Potential of packed column supercritical fluid chromatography for the separation of metoprolol from closely related compounds

Synthesis and characterization of cellulose 3,5-dimethylphenylcarbamate silica hybrid spheres for enantioseparation of chiral β-blockers

A cellulose derivative-based chiral stationary phase (CSP) is considered one of the most widely applied CSPs due to its powerful enantioseparation ability. The high loading capacity and mechanical strength of CSPs are crucial for their application in preparative chromatography, such as a simulated moving bed. Compared to traditional cellulose-based CSPs that have been adsorbed onto chromatographic supports, organic-inorganic hybrid CSPs exhibit a potentially higher loading capacity and mechanical strength by increasing the density of chiral recognition groups. A hybrid cellulose 3,5-dimethylphenylcarbamate chiral stationary phase (organic/inorganic: 70/30, w/w) was prepared via a sol-gel method and characterized with several analytical techniques, including Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and (29)Si cross polarization/magic angle spinning nuclear magnetic resonance ((29)Si CP/MAS NMR). In addition, the as-synthesized hybrid chiral silica spheres were treated with an end-capping process to mask the residual silica hydroxyl groups. Compared to a commercial Chiralpak IB column, better separation of β-blocker drugs, including pindolol (selectivity of 5.55), metoprolol (2.30), propranolol (1.96), bisoprolol (1.74) and atenolol (1.46), on the end-capped CSP was achieved using liquid chromatography, which suggests that the packing material synthesized in this work has sufficient chiral discriminating ability for the effective separation of β-blocker drugs.

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.

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.

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.

Reaction monitoring of aliphatic amines in supercritical carbon dioxide by proton nuclear magnetic resonance spectroscopy and implications for supercritical fluid chromatography

In the recent years, it has repeatedly been stated that amines react with CO2 and can therefore not be chromatographed under supercritical conditions with CO2. The aim of the present work is to elucidate the structural requirements and conditions that can lead to the reaction of an amine analyte with CO2 and, if this occurs, the structure of the formed product. The use of on-line nuclear magnetic resonance (NMR) spectroscopy with a flow probe for supercritical fluid chromatography (SFC) enables the investigation of these unstable analytes in supercritical mediums. Several alkyl-substituted secondary benzylamines and some primary aromatic amines were dissolved in supercritical CO2 and investigated by employing on-line SFC-1H NMR spectroscopy. It was found that the condition of carbamic acid formation depends on the steric properties of the substituents of the amine. A 2-isopropylamino alcohol compound, metoprolol, was also investigated with the setup. No carbamic acid could be detected with the present conditions.

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.

Comparison of drug substance impurity profiles generated with extended length columns during packed-column SFC

The current study assesses the effect of extending column length during gradient packed column sub/supercritical fluid chromatography (PCSFC) experiments on the detection of known and unknown impurities in a drug substance sample. Quantitative drug substance impurity profiles were generated and compared using multiple column PCSFC and HPLC conditions. Also, chromatographic figures of merit were estimated and compared for components of a standard mixture during PCSFC experiments, which used one column, four columns, and six columns in series.

Tuning of mobile and stationary phase polarity for the separation of polar compounds by SFC

The separation of polar compounds by supercritical-fluid chromatography (SFC) is reviewed. New developments in mobile and stationary phase tuning are reviewed for packed and packed capillary SFC. In terms of mobile phase polarity adjustment, new pure and multiple component fluids are presented. The approach of tuning the polarity of the stationary phase as a way of increasing the range of polar compounds analyzed by SFC using pure CO(2) is discussed using either silica-based or new materials as stationary phase. Chiral, liquid crystal and polymer-based stationary phases coated on particles are widely covered in this review as an interesting approach to separate polar compounds avoiding the major drawbacks associated to the use of modifiers in SFC.