Methods, devices, and systems for performing isoelectric focusing reactions are described. The systems or devices disclosed herein may comprise fixtures that have a membrane. In some instances, the disclosed devices may be designed to perform isoelectric focusing or other separation reactions followed by further characterization of the separated analytes using mass spectrometry. The disclosed methods, devices, and systems provide for fast, accurate separation and characterization of protein analyte mixtures or other biological molecules by isoelectric point.