The need for quick and accurate determination of trace but harmful impurities in radioactive iodine, to be used for medical purposes, led to establishing a spectrographic method of analysis using the copper-spark technique and based on the disappearing spectral line intensity principle. The least amounts of the relevant trace elements of harmful effect namely, Al, As, Ba, Be, B, Cd, Cr, Co, Fe, Pb, Mg, Mn, Hg, Ag, Na, Ni, Sn, and Te, which are permissible have been found, by determining the intensities of the spectral lines which just appear at such concentrations. The degree of purity of the radioactive iodine found spectrographically determines whether it is to be used in clinical or industrial purposes. Studies on the parameters governing the process of excitation in the spark plasma as well as the surface flatness of electrodes enabled achieving such high reliability in selection control.