Determination of cobalt, selenium and iodine by NAA in foodstuff by preconcentration of traces

Neutron activation analysis of cobalt, selenium and iodine requires pre-concentration of traces when the best possible conditions of sensitivity and accuracy are required at short irradiation times. The reason is 60mCo, 77mSe and 128I present half-lives of 10.5 m, 17.5 s and 25 m, respectively, which do not allow neither to dissolve the sample nor to perform any radiochemical separation after irradiation. On the other hand, their emissions are located in the beginning of the spectrum (59, 161 and 441 keV, respectively) where the Compton continuum makes difficult the identification and measurement of the peaks. This paper describes how foodstuff samples are dissolved in pure HNO3 by using conventional pressure pumps at 120-140 deg C during 3-4 hours. Once cooled down the acid solution, distilled water is added and pH fixed at 1-1.5 with ammonia. 20 mg of APDC are added while stirring during 2 minutes and the solution is passed through 50 mg of activated carbon, where the Se traces are trapped. Then the filtered solution is adjusted to pH 4-6 with ammonia and passed through a fresh identical activated carbon filter where the iodine traces are caught. Finally, to the filtered solution is added 20 mg of cupferron at same pH, stirring for 2 minutes and passed through a third identical carbon filter, where the cobalt traces remain. In this way, we have the cobalt, selenium and iodine traces in three different, extremely pure carbon matrices, with a small, known mass of cobalt, selenium and iodine as background. Each filter is ready to be irradiated during a suitable time, to calculate separately at maximum sensitivity and accuracy the concentration of these three trace elements so important to human nutrition. (author)