Investigation of retention behavior of drug molecules in supercritical fluid chromatography using linear solvation energy relationships

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.

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.

Linear solvation energy relationship (LSER) characterization of the normal phase retention mechanism on hypercrosslinked polystyrenes

Linear solvation energy relationships (LSERs) were applied to retention on hypercrosslinked polystyrene on silica (HC-Tol) to elucidate the type and relative importance of molecular interactions between model solutes and the HC-Tol stationary phase. Classical amino phase and another hypercrosslinked phase (5-HGN) were used as reference columns. On both the HC-Tol and amino, polar interactions predominate and contribute to retention. Solute volume V has no impact on retention on the amino column, while V has a slightly negative influence on retention for the HC-Tol column. The differences in coefficient v between the amino and the HC-Tol columns might explain why the HC-Tol is capable of group-type separations. 5-HGN phase has smaller a and b values compared to HC-Tol, which means that 5-HGN is not as basic or acidic in terms of hydrogen bonds as is HC-Tol. This suggests that the hydrogen bonding character of the HC-Tol phase arises from its silica substrate.

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.

Supercritical fluid chromatography/mass spectrometry in metabolite analysis

Supercritical fluid chromatography (SFC) owes many of its advantages to the properties of supercritical CO2, which possesses benefits as mobile phase. SFC has recently gained attention as a separation technique because it can be utilized for not only non-polar but also polar compound analysis. In addition, MS is widely adopted for SFC, and the options for MS are equivalent to liquid chromatography. Sensitive and selective detection is crucial in metabolite analysis. The SFC/MS system can be an alternative approach to liquid chromatography, as can metabolite analysis using packed-column SFC in biosamples. In this review we cover the fundamentals of SFC in combination with MS, and discuss the results of metabolite analysis using SFC/MS.

Evaluating the interactions of organic compounds with multi-walled carbon nanotubes by self-packed HPLC column and linear solvation energy relationship

Understanding the interactions between organic pollutants and carbon nanotubes (CNTs) is critical for fate assessment of both CNTs and organic pollutants. In this study, the chromatographic approach was introduced based on CNTs as stationary phase for the evaluation of such interactions. The pristine multi-walled carbon nanotubes (MWCNTs) were packed into columns of high-pressure liquid chromatography (HPLC) and the retention factors (k') were determined to characterize the adsorption affinity of organic compounds onto MWCNTs. Nine compounds were tested. The results showed that their lnk' values followed the order: benzene < toluene < phenol < chlorobenzene < bromobenzene < aniline < sulfamethoxazole < sulfadiazine ≈ sulfadimidine. The linear solvation energy relationship (LSER) theory was adopted to correlate lnk' with the molecular solvatochromic parameters. We found that lnk' of the studied compounds correlate positively with molecular polarizability (E) significantly, suggesting that the π-/n-electrons-dependent polarizable interactions play a major role for the adsorption. Moreover, the thermodynamic parameters calculated from van't Hoff equations revealed that the interactions between the compounds and MWCNTs were spontaneous and exothermic processes.