Radionuclide tracers, removal

The extent and rate of removal of radionuclide tracers from sea water can be ascertained through a knowledge of their residence time in the water column. The residence time, t, of a tracer due to its physical removal from one reservoir to another is defined [37,38,39,40,41] as ... 

Recognizing that scavenging of dissolved radionuclide tracers is a reversible process was a milestone in understanding the removal of particle-reactive substances from the ocean. This principle must be taken into account in using natural radionuclides as tracers of particle transport, and when using natural radionuclides to infer the rates of processes responsible for scavenging particle-reactive substances (e.g., trace metals) from the ocean. 

In the first set of experiments, six cells were ultrasonic-ally cleaned to remove rock powder produced during coring, and then flushed before tracer injection. Three were flushed with GGW and three with brine. A second set of experiments was designed to determine whether mineral alteration rates and/or solution temperature affect the rate of radionuclide interaction with granite. (Test conditions are given in Table VI of the Results Section). The 60°C temperature was maintained by submerging the cells in a water bath. Two cells were flushed with GGW for several weeks and then allowed to sit, filled with GGW, for approximately five months before the experiment was started. 

If no applicable analytical method is found, then individual separation steps have to be selected and combined sequentially, each step to remove one or more contaminants until all are removed to the extent necessary. The first step must match the sample form, each subsequent purification step must match the preceding step, and the final step must produce the counting source in its specified form. Each step must give high recovery of the radionuclide of interest and the required removal of interfering radionuclides. Suitably designed tracer tests can provide otherwise unavailable information. 

The radioactive tracer must not be contaminated with other radionuclides that interfere with the yield measurement. Some actinide-series tracers have radioactive progeny with slow ingrowth that should be identified from their decay scheme chain and require periodic purification to remove these progeny. 

The main radionuclides that are measured in this system are isotopes of thorium, uranium, and plutonium. Others are the longer-lived isotopes of heavy elements such as radium, protactinium, neptunium, americium, and curium. Conventionally, an isotopic tracer of known activity that emits alpha particles is added at the beginning of the radiochemical procedure (see Section 6.3.2). The solution from which the sample for counting will be deposited is thoroughly purified by the radiochemical procedure to remove interfering radionuclides and solids. 

Chemical separations may be specific for the analyte of interest (see Chapter 3), such as liquid or gas chromatography, or scavenging (such as by precipitation) to remove the major interfering substances. Addition of carrier, as practiced in radioanalytical chemistry to assist in purifying radionuclides, usually is not appropriate for mass spectrometric analysis. Such addition undermines the isotopic ratio measurements that are often at the heart of this procedure, and also overloads the system for ion generation and peak resolution (but carrier addition is used for accelerator mass spectrometry). Addition of tracers, known as isotope dilution, is often employed for yield determination (see Section 17.2.9). Interferences are distinctly different in radiometric and MS analyses of radionuclides, and may be the deciding factor in selecting one method versus the other. 

Studies on marine zooplankton element-elimination rate were conducted with zooplankton labeled with tracers and with fallout radionuclides (Kuenzler, 1969). The animals were placed in nonradioactive seawater, and after a period of time they were removed and the radioactivity of the water was determined. The tracer-labeled zooplankton were obtained by placing zooplankton in carrier-free solutions of Na I, CoCl2, or ZnCl2, retained in the solution until labeled adequately, washed, and placed in another container to study elimination. 

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