Unusual effects of separation conditions on chiral separations

Chiral liquid chromatography-tandem mass spectrometry analysis of superwarfarin rodenticide stereoisomers - Bromadiolone, difenacoum and brodifacoum - In human plasma

Superwarfarins are second-generation long-acting anticoagulant rodenticides that can cause unintended human and wildlife toxicity due, in part, to their prolonged half-lives. Commercially available superwarfarin rodenticides are synthesized as racemates with two asymmetric carbons, producing four stereoisomers. To support studies of human plasma half-lives of individual superwarfarin stereoisomers, a method was developed based on LC-MS/MS to separate and quantify stereoisomers of the commercially important superwarfarins bromadiolone, difenacoum and brodifacoum. Human plasma samples were prepared using protein precipitation and centrifugation. Chiral-phase HPLC separation was carried out on-line with tandem mass spectrometric quantitative analysis of the eluting stereoisomers using selected-reaction monitoring with positive ion electrospray on a triple quadrupole mass spectrometer. All four stereoisomers of each superwarfarin were resolved within 12.5 min with calibration curves spanning 2-3 orders of magnitude and lower limits of quantitation between 0.87 and 2.55 ng/mL. This method was used to determine the half-lives of superwarfarin stereoisomers in plasma from patients who had inhaled synthetic cannabinoid products contaminated with superwarfarins. These data may be used to guide the development of safer next generation anticoagulant rodenticides stereoisomers.

Enantioseparation and stereoselective dissipation of the novel chiral fungicide pydiflumetofen by ultra-high-performance liquid chromatography tandem mass spectrometry

Pydiflumetofen is a novel and efficient broad-spectrum chiral fungicide consisting of a pair of enantiomers. A simple and sensitive chiral analytical method was established to determine the enantiomers of this chiral fungicide in food and environmental samples by ultra-high-performance liquid chromatography tandem triple quadrupole mass spectrometry (UHPLC-MS/MS) using QuEChERS method coupled with octadecylsilane-dispersive solid-phase extraction (C₁₈-dSPE) as extraction procedure. The specific optical rotation and the absolute configuration of the enantiomers were identified by polarimetry and electronic circular dichroism (ECD). The elution order of the pydiflumetofen enantiomers on Lux Cellulose-2 was S-(-)-pydiflumetofen and R-(+)-pydiflumetofen. The average recoveries of eleven matrices ranged from 71.3% to 107.4%. The intraday relative standard deviations (RSDs) were less than 11.8%, and the interday RSDs were less than 12.6% for the two enantiomers. Stereoselective dissipation in pakchoi and soil were observed: S-(-)-pydiflumetofen was degraded faster than R-(+)-pydiflumetofen in pakchoi, causing the enantiomer fraction (EF) of the enantiomers to change from 0.50 to 0.42 in 7 days. However, R-(+)-pydiflumetofen was degraded faster than S-(-)-pydiflumetofen in soil, causing the EF of the enantiomers to change from 0.49 to 0.52 in 21 days. This study provides a method for monitoring pydiflumetofen enantiomer residues, which is crucial for improving risk assessments and the development of chiral pesticides.

Enantioseparation and determination of flumequine enantiomers in multiple food matrices with chiral liquid chromatography coupled with tandem mass spectrometry

The present work firstly described the enantioseparation and determination of flumequine enantiomers in milk, yogurt, chicken, beef, egg, and honey samples by chiral liquid chromatography-tandem mass spectrometry. The enantioseparation was performed under reversed-phase conditions on a Chiralpak IC column at 20°C. The effects of chiral stationary phase, mobile phase components, and column temperature on the separation of flumequine enantiomers have been studied in detail. Target compounds were extracted from six different matrices with individual extraction procedure followed by cleanup using Cleanert C18 solid phase extraction cartridge. Good linearity (R² >0.9913) was obtained over the concentration range of 0.125 to 12.5 ng g^-1 for each enantiomer in matrix-matched standard calibration curves. The limits of detection and limits of quantification of two flumequine enantiomers were 0.015-0.024 and 0.045-0.063 ng g^-1 , respectively. The average recoveries of the targeted compounds varied from 82.3 to 110.5%, with relative standard deviation less than 11.7%. The method was successfully applied to the determination of flumequine enantiomers in multiple food matrices, providing a reliable method for evaluating the potential risk in animal productions.

Stereoselective Separation of the Fungicide Bitertanol Stereoisomers by High-Performance Liquid Chromatography and Their Degradation in Cucumber

Bitertanol is a widely used triazole fungicide and consists of four stereoisomers. A new high-performance liquid chromatography (HPLC) method was developed for simultaneous analysis of the four stereoisomers in apple, pear, tomato, cucumber, and soil. The mechanism of separation was explained with molecular docking and effects of thermodynamic parameters on the resolution. The absolute configuration and optical rotation of four stereoisomers were confirmed by X-ray diffraction and HPLC tandem circular dichroism, respectively. A good linearity ( R² ≥ 0.999) was obtained for four stereoisomers in all matrix-matched calibration curves in the range of 0.02-10 mg/L. The mean recoveries of four stereoisomers in five matrices ranged from 74.6% to 101.0% with an intraday and interday relative standard deviation from 0.6% to 9.9%. Stereoselective degradation of bitertanol in cucumber was observed: (1 R,2 S)-bitertanol and (1 R,2 R)-bitertanol were preferentially degraded with enantiomeric fraction values from 0.5 to 0.43 at 7 d and 0.42 at 5 d, respectively. This research provides a useful tool for the analysis of bitertanol stereoisomers.

Enantioseparation, Stereochemical Assignment and Chiral Recognition Mechanism of Sulfoxide-Containing Drugs

The distinct pharmacodynamic and pharmacokinetic properties of enantiopure sulfoxide drugs have stimulated us to systematically investigate their chiral separation, stereochemical assignment, and chiral recognition mechanism. Herein, four clinically widely-used sulfoxide drugs were chosen and optically resolved on various chiral stationary phases (CSPs). Theoretical simulations including electronic circular dichroism (ECD) calculation and molecular docking were adopted to assign the stereochemistry and reveal the underlying chiral recognition mechanism. Our results showed that the sequence of calculated mean binding energies between each pair of enantiomers and CSP matched exactly with experimentally observed enantiomeric elution order (EEO). It was also found that the length of hydrogen bond might contribute dominantly the interaction between two enantiomers and CSP. We hope our study could provide a fresh perspective to explore the stereochemistry and chiral recognition mechanism of chiral drugs.

L(+) and D(-) lactate are increased in plasma and urine samples of type 2 diabetes as measured by a simultaneous quantification of L(+) and D(-) lactate by reversed-phase liquid chromatography tandem mass spectrometry

BACKGROUND

Plasma and urinary levels of D-lactate have been linked to the presence of diabetes. Previously developed techniques have shown several limitations to further evaluate D-lactate as a biomarker for this condition.

METHODS

D- and L-lactate were quantified using ultraperformance liquid chromatography tandem mass spectrometry with labelled internal standard. Samples were derivatized with diacetyl-L-tartaric anhydride and separated on a C(18)-reversed phase column. D- and L-lactate were analysed in plasma and urine of controls, patients with inflammatory bowel disease (IBD), and patients with type 2 diabetes (T2DM).

RESULTS

Quantitative analysis of D- and L-lactate was achieved successfully. Calibration curves were linear (r(2) > 0.99) over the physiological and pathophysiological ranges. Recoveries for urine and plasma were between 96% and 113%. Inter- and intra-assay variations were between 2% and 9%. The limits of detection of D-lactate and L-lactate in plasma were 0.7 μmol/L and 0.2 μmol/L, respectively. The limits of detection of D-lactate and L-lactate in urine were 8.1 nmol/mmol creatinine and 4.4 nmol/mmol creatinine, respectively. Plasma and urinary levels of D- and L-lactate were increased in patients with IBD and T2DM as compared with controls.

CONCLUSION

The presented method proved to be suitable for the quantification of D- and L-lactate and opens the possibility to explore the use of D-lactate as a biomarker.

Direct chiral resolution of metalaxyl and metabolite metalaxyl acid in aged mobile phases: the role of trace water

The separation of chiral transformation products greatly complements the understanding of the stereochemistry of chiral pollutants. In this study, direct enantiomeric resolution of metalaxyl and its main degradation product metalaxyl acid, often co-occurring in the environment, was carried out in normal-phase high-performance liquid chromatography with a Chiralcel OJ-H column. (R)-Metalaxyl acid and (S)-metalaxyl, which were almost parallel bonding to the chiral stationary phase, tended to separate, started to overlap, coeluted, and separated again with subtle changes of the mobile phase consisting of n-hexane, 2-propanol, acetic acid, and trace water. Their competition above hampered an acceptable direct separation in fresh mobile phases. Aged mobile phases with a storage period of 3-5 days, however, significantly improved their separation, in which trace water from moisture air diffusion was found to play a major role. Trace water differentially affected peak width and retention times and then induced enhanced peak separation, confirmed by deliberate addition of water to fresh mobile phases. Furthermore, none of the studied factors, involving temperature, concomitant analytes, and trace water, could cause changes of the configuration of the chiral stationary phase. Simultaneous enantiomeric separation of both compounds was achieved in aged or fresh mobile phases with adventitious or added water and gave satisfactory peak separation, all with Rs values of more than 1.20 in environmental samples.

Enantiomer separation of alpha-hydroxy acids in high-performance immunoaffinity chromatography

In this study, a monoclonal anti-d-hydroxy acid antibody was immobilized onto a synthetic high-flow-through chromatographic support material to produce a chiral stationary phase suitable for enantiomer separation of free alpha-hydroxy acids. Chiral separation of several aliphatic and aromatic members of this class of compounds was achieved in HPLC under mild isocratic buffer conditions using phosphate buffered saline, pH 7.4, as mobile phase. Due to the high degree of stereoselectivity exhibited by the immobilized antibody, in all cases the l-enantiomer eluted with the void volume, while the d-enantiomer was retained and eluted second. The effect of the mobile phase parameters flow rate, temperature, pH, and ionic strength on the enantiomer separation of the model analyte mandelic acid was investigated. While it was found that variations in the flow rate did not change the retention factor k2, dramatic effects on the interaction between the immobilized antibody and d-mandelic acid were observed when any of the other mobile phase parameters were modulated.

Temperature stability of reversed phase and normal phase stationary phases under aqueous conditions

In this study the temperature stability of several normal phase and RP columns was investigated using a water-only mobile phase. The temperature was adjusted to 120 degrees C for the bare silica stationary phases and to 185 degrees C for the metal oxide and carbon stationary phases. It could be shown that metal oxide stationary phases exhibited excellent thermal stability over the duration of the test period and are therefore suitable for high temperature LC applications.

Exploring solvent versatility in immobilized cellulose-based chiral stationary phase for the enantioselective liquid chromatographic resolution of racemates

Chiralpak IB, a new chiral stationary phase (CSP) containing cellulose tris(3,5-dimethylphenylcarabamate) immobilized onto silica gel, is investigated for the direct enantioselective separation of a set of racemic N-alkylated barbiturates and analogs of thalidomide alkylated in position 3 of the piperidin-2,6-dione ring using different nonstandard solvents such as dichloromethane (DCM), ethyl acetate, THF, methyl tert-butyl ether as an eluent and diluent, respectively, in HPLC. The separation, resolution, and elution order of the investigated compounds were compared on both immobilized and coated cellulose tris(3,5-dimethylphenylcarbamate) CSPs (Chiralpak IB and Chiralcel OD, respectively) using a mixture of n-hexane/2-propanol (90:10 v/v) as mobile phase with different flow-rates and fixed UV detection at 254 nm. The effect of the immobilization of the cellulose tris-(3,5-dimethylphenylcarbamate) CSP on silica (Chiralpak IB) on the chiral recognition ability was noted as the coated phase (Chiralcel OD) possesses a higher resolving power in some cases than the immobilized one (Chiralpak IB). However, a few racemates, which were not or poorly resolved on the immobilized Chiralpak IB or the coated Chiralcel OD when using standard solvents were most efficiently resolved on the immobilized Chiralpak IB upon using nonstandard solvents. Furthermore, the immobilized phase withstands the nonstandard (prohibited) HPLC solvents mentioned previously when used as eluents or as a dissolving agent for the analyte itself. An example of inversion or apparent inversion of elution order on Chiralpak IB is reported. The direct analysis of a spiked plasma sample extracted using DCM on Chiralpak IB is also shown.

Responses of enantioselective characteristics of imidazolinone herbicides and Chiralcel OJ column to temperature variations

Temperature affects not only the chromatographic characteristics of solute but may also alter the conformation of the stationary phase. However, temperature influences on enantioseparation of solute and conformation of chiral stationary phase (CSP) are seldom considered simultaneously. In this study, three temperature programs, a conventional heating procedure, a cyclic van't Hoff program, and a step-temperature program, were employed to evaluate temperature effects on enantioseparation of five imidazolinone herbicides on Chiralcel OJ column and the conformational state of the stationary phase. The van't Hoff plots of retention factor (k'), distribution constant (K) and separation factor (alpha) for imazapyr (1), imazapic (2), imazethapyr (3), and imazamox (4) were linear within 15-50 degrees C. Nonlinear van't Hoff plots of alpha were observed for imazaquin (5) with mobile phase of n-hexane (0.1% TFA)-2-propanol at 70/30 or 60/40 (v/v). The large molecular size of imazaquin (5) and van't Hoff plots of alpha were therefore more sensitive at detecting conformational changes of the stationary phase. Small but irreversible conformational changes occurred at 5-10 degrees C with the solvent ratio of 60/40. During the cyclic van't Hoff program, reversible conformational changes were observed at >or=15 degrees C. A switch was even visible at about 25 degrees C with the solvent ratio of 60/40 during the re-cooling cycle. The cyclic van't Hoff temperature program showed that using OJ column may yield satisfactory results at 15-50 degrees C but not at <or=15 degrees C. The step-temperature program showed that fast temperature changes over the 5-50 degrees C range may be performed on the coated OJ phase, and the recovery of enantioselectivity was relatively fast. This study shows that simultaneous evaluation of temperature effects on the chromatographic behavior of a chiral analyte and the conformational state of the chiral stationary phase provides more insights for understanding mechanisms of enantioseparation and selecting optimal separation conditions.

Determination of the enantiomer and positional isomer impurities in atomoxetine hydrochloride with liquid chromatography using polysaccharide chiral stationary phases

A normal-phase isocratic chiral liquid chromatographic method has been developed and validated for atomoxetine hydrochloride. In addition to the S-enantiomer of atomoxetine, the conditions separate both para and meta positional isomers and the phenyl des-methyl analog. Method development strategies included (a) evaluation of polysaccharide-based chiral stationary phases with nonaqueous mobile phases, (b) the use of an octyl stationary phase with a sulfated-beta-cyclodextrin mobile phase additive, and (c) capillary electrophoresis using a single isomer heptakis-6-sulfato-beta-cyclodextrin modifier. All three approaches yielded acceptable conditions for the separation of atomoxetine from related molecules with the former fully validated and the latter two held as alternatives if needed. The final method conditions employing a Chiralcel OD-H column and mobile phase of hexane/IPA/DEA/TFA (85/15/0.15/0.2, v/v/v/v) at 1.0 mL/min have been fully validated with acceptable specificity, linearity, accuracy, repeatability, intermediate precision and quantitation limit.