REGION-SPECIFIC TRITIUM ENRICHMENT, AND NOT DIFFERENTIAL BETA-ABSORPTION, IS THE MAJOR CAUSE OF QUENCHING IN FILM AUTORADIOGRAPHY

Tritium quenching refers to the situation in which estimates of tritiu m content generated by film autoradiography depend on the chemical com position of the tissue as well as on the concentration of the radioiso tope. When analysing thin brain sections, for example, regions rich in lipid content generate reduced optical densities on x-ray film compar ed with lipid-poor regions even when the total tissue concentration of tritium in those regions is identical. We hypothesize that the dried thickness of regions within sections depends upon the relative concent rations and types of lipid within the regions. Areas low in white matt er dry thinner than areas high in white matter, leading to a relative enrichment of tritium in the thinner regions. To test this model, a se ries of brain pastes were made with different concentrations of grey a nd white matter and impregnated with equal amounts of tritium. The thi ckness of dried sections was compared with percentage of white matter and apparent radioactive content as determined by autoradiogram analys is. The results demonstrated that thickness increased, and apparent ra dioactivity decreased, with higher percentages of white matter. In the second experiment, thickness measurements from dried sections were su ccessfully used to correct the apparent radioisotope content of autora diograms created from tritium containing white- and grey-matter tissue slices. We conclude that within-section thickness variation is the ma jor physical cause for 'tritium quenching'.