Instrumental Methods for Electrosorptive Detection in Liquid Chromatography

Abstract

Techniques and instrumentation have been developed for the electrosorptive detection of selected anions at mercury and silver in ion chromatography and of surface-active organic substances at mercury. Detection based on differential double-layer capacitance has proven most generally useful, although pulsebased methods are also beneficial, especially at silver where they have special utility in countering adsorption effects. An ancillary, and more specialized technique developed for the determination of ions senses adsorption, indirectly, relying on the kinetic facilitation imparted the rate of electroreduction of a transition-metal reactant by an adsorbed species. A less generally useful, but nevertheless complementary scheme based on the "tensammetric" peak may also be employed for the determination of organic species. A novel approach investigated for application Present address: The Upjohn Company, Kalamazoo, Michigan Present address: Isco Inc., Lincoln, Nebraska r 148 RAMSTAD AND MILNER to both ionic and neutral species is based on changes in the drop time of mercury caused by adsorption-induced changes in surface tension at the mercury/aqueous interface. Finally, more elaborate, "hyphenated" techniques -capacitive/capacitive and capacitive/amperometric detection -have also been developed for application to selected ions.

Digital Pulse ac Voltammetry for the Simultaneous Analysis of Electroactive and Electrosorptive Species in Flow Systems

“…From the literature, the change of double layer capacitance, Δ C dl , is proportional to the surface excess, Γ i of adsorbed species when the adsorption isotherm follows Henry's law. As the surface excess Γ i varies proportionally to the concentration of adsorbed species in solution, the change of charging current and differential capacitance are therefore proportional to the concentration of adsorbed species in solutions and can be used for the determination of electrosorptive species in solutions. − …”

“…The ac polarographic method is thus a useful method complementary to the very sensitive amperometric method for analyzing electroinactive species. From the change of differential capacitance or charging current, detection of electrosorptive species has been determined successfully with the conventional analog ac polarographic method. − However, instruments consisting of a small-amplitude ac signal source, a high-pass filter or turned amplifier input, a lock-in amplifier, a phase shifting circuitry, and a low-pass filter output are needed for performing the analog ac polarographic measurement. − To circumvent the demand of so many sophisticated instruments, a versatile digital ac polarographic method that can generate the fundamental and second harmonic ac polarograms in phase-sensitive or total current versions was developed by Bond et al − Currently, digital ac polarography has become one of the standard functions included in instruments of electrochemical analysis. But the functions of digital ac polarography embedded in those commercially available sophisticated instruments are often inflexible and limited.…”