In-depth characterization of six cellulose tris-(3,5-dimethylphenylcarbamate) chiral stationary phases in supercritical fluid chromatography

Investigation into the performance and stability of immobilized and coated polysaccharide columns in supercritical fluid chromatography

In this study, the performance of the widely used "golden four" coated chiral stationary phases (Chiralpak AD-3, Chiralcel OD-3, Chiralpak AS-3, and Chiralcel OJ-3) was compared with their corresponding immobilized versions (Chiralpak IA-3, Chiralpak IB-3, Chiralpak IB N-3, Chiralpak IH-3, and Chiralpak IJ-3) under supercritical fluid chromatography (SFC) conditions with a set of 30 racemic compounds. Using the traditional modifiers, methanol and isopropanol, the immobilized columns (Chiralpak IB N-3 and Chiralpak IH-3) showed an improved general ability to successfully resolve the enantiomers of the target analytes relative to their coated versions (Chiralcel OD-3 and Chiralpak AS-3), while the coated columns (Chiralpak AD-3, Chiralcel OD-3, and Chiralcel OJ-3) performed better than their immobilized versions (Chiralpak IA-3, Chiralpak IB-3, and Chiralpak IJ-3). An investigation of the non-traditional modifiers, dichloromethane, ethyl acetate, and tetrahydrofuran with immobilized columns, revealed a generally decreased ability to successfully resolve the enantiomers of the target analytes, relative to the use of the traditional modifiers, methanol and isopropanol. The stability of the coated columns (Chiralpak AD-H and Chiralcel OD-H) was evaluated by injecting "forbidden" solvents, including dichloromethane, dimethyl sulfoxide, and tetrahydrofuran. After 200 injections of these solvents on coated columns, the retention factors and resolutions slightly decreased, and a significant increase in column backpressure was observed, indicating some degree of stationary phase degradation.

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.

Comparison of dimethylated and methylchlorinated amylose stationary phases, coated and covalently immobilized on silica, for the separation of some chiral compounds in supercritical fluid chromatography

The chromatographic properties of a new coated amylose tris(3-chloro-5-methylphenylcarbamate) were evaluated in supercritical fluid chromatography for the separation of enantiomers of chiral 1-aryl-5-aryl-pyrrolidin-2-one derivatives, potential anticancer agents, and some commercial drugs. The mobile phase consisted of CO₂-modifier mixtures with 30% of either methanol or ethanol, the flow rate was 3 mL/min. The column oven temperature was 40 °C and the outlet pressure was 15 MPa, in order to limit the compressibility of the CO₂, thus limiting density variation along the column. The obtained results were then compared to those observed toward 3 other stationary phases: the coated amylose tris(3,5-dimethylphenylcarbamate), the immobilized amylose tris(3,5-dimethylphenylcarbamate) and the coated amylose tris(5-chloro-2-methylphenylcarbamate). It was shown that the new coated amylose tris(3-chloro-5-methylphenylcarbamate) was the most retentive column whatever the studied compounds, particularly for thalidomide and omeprazole with retention factors up to 73.3 and 29.5for the second enantiomer, respectively. Concerning the enantioselectivity, even most of the compounds are separated on all the four columns, the coated amylose tris(3-chloro-5-methylphenylcarbamate) allows the best resolution for most of the ten studied analytes (except omeprazole for which the resolution values are equal to 7.8 and 9.7 on the coated amylose tris(3-chloro-5-methylphenylcarbamate) and amylose tris(3,5-dimethylphenylcarbamate), respectively). Acting in complementary ways, the two chlorinated stationary phases permitted the complete separation of enantiomers of nine compounds out of the ten.

Preliminary kinetic evaluation of an immobilized polysaccharide sub-2μm column using a low dispersion supercritical fluid chromatograph

The performance of a 3×50mm, 1.6μm d_p column with an immobilized polysaccharide stationary phase (ChiralPak IA-U) was evaluated for efficiency, and pressure drop, with respect to flow rate and modifier concentration using supercritical fluid chromatography (SFC). This appears to be the first such report using such a column in SFC. A unique low dispersion (ultra-high performance) SFC was used for the evaluation. The minimum reduced plate height of 2.78, indicates that the maximum efficiency was similar to or better than coated polysaccharide columns. Selectivity was different from ChiralPak AD, with the same chiral selector, as reported by many others. At high flows and high methanol concentrations, pump pressures sometimes approached 600bar. With 5% methanol, pressure vs. flow rate was non-linear suggesting turbulent flow in the connector tubing. The optimum flow rate (F_opt) at 40% methanol was ≈0.8mL/min, where the column efficiency was highest. At 5% methanol, F_opt increased to ≈1.6mL/min, but efficiency degraded noticeably. The differences in F_opt suggests that the solute diffusion coefficients are a strong function of modifier concentration. Several sub-1min separations, including a 7.5s separation, are presented.

Chiral recognition in separation science - an update

Stereospecific recognition of chiral molecules is an important issue in various aspects of life sciences and chemistry including analytical separation sciences. The basis of analytical enantioseparations is the formation of transient diastereomeric complexes driven by hydrogen bonds or ionic, ion-dipole, dipole-dipole, van der Waals as well as π-π interactions. Recently, halogen bonding was also described to contribute to selector-selectand complexation. Besides structure-separation relationships, spectroscopic techniques, especially NMR spectroscopy, as well as X-ray crystallography have contributed to the understanding of the structure of the diastereomeric complexes. Molecular modeling has provided the tool for the visualization of the structures. The present review highlights recent contributions to the understanding of the binding mechanism between chiral selectors and selectands in analytical enantioseparations dating between 2012 and early 2016 including polysaccharide derivatives, cyclodextrins, cyclofructans, macrocyclic glycopeptides, proteins, brush-type selectors, ion-exchangers, polymers, crown ethers, ligand-exchangers, molecular micelles, ionic liquids, metal-organic frameworks and nucleotide-derived selectors. A systematic compilation of all published literature on the various chiral selectors has not been attempted.