Sensitivity and selectivity studies in capillary electrophoresis

Sammanfattning: Capillary electrophoresis is characterised by high separation efficiency, applicability to a wide range of compounds, small sample and solvent consumption and simple instrumentation. To improve sensitivity using on-line absorbance detectors, which are hampered by the short optical path length, sample volumes in the microlitre range were concentrated to nanolitre volumes either off-line or on-line. Sample enrichment in a single acoustically levitated droplet by solvent evaporation lowered the concentration detection limit by two orders of magnitude for dansylated amino acids. The sample was positioned in the levitator using a piezoelectric flow-through liquid microdispenser that ejected 100 droplets (each with a volume of 65 pL) per second. The levitated sample drop was then picked up using the separation capillary. Because levitation is a containerless technique, the sample was easily accessed and the risk of sample losses on container walls eliminated. Sample enrichment using miniaturised on-line solid-phase extraction lowered the concentration detection limit by almost four orders of magnitude using the basic drug terbutaline as model compound. A short length of capillary packed with reversed-phase sorbent particles was attached to the separation capillary inlet. The analytes were desorbed using an organic solvent, e.g. acetonitrile. The ratio between introduced sample volume and desorption volume was about 1000. The use of organic solvents promoted further enrichment by stacking. This contributed to the excellent separation efficiency and high concentration factors achieved. The concentration limit of detection was lowered by a factor of 7000, which allows low nanomolar concentrations to be detected using UV-absorbance detection. The selectivity in capillary electrophoresis was enhanced by addition of surfactant or cyclodextrin. Monomer and oligomers of a basic cyclic peptide were separated using micellar electrokinetic chromatography with sodium dodecyl sulphate. The complex cyclic peptide sample contained monomer, dimers, trimers and tetramers. Native and derivatised b -cyclodextrins were used for enantiomer separations of basic drugs (b -adrenergic antagonists) at low electrolyte pH. An entire enantiomer separation of a dansylated amino acid using b -cyclodextrin with the addition of organic solvent was captured in real-time using laser-induced fluorescence imaging detection. The migrating analytes were imaged in a 10 cm long detection window.