Strong and small: strong cation-exchange solid-phase extractions using porous polymer monoliths on a digital microfluidic platform

We present the first method for digital microfluidics-based strong cation-exchange solid-phase extractions. Digital microfluidics is a microscale fluid handling technique in which liquid droplets are actuated over an array of electrodes by electrodynamic forces. Strong cation exchange has gained considerable importance in the field of proteomics as a separation mode for protein and peptide extractions. The marriage of these two techniques is achieved by incorporating sulphonate-functionalised porous polymer monolith discs onto digital microfluidic chips. By manipulating sample and solvent droplets onto and off of these porous polymer monoliths, proteins and peptides are extracted by controlling solution pH and ionic strength. This novel microscale extraction method has efficiency comparable to commercially available strong cationexchange ZipTips and is highly effective for sample cleanup. We anticipate that this digital microfluidic strong cationexchange extraction technique will prove useful for microscale proteomic analyses and other applications requiring separation of cationic compounds.