The experimental GD-FI system , shown in Fig.1A, was linked to a nitrogen line, used for purging ammonia from the alkaline sample suspension during ammonium leaching. Nitrogen was first passed through a scrubber containing 0.5MH2SO4 solution for the removal of traces of ammonia or other basic gases. Traces of acidic gases (e.g. CO2) were absorbed in the alkaline sample suspension. The GD cell(Fig.1B) consisted of two Perspex disks(i.d.60mm),held together by stainless steel ring clamp sand four stainless steel bolts. Elliptical shape donor/acceptor chambers were machined in the blocks with major and minor axes of 30 and 5mm, respectively.