CEC: selected developments that caught my eye since the year 2000

Poly(carboxyethyl acrylate-co-ethylene glycol dimethacrylate) precursor monolith with bonded anthracenyl ligands for use in reversed-phase capillary electrochromatography based on hydrophobic and π-π interactions

In this research report, the post polymerization functionalization (PPF) of a carboxyethyl acrylate (CEA)-co-ethylene glycol dimethacrylate (EDMA) [poly-CEA-co-EDMA)] precursor monolith with 2-aminoanthracene was carried out in the presence of an organic solvent soluble carbodiimide, namely N,N´-dicyclohexylcarbodiimide (DCC), yielding the so-called anthracenyl-poly-CEA-co-EDMA monolith. This novel monolith proved to be an excellent monolithic stationary for reversed-phase capillary electrochromatography (RP-CEC) with hydrophobic and π-π interactions of a wide range of nonpolar solutes including those bearing aryl functional groups in their structures such as polycyclic aromatic hydrocarbons (PAHs), toluene derivatives and aniline derivatives as well as solutes carrying in their structures electron withdrawing substituents such as dinitrophenyl-amino acids (DNP-AAs) and di-DNP-AAs. The retention behaviors of the just mentioned solutes obtained on the anthracenyl-poly-CEA-co-EDMA monolithic column were compared to those obtained on octadecyl-poly-CEA-co-EDMA monolithic column prepared from the same carboxy-precursor monolith. The results demonstrated the superiority of anthracenyl column over the octadecyl column for the separation and enhanced selectivity for aromatic solutes since it provides not only hydrophobic interactions but also π-π interactions with aromatic nonpolar solutes.

Recent Advances in Capillary Electrochromatography of Proteins and Carbohydrates in the Biopharmaceutical and Biomedical Field

Capillary electrochromatography (CEC) is a powerful hybrid separation technique that combines capillary electrophoresis and capillary chromatography, capable to address the analytical challenges of proteomics and glycomics. The focus of this paper is to review the recent developments in capillary electrochromatography of proteins and carbohydrates. The different column types applied in capillary electrochromatography such as packed bed, open tubular and monoliths are conferred in detail with respective separation examples. A comprehensive comparison is also given listing the mostly utilized coating methods, stationary phase materials and column preparation methods. The choice of porogenic solvent combinations for monolithic column fabrication is thoroughly discussed, paying close attention to the fine tuning options for the separation driving electroosmotic flow. Application examples of CEC in process analytical technology for the biopharmaceutical and biomarker discovery in the biomedical fields are also given.

Electrochromatography on acrylate-based monolith in cyclic olefin copolymer microchip: an attractive technology

Electrochromatography (EC) on a porous monolithic stationary phase prepared within the channels of a microsystem is an attractive alternative for on-chip separation. It combines the separation mechanisms of electrophoresis and liquid chromatography. Moreover, the porous polymer monolithic materials have become popular as stationary phase due to the ease and rapidity of fabrication via free radical photopolymerization. Here, we describe a hexyl acrylate (HA)-based porous monolith which is simultaneously in situ synthesized and anchored to the inner walls of the channel of a cyclic olefin copolymer (COC) device in only 2 min. The baseline separation of a mixture of neurotransmitters including six amino acids and two catecholamines is realized.

Preparation of porous polymer monoliths featuring enhanced surface coverage with gold nanoparticles

A new approach to the preparation of porous polymer monoliths with enhanced coverage of pore surface with gold nanoparticles has been developed. First, a generic poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolith was reacted with cystamine followed by the cleavage of its disulfide bonds with tris(2-carboxylethyl)phosphine, which liberated the desired thiol groups. Dispersions of gold nanoparticles with sizes varying from 5 to 40 nm were then pumped through the functionalized monoliths. The materials were then analyzed using both energy dispersive X-ray spectroscopy and thermogravimetric analysis. We found that the quantity of attached gold was dependent on the size of nanoparticles, with the maximum attachment of more than 60 wt% being achieved with 40 nm nanoparticles. Scanning electron micrographs of the cross sections of all the monoliths revealed the formation of a non-aggregated, homogenous monolayer of nanoparticles. The surface of the bound gold was functionalized with 1-octanethiol and 1-octadecanethiol, and these monolithic columns were used successfully for the separations of proteins in reversed phase mode. The best separations were obtained using monoliths modified with 15, 20, and 30 nm nanoparticles since these sizes produced the most dense coverage of pore surface with gold.

Effect of ion adsorption on CEC separation of small molecules using hypercrosslinked porous polymer monolithic capillary columns

Both poly(styrene-co-vinylbenzyl chloride-co-divinylbenzene) and poly(4-methylstyrene-co-vinylbenzyl chloride-co-divinylbenzene) monolithic columns have been hypercrosslinked and for the first time used to achieve capillary electrochromatographic separations. Although these columns do not contain ionizable functionalities, electroosmotic flow was observed due to adsorption of ions from a buffer solution contained in the mobile phase on the surface of the hydrophobic polymer. An increase of more than one order of magnitude was observed with the use of both monolithic polymers. The hypercrosslinking reaction creates a large surface area thus enabling adsorption of a much larger number of ions. Alkylbenzenes were successfully separated using the hypercrosslinked monolithic columns.

Naphthyl methacrylate-phenylene diacrylate-based monolithic column for reversed-phase capillary electrochromatography via hydrophobic and π interactions

A neutral naphthyl methacrylate-phenylene diacrylate-based monolith (NPM) was introduced for RP-CEC of various neutral and charged solute probes via hydrophobic and π interactions. The NPM column was prepared by the in situ polymerization of naphthyl methacrylate as the functional monomer and 1,4-phenylene diacrylate (PDA) as the crosslinker in a ternary porogenic solvent containing cyclohexanol, dodecanol and water. The NPM column exhibited cathodal EOF despite the fact that it was devoid of any fixed charges. NPM exhibited stronger EOF than its counterpart naphthyl methacrylate monolith (NMM) made from the in situ polymerization of naphthyl methacrylate and trimethylolpropane trimethacrylate (TRIM). As for NMM, it is believed that the EOF arises from the adsorption of mobile phase ions onto the monolith surface. The higher EOF exhibited by NPM may be attributed to the acrylate nature of PDA as compared to the methacrylate nature of TRIM, and therefore PDA has a higher binding capacity for mobile phase ions due to its higher polarity than TRIM. The adsorption of mobile phase ions together with the additional π interactions offered by the aromatic rings of the NPM matrix modulated solute retention and separation selectivity. The applications of NPM were demonstrated by the separation of a wide range of small and large solutes including peptides, tryptic peptide maps and proteins.

Organic monoliths for hydrophilic interaction electrochromatography/chromatography and immunoaffinity chromatography

This article is aimed at providing a review of the progress made over the past decade in the preparation of polar monoliths for hydrophilic interaction LC (HILIC)/capillary electrochromatography (HI-CEC) and in the design of immuno-monoliths for immunoaffinity chromatography that are based on some of the polar monolith precursors used in HILIC/HI-CEC. In addition, this review article discusses some of the applications of polar monoliths by HILIC and HI-CEC, and the applications of immuno-monoliths. This article is by no means an exhaustive review of the literature; it is rather a survey of the recent progress made in the field with 83 references published in the past decade on the topics of HILIC and immunoaffinity chromatography monoliths.

TiO2-ZrO2 affinity chromatography polymeric microchip for phosphopeptide enrichment and separation

We fabricated a TiO(2)-ZrO(2) affinity chromatography micro-column on 2 mm PMMA plates, and demonstrated the enrichment and separation of (a) a standard mono- and tetra-phosphopeptide, and (b) phosphopeptides contained in a tryptic digest of β-Casein. The chromatography column consisted of 32 parallel microchannels with common input and output ports and was fabricated by lithography directly on the polymeric substrate followed by plasma etching (i.e. standard MEMS processing) and sealed with lamination. The liquid deposited TiO(2)-ZrO(2) stationary phase was characterized by X-ray diffraction and was found to be mostly TiO(2) and ZrO(2) in crystalline phases. Off-chip UV detection and MALDI MS identification of the separated effluents were used. The chip had a capacity of >1.4 μg (0.7 nmol) of a prototype mono-phosphopeptide and a recovery of 94 ± 3%, and can be used with small samples (less than 0.1 μL depending on the syringe pump used). The chip design allows an expansion of its capacity by means of increasing the number of parallel microchannels at a constant sample volume. Our approach provided an alternative to off-line extraction tips (with typical capacities of 1-2 μg and sample volumes of 1-10 μL), and to on-chip efforts based on packed bed and frit formats.

Capillary columns for reversed-phase CEC prepared via surface functionalization of polymer monolith with aromatic selectors

Macroporous crosslinked organic polymers based on N-acryloxysuccinimide (NAS) and ethylene dimethacrylate (EDMA) were prepared in the confines of 75 μm id fused-silica capillaries by photoinitiated free radical copolymerization in the presence of 2-2'-azobisisobutyronitrile as initiator and toluene as porogen. Monoliths with good mechanical strength, large porosity as well as surface reactive sites (succinimide leaving groups) could be obtained. Nucleophilic aromatic derivatives, namely benzylamine, phenylbutylamine and naphthylamine were grafted on the monolith surface to introduce π-conjugated ligands to develop particular selectivity. Successful achievement of the post-copolymerization functionalization was ascertained on the basis of in situ chemical characterization by means of Raman spectroscopy. Electrochromatographic properties of π-functionalized poly(NAS-co-EDMA) regarding alkylbenzenes, polycyclic aromatic hydrocarbons, anilines and phenols were evaluated in terms of retention, selectivity and resolution. The as-designed monolithic columns exhibited π-π interaction in addition to hydrophobic interaction due to the aromatic and non-polar nature of the surface-grafted aromatic selectors. One of the major results of this study is that monolithic columns with mixed selectivity providing high potentiality for the separation of solutes with varied chemical structure variation can be obtained by the surface grafting of the appropriate selector. Herein, an example is given for the phenylbutylamine functionalized poly(NAS-co-EDMA) where the butyl and phenyl fragments afford enhanced hydrophobic and π-selectivity, respectively.

Trends in nonpolar polymer-based monolithic columns for reversed-phase capillary electrochromatography

This review article is concerned with describing the various strategies that have been introduced for the preparation of nonpolar polymer-based monolithic columns for RP-CEC. First, the various traditional ways of generating the EOF that involved the introduction of fixed charges on the surface of the monoliths are reviewed. This is followed by a description of the development of neutral monoliths as the most promising monoliths for the separation of a wide range of neutral and charged species at a relatively moderate to strong EOF in the absence of electrostatic attraction or repulsion.

Porous polymer monolithic column with surface-bound gold nanoparticles for the capture and separation of cysteine-containing peptides

A new porous polymer monolithic capillary column modified with gold nanoparticles that enables the selective capture of cysteine-containing peptides has been developed to reduce the complexity of peptide mixtures generated in bottom-up proteomic analysis. The column is prepared from a poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolith through reaction of some of its epoxide moieties with cysteamine to afford a monolith rich in surface thiol groups. In situ reduction of chloroauric acid within the column is then used to form gold nanoparticles attached to the surface of the pores of the monolith. This process preserves the excellent hydrodynamic properties of the monolithic column while providing a means to selectively retain cysteine-containing peptides from an analyte due to their high affinity for gold. Release of the retained peptides is subsequently achieved with an excess of 2-mercaptoethanol. The loading capacity determined for l-cysteine using frontal elution is 2.58 mumol/m. Since the gold-thiol link is less stable at elevated temperatures, the adsorption capacity is recovered by washing the column at 80 degrees C for 2 h. While regeneration is easy, the multiplicity of bonds between the monolithic support and the gold nanoparticles prevents their elution even under harsh conditions such as treatment with pure 2-mercaptoethanol or treatment with boiling water for 5 h. Application of the gold modified monolith in tandem with a packed C18 capillary column is demonstrated with baseline separation of a peptide mixture achieved in a two step process. The first involves retention of cysteine-containing peptides in monolith with reversed phase separation of all other peptides, while the retained peptides are released from monolith and separated in the second step.

CEC of phytochemical bioactive compounds

Although there are many publications related to technological or methodological developments of CEC, few focus on the analysis of natural products, especially phytochemical bioactive compounds. This review summarized the application of CEC in the analysis of phytochemical bioactive components, including flavonoids, nucleosides, steroids, lignans, quinones and coumarins, as well as fingerprint analysis of herbs. The strategies for optimization of CEC conditions and detection were also discussed.

Porous polymer monoliths: amazingly wide variety of techniques enabling their preparation

The porous polymer monoliths went a long way since their invention two decades ago. While the first studies applied the traditional polymerization processes at that time well established for the preparation of polymer particles, creativity of scientists interested in the monolithic structures has later led to the use of numerous less common techniques. This review article presents vast variety of methods that have meanwhile emerged. The text first briefly describes the early approaches used for the preparation of monoliths comprising standard free radical polymerizations and includes their development up to present days. Specific attention is paid to the effects of process variables on the formation of both porous structure and pore surface chemistry. Specific attention is also devoted to the use of photopolymerization. Then, several less common free radical polymerization techniques are presented in more detail such as those initiated by gamma-rays and electron beam, the preparation of monoliths from high internal phase emulsions, and cryogels. Living processes including stable free radicals, atom transfer radical polymerization, and ring-opening metathesis polymerization are also discussed. The review ends with description of preparation methods based on polycondensation and polyaddition reactions as well as on precipitation of preformed polymers affording the monolithic materials.