephedrine, synephrine, epinephrine, norepinephrine and norphenylephrine were separated by changing experimental parameters such as stationary phase, acetonitrile (ACN) content, buffer pH and concentration, column temperature. Four polar stationary phases (i.e. cyano-, diol-, aminopropyl-silica and Luna HILIC, a cross-linked diol phase) were selected and packed into fused silica capillary columns of 100 μm internal diameter (i.d.). Among the four stationary phases investigated a complete separation of the all studied compounds was achieved with aminopropyl silica and Luna HILIC stationary phases only. Best chromatographic results were obtained employing a mobile phase composed by ACN/water (92/8, v/v) containing 10 mM ammonium formate buffer pH 3. The influence of the capillary temperature on the resolution of the polar basic drugs was investigated in the range between 10 and 50 °C. Linear correlation of ln k vs. 1/T was observed for all the columns; ΔH° values were negative with Luna HILIC and positive with aminopropyl- and diol-silica stationary phases, demonstrating that different mechanisms were involved in the separation.To compare the chromatographic performance of the different columns, Van Deemter curves were also investigated.     

Novel guanidinium-based ionic liquids as stationary phases for capillary gas chromatography

<正>The present study describes guanidinium-based ionic liquids(GBILs) as stationary phases for capillary gas chromatography (CGC) and to the best of our knowledge,no related reports are available up to now.In this study,a hexaalkylguanidinium ionic liquid(DOTMG-NTf_2) was synthesized and coated statically onto capillary columns.Selectivity of the stationary phase was evaluated by separating Grob test mixture,test mixture,alcohols mixture,and fatty acid methyl esters mixture,and thermal stability was investigated as well.The present study demonstrates that GBILs as CGC stationary phases exhibit satisfactory selectivity and thermal stability and have a great potential as new candidates for CGC stationary phases.     

Recent development trends for chiral stationary phases based on chitosan derivatives,cyclofructan derivatives and chiral porous materials in high performance liquid chromatography

The separation of enantiomers by chromatographic methods, such as gas chromatography, high‐performance liquid chromatography and capillary electrochromatography, has become an increasingly significant challenge over the past few decades due to the demand of pharmaceutical, agrochemical, and food analysis. Among these chromatographic resolution methods, high‐performance liquid chromatography based on chiral stationary phases has become the most popular and effective method used for the analytical and preparative separation of optically active compounds. This review mainly focuses on the recent development trends for novel chiral stationary phases based on chitosan derivatives, cyclofructan derivatives, and chiral porous materials that include metal‐organic frameworks and covalent organic frameworks in high‐performance liquid chromatography. The enantioseparation performance and chiral recognition mechanisms of these newly developed chiral selectors toward enantiomers are discussed in detail.     

Molecular imprinting technology in capillary electrochromatography

Molecularly imprinted polymers (MIPs) are tailor-made synthetic polymers with a predetermined selectivity for a given analyte, or group of structurally related compounds, that make them ideal materials for use as stationary phases in affinity chromatography. However, extensive peak broadening and tailing, especially of the more retained compound (normally the template) are often observed. Thus, huge efforts have been made during recent years to use MIPs in capillary electrochromatography, which is inherently a more efficient chromatographic technique than conventional HPLC. Accordingly, this paper gives an overview of the attempts carried out in the recent past to improve the chromatographic performance of MIPs in capillary electrochromatography as well as more recent applications. It is concluded that MIPs are very promising materials for use as selective stationary phases in CEC.     

Incorporation of carbon nanotubes in porous polymer monolithic capillary columns to enhance the chromatographic separation of small molecules

Multiwalled carbon nanotubes have been entrapped in monolithic poly(glycidyl methacrylate-co-ethylene dimethacrylate) capillary columns to afford stationary phases with enhanced liquid chromatographic performance for small molecules in the reversed phase. While the column with no nanotubes exhibited an efficiency of only 1800 plates/m, addition of a small amount of nanotubes to the polymerization mixture increased the efficiency to over 15,000 and 35,000 plates/m at flow rates of 1 and 0.15 μL/min, respectively. Alternatively, the native glycidyl methacrylate-based monolith was functionalized with ammonia and, then, shortened carbon nanotubes, bearing carboxyl functionalities, were attached to the pore surface through the aid of electrostatic interactions with the amine functionalities. Reducing the pore size of the monolith enhanced the column efficiency for the retained analyte, benzene, to 30,000 plates/m at a flow rate of 0.25 μL/min. Addition of tetrahydrofuran to the typical aqueous acetonitrile eluents improved the peak shape and increased the column efficiency to 44,000 plates/m calculated for the retained benzene peak.     

Characterization of High‐Pressure Liquid Chromatography Columns using Chromatographic Methods

Abstract

A great variety of columns for liquid chromatography (LC) are available in dimensions ranging from industrial scale to micro‐bore, nano‐bore, and capillary size, and on‐chip columns. The columns may be used in various liquid chromatography modes or in capillary electrochromatography, depending on the support materials and stationary phase chemistry. Every year many new column types are introduced on the market, with improved selectivity and efficiency, long lifetime, and mobile phase compatibility, intended for general use, for liquid chromatography/mass spectrometry (LC/MS) applications, proteomic research, or for the analysis of other specific sample types. Considerable improvement in pH, high‐temperature, and high‐pressure stability of new column types, together with advances in the instrumentation, enabled introduction of capillary, high‐temperature, and ultra‐high‐pressure HPLC into routine practice. Even though reversed‐phase mode is still by the most widely used in contemporary LC, applications of other separation modes (such as ion, normal‐phase, or high‐interaction liquid chromatography (HILC)) have become more frequent recently, because of unique separation selectivity for certain sample types.

Characterization of column quality is not a simple task, because a number of factors should be taken into account, that affect the selectivity, efficiency and resolution of sample separation and the reproducibility of chromatographic data. These include the type of the support, the arrangement and density of the stationary phase on the adsorbent surface, the homogeneity of the chromatographic bed, etc. Various physicochemical techniques are used for characterization of the properties of column packings however, most of them are suitable for bulk materials only and cannot be directly applied for commercial columns without damaging them. Not to destroy the columns, often precious and expensive, practicing chromatographers can apply chromatographic methods to characterize columns and evaluate their analytical suitability under real‐life conditions, where the intermolecular interactions between the analytes, the stationary phase, and the mobile phases affect the retention. The present review reports various chromatographic tests and strategies available for column evaluation.     

Liquid and gas chromatographic separation of isomeric methylated dibenzothiophenes

Trace amounts of a dibenzothiophene (DBT) series in complex matrices were detected by gas chromatography with flame photometric detection (FPD) after purification and isolation by normal-phase high performance liquid chromatography. The gas chromatographic behavior of eleven methylated DBTs was studied on six common stationary phases. Complete resolution of the four monomethyl DBT isomers was only achieved on a capillary column coated with a methylphenylsilicone phase gummified in situ by heat-curing. The determination of the relative distribution of the four monomethylated isomers is proposed as a powerful method for monitoring oil pollution in marine environment.     

Supercritical fluid chromatographic solvatochromic modifier polarity studies

To understand the chromatographic process as a whole, whether it be for gas chromatography (GC), liquid chromatography (LC), or supercritical fluid chromatography (SFC), one needs to know the chemical and physical nature of the mobile and stationary phases and also the interactions that take place between analytes (solutes) and the two phases. An approach towards Investigating the ways that stationary and mobile phases contribute to chromatographic retention Involves exploring the effects of solvent polarity on the strength of the mobile phase. In SFC this could involve determining the polarity of several different modifier/carbon dioxide mobile phases. In this paper, the use of a solvatochromic indicator to learn more about the effects of SFC modifier/mobile phase polarity will be investigated and discussed using several different modifiers and a diolmodified silica column.     

Applications of monolithic columns in gas chromatography and supercritical fluid chromatography

Porous monoliths are well‐known stationary phases in high‐performance liquid chromatography and capillary electrochromatography. Contrastingly, their use in other types of separation methods such as gas or supercritical fluid chromatography is limited and scarce. In particular, very few studies address the use of monolithic columns in supercritical fluid chromatography. These are limited to silica‐based monoliths and will be covered in this review together with an underlying reason for this trend. The application of monoliths in gas chromatography has received much more attention and is well documented in two reviews by Svec and Kurganov published in 2008 and 2013, respectively. The most recent studies, covered in this review, build on the previous findings and on further understanding of the influence of preparation conditions on porous properties and chromatographic performance of poly(styrene‐co‐divinylbenzene), polymethacrylate, and silica‐based monolithic columns while expanding to polymer‐based monoliths with incorporated metal organic frameworks and to vinylized hybrid silica monoliths. In addition, the potential application of porous layer open tubular monolithic columns in low‐pressure gas chromatography will be addressed.