Highly sensitive detection of chiral amino acids by CE based on on-line stacking techniques

We report a novel means for chiral separation and stacking of amino acids by MEKC with LED-induced fluorescence detection. Naphthalene-2,3-dicarboxaldehyde, hydroxypropyl-beta-cyclodextrin (Hp-beta-CD), SDS, and poly(ethylene oxide) (PEO) serve as a derivatized agent, chiral selector, pseudostationary phase, and concentrated medium, in sequence. To improve speed, resolution, and stacking efficiency, the analysis of chiral amino acids was performed under discontinuous conditions--the capillary was filled with a solution of 100 mM Tris-borate, 150 mM SDS, and 50 mM Hp-beta-CD, whereas buffer vials contain 20 mM Tris-borate, 150 mM SDS, 50 mM Hp-beta-CD, and 0.5% w/v PEO. A solution of nonionic PEO enters the capillary with the help of EOF during the separation. Through interaction of SDS micelles/Hp-beta-CD and chiral amino acid, the negatively charged complexes migrated into the PEO solution and stacked at the boundary between the sample zone and the PEO solution. Compared with normal sample injection (10 nL sample volume), a several hundred-fold sensitivity improvement for chiral amino acids was obtained under the injection of 270 nL sample volume (30% of the capillary volume). Meanwhile, the LOD at S/N of three for DL-amino acids were in the range of 0.18-0.22 nM. The proposed method has been applied for the determination of DL-leucine in urine and plasma samples.