Pirkle-type chiral stationary phase on core–shell and fully porous particles: Are superficially porous particles always the better choice toward ultrafast high-performance enantioseparations?

Chiral zwitterionic stationary phases based on Cinchona alkaloids and dipeptides: Application in chiral separation of dipeptides under reversed phase conditions

Chromatographic behavior of novel chiral stationary phases with bonded selectors based on Cinchona alkaloids modified with dipeptides was studied using dipeptides as probe molecules. Buffer-free and salt containing hydro-organic solutions were used as the mobile phases. The selectors exhibit pseudoenantiomeric behavior with respect to the L/D or LL/DD enantiomers and do not behave so with respect to the LD/DL enantiomers. The alkaloid part of the selectors is the driver of enantioselectivity, while the dipeptide substituent plays a modulating role. The quinidine-based selectors demonstrate stronger adsorption affinity and higher enantioselectivity as compared to the quinine-based selectors. The dipeptide analytes containing a glycyl fragment are weaker retained and their enantiomers are worse separated comparing to dipeptides with both units being larger amino acids. Moreover, a phenyl group in the structure of a dipeptide analyte facilitates enantioseparation. The effect of the mobile phase composition on retention depends on the hydrophobicity of an analyte. Hydrophobic dipeptides are better eluted by methanol-rich solvents, hydrophilic dipeptides are better eluted with water-rich solvents, and dipeptides with an intermediate hydrophobicity demonstrate a U-shaped or more complicated dependence of the retention factor on the percentage of methanol. Even a small buffer addition to the mobile phase decreases retention, but the ion-exchange mechanism was not confirmed. The effect of an electrolyte is rather due to the shielding of the charged groups of the selector reducing thereby electrostatic interaction between the selector and analyte. Efficiency of the novel columns is comparable to that of other brush-type chiral columns, the highest achieved number of the theoretical plates per 1 m varying between 30,000 and 40,000.

Novel insights into the dependence of adsorption-desorption kinetics on particle geometry in chiral chromatography

The existence of slow adsorption-desorption kinetics in chiral liquid chromatography is common knowledge. This may significantly contribute to worsening the efficiency and kinetic performance of a chromatographic run, especially when high flow rates are employed. Many attempts and protocols have been proposed to access this term, the so-called c ads , but they are based on different (theoretical) assumptions. As a consequence, no official method is available for the estimation of the adsorption-desorption kinetics term. In this work, a novel approach to access c ads is presented. This procedure combines experimental results obtained with kinetic and thermodynamic measurements. The investigations have been performed on two zwitterionic teicoplanin chiral stationary phases (CSPs) based on 1.9 μ m fully porous and 2.0 μ m superficially porous particles (FPPs and SPPs), using Z-D,L-Methionine as probe molecule. Kinetic studies have been performed through the combination of both stop-flow and dynamic measurements, while adsorption isotherms have been calculated through Inverse Method. This study has confirmed that, on both particle formats, analyte diffusion on the surface of the particle is negligible, meaning that adsorption is localized, and it has been demonstrated that adsorption-desorption kinetics is strongly dependent on particle geometry and, in particular, on the loading of chiral selector. These findings are fundamental not only to unravel novel aspects of the complex enantiorecognition mechanism but also to optimize the employment of CSPs for ultra-fast and preparative applications.

Exploring the application limits of different hold-up time markers in supercritical fluid chromatography

The study focuses on the application range of nitrous oxide as a hold-up time marker in supercritical fluid chromatography (SFC). This compound has been suggested a decade ago to be used as unretained marker, something that the field of SFC was missing for a long time, since its beneficial properties make it an ideal candidate as hold-up time marker. Determination of the hold-up volume and actual volumetric flow rates have always been problematic in SFC due to the compressibility of carbon dioxide and one part of this is the difficulty of hold-up time measurements. Depending on the mobile phase, different methods have been used to measure the hold-up time with varying results. Nitrous oxide and other molecules have been compared in different conditions, mobile phases and stationary phases. In all cases, nitrous oxide gave the lowest elution times. However, detection was difficult in mobile phases containing 10% or more of organic modifier, because most solvents mask the signal of nitrous oxide. Interestingly, the choice of stationary phase also had a slight effect on detection, while different pressure and temperature settings affected each compound in a different manner.

Recent developments in the high-throughput separation of biologically active chiral compounds via high performance liquid chromatography

Bioactive compounds, including active pharmaceutical ingredients (APIs), are often chiral molecules where stereoisomers have different biological and therapeutic activity. Nevertheless, the preparation of these molecules can lead to racemic or scalemic mixtures (it is not trivial to produce just the optically pure compound). The evaluation of the enantiomeric purity of bioactive compounds, and therefore quality, is indeed of fundamental importance for regulatory scopes. Chiral high performance liquid chromatography (HPLC) is the gold standard technique to separate and to purify enantiomers. This comes from the wide availability of commercial chiral stationary phases (CSPs) and operational modes, which makes the technique extremely versatile. In recent years, the most relevant trend in the field of chiral analytical HPLC has been the development of CSPs suitable for fast or even ultrafast separations, thus favoring the high throughput screening of biologically active chiral compounds. This process has somehow lagged behind compared to achiral HPLC, due to a series of practical and fundamental issues. The experience has shown how in chiral chromatography even very basic concepts, such as the supposed kinetic superiority of core-shell (pellicular) particles over fully porous ones to improve the chromatographic efficiency, cannot be taken for granted. In this review, the most relevant fundamental and practical features that must be taken into consideration to design successful high-throughput, fast enantioseparations will be discussed. Afterwards, the main classes of CSPs and the most relevant, recent (last five-year) high-throughput applications in the field of the separation of chiral bioactive compounds (for pharmaceutical, forensic, food, and omics applications) will be considered.

Preparation and chromatographic performance of chiral peptide-based stationary phases for enantiomeric separation

This study presents the development of three new chiral stationary phases. They are based on silica modified with peptides containing phenylalanine and proline. Successful analyses and characterizations were conducted using Fourier transform infrared spectra, elemental analysis, and thermogravimetric analysis. After this, the enantioselective performance of the three chiral peptide-based columns was evaluated. The evaluation used 11 racemic compounds under normal-phase high performance liquid chromatography mode. Optimized enantiomeric separation conditions were established. Under these conditions, the enantiomers of flurbiprofen and naproxen were successfully separated on CSP-1 column: the separation factor of these was 1.27 and 1.21, respectively. In addition, the reproducibility of the CSP-1 column was also investigated. The results of the investigation illustrated that the stationary phases have good reproducibility (RSD = 0.73%, n = 5).

Boosting the enantioresolution of zwitterionic-teicoplanin chiral stationary phases by moving to wide-pore core-shell particles

A novel zwitterionic-teicoplanin chiral stationary phase (CSP), based on superficially porous particles (SPPs) of 2.7 µm particle diameter and 160 Å pore size, has been prepared and evaluated towards the enantioseparation of important classes of compounds, including chiral drugs, pesticides, and N-derivatized amino acids. The comparison with two analogous CSPs prepared on SPPs with 2.7 and 2.0 µm particle diameter and 90 Å pore size has revealed that the use of large-pore particles allows to dramatically improve both the enantioselectivity and the resolution-per-analysis-time, at the point that the column prepared with the new CSP outperformed the one packed with the finest particles. On the novel wide-pore CSP, the separation of fifteen racemates of pratical importance was significantly improved in terms of both enantioselectivity and resolution-per-analysis time-compared to the CSPs based on SPPs with smaller pores (90 Å). Such a CSP would be suitable for very fast enantioseparations allowing the saving of solvent for greener high-efficiency/high-throughput applications.

Controllable organosilane monolayer density of surface bonding using silatranes for thiol functionalization of silica particles for liquid chromatography and validation of microanalytical method for elemental composition determination

The present work systematically investigates a new strategy for the functionalization of silica gel using alkyl silatrane chemistry instead of alkylsilanes for synthesis of chromatographic stationary phases. In this work, silica was chemically modified for further functionalization by a thiol-ene click reaction. Thus, 3-mercaptopropylsilatrane (MPS) was used which is capable to form self-assembled monolayers (SAM) on top of silanol surfaces in a controlled manner as previously shown for silicon wafers. The utility of this chemistry for stationary phase synthesis in liquid chromatography was not evaluated yet. Hence, silica surface modifications using MPS were studied in comparison to established 3-mercaptopropyltrimethoxysilane (MPTMS) chemistry. First, the employed elemental analysis method was validated and it showed excellent intra-day and inter-day precisions (typically less than 5% RSD). It could be shown that the reaction kinetics of MPS was roughly 35-times faster than with MPTMS. After 30 min reaction time with MPS, the thiol content reached 74% of the maximal coverage. Due to controlled chemistry with MPS, which does not lead to oligomeric siloxane network at the silica surface, the ligand coverage was lower. However, multiple silanization cycles with MPS led to a dense surface coverage (around 4 µmol m^-2). ²⁹Si cross polarization/magic angle spinning (CP/MAS) solid-state NMR revealed distinct T¹/T²/T³ ratios for MPS and MPTMS materials with up to 80% T³ (indicative for trifunctional siloxane linkage) for MPS and around 20% T³ for MPTMS. This indicates a more homogeneous, thinner monolayer film of MPS on the silica surface, as compared to an irregular thick oligomeric siloxane network with MPTMS. Bonding of quinine carbamate as chiral selector afforded an efficient chiral stationary phase (CSP) for chromatographic enantiomer separation. Separation factors were comparable to MPTMS-bonded CSP, however, chromatographic efficiency was much better for the MPS-bonded CSP. H/u curves indicated a reduced mass transfer resistance by roughly factor 3 for MPS- compared to MPTMS-bonded CSP. This confirms better chromatographic performance of surfaces with homogeneous monolayer compared to network structures on the silica surface which suffer from poor stationary phase mass transfer.

Potency testing of cannabinoids by liquid and supercritical fluid chromatography: Where we are, what we need

Hemp and cannabis industry is undergoing a renewed interest due to legalization of marijuana (a topic that all countries are discussing, especially in recent years) and the growing importance of therapeutic properties of cannabinoids. Together with an increment in the production of hemp and recreational cannabis, there has been an increasing demand for accurate potency testing of products (i.e. quantification of main cannabinoids present in the plant in terms of weight percentage) prior commercialization. This translates in an urgent need of reliable analytical methods to characterize cannabis and hemp samples. Cannabis and hemp preparations are commercialized under various forms (e.g., flowers, oils, candies or even baked goods) usually containing a large number of often very similar compounds making their separation very challenging. Strictly connected to this, another emerging topic concerns the need for the developing of large scale separation techniques for the purification of cannabinoids from complex matrices and for the preparation of analytical-grade standards (including the chiral ones). This paper reviews the most recent achievements in both these aspects. Cutting-edge applications and novel opportunities in potency testing by high performance liquid chromatography (HPLC) with UV detection (which is becoming the golden standard, according to several pharmacopeias, for this kind of measurements) are discussed. The focus has been given to the very important topic of enantio-discrimination of chiral cannabinoids, for which supercritical fluid chromatography (SFC) appears to be particularly suitable. The last part of the work covers the purification of cannabinoids through preparative chromatography. In this regard, particular attention has been given to the most innovative multi-column techniques allowing for the continuous purification of target molecules. The most recent advancements and future challenges in this field are discussed.

Fundamental to achieving fast separations with high efficiency: A review of chromatography with superficially porous particles

Types of particles have been fundamental to LC separation technology for many years. Originally, LC columns were packed with large-diameter (>100 μm) calcium carbonate, silica gel, or alumina particles that prohibited fast mobile-phase speeds because of the slow diffusion of sample molecules inside deep pores. During the birth of HPLC in the 1960s, superficially porous particles (SPP, ≥30 μm) were developed as the first high-speed stationary-phase support structures commercialized, which permitted faster mobile-phase flowrates due to the fast movement of sample molecules in/out of the thin shells. These initial SPPs were displaced by smaller totally porous particles (TPP) in the mid-1970s. But SPP history repeated when UHPLC emerged in the 2000s. Stationary-phase support structures made from sub-3-μm SPPs were introduced to chromatographers in 2006. The initial purpose of this modern SPP was to enable chromatographers to achieve fast separations with high efficiency using conventional HPLCs. Later, the introduction of sub-2-μm SPPs with UHPLC instruments pushed the separation speed and efficiency to a very fast zone. This review aims at providing readers a comprehensive and up-to-date view on the advantages of SPP materials over TPPs historically and theoretically from the material science angle.

Mass transfer kinetics on modern Whelk-O1 chiral stationary phases made on fully- and superficially-porous particles

In this work, a detailed study of mass transfer properties of trans-stilbene oxide (TSO) enantiomers on two Whelk-O1 chiral stationary phases (CSPs) has been performed. The CSPs were prepared by using both fully-porous silica particles of 2.5 μm particle diameter and superficially-porous ones of 2.6 μm particle diameter as base materials. By combining stop-flow and dynamic measurements in normal-phase conditions, the different contributions to mass transfer have been estimated. The study of intraparticle diffusion has revealed that the adsorption of both enantiomers is localized (i.e., characterized by absence of surface diffusion). The determination of thermodynamic binding constants (measured through adsorption isotherms) supports this finding.

Shedding light on mechanisms leading to convex-upward van Deemter curves on a cellulose tris(4-chloro-3-methylphenylcarbamate)-based chiral stationary phase

An unusual convex-upward van Deemter curve was observed for the more retained enantiomer of a chiral sulfoxide (2-(benzylsulfinyl)benzamide) on a cellulose tris(4-chloro-3-methylphenylcarbamate)-based chiral stationary phase (CSP), prepared on silica particles of 1000 Å pore size. In contrast, the firstly eluted enantiomer of the same molecule exhibited the traditional convex-downward van Deemter curve. A detailed kinetic and thermodynamic investigation has revealed that this unusual phenomenon, which however has already been observed in chiral chromatography, originates when the adsorption of the compound is very strong and the solid-phase diffusion negligible. Experimentally, the intraparticle diffusion of the more retained enantiomer of the sulfoxide was found to be one order of magnitude smaller than that of the first eluted one. Overall, this translates into very little longitudinal diffusion (b-term of van Deemter curve) accompanied by high solid-liquid mass transfer resistance (c-term). Finally the comparison with another, differently-substituted chiral sulfoxide (whose enantiomers both exhibit traditional van Deemter curve behavior) has allowed to correlate these findings to the specific characteristics of the molecule.

High-throughput enantioseparation of Nα-fluorenylmethoxycarbonyl proteinogenic amino acids through fast chiral chromatography on zwitterionic-teicoplanin stationary phases

In this study, 31 racemates of Nα-FMOC (fluorenylmethoxycarbonyl) amino acids (AAs) with different chemico-physical characteristics (neutral nonpolar, neutral polar, acidic and basic) have been successfully resolved in fast enantioselective chromatography on recently-developed zwitterionic-teicoplanin chiral stationary phases (CSPs). The CSPs were prepared by covalently bonding the teicoplanin selector on fully-porous particles of narrow dispersion particle-size distribution (particle diameter 1.9 µm) and superficially-porous particles (2.0 µm). Both the zwitterionic-teicoplanin CSPs have proved to be ideal media for the separation of this important class of compounds. In particular, the zwitterionic CSP prepared on superficially-porous particles exhibited superior enantioselectivity and resolution, compared to that made of fully porous particles, in virtue of more favorable thermodynamics. The zwitterionic nature of these CSPs allowed avoiding the annoying effect of Donnan's exclusion of enantiomers from the stationary phase. This effect, on the opposite, was frequently observed on a commercial teicoplanin CSP (Teicoshell) employed for comparative purposes. Noticeably, on the zwitterionic-teicoplanin CSPs, by using either acetonitrile- or methanol-rich mobile phases (MPs), it was possible to favor speed over enantioresolution and vice versa. This work gives further replies to the request for rapid determination of enantiomeric excess of Nα-FMOC proteinogenic (and non-proteinogenic) AAs, typically used as preferred chiral synthons in the solid-phase synthesis of therapeutic peptides.

A novel C2 symmetric chiral stationary phase with N-[(4-Methylphenyl)sulfonyl]-l-leucine as chiral side chains

In this study, a series of chiral stationary phases based on N-[(4-methylphenyl)sulfonyl]-l-leucine amide, whose enantiorecognition property has never been studied, were synthesized. Their enantioseparation abilities were chromatographically evaluated by 67 enantiomers. The chiral stationary phase derived from N-[(4-methylphenyl)sulfonyl]-l-leucine showed much better enantioselectivities than that based on N-(4-methylbenzoyl)-l-leucine amide. The construction of C₂ symmetric chiral structure greatly improved the enantiorecognition performance of the stationary phase. The C₂ symmetric chiral stationary phase exhibited superior enantioresolutions to other chiral stationary phases for most of the chiral analytes, especially for the chiral analytes with C₂ symmetric structures. By comparing the enantioseparations of the enantiomers with similar structures, the importance of hydrogen bond interaction, π-π interaction, and steric hindrance on enantiorecognition was elucidated. The enantiorecognition mechanism of trans-N,N'-(1,2-diphenyl-1,2-ethanediyl)bis-acetamide, which had an excellent separation factor on the C₂ symmetric chiral stationary phase, was investigated by ¹ H-NMR spectroscopy and 2D ¹ H-¹ H nuclear overhauser enhancement spectroscopy.

Ultrafast Gradient Separation with Narrow Open Tubular Liquid Chromatography

Separation speed and resolution are two important figures of merit in chromatography. Often, one gains the speed at the cost of the resolution, and vice versa. Scientists have employed short-packed columns for ultrafast separations but encountered challenges such as limited mobile phase velocity, extra-column effect caused band broadening, and column packing difficulty. We have recently demonstrated ultrahigh resolutions of narrow open tubular liquid chromatography (NOTLC); this allows us to trade some of the resolution for speed. In this work, we explored NOTLC for ultrafast LC separations. We used a 2.7 cm (effective length) narrow open tubular (NOT) column and showed a baseline separation of 6 amino acids in less than 700 ms. Ways to further increase the speed were discussed. Using short narrow open tubular (NOT) columns to perform ultrafast separation we overcame the challenges from using short packed columns. To demonstrate the feasibility of using this ultrafast separation technique for practical applications, we separated complex protein digests; peptides were nicely resolved in ∼1 min.

New Brush-Type Chiral Stationary Phases for Enantioseparation of Pharmaceutical Drugs

The importance of chirality in drug development is unquestionable, with chiral liquid chromatography (LC) being the most adequate technique for its analysis. Among the various types of chiral stationary phases (CSPs) for LC, brush-type CSPs provide the base for interaction analysis of CSPs and enantiomers, which provide valuable results that can be applied to interaction studies of other CSP types. In order to analyze the influence of aromatic interactions in chiral recognition, we designed a set of ten new brush-type CSPs based on (S)-N-(1-aryl-propyl)-3,5-dinitrobenzamides which differ in the aromatic unit directly linked to the chiral center. Thirty diverse racemates, including several nonsteroidal anti-inflammatory drugs and 3-hydroxybenzodiazepine drugs, were used to evaluate the prepared CSPs. Chromatographic analysis showed that the three new CSPs separate enantiomers of a wide range of compounds and their chromatographic behavior is comparable to the most versatile brush-type CSP—Whelk-O1. The critical role of the nonbonding interactions in positioning of the analyte (naproxen) in the cleft of CSP-6, as well as the analysis of interactions that make enantioseparation possible, were elucidated using computational methods. Furthermore, the influence of acetic acid as a mobile phase additive, on this enantiorecognition process was corroborated by calculations.

Detailed efficiency analysis of columns with a different packing quality and confirmation via total pore blocking

We report on a systematic study involving columns with a clearly different efficiency (4 distinct quality groups) obtained by packing the columns that were C₁₈ bonded and endcapped with a different carbon loading. Using B-term analysis (via peak parking) and theoretical models to estimate the magnitude of the C_m- and C_s-term contributions, it could be concluded that the difference in efficiency among the groups was entirely due to a difference in eddy dispersion. As such, the columns provided an ideal testing ground to verify how well the total pore blocking (TPB)-method can be used to probe differences in packing heterogeneity. In agreement with earlier literature observations, it turns out the TPB-method is much more sensitive to packing heterogeneities than the eddy dispersion (H_eddy)-contribution measured under open-pore conditions via B- and C- term subtraction. Typically, differences in H_eddy on the order of 0.1-0.5μm translate into a difference on the order of 0.5-2μm in the TPB mode. This confirms the TPB as a powerful technique to make very sensitive measurements of the homogeneity of packed beds.

Direct analysis of chiral active pharmaceutical ingredients and their counterions by ultra high performance liquid chromatography with macrocyclic glycopeptide-based chiral stationary phases

In this work the simultaneous separation of chiral active pharmaceutical ingredients (API) in salt form from their counterions has been performed by using different high-efficiency macrocyclic glycopeptide-based chiral stationary phases (CSPs). Not only a new zwitterionic vancomycin-based CSP has been prepared (similarly to what was done for teicoplanin) but macrocyclic selectors have also been bonded to sub-2 μm fully porous silica particles through traditional ureidic linkage to obtain versions of CSPs suitable for ultra-high performance applications. The direct separation of chiral APIs and counterions is particularly attracting since it simplifies the workflow traditionally used with reduction of analysis time and costs. The wide selection of macrocyclic antibiotics CSPs now available has allowed to manage different cases that can happen in the simultaneous separation of APIs and their counterions (either cations or anions). Indeed, while inorganic cations are retained on traditional vancomycin- and teicoplanin-based CSPs, inorganic anions are almost unretained (due to Donnan's effect). On the other hand, cations and anions can be both retained on the zwitterionic versions of these CSPs. Afterwards, zwitterionic CSPs allowed the separation of other compounds including N-derivative amino-acids, profens, polyols, sugar anomers, oligosaccharides and inorganic anions/cations opening new perspectives in the use of this family of CSPs.

On our way to sub-second separations of enantiomers in high-performance liquid chromatography

In this study our preliminary attempt for obtaining fast and highly efficient separations of enantiomers in high-performance liquid chromatography with slightly modified state-of-the-art commercial instrumentation is described. In order to reach this goal after careful selection of chiral analytes, the preparation of chiral stationary phase (CSP), mobile phase composition and column dimensions were optimized. The concept of segmented chiral-achiral column was introduced. As the result of these optimizations baseline separation of enantiomers was achieved with the analysis time between 1-2 s.

Application of cellulose 3,5-dichlorophenylcarbamate covalently immobilized on superficially porous silica for the separation of enantiomers in high-performance liquid chromatography

Our earlier studies have demonstrated the applicability of polysaccharide-based chiral selectors in combination with superficially porous (or core-shell) silica (SPS) particles for the preparation of highly efficient chiral stationary phases (CSP). In earlier studies, CSPs were prepared by coating (adsorption) of the chiral selector onto the surface of silica. In this study we report for the first time the CSP obtained by covalent immobilization of a chiral selector onto the surface of SPS particles. The applicability of this CSP for the separation of enantiomers in pure methanol and acetonitrile, as well as in n-hexane/2-propanol mobile phases is shown. The effect of the injected sample amount, mobile phase flow rate and detection frequency on separation performance were studied, as well as high efficiency separation of enantiomers with the analysis time less than 30 s was attempted.

Enantioselective ultra high performance liquid and supercritical fluid chromatography: The race to the shortest chromatogram

The ever-increasing need for enantiomerically pure chiral compounds has greatly expanded the number of enantioselective separation methods available for the precise and accurate measurements of the enantiomeric purity. The introduction of chiral stationary phases for liquid chromatography in the last decades has revolutionized the routine methods to determine enantiomeric purity of chiral drugs, agrochemicals, fragrances, and in general of organic and organometallic compounds. In recent years, additional efforts have been placed on faster, enantioselective analytical methods capable to fulfill the high throughput requirements of modern screening procedures. Efforts in this field, capitalizing on improved chromatographic particle technology and dedicated instrumentation, have led to highly efficient separations that are routinely completed on the seconds time scale. An overview of the recent achievements in the field of ultra-high-resolution chromatography on column packed with chiral stationary phases, both based on sub-2 μm fully porous and sub-3 μm superficially porous particles, will be given, with an emphasis on very recent studies on ultrafast chiral separations.

Macrocyclic glycopeptide chiral selectors bonded to core-shell particles enables enantiopurity analysis of the entire verubecestat synthetic route

Verubecestat is an inhibitor of β-site amyloid precursor protein cleaving enzyme 1 (BACE1) being evaluated in clinical trials for the treatment of Alzheimer's disease. Synthetic route development involves diastereoselective transformations with a need for enantiomeric excess (ee) determination of each intermediate and final active pharmaceutical ingredient (API). The analytical technical package of validated methods relies on enantioselective SFC and RPLC separations using multiple 3 and 5 μm coated polysaccharide-based chiral stationary phases (CSPs) and mobile phases combinations. Evaluation of recently developed chiral columns revealed a single chiral selector (Teicoplanin) bonded to 2.7 μm core-shell particles using H₃PO₄ in H₂O/ACN and triethylammonium acetate: methanol based eluents at different isocratic compositions allowed good enatioseparation of all verubecestat intermediates. EE determination of verubecestat is easily performed on NicoShell, another macrocyclic glycopeptide chiral selector bonded to 2.7 μm superficially porous particles. This approach enables fast and reliable enantiopurity analysis of the entire verubecestat synthetic route using only two chiral columns and mobile phases on a conventional HPLC system, simplifying technical package preparation, method validation and transfer to manufacturing facilities.

A cationic cyclodextrin clicked bilayer chiral stationary phase for versatile chiral separation in HPLC

A cationic cyclodextrin (CD) clicked bilayer chiral stationary phase (CSP) was developed via click chemistry for chiral separations in multimode high-performance chromatography (HPLC).

Future perspectives in high efficient and ultrafast chiral liquid chromatography through zwitterionic teicoplanin-based 2-μm superficially porous particles

With the aim of pushing forward the limits of high efficient and ultrafast chiral liquid chromatography, a new Chiral Stationary Phase (CSP) has been prepared by covalently bonding the teicoplanin selector on 2.0μm Superficially Porous Particles (SPPs). An already validated bonding protocol, which permits to achieve teicoplanin-based CSPs exhibiting zwitterionic behaviour, has been employed to prepare not only the 2.0μm version of the CSP but also two other analogous CSPs based, respectively, on 2.7μm SPPs and 1.9μm Fully Porous Particles (FPPs). The kinetic performance of these CSPs has been compared through the analysis of both van Deemter curves and kinetic plots by employing in-house packed columns of 4.6mm internal diameter and different lengths (20, 50 and 100mm). In particular on the columns packed with 2.0μm SPPs, extremely large efficiencies were observed for both achiral (>310,000 theoretical plates/meter, N/m; h_r: 1.61) and chiral compounds (>290,000 N/m; h_r: 1.72) in HILIC conditions. Thanks to their efficiency and enantioselectivity, these CSPs were successfully employed in ultrafast chiral separations. As an example, the enantiomers of haloxyfop were baseline resolved in about 3s, with a resolution higher than 2.0, (flow rate: 8mL/min) on a 2cm long column packed with the 2.0μm chiral SPPs.

Are We Approaching a Speed Limit for the Chromatographic Separation of Enantiomers?

As the speed of chromatographic enantioseparations advances to the point where the enantiomers of most chiral compounds can be resolved in less than a minute, some in less than a second, we pose the question of how this field is likely to develop over the next few years. Are we approaching a fundamental speed limit, or will further technological advances continue to deliver faster and faster separations? Are faster separations even needed for chemical research, and if so, how will they help? We herein examine this trend, investigating the barriers that currently limit speed and offering some insights into the continued evolution of fast chromatographic enantioseparations.

Preliminary kinetic evaluation of an immobilized polysaccharide sub-2μm column using a low dispersion supercritical fluid chromatograph

The performance of a 3×50mm, 1.6μm d_p column with an immobilized polysaccharide stationary phase (ChiralPak IA-U) was evaluated for efficiency, and pressure drop, with respect to flow rate and modifier concentration using supercritical fluid chromatography (SFC). This appears to be the first such report using such a column in SFC. A unique low dispersion (ultra-high performance) SFC was used for the evaluation. The minimum reduced plate height of 2.78, indicates that the maximum efficiency was similar to or better than coated polysaccharide columns. Selectivity was different from ChiralPak AD, with the same chiral selector, as reported by many others. At high flows and high methanol concentrations, pump pressures sometimes approached 600bar. With 5% methanol, pressure vs. flow rate was non-linear suggesting turbulent flow in the connector tubing. The optimum flow rate (F_opt) at 40% methanol was ≈0.8mL/min, where the column efficiency was highest. At 5% methanol, F_opt increased to ≈1.6mL/min, but efficiency degraded noticeably. The differences in F_opt suggests that the solute diffusion coefficients are a strong function of modifier concentration. Several sub-1min separations, including a 7.5s separation, are presented.