A discussion of the possible ways to improve the performance of silica monoliths using a kinetic plot analysis of experimental and computational plate height data

Characterization of hybrid organo-silica monoliths for possible application in the gradient elution of peptides

We characterized thermally polymerized organo-silica hybrid monolithic capillaries to test their applicability in the gradient elution of peptides. We have used a single-pot approach utilizing 3-(methacryloyloxy)propyltrimethoxysilane (MPTMS), ethylene dimethacrylate (EDMA), and n-octadecyl methacrylate (ODM) as functional monomers. The organo-silica monolith containing MPTMS and EDMA was compared with the stationary phase prepared by adding ODM to the original polymerization mixture. Column prepared using a three-monomer system provided a lower accessible volume of flow-through pores, a higher proportion of mesopores, and higher efficiency. We utilized isocratic and gradient elution data to predict peak widths in gradient elution. Both protocols provided comparable results and can be used for peptide peak width prediction. However, applying gradient elution data for peak width prediction seems simpler. Finally, we tested the effect of gradient time on achievable peak capacity in the gradient elution of peptides with a column prepared with a three-monomer system providing a higher peak capacity. However, the performance of hybrid organo-silica monolithic stationary phases in gradient elution of peptides must be improved compared to other monolithic stationary phases. The limiting factor is column efficiency in highly aqueous mobile phases, which needs to be focused on.

Customizable Heterogeneous Catalysts: Nonchanneled Advanced Monolithic Supports Manufactured by 3D-Printing for Improved Active Phase Coating Performance

Three-dimensional (3D)-printed catalysts are being increasingly studied; however, most of these studies focus on the obtention of catalytically active monoliths, and thus traditional channeled monolithic catalysts are usually obtained and tested, losing sight of the advantages that 3D-printing could entail. This work goes one step further, and an advanced monolith with specifically designed geometry has been obtained, taking advantage of the versatility provided by 3D-printing. As a proof of concept, nonchanneled advanced monolithic (NCM) support, composed of several transversal discs containing deposits for active phase deposition and slits through which the gas circulates, was obtained and tested in the CO-PrOx reaction. The results evidenced that the NCM support showed superior catalytic performance compared to conventional channeled monoliths (CMs). The region of temperature in which the active phase can work under chemical control, and thus in a more efficient way, is increased by 31% in NCM compared to the powdered or the CM sample. Turbulence occurs inside the fluid path through the NCM, which enhances the mass transfer of reagents and products toward and from the active sites to the fluid bulk favoring the chemical reaction rate. The nonchanneled monolith also improved heat dispersion by the tortuous paths, reducing the local temperature at the active site. Thus, the way in which reactants and products are transported inside the monoliths plays a crucial role, and this is affected by the inner geometry of the monoliths.

Methods to determine the kinetic performance limit of contemporary chromatographic techniques

By combining separation efficiency data as a function of flow rate with the column permeability, the kinetic plot method allows to determine the limits of separation power (time vs. efficiency) of different chromatographic techniques and methods. The technique can be applied for all different types of chromatography (liquid, gas, or supercritical fluid), for different types of column morphologies (packed beds, monoliths, open tubular, micromachined columns), for pressure and electro-driven separations and in both isocratic and gradient elution mode. The present contribution gives an overview of the methods and calculations required to correctly determine these kinetic performance limits and their underlying limitations.

Correlation of chromatographic performance with morphological features of organic polymer monoliths

Monoliths are considered to be a low pressure alternative to particle packed columns for liquid chromatography (LC). However, the chromatographic performance of organic monoliths, in particular, has still not reached the level of particle packed columns. Since chromatographic performance can be attributed to morphological features of the monoliths, in-situ characterization of the monolith structure in three dimensions would provide valuable information that could be used to help improve performance. In this work, serial sectioning and imaging were performed with a dual-beam scanning electron microscope for reconstruction and quantitative characterization of poly(ethylene glycol) diacrylate (PEGDA) monoliths inside a capillary column. Chord lengths, homogeneity factors, porosities and tortuosities were calculated from three-dimensional (3D) reconstructions of two PEGDA monoliths. Chromatographic efficiency was better for the monolith with smaller mean chord length (i.e., 5.23μm), porosity (i.e., 0.49) and tortuosity (i.e., 1.50) compared to values of 5.90μm, 0.59 and 2.34, respectively, for the other monolithic column. Computational prediction of tortuosity (2.32) was found to be in agreement with the experimentally measured value (2.34) for the same column. The monoliths were found to have significant radial heterogeneity since the homogeneity factor decreased from 5.39 to 4.89 (from center to edge) along the column radius.

An overview of recent advances in HPLC instrumentation

This review highlights the fundamentals and the most prominent advances in the field of HPLC instrumentation over the last decades. Fundamental aspects and practical considerations of column switching, conventional (heart-cut) and comprehensive two-dimensional LC are presented. Different aspects of microcolumn- and nanoliquid-chromatography are reviewed. Recent progress in column technology and the demands and developments in instrumentation and accessories for miniaturized LC are also discussed. In the field of miniaturization, particularly in chip-based nano-LC systems, some aspects on micro-fluidic chip fabrication, using particle-packed HPLC microchips or polymer-based monoliths, are addressed. An introduction to ultra performance LC (UPLC) is also presented.

Use of kinetic plots for the optimization of the separation time in ultra-high-pressure LC

The kinetic-plot approach, in which experimental t(0) and N-values are extrapolated to the performance at maximum system pressure by increasing the column length, was validated by coupling 250×3 mm columns packed with 3 μm particles. The extra-column volume introduced by coupling columns could be neglected with respect to the peak volumes. Plate numbers of up to 132,000 were experimentally achieved by coupling four columns. The maximum deviation between the experimental and predicted plate numbers was 7% for two coupled columns, and decreasing to 0.1% for four coupled columns. Kinetic plots were used to find the conditions to separate a critical pair, with a preset value for the effective plate number, in the shortest possible time. For high-efficiency separations yielding 100,000 effective plates, the optimum critical-pair retention factor was around 4.5. Kinetic plots are presented to find the optimal column length to obtain the fastest possible 100,000 effective-plate separation, taking into account the effect of mobile-phase viscosity on column pressure, and consequently the optimum column length.

Fabrication and chromatographic performance of porous-shell pillar-array columns

We report on a new approach to obtain highly homogeneous silica-monolithic columns, applying a sol-gel fabrication process inside a rectangular pillar-array column (1 mm in width, 29 microm in height and 33.75 mm in length) having a cross-sectional area comparable to that of a 200 microm diameter circular capillary. Starting from a silicon-based pillar array and working under high phase-separation-tendency conditions (low poly(ethylene glycol) (PEG)-concentration), highly regular silica-based chromatographic systems with an external porosity in the order of 66-68% were obtained. The pillars, 2.4 microm in diameter, were typically clad with a 0.5 microm shell layer of silica, thus creating a 3.4 microm total outer pillar diameter and leaving a minimal through-pore size of 2.2 microm. After mesopore creation by hydrothermal treatment and column derivatization with octyldimethylchlorosilane, the monolithic column was used for chip-based liquid-chromatographic separations of coumarin dyes. Minimal plate heights ranging between 3.9 microm (nonretaining conditions) and 6 mum (for a retention factor of 6.5) were obtained, corresponding to domain-size-reduced plate heights ranging between 0.7 and 1.2. The column permeability was in the order of 1.3 x 10(13) m(2), lower than theoretically expected, but this is probably due to obstructions induced by the sol-gel process in the supply channels.

Optimization of the porous structure and polarity of polymethacrylate-based monolithic capillary columns for the LC-MS separation of enzymatic digests

The porous structure as well as the polarity of methacrylate ester-based monolithic stationary phases has been optimized to achieve the separation of various peptides originating from enzymatic digestion. The porous structure, determined by the size of both pores and microglobules, was varied through changes in the composition of porogenic solvents in the polymerization mixture, while the polarity was controlled through the incorporation of butyl, lauryl, or octadecyl methacrylate in the polymer backbone. Both the morphology and the chemistry of the monoliths had a significant effect on the retention and efficiency of the capillary columns. The best resolution of peptidic fragments obtained by digestion of Cytochrome c with trypsin in solution was obtained in a gradient LC-MS mode using a monolithic capillary column of poly(lauryl methacrylate-co-ethylene dimethacrylate) featuring small pores and small microglobules. Raising the temperature from 25 to 60 degrees C enabled separations to be carried out at 40% higher flow rates. Separations carried out at 60 degrees C with a steeper gradient proceeded without loss of performance in half the time required for a comparable separation at room temperature. Our preparation technique affords monolithic columns with excellent column-to-column and run-to-run repeatability of retention times and pressure drops.

Experimental investigation of the band broadening originating from the top and bottom walls in micromachined nonporous pillar array columns

We report on the experimental investigation of the effect of the top and bottom wall plates in micromachined nonporous pillar array columns. It has been found that their presence yields an additional c-term type of band broadening that can make up a significant fraction of the total band broadening (at least if considering nonporous pillars and a nonretained tracer). Their presence also induces a clear (downward) shift of the optimal velocity. These observations are, however in excellent quantitative agreement with the theoretical expectations obtained from a computational fluid dynamics study. The presently obtained experimental results, hence, demonstrate that the employed high aspect ratio Bosch etching process can be used to fabricate micromachined pillar arrays that are sufficiently refined to achieve the theoretical performance limit.

Understanding and design of existing and future chromatographic support formats

The present contribution reviews the use of alternative support formats as a means to surpass the chromatographic performance of the packed bed of spheres. First, a number of idealized structures are considered to obtain a general insight in how the performance of a chromatographic support depends on its shape and size, using the isocratic peak-capacity generation speed as the main performance indicator. Using this criterion, it is found that the packing density or, equivalently, the external porosity, is the most important of all geometrical shape factors. Depending on whether the sample consists of weakly or strongly retained components, the optimal external porosity can be expected to vary between 60% and a value near 100%. The optimal exploitation of a high external porosity, however, also requires overall shrinkage of the domain size, towards and into the sub-micron range. With the current fabrication technologies, this requirement seems difficult to achieve. In the presence of a lower limit on the characteristic support size, each range of desired plate numbers or peak capacities has its own optimal external porosity, ranging from a very low value (high packing density) for high speed, small peak capacity applications, to very high external porosities (low packing density) for applications requiring a very large peak capacity. Subsequently, the obtained theoretical insights are used to review and discuss the past and current research on alternative support formats. Finally, a number of emerging micro- and nano-fabrication technologies are introduced and their potential for the future production of supports with improved shape and homogeneity is discussed.