On-line electrodialytic salt removal in electrospray ionization mass spectrometry of proteins

Salts and buffers, commonly used in isolation and stabilization of biological analytes, have a deleterious effect on electrospray ionization mass spectrometry (ESI-MS). Excessive concentrations of salts lead to ion suppression and adduct formation, which mask or complicate ion signals. In this work, we describe a salt remover (SR), configured as a three-compartment flow-through system, where the central compartment is separated from the outer compartments by a cation-exchange membrane (CEM) and an anion-exchange membrane (AEM). One platinum electrode is placed in each of the outer compartments, where water or electrolyte is flowing. The CEM electrode is held at a negative potential relative to the AEM side; cations/anions migrate by electrophoresis to the CEM/AEM receiver liquids, respectively. Proteins have poorer electrophoretic mobility relative to the buffer components, permitting removal of the salt. The salt-free proteins proceed to the ESI source. The capillary scale SR (internal volume 2.5 μL) described here effectively desalted continuous flows of NaCl solutions (200 mequiv/L at 1 μL/min, equivalent to a flux of 200 nequiv/min with 88% efficiency) and achieved >99.8% salt removal with 154 mM NaCl (isotonic saline) at 1 μL/min. With optimized current, >80% of concurrently present 20 μM protein was transmitted. Desalting efficiency and analyte loss was evaluated with different salt concentration and flow rate combinations under different applied current. Good-quality ESI-MS spectra of cytochrome c, myoglobin, and lysozyme as test proteins in a saline solution, passed through the SR, are demonstrated.