Strong cation exchange-type chiral stationary phase for enantioseparation of chiral amines in subcritical fluid chromatography

Chiral recognition without π-π-interactions: Highly efficient chiral strong cation exchangers lacking an aromatic unit in the molecular structure

Current state-of-the-art chiral stationary phases (CSPs) enable chiral resolution of almost any racemic mixture of choice. The exceptions represent ionizable and ionized substances that fail at any attempts to resolve on commercially available CSPs. These compounds, however, can be efficiently separated on chiral ion exchangers. Commercially available Cinchona alkaloids-based chiral weak ion-exchangers are typically used for chiral resolution of organic acids, while zwitterion ion-exchangers are efficient in the resolution of acids, bases, and zwitterions. The latter possess in their structure a cation exchange unit, which alone can serve as a cornerstone of chiral strong cation exchangers facilitating chiral separation of various basic racemic mixtures. Although chiral strong cation exchangers (cSCX) are efficient CSPs, their structural variations have not been thoroughly studied so far. It was assumed that the mechanism of chiral recognition of basic compounds by cSCX is based predominantly on π-π-interactions, hydrogen bonding and steric interactions (CSP I). To verify this assumption, we aimed in our study on the design and synthesis of cSCX first lacking lateral polar substituents on the aromatic unit in the selector's structure (CSP II), and second, to replace the aromatic unit by a cyclohexane ring (CSP III and IV), thereby to omit completely the π-π-interactions. We hypothesized that this structural change should lead to a partial or complete loss of enantiorecognition power of the selectors. Surprisingly, the non-aromatic cSCXs have shown chiral recognition capability comparable to that of previously described chiral cation exchange-type CSPs: from 16 analytes screened, 11 analytes were baseline resolved and 5 partially resolved on CSP I, while non-aromatic CSP III resolved 10 analytes baseline and 6 partially. We discuss the structural motifs of the known cSCX and the novel non-aromatic selectors in a relationship with their chromatographic performance using a set of basic analytes. Moreover, we present a theory of an effective chiral recognition mechanism by two novel non-aromatic cSCXs based on the chromatographic results and quantum mechanical calculations.

Synthesis and evaluation of aromatic stationary phases based on linear solvation energy relationship model for expanded application in supercritical fluid chromatography

In the last decade, the separation application based on aromatic stationary phases has been demonstrated in supercritical fluid chromatography (SFC). In this paper, four aromatic stationary phases involving aniline (S-aniline), 1-aminonaphthalene (S-1-ami-naph), 1-aminoanthracene (S-1-ami-anth) and 1-aminopyrene (S-1-ami-py) were synthesized based on full porous particles (FPP) silica, which were not end-capped for providing extra electrostatic interaction. Retention mechanism of these phases in SFC was investigated using a linear solvation energy relationship (LSER) model. The aromatic stationary phases with five positive parameters (a, b, s, e and d+) can provide hydrogen bonding, π-π, dipole-dipole and cation exchange interactions, which belong to the moderate polar phases. The LSER results obtained using routine test solutes demonstrated that the aforementioned interactions of four aromatic stationary phases were influenced by the type and bonding density of the ligand, but to a certain extent. Furthermore, the LSER data verified that the S-1-ami-anth column based on full porous particles silica had higher cation exchange capacity (d+ value), compared to the commercialized 1-AA column (based on the ethylene-bridged hybrid particles). The relationship between the d+ value and SFC additive was quantitatively proved so as to regulate electrostatic interaction reasonably. This value was greatly increased by phosphoric acid, slightly increased by trifluoroacetic acid and formic acid, but significantly reduced by ammonium formate and diethylamine. Taking the S-1-ami-naph column as an example, better peek shape of the flavonoids was obtained after the addition of 0.1 % phosphoric acid in MeOH while isoquinoline alkaloids were eluted successfully within 11 min after adding 0.1 % diethylamine in MeOH. Combined with the unique π-π interaction and controllable electrostatic interaction, the aromatic stationary phases in this study have been proven to have expandable application potential in SFC separation.

Supercritical fluid-based method for selective extraction and analysis of indole alkaloids from Uncaria rhynchophylla

The development of an approach based on simultaneous supercritical fluid extraction-sample cleanup, followed by supercritical fluid chromatography/tandem mass spectrometry (SFE-SFC-MS/MS) was as a tool for the extraction, separation and characterization of indole alkaloids of Uncaria rhynchophylla. A two-step SFE method was designed. A mixture of the U. rhynchophylla sample and an adsorbent named C18SCX with the ratio of 1:1 (w/w) was placed into an extraction cell. The extraction temperature was 40 °C and the pressure was 25 Mpa. In the first step, 10 % EtOH as the co-solvent was used to extract for 60 min, which was considered as a cleanup process to remove non-alkaloid components. In the second step, 0.1 % DEA was added to 10 % EtOH and it extracted for 60 min to obtain the desired extract. By introducing an additional adsorbent, the specificity of SFE towards alkaloids was greatly improved. An SFC-MS/MS method was then utilized for analysis of the SFE extract. Using 2-EP as stationary phase with the gradient elution of 0-10 min, 5-25 % EtOH (+0.05 % DEA) in CO2, column temperature 40 °C, and back pressure 13.8 Mpa, 10 peaks were separated within 8 min. Further MS/MS analysis confirmed that nine of the 10 peaks in the SFE extract were indole alkaloids. This study developed a supercritical fluid-based method specifically towards extraction and analysis of alkaloids, which is helpful to the study of alkaline compounds in complex samples.

Supercritical fluid chromatography based on reversed-phase/ ion chromatography mixed-mode stationary phase for separation of spirooxindole alkaloids

The present paper illustrates the versatility of the supercritical fluid chromatography (SFC) since, for the first time, four spirooxindole alkaloids (SOAs) including two pairs of isomers were separated by using two types of reversed-phase/ ion chromatography (RP/IC) mixed-mode stationary phases. Two mixed-mode stationary phases (C8SAX and C8SCX) was simultaneously provided dispersive and electrostatic interactions, which were suitable for the separation of such alkaloids. This study tried to provide an in-depth understanding of the SFC separation mechanism of the mixed-mode stationary phase through investigation of the impact of changes in mobile phase composition on alkaloids' retention behavior. On C8SAX, due to the strong electrostatic repulsion, there was a very narrow elution window of the alkaloids, of which behaviors were hardly affected by adding diethylamine in mobile phase. When adding formic acid or acidic ammonium formate, the prolonged retention time of alkaloids was presented because of the shielded effect of formate anions on the electrostatic repulsion. In particular, better peak shape and improved resolution were obtained by using acidic ammonium formate due to the deactivation of silanol groups by ammonium cations. On the other hand, both formic acid and acidic ammonium formate can strengthen the electrostatic attraction of C8SCX, causing difficult elution of the alkaloids. Ammonium cations from either the protonated diethylamine or the ionized ammonium formate, were considered as counter ions to effectively mask the electrostatic attraction of C8SCX, to significantly reduce the retention of alkaloids, but improve the resolution. Finally, utilizing two developed SFC methods, i.e., C8SAX with EtOH+ 10 mM acidic ammonium formate in CO2, or C8SCX with EtOH+0.1% diethylamine in CO2, the baseline separation of corynoxeine and isocorynoxeine, rhynchophylline and isorhynchophylline was achieved within 5 min.

An in-depth investigation of supercritical fluid chromatography retention mechanisms by evaluation of a series of specially designed alkylsiloxane-bonded stationary phases based on linear solvation energy relationship

Fundamental research on supercritical fluid chromatography (SFC) has gained considerable interest, with many studies focusing on its retention mechanism based on the linear solvation energy relationship (LSER) model. In this paper, a series of alkylsiloxane-bonded stationary phases were specifically designed and synthesized, then evaluated using the mobile phase composed of CO₂ with 10% (v/v) methanol. The study demonstrated the close relationship between the interactions (manner and magnitude) of stationary phases and the C-chain length, bonding density and the endcapping treatment. All C8 phases provide positive e, v and negative s, whose magnitude was regularly affected by bonding density. It was worth mentioning the non-endcapped C8 phases could provide H-bonding (positive a and b) by reducing the bonding density of the alkyl chain. Once it was endcapped, the interaction manner did not vary with bonding density adjustment. The non-endcapped C4 phases with higher bonding density could establish additional dispersion interaction (positive v). It can be seen that two synthesis strategies, 1) non-endcapped, long C-chain (C8) combined with low bonding density, and 2) non-endcapped, short C-chain (C4) combined with high bonding density, can obtain the alkylsiloxane-bonded stationary phases (C8-1 and C4-3) to provide both polar and dispersion interactions, showing different separation selectivity. Furthermore, the LSER model with ionic terms was applied to evaluate partial C8 columns, and its rationality was verified. The non-endcapped C8 showed great d⁺ values, which originated from the silanol groups. C8SCX also possessed a great d⁺ value due to the benzenesulfonic acid groups. A remarkable result showed that C8SAX exhibited prominent d^- and d⁺ values simultaneously due to the combined effect of silanol and quaternary ammonium groups, which indicates the unique selectivity when separating ionic compounds. This study provides in-depth insights into the retention mechanism of alkylsiloxane-bonded stationary phases in SFC, as well as a reference for the design of SFC stationary phases.

A Real-Time Strategy for Chiroptical Sensing and Enantiomeric Excess Determination of Primary Amines via an Acid-Base Reaction

Two achiral aromatic carboxylic acids that included the 1,8-naphthalimide group and an imidazolium cation were synthesized and exploited as chiroptical sensors. These compounds showed the real-time discrimination and enantiomeric excess determination of chiral amines and amino alcohols via an acid-base interaction, especially for UV-silent chiral compounds.

Supercritical Fluid Chromatography for Chiral Analysis, Part 1: Theoretical Background

The quantification of the enantiomers of racemic substances is of great importance in the development and regulation of pharmaceutical compounds. Active ingredients are often chiral; typically, only one of the stereoisomers has the desired pharmacokinetic and/or pharmacodynamic properties. Therefore, the stereoisomer distribution of chiral drug products must be characterized and evaluated during the drug discovery and development pipeline. Moreover, various chiral drugs present a stereoselective metabolism, highlighting the need for appropriate analytical strategies for the stereoselective analysis of metabolites, for example, in clinical and environmental studies. Due to its ease of use, robustness, and transferability, chiral liquid chromatography (LC) is the most common approach used in pharmaceutical analysis. Compared with LC, supercritical fluid chromatography (SFC) allows higher linear flow velocities while maintaining high chromatographic efficiency, often enabling the reduction of analysis time. In addition, SFC provides enhanced or complementary chiral selectivity and avoids or reduces toxic solvents, such as those used in normal-phase LC. In the first part of this review article the theoretical advantages, technological developments, and common practices in chiral SFC are discussed. This will be followed by a contribution discussing recent applications in pharmaceutical, clinical, forensic, and environmental analysis.

Chiral fungicide penconazole: Absolute configuration, bioactivity, toxicity, and stereoselective degradation in apples

Traditional evaluation of chiral pesticides can lead to inaccurate results, as their enantiomers may show different properties. Penconazole, a chiral triazole fungicide with two enantiomers, is widely applied to protect against phytopathogens. In this study, its absolute configuration, bioactivity, ecotoxicity, and stereoselective degradation were investigated at the enantiomeric level in detail. The absolute configuration of the two enantiomers (R-(+)-penconazole and S-(-)-penconazole) was first confirmed by electronic circular dichroism (ECD), and their enantioseparation method was developed and optimized using UPLC-MS/MS. S-(-)-penconazole showed high bioactivity, as its fungicidal activity against four target phytopathogens (Alternaria alternate f. sp. mali, Botryosphaeria berengeriana f. sp. piricola, Colletotrichum gloeosporioides, and Fusarium oxysporum) was 1.8-4.4 times higher than that of R-(+)-penconazole. The results of an acute toxicity test showed that the LC₅₀ values of S-(-)-penconazole against Daphnia magna were 32.5 times higher than those of R-(+)-penconazole at 24 h during the test period. Stereoselective degradation behaviors were found in nonbagging and bagging Fuji apples collected from three major apple-producing regions in China, with half-lives of 23.5-51.6 d (nonbagging treatment) and 23.0-57.5 d (bagging treatment) for R-(+)-penconazole and 41.1-60.9 d (nonbagging treatment) and 52.5-91.2 d (bagging treatment) for S-(+)-penconazole, respectively. This study provided new insights into the bioactivity, ecotoxicity, and stereoselective degradation of penconazole enantiomers. The above results also emphasized the importance of risk assessments of chiral pesticides at the enantiomeric level.

A Review of Modifications of Quinoline Antimalarials: Mefloquine and (hydroxy)Chloroquine

Late-stage modification of drug molecules is a fast method to introduce diversity into the already biologically active scaffold. A notable number of analogs of mefloquine, chloroquine, and hydroxychloroquine have been synthesized, starting from the readily available active pharmaceutical ingredient (API). In the current review, all the modifications sites and reactivity types are summarized and provide insight into the chemistry of these molecules. The approaches include the introduction of simple groups and functionalities. Coupling to other drugs, polymers, or carriers afforded hybrid compounds or conjugates with either easily hydrolyzable or more chemically inert bonds. The utility of some of the compounds was tested in antiprotozoal, antibacterial, and antiproliferative assays, as well as in enantiodifferentiation experiments.

'Ab Ovo' Chiral Phases and Chiral Reagents for Liquid Chromatographic Separation and Isolation of Enantiomers

The de-novo approach of mixing chirally pure reagents or Cu(II)-L-amino acid complexes in the slurry of silica gel for preparing TLC plates was reported from author's laboratory and was successful for separation and isolation of enantiomers. Using high molar absorptivity molecules, e. g., 1,5-difluoro-2,4-dinitrobenzene and cyanuric chloride, more than 38 new chiral derivatizing reagents were synthesized in our laboratory by straightforward nucleophilic substitution with simple chiral auxiliaries. Besides, (S)-naproxen, (S)-ketoprofen, and (S)-levofloxacin were used as chiral platforms. A conceptual approach using both achiral phases in chromatography for enantioseparation was also adopted. ¹ H NMR and DFT based software were used to explain structures of non-covalent and covalent diastereomeric pairs and determination of configuration and separation mechanism. The methods can be easily used to determine and control enantiomeric purity with advantages over a variety of commercial chiral phases.

Design and synthesis of naphthalene-based chiral strong cation exchangers and their application for chiral separation of basic drugs

In continuation of our efforts to synthesize a highly dedicated strong cation exchanger, we introduce four chiral stationary phases based on a laterally substituted naphthalene core featuring chiral 2-aminocyclohexansulfonic acid as the chiral cation-exchange site. The selectors were modified with two different terminal units, which enabled immobilization to the silica support by thiol-ene radical reaction or azide-yne click chemistry. The chromatographic parameters of these chiral stationary phases were determined using a set of chiral amines, mainly from the family of β-blocker pharmaceuticals. The chiral stationary phases immobilized by means of click chemistry were found to be superior to those possessing the sulfide linker to the silica support. The chromatographic results and visualization of density functional theory-calculated conformations of the selectors hint at a combination of a steric and electronic effect of the triazole ring in the course of chiral resolution of the target analytes.

Study on the stereoselective behaviors of fosthiazate stereoisomers in legume vegetables by supercritical fluid chromatography-tandem mass spectrometry (SFC-MS/MS)

A separation and analysis method of fosthiazate stereoisomers was established utilizing supercritical fluid chromatography-tandem mass spectrometry (SFC-MS/MS) with a CHIRALPAK AD-3 column. The determination of the four fosthiazate stereoisomers could be completed within 6 min. The environmental behaviors of fosthiazate stereoisomers were studied in legume vegetables. After applying fosthiazate granules to soil, the concentrations of fosthiazate stereoisomers in the legume vegetables increased with time, reached maximum values in 7-10 days, and then decreased gradually in all legumes except for in Glycine max. No obvious dissipation behaviors were observed in Glycine max. Interestingly, the stereoselective behaviors were species-specific. A-(-), B-(-) and B-(±)-fosthiazate were preferentially enriched in Phaseolus vulgaris Linn and Vigna unguiculata, while A-(+) and A-(±)-fosthiazate preferentially accumulated in Vicia faba Linn, Pisum sativum Linn and G. max. The opposite stereoselectivity of B-(±)-fosthiazate was observed in different growth stage of G. max. No stereoselective dissipation occurred in soil.

Enantioseparation of 2-(4-chlorophenyl)succinic acid by countercurrent chromatography and investigation of injection volume on resolution

2-(4-Chlorophenyl)succinic acid was successfully enantioseparated by countercurrent chromatography using hydroxypropyl-β-cyclodextrin as chiral selector. A two-phase solvent system composed of n-hexane-ethyl acetate-0.1 mol/L phosphate buffer with pH 2.65 (5:5:10, v/v) was selected. Enantioselective liquid-liquid extraction was used to optimize the enantioseparation conditions. Meanwhile, the influence of injection volume on resolution in countercurrent chromatography was investigated and a linear relationship between the inflection point of injection volume and sample loading was tentatively obtained. The peak resolution will decrease significantly when the injection volume over the inflection point was used. In addition, it could be found that the smaller amount of sample loading, the larger impact of injection volume on resolution could be observed, which might serve as a good reference for the selection of sample volume in enantioseparations by countercurrent chromatography. Under optimized conditions, 20 mg of 2-(4-chlorophenyl)succinic acid racemate dissolved in 10 mL of aqueous phase was successfully enantioseparated by countercurrent chromatography. The recovery for both of the enantiomer of (±)-2-(4-chlorophenyl)succinic acid reached within 70-75% with a purity of 99.0%.

Strong cation- and zwitterion-exchange-type mixed-mode stationary phases for separation of pharmaceuticals and biogenic amines in different chromatographic modes

A set of new mixed-mode ion-exchange stationary phases is presented. The backbone of organic selectors is formed by a linear hydrocarbon chain, which is divided into two parts of various lengths by a heteroatom (oxygen or nitrogen). In all studied cases, there is a sulfonic acid moiety as the terminal group. Therefore, selectors bearing oxygen gave rise to strong cation ion-exchange stationary phases, while selectors with an embedded nitrogen atom (inducing a weak anion exchange capacity) were used to create zwitterion ion-exchange stationary phases. The new mixed-mode stationary phases were chromatographically evaluated in high performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC) using isocratic elution conditions to disclose their chromatographic potential. In HPLC mode, aqueous-rich reversed phase chromatography, acetonitrile-rich hydrophilic interaction liquid chromatography and methanolic ion-exchange chromatography mobile phases were employed. In these chromatographic modes, retention factors and selectivity values for a test set of basic and zwitterionic analytes were determined. The results were compared and principal component analysis for each chromatographic mode was performed. For all chromatographic modes, the component 1 in the principal component analysis reflected the elution order. The application of different mobile phases on a particular column resulted not only in variation in retention, but also in modified selectivity, and different elution order of the analytes. The orthogonality of the elution order depending on the employed mobile phase conditions was especially reflected for structurally closely related analytes, such as melatonin and N-acetyl-serotonin, tryptamine and serotonin or noradrenalin and octopamine. However, ion-exchange interactions remain the main driving force for retention. From all investigated stationary phases, the SCX 2 (C5-linker and C4-spacer) seems to be the best choice for the separation of basic analytes using different mobile phase conditions.

Etoxazole stereoselective determination, bioaccumulation, and resulting oxidative stress in Danio rerio (zebrafish)

An environmentally-friendly and fast analytical method for the stereoselective determination of etoxazole was developed and then applied to estimate stereoselective bioaccumulation and elimination in zebrafish using SFC-MS/MS. Optimal enantioseparation conditions were determined using a Chiralpak IG-3 column and CO₂/MeOH mobile phase (80/20, v/v), at 3.0 mL/min within 1 min, 30°Me and 18 MPa. A modified QuEChERS method was developed for zebrafish sample pretreatment, and mean recoveries were 88.43-110.12% with relative standard deviations ranging from 0.32 to 5.34%. The enantioselectives of etoxazole enantiomers in zebrafish during uptake and depuration phases were evaluated. Significant enantioselective bioaccumulation was observed, with preferential accumulation of (-)-R-etoxazole compared to its antipode, during uptake at both low and high exposure concentrations. The toxic effects of etoxazole on zebrafish were further explored, and activities of antioxidant enzymes were determined in liver of zebrafish. Significant changes were observed in the SOD and GST activities and in the MDA levels, which indicated the occurrence of oxidative stress in liver of zebrafish. The toxic effects exhibited time- and dose-dependent properties. These results can facilitate the accurate risk evaluation of etoxazole and provide basic knowledge for further study of biotoxicity mechanisms.

Chaotropic Effects in Sub/Supercritical Fluid Chromatography via Ammonium Hydroxide in Water-Rich Modifiers: Enabling Separation of Peptides and Highly Polar Pharmaceuticals at the Preparative Scale

Chromatographic separation, analysis and characterization of complex highly polar analyte mixtures can often be very challenging using conventional separation approaches. Analysis and purification of hydrophilic compounds have been dominated by liquid chromatography (LC) and ion-exchange chromatography (IC), with sub/supercritical fluid chromatography (SFC) moving toward these new applications beyond traditional chiral separations. However, the low polarity of supercritical carbon dioxide (CO₂) has limited the use of SFC for separation and purification in the bioanalytical space, especially at the preparative scale. Reaction mixtures of highly polar species are strongly retained even using polar additives in alcohol modifier/CO₂ based eluents. Herein, we overcome these problems by introducing chaotropic effects in SFC separations using a nontraditional mobile phase mixture consisting of ammonium hydroxide combined with high water concentration in the alcohol modifier and carbon dioxide. The separation mechanism was here elucidated based on extensive IC-CD (IC couple to conductivity detection) analysis of cyclic peptides subjected to the SFC conditions, indicating the in situ formation of a bicarbonate counterion (HCO₃^-). In contrast to other salts, HCO₃^- was found to play a crucial role acting as a chaotropic agent that disrupts undesired H-bonding interactions, which was demonstrated by size-exclusion chromatography coupled with differential hydrogen-deuterium exchange-mass spectrometry experiments (SEC-HDX-MS). In addition, the use of NH₄OH in water-rich MeOH modifiers was compared to other commonly used basic additives (diethylamine, triethylamine, and isobutylamine) showing unmatched chromatographic and MS detection performance in terms of peak shape, retention, selectivity, and ionization as well as a completely different selectivity and retention behavior. Moreover, relative to ammonium formate and ammonium acetate in water-rich methanol modifier, the ammonium hydroxide in water additive showed better chromatographic performance with enhanced sensitivity. Further optimization of NH₄OH and H₂O levels in conjunction with MeOH/CO₂ served to furnish a generic modifier (0.2% NH₄OH, 5% H₂O in MeOH) that enables the widespread transition of SFC to domains that were previously considered out of its scope. This approach is extensively applied to the separation, analysis, and purification of multicomponent reaction mixtures of closely related polar pharmaceuticals using readily available SFC instrumentation. The examples described here cover a broad spectrum of bioanalytical and pharmaceutical applications including analytical and preparative chromatography of organohalogenated species, nucleobases, nucleosides, nucleotides, sulfonamides, and cyclic peptides among other highly polar species.

Determination of Optical Purity of Lactic Acid-Based Chiral Liquid Crystals and Corresponding Building Blocks by Chiral High-Performance Liquid Chromatography and Supercritical Fluid Chromatography

Liquid crystals (LCs) are among the most prominent materials of the current information age, mainly due to their well-known application in liquid crystal displays (LCDs). Their unique electro-optical properties stem from their ability to form organised structures (mesophases) on the transition from solid state to isotropic liquid. Molecules of LCs in a mesophase still maintain the anisotropy of solid crystals, while simultaneously exhibiting the fluidity of liquids, which gives the system the ability to react immediately to external stimuli such as electric or magnetic fields, light, mechanical stress, pressure and, of course, temperature. For the proper function of LC-based devices, not only chemical, but also optical purity of materials is strongly desirable, since any impurity could be detrimental to the self-assembly of the molecules. Therefore, in this study we aimed to verify synthetic methods published in the literature, which are used nowadays to prepare chiral building blocks based on lactic acid, for their enantioselectivity. Moreover, we have focused on the development of an analytical chiral separation method for target liquid crystalline materials. Using a chiral polysaccharide-based column operated in liquid chromatography mode, we show that not all published methods of LC synthesis are enantioselective, which could lead to significant differences in the properties of the resulting materials. We show that high-performance liquid chromatography with UV detection and supercritical fluid chromatography with UV and mass spectrometry detection enable full control over the chemical and optical purity of the target LCs and the corresponding chiral building blocks. For the first time, we utilise supercritical fluid chromatography with mass detection for the direct chiral analysis of liquid crystalline materials and impurities formed during the synthesis.

Stereoselective Analysis and Dissipation of Propiconazole in Wheat, Grapes, and Soil by Supercritical Fluid Chromatography-Tandem Mass Spectrometry

An efficient and sensitive chiral analytical method was established for the determination of propiconazole stereoisomers by supercritical fluid chromatography-tandem mass spectrometry (SFC-MS/MS). Stereoisomeric separation was performed on a Chiralpak AD-3 column with CO₂/ethanol (93:7) as the mobile phase. The four propiconazole stereoisomers were well separated in 4.7 min with resolutions above 2.0. The specificity, linearity, matrix effects, accuracy, precision, and stability of the developed method were evaluated. The stereoselective dissipation of propiconazole in wheat straw, grape, and soil samples was investigated according to the proposed method. The results indicated that significant stereoselective degradation occurred in wheat straw and grapes, with preferential degradation of (-)-propiconazole A and (+)-propiconazole B in wheat straw and the opposite case in grapes. No enantioselectivity was observed in soil, although diastereoisomer A degraded more rapidly than diastereoisomer B. These results could contribute to a more accurate assessment of the environmental risk and food safety of propiconazole.

A Comparative Study of Enantioseparations of Nα-Fmoc Proteinogenic Amino Acids on Quinine-Based Zwitterionic and Anion Exchanger-Type Chiral Stationary Phases under Hydro-Organic Liquid and Subcritical Fluid Chromatographic Conditions

The focus of this contribution is a comparative investigation of enantioseparations of 19 N^α-Fmoc proteinogenic amino acids on Quinine-based zwitterionic and anion-exchanger type chiral stationary phases employing hydro-organic and polar-ionic liquid and subcritical fluid chromatographic conditions. Effects of mobile phase composition (including additives, e.g., water, basis and acids) and nature of chiral selectors on the chromatographic performances were studied at different chromatographic modes. Thermodynamic parameters of the temperature dependent enantioseparation results were calculated in the temperature range 5–50 °C applying plots of lnα versus 1/T. The differences in standard enthalpy and standard entropy for a given pair of enantiomers were calculated and served as a basis for comparisons. Elution sequence in all cases was determined, where a general rule could be observed, both in liquid and subcritical fluid chromatographic mode the d-enantiomers eluted before the L ones. In both modes, the principles of ion exchange chromatography apply.

Stereoselective analysis of novel chiral fungicide pyrisoxazole in cucumber, tomato and soil under different application methods with supercritical fluid chromatography/tandem mass spectrometry

Various new chiral pesticides have been registered and used in crop yields. However, few studies have focused on the environmental behavior of such new registered chiral compounds on the stereoisomer level. In this study, an effective and sensitive chiral analytical method was first developed to detect pyrisoxazole stereoisomers and then further applied to investigate the stereoselective dissipation in vegetables and soil using supercritical fluid chromatography/tandem triple quadrupole mass spectrometry. Optimal separation condition was achieved with IA column using CO2/MeOH (75:25) as mobile phase at 2.0 mL/min in 5 min, 35 °C and 2400 psi. The average recoveries in all of the matrices at four spiking levels ranged from 84.0% to 105.6%. Significant stereoselective dissipation was observed in cucumber and tomato under both application modes. (-) Pyrisoxazole A and (-) pyrisoxazole B were preferentially degraded in cucumber under foliar spraying mode. In contrast, (+) pyrisoxazole A and (-) pyrisoxazole B were preferentially degraded in cucumber under soil irrigation mode. (-) Pyrisoxazole A and (-) pyrisoxazole B were degraded faster than their antipodes in tomato under both application modes. However, no significant stereoselectivity was observed in soil. The results of this study could help facilitate more accurate risk assessments of pyrisoxazole.