Hydroquinone as a Buffer Additive for Suppression of Bubbles Formed by Electrochemical Oxidation of the CE Buffer at the Outlet Electrode in Capillary Electrophoresis/Electrospray Ionization-Mass Spectrometry

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Abstract
Hydroquinone was found to suppress bubble formation at the outlet electrode of a sheathless capillary electrophoresis/electrospray ionization-mass spectrometer by replacing the oxidation of water (2H2O(l) ↔ O2(g) + 4H+ + 4e) with that of more easily oxidized hydroquinone (hydroquinone ↔ p-benzoquinone + 2H+ + 2e). Formation of p-benzoquinone replaces the formation of oxygen gas, effectively suppressing gas bubble formation. Several electrode materials, including platinum, gold-coated stainless steel, and stainless steel wires, were tested. However, hydroquinone suppressed bubbles only at the platinum electrode. Combination of the in-capillary electrode sheathless interface using platinum wire, hydroquinone as a buffer additive, and pressure programming at the inlet of the capillary electrophoresis provided a rugged high efficiency interface for analysis of protein digests using CE/ESI-MS.