Improved flow coulometric detector and its application to liquid chromatography

Application of controlled potential coulometry to the automatic recording of liquid chromatography. V

Abstract

A new method of detection based on controlled potential coulometry for liquid chromatography has been developed. This paper presents an improvement of the electrolytic flow cell and a method for detecting metal ions without interference of the dissolved oxygen in the column effluent.
In principle, the technique depends on introducing the column effluent into a cell where electrochemical reaction of almost all of the constituents take place at a working electrode of a constant potential. The potential is selected so that it is sufficient to electrolyse the ions to be determined but insufficient to electrolyse the eluate substrate. The electrolytic current measured is recorded on a recorder and the concentrations or the amounts of the constituents are calculated by Faraday's law.
Three types of the cell were designed and tested.Each of them consists of a working electrode of large surface area placed between two auxiliary electrodes. The working electrode and the auxiliary electrodes are separated by diaphragms of ion-exchange membrane. The working electrode is made of carbon-cloth and the auxiliary electrodes are silver-silver iodide wire netting of about 40 mesh. The electrodes are tightly fitted in openings of silicone rubber plates, which are arranged and bound with clamping bolts into a liquid-tight cell. One of these cells has a speed of response of less than 1 second and can be used in the flow rate of the effluent as large as 6 ml/min with the electrolytic efficiency of more than 99.5%. It has also a high sensitivity and is suitable for the high speed analysis in liquid chromatography.
The electrochemical reaction applied to the detection of a metal ion, Mⁿ⁺, was as follows:
[Hg-DTPA]^3-+Mⁿ⁺+2e
→[M-DTPA]^(5-n)-+Hg
Ag⁺, Au(III), Bi³⁺, Cd²⁺, Co²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, Mn²⁺, Ni²⁺, Pb²⁺, VO²⁺, Zn²⁺, and ions of alkaline earth metals and rare earth metals were detected quantitatively with the electrolytic cell. Aluminum and antimony ions were also detected but not quantitatively.
The method was applied successfully to the detection of alkaline earth metal ions, Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ separated by cation exchange chromatography.