Properties of Monolithic Silica Columns for HPLC

Mixed-mode chromatographic performance using nicotinic acid-functionalized chito-oligosaccharide-bonded Ti/Si hybrid monolithic capillary columns

This article describes the fabrication of porous nicotinic acid-functionalized chito-oligosaccharide-bonded titania/silica hybrid monoliths (TiO2/SiO2@ChO-N) through a co-gelation sol-gel process. A capillary monolith with a well-defined and homogeneous structure was obtained by controlling the hydrolysis speed of titanium alkoxides in a sol mixture by using glycerol and acetylacetone. As a result of the functionalization with chito-oligosaccharides (ChO)-modified nicotinic acid, the obtained stationary phase provides superior physiochemical properties, such as a cationic hydrophilic surface, porosity, and mechanical strength. Scanning electron microscope and attenuated total reflectance-infrared spectroscopy were used to characterize the functionalized monolithic columns. The produced capillary columns showed high chromatographic performance with acceptable selectivity for charged analytes as well as organic polar compounds such as nucleic bases, nucleosides, carbamate pesticides, and strobilurin fungicides. The obtained results also indicated that the functionalized ChO's amino, amide, hydroxyl, and pyridinium ring moieties served as hydrophilic electrostatic traps for charged substances, in addition to stroing π-π interaction with the carbamate pesticides and strobilurin fungicides analytes via hydrogen bonding.

Efficient Sub-1 Minute Analysis of Selected Biomarker Catecholamines by Core-Shell Hydrophilic Interaction Liquid Chromatography (HILIC) with Nanomolar Detection at a Boron-Doped Diamond (BDD) Electrode

A rapid, sensitive method for the separation of catecholamine biomarkers (CAs), of importance in traumatic brain injury (TBI) and in Parkinson’s disease (PD), has been successfully developed using hydrophilic interaction liquid chromatography (HILIC). Dopamine (DA), epinephrine (EPI), and norepinephrine (NE) are known to be three to fivefold elevated above normal in traumatic brain injury (TBI) patients. HILIC facilitates the rapid and efficient separation of these polar biomarkers, which can be poorly retained by reversed-phase liquid chromatography (RPLC), while electrochemical detection (ECD) at the boron-doped diamond (BDD) electrode provides enhanced nanomolar detection. Three HILIC columns were compared, namely the superficially porous (core-shell) Z-HILIC column and the Z-cHILIC and Z-HILIC fully porous columns. The core-shell Z-HILIC showed the highest efficiency with a rapid separation within 60 s. The HILIC method utilizing the core-shell Z-HILIC column was initially optimized for the simultaneous analysis of DA, EPI, and NE using UV detection. The advantages of using the BDD electrode over UV detection were explored, and the improved limits of detection (LODs, S/N = 3) measured were 40, 50, and 50 nM for DA, EPI, and NE, respectively. Method validation is reported in terms of the linearity, repeatability, reproducibility, and LODs. Furthermore, the proposed method was successfully applied to the real sample analysis of urinary CAs following phenylboronic acid (PBA) solid phase extraction (SPE) pretreatment.

Nanoscale Solid-Phase Isobaric Labeling for Multiplexed Quantitative Phosphoproteomics

We established a workflow for highly sensitive multiplexed quantitative phosphoproteomics using a nanoscale solid-phase tandem mass tag (TMT) labeling reactor. Phosphopeptides were first enriched by titanium oxide chromatography and then labeled with isobaric TMT reagents in a StageTip packed with hydrophobic polymer-based sorbents. We found that TMT-labeled singly phosphorylated peptides tend to flow through the titanium oxide column. Therefore, TMT labeling should be performed after the enrichment step from tryptic peptides, resulting in the need for microscale reactions with small amounts of phosphopeptides. Using an optimized protocol for tens to hundreds of nanograms of phosphopeptides, we obtained a nearly 10-fold increase in sensitivity compared to the conventional solution-based TMT protocol. We demonstrate that this nanoscale phosphoproteomics protocol works for 50 μg of HeLa proteins treated with selumetinib, and we successfully quantified the selumetinib-regulated phosphorylated sites on a proteome scale. The MS raw data files have been deposited with the ProteomeXchange Consortium via the jPOST partner repository (https://jpostdb.org) with the data set identifier PXD025536.

Three-dimensional graphing representing six variables for speed and separation performance in liquid chromatography

The two variables, flow rate and column length, enable naive determination of the number of theoretical plates (N) in isocratic elution; this, in turn, enables the formation of a three-dimensional graph with N as the z-axis. An alternate three-dimensional graph with N as the z-axis can be drawn, then, with the alternate basal plane illustrating the pressure drop and hold-up time. In this article, the pressure drop and hold-up time are formulated so as to be represented unitarily in the former graph, because the flow rate and column length interact simultaneously as operational variables. This formulation manipulates both the pressure drop and the hold-up time as logarithmic axes, to evaluate the landscape. Also of use is the representation, in the same graph, of the height equivalent to a theoretical plate, as the fundamental property of the packing supports. For this purpose, the number of theoretical plates per unit length are here introduced as the sixth variable, instead of the height equivalent to a theoretical plate. Representing the six variables in three-dimensional graphs enables a clear understanding both of the separation condition optimization methods and the relation among variables for the speed and separation performance. The linear velocity, column length, N, velocity-length product, hold-up time, and number of theoretical plates per unit length, are here selected as the six elementary variables for the three-dimensional graphs; and, based on the packing supports of 2, 3, and 5-μm particle and monolithic columns. Finally, the usage of logarithmic three-dimensional graph is illustrated for understanding the speed and separation performance.

The theory and practice of ultrafast liquid chromatography: A tutorial

Modern high-throughput experimentation and challenging analytical problems of academic/industrial research have put the responsibility on separation scientists to develop new fast separation approaches. With the availability of high-pressure pumps, small particles with hydrolytically stable surface chemistries, reduced extra-column band broadening, and low volume detectors with fast signal processing, it is now feasible to do sub-minute to sub-second chromatography. Herein, the fundamental theoretical principles of ultrafast chromatography, along with practical solutions, are reviewed. Approaches for rapid separations in packed beds, narrow open tubular columns, and monoliths are demonstrated, along with the challenges that were faced. The instrumentation requirements (pumps, injection systems, detectors, column packing process) for using short columns ranging from 0.5 to 5 cm are examined, followed by real applications. One of the main problems in ultrafast chromatography is partial or complete peak overlap. As per Gidding's statistical overlap theory, peak overlap cannot be avoided for a completely random sample for a column with a given peak capacity. Signal processing techniques based on Fourier transform deconvolution of band broadening, power laws, derivatives, and iterative curve fitting are explained to help improve the chromatographic resolution. An example of ten peaks separated in under a second is shown and discussed. Other ultrafast separations in supercritical fluid chromatography or capillary electrophoresis are briefly mentioned to provide a complete understanding of this emerging field.

Three approaches to improving performance of liquid chromatography using contour maps with pressure, time, and number of theoretical plates

Attempts to improve HPLC performance often focus on increasing the speed or separation performance. In this article, both the flow rate and column length are optimized as separation conditions, while observing the number of theoretical plates and hold-up time with isocratic elutions. In addition, the upper pressure limit must be simultaneously considered as the boundary condition. Approaches based on the optimal velocity (Opt.) are often adopted; but the kinetic performance limit (KPL) in Desmet's method can also be utilized for three-dimensional graphing with axes of pressure, time, and number of theoretical plates. Here, two approaches involving pressure increase are introduced, beginning with the condition of optimal linear velocity: one aimed at greater speed and the other at higher resolution. Coefficients of pressure-application are derived to measure the effectiveness of the intermediate conditions between the Opt. and KPL methods. In the third approach, the hold-up time is extended while maintaining a fixed pressure. Coefficients of time-extension are also derived, to determine the effectiveness to improve the separation performance.

On-chip electrochemical detection of glucose towards the miniaturised quality control of carbohydrate-based radiotracers

We have developed two microfluidic platforms for the electrochemical detection of glucose, using either a screen-printed electrode or wire electrodes, towards the quality control testing of carbohydrate-based radiotracers used in medical imaging.

Challenges and Advances in the Fabrication of Monolithic Bioseparation Materials and their Applications in Proteomics Research

High-performance liquid chromatography integrated with tandem mass spectrometry (HPLC-MS/MS) has become a powerful technique for proteomics research. Its performance heavily depends on the separation efficiency of HPLC, which in turn depends on the chromatographic material. As the "heart" of the HPLC system, the chromatographic material is required to achieve excellent column efficiency and fast analysis. Monolithic materials, fabricated as continuous supports with interconnected skeletal structure and flow-through pores, are regarded as an alternative to particle-packed columns. Such materials are featured with easy preparation, fast mass transfer, high porosity, low back pressure, and miniaturization, and are next-generation separation materials for high-throughput proteins and peptides analysis. Herein, the recent progress regarding the fabrication of various monolithic materials is reviewed. Special emphasis is placed on studies of the fabrication of monolithic capillary columns and their applications in separation of biomolecules by capillary liquid chromatography (cLC). The applications of monolithic materials in the digestion, enrichment, and separation of phosphopeptides and glycopeptides from biological samples are also considered. Finally, advances in comprehensive 2D HPLC separations using monolithic columns are also shown.

Binary colloidal crystals (BCCs): Interactions, fabrication, and applications

The organization of matter into hierarchical structures is a fundamental characteristic of functional materials and living organisms. Binary colloidal crystal (BCC) systems present a diversified range of nanotopographic structures where large and small colloidal particles simultaneously self-assemble into either 2D monolayer or 3D hierarchical crystal lattices. More importantly, understanding how BCCs form opens up the possibility to fabricate more complex systems such as ternary or quaternary colloidal crystals. Monolayer BCCs can also offer the possibility to achieve surface micro- and nano-topographies with heterogeneous chemistries, which can be challenging to achieve with other traditional fabrication tools. A number of fabrication methods have been reported that enable generation of BCC structures offering high accuracy in growth with controllable stoichiometries; however, it is still a challenge to make uniform BCC structures over large surface areas. Therefore, fully understand the mechanism of binary colloidal self-assembly is crucial and new/combinational methods are needed. In this review, we summarize the recent advances in BCC fabrication using particles made of different materials, shapes, and dispersion medium. Depending on the potential application, the degree of order and efficiency of crystal formation has to be determined in order to induce variability in the intended lattice structures. The mechanisms involved in the formation of highly ordered lattice structures from binary colloidal suspensions and applications are discussed. The generation of BCCs can be controlled by manipulation of their extensive phase behavior, which facilitates a wide range potential applications in the fields of both material and biointerfacial sciences including photonics, biosensors, chromatography, antifouling surfaces, biomedical devices, and cell culture tools.

Intelligent peak deconvolution through in-depth study of the data matrix from liquid chromatography coupled with a photo-diode array detector applied to pharmaceutical analysis

Multivariate curve resolution-alternating least squares (MCR-ALS) method was investigated for its potential to accelerate pharmaceutical research and development. The fast and efficient separation of complex mixtures consisting of multiple components, including impurities as well as major drug substances, remains a challenging application for liquid chromatography in the field of pharmaceutical analysis. In this paper we suggest an integrated analysis algorithm functioning on a matrix of data generated from HPLC coupled with photo-diode array detector (HPLC-PDA) and consisting of the mathematical program for the developed multivariate curve resolution method using an expectation maximization (EM) algorithm with a bidirectional exponentially modified Gaussian (BEMG) model function as a constraint for chromatograms and numerous PDA spectra aligned with time axis. The algorithm provided less than ±1.0% error between true and separated peak area values at resolution (R_s) of 0.6 using simulation data for a three-component mixture with an elution order of a/b/c with similarity (a/b)=0.8410, (b/c)=0.9123 and (a/c)=0.9809 of spectra at peak apex. This software concept provides fast and robust separation analysis even when method development efforts fail to achieve complete separation of the target peaks. Additionally, this approach is potentially applicable to peak deconvolution, allowing quantitative analysis of co-eluted compounds having exactly the same molecular weight. This is complementary to the use of LC-MS to perform quantitative analysis on co-eluted compounds using selected ions to differentiate the proportion of response attributable to each compound.

Positron detection in silica monoliths for miniaturised quality control of PET radiotracers

Real-time, high S/N radiodetection of the PET radiotracer, ⁶⁸Ga-citrate, was achieved on a monolithic column using a miniaturised positron sensor.

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.

Retention Study of Flavonoids Under Different Chromatographic Modes

The goal of this study was to investigate the chromatographic behavior of selected flavonoids from their different subgroups (flavonols, flavanones, flavones and isoflavones) in hydrophilic interaction liquid chromatography (HILIC). Chromatographic measurements were made on two different HILIC columns: cross-linked DIOL (Luna HILIC) and zwitterionic sulfoalkylbetaine (SeQuant ZIC-HILIC). Separation parameters such as the content of acetonitrile and pH of an eluent were studied. On the ZIC column, the retention factors of flavonoids increased with decreasing water content in the mobile phase. The increase in pH of the aqueous component mainly affects the polarity of the analytes. DIOL stationary phase shows more or less apparent dual retention mechanism, HILIC at the acetonitrile (ACN) content ≥75% and reversed phase (RP) with lower content of organic modifier. In the presence of ammonium acetate in the mobile phase, the retention of flavonoids onto the DIOL column increases without change in the selectivity of the separations. The similar effect, but considerably smaller was observed for aglycones on the ZIC column. The retention of studied glycosides (hesperidin, rutin) decreases in the presence of salt in the mobile phase. The significantly higher mass spectrometry sensitivity was observed under HILIC conditions in comparison with the most often used RP LC due to much higher content of ACN in the mobile phase. Finally, under optimal chromatographic conditions, the method was validated and applied for the determination of flavonoids in chamomile (Matricaria chamomilla L.) infusion.

Description of the interaction between Candida albicans and macrophages by mixed and quantitative proteome analysis without isolation

Candida albicans is an opportunistic pathogen that causes fatal diseases in immunocompromised hosts. Host resistance against C. albicans relies on ingestion of the pathogen by macrophages. Analysis of the escaping behavior of C. albicans from macrophages is required to understand the onset of systemic candidiasis. In this study, native interactions of C. albicans with macrophages were investigated by proteome analysis using high efficiency of long monolithic silica capillary column. Using this system, we developed a method of “mixed and quantitative proteome analysis” in which C. albicans and macrophages were simultaneously analyzed by nanoLC–MS/MS without the need to isolate the two individual living cells. Two hundred twenty-seven proteins from C. albicans and five proteins from macrophages were identified as candidate interaction-specific molecules. C. albicans seemed to produce glucose through a β-oxidation pathway, a glyoxylate cycle, and gluconeogenesis for escape from macrophages. Up-regulation of stress-related and candidate pathogenic proteins in C. albicans indicated how C. albicans endured the harsh environment inside the macrophages. Down-regulation of apoptosis-associated protein NOA1- and chaperone HSPA1A-syntheses in macrophage indicated that C. albicans was able to escape from macrophages in part by suppressing the production of these macrophage proteins.

Tuning preparation conditions towards optimized separation performance of thermally polymerized organo-silica monolithic columns in capillary liquid chromatography

Tuning of preparation conditions, such as variations in the amount of a porogen, concentration of an aqueous acid catalyst, and adjustment in polymerization temperature and time, towards optimized chromatographic performance of thermally polymerized monolithic capillaries prepared from 3-(methacryloyloxy)propyltrimethoxysilane has been carried out. Performance of capillary columns in reversed-phase liquid chromatography was assessed utilizing various sets of solutes. Results describing hydrophobicity, steric selectivity, and extent of hydrogen bonding enabled comparison of performance of hybrid monolithic columns prepared under thermal (TSG) and photopolymerized (PSG) conditions. Reduced amounts of porogen in the polymerization mixture, and prolonged reaction times were necessary for the preparation of monolithic columns with enhanced retention and column efficiency that reached to 111,000 plates/m for alkylbenzenes with shorter alkyl chains. Both increased concentration of catalyst and higher temperature resulted in faster polymerization but inevitably in insufficient time for pore formation. Thermally polymerized monoliths produced surfaces, which were slightly more hydrophobic (a methylene selectivity of 1.28±0.002 TSG vs 1.20±0.002 PSG), with reduced number of residual silanols (a caffeine/phenol selectivity of 0.13±0.001 TSG vs 0.17±0.003 PSG). However, steric selectivity of 1.70±0.01 was the same for both types of columns. The batch-to-batch repeatability was better using thermal initiation compared to monolithic columns prepared under photopolymerized conditions. RSD for retention factor of benzene was 3.7% for TSG capillaries (n=42) vs. 6.6% for PSG capillaries (n=18). A similar trend was observed for columns prepared within the same batch.

Effective and reusable monolith capillary trap of nitrosamine extraction by superheated water from frankfurter sausage

A novel, simple, rapid, and inexpensive method of extraction and cleanup of nitrosamines from frankfurter sausage was achieved with a capillary filled with monolith of either polystyrene-co-divinylbenzene (PS-DVB), Polydivinylbenzene (P-DVB), or silica that had been fabricated. The study of capability in trapping nonpolar matrix and monolith capillaries with varied lengths revealed that a silica monolith gave the best result for nitrosamine determination. With an online coupling between superheated water extraction (SWE) and silica monolith capillary connected to a 5% phenyl-methylpolysiloxane column, factors affecting the extraction and determination, namely, sensitivity with and without the monolith, reusability, injection-injection repeatability, capillary-capillary precision, and chromatographic separation, were investigated. This confirmed the feasibility of the method. The optimal length of silica monolith capillary was 30 mm, offering reuse more than 20 times. Separation and quantification of selected volatile nitrosamines were carried out using gas chromatography (GC) coupled with either a flame ionization detector (FID) or mass spectrometer (MS). The overall extraction and determination method determined by GC-MS allowed for a recovery of 75-88% with a <5% relative standard deviation (RSD) and detection limit of 2-5 ng of injected nitrosamine.

Agglomerated silica monolithic column for hydrophilic interaction LC

Hydrophilic interaction LC (HILIC) has gained wide acceptance in recent years due to its ability to retain and separate polar compounds such as pharmaceuticals. Most commercial HILIC phases are particle based, which limit the speed with which HILIC separations can be performed. Herein, agglomerated silica monolithic columns are prepared by electrostatically attaching polyionic latex particles onto a silica monolith by simply flushing a suspension of the ionic latex through a silica monolith. Such phases retain the high efficiency and permeability of the native silica monolith, while the agglomerated phase is easy to introduce and provides excellent mass transfer. High %ACN in the mobile phase dramatically increases the efficiency and retention, consistent with HILIC behavior. Test analytes such as benzoates, nucleotides and amino acids are separated with plate heights of 25-110 microm. The high permeability of monoliths allows HILIC separations to be performed with baseline resolution in less than 15 s. Electrostatic repulsion-hydrophilic liquid interaction chromatographic retention behavior of the latex-coated monoliths is verified using amino acids as test analytes.

HPLC method for simultaneous determination of retinoids and tocopherols in human serum for monitoring of anticancer therapy

A simple and rapid HPLC method requiring small volumes (250 microL) of human serum after C18 SPE sample preparation was developed using monolithic technology for simultaneous determination of all-trans-retinoic acid, 13-cis-retinoic acid, retinol, gamma- and alpha-tocopherol. The monolithic column, Chromolith Performance RP-18e (100x4.6 mm), was operated at ambient temperature. The mobile phase consisted of a mixture of acetonitrile (ACN) and 1% ammonium acetate in water (AMC) at pH 7.0. The mobile phase started at 98:2 (v/v) ACN/AMC (column pre-treatment) at a flow rate of 2 mL/min, then changed to 95:5 (v/v) ACN/AMC for 4 min at a flow rate of 1.5 mL/min and a further 3 min at a flow rate of 3.2 mL/min. Detection and identification were performed using a photodiode array detector. Retinol, 13-cis- and all-trans-retinoic acid were monitored at 325 nm. Both alpha- and gamma-tocopherol were detected at 295 nm. The total analysis time was 7.2 min. Tocol (synthesized tocopherol, not occurring in humans) was used as internal standard. The method was linear in the range of 0.125-10.00 micromol/L for all-trans-retinoic acid, 0.125-5.00 micromol/L for 13-cis-retinoic acid, 0.25-10.00 micromol/L for retinol, 0.5-50.00 micromol/L for gamma-tocopherol, and 0.5-50.00 micromol/L for alpha-tocopherol. The present method may be useful for monitoring of retinoids and tocopherols in clinical studies.

Separation efficiencies in hydrophilic interaction chromatography

Hydrophilic interaction chromatography (HILIC) is important for the separation of highly polar substances including biologically active compounds, such as pharmaceutical drugs, neurotransmitters, nucleosides, nucleotides, amino acids, peptides, proteins, oligosaccharides, carbohydrates, etc. In the HILIC mode separation, aqueous organic solvents are used as mobile phases on more polar stationary phases that consist of bare silica, and silica phases modified with amino, amide, zwitterionic functional group, polyols including saccharides and other polar groups. This review discusses the column efficiency of HILIC materials in relation to solute and stationary phase structures, as well as comparisons between particle-packed and monolithic columns. In addition, a literature review consisting of 2006-2007 data is included, as a follow up to the excellent review by Hemström and Irgum.