Radionuclides natural occurrence

Over the last few decades, safety has become one of the most important topics related to food. From this view, quality control of vegetable raw materials has at first to cover the following issues natural and anthropogenic contaminants (e.g. heavy metals, pollution from industrial and private combustions, not professionally deposited waste products, radionuclides), residues of fertilisers (e.g. nitrate), plant-conditioning and plant-protective agents, filth, pests, the microbial status and the occurrence of microbial toxins. It is not possible to discuss all these aspects in detail however, with a focus on herbs and spices, two of them should be stressed more thoroughly. For further information, see [8]. 

Samples that contain numerous radionuclides from certain portions of the periodic table with short (1 h-lday) and intermediate half-lives. This mixture is found in monitoring process streams at operating nuclear facilities, fallout from nuclear tests, or accidents within hours to days after occurrence. Subcategories include samples from activation analysis, nuclear chemistry studies, and nature. 

Sources conventionally are prepared by precipitation to permit gravimetric measurement of stable isotopic carrier yield, as discussed in Section 6.3. The isotopic carrier must be in the same chemical state as the radionuclide of interest, or the sample must be processed to achieve this requirement when carrier is added. The precipitating agent is selected to obtain a pure precipitate of the radioelement with a large, but not necessarily quantitative, yield and reproducible weight. If this precipitate does not completely purify the sample, as is often the case, then previous separation steps should have done so. The various purification steps must eliminate extraneous solids that will add to the carrier yield and contaminant radionuclides that will add to the count rate. The purification steps must reduce such contaminants to a small fraction of the amount to be weighed and counted. Occurrence in nature of significant amounts of the isotopic carrier in the sample must be determined in control samples to correct the yield value. 

In addition to the previously mentioned artificial radionuclides the accumulation of natural radionuclides of "Po and "Pb has been observed in this same food chain. The occurrence of these radionuclides in lichens will be described later. 

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