An examination of the effects of water on normal phase enantioseparations

Fast, sensitive LC-MS resolution of α -hydroxy acid biomarkers via SPP-teicoplanin and an alternative UV detection approach

Enantioseparation of α -hydroxy acids is essential since specific enantiomers of these compounds can be used as disease biomarkers for diagnosis and prognosis of cancer, brain diseases, kidney diseases, diabetes, etc., as well as in the food industry to ensure quality. HPLC methods were developed for the enantioselective separation of 11 α -hydroxy acids using a superficially porous particle-based teicoplanin (TeicoShell) chiral stationary phase. The retention behaviors observed for the hydroxy acids were HILIC, reversed phase, and ion-exclusion. While both mass spectrometry and UV spectroscopy detection methods could be used, specific mobile phases containing ammonium formate and potassium dihydrogen phosphate, respectively, were necessary with each approach. The LC-MS mode was approximately two orders of magnitude more sensitive than UV detection. Mobile phase acidity and ionic strength significantly affected enantioresolution and enantioselectivity. Interestingly, higher ionic strength resulted in increased retention and enantioresolution. It was noticed that for formate-containing mobile phases, using acetonitrile as the organic modifier usually resulted in greater enantioresolution compared to methanol. However, sometimes using acetonitrile with high ammonium formate concentrations led to lengthy retention times which could be avoided by using methanol as the organic modifier. Additionally, the enantiomeric purities of single enantiomer standards were determined and it was shown that almost all standards contained some levels of enantiomeric impurities.

Liquid Chromatographic Enantioseparation of Newly Synthesized Fluorinated Tryptophan Analogs Applying Macrocyclic Glycopeptides-Based Chiral Stationary Phases Utilizing Core-Shell Particles

Due to the favorable features obtained through the incorporation of fluorine atom(s), fluorinated drugs are a group with emerging pharmaceutical importance. As their commercial availability is still very limited, to expand the range of possible candidates, new fluorinated tryptophan analogs were synthesized. Control of enantiopurity during the synthesis procedure requires that highly efficient enantioseparation methods be available. In this work, the enantioseparation of seven fluorinated tryptophans and tryptophan was studied and compared systematically to (i) develop analytical methods for enantioselective separations and (ii) explore the chromatographic features of the fluorotrytophans. For enantioresolution, macrocyclic glycopeptide-based selectors linked to core-shell particles were utilized, applying liquid chromatography-based methods. Application of the polar-ionic mode resulted in asymmetric and broadened peaks, while reversed-phase conditions, together with mobile-phase additives, resulted in baseline separation for all studied fluorinated tryptophans. The marked differences observed between the methanol and acetonitrile-containing eluent systems can be explained by the different solvation abilities of the bulk solvents of the applied mobile phases. Among the studied chiral selectors, teicoplanin and teicoplanin aglycone were found to work effectively. Under optimized conditions, baseline separations were achieved within 6 min. Ionic interactions were semi-quantitatively characterized and found to not influence enantiorecognition. Interestingly, fluorination of the analytes does not lead to marked changes in the chromatographic characteristics of the methanol-containing eluents, while larger differences were noticed when the polar but aprotic acetonitrile was applied. Experiments conducted on the influence of the separation temperature indicated that the separations are enthalpically driven, with only one exception. Enantiomeric elution order was found to be constant on both teicoplanin and teicoplanin aglycone-based chiral stationary phases (L < D) under all applied chromatographic conditions.

Effect of water and protic solvents on polysaccharide-based column efficiency

In the present study, polysaccharide-based columns were used to evaluate the efficiency of columns in response to the introduction of water and protic solvents (methanol and ethanol) into the mobile phase, replacing acetonitrile. While increasing water content frequently enhances enantiomer resolution, the inclusion of water, particularly when combined with methanol and ethanol in the mobile phase, has an adverse impact on mass transfer, thus influencing the column plate height. These effects are more pronounced with ethanol, and in many cases, van Deemter plots exhibit the absence of a minimum point optimal in the explored range. Consequently, acetonitrile and its water mixtures are the preferred choices to mitigate these effects for situations in which the chiral column is operated at a relatively high flow rate (> 1 mL/min in a 4.6 mm column).

Enantioseparation of a-substituted proline analogs with macrocyclic glycopeptide-based chiral stationary phases immobilized on superficially porous particles of silica applying liquid chromatography with ultraviolet and mass spectrometric detection

In this study, the liquid chromatography-based direct enantioseparation of the stereoisomers of α-substituted proline analogs has been investigated utilizing chiral stationary phases with UV and/or mass spectrometric (MS) detection. Macrocyclic antibiotics, such as vancomycin, teicoplanin, modified teicoplanin, and teicoplanin aglycone, all covalently immobilized to 2.7 μm superficially porous silica particles have been applied as stationary phases. Mobile phases utilizing mixtures of methanol and acetonitrile with different additives (polar-ionic mode) were optimized during method development. Best separations were achieved with mobile phases of 100% MeOH containing either 20 mM acetic acid or 20 mM triethylammonium acetate. Special attention was given to the applicability of MS-compatible mobile phases. Acetic acid was found to be advantageous as a mobile phase additive for MS detection. Enantioselective chromatographic behaviors are interpreted based on the explored correlations between the analytes' structural features and those of the applied chiral stationary phases. For the thermodynamic characterization, separations were studied in the temperature range of 5-50 °C. Generally, retention and selectivity decreased with increasing temperature, and in most cases, enthalpy-driven enantiorecognition was observed, but entropic contributions also were present. Unexpectedly, unusual shapes for the van Deemter curves were registered in the kinetic evaluations. General trends could be observed in the enantiomeric elution orders: S < R on VancoShell and NicoShell, and opposite R < S on TeicoShell and TagShell columns.

Synthesis and evaluation of ginsenosides imprinted polymer-based chromatographic stationary phase

The molecular imprinting technique has aroused great interest in preparing novel stationary phases, and the resulting materials named molecularly imprinted polymers coated silica packing materials exhibit good performance in separating diverse analytes based on their good characteristics (including high selectivity, simple synthesis, and good chemical stability). To date, mono-template is commonly used in synthesizing molecularly imprinted polymers-based stationary phases. The resulting materials always own the disadvantages of low column efficiency and restricted analytes, and the price of ginsenosides with high purity was very high. In this study, to overcome the weaknesses of molecularly imprinted polymers-based stationary phases mentioned above, the multi-templates (total saponins of folium ginseng) strategy was used to prepare ginsenosides imprinted polymer-based stationary phase. The resulting ginsenosides imprinted polymer-coated silica stationary phase has a good spherical shape and suitable pore structures. Additionally, the total saponins of folium ginseng were cheaper than other kinds of ginsenosides. Moreover, the ginsenosides imprinted polymer-coated silica stationary phase-packed column performed well in the separation of ginsenosides, nucleosides, and sulfonamides. The ginsenosides imprinted polymer-coated silica stationary phase possesses good reproducibility, repeatability, and stability for seven days. Therefore, a multi-templates strategy for synthesizing the ginsenosides imprinted polymer-coated silica stationary phase is considered in the future.

Comparison of core-shell versus fully porous particle packings for chiral liquid chromatography productivity

A loading and productivity study was done using three racemates on vancomycin and teicoplanin-bonded chiral stationary phases of different particle formats. Two columns were packed with 2.7 μm superficially porous particles and two columns were packed with identically bonded 5 μm fully porous particles. The last two columns were packed with specially synthesized 4.5 μm vancomycin and teicoplanin superficially porous particles. The loading of different chiral compounds showed that the columns filled with 2.7-μm chiral stationary phases were inappropriate for preparative separations due to their very low permeability which precluded high flow rates. However, columns containing 4.5 μm superficially porous (core-shell) particles were as effective for small-scale preparative chiral separations as columns filled with classical 5 μm fully porous particles. Comparing the 4.5 μm superficially porous particles and 5 μm fully porous particles teicoplanin columns, the observed respective productivities of 270 and 265 mg/g chiral phase/h for 5-methyl-5-phenyl hydantoin enantiomers were obtained. Particular attention was given to the peculiar case of the mianserin enantiomeric separation on vancomycin columns that gave observed productivities of 200 and 205 mg/g chiral phase/h on the 4.5 μm superficially porous particles and 5 μm fully porous particles, respectively.

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.