Contamination Scenerios

The problem of contamination arises because biochemists in their “spiking” experiments handle amounts of radioactive tracers, mainly 14C, 3H and 35S in mCi quantities, and the oceanographers studying the natural distribution of these isotopes, sample seawater and make measurements with a sensitivity of about 0.01 pCi (1 pCi=2.22 dpm). Thus, the spiking experiments deal with quantities of these isotopes that are 1011 times as large as the detection sensitivity of the natural tracer measurements. Spills many orders of magnitude below health hazard limits could still be a major contamination threat in the studies of oceanic tracer distributions.

The contamination pathways of 14C, 35S and 3H on board a ship may be different. 14C is usually used as a carbonate solution that, if spilled on the floor, will dry up. The solid residue will form dust, which will be carried around on shoes, and some minute particles might end up in ocean water samples. Assume that in a series of experiments with a total of 10 mCi of 14C, one of a hundred bottles is dropped on the floor and 99% of the spill is wiped up and secured. On the floor is still 1 µCi. Let us say that 1/1000 of that residue is carried out on shoes to the critical area; the contamination there is now 1000 pCi compared to a measurement resolution of 0.02 pCi. If 1/10000 of that quantity ends up in an oceanic 14C sample, it increases its activity by 5 times the accuracy of measurement and thus effectively ruins the study.

Sulfur-35, which is counted as 14C using our counting procedure, is usually used to label amino acids. These amino acid solutions can be spilled on the floor, dry up, and be transported as organic dust.

For 3H, the pathway via dust (organics) is also possible, as is the one via atmospheric water vapor, as HTO. Any organic component with a high activity of 3H will auto-oxidate and exchange H/T and, therefore, will release HTO to the air. Since seawater samples for tritium are usually transferred out on deck, the danger of contamination is less than that for 14C samples, which are usually processed in the ship's lab. Incidentally, the air in large laboratory buildings usually has highly elevated 3H levels. Sources of such contamination are autoradiography plates, luminescent dials on divers' watches, pressure gauges, and ordinary watches — all these emit 3H in comparatively large quantities as water vapor to the atmosphere.