Molecular Size-Selective Electrodialytic Desalters for Electrospray Ionization-Mass Spectrometry

A membrane-based electrodialytic desalter has been developed for the selective removal of buffer/salt constituents from a fluid stream while retaining larger charged molecules such as proteins prior to mass spectrometric (MS) detection. The salts are deleterious to MS causing signal suppression, formation of adducts, and eventual contamination of the inlet. The new device uses dialysis membranes (DMs) paired with ion exchange membranes (IEMs) flanking the carrier flow channel in a planar configuration. The DMs contact the carrier channel preventing adsorptive losses of large, charged molecules to the IEMs. Ions are removed under an applied electric field using four pairs of electrodes along the flow channel. Removal of both anions and cations is more energy intensive than conversion of a suitable MS friendly salt into its respective acid or base, for example, ammonium acetate into acetic acid. The energetics and optimal voltage profiles for both scenarios have been thoroughly investigated. The DMs resulted in nonlinear increases in energy required for desalting over standard IEM devices due to electroosmotic flow of water into the interstitial space between the membranes. For a device channel with nominal volume of 15.2 μL, a maximum concentration of 200 mM ammonium acetate flowing at 0.25 mL/min was converted into acetic acid. Recovery of bovine serum albumin measured at 280 nm was 67%-96% at tested salt concentrations, and dispersion volumes were less than 200 μL² and may be suitable for coupling to liquid chromatography.