A method of online sample processing for electrospray ionization, comprising: providing a microfluidic device comprising: a fluidic sample separation channel; an ESI emitter in communication with the sample separation channel; a sample reservoir in communication with the sample separation channel; a background electrolyte reservoir; and a background electrolyte channel in fluid communication with the BGE reservoir, wherein the BGE channel is connected to the sample separation channel at a junction proximate to the ESI emitter; and selectively transporting at least one target analyte of the sample from the sample reservoir to the ESI emitter; characterized in that selectively transporting the at least one target analyte comprises generating a first axial electroosmotic flow along an axial direction in the sample separation channel in a direction away from the ESI emitter, and generating a second axial EOF in the BGE channel along an axial direction in the BGE channel in a direction toward the ESI emitter, the second axial EOF having a magnitude that is greater than a magnitude of the first axial EOF thereby inhibiting transport of contaminant materials contained in the sample reservoir. providing a microfluidic device comprising: a fluidic sample separation channel; an ESI emitter in communication with the sample separation channel; a sample reservoir in communication with the sample separation channel; a background electrolyte reservoir; and a background electrolyte channel in fluid communication with the BGE reservoir, wherein the BGE channel is connected to the sample separation channel at a junction proximate to the ESI emitter; and selectively transporting at least one target analyte of the sample from the sample reservoir to the ESI emitter; characterized in that selectively transporting the at least one target analyte comprises generating a first axial electroosmotic flow along an axial direction in the sample separation channel in a direction away from the ESI emitter, and generating a second axial EOF in the BGE channel along an axial direction in the BGE channel in a direction toward the ESI emitter, the second axial EOF having a magnitude that is greater than a magnitude of the first axial EOF thereby inhibiting transport of contaminant materials contained in the sample reservoir. a fluidic sample separation channel; an ESI emitter in communication with the sample separation channel; a sample reservoir in communication with the sample separation channel; a background electrolyte reservoir; and a background electrolyte channel in fluid communication with the BGE reservoir, wherein the BGE channel is connected to the sample separation channel at a junction proximate to the ESI emitter; and selectively transporting at least one target analyte of the sample from the sample reservoir to the ESI emitter; characterized in that selectively transporting the at least one target analyte comprises generating a first axial electroosmotic flow along an axial direction in the sample separation channel in a direction away from the ESI emitter, and generating a second axial EOF in the BGE channel along an axial direction in the BGE channel in a direction toward the ESI emitter, the second axial EOF having a magnitude that is greater than a magnitude of the first axial EOF thereby inhibiting transport of contaminant materials contained in the sample reservoir.