The detection of a range of neutral organic compounds in liquid chromatography based on decreases in differential double-layer capacitance, ΔCd, at a mercury—aqueous interface caused by analyte specific adsorption is described. The arrangement employs alternating current phase-selective measurements using a large-volume wall-jet configuration. The low-molecular-weight organic solutes examined include aliphatic alcohols, diols, mono- and dicarboxylic acids, amines and alkanolamines. The differential capacitance measurements were conducted close to the potential of zero charge, where adsorption of such species is most extensive. Plots of ΔCdversus analyte concentration were generally sigmoidal, in accordance with expectations from the Frumkin adsorption isotherm. A potential-step coulometric method, where variations in the non-faradaic charge, Δ(Δq), are measured, was found to be a useful alternative detection scheme. Methods based on measuring transient capacitive currents associated with tensammetric adsorption—desorption behavior were also briefly investigated. A virtue of these double-layer capacitance detection schemes is that the magnitude of the ΔCd or Δ(Δq) response exhibits a clear sensitivity to the organic molecular structure, as anticipated in view of the known dependence of the adsorption thermodynamic upon solute hydrophobicity.
Read full abstract