Hiroyuki Nishi

Chiral separation by cyclodextrin-modified micellar electrokinetic chromatography

Chiral separation by micellar electrokinetic chromatography (MEKC), which permits the separation of uncharged or electrically neutral compounds by the electrophoretic technique, was achieved through the addition of cyclodextrins (CDs) to sodium dodecyl sulphate (SDS) micelle solution. In this cyclodextrin-modified MEKC (CD-MEKC), CDs cannot be solubilized to the SDS micelle and migrate with the same velocity as that of the electroosmotic flow. The solutes are distributed among three phases, an aqueous phase, the micelle and the CD. Chiral recognition depended on the type of CD; in particular, γ-CD was effective for the chiral separation in this method. The addition of an organic solvent or a chiral compound such as sodium d-camphor-10-sulphonate or l-menthoxyacetic acid to the SDS micelle solution containing CDs improved the enantioselectivity. The addition of CDs reduced the capacity factors of solutes; in contrast, chiral additives increased them. The resolution was optimized by changing the concentrations of CDs and chiral additives. The chiral separation mechanism is also briefly discussed.

Separation of enantiomers by capillary electrophoretic techniques

Abstract This review surveys the separation of enantiomers by capillary electrophoretic (CE) techniques for those who are not highly acquainted with CE techniques. The CE techniques described are capillary zone electrophoresis, electrokinetic chromatography, isotachophoresis, capillary gel electrophoresis and electrochromatography. The fundamental separation theory of each CE technique is discussed from the viewpoint of the optimization of selectivity and resolution. Enantiomeric separations by the CE techniques are mostly based on complex formation between the analytes and the chiral selectors added to the separation system, and therefore the chiral additives and their effects are discussed, including interaction models. The indirect separation of enantiomers by the prederivatization to diastereomers is also briefly discussed.

Separation and determination of lipophilic corticosteroids and benzothiazepin analogues by micellar electrokinetic chromatography using bile salts.

The separation of corticosteroids and benzothiazepin analogues by micellar electrokinetic chromatography (micellar EKC) was studied in comparison with capillary zone electrophoresis. The separation of these substances was not successful under neutral and alkaline conditions because they migrated with the same velocity as that of the electroosmotic flow. Micellar EKC with sodium dodecyl sulphate (SDS) solutions was also not successful because these substances migrated with almost the same velocity as that of the SDS micelle, owing to their high lipophilicity. The use of bile salts, which have a similar skeleton to corticosteroids, as the micellar phase permitted the separation of these substances with high theoretical plate numbers (150,000-350,000) within a short time (ca. 15 min). Sodium cholate was particularly useful. The effects of bile salt concentration, pH and the addition of methanol were investigated. Micellar EKC was also applied to the determination of the drug substances in tablets and cream using the internal standard method and to purity testing of drug substances and tablets.

Separation of water-soluble vitamins by micellar electrokinetic chromatography

The retention behaviour of eleven water-soluble vitamins in micellar electrokinetic chromatography (micellar EKC) was investigated in comparison with capillary zone electrophoresis. Sodium dodecyl sulphate (SDS) and sodium lauroylmethyl taurate were used as the anionic surfactants at concentrations of 0.05-0.2 M in micellar EKC. The retention times of cationic substances increased more rapidly with increasing concentration of the anionic surfactant than those of other substances. This result suggests that ion-pair formation between cationic substances and anionic surfactants contributes to the retention of the former. The difference in the structures of the two surfactants affects the retention behaviour of solutes, especially cationic substances. To clarify the effect of the micelle, an ion-pairing agent that does not form the micelle structure was employed. All solutes were successfully separated within 15 min by using a 650 mm x 0.05 mm I.D. fused-silica tube with a 0.05 M SDS solution (pH 9.0) to give theoretical plates ranging from 100,000 to 350,000.

Separation of β-lactam antibiotics by micellar electrokinetic chromatography

Abstract The retention behaviour of β-lactam antibiotics in micellar electrokinetic chromatography (EKC) was investigated. Sodium dodecyl sulphate (SDS) and sodium N-lauroyl-N-methyltaurate were used as anionic surfactants at concentrations of 0.05–0.3 M. It was found that the retention of ionic substances in micellar EKC is determined by the following three factors: the electrophoretic migration of the ionic substances, the interaction between the ionic substances and ionic surfactants and solubilization of the solute by the micellar phase. A difference in the retention behaviours of cationic substances was observed between the two anionic surfactants, which have different groups neighbouring the charge-bearing groups. The effect of an ion-pairing reagent was also investigated to make the effect of the micelle clearer. All test solutes were successfully separated by micellar EKC at SDS concentrations above 0.1 M, with theoretical plate numbers ranging from 70 000 to 260 000.

Enantiomer separation of amino compounds by a novel chiral stationary phase derived from crown ether

Abstract A novel chiral stationary phase (CSP-18C6I) was prepared by immobilizing (+)-18-crown-6 tetracarboxylic acid on 3-aminopropylsilanized silica-gel to separate enantiomers of drugs having a primary amino group. The chiral crown ether was combined with 3-aminopropyl silica gel (0.85 mmol of amine per gram of gel) to provide CSP-18C6I having a chiral selector loading of 0.26 mmol per gram of gel. This CSP-18C6I showed good chiral recognition for thirteen out of eighteen dl -amino acids and seven racemic aminoalcohols using a dilute aqueous solution of perchloric acid as the eluent. Afloqualone (a muscle relaxant), primaquine (an antimalarial), and 1-(1-naphthyl)ethylamine (1-NEA) were resolved on the CSP-18C6I. Furthermore, alanine-β-naphthylamide (Ala-β-NA), which is hydrophobic and did not elute within 60 min by the commercially available CROWNPAK CR(+) with 15% methanol and a column temperature of 40°C, was successfully enantioseparated by the novel CSP-18C6I.

Separation and determination of the ingredients of a cold medicine by micellar electrokinetic chromatography with bile salts

The separation of fourteen active ingredients used in a cold medicine was investigated by micellar electrokinetic chromatography (EKC) employing bile salts. Basic drugs were also successfully separated by micellar EKC using bile salts with high theoretical plate numbers (2.0 x 10(5)-3.5 x 10(5)) within a relatively short time (ca. 20 min). The separation of these solutes by micellar EKC was not successful using sodium dodecyl sulphate. The effects of micellar concentration, pH and organic modifier content on migration times and selectivity were investigated. This technique was also applied to the determination of several active ingredients combined in commercial preparations by an internal standard method.

Separation and determination of aspoxicillin in human plasma by micellar electrokinetic chromatography with direct sample injection

Both the separation and determination of aspoxicillin in human plasma by micellar electrokinetic chromatography (MEKC) were investigated. Selectivity in the separation of seven penicillin antibiotics was improved by using MEKC in comparison with capillary zone electrophoresis. Plasma proteins, which might interfere with drug analysis in conventional chromatography, were solubilized by the micelles employed in MEKC and eluted later than the drugs. This permitted the determination of the drugs in plasma by a direct sample injection method. One analysis of a plasma sample was performed within ca. 20 min without pretreatment. Good linearity and recovery were also obtained in the range of plasma levels usually encountered in clinical analysis with a correlation coefficient r = 0.999 and 94-104% recovery. The limit of detection for aspoxicillin was 1.3 micrograms ml-1 at a signal-to-noise ratio of 3.

Separation of corticosteroids and aromatic hydrocarbons by cyclodextrin modified micellar electrokinetic chromatography

Abstract Micellar electrokinetic chromatography (MEKC) permits the separation of uncharged or electrically neutral compounds by the electrophoretic technique, but highly lipophilic compounds, e.g., corticosteroids and aromatic hydrocarbons could not be resolved by MEKC with sodium dodecyl sulfate (SDS) solutions because such solutes migrated with almost the same velocity as that of the SDS micelle, owing to their large micellar solubilization. the addition of cyclodextrins (CDs) to the SDS solution, that is, cyclodextrin-modified micellar electrokinetic chromatography (CD/MEKC), remarkably improved the resolution of these lipophilic compounds. the effects of concentration and the type of CDs on the separation of corticosteroids and aromatic hydrocarbons were investigated.

Chiral separation of trimetoquinol hydrochloride and related compounds by micellar electrokinetic chromatography using sodium taurodeoxycholate solutions and application to optical purity determination

Abstract The chiral separation of trimetoquinol hydrochloride, which is a bronchodilator (Inolin), and three related compounds by micellar electrokinetic chromatography was investigated using a bile salt as a chiral surfactant. Enantiomers of these compounds, except laudanosoline, were successfully separated within 12 min using a separation tube of effective length 500 mm × 0.05 rum i.d. and a 0.05 M sodium taurodeoxycholate solution of pH 7.0. The observed theoretical plate numbers of the peaks were ca. 150000. Chiral recognition was affected by the structure of bile salts, the pH of the buffer solutions used and the structure of the solutes. Of four kinds of bile salts, successful chiral separation was achieved only using sodium taurodeoxycholate solution under neutral conditions. The method was applied to the optical purity determination of trimetoquinol hydrochloride. The effects of surfactant concentrations and some additives to the micellar solution are briefly described.