Porous Electrodes Supported on Ion-Exchange Membranes as Electrochemical Detectors for Supercritical Fluid Chromatography

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Abstract
A conveniently assembled electrochemical cell, exploiting a porous electrode supported on a moist perfluorinated ion-exchange polymer, is proposed for profitable electrochemical detection in supercritical fluid chromatography. It consists of a porous Pt working electrode, contacted by the mobile phase from the chromatographic column, which is chemically deposited onto one side of a Nafion membrane. The rear uncoated side of this membrane, acting as a solid polymer electrolyte, is contacted by an electrolyte solution (1 M NaCl) contained in an internal compartment equipped with a Pt counter electrode and a Ag/AgCl, Cl- 1 M reference electrode. Ferrocene, eluted with supercritical carbon dioxide through a Spherisorb column installed in a supercritical fluid chromatographic system, was used as electroactive prototype analyte to test the performance of this detector, which turned out to be quite better than that provided by a conventional on-line UV absorbance detector. The recorded peaks were characterized by both a good reproducibility (4.5%) and a linear dependence of their height and area, which extended over a wide concentration range (∼3 orders of magnitude). Moreover, they were not interfered by possible solvent front, unlike peaks recorded by the UV detector. The detection limit, estimated for a signal-to-noise ratio of 3 (4.2 × 10-11 mol), was lower by ∼1 order of magnitude than that found for the UV detector. Finally, the long-term stability of this detector was satisfactory in that only a ∼6% decrease in the current responses was observed after a rather long period (2 months) of continuous use.