Coupled achiral/chiral column techniques in subcritical fluid chromatography for the separation of chiral and nonchiral compounds

The effect of tandem coupling of NicoShell and TeicoShell columns in sub/supercritical fluid chromatography on enantioresolution

The coupling of columns in sub/supercritical fluid chromatography presents a great opportunity for influencing the separation efficiency and extending the selectivity of the separation system. Combinations of different types of chiral stationary phases could positively affect the enantioresolution if single ones are complementary to each other. In this work, two superficially porous particle (2.7 μm) macrocyclic glycopeptide-based columns, namely TeicoShell and NicoShell, were serially coupled and tested in sub/supercritical fluid chromatography for the first time. The influence of the column arrangement on the enantioseparation of structurally diverse biologically active compounds was examined. The obtained results showed how the column order crucially affected the enantioresolution of compounds tested, but the retention was negligibly affected in most cases. We also demonstrated that single TeicoShell and NicoShell columns are very promising towards the development of highly efficient and fast/ultrafast sub/supercritical fluid chromatography methods for structurally different chiral compounds. The optimized methods for sub-minute enantioselective separation of certain biologically important compounds were proposed.

Supercritical fluid chromatography and liquid chromatography for isomeric separation of a multiple chiral centers analyte

The development of a chiral separation strategy has always been a challenge of crucial importance, particularly in the pharmaceutical field. Chromatographic methods have become popular, particularly High Performance Liquid Chromatography and Supercritical Fluid Chromatography from a preparative scale point of view. A bioactive compound bearing three stereogenic centers was entrusted in our laboratory and the aim of this work was to obtain the complete resolution of the eight stereoisomers. Nine different polysaccharide-based columns were tested in SFC under various carbon dioxide-based mobile phases. The use of a single chiral column Lux Cellulose-2 under 30% 2-PrOH in carbon dioxide, at a flow-rate of 1 mL/min, column temperature of 40°C, 120 bar outlet pressure allowed the obtention of eight peaks. To further improve the resolution of the two last isomers, two columns were serially coupled . The results obtained with the six different combinations are discussed. The tandem column supercritical fluid chromatography has demonstrated to be a useful technique to resolve the eight stereoisomers on Lux Cellulose-2//Cellulose-2 tandem of coupled columns with 30% 2-PrOH in carbon dioxide, at a flow-rate of 1 mL/min, column temperature of 40°C and 120 bar outlet pressure, despite a long analysis time. In order to compare the two methods (i.e supercritical and liquid), chiral liquid chromatography under polar aqueous-organic mode, polar organic mode and normal-phase mode, was implemented. The last mode allowed the full baseline resolution of the eight isomers on Cellulose-5 CSP, with 20% 2-PrOH in n-heptane at a flow-rate of 0.8 mL/min, at 25°C, λ = 220 nm. The limits of detection and of quantification were determined for this method and the best values obtained for isomer 8 were equal to 2.84 and 9.37 nM respectively. Finally, a small-scale preparative separation of the multiple chiral centers compound was implemented on Cellulose-5 CSP within 10% 2-PrOH in n-heptane in order to study the stereoisomer elution order on Cellulose-2, Cellulose-5 and Chiralpak AD-H, under EtOH or 2-PrOH in n-heptane mobile phases, and partial reversal elution orders were observed.

Supercritical fluid chromatography of metoprolol and analogues on aminopropyl and ethylpyridine silica without any additives

Metoprolol and a number of related amino alcohols and similar analytes have been chromatographed on aminopropyl (APS) and ethylpyridine (EPS) silica columns. The mobile phase was carbon dioxide with methanol as modifier and no amine additive was present. Optimal isocratic conditions for the selectivity were evaluated based on experiments using design of experiments. A central composite circumscribed model for each column was used. Factors were column temperature, back-pressure and % (v/v) of modifier. The responses were retention and selectivity versus metoprolol. The % of modifier mainly controlled the retention on both columns but pressure and temperature could also be important for optimizing the selectivity between the amino alcohols. The compounds could be divided into four and five groups on both columns, with respect to the selectivity. Furthermore, on the aminopropyl silica the analytes were more spread out whereas on the ethylpyridine silica, due to its aromaticity, retention and selectivity were closer. For optimal conditions the column temperature and back-pressure should be high and the modifier concentration low. A comparison of the selectivity using optimized conditions show a few switches of retention order between the two columns. On aminopropyl silica an aldehyde failed to be eluted owing to Schiff-base formation. Peak symmetry and column efficiency were briefly studied for some structurally close analogues. This revealed some activity from the columns that affected analytes that had less protected amino groups, a methyl group instead of isopropyl. The tailing was more marked with the ethylpyridine column even with the more bulky alkyl substituents. Plate number N was a better measure than the asymmetry factor since some analyte peaks broadened without serious deterioration of symmetry compared to homologues.

Solving multicomponent chiral separation challenges using a new SFC tandem column screening tool

A tool for improved tandem column chiral supercritical fluid chromatography (SFC) method development screening was prepared by modification of a commercial analytical SFC instrument with two different software-controllable, six position high-pressure column selection valves, each controlling a bank of five different columns and a pass through line. The resulting instrument, which has the ability to screen 10 different individual columns and 25 different tandem column arrangements, is a useful tool for facilitating the screening of tandem column SFC arrangements for separation of complex mixtures of stereoisomers or other multicomponent mixtures. Strategies for optimal use of the instrument are discussed, and several examples of the use of the instrument in developing tandem SFC methods for resolution of multicomponent mixtures are presented.

Use of Achiral/Chiral SFC/MS for the Profiling of Isomeric Cinnamonitrile/Hydrocinnamonitrile Products in Chiral Drug Synthesis

A directly coupled achiral/chiral SFC/MS method has been developed for the profiling of a three-step stereoselective synthesis of cinnamonitrile and hydrocinnamonitrile intermediates. Semipurified reaction mixtures were screened in one step to determine the diastereomeric/enantiomeric composition of the final product as well as to identify any remaining E/Z isomers present from the starting material. The coupled achiral/chiral column combination was found to significantly enhance the separation of both enantiomers and diastereomers, without adding significantly to the overall analysis time. This analytical technique should prove to be generally useful for the profiling of isomeric reaction products in chiral drug synthesis.

Supercritical fluid chromatography tandem-column method development in pharmaceutical sciences for a mixture of four stereoisomers

A tandem-column method using Chiralpak AD-H and Chiralcel OD-H columns was achieved for baseline separation of a mixture of chiral pharmaceutical compounds (i.e., four stereoisomers) via supercritical fluid chromatography (SFC) with a mobile phase consisting of 90% liquid carbon dioxide and 10% ethanol:isopropanol (50:50 v/v). On the contrary, this mixture (mixture A) could not be baseline separated by SFC conditions explored with individual Chiralpak AD-H and Chiralcel OD-H columns. The effects of various mobile phases on elution order, capacity factor, selectivity, and resolution were determined with mixture A on the individual aforementioned columns to develop the tandem-column method.

Stereoselective chromatography of cardiovascular drugs: an update

This review reports the latest achievements in chromatographic enantioseparations of various classes of cardiovascular drugs and selected applications of these methods in pharmaceutical and clinical analysis. The use of these drugs as test compounds for new chiral stationary phases and different parameters of chromatographic processes is also presented.

Enantioseparations in super- and subcritical fluid chromatography

The separation of chiral compounds by sub- and supercritical fluid chromatography has been a field of great progress since the first demonstration of a chiral separation by SFC in 1985. Almost all of the chiral selectors used in gas or liquid chromatography have been successfully applied to sub-/supercritical chromatography. Easier and faster method development, high efficiency, superior and rapid separations of a wide variety of analytes, extended-temperature capability, analytical and preparative-scale equipment improvements and a selection of detection options have been reported.

Comparison of packed-column supercritical fluid chromatography--tandem mass spectrometry with liquid chromatography--tandem mass spectrometry for bioanalytical determination of (R)- and (S)-ketoprofen in human plasma following automated 96-well solid-phase extraction

The popularity of packed-column supercritical fluid, subcritical fluid, and enhanced fluidity liquid chromatographies (pcSFC) for enantiomeric separations has increased steadily over the past few years. The addition of a significant amount (typically 20-95%) of a viscosity lowering agent, such as carbon dioxide, to the mobile phase provides a number of advantages for chiral separations. For example, higher mobile-phase flow rates can often be attained without a concomitant loss in chromatographic efficiency since diffusion coefficients, and optimum velocities, are typically higher in pcSFC. Ultratrace enantioselective quantitation of drugs in biomatrixes is an ideal application for these chromatographic attributes. To demonstrate the utility of this approach, a pcSFC tandem mass spectrometry (pcSFC-MS/MS) method was compared to a LC-MS/MS method for quantitation of the (R)- and (S)-enantiomers of ketoprofen (kt), a potent nonsteroidal, anti-inflammatory drug, in human plasma. After preparation using automated solid-phase extraction in the 96-well format, kt enantiomers were separated on a Chirex 3005 analytical column using isocratic conditions. Validation data and study sample data from patients dosed with either orally or topically administered ketoprofen were generated using both pcSFC and LC as the chromatographic methods to compare and contrast these analytical approaches. Generally, most analytical attributes, including specificity, linearity, sensitivity, accuracy, precision, and ruggedness, for both of these methods were comparable with the exception that the pcSFC separation provided a roughly 3-fold reduction in analysis time. A 2.3-min pcSFC separation and a 6.5-min LC separation provided equivalent, near-baseline-resolved peaks, demonstrating a significant time savings for analysis of large batch pharmacokinetic samples using pcSFC.

Progress in packed column supercritical fluid chromatography: materials and methods

This article summarizes recent developments in packed column supercritical fluid chromatography. Silica-based chemically bonded sorbents, similar to those used for HPLC, are widely used with solvent-modified fluids containing additives to suppress undesirable solute-sorbent interactions that lead to poor peak shapes. Composition programming is the most useful approach to gradient elution separations since solvent-modified fluids have low compressibility. Packed column SFC is most useful for the separation of mixtures usually separated by normal-phase HPLC. Compared to normal-phase HPLC it offers faster separations, higher efficiencies, faster column re-equilibration, and a wider range of experimental variables for optimization. Packed column SFC is being increasingly selected for the analytical and preparative separation of racemic mixtures using enantiomer-selective sorbents.

Application of the thermally tuned tandem column concept to the separation of several families of environmental toxicants

Separations of several families of environmental toxicants were optimized by means of the thermally tuned tandem column (T3C) concept. We use a tandem combination of an octadecylsilane (ODS) and a carbon-coated zirconia (C-ZrO2) column; and tune the selectivity by independently adjusting the isothermal temperatures of the two columns. This results in the change in the contribution that each column makes to the overall retention and selectivity. The separation was optimized by locating the optimum pair of column temperatures which give the best separation of the critical solute pair. For both triazine herbicides and carbamate pesticides samples, dramatically different selectivities and different critical pairs were observed for the two types of phases. Although neither individual phase gave adequate separation, the T3C approach provided baseline separations using only four preliminary trial separations. We also showed that, for the triazine samples, the T3C approach gave a better separation than did conventional mobile phase optimization with an ODS column. The combination of superior selectivity of T3C and high flow rate allows the baseline separation of complex mixtures in just a few minutes.