Accurate Determination of ^113mCd in Environmental Samples

Abstract

^113mCd(t _1/214.6y,β⁻) entered the environment during testing of thermonuclear weapons in the atmosphere. It has been detected in water and sediments of the Laurentian Great Lakes, and in marine biota both near to and remote from the test sites. The isotope has potential as a biogeochemical tracer for natural Cd, provided it can be determined accurately. Because ^113mCd decays by emission of low energy β⁻ particles, and because its concentration in environmental samples is extremely low, earlier reports of ^113mCd detection were based on chemical inference alone. We have now demonstrated conclusively that ^113mCd may be determined accurately at levels on the order of 100 μBq ⁻¹ in water and 1 mBq g⁻¹ in sediments. Our evidence consists of reverse tracer analysis in two different systems, and modified Feather analysis using graded Al absorbers.

Dithiolate complexes of Cd were partitioned between CHCl₃ and water, and between pH 6.3 buffer and ion exchange resin. In both systems, phase distribution of the radio-cadmium was statistically indistinguishable from that of added stable Cd tracer. In Al absorbers, the range-log countrate curve for suspected ^113mCd isolates yielded E_max for the β⁻ particles of 0.66 (±0.07) MeV, compared to the accepted value of 0.59 MeV.

Key Words:

• Cadmium
• biogeochemical tracer
• water sediments