A CE/ESI-MS Interface for Stable, Low-Flow Operation

A rugged, stable liquid sheath interfacing procedure for on-line CE/MS is described. This procedure combines optimized component sizes, tapered capillary tips, and adjustment of the capillary with respect to the liquid sheath tube during electrospray operation (active capillary positioning) to establish a stable electrospray quickly and reliably. The interface is especially effective at low flow rates, and CE/MS with a liquid sheath flow rate of 250 nL/min has been achieved. Active capillary positioning also allows on-line capillary isoelectric focusing mass spectrometry with in-probe focusing. In this application, the capillary is retreated into the liquid sheath tube, which becomes a microreservoir. After focusing, the capillary is returned to its proper position for electrospray, and focused zones are mobilized into the mass spectrometer for on-line detection. Both active capillary positioning and in-probe focusing readily lend themselves to automation.