Coupling of a Gas-Phase Chemiluminescence Nitrogen Detector and a Capillary Electrophoretic System

Abstract

A capillary electrophoretic system has been successfully connected to a gas-phase chemiluminescence nitrogen detector via a pneumatic nebulizer interface. The interface, built in-house, consists of a nebulizing gas delivery subsystem, a sheath liquid subsystem, a short spray tube, and a liquid gap. The liquid gap is formed at the point where the spray tube, the separation capillary, and the sheath liquid subsystem meet. The sheath liquid subsystem consists of a grounding connection to complete the electric circuit for the electrophoretic system, a sheath liquid delivery pump, a sheath liquid overflow outlet maintained at atmospheric pressure, and a sheath liquid conduit with a hydrodynamic resistance that is much smaller than that of the separation capillary. The interface operates at the natural self-aspiration rate of the short spray tube. The design ensures that the natural self-aspiration rate of the nebulizer is higher than the maximum electroosmotic flow rate that can be produced in the separation capillary. The flow difference is made up by the sheath liquid which, due to the hydrodynamic resistance differences, is sucked into the liquid gap preferentially from the sheath liquid conduit. Thus, the spray tube and the separation capillary are decoupled from each other hydrodynamically, and any laminar flow-induced additional band broadening in the separation capillary is avoided. Using the combined electrophoretic separation and gas-phase chemiluminescence nitrogen detector system, the mass detection limit for five nucleotide bases used as test compounds was found to be about 10 pmol nitrogen.