Foods, radionuclide analyses

One common distinction between processing food and vegetation samples is that the edible portion in food is analyzed separately to provide information on radionuclide consumption. Additional treatment of food before analysis may include washing and other preparation steps. 

In this discussion, we follow lAEA-Report 295 (1989) and describe samples and pathways relevant to the analysis of radionuclides in foods, and of environmental materials that are part of the immediate pathways leading to contamination of food. 

It is recommended that food analyses be based on the determination of radionuclides in individual food items rather than a mixed diet sample. Only the analysis of individual foodstuffs can indicate whether and which countermeasures should be taken to reduce doses. Food sampling for estimation of total consumption should be carried out at the retail level when appropriate otherwise, it should be carried out at the consumption level. The selection of foods to be sampled can be based on individual diet or food consumption statistics. Analyses of individual foodstuffs should preferably be performed after preparation, taking into account the effect of kitchen activities such as washing, cleaning and cooking. 

Analysis of herbage (of forage) has sometimes been used to detect and identify radionuclides deposited from the atmosphere (Jackson et al., 1981). However, the problem arises that when the deposition rate is low, large areas of vegetation need to be sampled for detection. In the case of plutonium, an alternative is to collect the faeces of grazing animals such as cows, sheep and rabbits. Plutonium is very poorly absorbed by the mammalian gut and so virtually all that is ingested by an animal will appear in its faeces. Also, if the species selected obtains its food entirely by grazing, then the isotopic ratio Pu will be the same in the faeces as deposited on the... 

Milk is frequently analyzed for radionuclides in a monitoring program because it is one of the few foods that reaches the market soon after collection. Milk commonly is consumed within 7-11 days after milking and occasionally reaches the consumer within 2 days. It may contain relatively short-lived radionuclides and be a dietary source of fission-produced Sr, °Sr, 1, Cs, and as well as naturally occurring A 4-1 sample usually is collected for analysis. Cow s milk samples can be collected to represent a specific herd in the form of raw milk, or a regional pool of pasteurized milk. Goat s milk is collected when this medium may... 

Once the radionuclides of interest are identified, a sample collection pattern should be developed to delineate the region of contamination and the radionuclide concentrations that affect radiation exposure. The usual concern is to identify locations of external exposure by direct radiation and internal exposure by inhalation and food consumption. The lower limits of detection required for analysis are specified by the customer to assure reliable detection in accord with the regulation or guidance that limits the concentration of each radionuclide. The lower limit of detection usually is required to be 2-10 times below the concentration limit to assure reliable measurement at the limit. 

The analysis of natural radioactive materials is relatively straightforward, since it usually involves the determination of a small number of well-known radioisotopes of a few elements. The determination of artificial radionuclides in environmental samples, food samples, radiopharmaceuticals, etc., is more complicated, because several radionuclides of a number of elements may be involved. 

All chromatographic methods (liquid, liquid-liquid, liquid-solid, gas, gas-liquid, and gas-solid chromatography) are used in radiochemical analysis of food and environmental samples. Equipment for the evaluation of paper and thin-layer radiochromatograms is commercially available. The radiation detectors used vary according to the type and energy of the radiation emitted by the measured radionuclides. Use is made of ionization chambers, proportional counters, and Geiger-Miiller, scintillation, and semiconductor detectors. 

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