Clinical Chemistry, Vol 8, 318-332, Copyright © 1962 by the American Association for Clinical Chemistry

Beta Radiation Self Absorption of Precipitates Collected on Filter Paper

Studies with S³⁵-Barium Sulfate

¹ Cancer and Geriatric Research Laboratories, Veterans Administration Center, Martinsburg, W. Va., and the Department of Pathology, George Washington University School of Medicine, Washington, D. C.

Different yields of precipitate and different types of self absorption curves were obtained when radioactive sulfur was precipitated as BaSO₄ in the presence and in the absence of salt. An equation was derived for expressing the self absorption of precipitates collected on filter paper which takes into account the yield of precipitate and the partial penetration of the precipitate into the filter paper. From the computed estimates of the parameters, it was concluded that the precipitate obtained in the absence of salt penetrates into the filter paper. This conclusion was supported by microscopic observations.

The Denis method of oxidation followed by barium precipitation and filtration is well suited for the determination of total and isotopic sulfur because of complete recovery and lack of penetration of the precipitate into the filter paper. The count rates of radiosulfur treated by this method fit the usual equation reflecting an exponential absorption of beta radiation better than empirical linear relationships. A filtration apparatus is described which permits the determination of the radioactivity of a precipitate without removing the filter paper from its support.