Development of an amperometric detector for packed capillary column supercritical fluid chromatography

Amperometric detection at a naked platinum microelectrode is shown to be compatible with pressure-programmed packed capillary column supercritical fluid chromatography (SFC) using carbon dioxide modified with as little as 1% acetonitrile. Amperometric detection in modified carbon dioxide is, hence, possible without addition of salts, which otherwise may limit the stability of the chromatographic system. The detection, which can been based on either oxidations or reductions, is also compatible with the use of methanol as a sole modifier. Picogram amounts of ferrocene could be detected after separations both at a constant pressure and under pressure-programmed conditions. The detection limit for ferrocene, at a constant pressure of 200 atm, was found to be approximately 20 pg. The response of the amperometric detector was observed to increase with decreased mobile phase density and increased amount of added modifier. The injection of increasing amounts of analytes resulted in increased peak tailing, most likely due to the limited solubility of the oxidation products in the mobile phase. The influence of this effect was, however, small for the amounts of analytes relevent in capillary column SFC.