Determination of natural radioactive tracers

Rates of many transport and reaction processes in the ocean can be evaluated with the natural clocks provided by the radioactive decay series of three primordial radionuclides present in the earth s crust Th, and (detailed review in Ivanovich and Harmon, 1992). In all three decay series, isotopes of relatively soluble elements such as U, Ra and Rn, decay to isotopes of highly particle-reactive elements (Th, Pa, Po, Pb) and vice versa. The resulting disequilibria in the decay chains can be interpreted as a measure of transport rates of the particulate and liquid phases relative to each other (Ivanovich and Harmon, 1992). 

The highly different ocean chemistries of a large spectrum of parent-daughter pairs have been reviewed by Cochran (1992). The tracer pairs can be subdivided into three categories Rutgers van der Loeff and Boudreau, 1997). 

Pairs with a particle-reactive parent and a mobile daughter, suitable to study dispersion of material released from the seafloor into the bottom water. Applications include deep-sea mixing ( Rn, Ra) and the advective transport between shelves and the inner ocean ( Ra). 

In this chapter we will focus on the determination of those nuclides that are widely used as tracers, and where the analysis can be performed without access to highly specialized equipment such as AMS (accelerator mass spectrometry) or TIMS (thermal ionisation mass spectrometry). We describe detailed analytical methods for the nuclides Th, Th, °Po, °Pb, Be, Ra, Ra, Ra, Ra and Rn. For the determination of other nuclides key references have been listed in Tables 13-1 and 13-2. 

A general review of chemical procedures for the separation of U-series nuclides has been given by Lally (1992) and for mass spectrometric determinations by Chen et ai (1992). 

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Isotope ratios, as measured by mass spectrometry, along with the known amount of the tracer, are used to determine the amount of the analyte in the sample. Isotopically altered elements (sometimes consisting of nearly pure stable isotopes) and radioactive isotopes are commercially available for use as tracers. Nearly every element can be purchased as a solution whose atom concentration is known and traceable to NIST. Standard materials are also available with isotopic composition determined to a high degree of accuracy. Radioactive tracers, such as 233U and 244Pu, are available for tracing actinide elements that do not have stable isotopes. Because many of the samples analyzed in radionuclide determinations already have altered isotopic ratios, many times it is possible to use a natural isotope as the tracer. 

Membrane Diffusion in Dilute Solution Environments. The measurement of ionic diffusion coefficients provides useful information about the nature of transport processes in polymer membranes. Using a radioactive tracer, diffusion of an ionic species can be measured while the membrane is in equilibrium with the external solution. This enables the determination of a selfdiffusion coefficient for a polymer phase of uniform composition with no gradients in ion or water sorption. In addition, selfdiffusion coefficients are more straightforward in their interpretation compared to those of electrolyte flux experiments, where cation and anion transport rates are coupled. 

The first experiments with radioactive tracers were conducted in 1913 by de Hevesy and Paneth who determined the solubility of lead salts by using one of the naturally occurring radioactive isotopes of lead. Later, after discovery of induced radioactivity, de Hevesy and Chiewitz in 1935 synthesized (j8 14.3 d) and used this tracer in biological studies. 

The analytical application of radionuclides, along with other applications in radioanalysis, results directly from Hevesy and Paneth s invention of radio-tracer and radio-indication techniques in 1912. They pointed out that the addition to a solution of an element of its radioactive isotope makes possible the identification and determination of the element. In their first application they labeled a solution of lead with radium-D, a natural radioactive lead isotope and determined the solubility of sparingly soluble lead salts. 

As a measure for the mixture homogeneity the relative standard deviation of the measured count rates can be used. It is important to subtract the variations due to the statistical nature of radioactive decay, which would otherwise contribute to the measured variance. It has to be noted that the determination of homogeneity is not unambiguous. The variations in tracer concentration of samples depend on the sample size and on the microstructure of the material. 

Solubility. The first radioactive tracer study was the determination of the solubility of poorly soluble PbCr04 by Hevesy and Paneth (1913). Lead chromate labeled with the naturally occurring Pb was prepared, shaken with water, and filtered. The filtrate was evaporated to dryness on a nickel plate and the P activity of Bi in equilibrium with Pb was measured. 

Water is poorly soluble in hydrocarbons and determination of its solubility by ordinary chemical analysis requires a large amount of hydrocarbon samples. The solubility of water in benzene (Joris and Taylor 1948) and other hydrocarbons (Black et al. 1948) was determined with the use of tritium ( H, T) as tracer. These studies give an example where correction for the mass difference between an element of natural isotopic composition and its radioactive tracer is necessary. Water labeled with T was prepared by bombardment of heavy water (D2O) with... 

The vitamin B12 activity of a given sample may be determined by biological, chemical, microbiological or physical methods including radioactive tracer, spectrophotometric, and partition techniques. The nature of the sample, the concentration of activity, precision, and time required will usually determine the assaj of choice. 

Since the advent of radioactive tracer techniques and their first application in lichen physiology by Smith (1961), much information has accumulated on the interaction between alga and fungus. About 35 lichen species with 12 different types of algal partners have been studied to determine the nature of the substances which pass from the autotrophic alga to the fungus. When a lichen is allowed to photosynthesize in the presence of radioactive (or NaH COg solution) the following events occur. 

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