Enantiorecognition ability of different chiral selectors for separation of liquid crystals in supercritical fluid chromatography; critical evaluation

Advantages of polar organic solvent chromatography for enantioseparation of chiral liquid crystals

Three sets of fluorinated chiral liquid crystals were used to explore the polar organic solvent chromatography mode for their enantioseparation. The materials include a set of newly synthesized compounds with chiral center derived from 2-hexanol and two sets of compounds with chiral center derived from 2-/3-octanol. Baseline enantioseparation of all materials was achieved using binary mobile phases without additives. For some of the compounds exceedingly high values of enantioresolution (> 20) and enantioselectivity (> 4) were found. The chromatographic behavior of the sample set was studied on three different polysaccharide-based chiral columns - Chiralpak IA-U, IG-U and IB-U. Comparison of results from Chiralpak IA-U and IB-U shows the effect of amylose vs. cellulose polysaccharide backbone while comparison of Chiralpak IA-U and IG-U reveals the effect of 3,5-dimethylphenylcarbamate vs. 3‑chloro-5-methylphenylcarbamate substituent. The mobile phases tested included whole range of acetonitrile/methanol mixtures to demonstrate that acetonitrile-rich and alcohol-rich mobile phases offer different enantiorecognition mechanisms and can provide complementarity to some extent. The effect of temperature on enantioseparation was investigated on Chiralpak IA-U by constructing van't Hoff plots for selected liquid crystals in pure acetonitrile and pure methanol as mobile phases.

Evaluation of chiral triticonazole in three kinds of fruits: enantioseparation, degradation, and dietary risk assessment

The enantioselective behaviors of chiral pesticides would affect the accuracy of risk assessment. This study evaluated the enantioselectivity of chiral triticonazole (a widely used fungicide) in three kinds of fruits. Firstly, the enantioseparation of triticonazole enantiomers was carried out within 1.2 min utilizing CHIRALPAK OJ-3 column with a mixture of CO₂ and methanol (93:7, v/v) using SFC-MS/MS. Secondly, field trials were conducted to clarify the enantioselective degradation and residue of S-( +)-triticonazole and R-(-)-triticonazole in fruits. The initial concentrations of rac-triticonazole were 25.1-93.1 ng/g, and enantioselective degradation was observed in pear, peach, and jujube after 2 h, 10 days, and 3 days, respectively. The degradation of S-( +)-triticonazole was fastest in pear (T_1/2, 2.01 days), while the T_1/2 of R-(-)-triticonazole was 5.02 days. The residue concentrations of rac-triticonazole were less than the MRL set by EU (10 ng/g) on the 3rd and 21st day in pear and peach, respectively, which were lower than 10 ng/g in jujube on the 30th day (no MRL). Finally, we found that the dietary intake risks of rac-triticonazole in fruits were low for 2-7 age, 20-50 age/female, and 20-50 age/male. The current study could provide complimentary references for the rational usage, MRL formulation, and risk assessment of chiral triticonazole.

The effect of tandem coupling of NicoShell and TeicoShell columns in sub/supercritical fluid chromatography on enantioresolution

The coupling of columns in sub/supercritical fluid chromatography presents a great opportunity for influencing the separation efficiency and extending the selectivity of the separation system. Combinations of different types of chiral stationary phases could positively affect the enantioresolution if single ones are complementary to each other. In this work, two superficially porous particle (2.7 μm) macrocyclic glycopeptide-based columns, namely TeicoShell and NicoShell, were serially coupled and tested in sub/supercritical fluid chromatography for the first time. The influence of the column arrangement on the enantioseparation of structurally diverse biologically active compounds was examined. The obtained results showed how the column order crucially affected the enantioresolution of compounds tested, but the retention was negligibly affected in most cases. We also demonstrated that single TeicoShell and NicoShell columns are very promising towards the development of highly efficient and fast/ultrafast sub/supercritical fluid chromatography methods for structurally different chiral compounds. The optimized methods for sub-minute enantioselective separation of certain biologically important compounds were proposed.

How mobile phase composition and column temperature affect enantiomer elution order of liquid crystals on amylose tris(3-chloro-5-methylphenylcarbamate) as chiral selector

A comprehensive study into the effects of mobile phase composition and column temperature on enantiomer elution order was conducted with a set of chiral rod-like liquid crystalline materials. The analytes were structurally similar and comprised variances such as length of terminal alkyl chain, presence of chlorine, number of phenyl rings, and type of chiral center. Experiments were carried out in polar organic and reversed-phase modes using amylose tris(3-chloro-5-methylphenylcarbamate) immobilized on silica gel as the chiral stationary phase. For all liquid crystals, reversal of elution order of enantiomers was observed based on type of used cosolvent and/or its content in the mobile phase; for some of the liquid crystals a temperature-induced reversal was also observed. Both linear and nonlinear dependencies of natural logarithm of enantioselectivity on temperature were found. Tested mobile phases comprised pure organic solvents and binary and tertiary mixtures of acetonitrile with organic solvents and/or water. Effect of acidic/basic mobile phase additives was also tested. Effect of structure of chiral selector is briefly discussed.

Enantioseparation performance of superficially porous particle vancomycin-based chiral stationary phases in supercritical fluid chromatography and high performance liquid chromatography; applicability for psychoactive substances

Novel psychoactive substances (NPS) are synthetic compounds that have been designed to produce the physiological and psychological effects of known recreational drugs, while circumventing current drug control laws and scheduling guidelines. Such "designer drugs" pose problems in detection and prevention of use, and they are no less dangerous than known controlled substances. Among the various classes of NPS, many are chiral. As they are synthetic products, most are racemates. Not unexpectedly, there is limited information about different the pharmacological and toxicological properties of their pure enantiomers. Hence, fast and reliable enantioselective methods are of great interest. In this work, superficially porous particle (SPP) vancomycin-based chiral stationary phases were used for development of fast enantioselective separation methods for various classes of NPS in supercritical fluid chromatography and liquid chromatography. The NPS tested included pyrovalerones, benzofurans, phenidines and phenidates. The effect of mobile phase composition on the retention and resolution of NPS in supercritical fluid chromatography was examined. The amount as well as the ratios of additives used is crucial for enantioseparation efficiency. Results showed the high enantioselective potential of vancomycin-based columns in both chromatographic techniques; 88% of NPS tested were enantioseparated in supercritical fluid chromatography and 69% of NPS tested were enantioseparated in liquid chromatography. Moreover, under optimized conditions, simultaneous enantioseparations of some NPS were achieved, which indicates great suitability of vancomycin-based columns for this purpose. The proposed methods can serve as guides for method development and for enantioseparation of further upcoming NPS.