Temperature Responsive × Fast Chiral Comprehensive Liquid Chromatography: a New 2D-LC Platform for Resolving Mixtures of Chiral Isomers

Combining per-aqueous and chiral reversed phase separation modes towards an enhanced comprehensive 2-dimensional liquid chromatographic based chiral screening platform

The differentiation of enantiomers in complex mixtures is crucial in various fields such as food science, pharmaceuticals, and environmental studies. While this is achievable in principle through comprehensive two-dimensional liquid chromatography (LC × LC), practical challenges emerge when the mobile phase used in the first dimension (1D) is too strong or incompatible with the one in the second dimension (2D), leading to peak broadening and reduced resolution. These drawbacks become particularly evident when analyzing a diverse range of chiral compounds. In order to obtain timely elution of such compounds, typically spanning a range of hydrophobicity, the use of gradients (in both dimensions) is inevitable. This in turn leads to changing mobile phase compositions transferred from 1D to 2D, and hence to variations in the effectiveness of the modulation process. Lowering of the eluotropic strength of the mobile phase used in 1D allows to mitigate such problems. In this study, a novel achiral × chiral platform for fully automated screening of chiral compounds was developed. In the 1D of this platform, reversed HILIC (or per-aqueous liquid chromatography (PALC)) was employed, utilizing a commercially available HILIC column, which ensures robust and reproducible results with a water-rich mobile phase. In 2D, chiral chromatography with a broad range of gradients was utilized. Moreover, the impact of varying concentrations of organic solvent transferred to the 2D on enantioseparation was investigated. The water-rich mobile phase in the 1D facilitates the complete refocusing of organic solutes before entering the second dimension, preventing the loss of resolution in the 2D. This also allows for longer sampling times, consequently longer 2D running times, reducing the need for ultra-fast columns in the 2D. Furthermore, it enables the application of various mobile phase compositions in the 2D. Finally, this tool was successfully used to analyze compounds in urine and plasma matrices.

Exploring the Effects of Optically Active Solvents in Chiral Chromatography on Polysaccharide-Based Columns

Exploring the effectiveness of optically active solvents as mobile-phase modifiers in chiral liquid chromatography (LC) can offer an additional new tool to tune the chiral selectivity. Hence, the potential of l-ethyl lactate (LEL), a biobased solvent of this nature, was explored for its distinctive interactions with both the mobile phase and analytes, as anticipated from its chiral nature. The findings reveal that LEL provides distinct selectivity compared to commonly used modifiers in chiral LC. Reversed-phase LC (RPLC)-type chiral separations were therefore compared under various conditions whereby LEL was partially or completely replacing common achiral solvents such as acetonitrile (ACN) and methanol (MeOH). An increase in chiral resolution was obtained in 8 of 16 test compounds. For 5 of them a decrease was obtained, and 3 test solutes did not offer satisfactory results under any of the tested conditions on the polysaccharide columns. When LEL was combined with methanol instead of ACN, worse results were obtained, presumably due to its protic nature. Moreover, LEL demonstrates excellent compatibility with salt additives and is fully miscible with aqueous phases. Interestingly, a steeper increase in chiral resolution is observed for LEL, as compared to ACN at lower temperatures. While LEL is somewhat hindered by its higher UV absorbance, it paves the way toward more simplified chiral screening platforms, whereby chiral solutions can be found for fewer columns and greener solvents such as LEL are incorporated. Finally, to elucidate the impact of chiral interactions between the solvent and analytes, the influence of d-ethyl lactate (DEL) was compared with that of LEL. The results revealed different interactions between the stereoisomers of ethyl lactate (EL) and chiral analytes, demonstrating an influence of optically active solvents on enantioseparations.

Temperature-responsive comprehensive two-dimensional liquid chromatography coupled to high resolution mass spectrometry for the elucidation of the oxidative degradation processes of chemicals of environmental concern

This study explores the possibilities offered by temperature-responsive liquid chromatography (TRLC) based comprehensive 2-dimensional liquid chromatography in combination with reversed-phase liquid chromatography (RPLC) for the analysis of degradation products formed upon oxidative treatment of persistent organic pollutants, in this case exemplified through carbamazepine (CBZ). The TRLC×RPLC combination offers the possibility to overcome peak overlap and incomplete separation encountered in 1D approaches, while the transfer of the purely aqueous mobile phase leads to refocusing of all analytes on the second dimension column. Consequently, this allows for about method-development free and hence, easier LC×LC. The study focuses on the oxidative degradation of CBZ, a compound of environmental concern due to its persistence in water bodies. The TRLC×RPLC combination effectively separates and identifies CBZ and its degradation products, while offering improved selectivity over the individual TRLC or RPLC separations. This allows gathering more understanding of the degradation cascade and allows real-time monitoring of the appearance and disappearance of various degradation products. The compatibility with high-resolution mass spectrometry is last shown, enabling identification of 21 CBZ-related products, nine of which were not previously reported in CBZ degradation studies. The approach's simplicity, optimization-free aspects, and ease of use make it a promising tool for the analysis of degradation pathways in environmental contaminants.

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.