Automatic chemical current determination in coulometric stripping potentiometry facilitating calibration-free trace metal determinations

The theory of an improved coulometric stripping potentiometry method, involving automatic determination of the chemical current for each sample, and thus suitable For calibration-free determination of trace elements in microliter samples is described. The method has been applied to the determination of Zn-II, Cd-II. Pb-II,Tl-I, Cu-II and Bi-III by reductive in situ amalgamation and of As-V, Sn-IV and Cu-II by reductive in situ dissolution on a gold film followed by three consecutive constant current stripping potentiometry measurements permitting the determination of the chemical current. Quantitative reduction (greater than or equal to 99%) of the analyte ions was obtained after four minutes of vibrationally enhanced reductive electrolysis of the 12 mu L samples placed on top of an inverted glassy carbon disk electrode combined with Ag and Ag/AgCl counter and reference electrodes. The concentration detection limits ranged from 11 nM for elements oxidized to the monovalent state to 3 nM for elements oxidized to the trivalent state. The mass detection limits ranged from 4.5 pg for Zn-II to 28 pg for Tl-I.