Radiocolloids

Radiocolloids are colloidal forms of microamounts of radioactive substances. Their formation was first observed by Paneth (1913) in his research on the separation of 2 °Bi and Po. Radiocolloids can be separated from aqueous solutions by ultrafiltration, centrifugation, dialysis and electrophoresis. They can be detected with high sensitivity by autoradiography. As an example, the autoradiography of a radiocolloid of Th is shown in Fig. 13.4. 

In order to understand the nature of radiocolloids, knowledge of the general properties of colloids is needed. Colloids are finely dispersed particles in a liquid phase, a gas phase or a solid. The size of colloidal particles is in the range between that of molecules or ions and that of particles visible by means of a light microscope, i.e. between about 1 nm and about 0.45 pm. The upper value corresponds to the mean wavelength of visible light. Large molecules, in particular polymers and biomolecules, approach or exceed the upper value and may also form colloids. 

Like ions and small molecules, colloids are considered to be components of the phase in which they are suspended. In general, the metastable colloidal state exists for longer periods of hme (up to several months) and colloidal particles may be transported with water or air over long distances. 

Many organic molecules are large enough to form colloids, and organic colloids are frequently encountered in the life sciences. Humic substances are found in natural waters and may form complexes with radionuclides. 

It is obvious that the formation of radiocolloids will be observed 

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For patients with melanomas that are at risk of spreading to the lymph nodes, a sentinel lymph node (SLN) biopsy is performed. The SLN, the first lymph node to receive lymph draining from the tumor, is identified by injecting a radioactive material, technetium-99m-labeled radiocolloids, and vital... 

When not complexed, lanthanide ions have a high affinity for bone in vivo because they act as calcium ion mimics. Because the lanthanides undergo hydrolysis above a pH of 4, they readily form radiocolloids when not complexed, and are then taken up by the liver. This bone and liver uptake results in non-specific radiation doses to non-target (normal) tissues and organs and is undesirable.91 The polyaminocarboxylate class of ligands are considered to be the optimal choice for the basis of BFCAs for the+3 metal cations, including the lanthanides. It is essential that the... 

Kahn, M., Coprecipitation, Deposition, and Radiocolloid Formation of Carrier-Free Tracers, Radioactivity applied to Chemistrv(A. C. Wahl, and N. A. Bonner, ed) pp. 403-433, John Wiley Sons, Inc., New York (1951). 

Schweitzer, G. K. (1956). The radiocolloid properties of the rare earth elements, page 31 in Rare Earths in Biochemical and Medical Research A Conference Sponspored by the Medical Division, Oak Ridge Institute of Nuclear Studies, October 1955, Report No. ORINS-12, Kyker, G. C. and Anderson, E. B., Eds. (Office of Technical Services, Washington). 

In one of the few studies of curium in soils and plants Thomas and Jacobs (43) came to the conclusion that radiocolloid particles must be formed they were of the opinion that ion exchange was unimportant in the soil chemistry of curium. 

It has been established that plutonium hydrolysis products exhibit colloidal behaviour (147-151) and may adsorb onto minerals and other surfaces to form radiocolloids. However, it is difficult to determine whether a radiocolloid is a true colloid or a pseudocolloid formed by adsorption of the plutonium species onto other colloidal impurities in the solution (152). In some cases both forms may be present... 

Even though the solubility product of Pu(OH)4 is 1 X 10 56, some Pu4+ must remain in solution as the equilibrium is established. The monomeric Pu(OH)4 and the very low molecular weight polymeric species are able to pass through an ultrafilter, and Lindenbaum and West-fall (22) found that as much as 5% of the hydrolysis species remained ultrafilterable after 72 hours at pH 11. These unfilterable species may be either true radiocolloids or pseudocolloids. The latter likely occur as a result of minute impurities in the solutions which act as nuclei on which the polymeric or ionic species adsorb (14). However, this point has been the subject of extensive debate (36, 37, 39), and opinions vary as to whether pseudocolloids form in this manner, or in fact whether there are such species at all. In general the term colloidal plutonium will be used throughout this paper to indicate all of the insoluble plutonium hydrolysis products and polymeric species of colloidal size. 

To determine if bicarbonate has similar effects on the nature of the radiocolloid, the plutonium size distributions were studied at pH 7 as a function of bicarbonate concentration, using bicarbonate only from atmospheric CO.j and in solutions with [CH(V] equal to 10"2Af. The latter were prepared by adding NaHCOa. Two different ionic strengths were used to distinguish between the effects of added bicarbonate and ionic strength. The distributions of identical solutions which were shaken in flasks with and without granular silica were determined initially and... 

The entire picture is still more confusing because of the fact that several different types of colloids are distinguished—i.e., radiocolloids, pseudo-colloids (7, 8, 28, 33), and true colloids. Radio-colloids refer to systems of radiotracers which appear to be in colloidal form although they are in concentrations well below their ionic solubility (25, 26). The term pseudo-colloid is used to describe the formation of a colloid system... 

Precipitation of Hafnium Hydroxide. In order to interpret the adsorption data it was necessary to determine the conditions which lead to the precipitation of hafnium hydroxide. It is not usually advisable to depend on the solubility product because the information on this quantity is often unreliable for hydroxides of polyvalent metal ions. In addition, "radiocolloids may apparently form much below saturation conditions in radioactive isotope solutions. In the specific case of hafnium hydroxide only two measurements of the solubility seem to have been reported. According to Larson and Gammill (16) K8 = [Hf(OH)22+] [OH ]2 — 4 X 10"26 assuming the existence of only one hydrolyzed species Hf(OH)22+. The second reported value is Kso = [Hf4+] [OH-]4 = 3.7 X 10 55 (15). If one uses the solubility data by Larson and Gammill (Ref. 16, Tables I and III) and takes into consideration all monomeric hafnium species (23) a KBO value of 4 X 10 58 is calculated. 

Strand, S.-E. and B. R. R. Petsson. 1979. Quantitative lymphoscintigraphy I basic concepts for optimal uptake of radiocolloids in the parasternal lymph nodes of rabbLt lucl. Med20 1038-1046. 

Other solid particles (S- or Au-colloids, gelatin, targesin, etc.) have similarly been examined for their ability to transfer the 211 At isotope to tumour bearing tissues46,97,99. Application of solid carriers is, however, limited by their lack of biological specificity. Therefore radiocolloids or labelled microspheres are primarily used for local administration into the tumourous tissue itself. 

Thus, the ions in very low concentration can be precipitated even when the concentrations do not reach the solubility product (i.e., normally, they would stay in solution) and/or radiocolloids may form. 

Colloids are found in many systems, e.g. in natural waters and in the air. Traces of colloids formed by dust particles or by particles given olf from the walls of containers are practically omnipresent. They can only be removed by careful ultrafiltration. If a radionuclide or a labelled compound enters such a system, there is a high probability that it will be sorbed on the colloids, provided that the competition of other ions or molecules is not too strong. Only if the presence of colloids from other origins can be excluded and if the solubilities of relevant species are not exceeded, formation of radiocolloids by microamounts of radionuclides can be neglected. 

Like normal colloids, radiocolloids show different behaviour from that of ions or molecules. For instance, they are generally not sorbed on ion exchangers or chromatographic columns. 

Formation of intrinsic colloids in natural waters can be excluded for radioisotopes of elements of groups 0, I and VII, and the probability that they may be formed is small for radioisotopes of elements of other groups as long as the concentration of the elements is low. In general, formation of carrier colloids by interaction of radionuclides with colloids already present in natural waters is most probable. Thus, clay particles have a high affinity for heavy alkali and alkaline-earth ions, which are bound by ion exchange. This leads to the formation of carrier colloids with Cs, Ra and °Sr. Formation of radiocolloids with hydrolysing species has already been discussed (section 13.4). 

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