Preparation and evaluation of zirconia-coated silica monolith for capillary electrochromatography

Applications of hierarchically structured porous materials from energy storage and conversion, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine

A comprehensive review of the recent progress in the applications of hierarchically structured porous materials is given.

Development of an Automated Column Solid-Phase Extraction Cleanup of QuEChERS Extracts, Using a Zirconia-Based Sorbent, for Pesticide Residue Analyses by LC-MS/MS

A new, automated, high-throughput, mini-column solid-phase extraction (c-SPE) cleanup method for QuEChERS extracts was developed, using a robotic X-Y-Z instrument autosampler, for analysis of pesticide residues in fruits and vegetables by LC-MS/MS. Removal of avocado matrix and recoveries of 263 pesticides and metabolites were studied, using various stationary phase mixtures, including zirconia-based sorbents, and elution with acetonitrile. These experiments allowed selection of a sorbent mixture consisting of zirconia, C18, and carbon-coated silica, that effectively retained avocado matrix but also retained 53 pesticides with <70% recoveries. Addition of MeOH to the elution solvent improved pesticide recoveries from zirconia, as did citrate ions in CEN QuEChERS extracts. Finally, formate buffer in acetonitrile/MeOH (1:1) was required to give >70% recoveries of all 263 pesticides. Analysis of avocado extracts by LC-Q-Orbitrap-MS showed that the method developed was removing >90% of di- and triacylglycerols. The method was validated for 269 pesticides (including homologues and metabolites) in avocado and citrus. Spike recoveries were within 70-120% and 20% RSD for 243 of these analytes in avocado and 254 in citrus, when calibrated against solvent-only standards, indicating effective matrix removal and minimal electrospray ionization suppression.

Graphene oxide and reduced graphene oxide as novel stationary phases via electrostatic assembly for open-tubular capillary electrochromatography

We present here the application of graphene oxide (GO) and reduced graphene oxide (GOOH) sheet as novel stationary phases for open-tubular CEC (OTCEC) separation based on electrostatic assembly. The inner walls of a bare capillary column was first modified by ionic assembly of poly (diallyldimethylammonium chloride) (PDDA), and then negatively charged GO or GOOH was easily assembled on a positively charged interior walls of the capillary by electrostatic force. Scanning Electron Microscope images showed that GO and GOOH can still maintain sheet-layer-like structure when coated onto the capillary via electrostatic assembly. The chromatographic properties of the GO and GOOH coated columns were evaluated via OTCEC separations of various kinds of analytes, including three acid nitrophenol isomers, three basic nitroaniline isomers, and four neutral PAHs. Efficient separations of all the analytes were achieved with optimized buffer pH and organic additive. The reproducibility and stability of the GO or GOOH coated columns were investigated. Our results indicate the capability of application GO or GOOH sheet in OTCEC separation, which can be coated on the inner wall of fused-silica capillary via electrostatic assembly.

Determination of pesticide residues in high oil vegetal commodities by using various multi-residue methods and clean-ups followed by liquid chromatography tandem mass spectrometry

Several extraction methods were evaluated in terms of recoveries and extraction precision for 113 pesticides in avocado: QuEChERS with various d-SPE clean-ups (Z-Sep, Z-Sep+, PSA+C18 and silica), miniLuke and ethyl acetate. Extracts were analysed using liquid chromatography coupled with triple quadrupole mass spectrometer working in multi-reaction monitoring mode. Z-Sep and Z-Sep+ are new types of material for high lipid matrices - these two sorbents contain ZrO2, which improves fat removal from the extracts. The QuEChERS protocol with Z-Sep provided the highest number of pesticides with recoveries in the 70-120% range along with the lowest amount of coextracted matrix compounds. Subsequently, this method was validated in two matrices - avocado and almonds. In the validation recoveries at two levels - 10 and 50μg/kg - limit of quantitation, linearity, matrix effects, as well as the inter- and intraday precision were studied. In the avocado samples, 107 analytes had LOQs equal to 10μg/kg (signal to noise of quantitative transition was equal 20 or more). In the almond samples, 92 pesticides had LOQs equal to 10μg/kg (S/N≥20) and 2 pesticides at 50μg/kg. The validated method was employed in the analysis of real avocado and almond samples.

In situ sol-gel preparation of porous alumina monoliths for chromatographic separations of adenosine phosphates

A method enabling the in situ preparation of porous alumina monoliths within 100 μm i.d. fused silica capillaries has been developed. These monoliths were prepared using the sol-gel process from a mixture consisting of an inorganic aluminum salt, a porogen, an epoxide, and a solvent. We investigated the effects of varying the preparation conditions on the physical characteristics of the monoliths with respect to their potential application in chromatographic separations. The best columns were obtained from a mixture of aluminum chloride hexahydrate, N,N-dimethylformamide, water, ethanol and propylene oxide. Adenosine phosphates were then separated in the optimized column with retention increasing according to number of phosphate functionalities.

Influence of the hydrothermal treatment on the chromatographic properties of monolithic silica capillaries for nano-liquid chromatography or capillary electrochromatography

In the last decade, silica monolithic capillaries have focused more and more attention on miniaturized separation techniques like capillary electrochromatography (CEC), nano-liquid chromatography (nano-LC) and chip electrochromatography owing to their unique chromatographic properties and their simplified preparation compared with packed columns. They are synthesized according to a sol-gel multi-step process that includes, after a gelation step at 40 degrees C leading to the formation of the macropores network and the silica skeleton, a post-gelation step (hydrothermal treatment at 120 degrees C in basic medium) that allows to tailor the mesopores and finally a calcination or a washing step to remove remaining polymers. In order to reduce the synthesis time, the number of synthesis steps and above all the temperature synthesis, to adapt the synthesis of such silica monoliths in polymeric microsystem devices, we extensively studied the influence of the hydrothermal treatment and its duration on textural (pore size distribution) and chromatographic properties (retention, efficiency) of in situ-synthesized capillary monoliths in nano-LC and CEC. This study was performed on pure silica and octyl chains grafted silica monoliths. Untreated monoliths show small pores (<6 nm), whereas hydrothermally treated monoliths exhibit medium and large mesopores (8-17 nm). It was demonstrated that the hydrothermal treatment at 120 degrees C was not necessary for pure silica monolithic capillaries dedicated to normal phase liquid chromatography or hydrophilic interaction liquid chromatography (HILIC) and electrochromatography: the suppression of the hydrothermal treatment did not impair efficiencies in CEC and in nano-LC but contributed to increase in retention factors. Minimal plate heights of ca. 5 microm in CEC and 6 microm in nano-LC were obtained with or without hydrothermal treatment with bare silica. In the same way, the hydrothermal treatment was not necessary for grafted silica monoliths only dedicated to CEC. However, the results clearly indicate that the hydrothermal treatment becomes essential before grafting in order to preserve the efficiency of the monolithic silica capillaries dedicated to nano-LC: in this particular case, the suppression of the hydrothermal treatment leads approximately to a loss of a factor two in efficiency.

Recent progress in polar stationary phases for CEC

This review summarizes most of the recent developments in the preparation and application of polar stationary phases for CEC covering the literature published since the year 2004. These polar stationary phases have been adopted for separation of analytes by the modes of packing column CEC, open-tubular CEC (o-CEC) and monolithic column CEC. Currently, development of o-CEC using biomolecules, such as protein and DNA, as the immobilized ligands is highlighted partly due to the simplicity of preparation. Furthermore, monolithic columns have been extended quickly, particularly inorganic materials-based monoliths, such as silica, zirconia, hafnium, etc., as an alternative to packed columns have been developed quickly.

An overview of the chromatographic properties and stability of C18 titanized phases

Phases based on titanized silicas are an alternative in the search for HPLC stationary phases with enhanced pH stability. This technology explores the chemical modification of the bare silica surface by grafting a titanium oxide layer with the objective of retarding the dissolution of the support in alkaline mobile phases, followed by C18 silanization. The present manuscript describes recent work on the development of chemically bonded titanized phases, including phases containing embedded urea groups, and phases prepared both in the absence and in the presence of a monolayer of water preadsorbed onto the bare silica. The advantages and disadvantages of these alternative C18 titanized phases are discussed, taking into account their chromatographic properties accessed by some common test procedures. Column lifetimes, measured by accelerated aging tests using aggressive conditions, such as high-pH phosphate mobile phases and elevated temperature, are also discussed.

Recent advances in capillary electrophoresis and capillary electrochromatography of pollutants

Recent advances in the CE and CEC separation, detection, and sample preparation methodologies applied to the determination of a variety of compounds having current or potential environmental relevance have been overviewed. The reviewed literature has illustrated the wide range of CE applications, indicating the continuing interest in CE and CEC in the environmental field.

Synthesis of zirconia monoliths for chromatographic separations

The aim of this work is to join the advantages of two different kinds of stationary phases: monolithic columns and zirconia-based supports. On the one hand, silica monolithic columns allow a higher efficiency with a lower back-pressure than traditional packed columns. On the other hand, chromatographic stationary phases based on zirconia have a higher thermal and chemical stability and specific surface properties. Combining these advantages, a zirconia monolith with a macroporous framework could be a real improvement in separation sciences. Two main strategies can be used in order to obtain a zirconia surface on a monolithic skeleton: coating or direct synthesis. The coverage by a zirconia layer of the surface of a silica-based monolith can be performed using the chemical properties of the silanol surface groups. We realized this coverage using zirconium alkoxide and we further grafted n-dodecyl groups using phosphate derivatives. Any loss of efficiency was observed and fast separations have been achieved. The main advance reported in this paper is related to the preparation of zirconia monoliths by a sol-gel process starting from zirconium alkoxide. The synthesis parameters (hydrolysis ratio, porogen type, precursor concentration, drying step, etc.) were defined in order to produce a macroporous zirconia monoliths usable in separation techniques. We produced various homogeneous structures: zirconia rod 2 cm long with a diameter of 2.3 mm, and zirconia monolith inside fused silica capillaries with a 75 microm I.D. These monoliths have a skeleton size of 2 microm and have an average through pore size of 6 microm. Several separations have been reported.

Silica monolithic columns: Synthesis, characterisation and applications to the analysis of biological molecules

In recent years, proteomics has been a subject of intense research. The complexity of proteomics samples has fostered technological developments. One of these addresses the need for more efficient and faster separations. Monolithic columns prepared from organic and silica monomers offer very efficient separations at low back-pressure. Silica-based monoliths have small-sized skeletons and a bimodal pore size distribution with microm-sized throughpores and nm-sized mesopores. This gives silica-based monoliths favourable properties for high-efficiency, fast separations, like a low-pressure drop across the column, fast mass transfer kinetics and a high binding capacity.

Utilization of zirconia stationary phase as a tool in drug control

Zirconia-based stationary phases represent an interesting alternative to silica-based materials. Two zirconia-based stationary phases were studied as an option for use in drug analysis. The different properties of zirconia material, distinct from RP silica-columns, were employed for the development of a novel and rapid stability monitoring HPLC method. This method enables simultaneous control of possible degradation processes of active substance (ibuprofen) as well as antimicrobial excipients (methyl-and propylparaben). The separation of ibuprofen, its two main degradation products 2-(4-isobutyrylphenyl)propionic acid and 4-isobutylacetophenone, parabens, and 4-hydroxybenzoic acid as their degradation product was successfully accomplished on a Zr-CarbonC18 column using a mobile phase consisting of acetonitrile-phosphate buffer (pH 4.8)-propan-2-ol (27:56:17, v/v/v). Detection was performed at 258 nm and the analysis was completed within 17 minutes.

High-performance liquid chromatography of some basic drugs on an-octadecylphosphonic acid modified magnesia-zirconia stationary phase

The high-performance liquid chromatographic behavior of some basic drugs was studied on a n-octadecylphosphonic acid modified magnesia-zirconia (C18PZM) stationary phase. The effect of mobile phase variables such as methanol content, ionic strength, and pH on their chromatographic behavior was investigated. The retention mechanism of basic drugs on the stationary phase was elucidated. The results indicate that both hydrophobic and cation-exchange interactions contribute to solute retention under most chromatographic conditions. The inherent Brönsted-acid sites and also the adsorbed Lewis base anionic buffer constituents on accessible ZM surface Lewis acid sites play a role in the retention of ionized solutes by cation-exchange interaction. However, especially at high mobile phase pH, the retention of basic drugs depends mainly on hydrophobic interactions between solutes and support. Separations of the basic drugs on the C18PZM phase by a predominantly reversed-phase retention mode were very promising. The mixed-mode retention feature on this phase, as a result of the adsorbed Lewis base anionic buffer constituents acting as sites for cation-exchange, could also be very useful, e.g. for enhancing the chromatographic selectivity of such analytes. The C18PZM seems to be an excellent alternative to silica-based reversed-phase stationary phase for the separation of strongly basic solutes.

Recent developments in the field of monolithic stationary phases for capillary electrochromatography

This review summarizes the contributions to the rapidly growing area of monolithic columns based on both silica and synthetic polymers for capillary electrochromatography and chip electrochromatography, with a focus on those published during the year 2004. A wide variety of both modified approaches to the "old" monoliths and new monoliths have been reported despite the very short period of time covered. This demonstrates that monolithic stationary phases have become a well-established format in the field of electrochromatography. The simplicity of their preparation as well as the good control over their porous properties and surface chemistries make the monolithic separation media an attractive alternative to capillary columns packed with particulate materials.

Metal oxide monolithic columns

Metal oxide monoliths composed of ZrO2 and HfO2 have been synthesized in situ inside capillary columns. The material shows globular-like structure and through pores. Capillary electrochromatography and capillary liquid chromatography were performed in a monolithic column with the HfO2 material. Separation of a simple sample mixture showed the potential of the new metal oxide monolithic columns.

Titania-coated monolithic silica as separation medium for high performance liquid chromatography of phosphorus-containing compounds

A method of preparing titania-coated monolithic silica stationary phase has been developed to achieve liquid chromatographic separation of phosphorus-containing compounds, which have recently been attracting increasing attention in biochemical research. The titania-coated silica columns exhibited efficient separation with low pressure drop, which is a typical feature of monolithic structures, and also possessed phospho-selectivity, which is a unique property of the titania surface. The material characteristics of titania-coated monolithic silica were examined, and then resin-clad columns were applied to the HPLC analysis of phosphorylated compounds. Highly efficient separation of phosphorylated substances indicated that the novel titania-coated monolithic silica column will find applications as a useful tool in the field of biochemistry, especially in post-genomic analyses.