Chemical Modification of Carbonaceous Stationary Phases by the Reduction of Diazonium Salts

This paper describes a new strategy for the creation of chemically modified carbonaceous stationary phases. The strategy exploits the electroreduction of arenediazonium salts as a means for functionalizing the surface of glassy carbon (GC) and porous graphitic carbon (PGC) stationary phases. The one-electron reduction of these salts forms an arene radical which then couples via a carbon−carbon linkage to the carbon framework at the surface of the stationary phase. Two arenediazonium-based modifiers were used in evaluating the potential utility of this strategy: 4-nitrobenzenediazonium tetrafluoroborate for the GC and PGC phases and 4-hexylbenzenediazonium tetrafluoroborate for only the PGC phases. Modifications were carried out by packing the phases into a column used for electrochemically modulated liquid chromatography. The effectiveness of the modifications was assessed by X-ray photoelectron spectroscopy and by comparing the liquid separation of a series of mixtures before and after coating deposition. For the nitrobenzyl-modified GC phase, the test mixture contained both anisole and fluoranthene. The performance of the nitrobenzyl- and hexylbenzyl-modified PGC stationary phases was characterized by the separations of substituted phenols (i.e., nitrophenol and resorcinol) and a few important pharmaceutical agents (i.e., hexobarbital, oxazepam, and nitrazepam). The potential utility of this modification procedure to form stationary phases that are stable upon extended exposure to aggressive mobile phases is discussed and briefly examined.