Comparison between the efficiencies of columns packed with fully and partially porous C18-bonded silica materials

Novel insights into the dependence of adsorption-desorption kinetics on particle geometry in chiral chromatography

The existence of slow adsorption-desorption kinetics in chiral liquid chromatography is common knowledge. This may significantly contribute to worsening the efficiency and kinetic performance of a chromatographic run, especially when high flow rates are employed. Many attempts and protocols have been proposed to access this term, the so-called c ads , but they are based on different (theoretical) assumptions. As a consequence, no official method is available for the estimation of the adsorption-desorption kinetics term. In this work, a novel approach to access c ads is presented. This procedure combines experimental results obtained with kinetic and thermodynamic measurements. The investigations have been performed on two zwitterionic teicoplanin chiral stationary phases (CSPs) based on 1.9 μ m fully porous and 2.0 μ m superficially porous particles (FPPs and SPPs), using Z-D,L-Methionine as probe molecule. Kinetic studies have been performed through the combination of both stop-flow and dynamic measurements, while adsorption isotherms have been calculated through Inverse Method. This study has confirmed that, on both particle formats, analyte diffusion on the surface of the particle is negligible, meaning that adsorption is localized, and it has been demonstrated that adsorption-desorption kinetics is strongly dependent on particle geometry and, in particular, on the loading of chiral selector. These findings are fundamental not only to unravel novel aspects of the complex enantiorecognition mechanism but also to optimize the employment of CSPs for ultra-fast and preparative applications.

Comprehensive analysis of alkenones by reversed-phase HPLC-MS with unprecedented selectivity, linearity and sensitivity

Alkenones are among the most widely used paleotemperature biomarkers. Traditionally, alkenones are analyzed using gas chromatography-flame ionization detector (GC-FID), or GC-chemical ionization-mass spectrometry (GC-CI-MS). However, these methods encounter considerable challenges for samples that exhibit matrix interference or low concentrations, with GC-FID requiring tedious sample preparations and GC-CI-MS suffering from nonlinear response and a narrow linear dynamic range. Here we demonstrate that reversed-phase high pressure liquid chromatography-mass spectrometry (HPLC-MS) methods provide excellent resolution, selectivity, linearity and sensitivity for alkenones in complex matrices. We systematically compared the advantages and limitations of three mass detectors (quadrupole, Orbitrap, and quadrupole-time of flight) and two ionization modes (electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI)) for alkenone analyses. We demonstrate that ESI performs better than APCI as response factors of various unsaturated alkenones are similar. Among the three mass analyzers tested, orbitrap MS provided the lowest limit of detection (0.4, 3.8 and 8.6 pg injected masses for Orbitrap, qTOF and single quadrupole MS, respectively) and the widest linear dynamic range (600, 20 and 30 folds for Orbitrap, qTOF and single quadrupole MS, respectively). Single quadrupole MS operated in ESI mode provides accurate quantification of proxy measurements over a wide range of injection masses, and with its modest instrument cost, represents an ideal method for routine applications. Analysis of global core-top sediment samples confirmed the efficacy of HPLC-MS methods for the detection and quantification of paleotemperature proxies based on alkenones and their superiority over GC-based methods. The analytical method demonstrated in this study should also allow highly sensitive analyses of diverse aliphatic ketones in complex matrices.

Reversed-Phase Liquid Chromatography of Peptides for Bottom-Up Proteomics: A Tutorial

The performance of the current bottom-up liquid chromatography hyphenated with mass spectrometry (LC-MS) analyses has undoubtedly been fueled by spectacular progress in mass spectrometry. It is thus not surprising that the MS instrument attracts the most attention during LC-MS method development, whereas optimizing conditions for peptide separation using reversed-phase liquid chromatography (RPLC) remains somewhat in its shadow. Consequently, the wisdom of the fundaments of chromatography is slowly vanishing from some laboratories. However, the full potential of advanced MS instruments cannot be achieved without highly efficient RPLC. This is impossible to attain without understanding fundamental processes in the chromatographic system and the properties of peptides important for their chromatographic behavior. We wrote this tutorial intending to give practitioners an overview of critical aspects of peptide separation using RPLC to facilitate setting the LC parameters so that they can leverage the full capabilities of their MS instruments. After briefly introducing the gradient separation of peptides, we discuss their properties that affect the quality of LC-MS chromatograms the most. Next, we address the in-column and extra-column broadening. The last section is devoted to key parameters of LC-MS methods. We also extracted trends in practice from recent bottom-up proteomics studies and correlated them with the current knowledge on peptide RPLC separation.

Insights into enantioselective separations of ionic metal complexes by sub/supercritical fluid chromatography

Sub/supercritical fluid chromatography (SFC) is a green separation technique that has been used to separate a wide variety of compounds and is proven to be immensely useful for chiral separations. However, SFC is currently not thought to be applicable for ionic compounds due to their low solubility in CO2, even with additives and organic modifiers. Recently, a large amount of research has been centered on octahedral complexes of Ru(II) and Os(II) with bidentate polypyridyl ligands due to their ability to serve in cancer treatment and other biological activities. These compounds exist as the delta (Δ) and lambda (Λ) enantiomers. Previously, similar compounds have been enantiomerically separated using HPLC and capillary electrophoresis, but never with SFC. Cyclofructan-6 (CF6) derivatized with (R)-naphthyl ethyl (RN) groups has been proven to be an effective chiral stationary phase for these separations in HPLC. This column chemistry was expanded to SFC to provide the first chiral separation of a wide variety (23 complexes in total) of ionic octahedral polypyridyl complexes. Unexpected behavior for mixing methanol and acetonitrile as the organic modifier will be discussed, along with the effects of additives. Enantioselectivity on CF6-RN chemistry is shown to be dependent on the conjugation level and rigidity of the metal complexes. Mass transfer kinetic behavior is also shown, and high-efficiency baseline resolved rapid separations are shown for fast screening or quantitation of representative coordination complexes.

Expanding Antineoplastic Drugs Surface Monitoring Profiles: Enhancing of Zwitterionic Hydrophilic Interaction Methods

Antineoplastic drugs are a wide and heterogeneous group of substances that, as universally known, can cause highly severe toxic effects to whoever is exposed. From an occupational safety point of view, surface contaminations inside preparation and administration units are a growing issue and therefore require the development and implementation of sensible and fast monitoring methods. The unlikelihood of a unique all-embracing chromatography, able to correctly retain and separate each analyte led to the need to create an orthogonal normal phase analysis, which might be able to fill the gaps in the more common reversed-phase ones. An existing hydrophilic interaction method has thus been expanded to 6 other drugs and applied to real samples after an evaluation of its performances. The experimental data were then used to evaluate the possibility of estimating reliable relationships between the chromatographic retention and the chemical-structural features of the drugs under analysis.

Taylor-Aris methodology for the experimental determination of molecular diffusion coefficients: Tutorial with focus on large biomolecules

High-Performance Liquid Chromatography (HPLC) is a key technique in the evaluation of biopharmaceuticals. To improve the separation of biopharmaceuticals, it is crucial to improve the fundamental understanding of the parameters governing their band broadening behavior. This can be obtained by a detailed assessment of the individual contributions to their mass transfer. For this purpose, a precise knowledge of the molecular diffusion coefficient (D_m) of biopharmaceuticals is required. Only little experimental data is available for the D_m-values of biopharmaceuticals under HPLC relevant conditions. Furthermore, none of the available equations that can be used to calculate D_m-values, allows to account for any conformational changes that might occur. The Taylor-Aris method is a very simple and absolute method that is often employed to determine D_m-coefficients. The Taylor-Aris method measures the band broadening of an analyte in an open tube under laminar conditions, wherein (1) longitudinal diffusion can be ignored, (2) the sample is fully radially equilibrated and (3) the contribution of the extra-column variance to the total variance is negligible. Moreover, since the open tubes are typically coiled for practical reasons, (4) the influence of secondary flows on the band broadening should be insignificant. In this tutorial paper, the impact of the four conditions mentioned above on the accuracy of the obtained D_m values is revisited. For this purpose, D_m values are measured for two representative compounds (Bovine Serum Albumin and Thiourea), and the obtained values are compared with literature data and theoretical recommendations. Based on these observations, a set of 'rules' for accurate and fast D_m measurements is put forward. Finally, an Interactive Tool (IT), combining these rules in a comprehensive way, is introduced and can be used to set up TA experiments.

Polystyrene Immobilized Sol-Gel Ground Silica Monolith Particles Using One-Pot Reaction of Enhanced Separation Efficiency

The stationary phase based on sol-gel ground silica monolith particles has been produced by one-pot polymerization method incorporation of styrene and ethylene dimethacrylate. First, the ground silica monolith particles were prepared by a sol-gel process followed by sedimentation. The particles were then subjected to modify with styrene ligand via one-pot polymerization, whereas ethylene dimethacrylate was used as the cross-linker. The glass lined stainless steel columns (1 mm internal diameter, 150 mm length) were packed with the above phase for estimation of the chromatographic performance in high-performance liquid chromatography. An average number of theoretical plates of as high as 39,300 plates/column was obtained under the optimized elution condition. The column-to-column reproducibility was proved satisfactory in separation efficiency and retention factor. The experimental results indicate that sol-gel ground silica particles prepared by an aid of one-pot modification can provide a better way for preparation of highly efficient stationary phase.

Fundamental to achieving fast separations with high efficiency: A review of chromatography with superficially porous particles

Types of particles have been fundamental to LC separation technology for many years. Originally, LC columns were packed with large-diameter (>100 μm) calcium carbonate, silica gel, or alumina particles that prohibited fast mobile-phase speeds because of the slow diffusion of sample molecules inside deep pores. During the birth of HPLC in the 1960s, superficially porous particles (SPP, ≥30 μm) were developed as the first high-speed stationary-phase support structures commercialized, which permitted faster mobile-phase flowrates due to the fast movement of sample molecules in/out of the thin shells. These initial SPPs were displaced by smaller totally porous particles (TPP) in the mid-1970s. But SPP history repeated when UHPLC emerged in the 2000s. Stationary-phase support structures made from sub-3-μm SPPs were introduced to chromatographers in 2006. The initial purpose of this modern SPP was to enable chromatographers to achieve fast separations with high efficiency using conventional HPLCs. Later, the introduction of sub-2-μm SPPs with UHPLC instruments pushed the separation speed and efficiency to a very fast zone. This review aims at providing readers a comprehensive and up-to-date view on the advantages of SPP materials over TPPs historically and theoretically from the material science angle.

Shedding light on mechanisms leading to convex-upward van Deemter curves on a cellulose tris(4-chloro-3-methylphenylcarbamate)-based chiral stationary phase

An unusual convex-upward van Deemter curve was observed for the more retained enantiomer of a chiral sulfoxide (2-(benzylsulfinyl)benzamide) on a cellulose tris(4-chloro-3-methylphenylcarbamate)-based chiral stationary phase (CSP), prepared on silica particles of 1000 Å pore size. In contrast, the firstly eluted enantiomer of the same molecule exhibited the traditional convex-downward van Deemter curve. A detailed kinetic and thermodynamic investigation has revealed that this unusual phenomenon, which however has already been observed in chiral chromatography, originates when the adsorption of the compound is very strong and the solid-phase diffusion negligible. Experimentally, the intraparticle diffusion of the more retained enantiomer of the sulfoxide was found to be one order of magnitude smaller than that of the first eluted one. Overall, this translates into very little longitudinal diffusion (b-term of van Deemter curve) accompanied by high solid-liquid mass transfer resistance (c-term). Finally the comparison with another, differently-substituted chiral sulfoxide (whose enantiomers both exhibit traditional van Deemter curve behavior) has allowed to correlate these findings to the specific characteristics of the molecule.

Online Comprehensive High pH Reversed Phase × Low pH Reversed Phase Approach for Two-Dimensional Separations of Intact Proteins in Top-Down Proteomics

A proof-of-concept study is presented on the use of comprehensive two-dimensional liquid chromatography mass spectrometry (LC × LC-MS) for the separation of intact protein mixtures using a different mobile phase pH in each dimension. This system utilizes mass spectrometry (MS) friendly pH modifiers for the online coupling of high pH reversed phase liquid chromatography (HPH-RPLC) in the first dimension (¹D) followed by low pH reversed phase liquid chromatography (LPH-RPLC) in the second dimension (²D). Owing to the ionic nature of proteins, the use of a different mobile phase pH was successful to provide altered selectivity between the two dimensions, even for closely related protein variants, such as bovine cytochrome c and equine cytochrome c, which differ by only three amino acids. Subminute gradient separation of proteins in the second dimension was successful to minimize analysis time, while maintaining high peak capacity. Unlike peptides, the elution order of studied proteins did not follow their isoelectric points, where acidic proteins would be expected to be more retained at low pH (and basic proteins at high pH). The steep elution isotherms (on-off retention mechanism) of proteins and the very steep gradients utilized in the second-dimension column succeeded in overcoming pH and organic solvent content mismatch. The utility of the system was demonstrated with a mixture of protein standards and an Escherichia coli protein mixture.

Isolation of intact bacteria from blood by selective cell lysis in a microfluidic porous silica monolith

Rapid and efficient isolation of bacteria from complex biological matrices is necessary for effective pathogen identification in emerging single-cell diagnostics. Here, we demonstrate the isolation of intact and viable bacteria from whole blood through the selective lysis of blood cells during flow through a porous silica monolith. Efficient mechanical hemolysis is achieved while providing passage of intact and viable bacteria through the monoliths, allowing size-based isolation of bacteria to be performed following selective lysis. A process for synthesizing large quantities of discrete capillary-bound monolith elements and millimeter-scale monolith bricks is described, together with the seamless integration of individual monoliths into microfluidic chips. The impact of monolith morphology, geometry, and flow conditions on cell lysis is explored, and flow regimes are identified wherein robust selective blood cell lysis and intact bacteria passage are achieved for multiple gram-negative and gram-positive bacteria. The technique is shown to enable rapid sample preparation and bacteria analysis by single-cell Raman spectrometry. The selective lysis technique presents a unique sample preparation step supporting rapid and culture-free analysis of bacteria for the point of care.

The Way to Ultrafast, High-Throughput Enantioseparations of Bioactive Compounds in Liquid and Supercritical Fluid Chromatography

Until less than 10 years ago, chiral separations were carried out with columns packed with 5 or 3 μm fully porous particles (FPPs). Times to resolve enantiomeric mixtures were easily larger than 30 min, or so. Pushed especially by stringent requirements from medicinal and pharmaceutical industries, during the last years the field of chiral separations by liquid chromatography has undergone what can be defined a “true revolution”. With the purpose of developing ever faster and efficient method of separations, indeed, very efficient particle formats, such as superficially porous particles (SPPs) or sub-2 μm FPPs, have been functionalized with chiral selectors and employed in ultrafast applications. Thanks to the use of short column (1–2 cm long), packed with these extremely efficient chiral stationary phases (CSPs), operated at very high flow rates (5–8 mL/min), resolution of racemates could be accomplished in very short time, in many cases less than 1 s in normal-, reversed-phase and HILIC conditions. These CSPs have been found to be particularly promising also to carry out high-throughput separations under supercritical fluid chromatography (SFC) conditions. The most important results that have been recently achieved in terms of ultrafast, high-throughput enantioseparations both in liquid and supercritical fluid chromatography with particular attention to the very important field of bioactive chiral compounds will be reviewed in this manuscript. Attention will be focused not only on the latest introduced CSPs and their applications, but also on instrumental modifications which are required in some cases in order to fully exploit the intrinsic potential of new generation chiral columns.

Transport properties and size exclusion effects in wide-pore superficially porous particles

The effects of hydrodynamic radius on the transport of solute molecules in packed beds of wide-pore superficially porous particles (SPP) are studied using pore-scale simulation. The free molecular diffusion rate varies with radius through the Stokes-Einstein relation. Lattice Boltzmann and Langevin methods are used to model fluid motion and the transport of an ensemble of solute molecules in the fluid, providing statistics on solute concentration, flux, molecule age and residence time, as a function of depth in the SPP. Intraparticle effective diffusion and bed dispersion coefficients are calculated and correlated with the hydrodynamic radius and accessible porosity. The relative importance of convection and diffusion are found to depend on the molecule (tracer) size through the diffusion rate, and convection effects are more significant for larger, slower-diffusing molecules. When larger molecules are utilized, the intraparticle concentration is reduced in proportion to the local particle porosity, leading to a natural definition of the accessible porosity used in size exclusion chromatography (SEC). Although the pore shape is complex, the SEC constant K can be calculated directly from simulation. Simulation demonstrates that the effective diffusion coefficient is elevated near the particle hull, which is largely open to interstitial flow, and decreases with depth into the particle. All molecules studied here have transport access to the entire particle depth, although the accessible volume at a given depth depends on their size. The first passage time into the particle is well predicted by the diffusion rate, but residence time is influenced by convection, shortening the average visit duration. These results are of interest in "perfusion" chromatography where convection is thought to increase separation efficiency for large biomolecules.

Plant Metabolomics: Maximizing Metabolome Coverage by Optimizing Mobile Phase Additives for Nontargeted Mass Spectrometry in Positive and Negative Electrospray Ionization Mode

Nontargeted screening methods with ultrahigh-performance liquid chromatography-electrospray ionization/quadrupole-time-of-flight mass spectrometry have been extensively applied to plant metabolomics to very diverse scientific issues in plant metabolomics. In this study, different mobile phase additives were tested in order to improve the electrospray ionization process and to detect as many metabolites as possible with high peak intensities in positive and negative ionization mode. Influences of modifiers were examined for nonpolar and polar compounds, as optimal conditions are not always the same. By combining different additives, metabolite coverage could be significantly increased. The best results for polar metabolites in positive ionization mode were achieved by using 0.1% acetic acid and 0.1% formic acid in negative ionization mode. For measurements of nonpolar metabolites in positive ionization mode, the application of 10 mmol/L ammonium formate led to the best findings, while the use of 0.02% acetic acid was more appropriate in negative ionization mode.

Future perspectives in high efficient and ultrafast chiral liquid chromatography through zwitterionic teicoplanin-based 2-μm superficially porous particles

With the aim of pushing forward the limits of high efficient and ultrafast chiral liquid chromatography, a new Chiral Stationary Phase (CSP) has been prepared by covalently bonding the teicoplanin selector on 2.0μm Superficially Porous Particles (SPPs). An already validated bonding protocol, which permits to achieve teicoplanin-based CSPs exhibiting zwitterionic behaviour, has been employed to prepare not only the 2.0μm version of the CSP but also two other analogous CSPs based, respectively, on 2.7μm SPPs and 1.9μm Fully Porous Particles (FPPs). The kinetic performance of these CSPs has been compared through the analysis of both van Deemter curves and kinetic plots by employing in-house packed columns of 4.6mm internal diameter and different lengths (20, 50 and 100mm). In particular on the columns packed with 2.0μm SPPs, extremely large efficiencies were observed for both achiral (>310,000 theoretical plates/meter, N/m; h_r: 1.61) and chiral compounds (>290,000 N/m; h_r: 1.72) in HILIC conditions. Thanks to their efficiency and enantioselectivity, these CSPs were successfully employed in ultrafast chiral separations. As an example, the enantiomers of haloxyfop were baseline resolved in about 3s, with a resolution higher than 2.0, (flow rate: 8mL/min) on a 2cm long column packed with the 2.0μm chiral SPPs.

Recent advances in capillary ultrahigh pressure liquid chromatography

In the twenty years since its initial demonstration, capillary ultrahigh pressure liquid chromatography (UHPLC) has proven to be one of most powerful separation techniques for the analysis of complex mixtures. This review focuses on the most recent advances made since 2010 towards increasing the performance of such separations. Improvements in capillary column preparation techniques that have led to columns with unprecedented performance are described. New stationary phases and phase supports that have been reported over the past decade are detailed, with a focus on their use in capillary formats. A discussion on the instrument developments that have been required to ensure that extra-column effects do not diminish the intrinsic efficiency of these columns during analysis is also included. Finally, the impact of these capillary UHPLC topics on the field of proteomics and ways in which capillary UHPLC may continue to be applied to the separation of complex samples are addressed.

Separation of peptides on superficially porous particle based macrocyclic glycopeptide liquid chromatography stationary phases: consideration of fast separations

Macrocyclic glycopeptide based liquid chromatography stationary phases are known for their highly selective peptide separations. Fast and ultrafast (t _R < 1 min) high-efficiency separations were achieved with superficially porous particle (SPP)-based stationary phases. Separations of pharmaceutically important classes of peptides such as enkephalins and bradykinins have been achieved in less than 5 min in isocratic elution modes. Selectivity for peptides structurally similar to one another was increased with use of teicoplanin-based stationary phases compared with commercial C₁₈ stationary phases. Ultrafast isocratic separations of structurally related peptides were achieved with teicoplanin- and vancomycin-based short SPP columns. Acidic mobile phases produced better separations. Ammonium formate was the optimal mobile phase buffer additive. Use of an appropriate combination of a macrocyclic glycopeptide stationary phase and a mobile phase permits faster and more electrospray ionization mass spectrometry compatible isocratic separations than previous gradient approaches. The tryptic peptide separation characteristics of the teicoplanin stationary phase are demonstrated. Additionally, compared with commercial C₁₈ stationary phases, teicoplanin showed tryptic peptide separations with different selectivities. Graphical Abstract Ultrafast separation of enkephalin peptide epimers.

Recent advancements and future directions of superficially porous chiral stationary phases for ultrafast high-performance enantioseparations

This review focuses on the use of superficially porous particles (SPPs) as chiral stationary phases for ultra-high performance liquid enantioseparations.

Different Stationary Phase Selectivities and Morphologies for Intact Protein Separations

The central dogma of biology proposed that one gene encodes for one protein. We now know that this does not reflect reality. The human body has approximately 20,000 protein-encoding genes; each of these genes can encode more than one protein. Proteins expressed from a single gene can vary in terms of their post-translational modifications, which often regulate their function within the body. Understanding the proteins within our bodies is a key step in understanding the cause, and perhaps the solution, to disease. This is one of the application areas of proteomics, which is defined as the study of all proteins expressed within an organism at a given point in time. The human proteome is incredibly complex. The complexity of biological samples requires a combination of technologies to achieve high resolution and high sensitivity analysis. Despite the significant advances in mass spectrometry, separation techniques are still essential in this field. Liquid chromatography is an indispensable tool by which low-abundant proteins in complex samples can be enriched and separated. However, advances in chromatography are not as readily adapted in proteomics compared to advances in mass spectrometry. Biologists in this field still favour reversed-phase chromatography with fully porous particles. The purpose of this review is to highlight alternative selectivities and stationary phase morphologies that show potential for application in top-down proteomics; the study of intact proteins.

Core-Shell in Liquid Chromatography: Application for Determining Sulphonamides in Feed and Meat Using Conventional Chromatographic Systems

A C18 column packed with core-shell particles was used for the chromatographic separation of sulphonamides in feed and meat by a conventional high performance liquid chromatography system coupled with a diode array detector. Two analytical methods, already used in our laboratory, have been modified without any changes in the extraction and clean-up steps and in the liquid chromatography instrumentation. Chromatographic conditions applied on a traditional 5-µm column have been optimized on a column packed with 2.6 µm core-shell particles. A binary mobile phase [acetate buffer solution at pH 4.50 and a mixture of methanol acetonitrile 50: 50 (v/v)] was employed in gradient mode at the flow rate of 1.2 mL with an injection volume of 6 µL. These chromatographic conditions allow the separation of 13 sulphonamides with an entire run of 13 minutes. Preliminary studies have been carried out comparing blanks and spiked samples of feed and meat. A good resolution and the absence of interferences were achieved in chromatograms for both matrices. Since no change was made to the sample preparation, the optimized method does not require a complete revalidation and can be used to make routine analysis faster.