Electrospray from a droplet

A method for achieving electrospray ionization directly from a droplet is reported. The electrospray is formed at the tip of a capillary wick bonded to a glass substrate with patterned electrodes. The droplet of sample solution is delivered to the base of the wick, forming a reservoir. Capillary action draws the solution from the reservoir to the tip of the wick, continuously replenishing the sample as it is emitted via electrospray. No external pressure source is required to drive the flow. The glass surface of the substrate is made hydrophobic to ensure the integrity of the reservoir. An 8 μL droplet of 0.4% (w/v) bovine serum albumin solution maintains a steady spray for 1–2 min operating in cone-jet mode. The electrospray emitter is compatible with digital microfluidic schemes that employ single-plate droplet actuation.