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

Supercritical fluid chromatography is not (only) normal-phase chromatography

Supercritical fluid chromatography (SFC), now using carbon dioxide as a major component of the mobile phase, has been known for over 60 years but still some misunderstandings remain about its capabilities. Amongst them, SFC is often described as a normal-phase chromatographic technique, based on different considerations: polarity of the stationary phase, elution order of the analytes, relative non-polarity of the mobile phase, non-linear retention behaviour, or adsorption retention mechanisms. All of these assumptions are true to a certain extent, and in certain circumstances. But also, all of these assumptions are wrong in different circumstances. In this paper, the criteria to categorize SFC as a normal-phase chromatographic method will be examined individually, considering all knowledge acquired from the early years of its development. Finally, it will appear that the "normal-phase" glass lens is greatly reducing the true extent of SFC's possibilities.

Ion pair-based mobile phase additives to improve the separation of alkaloids in supercritical fluid chromatography

In this study, a supercritical fluid chromatography (SFC) method based on ion pair reagents was used to separate alkaloids. The chromatographic parameters, including the stationary phase, additive type, additive concentration, outlet pressure, temperature and flow rate, were optimized. Baseline separation was completed in 20 min on an Agilent Pursuit 5 PFP column (4.6 × 150 mm) using carbon dioxide as the mobile phase and 7.5 mM sodium 1-pentanesulfonate as an additive with gradient elution at 140 bar, 60 °C, and a flow rate of 1.5 mL/min. The retention rate and resolution of the analytes were satisfactory. The limits of detection were 27.04-298.03 ng/mL, and the limits of quantification were 90.15-993.42 ng/mL. The recoveries of low and high concentrations were 77.46-111.86% and 83.84-111.00%, respectively. This ion pair additive greatly improved the separation efficiency of alkaloids. Consequently, this SFC method was successfully applied to the separation of alkaloids from Rhizoma corydalis.

Evaluation of chromatographic parameters in supercritical fluid chromatography of amino acids as model polar analytes and extended to polypeptide separations

Chromatographic parameters have been evaluated regarding the supercritical fluid chromatography of amino acids as model compounds for polar and chargeable substances of biochemical and biopharmaceutical interest. Their inherent hydrophilicity requires suitable chromatographic conditions regarding stationary and mobile phase to obtain suitable solubility and peak performance. Ten stationary phases differing in polarity and type of ligands including neutral and charged ones were investigated regarding selectivity, peak performance and complementarity/orthogonality. The mobile phase composition regarding solvent and type of buffer additives by varying the degree of hydrophobicity of the acids and bases and degree of substitution in case of the basic amines. Addition of water to the methanol solvent was furthermore found crucial for obtaining a high peak performance. Some complementary selectivity could be obtained depending on a choice of column. Temperature and pressure variations basically only influenced retention with no selectivity changes. Suitable conditions obtained for the amino acids were then successfully applied to oligo- and polypeptides enabling a separation of 4 polypeptides and differing in only one of the 39 amino acids, thereby mimicking analysis of biotransformation reaction products. The utilization of water as mobile phase additive and sulfonic acids as buffer agents could serve as a generic methodology for polar and charged compounds such as amino containing biomolecules.

Bioanalytical applications using supercritical fluid techniques

The bioanalytical applications of supercritical fluid techniques, such as supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC), are of increasing interest. The main role of these techniques is in the sample preparation and separation of biologically active compounds, particularly drugs and their metabolites, as well as endogenous compounds. An insight is given into the different types of extracting fluids and modifiers, detectors, stationary phases, mobile phases and collection strategies. A critical discussion is presented on the existing state of the art concerning the applications of SFC and SFE with a specific focus on its advantages and limitations in the bioanalytical field. New developments and the possibilities for routine work in the near future are also covered.

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.