Preparative enantioseparations using supercritical fluid chromatography

A Dual Column pH Switchable Water Stationary Phase System for Separation Control in Supercritical Fluid Chromatography

A dual column system comprised of a pH switchable water stationary phase column and a conventional non-polar capillary column is introduced for use in Supercritical Fluid Chromatography (SFC). By removing or adding NH4OH to the system hydration source, the water stationary phase pH can be rapidly switched between acidic (measured at pH∼3) and basic (measured at pH∼9) in seconds, while the operating character of the conventional column is unchanged. This switch modulates the velocity of ionizable analytes about 20-fold in the system, whereas non-ionizable analytes are not affected. In this way, the retention time of acids and/or bases can be reproducibly altered (<1% RSD; n = 3) in SFC separations. As a result, analyte selectivity and resolution can be readily controlled during analyses. For example, a selectivity reversal (alpha from 0.4 to 1.6) and a resolution increase (from 0 to 13) are demonstrated. Rapid stationary phase pH switching also allows multiple acids, bases, and/or neutral analytes to be determined simultaneously. Applications demonstrate that this method can greatly simplify complex mixture analysis in SFC by helping to separate target analytes from interfering matrix components.

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.

Highly sensitive determination of multiple pesticide residues in foods by supercritical fluid chromatography coupled with ion mobility quadrupole time-of-flight mass spectrometry

This experiment aimed to establish a green, simple and highly sensitive method (supercritical fluid chromatography (SFC) coupled with ion mobility quadrupole time-of-flight mass spectrometry (IM-Q-TOF/MS)) for the detection of multiple pesticides in foods. During the experiments, several important SFC parameters, such as stationary phase, modifier, make-up solution, back-temperature and back-pressure were optimized. Here, single-field collision cross section (CCS) values and multifield CCS values of 20 pesticides were examined by IM-Q-TOF/MS as highly specific parameters with excellent experimental precision. In addition, based on accurate mass matching and fragment ion comparison, mass fragments were obtained by IM-Q-TOF/MS, which elucidated the regularities of compound structure and characteristic fragment ions. Under the optimized conditions, satisfactory linearity (R2 ≥ 0.9989) and recoveries (79.60 % to 112.97 %) were obtained. The intra- and interday precisions were favorable, with RSDs lower than 4.91 and 7.65 %, respectively. Additionally, the method showed low limits of detection (0.1-8.8 ng/mL). The proposed method has been successfully applied to the highly sensitive detection of phenylurea herbicide, triazine herbicides, organophosphorus pesticide, pyrethroid insecticide and acaricide in yam and potato.

Analytical and preparative chiral supercritical fluid chromatography resolutions using crown ether-derived column

In this study, crown ether-derived column Crownpak® CR-I (+) was evaluated under SFC conditions using 12 primary amines, and the chromatographic results were compared against eight immobilized polysaccharide-based columns. Crownpak® CR-I (+) achieved a significantly higher success rate. It was found that the addition of 5% water to the modifier dramatically improved the peak shape for chiral separation of primary amines on Crownpak® CR-I (+). The first reported preparative SFC separations on Crownpak® CR-I (+) are shown, offering a new approach for the preparative resolution of primary amines. The case studies demonstrate that Crownpak® CR-I (+) is a very useful column in the chiral separation of challenging compounds that contain a primary amine group in the pharmaceutical industry.

A novel switchable water stationary phase for supercritical fluid chromatography

A novel pH switchable water stationary phase is presented for use in supercritical fluid chromatography (SFC). By adding NH4OH to the water coating and system hydration, changes in CO2 pressure and temperature allow a wide range of stationary phase pH conditions (∼3-9) to be achieved, which impact analyte retention properties. For example, 100 atm and 50 °C produces an acidic water stationary phase (pH near 4.0) where octanoic acid readily elutes while the base caffeine does not. Conversely, at 80 atm and 120 °C a basic water stationary phase (pH near 8.0) is obtained and the opposite occurs. Further, under constant pressure and temperature conditions, simply adding or removing NH4OH from the system is also found to readily allow switching between the basic and acidic water stationary phase modes and demonstrates control over ionizable analyte elution. For instance, hexanoic acid elution is near 40 times more delayed on a basic water stationary phase and, as such, it can be eluted at later points in time as desired by removing the NH4OH and switching to an acidic stationary phase. Experiments indicate that stationary phase pH switching occurs uniformly across the 15 m column length within about 18 s and that analyte retention times are very reproducible upon performing a switch (1.4% RSD; n = 3). Results demonstrate the selectivity factor between acidic and neutral analytes can be reversed and increased about 35 times, while in other trials resolution also similarly increased near 40-fold. By rapidly switching the stationary phase pH back and forth between acidic and basic modes, the selectivity between ionizable analytes could also be increased as desired. Various applications with the system show that it can vastly increase the separation between target analytes and matrix components as required by the dynamics of a particular separation.

Supercritical Fluid Nanospray Mass Spectrometry

Supercritical fluids are typically electrosprayed using an organic solvent makeup flow to facilitate continuous electrical connection and enhancement of electrospray stability. This results in sample dilution, loss in sensitivity, and potential phase separation. Premixing the supercritical fluid with organic solvent has shown substantial benefits to electrospray efficiency and increased analyte charge state. Presented here is a nanospray mass spectrometry system for supercritical fluids (nSF-MS). This split flow system used small i.d. capillaries, heated interface, inline frit, and submicron emitter tips to electrospray quaternary alkyl amines solvated in supercritical CO₂ with a 10% methanol modifier. Analyte signal response was evaluated as a function of total system flow rate (0.5-1.5 mL/min) that is split to nanospray a supercritical fluid with linear flow rates between 0.07 and 0.42 cm/sec and pressure ranges (15-25 MPa). The nSF system showed mass-sensitive detection based on increased signal intensity for increasing capillary i.d. and analyte injection volume. These effects indicate efficient solvent evaporation for the analysis of quaternary amines. Carrier additives generally decreased signal intensity. Comparison of the nSF-MS system to the conventional SF makeup flow ESI showed 10-fold signal intensity enhancement across all the capillary i.d.s. The nSF-MS system likely achieves rapid solvent evaporation of the SF at the emitter point. The developed system combined the benefits of the nanoemitters, sCO₂, and the low modifier percentage which gave rise to enhancement in MS detection sensitivity.

Insights into enantioselective separations of ionic metal complexes by sub/supercritical fluid chromatography

Sub/supercritical fluid chromatography (SFC) is a green separation technique that has been used to separate a wide variety of compounds and is proven to be immensely useful for chiral separations. However, SFC is currently not thought to be applicable for ionic compounds due to their low solubility in CO2, even with additives and organic modifiers. Recently, a large amount of research has been centered on octahedral complexes of Ru(II) and Os(II) with bidentate polypyridyl ligands due to their ability to serve in cancer treatment and other biological activities. These compounds exist as the delta (Δ) and lambda (Λ) enantiomers. Previously, similar compounds have been enantiomerically separated using HPLC and capillary electrophoresis, but never with SFC. Cyclofructan-6 (CF6) derivatized with (R)-naphthyl ethyl (RN) groups has been proven to be an effective chiral stationary phase for these separations in HPLC. This column chemistry was expanded to SFC to provide the first chiral separation of a wide variety (23 complexes in total) of ionic octahedral polypyridyl complexes. Unexpected behavior for mixing methanol and acetonitrile as the organic modifier will be discussed, along with the effects of additives. Enantioselectivity on CF6-RN chemistry is shown to be dependent on the conjugation level and rigidity of the metal complexes. Mass transfer kinetic behavior is also shown, and high-efficiency baseline resolved rapid separations are shown for fast screening or quantitation of representative coordination complexes.

Large-scale supercritical fluid chromatography purification of unstable STING agonist intermediates

A regioisomeric mixture of the nucleoside derivative, Intermediate 1, required resolution by preparative supercritical fluid chromatography (SFC) in order to obtain the desired regioisomer as a key intermediate in a STING agonist program. Various chiral columns and solvents including methanol, acetonitrile, isopropanol, and the mixture of acetonitrile and isopropanol as organic modifiers in carbon dioxide at different temperatures were screened to obtain the best regioisomeric resolution. A key issue associated with interconversion between the regioisomers via silyl migration during purification was investigated in methanol, acetonitrile, and the mixture of acetonitrile and isopropanol, and the optimal organic modifier in CO₂ was established to mitigate the interconversion to an acceptable level (<5%). Taking into account peak resolution, throughput, interconversion and operation robustness, an efficient SFC method for large-scale purification was successfully developed and scaled up onto a 5 cm I. D. Chiralcel OJ-H column using 25% acetonitrile: isopropanol [1:1 (v/v)] with 0.1% ammonium hydroxide as the modifier in CO₂ at a total flow rate of 270 mL/min and a temperature of 30°C. In addition, continual evaporation (i.e. every hour) of the desired isomer fraction stream post-separation ensured minimal further interconversion. A total of 258 grams were separated at a high throughput of 8.6 g/h. Regioisomeric purity of the desired isomer of Intermediate 1 was ≥98.2% and the recovery was ≥90.2%. A similar purification strategy was applied to the regioisomeric resolution of Intermediate 2, an analog of Intermediate 1. In total, 1028 grams of Intermediate 2 were processed at a high throughput of 12.5 g/h on a Viridis BEH 2-EP column. The regioisomeric purity of the desired isomer was ≥96.8% and the recovery was ≥90.7%.

Reverse-phase HPLC purification for an extremely unstable glucuronide metabolite

A pure β-​D-​Glucopyranosiduronic acid metabolite (≥98.0 % purity and a single impurity ≤0.50 %) was requested for biological studies. Due to its unusual instability, the purification of the glucuronide metabolite was extremely challenging. Initially, the crude sample (89 % HPLC area purity) was purified on a Waters SunFire C₈ OBD column with 40 mM ammonium acetate buffer and acetonitrile as the mobile phase under a gradient program. The purified glucuronide metabolite solid was obtained by evaporation and lyophilization. However, this procedure yielded the target compound with 97.6 % HPLC area purity and did not meet the requirements. Through the investigation, lyophilization was identified as the key step for the purity of the metabolite, and further lyophilization resulted in an increased amount of the degraded impurities. To better understand the compound, stability studies of the purified metabolite were conducted under sample media, organic solvent, acid, base, and light exposure. The compound was observed to be extremely unstable in water, acid, base and methanol, and sensitive to light, but relatively stable in ammonium acetate buffer (pH 5.0). Taking into account compound stability and the initial purification method, the improved purification procedure was successfully developed and the purified glucuronide metabolite was obtained with 99.2 % HPLC area purity and 0.39 % of the largest single impurity.

Chiral Chromatographic Isolation on Milligram Scale of the Human African Trypanosomiasis Treatment d- and l-Eflornithine

Eflornithine is a recommended treatment against the otherwise fatal parasitic disease late stage human African trypanosomiasis (HAT), also known as Gambian sleeping sickness. It is administered repeatedly as a racemic mixture intravenously (IV) together with oral nifurtimox. Racemic eflornithine has been investigated in clinical trials for oral dosing. However, due to low systemic exposures at a maximum tolerated oral dose, the drug is continued to be administered IV. The eflornithine enantiomers, d- and l-eflornithine, have different affinities to the target enzyme ornithine decarboxylase, suggesting that the pharmacodynamics of the enantiomers may differ. The aim of this study was to develop a method for isolation of d- and l-eflornithine from a racemic mixture. Several chiral stationary phases (CSPs) were evaluated for enantioselectivity using supercritical fluid chromatography (SFC) or high-performance liquid chromatography (HPLC). None of the tested CSPs rendered separation of the enantiomers in SFC mode. Separation of the enantiomers with SFC on the CSP Chiralpak IG was only achieved on an analytical scale after derivatization with ortho-phthalaldehyde (OPA). This was the first reported enantioselective SFC method for an eflornithine derivate. However, due to poor stability, the eflornithine-OPA derivates degraded and no chemically pure enantiomers were obtained. The CSP that showed enantioselectivity in HPLC mode was Chirobiotic R, which resulted in a successful isolation on a semipreparative milligram scale. The isolated eflornithine enantiomers will be tested in nonclinical in vitro and in vivo studies to support and assess the feasibility of a future clinical program with an oral HAT treatment.

Trends in sample preparation and separation methods for the analysis of very polar and ionic compounds in environmental water and biota samples

Recent years showed a boost in knowledge about the presence and fate of micropollutants in the environment. Instrumental and methodological developments mainly in liquid chromatography coupled to mass spectrometry hold a large share in this success story. These techniques soon complemented gas chromatography and enabled the analysis of more polar compounds including pesticides but also household chemicals, food additives, and pharmaceuticals often present as traces in surface waters. In parallel, sample preparation techniques evolved to extract and enrich these compounds from biota and water samples. This review article looks at very polar and ionic compounds using the criterion log P ≤ 1. Considering about 240 compounds, we show that (simulated) log D values are often even lower than the corresponding log P values due to ionization of the compounds at our reference pH of 7.4. High polarity and charge are still challenging characteristics in the analysis of micropollutants and these compounds are hardly covered in current monitoring strategies of water samples. The situation is even more challenging in biota analysis given the large number of matrix constituents with similar properties. Currently, a large number of sample preparation and separation approaches are developed to meet the challenges of the analysis of very polar and ionic compounds. In addition to reviewing them, we discuss some trends: for sample preparation, preconcentration and purification efforts by SPE will continue, possibly using upcoming mixed-mode stationary phases and mixed beds in order to increase comprehensiveness in monitoring applications. For biota analysis, miniaturization and parallelization are aspects of future research. For ionic or ionizable compounds, we see electromembrane extraction as a method of choice with a high potential to increase throughput by automation. For separation, predominantly coupled to mass spectrometry, hydrophilic interaction liquid chromatography applications will increase as the polarity range ideally complements reversed phase liquid chromatography, and instrumentation and expertise are available in most laboratories. Two-dimensional applications have not yet reached maturity in liquid-phase separations to be applied in higher throughput. Possibly, the development and commercial availability of mixed-mode stationary phases make 2D applications obsolete in semi-targeted applications. An interesting alternative will enter routine analysis soon: supercritical fluid chromatography demonstrated an impressive analyte coverage but also the possibility to tailor selectivity for targeted approaches. For ionic and ionizable micropollutants, ion chromatography and capillary electrophoresis are amenable but may be used only for specialized applications such as the analysis of halogenated acids when aspects like desalting and preconcentration are solved and the key advantages are fully elaborated by further research. Graphical abstract.

Chiral Separations in Preparative Scale: A Medicinal Chemistry Point of View

Enantiomeric separation is a key step in the development of a new chiral drug. Preparative liquid chromatography (LC) continues to be the technique of choice either during the drug discovery process, to achieve a few milligrams, or to a scale-up during the clinical trial, needing kilograms of material. However, in the last few years, instrumental and technical developments allowed an exponential increase of preparative enantioseparation using other techniques. Besides LC, supercritical fluid chromatography (SFC) and counter-current chromatography (CCC) have aroused interest for preparative chiral separation. This overview will highlight the importance to scale-up chiral separations in Medicinal Chemistry, especially in the early stages of the pipeline of drugs discovery and development. Few examples within different methodologies will be selected, emphasizing the trends in chiral preparative separation. The advantages and drawbacks will be critically discussed.

Preparative supercritical fluid chromatography for lipid class fractionation-a novel strategy in high-resolution mass spectrometry based lipidomics

In this work, a lipidomics workflow based on offline semi-preparative lipid class-specific fractionation by supercritical fluid chromatography (SFC) followed by high-resolution mass spectrometry was introduced. The powerful SFC approach offered separation of a wide polarity range for lipids, enabled enrichment (up to 3 orders of magnitude) of lipids, selective fractionation of 14 lipid classes/subclasses, and increased dynamic range enabling in-depth characterization. A significantly increased coverage of low abundant lipids improving lipid identification by numbers and degree (species and molecular level) was obtained in Pichia pastoris when comparing high-resolution mass spectrometry based lipidomics with and without prior fractionation. Proof-of-principle experiments using a standard reference material (SRM 1950, NIST) for human plasma showed that the proposed strategy enabled quantitative lipidomics. Indeed, for 70 lipids, the consensus values available for this sample could be met. Thus, the novel workflow is ideally suited for lipid class-specific purification/isolation from milligram amounts of sample while not compromising on omics type of analysis (identification and quantification). Finally, compared with established fractionation/pre-concentration approaches, semi-preparative SFC is superior in terms of versatility, as it involved only volatile modifiers and salt additives facilitating any follow-up use such as qualitative or quantitate analysis or further purification down to the single lipid species level. Graphical Abstract.

Defining a system suitability limit to decide on column deterioration and to facilitate column transfers in chiral supercritical fluid chromatography

The separation of enantiomers is an important requirement during the entire drug life cycle in the pharmaceutical industry. High-performance liquid chromatography and supercritical fluid chromatography (SFC) are the main chromatographic techniques used to separate enantiomers. Since chiral stationary phases are often extensively used once a method has been developed, columns will age and must be replaced after a certain period. However, no practical guidelines exist to determine when a column is deteriorated or to decide whether a transfer to another column (with the same chiral selector) is successful. In this study, a system suitability limit for resolution was defined, based on an intermediate (time-different) precision study in SFC on four immobilized polysaccharide-based columns that only differed in manufacturer or particle size. This system suitability limit could be used to decide on column deterioration or as a requirement to evaluate whether a separation transfer was successful. Some method adaptations may be necessary to obtain successful transfers. An approach was proposed, which helped the analyst to make successful transfers. Graphical abstract.

Supercritical fluid chromatography-a technical overview and its applications in medicinal plant analysis: an update covering 2012-2018

Medicinal plants with complex matrices are endowed with a wide scope of biological activities. The separation, quantification, characterization and purification of bioactive components from herbal medicine extracts have always challenged analysts. Fortunately, the advancement of various emerging techniques has provided potent support for improving the method selectivity, sensitivity and run speeds in medicinal plant analyses. In recent years, the advent of new-generation supercritical fluid chromatography (SFC) instruments and a wide diversity of column chemistries, coupled with the intrinsic technical features of SFC, have made it an alternative and prominent analytical platform in the medicinal plant research area. This work aims to give a comprehensive overview of the fundamentals, technical advancement and investigating parameters of SFC in combination with three prevalent detectors. Moreover, the latest research progress of SFC applications in medicinal plant analyses is illuminated, with focus on herbal medicine-related SFC papers on the analytical and preparative scale that were published during the period of 2012 to December 2018. The most relevant applications were classified based on the constituents to be analysed. As for the respective research cases, analytical protocols and data processing strategies were provided, along with the indicated restrictions or superiority of the method; thus, the current status of SFC in medicinal plant analysis was presented.

Density dependence of retention factors of trans-stilbene oxide for chiral separation by supercritical fluid chromatography

Retention factors for two enantiomers of trans-stilbene oxide, k₁ and k₂, were measured with a chiral AD-H column using two syringe pumps to feed CO₂ and methanol as a co-solvent at various temperatures, pressures and co-solvent mole fractions to determine the effects of these operating conditions on the retention factors. The retention factors k₁ and k₂ are for the (R,R)- and (S,S)-forms, respectively. When the isothermal compressibilities of a mixture of CO₂ and MeOH were lower than 0.01, far from the critical locus of the CO₂ and methanol mixture, both retention factors were well expressed with the solvent density and temperature with an average absolute relative deviation of 1-2%. In the vicinity of the critical locus, however, where the isothermal compressibilities were much larger than 0.01, the relationship between retention factor and density was complicated. Both retention factors were proportional to the isothermal compressibility, irrespective of methanol mole fraction at each temperature.

Rapid purification of diastereoisomers from Piper kadsura using supercritical fluid chromatography with chiral stationary phases

Supercritical fluid chromatography (SFC) with chiral stationary phases (CSPs) is an advanced solution for the separation of achiral compounds in Piper kadsura. Analogues and stereoisomers are abundant in natural products, but there are obstacles in separation using conventional method. In this paper, four lignan diastereoisomers, (-)-Galbelgin, (-)-Ganschisandrin, Galgravin and (-)-Veraguensin, from Piper kadsura were separated and purified by chiral SFC. Purification strategy was designed, considering of the compound enrichment, sample purity and purification throughput. Two-step achiral purification method on chiral preparative columns with stacked automated injections was developed. Unconventional mobile phase modifier dichloromethane (DCM) was applied to improve the sample solubility. Four diastereoisomers was prepared at the respective weight of 103.1mg, 10.0mg, 152.3mg and 178.6mg from 710mg extract with the purity of greater than 98%.

Chiral separation of new sulfonamide derivatives and evaluation of their enantioselective affinity for human carbonic anhydrase II by microscale thermophoresis and surface plasmon resonance

The aim of this study was to develop a method combining chiral separation and biophysical techniques to evaluate the enantioselective affinity of original sulfonamide derivatives towards their therapeutic target, the human carbonic anhydrase II (hACII). The first step consisted in the preparation of the enantiomers by chromatographic separation. The performances of HPLC and Supercritical Fluid Chromatography (SFC) were studied at the analytical scale by optimization of various experimental conditions using adsorbed polysaccharide chiral stationary phases (amylose AD-H and cellulose OD-H). Since SFC allowed obtaining higher enantioresolutions per time unit, it was selected for the semi-preparative scale and successfully used to isolate each enantiomer with a satisfactory enantiomeric purity (>98%). Secondly, microscale thermophoresis (MST) method and surface plasmon resonance (SPR) used as reference method were developed to measure potential enantioselective affinities of these enantiomers towards the hACII. The optimizations of both methods were performed using a reference compound, i.e. acetazolamide, which affinity for hCAII has previously been demonstrated. For all compounds, K_D values obtained using MST and SPR were in good agreement, leading to similar affinity scales despite both approaches totally differ (labeling for MST versus immobilization of the protein for SPR). The equilibrium dissociation constants of our original compounds for the hCAII were in the range 100-1000nM and an enantioselectivity was observed using the MST and SPR methods for the diarylpyrazole 2. Finally, by comparing the MST and SPR techniques, MST appears especially adapted for further screening of a series of sulfonamide derivatives due to the lower time required to estimate a binding constant while consuming as little hCAII as SPR.

Cardiovascular drug determination in bioanalysis: an update

The great impact of cardiovascular diseases in human health has led to the development of a huge number of drugs and therapies to improve the treatment of these diseases. Cardiovascular drug analysis in biological fluids constitutes an important challenge for analytical scientists. There is a clear need for reliable methods to carry out both qualitative and quantitative analysis in a short time of analysis. Different problems such as drug monitoring, analysis of metabolites, study of drugs interactions, drugs residues or degradation products, chiral separation, and screening and confirmation of drugs of abuse in doping control must be solved. New trends in sample preparation, instrumental and column technology advances in LC and innovations in MS are described in this work.