C14-labelled phytoplankton on filter papers were counted with a gas-flow geiger counter, then combusted in oxygen combustion flasks. C14C>2 was recovered with hyamine, and the absolute activities of the original samples were determined by liquid scintillation counting. Geiger counting efficiency, based on the absolute activities, was 25-8 % ± 1-19. Intact filter papers containing C14-phytoplankton also were counted directly by liquid scintillation. Comparison of liquid scintillation and geiger activities for the same samples showed that the absolute activity of C14-phytoplankton on filter paper can be determined by direct liquid scintillation counting of the intact sample.
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Wolfe et al. (1967) studied this question.