A method for determining low levels of tritium radioactivity in biological samples by means of a one-step combustion operation and a low background counting tube was described by Koenig and Brattghrd in 1963 (1). This technique has been applied in several cases in which low sample activity prevented the use of more conventional measuring methods. It enabled Daneholt and Brattgbrd, in 1966 (2), to determine the activity of the four RNA bases after electrophoretic separation of samples from hypoglossal nuclei of rabbits, labeled in viva with [H3]adenine and [H3]cytidine. They showed that it was possible to measure the incorporation of RNA precursors in very small groups of cells and in glia at the cellular level.
Hyden, Bjurstam, and McEwen, in 1966 (3), used this micro radiometric method for determining the specific activities of tritium-labeled proteins in discs which were cut out of polyacrylamide gels after electrophoretic separation of lo-? to 1O-9 gm of proteins obtained from isolated cortical neurons of rats.
Hyden and Lange, in 1968 (4), employed this method for studying protein synthesis in hippocampal pyramidal cells during a behavioral test by making radiometric determinations of the various protein fractions after elect8rophoretic separation. Hyden, in 1968 (5), described a method for separation and determination of RNA in 1O-9 to 1O-7 gm amounts by disc electrophoresis. The specific activity per RNA micro fraction was determined by the one-step combustion method.
In all these applications the activity of the samples was so low that a measuring system with high efficiency and low background, i.e., an instrument with a high "figure of merit" was required. The "figure of merit," F/B (E = efficiency, B = background), is used for a coarse quantitative evaluation of the performance of radiometric instruments. A high E2/B value indicates that the instrument can achieve a desired statistical accuracy in a short counting time. Table 1 lists the efficiency, background, and 'Yigures of merit" of a number of instruments. The instruments numbered l-3 in the table represent the three different types of detectors and their corresponding counters which have been in use in this laboratory, 245
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