Disclosed are an apparatus, system, and method for performing electrospray of biomolecules, particularly peptides, polypeptides, and proteins. The apparatus comprises at least (1) a microfluidic substrate for containing an electrospray microchannel for delivering analyte molecules to a side edge of the substrate, and (2) a porous membrane attached to the side edge for performing electrospray from the exposed membrane surface. In one preferred embodiment, the exposed membrane surface is positioned above a target surface for depositing analyte molecules onto the target surface by electrospray. In another preferred embodiment, a proteolytic enzyme is bound to the porous membrane for performing protein digestion during electrospray.