Alexandre Grand-Guillaume Perrenoud

Modern analytical supercritical fluid chromatography using columns packed with sub-2 μm particles: a tutorial.

This tutorial provides an overview of the possibilities, limitations and analytical conditions of modern analytical supercritical fluid chromatography (SFC) using columns packed with sub-2 μm particles. In particular, it gives a detailed overview of commercially available modern SFC instrumentation and the detectors that can be employed (UV, MS, ELSD, FID, etc.). Some advice on the choice of the stationary phase dimensions and chemistries, the nature of the mobile phase (choice of organic modifier and additives) and its flow rate as well as the backpressure and temperature are also provided. Finally, several groups of potentially problematic compounds, including lipophilic compounds, hydrophilic substances and basic drugs, are discussed in detail. All these families of analytes can be resolved with SFC but require specific analytical conditions.

Comparison of ultra-high performance supercritical fluid chromatography and ultra-high performance liquid chromatography for the analysis of pharmaceutical compounds.

Currently, columns packed with sub-2 μm particles are widely employed in liquid chromatography but are scarcely used in supercritical fluid chromatography. The goal of the present study was to compare the performance, possibilities and limitations of both ultra-high performance liquid chromatography (UHPLC) and ultra-high performance supercritical fluid chromatography (UHPSFC) using columns packed with sub-2 μm particles. For this purpose, a kinetic evaluation was first performed, and van Deemter curves and pressure plots were constructed and compared for columns packed with hybrid silica stationary phases composed of 1.7 and 3.5 μm particles. As expected, the kinetic performance of the UHPSFC method was significantly better than that of the UHPLC. Indeed, the h(min) values were in the same range with both strategies and were between 2.2 and 2.8, but u(opt) was increased by a factor of >4 in UHPSFC conditions. Another obvious advantage of UHPSFC over UHPLC is related to the generated backpressure, which is significantly lower in the presence of a supercritical or subcritical fluid. However, the upper pressure limit of the UHPSFC system was only ∼400 bar vs. ∼1000 bar in the UHPLC system, which prevents the use of highly organic mobile phases at high flow rates in UHPSFC. Second, the impact of reducing the particle size (from 3.5 to 1.7 μm) was evaluated in both UHPLC and UHPSFC conditions. The effect of frictional heating on the selectivity was demonstrated in UHPLC and that of fluid density or decompression cooling was highlighted in UHPSFC. However, in both cases, a change in selectivity was observed for only a limited number of compounds. Third, various types of column chemistries packed with 1.7 μm particles were evaluated in both UHPLC and UHPSFC conditions using a model mixture of acidic, neutral and basic compounds. It has been shown that more drastic changes in selectivity were obtained using UHPSFC columns compared to those obtained by changing UHPLC columns. In addition, there was a good complementarity between the two separation modes. Finally, by combining the use of small particles with supercritical fluids as a mobile phase, it was possible to achieve the analysis of pharmaceutical compounds in less than 1 min or to attain a peak capacity of more than 250 in approximately 40 min, both with a high degree of repeatability.

Analysis of basic compounds by supercritical fluid chromatography: Attempts to improve peak shape and maintain mass spectrometry compatibility

While neutral and acidic compounds are well separated by supercritical fluid chromatography (SFC), basic analytes are more challenging to separate and often problems occur with their peak shapes. Two different methods were explored in the present paper to reduce these problems and maintain compatibility with mass spectrometry (MS). Five different, commercially available 2-ethylpyridine (2-EP) stationary phases were tested without a mobile phase additive using 92 pharmaceutical compounds with basic properties. The kinetic performances of the 5 columns were nearly identical, but the peak shapes of the basic drugs were strongly affected by the stationary phase. The PrincetonSFC 2-EP and Zymor Pegasus 2-EP phases clearly outperformed the other stationary phases, with 77% and 69% of the compounds having Gaussian peaks (and asymmetries between 0.8 and 1.4), respectively. Comparatively, the Waters Viridis Silica 2-EP, Waters Viridis BEH 2-EP and ES industries GreenSep 2-EP phases provided only 52%, 44% and 22% of the compounds with Gaussian peaks, respectively. These differences were attributed to the significant dissimilarities in their silica matrix properties. An alternative strategy was also performed with a hybrid silica stationary phase, Viridis BEH, using 20mM ammonium hydroxide in the mobile phase, which was a mixture of CO(2) and MeOH. With these conditions, 81% of the peaks observed for the basic analytes were Gaussian; however, this value dropped to 17% and 10% in the absence of additive and in the presence of 20mM formic acid, respectively. Finally, the use of a hybrid bare silica stationary phase in the presence of 20mM ammonium hydroxide is quite an interesting solution as this system is compatible with both ultra high performance SFC (UHPSFC) columns packed with sub-2 μm particles and with MS detection. The overall applicability of this system was demonstrated with various mixtures of basic drugs.

Coupling state-of-the-art supercritical fluid chromatography and mass spectrometry: From hyphenation interface optimization to high-sensitivity analysis of pharmaceutical compounds

The recent market release of a new generation of supercritical fluid chromatography (SFC) instruments compatible with state-of-the-art columns packed with sub-2μm particles (UHPSFC) has contributed to the reemergence of interest in this technology at the analytical scale. However, to ensure performance competitiveness of this technique with modern analytical standards, a robust hyphenation of UHPSFC to mass spectrometry (MS) is mandatory. UHPSFC-MS hyphenation interface should be able to manage the compressibility of the SFC mobile phase and to preserve as much as possible the chromatographic separation integrity. Although several interfaces can be envisioned, each will have noticeable effects on chromatographic fidelity, flexibility and user-friendliness. In the present study, various interface configurations were evaluated in terms of their impact on chromatographic efficiency and MS detection sensitivity. An interface including a splitter and a make-up solvent inlet was found to be the best compromise and exhibited good detection sensitivity while maintaining more than 75% of the chromatographic efficiency. This interface was also the most versatile in terms of applicable analytical conditions. In addition, an accurate model of the fluidics behavior of this interface was created for a better understanding of the influence of chromatographic settings on its mode of operation. In the second part, the most influential experimental factors affecting MS detection sensitivity were identified and optimized using a design-of-experiment approach. The application of low capillary voltage and high desolvation temperature and drying gas flow rate were required for optimal ESI ionization and nebulization processes. The detection sensitivity achieved using the maximized UHPSFC-ESI-MS/MS conditions for a mixture of basic pharmaceutical compounds showed 4- to 10-fold improvements in peak intensity compared to the best performance achieved by UHPLC-ESI-MS/MS with the same MS detector.

Maximizing kinetic performance in supercritical fluid chromatography using state-of-the-art instruments

Recently, there has been a renewed interest in supercritical fluid chromatography (SFC), due to the introduction of state-of-the-art instruments and dedicated columns packed with small particles. However, the achievable kinetic performance and practical possibilities of such modern SFC instruments and columns has not been described in details until now. The goal of the present contribution was to provide some information about the optimal column dimensions (i.e. length, diameter and particle size) suitable for such state-of the-art systems, with respect to extra-column band broadening and system upper pressure limit. In addition, the reliability of the kinetic plot methodology, successfully applied in RPLC, was also evaluated under SFC conditions. Taking into account the system variance, measured at ∼85μL(2), on modern SFC instruments, a column of 3mm I.D. was ideally suited for the current technology, as the loss in efficiency remained reasonable (i.e. less than 10% decrease for k>6). Conversely, these systems struggle with 2.1mm I.D. columns (55% loss in N for k=5). Regarding particle size, columns packed with 5μm particles provided unexpectedly high minimum reduced plate height values (hmin=3.0-3.4), while the 3.5 and 1.7μm packing provided lower reduced plate heights hmin=2.2-2.4 and hmin=2.7-3.2, respectively. Considering the system upper pressure limit, it appears that columns packed with 1.7μm particles give the lowest analysis time for efficiencies up to 40,000-60,000 plates, if the mobile phase composition is in the range of 2-19% MeOH. The 3.5μm particles were attractive for higher efficiencies, particularly when the modifier percentage was above 20%, while 5μm was never kinetically relevant with modern SFC instruments, due to an obvious limitation in terms of upper flow rate value. The present work also confirms that the kinetic plot methodology could be successfully applied to SFC, without the need for isopycnic measurements, as the difference in plate count between predicted and experimental values obtained by coupling several columns in series (up to 400mm) was on average equal to 3-6% and with a maximum of 13%.

Ultra high performance supercritical fluid chromatography coupled with tandem mass spectrometry for screening of doping agents. II: Analysis of biological samples

The potential and applicability of UHPSFC-MS/MS for anti-doping screening in urine samples were tested for the first time. For this purpose, a group of 110 doping agents with diverse physicochemical properties was analyzed using two separation techniques, namely UHPLC-MS/MS and UHPSFC-MS/MS in both ESI+ and ESI- modes. The two approaches were compared in terms of selectivity, sensitivity, linearity and matrix effects. As expected, very diverse retentions and selectivities were obtained in UHPLC and UHPSFC, proving a good complementarity of these analytical strategies. In both conditions, acceptable peak shapes and MS detection capabilities were obtained within 7 min analysis time, enabling the application of these two methods for screening purposes. Method sensitivity was found comparable for 46% of tested compounds, while higher sensitivity was observed for 21% of tested compounds in UHPLC-MS/MS and for 32% in UHPSFC-MS/MS. The latter demonstrated a lower susceptibility to matrix effects, which were mostly observed as signal suppression. In the case of UHPLC-MS/MS, more serious matrix effects were observed, leading typically to signal enhancement and the matrix effect was also concentration dependent, i.e., more significant matrix effects occurred at the lowest concentrations.

Ultra high performance supercritical fluid chromatography coupled with tandem mass spectrometry for screening of doping agents. I: Investigation of mobile phase and MS conditions

The conditions for the analysis of selected doping substances by UHPSFC-MS/MS were optimized to ensure suitable peak shapes and maximized MS responses. A representative mixture of 31 acidic and basic doping agents was analyzed, in both ESI+ and ESI- modes. The best compromise for all compounds in terms of MS sensitivity and chromatographic performance was obtained when adding 2% water and 10mM ammonium formate in the CO2/MeOH mobile phase. Beside mobile phase, the nature of the make-up solvent added for interfacing UHPSFC with MS was also evaluated. Ethanol was found to be the best candidate as it was able to compensate for the negative effect of 2% water addition in ESI- mode and provided a suitable MS response for all doping agents. Sensitivity of the optimized UHPSFC-MS/MS method was finally assessed and compared to the results obtained in conventional UHPLC-MS/MS. Sensitivity was improved by 5-100-fold in UHPSFC-MS/MS vs. UHPLC-MS/MS for 56% of compounds, while only one compound (bumetanide) offered a significantly higher MS response (4-fold) under UHPLC-MS/MS conditions. In the second paper of this series, the optimal conditions for UHPSFC-MS/MS analysis will be employed to screen >100 doping agents in urine matrix and results will be compared to those obtained by conventional UHPLC-MS/MS.

Evaluation and comparison of various separation techniques for the analysis of closely-related compounds of pharmaceutical interest.

The aim of the present work was to compare various separation techniques for the fast analysis of closely-related compounds, including structurally-related compounds, positional isomers, diastereoisomers, Z/E isomers. Three analytical techniques were evaluated, namely ultra high performance liquid chromatography (UHPLC), ultra high performance supercritical fluid chromatography (UHPSFC), both with sub-2μm particles, and capillary electrophoresis (CE) using non-aqueous solvents. To fairly compare the three analytical techniques, only two starting conditions for further method development were considered. All the selected mobile phases or background electrolyte were MS-compatible. As expected, CE often provided excellent results for the analysis of basic compounds but it was difficult to find out conditions that could be widely applied. On the other hand, UHPLC and UHPSFC were more generic and the performance was better than CE for the analysis of neutral and acidic compounds. In all cases, the analysis time was systematically lower than 3min. In conclusion, UHPLC was the most versatile strategy for the analysis of closely-related compounds and should be tested in a first instance. UHPSFC and CE approaches offered some drastic changes in selectivity and should be considered a second choice to reach alternative selectivity as they also allow high throughput separations.

Evaluation of various chromatographic approaches for the retention of hydrophilic compounds and MS compatibility

The goal of this study was to compare the performance of three separation techniques for the analysis of 57 hydrophilic compounds. RPLC, hydrophilic interaction liquid chromatography (HILIC) and subcritical fluid chromatography (SFC) were tested. The comparison was based on the retention, selectivity, peak shape (asymmetry and peak width) and MS sensitivity. As expected, RPLC had some obvious limitations for such classes of compounds, and on average the %ACN required to elute these hydrophilic substances was 4, 7, and 11% ACN at pH 3, 6, and 9, respectively. However, a hybrid polar-embedded C18 phase with an appropriate mobile phase could represent a viable strategy for hydrophilic basic compounds with log D greater than -2 on average. HILIC and SFC were found to be more appropriate for analyzing a large majority of these hydrophilic analytes (~60 and 70% of compounds eluted during the gradient in HILIC and SFC), while maintaining good MS sensitivity. Finally, this work demonstrated the complementarity of the three analytical techniques and showed that the selection of a suitable strategy should mostly be based on physicochemical properties of the analytes (pKa, log D, H-bonding capability, etc.).

Comparison of liquid chromatography and supercritical fluid chromatography coupled to compact single quadrupole mass spectrometer for targeted in vitro metabolism assay

The goal of this study was to evaluate the combination of powerful chromatographic methods and compact single quadrupole MS device for simple in vitro cytochrome P450 (CYP) inhibition assay, instead of more expensive triple quadrupole MS/MS detectors. For this purpose, two modern chromatographic approaches (ultra-high pressure liquid chromatography (UHPLC) and ultra-high performance supercritical fluid chromatography (UHPSFC)) were tested in combination with simple MS detector. To show the applicability for an in vitro CYP-mediated metabolism assay using the cocktail approach, a method was first developed in UHPLC-MS to separate a mixture of 8 probe substrates and 8 CYP-specific metabolites. A screening procedure was initially applied to determine the best combination of a column, an organic modifier and a mobile-phase pH, followed by fine tuning of the conditions (i.e., gradient profile, temperature and pH) using HPLC modelling software. A similar sequential method development procedure was also evaluated for UHPSFC-MS. For method development, where peak tracking is necessary, the use of single quadrupole MS was found to be extremely valuable for following the investigated analytes. Ultimately, a baseline separation of the 16 compounds was achieved in both UHPLC-MS and UHPSFC-MS with an analysis time of less than 7 min. In a second series of experiments, sensitivity was evaluated, and LOQ values were between 2 and 100 ng/mL in UHPLC-MS, while they ranged from 2 to 200 ng/mL in UHPSFC-MS. Based on the concentrations employed for the current in vitro phase I metabolism assay, these LOQ values were appropriate for this type of application. Finally, the two analytical methods were applied to in vitro CYP-dependent metabolism testing. Two well-known phytochemical inhibitors, yohimbine and resveratrol, were investigated, and reliable conclusions were drawn from these experiments with both UHPLC-MS and UHPSFC-MS. At the end, the proposed strategy of optimized chromatography combined with simple MS device has been shown to potentially compete with the widely used combination of generic chromatography and highly selective MS/MS device for simple in vitro CYP inhibition assays. In addition, our analytical method may be easier to use in a routine environment; the instrument cost is significantly reduced and the two developed methods fit for purpose.