Charlene Galea

Method development for impurity profiling in SFC: The selection of a dissimilar set of stationary phases

Supercritical fluid chromatography (SFC) is drawing considerable interest as separation technique in the pharmaceutical industry. The technique is already well established in chiral separations both analytically and on a preparative scale. The use of SFC as a technique for drug impurity profiling is examined here. To define starting conditions in method development for drug impurity profiling, a set of dissimilar stationary phases is screened in parallel. The possibility to select a set of dissimilar columns using the retention factors (k-values) for a set of 64 drugs measured on 27 columns in SFC was examined. Experiments were carried out at a back-pressure of 150 bar and 25 °C with a mobile phase consisting of CO2 and methanol with 0.1% isopropylamine (5-40% over 10 min) at a flow rate of 3 mL/min. These k-values were then used to calculate correlation coefficients on the one hand and to perform a principal component analysis on the other. The Kennard and Stone algorithm, besides dendrograms and correlation-coefficient colour maps were used to select a set of 6 dissimilar stationary phases. The stationary phase characterization results from this study were compared to those from previous studies found in the literature. Retention mechanisms for compounds possessing different properties were also evaluated. The dissimilarity of the selected subset of 6 stationary phases was validated using mixtures of compounds with similar properties and structures, as one can expect in a drug impurity profile.

Characterization and classification of stationary phases in HPLC and SFC – a review.

Packed column supercritical fluid chromatography (pSFC) is an attractive technique in drug discovery related analysis because it offers several advantages over the more commonly used high-performance liquid chromatography (HPLC) technique. The environmental-friendly CO2 mobile phase, the high-throughput capacity, the increased efficiency and the lower operational costs give SFC additional benefits over HPLC in analysis related to drug development. The latter technique is well established and has been used for decades in the pharmaceutical industry. On the other hand, SFC is still in its infancy, even though the technique has been known for decades and researchers are still discovering the possibilities and limitations of this technique. Column characterization aims to obtain a quantitative understanding of the properties of a column that influence the selectivity of a separation. Columns have been extensively characterized in HPLC using several approaches. However, limited column characterization has been done in SFC. Determining column properties allows the rapid selection of appropriate columns for method development for a particular application. Hence, they also show which columns will provide similar or dissimilar separations. The aim of this review is to compare and contrast approaches used to characterize stationary phases in HPLC and SFC, and to highlight points that still may need to be researched further.

Is the solvation parameter model or its adaptations adequate to account for ionic interactions when characterizing stationary phases for drug impurity profiling with supercritical fluid chromatography

Nine commercially available polar and aromatic stationary phases were characterized under supercritical fluid chromatographic (SFC) conditions. Retention data of 64 pharmaceutical compounds was acquired to generate models based on the linear solvation energy relationship (LSER) approach. Previously, adaptation of the LSER model was done in liquid chromatography by the addition of two solute descriptors to describe the influence of positive (D(+)) and negative (D(-)) charges on the retention of ionized compounds. In this study, the LSER models, with and without the ionization terms for acidic and basic solutes, were compared. The improved fits obtained for the modified models support inclusion of the D(+) and D(-) terms for pharmaceutical compounds. Moreover, the statistical significance of the new terms in the models indicates the importance of ionic interactions in the retention of pharmaceutical compounds in SFC. However, unlike characterization through the retention profiles, characterization of the stationary phases by modelling never explains the retention variance completely and thus seems less appropriate.

Investigation of the effect of mobile phase composition on selectivity using a solvent-triangle based approach in achiral SFC

HighlightsA solvent‐triangle based approach in SFC.Effect of modifier composition on selectivity.Optimization of mobile phase composition in achiral SFC. ABSTRACT Defining a method development methodology for achiral drug impurity profiling in SFC requires a number of steps. Initially, diverse stationary phases are characterized and a small number of orthogonal or dissimilar phases are selected for further method development. In this paper, we focus on a next step which is the investigation of the modifier composition on chromatographic selectivity. A solvent‐triangle based approach is used in which blends of organic solvents, mainly ethanol (EtOH), propanol (PrOH), acetonitrile (ACN) and tetrahydrofuran (THF) mixed with methanol (MeOH) are tested as modifiers on six dissimilar stationary phases. The tested modifier blends were composed to have equal eluotropic strengths as calculated on bare silica. The modifier leads to minor changes in terms of elution order, retention and mixture resolution. However, varying only the modifier composition on a given stationary phase does not lead to the creation of dissimilar systems. Therefore the modifier composition is an optimization parameter, with the stationary phase being the factor determining most the selectivity of a given mixture in achiral SFC.

Investigation of the effect of column temperature and back-pressure in achiral supercritical fluid chromatography within the context of drug impurity profiling

Twenty commercially available stationary phases were characterized in supercritical fluid chromatography (SFC) using a diverse set of pharmaceutical compounds. Six dissimilar phases were selected, and a benzodiazepine and a trimethoprim impurity mixture were screened on these phases. Two stationary phases were then selected for each mixture to study the effect of temperature and back-pressure on retention, separation and chromatographic efficiency using a response surface design approach. The maximal feasible domain for each phase was examined and column performance was monitored for stability during the duration of the study. Chromatographic responses of the individual mixture components, such as retention time, peak width and apparent plate count, were modelled as a function of temperature and back-pressure. The use of high temperatures led to improved separations and higher efficiencies while high back-pressures resulted in reduced retention. For the two mixtures, the response surface plots of the resolution of the worst-separated peak pair over the experimental domain allowed the identification of the temperature and back-pressure leading to the maximal resolution for the worst-separated peak pair. For the mixtures investigated, the use of high temperatures led to improved separations and high efficiencies, while high back-pressures resulted in reduced retention. These factors are fine-tuning parameters in SFC, and similarly to the modifier composition, they lead to local, rather than global selectivity differences.

Method optimization for drug impurity profiling in supercritical fluid chromatography: Application to a pharmaceutical mixture

A supercritical chromatographic method for the separation of a drug and its impurities has been developed and optimized applying an experimental design approach and chromatogram simulations. Stationary phase screening was followed by optimization of the modifier and injection solvent composition. A design-of-experiment (DoE) approach was then used to optimize column temperature, back-pressure and the gradient slope simultaneously. Regression models for the retention times and peak widths of all mixture components were built. The factor levels for different grid points were then used to predict the retention times and peak widths of the mixture components using the regression models and the best separation for the worst separated peak pair in the experimental domain was identified. A plot of the minimal resolutions was used to help identifying the factor levels leading to the highest resolution between consecutive peaks. The effects of the DoE factors were visualized in a way that is familiar to the analytical chemist, i.e. by simulating the resulting chromatogram. The mixture of an active ingredient and seven impurities was separated in less than eight minutes. The approach discussed in this paper demonstrates how SFC methods can be developed and optimized efficiently using simple concepts and tools.

Separation of Stereoisomers

Stereoisomers are isomeric molecules that have the same molecular constitution, but a different three-dimensional spatial arrangement of the atoms. These could be broadly classified as enantiomers and diastereomers. Different enantiomers bind to different receptors and possibly bring about different reactions. For this reason, stereoselective separation methods are valuable tools in the pharmaceutical, food, and agricultural fields. Liquid chromatography and supercritical fluid chromatography are the most popular techniques used for chiral separations. This chapter presents an overview of the different chromatographic parameters that are important when developing a chiral method in SFC. Different stationary phases and their properties are described and a comparison of SFC with other techniques, also suitable for enantioselective separations, is made. An overview of applications in the pharmaceutical, agricultural, and food industries, using SFC, from the last 10 years is presented. Diastereomers can be separated using classical separation methods, and a brief mention of these applications is also made.

First inter-laboratory study of a Supercritical Fluid Chromatography method for the determination of pharmaceutical impurities

Graphical abstract Figure. No Caption available. HighlightsInter‐laboratory study was conducted for the first time in SFC.This study involved 19 participating laboratories from 4 continents and 9 different countries.Results consistencies within‐ and between‐laboratories were deeply examined.The method reproducibility was estimated taking into account variances in replicates, between‐days and between‐laboratories.Repeatability and reproducibility variances were found to be similar or better than those described for LC methods. ABSTRACT Supercritical Fluid Chromatography (SFC) has known a strong regain of interest for the last 10 years, especially in the field of pharmaceutical analysis. Besides the development and validation of the SFC method in one individual laboratory, it is also important to demonstrate its applicability and transferability to various laboratories around the world. Therefore, an inter‐laboratory study was conducted and published for the first time in SFC, to assess method reproducibility, and evaluate whether this chromatographic technique could become a reference method for quality control (QC) laboratories. This study involved 19 participating laboratories from 4 continents and 9 different countries. It included 5 academic groups, 3 demonstration laboratories at analytical instrument companies, 10 pharmaceutical companies and 1 food company. In the initial analysis of the study results, consistencies within‐ and between‐laboratories were deeply examined. In the subsequent analysis, the method reproducibility was estimated taking into account variances in replicates, between‐days and between‐laboratories. The results obtained were compared with the literature values for liquid chromatography (LC) in the context of impurities determination. Repeatability and reproducibility variances were found to be similar or better than those described for LC methods, and highlighted the adequacy of the SFC method for QC analyses. The results demonstrated the excellent and robust quantitative performance of SFC. Consequently, this complementary technique is recognized on equal merit to other chromatographic techniques.

Chromatography: Supercritical Fluid Chromatography

Supercritical fluid chromatography is an analytical technique being used in several processes relating to food analysis and health analysis. This article explains the principles of the technique and goes into more detail regarding the analysis of lipids, fat-soluble vitamins, nutraceuticals, pesticides, and medicines. An overview of the technique at preparative scale and of supercritical fluid extraction is also included.