Sub- and supercritical fluid chromatography on packed columns: a versatile tool for the enantioselective separation of basic and acidic drugs

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.

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.

Role of Stereoselective Assays in Bioequivalence Studies of Racemic Drugs: Have We Reached a Consensus?

The existence of stereoselectivity in metabolism and drug disposition, coupled with the existence of genetic polymorphisms and modulation of enantiomeric kinetics via special delivery systems, provides some compulsion to assess bioequivalence using stereoselective data. However, examination of the literature suggests that nonstereoselective data are commonly used for the bioequivalence assessment of drug racemates.

Ion-pair supercritical fluid chromatography of metoprolol and related amino alcohols on diol silica

In this paper, a chromatographic system based on carbon dioxide with methanol as mobile phase, and diol silica as stationary phase has been investigated for metoprolol and related amino alcohols by addition of strong acids to systems with triethylamine base as primary additive. Standard conditions used were 10% of methanol, containing 24 mM of acid and 18 mM of triethylamine, in carbon dioxide with a flow rate of 1.5 ml min(-1). The column dimensions were 125 mm x 4 mm I.D. and kept at 40 degrees C with a back pressure of 150 bar. Effects on selectivity were stronger with trifluoroacetic acid than with ethanesulfonic acid. From a large set of related analytes, it was shown that selectivity changes were significant when the structure close to the nitrogen of the amino alcohol analyte differed. The stability of the column in the short time perspective was examined and it showed negligible changes. For a diastereoisomeric pair, not resolved in a basic system with triethylamine nor by addition of ethanesulfonic acid, resolution improved to about 2.1 with trifluoroacetic acid. The described approach offers a way to tune the selectivity of SFC systems when amines are analyzed without the need to change stationary phase for the chromatographic separation.

Chiral separation of amines in subcritical fluid chromatography using polysaccharide stationary phases and acidic additives

The chiral separation of basic compounds by subcritical fluid chromatography (SFC) is often unsuccessful, due possibly to multiple interactions of the analyte with the mobile and stationary phase. Incorporation of a strong acid, ethanesulfonic acid (ESA), into the sample diluent and mobile phase modifier gives a dramatic improvement in these separations. Screening with ethanol containing 0.1% ESA on CHIRALPAK AD-H gave separation of 36 of 45 basic compounds previously not separated in SFC. The mechanism appears to involve the separation of an intact salt pair formed between the basic compound and ESA. Other modifiers, other acids and one additional stationary phase were examined and found to yield additional separations.

Sub- and Supercritical Chiral Separation of Racemic Compounds on Columns with Stationary Phases Having Different Functional Groups

Separation of the enantiomers of each of three different racemates, neutral rac-alpha-tetralol, acidic rac-2-phenylpropionic acid, and basic rac-1-phenylethylamine, using subcritical and supercritical fluid chromatography with two different chiral stationary phases, heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (Sumichiral OA-7500 column) and tris(3,5-dimethylphenylcarbamate) of amylose (Chiralpak AD-H column), was compared. The elution order of the enantiomers of the three racemates was determined, and the effects of the type of alcohol modifier, column oven temperature, mobile phase composition, flow rate, and pressure were examined. The most appropriate column oven temperature depended on both the type of alcohol modifier and the compound analyzed. Lower alcohol content improved the peak separation of both rac-alpha-tetralol on the Sumichiral OA-7500 column and rac-1-phenylethylamine on the Chiralpak AD-H column, while the same phenomenon was not observed with either rac-alpha-tetralol or rac-2-phenylpropionic acid on the Chiralpak AD-H column. Decreasing outlet pressure improved the peak separation obtained with rac-2-phenylpropionic acid, but had little effect on either rac-alpha-tetralol or rac-1-phenylethylamine.

Reversal of elution order for profen acid enantiomers in packed-column SFC on Chiralpak AD

Enantiomeric separations of four 2-substituted propionic acid drugs have been studied using packed-column supercritical fluid chromatography (SFC) with amylose tris(3,5-dimethylphenylcarbamate) coated on silica as support (Chiralpak AD). Under standard conditions (i.e., flow rate, 1.5 ml/min; column temperature, 30 degrees C; back-pressure, 150 bar), the order of elution could be reversed when the polar alcohol modifier methanol in carbon dioxide was replaced by 2-propanol for ibuprofen, ketoprofen, and naproxen. For flurbiprofen, with the highest selectivity factor, no reversal was observed, although selectivity was reduced significantly with higher alcohols. Naproxen and flurbiprofen were also investigated with 2-butanol and 2-pentanol. The former showed reversal of elution order but not the latter. For higher alcohol modifiers, including 2-propanol, the peak symmetry was poor but could be improved by addition of citric acid in the alcohol modifier. These results stress the importance to investigate enantiomer elution order during the development of enantioselective methods and when chromatographic conditions are optimized. Preliminary experiments with column temperatures over the range of -15 to 45 degrees C revealed that, in a few cases, reversal took place with a change in temperature only.

Effect of amine mobile phase additives on chiral subcritical fluid chromatography using polysaccharide stationary phases

Increased retention and selectivity in the subcritical fluid chromatography (SFC) of various amine compounds on polysaccharide chiral stationary phases (CSP) was observed upon incorporation of cyclic amines into the modifier. The retention increases are most pronounced with 2-propanol and are almost absent when methanol is used as modifier. This suggests that the effect may arise from a restriction to the modifier access to the binding site required to effect elution. The effect of the amine additives in SFC does not remain after their removal from the mobile phase. Findings were applied to the development of a 5 min separation of amphetamine and methamphetamine enantiomers.

Packed column supercritical fluid chromatography of a peroxysome proliferator-activating receptor agonist drug

This paper describes packed column supercritical fluid chromatography (SFC) for the analysis of a peroxysome proliferator-activating gamma-receptor agonist that is a carboxylic acid. Evaluation of conditions for the separation of this candidate drug and related compounds in bulk substance is described. A Chiralcel OD column was used for this purpose due to its high selective retention of related substances and relative inertness, though the enantioselectivity was negligible, with methanol as polar modifier. A high enantioselectivity was obtained on Chiralpak AD and it was possible to determine the enantiomeric purity within 10 min on a 5 cm short column. Both the achiral and the chiral systems were run without acid additive in the mobile phase and the level of detection of impurities by area was about 0.1%. For the analysis of samples dissolved in water, without any isolation step, 2-propanol was used as modifier. Due to the column surface activity, evidently generated by injected water, citric acid 1 mM was included as additive in the 2-propanol in order to maintain symmetric and undistorted peak shape. The detection limit for the assay was 21 microg mL(-1) (50 nmol mL(-1)) for 5 microL injected (R.S.D. 6.4%, n = 8). A 5 cm short Chiralcel OD column was used. Determination of enantiomeric purity of the drug in aqueous samples required increased sensitivity. The sample was acidified and extracted into a small volume of 1-pentanol, out of which 25 microl was analyzed by SFC. The minor enantiomer at the 3% (w/w) level added could be confirmed. Its ratio remained constant during the procedure as measured relative to a reference solution in organic media.

Enantiomeric separations of amino alcohols by packed-column SFC on Hypercarb with L-(+)-tartaric acid as chiral selector

The use of L-(+)-tartaric acid as a chiral mobile phase additive (CMPA) has been investigated in a packed-column SFC system. The CMPA, carbon dioxide, and methanol, containing a high concentration of aliphatic amine additive, were used as the mobile phase and Hypercarb as support [Gyllenhaal O., Karlsson A., SFC of metoprolol and other amino alcohols on Hypercarb (in preparation)]. Good enantioselectivities were obtained for tertiary amine homologues of 2-amino alcohols, used as beta-adrenoreceptor-blocking drugs. Moderate selectivities were observed for aromatic compounds having a second substituent in the ortho-position. The overall retention was influenced by the aromaticity of the analytes as well as the presence of free electron pairs in the molecule. Increased concentrations of CMPA gave higher retention and also increased the enantioselectivity. The practical utility of this present enantioselective system was demonstrated on one batch of (S)-metoprolol that was N-methylated with methyl iodide. The enantiomeric separation was accomplished within 10 min.

Separation of albendazole sulfoxide enantiomers by chiral supercritical-fluid chromatography

The enantioseparation of albendazole sulfoxide (ABZSO) by chiral supercritical-fluid chromatography (SFC) on two columns, based on the polysaccharide derivatives Chiralpak AD and Chiralcel OD, was studied. The effect of different modifiers, methanol, ethanol, 2-propanol, and acetonitrile, was examined. The results showed that ABZSO can be separated on both columns, using an alcohol-type modifier. Using the Chiralpak AD column, the best results were obtained with 2-propanol and, in the case of the Chiralcel OD, with methanol.

Determination of the enantiomerization energy barrier of some 3-hydroxy-1,4-benzodiazepine drugs by supercritical fluid chromatography

The first-order kinetic equation for irreversible reactions was used to determine the enantiomerization barrier of some of 3-hydroxy-1,4-benzodiazepine enantiomers by supercritical fluid chromatography (SFC). The racemates of lorazepam, oxazepam and temazepam were separated by SFC on chiral (R,R)-Whelk-O1 column with supercritical carbon dioxide containing 12.5% methanol and 0.5% diethylamine as a mobile phase. Peak areas of enantiomers prior to (A(A0), A(B0)) and after the separation (A(A), A(B)), used for calculation of the enantiomerization barrier, were determined by computer-assisted peak deconvolution of peak clusters from the chromatograms. It was demonstrated for the first time that using a model for a four-peak cluster produces height precise results, and most closely approximates the published results. The kinetic equation for irreversible reactions was used to determine apparent enantiomerization rate constants. The dependence of the apparent enatiomerization barrier (deltaG(app)(A-->B), deltaG(app)(B-->A)) on temperature was used to determine apparent activation enthalpy (deltaH(app)(R-->S), deltaH(app)(S-->R)) and entropy (deltaS(app)(R-->S), deltaS(app)(S-->R)) for all studied benzodiazepines.

Separation of enantiomers of ibuprofen on chiral stationary phases by packed column supercritical fluid chromatography

A packed column supercritical fluid chromatography (SFC) method for the separation of ibuprofen enantiomers on a chiral stationary phase and CO2 with modifier as mobile phase has been developed at an analytical scale. Among 11 different stationary phases the Kromasil CHI-TBB phase showed by far the best separation properties. The influence of different modifiers, injection solvents, temperature, and pressure, and density of the fluid, respectively, on the separation behavior has been studied. It was found that the separation behavior strongly depends on the type of modifier and the modifier content. Temperature and pressure are of less influence.

Applications of the Chiralpak AD and Chiralcel OD chiral columns in the enantiomeric separation of several dioxolane compounds by supercritical fluid chromatography

Two chiral columns based on polysaccharide derivatives (Chiralpak AD and Chiralcel OD) have been tested for the chiral separation of several dioxolane compounds, using supercritical fluid chromatography. The compounds studied included ketoconazole and some of its precursors. The effect of the different modifiers and the pressure, on the chromatographic parameters was also evaluated. In general, the alcohol modifiers provided better results than acetonitrile, and all the compounds could be separated with these two columns, but the selection of the column depends on the kind of compound.

Fast enantioseparations of basic analytes by high-performance liquid chromatography using cellulose tris(3,5-dimethylphenylcarbamate)-coated zirconia stationary phases

In this work, we study the influence of the mobile phase and column temperature on the enantioresolution of basic compounds on microparticulate porous zirconia coated with cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC). The chiral analytes are amino compounds, including a number of beta-blockers. Analytes are eluted with hexane-alcohol mobile phases. We investigated the effect of alcohol (type and concentration), basic eluent additives, and column temperature on the parameters that control resolution (column efficiency, retention and selectivity). Conditions for achieving an adequate separation in the least time have been determined for numerous racemic mixtures. For most solutes, baseline resolution of the enantiomeric pair was achieved in less than 1 min; 12 of 13 pairs were separated in less than 2 min.

Development of a packed-column supercritical fluid chromatography/atmospheric pressure chemical-ionisation mass spectrometric technique for the analysis of atropine

A packed-column supercritical fluid chromatography/atmospheric pressure chemical-ionisation mass spectrometry (pSFC-APCI/MS) method has been developed for the determination of atropine from Atropa belladonna L extracts. The technique does not require any kind of derivatisation prior to the analysis. The optimum conditions were studied by using the pure substance in methanol (MeOH). All samples were simply dissolved in MeOH and injected into the mobile phase. Detection was achieved by using mass spectrometry (MS) with atmospheric pressure chemical ionisation (APCI). Terbutaline was used as an internal standard for the determination of the analytical reproducibility. The supercritical carbon dioxide (scCO(2)) mobile phase was modified by 15% MeOH containing 0.5% trifluoroacetic acid (TFAA) and 0.5% diethylamine (DEA) additives. Concentrations of atropine were determined with a relative standard deviation of less than 1% by the pSFC-APCI/MS procedure for a sample containing atropine and terbutaline. The correlation coefficient was 0.997 and detection limit 700 pg. The absolute retention time was 9.87 min with a standard deviation of 5.2x10(-3) min and a relative standard deviation of 0.61% with respect to terbutaline.

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.

Determination of the 2-bromomethyl-2-(2,4-dichlorophenyl)-1, 3-dioxolan-4-yl)methyl benzoate diastereoisomers by supercritical fluid chromatography

The study of the separation and determination of the two 2-bromomethyl-2-(2,4-dichlorophenyl)-1,3-dioxolan-4-yl-methyl benzoate diastereoisomers is presented in this work. The separation was achieved using packed column supercritical fluid chromatography. Three stationary phases were checked, the C(18) packing providing the best results. The influence of several parameters (nature and percentage of modifier, temperature and pressure) on the separation were also evaluated. The baseline separation (R(s)=1.78) of the compounds was obtained in only 2 min, using a C(18) column, a pressure of 100 bar, a temperature of 35 degrees C, a flow-rate of 2. 5 ml/min and 5% of 2-propanol as organic modifier. Under these conditions the unwanted diastereoisomer (trans) was eluted before the main compound (cis), so the determination of the purity was more accurate.

Separation of drugs by packed-column supercritical fluid chromatography

Packed-column supercritical fluid chromatography (pSFC) has been expected to analyze various kinds of compounds. Many researchers have expected a new chromatographic technique that overcomes the limitations of other techniques, HPLC and GC. In pharmaceutical development, chromatography plays an important role in the evaluation of safety and efficacy of a new compound. This article provides an overview of the separation of drugs by pSFC. The effects of the chromatographic parameters were studied for the separation of steroids. In chiral separation, the successful results were shown and compared with HPLC.

Separation of ketoconazole enantiomers by chiral subcritical-fluid chromatography

The separation of ketoconazole enantiomers by subcritical-fluid chromatography using an amylose-based column is described. Drastic changes in the resolution have been obtained for the different organic modifiers evaluated, with ethanol providing the best results. Other chromatographic parameters such as temperature, pressure and flow-rate have also been studied. The best results in terms of resolution and analysis time were obtained using 30% ethanol (containing 0.1% triethylamine and 0.1% trifluoroacetic acid), a pressure of 300 bar, a temperature of 35 degrees C and a flow-rate of 3 ml/min. Under these conditions the ketoconazole enantiomers are resolved in a short time (less than 7 min) and with high resolution (4.29).

Chiral separation of four 1,3-dioxolane derivatives by supercritical fluid chromatography on an amylose-based column

The chiral separation of four 1,3-dioxolane derivatives by supercritical fluid chromatography on an amylose-based column is described. The effects of mobile phase composition, temperature and pressure have been investigated. The nature of the modifier is the parameter which has the highest impact on the chiral resolution and it is more important than the polarity of the mobile phase. The organic modifier used for the best enantiomeric separation was different for each compound, because it depends strongly on the molecular structure of the compound.

Direct, preparative enantioselective chromatography of propranolol hydrochloride and thioridazine hydrochloride using carbon dioxide-based mobile phases

In this paper, we describe the direct, preparative enantioselective chromatography of racemic (rac)-propranolol hydrochloride (HCI) and rac-thioridazine.HCl using Chiralpak AD chiral stationary phase and mobile phase systems containing carbon dioxide and methanol without the use of basic or acidic additives. Isolated fractions of propranolol.HCl were positively identified by mass spectrometry, Beilstein flame test, melting point, and chemical analysis to be HCI enantiomers of propranolol-HCl salts exhibited characteristic mass spectra peaks at 36 and 38 mass-to-charge ratio in the expected 3:1 isotopic ratio for the solute that were absent in the mass spectra for the free-base forms. To our knowledge, the direct, preparative enantioselective isolation of HCI enantiomeric salts of rac-propranolol and of rac-thioridazine have not been previously demonstrated and published.

Supercritical fluids in separation science--the dreams, the reality and the future

The last 20 years have seen an intense interest in the use of supercritical fluids in separation science. This started with the introduction of commercial instruments first for packed and then for capillary chromatography and it looked as if this would be a technique to rival gas-liquid chromatography and HPLC. The activity developed quite rapidly into packed column supercritical fluid separations then into supercritical fluid extraction. However, in recent years there has been a decline in publications. These later techniques continue to be used but are now principally applied to a limited group of applications where they offer significant advantages over alternative techniques. This review looks back over this period and analyses how these methods were developed and the fluids, detectors and applications that were examined. It suggests why many of the initial applications have vanished and why the initial apparent promise was not fulfilled. The rise and fall of supercritical fluids represents a lesson in the way analysts approach new techniques and how we might view other new separation developments at the end of this millennium. The review looks forward to the future of supercritical fluids and their role at the end of the first century of separation science. Probably the most important idea that supercritical fluids have brought to separation science is a recognition that there is unity in the separation methods and that a continuum exists from gases to liquids.

Recent chromatographic and electrophoretic enantioseparations of cardiovascular drugs

The chromatographic and electrophoretic enantiomeric separation and analysis of several clinically used cardiovascular drugs have been reviewed. Several examples of recently reported applications of enantioselective analysis and various cardiovascular agents are presented.

Supercritical fluid chromatography in forensic science: a critical appraisal

The application of supercritical fluid chromatography (SFC) in forensic science is reviewed. The applications centre on the analysis of explosives and of drugs of abuse. The systems employed are discussed in the context of comparison with gas chromatography and high-performance liquid chromatography methods which are traditionally used for such analyses. The advantages and disadvantages of SFC over these methods are discussed. Recommendations are made for the developments which are required in SFC technology if it is to find greater application in forensic science.