Characterisation of stationary phases in subcritical fluid chromatography with the solvation parameter model

An improved subtraction model applied in supercritical fluid chromatography to characterise polar stationary phases

Herein, an improved subtraction model was proposed to characterise the polar stationary phases in supercritical fluid chromatography (SFC). Fifteen stationary phases were selected, including two types of aromatic columns, Waters Torus and Viridis series columns, as well as silica and amino columns. Ethylbenzene and Torus 1-AA were defined as the reference solute and column, respectively. Identifying the interaction with the maximum contribution to retention in SFC separation and using it as the initial term is a key step in modelling. The dipole, or induced dipole interaction (θ'P), replaced the hydrophobic interaction (η'H) as the starting term. The improved model was expressed as logα=η'H+β'A+α'B+κ'C+θ'P+ε'E+σ'S, where the term ε'E indicated that anion exchange interaction was intentionally supplemented. A 7-step modelling process, including bidirectional fitting and residual analysis, was proposed. The obtained column parameters had reasonable physical significance, with the adjusted determination coefficient (R2adj) greater than 0.999 and the standard error (SE) less than 0.029. Methodological validation was further performed using the other four columns and 12 solutes that were not involved in the modelling. The result revealed good predictions of solutes' retention, as demonstrated by R2adj from 0.9923 to 0.9979 and SE from 0.0636 to 0.1088. This study indicated the feasibility of using the improved subtraction model to characterise polar stationary phases in SFC, with the most crucial being the determination of an initial term, followed by the addition of a new descriptor and the selection of an appropriate reference column. The study expanded the application scope of the subtraction model in SFC, which will help gain an in-depth understanding of the SFC separation mechanism.

Columns in analytical-scale supercritical fluid chromatography: From traditional to unconventional chemistries

Within this review, we thoroughly explored supercritical fluid chromatography (SFC) columns used across > 3000 papers published from the first study carried out under SFC conditions in 1962 to the end of 2022. We focused on the open tubular capillary, packed capillary, and packed columns, their chemistries, dimensions, and trends in used stationary phases with correlation to their specific interactions, advantages, drawbacks, used instrumentation, and application field. Since the 1990s, packed columns with liquid chromatography and SFC-dedicated stationary phases for chiral and achiral separation are predominantly used. These stationary phases are based on silica support modified with a wide range of chemical moieties. Moreover, numerous unconventional stationary phases were evaluated, including porous graphitic carbon, titania, zirconia, alumina, liquid crystals, and ionic liquids. The applications of unconventional stationary phases are described in detail as they bring essential findings required for further development of the supercritical fluid chromatography technique.

An attempt to apply a subtraction model for characterization of non-polar stationary phase in supercritical fluid chromatography

This study verified the feasibility of using a subtraction model to characterize the non-polar stationary phases (including C4, C8, and phenyl-type) in supercritical fluid chromatography (SFC). The model with 6 terms was expressed as log α = η'H + θ'P + β'A + α'B + κ'C + σ'S, where a term θ'P indicating dipole or induced dipole interaction was intentionally supplemented. Ethylbenzene and SunFire C8 were respectively defined as the reference solute and column. A 7-step modeling procedure was proposed: in the first 6 steps, except σ'S, by the use of a bidirectional fitting method, other parameters were calculated based on the equation: log α = log (k_i/k_ref) ≈ η'H + θ'P + β'A + α'B + κ'C; and in the 7th step, residual analysis was employed to describe the σ'S term according to the equation: σ'S = log α_exp. - log α_pre. Furthermore, six columns that were not involved in modeling process and 12 compounds with unknown retention were used for methodology validation. It showed good predictions of log k, as demonstrated by adjusted determination coefficient (R²_adj) from 0.9927 to 0.9998 (column) and from 0.9940 to 0.9999 (compound), respectively. The subtraction model emphasized the contribution of dipole or induced dipole interaction to the retention in SFC, and it obtained the σ'S term through residual analysis. Moreover, it made reasonable physical-chemical sense as the linear solvation energy relationship (LSER) model did, with the distinct advantages of better fitting and more accurate prediction. This study provided some new insights into the characterization of non-polar stationary phases in SFC.

An in-depth investigation of supercritical fluid chromatography retention mechanisms by evaluation of a series of specially designed alkylsiloxane-bonded stationary phases based on linear solvation energy relationship

Fundamental research on supercritical fluid chromatography (SFC) has gained considerable interest, with many studies focusing on its retention mechanism based on the linear solvation energy relationship (LSER) model. In this paper, a series of alkylsiloxane-bonded stationary phases were specifically designed and synthesized, then evaluated using the mobile phase composed of CO₂ with 10% (v/v) methanol. The study demonstrated the close relationship between the interactions (manner and magnitude) of stationary phases and the C-chain length, bonding density and the endcapping treatment. All C8 phases provide positive e, v and negative s, whose magnitude was regularly affected by bonding density. It was worth mentioning the non-endcapped C8 phases could provide H-bonding (positive a and b) by reducing the bonding density of the alkyl chain. Once it was endcapped, the interaction manner did not vary with bonding density adjustment. The non-endcapped C4 phases with higher bonding density could establish additional dispersion interaction (positive v). It can be seen that two synthesis strategies, 1) non-endcapped, long C-chain (C8) combined with low bonding density, and 2) non-endcapped, short C-chain (C4) combined with high bonding density, can obtain the alkylsiloxane-bonded stationary phases (C8-1 and C4-3) to provide both polar and dispersion interactions, showing different separation selectivity. Furthermore, the LSER model with ionic terms was applied to evaluate partial C8 columns, and its rationality was verified. The non-endcapped C8 showed great d⁺ values, which originated from the silanol groups. C8SCX also possessed a great d⁺ value due to the benzenesulfonic acid groups. A remarkable result showed that C8SAX exhibited prominent d^- and d⁺ values simultaneously due to the combined effect of silanol and quaternary ammonium groups, which indicates the unique selectivity when separating ionic compounds. This study provides in-depth insights into the retention mechanism of alkylsiloxane-bonded stationary phases in SFC, as well as a reference for the design of SFC stationary phases.

A story of peptides, lipophilicity and chromatography - back and forth in time

Peptides, as part of the beyond the rule of 5 (bRo5) chemical space, represent a unique class of pharmaceutical compounds. Because of their exceptional position in the chemical space between traditional small molecules (molecular weight (MW) < 500 Da) and large therapeutic proteins (MW > 5000 Da), peptides became promising candidates for targeting challenging binding sites, including even targets traditionally considered as undruggable - e.g. intracellular protein-protein interactions. However, basic knowledge about physicochemical properties that are important for a drug to be membrane permeable is missing but would enhance the drug discovery process of bRo5 molecules. Consequently, there is a demand for quick and simple lipophilicity determination methods for peptides. In comparison to the traditional lipophilicity determination methods via shake flask and in silico prediction, chromatography-based methods could have multiple benefits such as the requirement of low analyte amount, insensitivity to impurities and high throughput. Herein we elucidate the role of peptide lipophilicity and different lipophilicity values. Further, we summarize peptide analysis via common chromatographic techniques, in specific reversed phase liquid chromatography, hydrophilic interaction liquid chromatography and supercritical fluid chromatography and their role in drug discovery and development process.

An Assessment of Stationary Phase Selectivity in SFC

Supercritical fluid chromatography (SFC) has seen a recent resurgence in interest following investment in the development of instrument technology by numerous instrument manufacturers. Increased focus on sustainability in chromatographic science, coupled with the orthogonality to reversed phase HPLC, is likely to further drive the uptake of SFC in many sectors. As with any form of chromatography, optimizing separation selectivity is a key variable in providing adequate resolution and accurate identification and quantification of target analytes. Stationary phase chemistry can be readily exploited to substantially alter the separation selectivity obtained. This article examines and characterizes the selectivity differences offered by three prototype SFC phases.

Characterization of stationary phases in supercritical fluid chromatography including exploration of shape selectivity

Achiral packed column supercritical fluid chromatography (SFC) has shown an important regain of interest in academic and industrial laboratories in the recent years. In relation to this increased concern, major instrument manufacturers have designed some stationary phases specifically for SFC use. SFC stationary phases have been widely examined over the last two decades, based on the use of linear solvation energy relationships (LSER), which relate analyte retention to its properties and to the interaction capabilities of the chromatographic system. The method provides some understanding on retention mechanisms (normal phase, reversed phase or mixed-mode) and the possibility to compare stationary phases on a rational basis, especially through a spider diagram providing a visual classification. The latter can be used as a primary tool to select complementary stationary phases to be screened for any separation at early stages of method development, before optimization steps. In this context, the characterization of the 14 columns from the Shim-pack UC series (Shimadzu Corporation, Kyoto, Japan), which are dedicated to SFC and more broadly to unified chromatography (UC), was performed, using the LSER methodology. As in previous works, seven descriptors, including five Abraham descriptors (E, S, A, B, V) and two descriptors describing positive and negative charges (D^- and D⁺) were first employed to describe interactions with neutral and charged analytes. Secondly, two more descriptors were introduced, which were previously employed solely for the characterization of enantioselective systems and expressing shape features of the analytes (flexibility F and globularity G). They brought additional insight into the retention mechanisms, showing how spatial insertion of the analytes in some stationary phases is contributing to shape separation capabilities and how folding possibilities in flexible molecules is unfavorable to retention in other stationary phases.

Chromatographic analysis of biomolecules with pressurized carbon dioxide mobile phases - A review

Biomolecules like proteins, peptides and nucleic acids widely emerge in pharmaceutical applications, either as synthetic active pharmaceutical ingredients, or from natural products as in traditional Chinese medicine. Liquid-phase chromatographic methods (LC) are widely employed for the analysis and/or purification of such molecules. On another hand, to answer the ever-increasing requests from scientists involved in biomolecules projects, other chromatographic methods emerge as useful complements to LC. In particular, there is a growing interest for chromatography with a mobile phase comprising pressurized carbon dioxide, which can be named either (i) supercritical (or subcritical) fluid chromatography (SFC) when CO₂ is the major constituent of the mobile phase, or (ii) enhanced fluidity liquid chromatography (EFLC) when hydro-organic or purely organic solvents are the major constituents of the mobile phase. Despite the low polarity of CO₂, supposedly inadequate to solubilize such biomolecules, SFC and EFLC were both employed in many occasions for this purpose. This paper specifically reviews the literature related to the SFC/EFLC analysis of free amino acids, peptides, proteins, nucleobases, nucleosides and nucleotides. The analytical conditions employed for specific molecular families are presented, with a focus on the nature of the stationary phase and the mobile phase composition. We also discuss the potential benefits of combining SFC/EFLC to LC in a single gradient elution, a method sometimes designated as unified chromatography (UC). Finally, detection issues are presented, and more particularly hyphenation to mass spectrometry.

Retention characteristics of silica materials in carbon dioxide/methanol mixtures studied by inverse supercritical fluid chromatography

In this work, inverse supercritical fluid chromatography was applied to characterize the surface of four silica materials (three commercial Kromasils and one silica aerogel) from chromatographic retention data. Retention factors at various pressures (150-300 bar), temperatures (25-60 °C) and modifier concentrations (5-20 vol.% methanol in CO₂) for a set of representative 17 solutes were correlated with the solute properties by the linear solvation energy relationships (LSER). Two types of the LSER models were identified based on different criteria. Firstly, a generally valid model with two descriptors concerning dipolarity/polarizability and solute hydrogen-bonding acceptor ability was constructed. Secondly, a group of specific models for each particular silica material was proposed. According to the statistical analysis of the modeling results, the acid-basic interactions were demonstrated to have a major contribution to the retention for all studied silicas. The intensity of these interactions decreases with increasing methanol concentration in the mobile phase, possibly due to the mixed mechanism of competitive adsorption of the modifier on silanol groups and modification of mobile phase property. Moreover, retention factors measured under constant conditions (p, T, methanol concentration) for a pair of the materials were found to be proportional in logarithmic scale implying the transferability of the adsorption free energies and the adsorption constants across four studied silica materials.

Development of a practical online supercritical fluid extraction-supercritical fluid chromatography/mass spectrometry system with an integrated split-flow method

Herein, we describe a practical online supercritical fluid extraction-supercritical fluid chromatography/mass spectrometry (SFE-SFC/MS) system with an integrated split-flow method and a pre-column trap method that is well suited for the continuous extraction and separation of a wide range of compounds, including hydrophilic ones. Although an SFE-SFC system with a splitting method is already commercially available, in this study, we added some new features to this system: 1) a splitting method that further reduces the amount of extractant introduced into SFC, 2) a trap column, connected before the analytical column, with a different separation mechanism than the analytical column in the system with the splitting method, and 3) a system for calculating the recovery rate of SFE during online SFE-SFC/MS. In the above setup, part of the analyzed extract is introduced into the separation section at a higher split ratio owing to the make-up pump flow rate, thus reducing the distortion of the target analyte peak shape caused by the use of a strong extractant. Furthermore, the separation efficiency is improved by the use of an additional pre-column capable of interacting with compounds weakly retained on the analytical column. Finally, we show that equalization of the SFE and autosampler injection conditions allows evaluation of the recovery rate of SFE during online SFE-SFC/MS.

Supercritical Fluid Chromatography as a Technique to Fractionate High-Valued Compounds from Lipids

Natural products are in high demand these days due to rising awareness among consumers. Healthy diets, especially those in emerging markets, growth in populations with nutritional deficiencies, and supporting government regulations provide high growth opportunities for these compounds. However, extraction of high-valued compounds from natural sources is not an easy task. Natural products are complex matrices, with relevant compounds present in small amounts and often mixed with other compounds of similar structures. Most of the applications are related to the pharmaceutical sector, but interest in food and natural products is growing fast. Lipid and carbohydrate extracts are examples of starting materials employed to purify these relevant compounds. At the same time supercritical fluid chromatography (SFC) is an emerging technique for preparative separation due to (1) use of supercritical fluids, commonly carbon dioxide, giving a large reduction in use of organic solvents; and (2) new hardware has been made commercially available recently that makes SFC a viable option. SFC fulfills high demands with respect to selectivity, versatility and sensibility. Fractionation or purification by SFC of high-valued compounds from natural sources is an interesting option, the relevance of which will increase in the future. This paper is a survey of trends and applications of SFC in the field of natural products purification.

Hydrophilic interaction liquid chromatography in the speciation analysis of selenium

The hydrophilic interaction liquid chromatography (HILIC) coupled to mass spectrometry was employed to study retention behavior of selected selenium compounds using two different HILIC stationary phases: silica and zwitterionic. Two organic solvents - acetonitrile and methanol - were compared as a component of mobile phase. Separation parameters such as a content of organic modifier, the eluent pH and inorganic buffer concentration were investigated. Based on all observations, methanol seems to be beneficial for the separation of studied compounds. The optimal HILIC separation method involved silica column and eluent composed of 85% MeOH and CH₃COONH₄ (8 mM, pH 7) was compared to RP method in terms of time of the single run, the separation efficiency and limit of detection.

A systematic investigation of sample diluents in modern supercritical fluid chromatography

This paper focuses on the possibility to inject large volumes (up to 10μL) in ultra-high performance supercritical fluid chromatography (UHPSFC) under generic gradient conditions. Several injection and method parameters have been individually evaluated (i.e. analyte concentration, injection volume, initial percentage of co-solvent in the gradient, nature of the weak needle wash solvent, nature of the sample diluent, nature of the column and of the analyte). The most critical parameters were further investigated using in a multivariate approach. The overall results suggested that several aprotic solvents including methyl tert-butyl ether (MTBE), dichloromethane, acetonitrile or cyclopentyl methyl ether (CPME) were well adapted for the injection of large volume in UHPSFC, while MeOH was generally the worst alternative. However, the nature of the stationary phase also had a strong impact and some of these diluents did not perform equally on each column. This was due to the existence of a competition in the adsorption of the analyte and the diluent on the stationary phase. This observation introduced the idea that the sample diluent should not only be chosen according to the analyte but also to the column chemistry to limit the interactions between the diluent and the ligands. Other important characteristics of the "ideal" SFC sample diluent were finally highlighted. Aprotic solvents with low viscosity are preferable to avoid strong solvent effects and viscous fingering, respectively. In the end, the authors suggest that the choice of the sample diluent should be part of the method development, as a function of the analyte and the selected stationary phase.

Supercritical Fluid Chromatography of Drugs: Parallel Factor Analysis for Column Testing in a Wide Range of Operational Conditions

Retention mechanisms involved in supercritical fluid chromatography (SFC) are influenced by interdependent parameters (temperature, pressure, chemistry of the mobile phase, and nature of the stationary phase), a complexity which makes the selection of a proper stationary phase for a given separation a challenging step. For the first time in SFC studies, Parallel Factor Analysis (PARAFAC) was employed to evaluate the chromatographic behavior of eight different stationary phases in a wide range of chromatographic conditions (temperature, pressure, and gradient elution composition). Design of Experiment was used to optimize experiments involving 14 pharmaceutical compounds present in biological and/or environmental samples and with dissimilar physicochemical properties. The results showed the superiority of PARAFAC for the analysis of the three-way (column × drug × condition) data array over unfolding the multiway array to matrices and performing several classical principal component analyses. Thanks to the PARAFAC components, similarity in columns' function, chromatographic trend of drugs, and correlation between separation conditions could be simply depicted: columns were grouped according to their H-bonding forces, while gradient composition was dominating for condition classification. Also, the number of drugs could be efficiently reduced for columns classification as some of them exhibited a similar behavior, as shown by hierarchical clustering based on PARAFAC components.

Development of a supercritical fluid chromatography method with ultraviolet and mass spectrometry detection for the characterization of biomass fast pyrolysis bio oils

The characterization of complex mixtures is a challenging issue for the development of innovative processes dedicated to biofuels and bio-products production. The huge number of compounds present in biomass fast pyrolysis oils combined with the large diversity of chemical functions represent a bottleneck as regards analytical technique development. For the extensive characterization of complex samples, supercritical fluid chromatography (SFC) can be alternative to usual separation techniques such as gas (GC) or liquid chromatography (LC). In this study, an approach is proposed to define the best conditions for the SFC separation of a fast pyrolysis bio-oil. This approach was based on SFC data obtained directly from the bio-oil itself instead of selecting model compounds as usually done. SFC conditions were optimized by using three specific, easy-to-use and quantitative criteria aiming at maximizing the separation power. Polar stationary phases (ethylpyridine bonded silica) associated to a mix of acetonitrile and water as polarity modifier provided the best results, with more than 120 peaks detected in SFC-UV.

Predicting Partitioning and Diffusion Properties of Nonpolar Chemicals in Biotic Media and Passive Sampler Phases by GC × GC

The chemical parameters needed to explain and predict bioavailability, biodynamics, and baseline toxicity are not readily available for most nonpolar chemicals detected in the environment. Here, we demonstrate that comprehensive two-dimensional gas chromatography (GC × GC) retention times can be used to predict 26 relevant properties for nonpolar chemicals, specifically: partition coefficients for diverse biotic media and passive sampler phases; aquatic baseline toxicity; and relevant diffusion coefficients. The considered biotic and passive sampler phases include membrane and storage lipids, serum and muscle proteins, carbohydrates, algae, mussels, polydimethylsiloxane, polyethylene, polyoxymethylene, polyacrylate, polyurethane, and semipermeable membrane devices. GC × GC-based chemical property predictions are validated with a compilation of 1038 experimental property data collected from the literature. As an example application, we overlay a map of baseline toxicity to fathead minnows onto the separated analyte signal of a polychlorinated alkanes (chlorinated paraffins) technical mixture that contains 7820 congeners. In a second application, GC × GC-estimated properties are used to parametrize multiphase partitioning models for mammalian tissues and organs. In a third example, we estimate chemical depuration kinetics for mussels. Finally, we illustrate an approach to screen the GC × GC chromatogram for nonpolar chemicals of potentially high concern, defined based on their GC × GC-estimated biopartitioning properties, diffusion properties, and baseline toxicity.

Evaluation of an amide-based stationary phase for supercritical fluid chromatography

A relatively new stationary phase containing a polar group embedded in a hydrophobic backbone (i.e., ACE®C18-amide) was evaluated for use in supercritical fluid chromatography. The amide-based column was compared with columns packed with bare silica, C18 silica, and a terminal-amide silica phase. The system was held at supercritical pressure and temperature with a mobile phase composition of CO2 and methanol as cosolvent. The linear solvation energy relationship model was used to evaluate the behavior of these stationary phases, relating the retention factor of selected probes to specific chromatographic interactions. A five-component test mixture, consisting of a group of drug-like molecules was separated isocratically. The results show that the C18 -amide stationary phase provided a combination of interactions contributing to the retention of the probe compounds. The hydrophobic interactions are favorable; however, the electron donating ability of the embedded amide group shows a large positive interaction. Under the chromatographic conditions used, the C18 -amide column was able to provide baseline resolution of all the drug-like probe compounds in a text mixture, while the other columns tested did not.

A polyvinyl alcohol-coated silica gel stationary phase for hydrophilic interaction chromatography

A facile and green way is proposed to prepare a polyvinyl alcohol coated silica gel stationary phase for HILIC. It demonstrated high separation efficiency and high chemical stability.