Sample enrichment in a single levitated droplet for capillary electrophoresis

This paper describes sample enrichment in a single levitated droplet for capillary electrophoresis (CE) analysis. The droplet was trapped in an acoustical field. The minute sample volumes needed for the enrichment procedure were precisely handled using a piezoelectric flow-through liquid microdispenser. Droplets with a volume of 65 pl were ejected from the device at a repetition rate ranging from one single droplet up to several hundreds per second. By counting the number of droplets ejected and accumulated in the levitated drop the sample volume was controlled. Through solvent evaporation the analytes were enriched in the diminishing droplet. The droplet was then injected into a CE capillary and the analytes, dansyl-Gly and dansyl-Val dissolved in ethanol, were separated in a 100 mM borate buffer (pH 9.0) utilising UV-absorption detection at 200 nm near the capillary outlet. Enrichment of 36000 sample droplets (2.3 microl) through solvent evaporation in the levitated drop resulted in a concentration limit of detection (CLOD) of 15 nM for the dansylated amino acids as compared to a CLOD of 2.5 microM which was achieved using standard hydrodynamic injection without preconcentration.