Monolithic LC columns

Plant metabolomics: potential for practical operation

In the postgenomic era, metabolomics is expected to be the newest useful omics science for functional genomics. However, in plant science, the present metabolomics technology cannot be considered a universal tool to perfectly elucidate perturbations imposed on sample plants although this is desired by plant physiologists. Despite it being an immature technology, metabolomics has already been used as a powerful tool for precise phenotyping, particularly for industrial application. Metabolomics is the best technology for the analysis of large mutant or transgenic libraries of model experimental plants, such as Arabidopsis, rice, etc. Here, we review the applications and technical problems of metabolomics. We also suggest the potential of metabolomics for plant post-genomic science.

Capillary-channeled polymer (C-CP) fibers as a stationary phase in microbore high-performance liquid chromatography columns

Microbore columns utilizing polypropylene capillary-channeled polymer (C-CP) fibers as the stationary phase in high-performance liquid chromatography (HPLC) have been investigated. The polypropylene C-CP fiber diameter is approximately 50 microm, with eight channels along the periphery of the fiber ranging in diameter from approximately 12 to 35 microm. The polypropylene C-CP fibers were packed into fluorinated ethylene propylene (FEP) tubing, 1.3 mm inner diameter, with lengths of 500, 750, and 1,000 mm, to examine the effects of increased column length with regards to plate height, resolution and analysis time. The low backpressures characteristic of the C-CP fiber stationary phases allow the length of the column to be increased without significantly decreasing the specific permeability. The high specific permeability (approximately 5x10(-8) cm2) of the C-CP packed microbore columns yields a relatively low backpressure of 2.35 MPa at the highest flow rate of 17 microL/s (54 mm/s) for a 1,000 mm column. Radial compression of the soft-walled FEP tubing is accomplished by pulling the 1.7 mm o.d. column through a 1.4 mm diameter orifice. Reducing the inner diameter of the column from 1.3 to 1.0 mm lowered the interstitial fraction from 47% to 42%, decreased the A-term contributions to band broadening, resulted in a significant decrease in average plate height (approximately 30%), and increased resolution (approximately 36%) at identical linear velocities. Although the lower void volume of the radially compressed column increased the backpressure from 0.57 to 2.11 MPa at a linear velocity of approximately 20 mm/s, the specific permeability only decreased from approximately 7x10(-8) to 4x10(-8) cm2.

Two-dimensional separation system of coupling capillary liquid chromatography to capillary electrophoresis for analysis of Escherichia coli metabolites

A two-dimensional (2-D) separation system of coupling chromatography to electrophoresis was developed for profiling Escherichia coli metabolites. Capillary liquid chromatography (LC) with a monolithic silica-octadecyl silica column (500 x 0.2 mm ID) was used as the first dimension, from which the effluent fractions were further analyzed by capillary electrophoresis (CE) acting as the second dimension. Field-enhanced stacking was selectively employed as a concentration strategy to interface the two dimensions, which proved to be beneficial for the detection of metabolites. An artificial sample containing 118 standards, some of which lack chromophores or have weak UV absorbance, was used to optimize the 2-D separation system. Under the optimum conditions, 63 components in the artificial sample having absorbance at 254 nm could be well resolved and detected. The utility of the system was demonstrated by comprehensive analysis of E. coli metabolites. Comparing with the previous 2-D separation system we published in Anal. Chem. 2004, 76, 1419-1428, using a longer monolithic column in the first dimension improved the separation efficiency and offered the possibility of increasing the injection volume without compromising the separation efficiency. In the second dimension, field-enhanced stacking was used to improve the concentration sensitivity of the metabolites, and more metabolites in E. coli cell extract were detected and identified using the developed 2-D separation system. In addition, preliminary investigation for future CE-mass spectrometry coupling was also made in the study by using volatile buffers in the capillary LC and CE techniques.

Micro-separation toward systems biology

Current biology is experiencing transformation in logic or philosophy that forces us to reevaluate the concept of cell, tissue or entire organism as a collection of individual components. Systems biology that aims at understanding biological system at the systems level is an emerging research area, which involves interdisciplinary collaborations of life sciences, computational and mathematical sciences, systems engineering, and analytical technology, etc. For analytical chemistry, developing innovative methods to meet the requirement of systems biology represents new challenges as also opportunities and responsibility. In this review, systems biology-oriented micro-separation technologies are introduced for comprehensive profiling of genome, proteome and metabolome, characterization of biomolecules interaction and single cell analysis such as capillary electrophoresis, ultra-thin layer gel electrophoresis, micro-column liquid chromatography, and their multidimensional combinations, parallel integrations, microfabricated formats, and nano technology involvement. Future challenges and directions are also suggested.

Separation of polyprenol and dolichol by monolithic silica capillary column chromatography

We attempted an analysis of naturally occurring polyprenol and dolichol using a monolithic silica capillary column in HPLC. First, the separation of the polyprenol mixture alone was performed using a 250 x 0.2 mm inner diameter (ID) octadecylsilyl (ODS)-monolithic silica capillary column. The resolution of the separation between octadecaprenol (prenol 18) and nonadecaprenol (prenol 19) exceeded by >or=2-fold the level recorded when using a conventional ODS-silica particle-packed column (250 x 4.6 mm ID) under the same elution conditions. Next, the mixture of the prenol type (polyprenol) and dolichol type (dihydropolyprenol) was subjected to this capillary HPLC system, and the separation of each homolog was successfully achieved. During the analysis of polyprenol fraction derived from Eucommia ulmoides leaves, dolichols were found as a single peak, including all-trans-polyprenol and cis-polyprenol previously identified. This sensitive high-resolution system is very useful for the analysis of compounds that are structurally close to polyprenols and dolichols and that have a low content.

Comparison of the performance of conventional microparticulates and monolithic reversed-phase columns for liquid chromatography separation of eleven pollutant phenols

The performance of isocratic separations of 11 pollutant phenols (PP) using monolithic (Chromolith RP-18e) and conventional reversed-phase 5 microm (Luna and Purospher C18) and 4 microm (Synergi C12) particulate size columns, selected from high purity silica materials, has been compared. The separations have been optimized based on a previously optimized separation in which a reversed-phase C18 Luna column and acetonitrile as organic modifier were used, allowing the separation of all phenols tested in 23 min. The optimization process was carried out for each column by studying the effect of the mobile phase (acetonitrile as organic modifier, pH, flow-rate) on phenols separation. Under the optimized separation conditions, all phenols were separated in less than 23 min for all columns tested. Asymmetry factors were further evaluated and used to estimate column efficiency using the Dorsey-Foley equation. The efficiency and asymmetry factors were lower for Chromolith than for Purospher and Luna columns respectively. The Chromolith column was finally selected, due to its lower flow resistance, analysis time and good efficiency and asymmetry factors. The PPs separation was achieved in 3 min. The asymmetry factors were in the range 0.9-1.5 using 50mM acetate buffer (pH = 5.25)-ACN (64:36, v/v) as mobile phase, T=45 degrees C and 4.0 ml min(-1) flow-rate.

Ion chromatography for determination of nitrite and nitrate in seawater using monolithic ODS columns

A fast and highly sensitive ion chromatographic method using monolithic ODS columns was developed for the determination of nitrite (NO2-) and nitrate (NO3-) in seawater. Two monolithic ODS columns (50 mm x 4.6 mm i.d. + 100 mm x 4.6 mm i.d.) connected in series were coated and equilibrated with 5 mM cetyltrimethylammonium chloride (CTAC) aqueous solution. The column efficiency with 0.5 M NaCl as the mobile phase did not decrease in spite of the increase in flow rate of the mobile phase. Thus, good chromatograms were obtained within 3 minutes for NO2- and NO3 in artificial seawater without interferences by coexisting ions. The detection limit (S/N = 3) with UV detection at 225 nm was 0.8 and 1.6 microg/L for NO2- and NO3-, respectively. The characteristics of the monolithic CTA(+)-coated ODS columns were discussed. The present method was successfully applied to the fast and sensitive determination of NO2- and NO3- in real seawater samples.

Numerical analysis of electroosmotic flow in dense regular and random arrays of impermeable, nonconducting spheres

We present a numerical scheme for analyzing steady-state isothermal electroosmotic flow (EOF) in three-dimensional random porous media, involving solution of the coupled Poisson, Nernst-Planck, and Navier-Stokes equations. While traditional finite-difference methods were used to resolve the Poisson-Nernst-Planck problem, the (electro)hydrodynamics has been addressed with high efficiency using the lattice-Boltzmann method. The developed model allows simulation of electrokinetic transport under most general conditions, including arbitrary value and distribution of electrokinetic potential at the solid-liquid interface, electrolyte composition, and pore space morphology. The approach provides quantitative information on a spatial distribution of simulated velocities. This feature was utilized to characterize EOF fields in regular and random, confined and bulk packings of hard (i.e., impermeable, nonconducting) spheres. Important aspects of pore space morphology (sphere size distribution), surface heterogeneity (mismatch in electrokinetic potentials at confining wall and sphere surface), and fluid phase properties (electrical double layer thickness) were investigated with respect to their influence on the EOF dynamics over microscopic and macroscopic spatial domains. Most important is the observation of a generally nonuniform pore-level EOF velocity profile in the sphere packings (even in the thin double layer limit) which is caused by pore space morphology and which is in contrast to the pluglike velocity distribution in a single, straight capillary under the same conditions.

Capillary-Scale Monolithic Immunoaffinity Columns for Immunoextraction with In-Line Laser-Induced Fluorescence Detection

A bimodal meso/macroporous monolithic silica capillary column containing an entrapped antibody was prepared by a biocompatible sol-gel process and used for nanoflow immunoaffinity chromatography and immunoextraction studies. Stationary phases were prepared by combining the protein-compatible silane precursor diglycerylsilane with an aqueous solution containing 10,000 Da poly(ethylene glycol) and the antibody. An analytical method was developed that was capable of determining both the dissociation constant and binding site content for the anti-fluorescein antibody within the stationary phase. The assay showed that while the antibody residing in macropores was easily removed, approximately 20% of initially loaded antibody remained active and accessible after several washes, consistent with the antibody being entrapped within the mesopores of the sol-gel matrix. The dissociation constants for fluorescein binding to the anti-fluorescein antibody were similar in solution and in the meso/macroporous silica, indicating that the entrapped antibody retained its native conformation within such a matrix. The mixture was loaded into a 250-microm-i.d. fused-silica capillary where the polymer phase separated from the silica followed by gelation of the silica. The capillary-scale immunoaffinity columns could be operated at low back pressure using a syringe pump and were capable of performing chromatographic separations that were dependent on the presence of the antibody within the stationary phase. Such columns could also be operated using in-line laser-induced fluorescence detection. The use of the capillary-scale monolithic columns for on-column immunoextraction and preconcentration is also demonstrated.

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.

Determination of 16 Phthalate Metabolites in Urine Using Automated Sample Preparation and On-line Preconcentration/High-Performance Liquid Chromatography/Tandem Mass Spectrometry

We developed an on-line solid-phase extraction (SPE) method, coupled with isotope dilution high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS) and with automated sample preparation, to simultaneously quantify 16 phthalate metabolites in human urine. The method requires a silica-based monolithic column for the initial preconcentration of the phthalate metabolites from the urine and a silica-based conventional analytical column for the chromatographic separation of the analytes of interest. It uses small amounts of urine (100 microL), is sensitive (limits of detection range from 0.11 to 0.90 ng/mL), accurate (spiked recoveries are approximately 100%), and precise (the inter- and intraday coefficients of variation are <10%). The method is not labor intensive, and, because pretreatment of the urine samples was performed automatically using an HPLC autosampler, involves minimal sample handling, thus minimizing exposure to hazardous chemicals. The method was validated on spiked, pooled urine samples and on urine samples from 43 adults with no known exposure to phthalates. The high sensitivity and high throughput (HPLC run time, including the preconcentration step, is 27 min) of this analytical method combined with the ease of use and effective automated sample preparation procedure make it suitable for large epidemiological studies to evaluate the prevalence of human exposure to phthalates.

Development of miniaturized multi-channel high-performance liquid chromatography for high-throughput analysis

We have developed miniaturized multi-channel high-performance liquid chromatography (HPLC) system. With this system, we can simultaneously separate multiple samples, using a single high-pressure gradient pump, a chip-based sample injection unit, a monolithic silica capillary column array, and a multi-channel UV detection unit based on fiber optics. The injection unit has a simplified structure composed of brass housing and a quartz microchip having microchannels and access ports, which enable a direct injection of sample to multi-channel by commercial multichannel micropipette. Moreover, that possesses a function of microvalve, and on-chip definition of sample injection plugs achieved with a cross channel injection method, providing each column of monolithic silica capillary array. The substances in channels were simultaneously detected with UV having multiple cells. Standard samples were analyzed for characterizing newly developed system, and sharp peaks were obtained with reproducibility data of < 0.9% (R.S.D.). Analysis of tryptic digestion of casein was also employed. These results show that the novel multi-channel HPLC system has the benefits for the high-throughput analysis in the post-genomic analysis/combinatorial chemistry.

Evaluation of a monolithic silica column operated in the hydrophilic interaction chromatography mode with evaporative light scattering detection for the separation and detection of counter-ions

In this work a monolithic silica column operated in the hydrophilic interaction chromatography (HILIC) mode in conjunction with an evaporative light scattering detector (ELSD) was investigated. Lithium, sodium and potassium were used as the test counter-ions for this evaluation. Chromatographic properties of this column operated in the HILIC mode were determined by varying key mobile phase parameters, such as pH, flow rate, buffer strength, acid and organic modifier. As organic content was increased from 60 to 90% acetonitrile, retention time increased on average by a factor of seven for the test cations listed above. Buffer concentration and pH were also observed to have an effect, although not as significant as the HILIC effect that was observed by changing organic content. Flow rates up to 5 mL/min were utilized to perform counter-ion separations in less than 3 min. After examining the changes in retention, resolution, and peak shape an optimized method was established and then further evaluated for linearity, reproducibility, and limit of detection (LOD) for sodium. Linearity was acceptable with an R2 value of 0.999 across the working-standard range and a LOD of 0.1 microg/mL was calculated. The reproducibility on the counter-ion determination from pharmaceutical sodium salts was 1.6% R.S.D. on average, and the accuracy of the counter-ion prediction was approximately 3% from theory when salt content was corrected for potency.

An electro-osmotic micro-pump based on monolithic silica for micro-flow analyses and electro-sprays

A high-pressure electro-osmotic micro-pump fabricated by a sol-gel process is reported as a fluid-driving unit in a flow-injection analysis (FIA) system. The micro FIA system consists of a monolithic micro-pump on a glass slide (2.5 x 7.5 cm), a micro-injector, and a micro-sensor (2.5 x 1.5 cm). The monolithic silica matrix has a continuous skeleton morphology with micrometer-sized through-pores. The micrometer-size pores with a large negative surface charge density build up a large pressure under a DC electric field to drive fluid through the downstream units. A novel Nafion joint for the downstream cathode eliminates flow into the electrode reservoir and further enhances pressure build-up. The measured pump-pressure curve indicated a maximum pressure of 0.4 MPa at flow rate of 0.4 microL min(-1) at 6 kV. Despite the large voltage, the small current transmission area through the monolith produced a negligible current (less than 100 microA) that did not generate bubbles or ion contaminants. The flow rate can be precisely controlled in the range 200 nL to 2.5 microL min(-1) by varying the voltage from 1 to 6 kV. The high pump pressure and the large current-free DC field also enabled the pump to act as an electro-spray interface with a downstream analytical instrument.

Monolithic Bioreactor Immobilizing Trypsin for High-Throughput Analysis

A miniaturized trypsin reactor was prepared by coating a trypsin-containing gel on a porous silica monolith. The trypsin-encapsulated gel was prepared by the sol-gel method. The sol-gel reaction was optimized so that the sol solution containing trypsin forms a thin film on the sol-gel monolith. The trypsin was encapsulated into the gel matrix without losing its activity. The silica monolith was fabricated to fit into a 96-well microtiter plate well and could then be easily removed. The trypsin-immobilized monolith was reacted in the 96-well microtiter plate. After the reaction, the monolith was removed, and the enzymatic activity was measured. The large surface area of the monolith enabled the immobilized trypsin to achieve a high catalytic turnover rate. Furthermore, the kinetic parameter of the immobilized trypsin indicates the absence of diffusional limitations. The durability and repeatability of the fabricated trypsin-coated monolith was tested and found to be satisfactory. The encapsulated trypsin exhibits an increased stability even after continuous use compared with that in free solution. Furthermore, this on-plate bioreactor was applicable to the digestion of protein with multiple cleavage sites.

Allyl-functionalized hybrid silica monoliths

A hybrid organosilica monolith was synthesized containing an allyl functionality. This provided a viable platform for producing silica-based, chromatographic, monolithic columns with the stationary phase bonded through a surface silicon-carbon bond rather than a conventional siloxane bond.

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.

A novel on-line solid-phase extraction approach integrated with a monolithic column and tandem mass spectrometry for direct plasma analysis of multiple drugs and metabolites

An on-line solid-phase extraction liquid chromatography/tandem mass spectrometry (SPE LC/MS/MS) assay using a newly developed SPE column and a monolithic column was developed and validated for direct analysis of plasma samples containing multiple analytes. This assay was developed in an effort to increase bioanalysis throughput and reduce the complexity of on-line SPE LC/MS/MS systems. A simple column-switching configuration that requires only one six-port valve and one HPLC pumping system was employed for on-line plasma sample preparation and subsequent gradient chromatographic separation. The resulting analytical method couples the desired sensitivity with ease of use. The method was found to perform satisfactorily for direct plasma analysis with respect to assay linearity, specificity, sensitivity, precision, accuracy, carryover, and short-term stability of an eight-analyte mixture in plasma. A gradient LC condition was applied to separate the eight analytes that cannot be distinctly differentiated by MS/MS. With a run time for every injection of 2.8 min, a minimum of 300 direct plasma injections were made on one on-line SPE column without noticeable changes in system performance. Due to the ruggedness and simplicity of this system, generic methods can be easily developed and applied to analyze a wide variety of compounds in a high-throughput manner without laborious off-line sample preparation.

Effect of the Silanization Conditions on Chromatographic Behavior of an Open-tubular Capillary Column Coated with a Modified Silica-gel Thin Layer

The performance of an open-tubular capillary column coated with a modified silica-gel thin layer was investigated, particularly concerning the effect of the silanization process on it. Although the increase in the octadecyltriethoxysilane (ODTES) concentration in the silanization process could enhance the retention factor of naphthalene, its theoretical plate number was significantly reduced (ODTES, 5 to 50%; k, 0.2 to 4.3; N, 79600 to 2600 m(-1)). Namely, the increase in the retention factor was accompanied by a decrease in the theoretical plate number. A similar phenomenon was also observed when octadecyldimethylchlorosilane (ODCS) was used as the silanization regent. However, increases in both the retention factor and the theoretical plate number could be achieved (sample, naphthalene; k, 0.05 to 0.09; N, 149000 to 220000 m(-1)) by a NaOH treatment to the fabricated thin porous silica-gel layer before silanization with ODCS. The electrochromatographic separation of proteins and peptides by using the NaOH-treated column could obtain more peaks than electrophoretic separation.

High-throughput liquid chromatography-tandem mass spectrometry determination of bupropion and its metabolites in human, mouse and rat plasma using a monolithic column

In the present work, a high-throughput LC/MS/MS method using a Chromolith RP-18 (50 mm x 4.6 mm) monolithic column was developed and partially validated for the determination of bupropion (BUP), an anti-depressant drug, and its metabolites, hydroxybupropion and threo-hydrobupropion (TB), in human, mouse, and rat plasma. A modern integrated liquid chromatograph and an LC/MS/MS system with a TurboIonSpray (TIS) interface were used for the positive electrospray selected reaction monitoring (SRM) LC/MS analyses. Spiked control plasma calibration standards and quality control (QC) samples were extracted by semi-automated 96-well liquid-liquid extraction (LLE) using ethyl acetate. A mobile phase consisting of 8mM ammonium acetate-acetonitrile (55:45, v/v) delivered isocratically at 5 ml/min, and split post-column to 2 ml/min directed to the TIS, provided the optimum conditions for the chromatographic separation of bupropion and its metabolites within 23s. The isotope-labeled D(6)-bupropion and D(6)-hydroxybupropion were used as internal standards. The method was linear over a concentration range of 0.25-200 ng/ml (bupropion and threo-hydrobupropion), and 1.25-1000 ng/ml (hydroxybupropion). The intra- and inter-day assay accuracy and precision were within 15% for all analytes in each of the biological matrices. The monolithic column performance as a function of column backpressure, peak asymmetry, and retention time reproducibility was adequately maintained over 864 extracted plasma injections.

Capillary columns in liquid chromatography: between conventional columns and microchips

Liquid chromatography on columns with small internal diameters has been reviewed as the intermediate technique between conventional liquid chromatography and microchip separations. The development of micro column separations in the early years has been described, starting with the papers of Horváth and co-workers and Ishii and co-workers, continuing into the first part of the eighties, then making a leap in time to recent innovations with small-bore columns. Based on internal diameters a classification of the different analytical HPLC columns has been suggested. The advantages of small-bore columns have been discussed, with particular emphasis on the advantage of coupling to concentration sensitive detectors when the sample amount is limited. Open tubular columns are treated as a part of the historic background. The recent developments include a brief look into the current status of monolithic columns, the use of packed nano columns and micro columns with electrospray mass spectrometry, and the potential of two-dimensional comprehensive liquid chromatography. Finally, the coupling of sample preparation to analytical columns and the future applications of the novel technological improvements to the microchip separation methods have been discussed.

Comparison of the efficiency of microparticulate and monolithic capillary columns

A comparison is made between the efficiency of microparticulate capillary columns and silica and polymer-based monolithic capillary columns in the pressure-driven (high-performance liquid chromatography) and electro-driven (capillary electrochromatography) modes. With packed capillary columns similar plate heights are possible as with conventional packed columns. However, a large variation is observed in the plate heights for individual columns. This can only be explained by differences in the quality of the packed bed. The minimum plate height obtained with silica monolithic capillary columns in the HPLC mode is approximately 10 microm, which is comparable to that of columns packed with 5-microm particles. The permeability of wide-pore silica monoliths was found to be much higher than that of comparable microparticulate columns, which leads to much lower pressure drops for the same eluent at the same linear mobile phase velocity. For polymer-based monolithic columns (acrylamide, styrene/divinyl benzene, methacrylate, acrylate) high efficiencies have been found in the CEC mode with minimum plate heights between 2 and 10 microm. However, in the HPLC mode minimum plate heights in the range of 10 to 25 microm have been reported.

How to utilize the true performance of monolithic silica columns

Ways of utilizing the true separation efficiency of monolithic silica (MS) columns were studied. The true performance of MS columns, both regular-sized (rod-type clad with PEEK resin, 4.6 mm ID, 10 cm) and capillary sized (in 100 or 200 microm ID fused silica capillary, 25-140 cm) was evaluated by calculating the contribution of extra-column effects. HETP values of 7-9 microm were observed for solutes having retention factors (kvalues) of up to 4 for rod columns and up to 15 for a capillary column. The high permeability of MS columns allowed the use of long columns, with several connected together in the case of rod columns. Narrow-bore connectors gave good results. Peak variance caused by a column connector ranges from 50 to 70% of that caused by one rod-type column for up to three connectors or four columns in 80% methanol, but the addition of a 4th or 5th connector to add a 5th and 6th column, respectively, caused a much greater increase in peak variance, especially for long-retained solutes, which is greater than the variance caused by one rod column. Rod columns seem to show slightly lower efficiency at a pressure higher than 10 MPa or so. The use of acetonitrile-water as a mobile phase better preserved the ability of individual rod columns to generate up to 100,000 theoretical plates with 14 columns connected. Methods for eliminating extra-column effects in micro-HPLC were also studied. Split injection and on-column detection resulted in optimum performance. A long MS capillary measuring 140 cm produced 160,000 theoretical plates. The column efficiency of a capillary column was not affected by the pressure, showing advantages over the rod columns that exhibited peak broadening caused by connectors and pressure.

Monolithic silica-based capillary reversed-phase liquid chromatography/electrospray mass spectrometry for plant metabolomics

Application of C18 monolithic silica capillary columns in HPLC coupled to ion trap mass spectrometry detection was studied for probing the metabolome of the model plant Arabidopsis thaliana. It could be shown that the use of a long capillary column is an easy and effective approach to reduce ionization suppression by enhanced chromatographic resolution. Several hundred peaks could be detected using a 90-cm capillary column for LC separation and a noise reduction and automatic peak alignment software, which outperformed manual inspection or commercially available mass spectral deconvolution software.

Hybrid organic–inorganic monolithic stationary phase for acidic compounds separation by capillary electrochromatography

A novel type of organic-inorganic hybrid porous silica-based monolithic stationary phase for capillary electrochromatography (CEC) has been developed by sol-gel chemistry combined with supramolecular template-based approach in a simple and rapid manner. Both chromatographic interaction and electrophoretic migration contribute to the separation of acidic compounds by the monolithic column. Eight organic acids were separated rapidly with column efficiency up to 267,000 theoretical plates/m. The influences of buffer concentration and organic modifier content on the separation have been investigated. In addition, the hybrid monolithic column was used to separate triterpenoids from Ganoderma lucidum.

High-performance liquid chromatographic enantioseparations on capillary columns containing monolithic silica modified with cellulose tris(3,5-dimethylphenylcarbamate)

Monolithic capillary columns containing native silica gel were modified with cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC) and used for enantioseparations in capillary liquid chromatography. The method adopted for in situ enantioselective modification of monolithic fused silica capillary columns by coating with CDPMC appears to be fairly simple and fast. High efficiency enantioseparations of test racemic compounds and s(everal chiral drugs were achieved in a short time. It was possible to increase the amount of chiral selector present by multiple coating of monoliths with CDMPC. The baseline enantioseparation of 2,2,2-trifluoro-1-(9-anthryl)ethanol was achieved in an analysis time less than 30 s with this capillary column. In addition, reproducible enantioseparations were obtained when the chiral selector was removed from the monolithic column by flushing it with appropriate solvent and the column recoated.

Structural formation of hybrid siloxane-based polymer monolith in confined spaces

Structural deformation of phase-separated methylsiloxane gel under the influence of a surface has been studied. Competitive wetting of siloxane gel phase on a surface during phase formation is found to significantly affect the final morphology in a confined space. When the spinodal wavelength is sufficiently shorter than the size of the available space, a uniform bicontinuous structure forms in confined geometry. However, gel skeletons in the vicinity of a surface are elongated with decreasing size of the space, and finally when the size of the space becomes shorter than the spinodal wavelength, all the gel phase wets on a surface, showing a "wetting transition". Homogeneous bicontinuous methylsiloxane gels were successfully prepared, avoiding such structural deformation, in a long cylindrical fused silica capillary and used for capillary HPLC. The capillary gels exhibited excellent separation efficiency of nitrobenzenes and it was found that the surface character can be altered by incorporating surfactants, which will enable more advanced and extended control of surface character, depending on the analytes.

Applications of silica-based monolithic HPLC columns

The recent invention and successive commercial introduction of monolithic silica columns has motivated many scientists from both academia and industry to study their use in HPLC. The first paper on monolithic silica columns appeared in 1996. Currently about 200 papers have been published relating to applications and characterization of monolithic silica columns, including monolithic capillaries. This review attempts to give an overview covering various aspects of this new column type in the field of high throughput analysis of drugs and metabolites, chiral separations, analysis of pollutants and food-relevant compounds, as well as in bioanalytical separations such as in proteomics. Some of the applications are described in greater detail. The numerous publications dealing with the physicochemical and chromatographic characterization of monolithic silica columns are briefly summarized.

Simple 2D-HPLC using a monolithic silica column for peptide separation

Separation of peptides by fast and simple two-dimensional (2D)-HPLC was studied using a monolithic silica column as a second-dimension (2nd-D) column. Every fraction from the first column, 5 cm long (2.1 mm ID) packed with polymer-based cation exchange beads, was subjected to separation in the 2nd-D using an octadecylsilylated (C18) monolithic sillica column (4.6 mm ID, 2.5 cm). A capillary-type monolithic silica C18column (0.1 mm ID, 10 cm) was also employed as a 2nd-D column with split flow/injection. Effluentof the first dimension (1st-D) was directly loaded into an injector loop of 2nd-D HPLC. UV and MS detection were successfully carried out at high linear velocity of mobile phase at 2nd-D using flow splitting for the 4.6 mm ID 2nd-D column, or with directconnection of the capillary column to the MS interface. Two-minute fractionation inthe 1st-D, 118-second loading, and 2-second injection by the 2nd-D injector, allowed one minute for gradient separation in the 2nd-D, resulting in a maximum peak capacity of about 700 within 40 min. The use of a capillary column in solvent consumption and better MS detectability compared to a larger-sized column. This kind of fast and simple 2D-HPLC utilizing monolithic silica columns will be useful for the separation of complex mixtures in a short time.

Development of a monolithic silica extraction tip for the analysis of proteins

In proteomics, pre-treatment of sample is the most important procedure to remove the matrix for interfacing with mass spectrometry (MS). Additionally, for the samples with low concentration, the process of pre-concentration is required before MS analysis. We have newly developed solid-phase extraction (SPE) tool with pipette-tip shape for purification of bio-samples of various characteristics, utilizing monolithic silica gel as medium. The monolithic silica surface was modified with a C18 phase or coated with titania phase. A C18-bonded tip and a non-modified tip were used for sample concentration, desaltination and removal of detergents from sample. A titania-coated tip was also applied for purification and concentration of phosphorylated peptides. This novel pre-treatment method using monolithic silica extraction tip is much effective and suitable for protein analysis.

Fast high-performance liquid chromatography method for quality control of soy extracts

Soy extracts contain a mixture of isoflavones belonging to the group of phytoestrogens. In the quality control of soy the amount of isoflavones, both aglycones and glycosides, is usually determined by means of reversed-phase HPLC-UV. On conventional C18-material columns, long analysis times are required in order to separate this complex mixture. In order to speed up analysis, the separation was optimized using two linked monolithic silica-based reversed-phase C18 columns. A spectacular decrease of the analysis time, i.e. almost three-fold, was achieved by applying a flow rate of 3-4 ml/min without loosing any separation efficiency. This analysis method for determination of isoflavones in soy extracts in less than 25 min was fully validated according to the ICH guidelines.

Capillary electrochromatography with monolithic stationary phases. II. Preparation of cationic stearyl-acrylate monoliths and their electrochromatographic characterization

A novel cationic monolithic stationary phase based on the co-polymerization of pentaerythritol diacrylate monostearate (PEDAS) with a selected quaternary amine acrylic monomer was designed for performing capillary electrochromatography at high flow velocity. While PEDAS functioned as both the ligand provider and the cross-linker, the quaternary amine acrylic monomer was introduced to control the magnitude of the electroosmotic flow (EOF). The fabrication of the cationic stearyl-acrylate monolith (designated as cationic C17 monolith) with controlled porosity was achieved by free radical polymerization using the initiator 2,2'-azobisisobutyronitrile in the presence of a ternary porogenic solvent composed of cyclohexanol, ethylene glycol and water. Four different quaternary amine acrylic monomers were investigated in order to find the optimum monomer for achieving maximum electroosmotic flow (EOF) velocity. Both photo- and thermally-initiated polymerization proved effective in producing the cationic C17 monolith, and the best monolith was achieved when [2-(acryloyloxy)ethyl]trimethyl ammonium methyl sulfate (AETA) was used as the quaternary amine acrylic monomer. Although the zeta potential of the resulting cationic C17 monolith is positive with respect to water, the magnitude and direction of the EOF was markedly affected by the nature of the electrolyte in the mobile phase. Consequently, anodal, zero or cathodal EOF was observed depending on the nature of the electrolyte, and this was attributed to the adsorption of the ionic components of the electrolyte on to the solid stationary phase, which is characterized by its amphiphilic nature consisting of C17 chains, ester functions, hydroxyl groups and quaternary amine moieties. Optimized PEDAS-AETA monoliths yielded columns with high separation efficiency and allowed rapid separations on the time scale of seconds to be achieved with short capillaries.

Practical aspects of fast reversed-phase high-performance liquid chromatography using 3μm particle packed columns and monolithic columns in pharmaceutical development and production working under current good manufacturing practice

The potential and limitations of fast reversed-phase high-performance liquid chromatographic separations for assay and purity of drug substances and drug products were investigated in the pharmaceutical industry working under current good manufacturing practice using particle packed columns and monolithic columns. On particle packed columns, the pressure limitation of commercially available HPLC systems was found to be the limiting factor for fast separations. On 3 microm particle packed columns, HPLC run times (run to run) for assay and purity of pharmaceutical products of 20 min could be achieved. As an interesting alternative, monolithic columns were investigated. Monolithic columns can be operated at much higher flow rates, thus allowing for much shorter run times compared to particle packed columns. Compared to particle packed columns, the analysis time could be reduced by a factor up to 6. However, some compounds investigated showed a dramatic loss of efficiency at higher flow rates. This phenomenon was observed for some larger molecules supporting the theory that mass transfer is critical for applications on monolithic columns. At flow rates above 3 ml/min some HPLC instruments showed a dramatic increase in noise, making quantifications at low levels impossible. For very fast separations on monolithic columns, the maximum data acquisition rate of the detector is the limiting factor.

Protein-Doped Monolithic Silica Columns for Capillary Liquid Chromatography Prepared by the Sol−Gel Method: Applications to Frontal Affinity Chromatography

The development of bioaffinity chromatography columns that are based on the entrapment of biomolecules within the pores of sol-gel-derived monolithic silica is reported. Monolithic nanoflow columns are formed by mixing the protein-compatible silica precursor diglycerylsilane with a buffered aqueous solution containing poly(ethylene oxide) (PEO, MW 10,000) and the protein of interest and then loading this mixture into a fused-silica capillary (150-250-microm i.d.). Spinodal decomposition of the PEO-doped sol into two distinct phases prior to the gelation of the silica results in a bimodal pore distribution that produces large macropores (>0.1 microm), to allow good flow of eluent with minimal back pressure, and mesopores (approximately 3-5-nm diameter) that retain a significant fraction of the entrapped protein. Addition of low levels of (3-aminopropyl)triethoxysilane is shown to minimize nonselective interactions of analytes with the column material, resulting in a column that is able to retain small molecules by virtue of their interaction with the entrapped biomolecules. Such columns are shown to be suitable for pressure-driven liquid chromatography and can be operated at relatively high flow rates (up to 500 microL x min(-1)) or with low back pressures (<100 psi) when used at flow rates of 5-10 microL x min(-1). The clinically relevant enzyme dihydrofolate reductase was entrapped within the bioaffinity columns and was used to screen mixtures of small molecules using frontal affinity chromatography with mass spectrometric detection. Inhibitors present in compound mixtures were retained via bioaffinity interactions, with the retention time being dependent on both the ligand concentration and the affinity of the ligand for the protein. The results suggest that such columns may find use in high-throughput screening of compound mixtures.

Simple and Comprehensive Two-Dimensional Reversed-Phase HPLC Using Monolithic Silica Columns

Simple and comprehensive two-dimensional (2D)-HPLC was studied in a reversed-phase mode using monolithic silica columns for second-dimension (2nd-D) separation. Every fraction from the first column, 15 cm long (4.6-mm i.d.), packed with fluoroalkylsilyl-bonded (FR) silica particles, was subjected to the separation in the 2nd-D using one or two octadecylsilylated (C(18)) monolithic silica columns (4.6-mm i.d., 3 cm). Monolithic silica columns in the 2nd-D were eluted at a flow rate of up to 10 mL/min with separation time of 30 s that meets the fractionation every 15-30 s at the first dimension (1st-D) operated at a flow rate of 0.4-0.8 mL/min. Three cases were studied. (1) In the simplest scheme of 2D-HPLC, effluent of the 1st-D was directly loaded into an injector loop of 2nd-D HPLC for 28 s, and 2 s was allowed for injection. (2) Two six-port valves each having a sample loop were used to hold the effluent of the 1st-D alternately for 30 s for one 2nd-D column to effect comprehensive 2D-HPLC without the loss of 1st-D effluent. (3) Two monolithic silica columns were used for 2nd-D by using a switching valve and two sets of 2nd-D chromatographs separating each fraction of the 1st-D effluent with the two 2nd-D columns alternately. In this case, two columns of the same stationary phase (C(18)) or different phases, C(18) and (pentabromobenzyloxy)propylsilyl-bonded (PBB), could be employed at the 2nd-D, although the latter needed two complementary runs. The systems produced peak capacity of approximately 1000 in approximately 60 min in cases 1 and 2 and in approximately 30 min in case 3. The three stationary phases, FR, C(18), and PBB, showed widely different selectivity from each other, making 2D separations possible. The simple and comprehensive 2D-HPLC utilizes the stability and high efficiency at high linear velocities of monolithic silica columns.

Model column structure for the analysis of the flow and band-broadening characteristics of silica monoliths

We report on the use of commercial computational fluid dynamics software to study the band broadening in a perfectly ordered three-dimensional model structure, the so-called tetrahedral skeleton column (TSC), selected for its close geometrical resemblance to the specific pore network topology of silica monoliths. Van Deemter plots are presented for the case of a species flow through a non-porous skeleton and for the case of a retained component (k' = 1) in a porous skeleton (mesopore porosity epsilon = 0.6 in both cases). Using the flow domain as the characteristic scaling dimension, the TSC model yields reduced plate heights as small as h(min) = 0.8 and separation impedances as small as Emin = 120 for a retained component with k' = 1. The very small reduced plate heights for the TSC model can without any doubt largely be attributed to the perfect homogeneity of the considered model structure: the B and C terms are similar to those obtained in real silica monoliths with similar external porosity, whereas the A term is significantly (about a factor of 10) smaller. The present study hence suggests that further experimental work to obtain more homogeneous silica networks could yield large gains in reduced plate height and separation impedance. Comparing the three-dimensional TSC model with a 2D array of cylinders, it was found that the use of the domain size as the characteristic dimension in the reduced plate height expression is much more appropriate than the use of the skeleton size, hence validating earlier approaches adopted in the literature.

Two-Dimensional Separation Method for Analysis of Bacillus subtilis Metabolites via Hyphenation of Micro-Liquid Chromatography and Capillary Electrophoresis

A novel two-dimensional separation method, which hyphenated chromatography and electrophoresis, was developed for analysis of Bacillus subtilis metabolites. Micro-liquid chromatography (LC) with a monolithic silica-ODS column was used as the first dimension, from which the effluent fractions were further analyzed by capillary electrophoresis (CE) acting as the second dimension. Concentration strategies, namely, dynamic pH junction and sweeping, were selectively employed to interface the two dimensions, which proved to be beneficial for the detection of metabolites. For system evaluation, an artificial sample containing 54 standard metabolites was separated according to their hydrophobicity by micro-LC with gradient mode. The early-eluting fractions were separated by capillary zone electrophoresis in combination with dynamic pH junction, while the late-eluting fractions were separated by sweeping micellar electrokinetic chromatography. The middle fractions were analyzed by both modes of CE. Under the optimum conditions, all the components in the artificial sample could be well resolved. The method was applied to profile B. subtilis metabolites. Some crucial metabolites were identified. This method provided great potential for resolving complex biological samples containing compounds having different characteristics.

Linking protein fractionation with multidimensional monolithic reversed-phase peptide chromatography/mass spectrometry enhances protein identification from complex mixtures even in the presence of abundant proteins

Recently, multidimensional shotgun proteomics has proven to be an alternative technology able to identify hundreds of proteins from single samples. Two major limitations of the technology are the presence of high abundance proteins (e.g. RUBISCO in plant leaf tissue) and the enormous number of co-eluting peptides that overstrain the loading and resolving capacity of conventional particle-packed columns as well as the capacity of electrospray ionisation due to ion suppression. Here, the coupling of fast performance liquid chromatography (FPLC) pre-fractionation of an Arabidopsis leaf protein extract and subsequent two-dimensional liquid chromatography/mass spectrometry with improved resolution using a monolithic silica C18 capillary column allowed the identification of 1032 unique proteins in a single 4 mg total protein plant leaf tissue sample. The reassignment of peptide IDs to distinct FPLC protein fractions enhances the identification procedure, especially in the case of present protein isoforms. The proposed strategy is useful to detect proteins otherwise not seen in conventional multidimensional chromatography/mass spectrometry approaches.