I've been using the silicon oil filtration method from the Heldt papers, which uses tritiated water and tritiated sorbitol to measure intracellular volume. The scintillation counter reads extremely low dpm when I cut out and measure the lower sorbitol fraction (approximately 150dpm instead of 50000dpm). I'm thinking it's a problem with my protocol, a more complete one (or one that someone has used successfully) may be the answer.