High-performance liquid chromatography study of the enantiomer separation of chrysanthemic acid and its analogous compounds on a terguride-based stationary phase

Stereo and enantioselective separation and identification of synthetic pyrethroids, and photolytical isomerization analysis

Permethrin, cypermethrin and cyfluthrin are three important pyrethroids with similar structures and contain four, eight and eight stereoisomers, respectively. All the stereoisomers were completely resolved by a combination of achiral and chiral high-performance liquid chromatography with hexane/isopropanol as the mobile phase. The absolute configurations for the stereoisomers were also assigned on the basis of enantioselective resolution of permethrinic acid. For cypermethrin and cyfluthrin, although there is the only difference being one F atom substitution in cyfluthrin, the cis-I diastereomer exhibited a reversal of enantiomer order. Three specific stereoisomers were further isolated to investigate the photolysis and chiral stability of synthetic pyrethroids at the enantiomeric level. The results clearly revealed that significant isomerization occurred along with the photolysis process. The isomerization occurred at chiral 1-C position, 3-C position or both in the cyclopropyl ring, and the chiral 3-C exhibited a higher inversion tendency.

Isomer- and enantioselective degradation and chiral stability of fenpropathrin and fenvalerate in soils

Fenpropathrin and fenvalerate are two widely used pyrethroid insecticides. Due to one and two asymmetrical centers, fenpropathrin and fenvalerate consist of two and four stereoisomers, respectively. In the present study, the degradation of fenpropathrin and fenvalerate in two soils, an alkaline soil and an acidic soil, was investigated using enantioselective high-performance liquid chromatography (HPLC) and the following results were obtained. (i) Degradation of fenpropathrin and fenvalerate in alkaline soil was slightly enantioselective with S-fenpropathrin and alphaS,2R-fenvalerate being faster degraded, respectively. (ii) Fenpropathrin and fenvalerate were found configurationally unstable in alkaline soil and significant racemization at the chiral alpha-C position took place along with the degradation process. The racemization is chemically induced since it occurred under both nonsterilized and sterilized conditions. (iii) No racemization was observed in acidic soil. The soil pH dependent effect of the racemization was further validated by incubating S-fenpropathrin and alphaS,2R-fenvalerate in methanol-buffer solutions with different pH values. (iv) Fenvaleric acid was identified as the main metabolite of fenvalerate, formed by cleavage of the ester bond, during fenvalerate degradation in alkaline soil. Formation of fenvaleric acid in alkaline soil was slightly enantioselective, resulting in an enrichment of S-fenvaleric acid in the soil.

CEC enantioseparations on chiral monolithic columns: A study of the stereoselective degradation of (R/S)-dichlorprop [2-(2,4-dichlorophenoxy)propionic acid] in soil

For the study of the stereoselective degradation of the herbicide 2-aryloxipropionic acid dichlorprop (DCPP) in soil, a porous monolithic chiral column (100 microm id) was prepared by in situ copolymerization of glycidyl methacrylate, methyl methacrylate and ethylene glycol dimethacrylate in the presence of formamide and 1-propanol as the porogen solvents. Subsequently, the epoxide groups at the surface of the monolith were reacted with (+)-1-(4-aminobutyl)-(5R,8S,10R)-terguride as the chiral selector. Optimum conditions for the herbicide resolution by CEC were found using mobile phases consisting of acetic acid/triethylamine mixtures in ACN-methanol (9:1 v/v). Under these conditions fully separation of DCPP enantiomers in the presence of clofibric acid (internal standard) was achieved in about 5 min. Experiments on the incubation of rac-DCPP in soil at room temperature showed the herbicide undergone during 23 incubation days to a degradation to levels </=20% of the initial concentration, with rates for (R)-DCPP slower than (S)-DCPP. More interesting results were observed when herbicide enantiomers were individually incubated. In both the experiments, the formation of the opposite isomer in the presence of the initial one, and reversed enantiomeric interconversion in the case of (S)-DCPP was observed. (R)-DCPP was found to be the most persistent isomer after incubation.

Enantioseparation of 2-aryloxypropionic acids on chiral porous monolithic columns by capillary electrochromatography

The enantioseparation of 2-aryloxypropionic acids by capillary electrochromatography was tested on columns with a monolithic stationary phase prepared from silanized fused-silica capillaries (100 microm I.D.) by in situ copolymerization of glycidyl methacrylate, ethylene glycol dimethacrylate and methyl methacrylate in the presence of formamide and 1-propanol as the porogen solvents. The porous chiral monolithic stationary phases were prepared by reaction of the epoxy-groups at the surface of the monolith with (+)-1-(4-aminobutyl)-(5R,8S,10R)-terguride. To attain the minimum HETP values for the enantiodiscrimination of 2-phenoxypropionic acid, the influence of the composition of polymerization solution on column total porosity and efficiency was investigated. Optimum mobile phase conditions were found for all analytes tested using acetonitrile-methanol mixtures containing triethylamine and acetic acid as the buffer components. Furthermore, the chemical and mechanical stabilities of the columns were satisfactory, allowing hundreds of analyses.

Direct high-performance liquid chromatographic method for enantioselective and diastereoselective determination of selected pyrethroic acids

This study reports on the direct HPLC stereoisomer separation of selected pyrethroic acids employing commercial cinchona alkaloid derived chiral stationary phases (CSPs). cis/trans-Chrysanthemic acid (cis/trans-CA), cis/trans-chrysanthemum dicarboxylic acid (cis/trans-CDCA), cis/trans-permethrinic acid (cis/trans-PA), and fenvaleric acid (FA) were resolved into the individual stereoisomers, i.e. enantiomers and diastereomers as well. To achieve satisfactory baseline separation an optimisation of the variables of the chromatographic method including chemical structure of the cinchona carbamate CSP, mobile phase composition, and flow rate was required. All four stereoisomers of PA were successfully separated in a single run (alphacis = 1.20, alphatrans = 1.26, critical Rs = 1.65) with an acetonitrile (ACN)-based polar-organic eluent. The complete baseline resolution of all CA stereoisomers succeeded in polar-organic (alphacis = 1.20, alphatrans = 1.35, critical Rs = 3.03) as well as in acetonitrile-based reversed-phase media (alphacis = 1.24, alphatrans = 1.22, critical Rs = 2.73). The latter elution mode was also found to be suitable for the enantio- as well as diastereoselective resolution of CDCA (alphacis = 1.09, alphatrans = 1.50, critical Rs = 1.43), which is to the best of our knowledge the first reported enantiomer separation of this analyte. The enantiomers of FA could be baseline separated employing also reversed-phase mode (alpha = 1.16, Rs = 2.91). These separation methods may be applied for quality control processes in the production of stereoisomerically pure insecticides as well as stereoselective toxicokinetic studies, as CDCA, PA, and FA are suitable biomarkers for monitoring human pyrethroid burden.

Fast and precise determination of phenthoate and its enantiomeric ratio in soil by the matrix solid-phase dispersion method and liquid chromatography

A fast and precise method was developed for the determination of phenthoate and its enantiomeric ratio (ER) in three soil samples. A recently developed sample pretreatment technology--matrix solid-phase dispersion (MSPD) was used to extract phenthoate simply and effectively. MSPD conditions, i.e. solid-phase Florisil amount, water content of Florisil-soil mixture, the constituent and volume of the eluting solvent, were optimized stepwise. The MSPD extract was directly used for quantitative determination of phenthoate by silica-based high-performance liquid chromatography (HPLC) with UV detection. The recoveries of phenthoate from three different types of soils fortified at three levels of 0.1, 1, 10 microg/g ranged from 75 to 94% with RSDs of 1.5-6.5%. On this basis, phenthoate was further isolated from the remainder of MSPD extract by silica-based HPLC and then ER determined on HPLC with cellulose tris-3, 5-dimethylphenylcarbamate as chiral stationary phase. The ERs determined in the soils spiked with racemic or enantiomer-enriched phenthoate agreed sufficiently well with those in the corresponding standard solutions. Finally, the proposed method was successfully applied to the study of enantioselective degradation of phenthoate in the three soils under laboratory conditions. High enantioselectivity was observed in the two alkaline soils with (+)-enantiomer degrading faster than the (-)-enantiomer, while there was little to no enantioselectivity in the acidic soil. The methodology can be used to study the enantioselective environmental behavior of chiral pollutants.

Chiral separation by chromatographic and electromigration techniques. A review

This review gives a survey of different chiral separation principles and their use in high-performance liquid chromatography (HPLC), gas chromatography (GC), supercritical fluid chromatography (SFC), thin-layer chromatography (TLC), capillary electrophoresis (CE) and capillary electrochromatography (CEC) highlighting new developments and innovative techniques. The mechanisms of the different separation principles are briefly discussed and some selected applications are shown.

High-performance liquid chromatography chiral stationary phases based on low-molecular-mass selectors

A review of HPLC chiral stationary phases (CSPs) based on low molecular mass selectors is given. The review is focused on brush- and monomeric-type CSPs obtained by covalent linkage of chiral selectors, with emphasis on those obtained by total synthesis. Emphasis is given to new, emerging aspects like enantioseparations on receptor-like chiral stationary phases and dynamic enantioselective chromatography of stereolabile compounds.