Adsorptive stripping voltammetry of fluorescein isothiocyanate, phenyl isothiocyanate, phenylthiohydantoin-tyrosine and methylthiohydantoin-glycine

Abstract

Fluorescein isothiocyanate (FITC) in pH 8.0 phosphate buffer gives a single adsorptive stripping voltammetric peak at a hanging mercury drop electrode at –0.88 V versus Ag-AgCl after adsorption at 0 V. This peak is due to reduction of the fluorescein moiety in the molecule. When accumulation was carried out at potentials more positive than 0 V a second peak was observed at –0.60 V owing to formation of mercury(II) sulphide during the accumulation period. Phenyl isothiocyanate (PITC) gave no adsorptive stripping peaks even when accumulation was attempted at positive potentials. Freshly prepared solutions of FITC and PITC in pH 8.0 phosphate buffer were shown to degrade to form sulphide. When these degraded solutions were used the adsorptive stripping peak at –0.60 V was obtained. For example, this peak was obtained when a PITC solution in pH 8.0 phosphate buffer was prepared from a stock PITC solution in acetone and then left for 30 min at room temperature. Two derivatives of isothiocyanates with amino acids, phenylthiohydantoin-tyrosine and methylthiohydantoin-glycine, were shown to give the –0.60 V peak when accumulation was effected at 0 V from pH 8.0 phosphate buffer. Calibration graphs for the determination of these molecules using this peak were rectilinear from 2 × 10^–8 to at least 2 × 10^–7M.