Separation of d/l-carnitine enatiomers by capillary electrophoresis

Robust discriminator of chiral molecules via a topological invariant

We propose a scheme for chirality discrimination via a topological invariant. The physical model is based on a three-level subspace of a molecule. By modulating the components of the control field with proper frequencies, two different two-level effective Hamiltonians are derived for the left-handed and the right-handed molecules. We parameterize the effective Hamiltonians with two angles and demonstrate that a topological quantum phase transition can be induced by tuning the effective Rabi frequency if the molecule is right-handed. This phenomenon provides a method to discriminate the chirality of the molecule by measuring a topological invariant, i.e., the Chern number, of the parametric manifold. Since the Chern number is robust against perturbations to the system, the scheme is insensitive to the systematic errors of the control fields, the deviations of the modulation frequencies, and decoherence of the molecule. Therefore, the scheme may provide useful perspectives to construct a robust discriminator of chiral molecules.

Discrimination of enantiomers for chiral molecules using analytically designed microwave pulses

We perform a theoretical exploration of quantum coherent control of enantio-selective state transfer (ESST) of chiral molecules with three rotational states connected by the a-type, b-type, and c-type components of the transition dipole moments. A pulse-area theorem based on a closed-loop three-level system is derived without applying the rotating-wave approximation and used to analytically design three linearly polarized microwave pulses with optimal amplitudes and phases. By utilizing two optimized microwaves to mix two excited rotational states into the maximal coherence, we find that the discrimination of enantiomers via ESST for chiral molecules can be achieved by controlling the delay time of the third optimized microwave pulse. We examine the robustness of such control schemes against the Rabi frequency and detuning errors and the environment effect through pure dephasing processes for practical applications. This work provides an alternative approach to analytically designing optimal control fields for quantum control of ESST by using complex pulse areas.

(+) or (-)-1-(9-fluorenyl)ethyl chloroformate as chiral derivatizing agent: A review

Over the last 30years, (±)-1-(9-fluorenyl)ethyl chloroformate ((±)-FLEC) was used as a chiral derivatizing agent in various analytical applications involving a wide range of endogenous, pharmaceutical and environmentally relevant molecules. This comprehensive review aims to present all the significant aspects related to the state of the art in FLEC labeling and subsequent chiral separation of the resulting diastereomers using LC, SFC and CE techniques.

Bioanalysis and enantioseparation of dl-carnitine in human plasma by the derivatization approach

BACKGROUND

L-carnitine is an over the counter drug, used to treat disorders like cardiomyopathy, skeletal myopathy, hypoglycemia and hyperammonemia. Preparations containing D-carnitine should be avoided by dialysis patients because it has toxic influence on biochemical processes by inhibiting the carnitine acetyltransferase. Therefore, it is of utmost importance to assess and control the content of D-carnitine.

METHODS

A HPLC method was developed and validated for determination and enantiomeric resolution of DL-carnitine in human plasma by derivatization approach. (S)-Naproxen-based three derivatizing reagents were synthesized and applied.

CONCLUSION

The limit of detection values were found to be 1.26 and 1.35 ng ml(-1) for the two isomers. The method is simple, reproducible, and can be used for routine analysis in laboratories for control of enantiomeric purity of carnitine.

Sensitive determination of D-carnitine as enantiomeric impurity of levo-carnitine in pharmaceutical formulations by capillary electrophoresis-tandem mass spectrometry

In this work, capillary electrophoresis-electrospray ionization-tandem mass spectrometry was applied to the determination of l- and d-carnitine in pharmaceutical formulations. A simple sample treatment procedure consisting of the use of a dilution or an extraction step with water was employed prior to derivatization with 9-fluorenylmethoxycarbonyl (FMOC). The method was validated in terms of selectivity, linearity, accuracy, precision and sensitivity, with a LOD of 10ngmL(-1) for each enantiomer, which was enough to detect enantiomeric impurities up to 0.002% of d-carnitine with respect to the main enantiomer (l-carnitine). Eleven pharmaceutical formulations were analyzed including ampoules, oral solutions, sachets, and tablets. Results showed contents for carnitine comprised between 77 and 101% with respect to the labeled ones in the case of those formulations marketed with the racemate, and from 97 to 102% in those cases where the single enantiomer (l-carnitine) was employed as active ingredient. Percentages for the enantiomeric impurity (d-carnitine) ranging from 0.6 to 1.3% were obtained exceeding the limits established for impurities in drug products. These results corroborate the need of validated analytical methodologies enabling the quality control of pharmaceutical formulations containing carnitine.

Achiral liquid chromatography with circular dichroism detection for the determination of carnitine enantiomers in dietary supplements and pharmaceutical formulations

A simple and enantioselective method for the separation and determination of carnitine enantiomers in dietary supplements and pharmaceutical formulation samples is proposed. This method is based on achiral liquid chromatographic separation of carnitine enantiomers from interferences and direct circular dichroism (CD) detection. The calibration curve of the anisotropy factor (g) versus the enantiomeric excess was linear, with a correlation coefficient (R(2)) of 0.996. The precision evaluated by UV peak area and CD peak area was suitable (RSD <5% in all cases). The usefulness of the proposed method was demonstrated by analysing natural dietary supplements and pharmaceutical formulation samples. This method has the advantages of being rapid and precise, without using an expensive chiral column. The method was suitable for the simultaneous determination of both enantiomers and for assessing the chemical purity of carnitine.

Direct determination of acylcarnitines in amniotic fluid by column-switching liquid chromatography with electrospray tandem mass spectrometry

A direct, simple, and simultaneous determination of acylcarnitines in amniotic fluid was developed using column-switching liquid chromatography/tandem mass spectrometry (LC/MS/MS). The analytes can be assayed within 20 min without any sample preparation process, and we monitored separated acylcarnitines with positive electrospray ionization (ESI)-MS/MS. The calibration ranges of acylcarnitines were 1 to 100 nmol/L. The linearity of the method was 0.992 to 0.999, and the limits of detection at a signal-to-noise ratio of 3 were 1 nmol/L. The coefficients of variation were in the range of 5.2 to 13.3% for within-day variation and 6.7 to 11.9% for day-to-day, respectively. We detected acylcarnitines in the amniotic fluid of 22 women in the early stages of their pregnancies in the range of 2.2 to 17.2 nmol/L. The proposed method could be applied to diagnosis, monitoring, and biomedical investigations of inborn errors of the organic acid and fatty acid metabolism of the embryo.

Capillary electrophoresis determination of carnitine in food supplements

L-Carnitine is a substance natural for human body which transfers fatty acids to the place of burning-mitochondria and aids the transformation of fats into energy and this way supports overweight reduction and immediate physical performance, increases resistance from physical load and protect heart from overload. In this study are described newly developed electrophoretic methods (ITP, CZE with direct and/or indirect UV detection) for carnitine determination in various samples. The results were compared with results obtained by validated HPLC method. All of these methods gave comparable results. The detection limits of the electrophoretic methods were between 2.4 and 4.7 microg/ml, reproducibility (relative standard deviation, RSD%) was between 1.2 and 4.4% and recoveries were between 91 and 113% in different samples. The shorter analysis and low running cost are the main advantages of CE methods.

Determination of l-carnitine in food supplement formulations using ion-pair chromatography with indirect conductimetric detection

A novel method for the determination of L-carnitine in food supplement formulations was developed and validated, using ion-pair chromatography with indirect conductimetric detection. The chromatographic method was based on a non-polar (C18) column and an aqueous octanesulfonate (0.64 mM) eluent, acidified with trifluoroacetic acid (5.2 mM). The retention time was 5.4 min and the asymmetry factor 0.65. A linear calibration curve from 10 to 1000 microg/ml (r= 0.99998), with a detection limit of 2.7 microg/ml (25 microl injection volume), a repeatability %RSD of 0.8 (40 microg/ml, n = 5) and reproducibility %RSD of 2.6 were achieved. The proposed method was applied for the determination of carnitine in oral solutions and capsules. No interference from excipients was found and the only pretreatment step required was the appropriate dilution with the mobile phase. Recovery from spiked samples was ranged from 97.7 to 99.7% with a precision (%RSD, n = 3) of 0.01-2.1%.

Determination of carnitine and acylcarnitines in urine by high-performance liquid chromatography-electrospray ionization ion trap tandem mass spectrometry

A high-performance liquid chromatography-mass spectrometry method has been developed for the simultaneous determination of native carnitine and eight acylcarnitines in urine. The procedure uses a solid-phase extraction on a cation-exchange column and the separation is performed without derivatization within 17 min on a reversed-phase C8 column in the presence of a volatile ion-pairing reagent. The detector was an ion trap mass spectrometer and quantification was carried out in the MS-MS mode. Validation was done for aqueous standards at ranges between 0.75 and 200 micromol/l, depending on the compound. Carnitine was quantified in urine and comparison with a radioenzymatic assay gave a satisfactory correlation (R2 = 0.981). The assay could be successfully applied to the diagnostic of pathological acylcarnitines profile of metabolic disorders in urines of patients suffering from different organic acidurias.

Derivatization trends in capillary electrophoresis

This survey gives an overview of recent derivatization protocols, starting from 1996, in combination with capillary electrophoresis (CE). Derivatization is mainly used for enhancing the detection sensitivity of CE, especially in combination with laser-induced fluorescence. Derivatization procedures are classified in tables in pre-, on- and postcapillary arrangements and, more specifically, arranged into functional groups being derivatized. The amine and reducing ends of saccharides are reported most frequently, but examples are also given for derivatization of thiols, hydroxyl, carboxylic, and carbonyl groups, and inorganic ions. Other reasons for derivatization concern indirect chiral separations, enhancing electrospray characteristics, or incorporation of a suitable charge into the analytes. Special attention is paid to the increasing field of research using on-line precapillary derivatization with CE and microdialysis for in vivo monitoring of neurotransmitter concentrations. The on-capillary derivatization can be divided in several approaches, such as the at-inlet, zone-passing and throughout method. The postcapillary mode is represented by gap designs, and membrane reactors, but especially the combination of separation, derivatization and detection on a chip is a new emerging field of research. This review, which can be seen as a sequel to our earlier reported review covering the years 1991-1995, gives an impression of current derivatization applications and highlights new developments in this field.

Analysis of carnitine and acylcarnitines in urine by capillary electrophoresis

A capillary electrophoresis method is described for the simultaneous analysis of carnitine and short-chain acylcarnitines in aqueous standard solutions and urine samples. Samples were worked up using silica gel extraction and derivatization with 4'-bromophenacyl trifluoromethanesulfonate. Separation was performed in less than 8 min using a binary buffer system containing phosphate/phosphoric acid and sodium dodecyl sulfate. 3-(2,2,2-Trimethylhydrazinium)propionate (mildronate) was used as an internal standard. The method was developed with aqueous standard solutions and then applied successfully to spiked and unspiked human urine samples. The limit of detection for both carnitine and acetylcamitine is 3 microM.

Mechanistic study of opposite migration order of dimethindene enantiomers in capillary electrophoresis in the presence of native beta-cyclodextrin and heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin

The possible mechanisms of the opposite affinity pattern of the enantiomers of dimethindene [(R,S)-N,N-dimethyl-3[1(2-pyridyl)ethyl]indene-2-ethylamine] (DIM) towards native beta-cyclodextrin (beta-CD) and heptakis(2,3,6-tri-O-methyl-)-beta-CD (TM-beta-CD) were studied using capillary electrophoresis (CE), NMR spectrometry, electrospray ionization mass spectrometry (ESI-MS) and X-ray crystallography. NMR spectrometry allowed to estimate the stoichiometry of the complex and to determine the binding constants. As found using ESI-MS, together with more abundant 1:1 complex, a complex with 1:2 stoichiometry may also be present in a rather small amount in a solution of DIM and beta-CD. One-dimensional ROESY experiments indicated that the geometry of the complexes of DIM with native beta-CD depends on the ratio of the components in the solution. In the 1:1 solution of DIM and beta-CD the complex may be formed by inclusion of the indene moiety of DIM into the cavity of beta-CD on the primary side and into the cavity of TM-beta-CD into the secondary side. The most likely structural reason for lower affinity of the enantiomers of DIM towards the cavity of TM-beta-CD compared to native beta-CD could be elucidated. The indene moiety does not enter the cavity of TM-beta-CD as deeply as the cavity of beta-CD. This may be the most likely explanation of significantly higher affinity constants of DIM enantiomers towards the latter CD compared to the former one. The marked difference between the structure of the complexes may also be responsible for the opposite affinity pattern of the DIM enantiomers towards beta-CD and TM-beta-CD.

Determination of carnitine and acylcarnitines in biological samples by capillary electrophoresis-mass spectrometry

Free carnitine and acylcarnitines (carnitine esters) play an important role in the metabolism of fatty acids. Metabolic disorders can be detected by abnormal levels of these compounds in biological fluids. Capillary electrophoresis-mass spectrometry has the advantage of combining an efficient separation technique with highly selective detection. Therefore, we have developed a method for the determination of carnitine and several of its esters implementing electrospray capillary electrophoresis-mass spectrometry in the positive ion selected reaction monitoring mode. A sheath-flow interface with a mixture of 2-propanol or methanol, water and acetic acid as sheath liquid and nitrogen as nebulizing gas was used. The zwitterionic analytes migrated as cations in the applied electric field using ammonium acetate-acetic acid or formic acid electrolytes. Separations were performed in aqueous, mixed organic-aqueous and non-aqueous media. The influence of the electrolyte composition on the separation efficiency was investigated. The electrospray conditions have been optimized regarding ion current stability and sensitivity. Ammonium acetate (10 mmol/l)-0.8% formic acid in water or 6.4% formic acid in acetonitrile-water (1:1) were used as running buffers for the determination of carnitine and acylcarnitines in human biological samples. Methanol extracts of dried blood spots were analyzed as well as urine and plasma following sample preparation via solid-phase or liquid-liquid extraction. Recoveries approaching 100% were achieved depending on the analytes and sample preparation procedures employed. Endogenous carnitine and acetylcarnitine were determined at concentrations between 2.7 and 108 nmol/ml in normal human urine and plasma. Other acylcarnitines were detected at levels of below the limit of detection to 12 nmol/ml. Good precision (0.8 to 14%) and accuracy (85 to 111%) were obtained; the achieved limits of quantitation (0.1 to 1 nmol/ml) are sufficient to characterize carnitine and acylcarnitine levels occurring as markers for metabolic disorders.

Direct chromatographic resolution of carnitine and O-acylcarnitine enantiomers on a teicoplanin-bonded chiral stationary phase

R-(-)-Carnitine (vitamin B(T)) plays an important role in human energy metabolism, by facilitating the transport of long-chained fatty acids across the mitochondrial membranes. Its (S)-enantiomer acts as a competitive inhibitor of carnitine acetyltransferase, causing depletion of the body R-(-)-carnitine stock. Consequently, the separation of carnitine enantiomers is very important both to study their biological activities and to control the enantiomeric purity of pharmaceutical formulations. In the present paper we describe an easy, fast and convenient procedure for the separation of the enantiomers of carnitine and O-acylcarnitines by enantioselective HPLC on a laboratory-made chiral column containing covalently bonded teicoplanin as selector. High enantioselectivity factors (alpha values ranging from 1.31 to 3.02) and short-time analyses characterize the analytical procedure; in addition, analytes are easily detected by evaporative light scattering with no need for preliminary derivatization. The effects of pH and ionic strength of the mobile phase and of the nature of the organic modifier on the enantioselective separations were also investigated.

Chiral separations in capillary electrophoresis

The marked increase in the number of communications on the utilization of electrophoresis for practical chiral separations within the last three years is the most evident, and the most important fact. It reveals that the basic period of intensive research in the field is finished. The search for chiral selectors discriminating racemates in a reasonably analytical manner and the study of both the mechanism and physicochemical aspects of the chiral discrimination process were the main features of that period. Here, we review the state of the art in the field and state the references of the related literature up to the end of 1998.

Enantiomeric separation of D- and L-carnitine by integrating on-line derivatization with capillary zone electrophoresis

A new capillary zone electrophoretic method has been developed for the enantiomeric separation and quantification of enantiomers of carnitine, D- and L-carnitine were derivatized with 9-fluorenylmethyl chloroformate in a flow system, working on-line with the capillary electrophoretic equipment. The separation was performed using a selective chiral buffer containing 2,6-dimethyl-beta-cyclodextrin (heptakis). Triethanolamine was used as electroosmotic modifier and the separation was carried out in a uncoated capillary. Under the optimal conditions the resolution between D- and L-carnitine was 1.2 and the limits of detection for both isomers were about 5.0 microM. The proposed method was applied to the determination of D-carnitine in excess of L-carnitine in synthetic samples, and the results demonstrated that the maximal D-:L-carnitine ratio determined was approximately 1:100.

Controlling enantioselectivity in chiral capillary electrophoresis with inclusion-complexation

The separation of chiral compounds is of key importance in different fields of application, e.g., pharmaceutical, industrial, forensic, biological, clinical etc. Capillary electrophoresis (CE) is a powerful analytical method applied in chiral analysis and inclusion-complexation is one of the most frequently used mechanism to improve the selectivity of the enantiomeric separation. Cyclodextrins and their derivatives or modified crown-ethers have been successfully applied in CE for the enantiomeric separation of a wide number of analytes. This review surveys the separation of enantiomers by CE when chiral selectors, forming inclusion-complexation, are used. The control of enantioselectivity can be done carefully by considering several experimental parameters such as chiral selector type and concentration, pH, ionic strength and concentration of the background electrolyte, electroosmotic flow, organic modifier etc. The review presents a list of the latest separation of enantiomers by CE where inclusion-complexation plays a key role in the stereoselective separation mechanism.

Capillary electrophoretic separations of enantiomers using cyclodextrin-containing background electrolytes

The 1996 primary literature papers which deal with the separation of enantiomers using cyclodextrins are reviewed here. Though the majority of the papers still use the neutral native cyclodextrins or the neutral derivatized cyclodextrins as resolving agents, there was a significant increase in number of separations which relied on charged cyclodextrins, both weak electrolytes and strong electrolytes, as resolving agents. Also, there was an increase in the number of papers which reported binding constants and correlated them with other physical or chemical characteristics of the analytes. Several successful minor enantiomer determinations were presented, pushing the reliable quantitation levels below 0.1%. Work continued on the simultaneous use of neutral and charged cyclodextrins to improve separation selectivity or peak resolution.