Stacking and separation of enantiomers by acetonitrile‐salt mixtures in micellar electrokinetic chromatography

Abstract

The feasibility of employing the "acetonitrile stacking" method in micellar electrokinetic chromatography (MEKC) for the on-line preconcentration and separation of enantiomers is demonstrated for the first time. The effects of various experimental parameters on the stacking and separation of three different pairs of optical isomers, i.e., two substituted naphthyl enantiomers and one dansylated-DL-amino acid, were examined. In particular, the effectiveness of the addition of acetonitrile and salt in the sample matrix to induce narrowing of the analyte bands was investigated in the presence of sodium cholate as the chiral surfactant micelle in the separation buffer. For example, it was found that the presence of both acetonitrile and 1% NaCl in the sample matrix (volume ratio = 2:1) led to a significant improvement of the peak height and resolution for the MEKC separation of a pair of R(-)/S(+)-1,1'-binaphthyl diyl hydrogen phosphate enantiomers when the injection sample size was relatively large (e.g., 12% capillary volume). Furthermore, the feasibility of combining salting-out solvent extraction (off-line) and acetonitrile stacking (on-line) as a novel approach for sample preconcentration in capillary electrophoresis was also demonstrated.

Sensitive chiral analysis by capillary electrophoresis

“…On the other hand, the effect of ACN and salt in the sample matrix on the stacking and separation of naphthyl derivatives has provided an improvement of peak efficiency and thus concentration detection sensitivity when the sample injection size was relatively large. However, further investigations may be done to clarify this effect which has not been observed for other molecules [101]. Finally, sweeping preconcentration of the phenoxyacid herbicide fenoprop enabled its detection at ppb levels (,10 28 M) [94].…”

MEKC: An update focusing on practical aspects

“…There have been several reports on the use of sample stacking in MEKC over the period covered in this review. In most cases, the normal stacking mode with SDS was used to improve the sensitivity in the determination of various analytes, such as Viagra and its metabolite [47], plant hormones [48], morphine [49], and ecdysteroids [50]; however, other stacking modes have also been employed such as stacking with reverse migration of micelles [51][52][53] and high-salt sample stacking [54] in addition to new approaches such as so-called "ACN stacking" (viz the addition of ACN and salts to the sample matrix in order to narrow analyte bands) [55] and the "orthogonal analyte stacking/injection system" proposed by Palmer et al [56].…”

Enhanced detection in capillary electrophoresis: Example determination of serum mycophenolic acid

“…The analytes stack temporarily between the leading ion and the acetonitrile, analogous to that in ITP, before they enter the separation buffer [2]. Several workers have confirmed this type of stacking for different compounds varying from drugs to peptides [4][5][6][7][8][9][10]. However, this stacking lacks the basic and theoretical studies necessary to maximize its full potential.…”