Tritium-labeled compounds I. Radioassay of tritium-labeled compounds in ""infinitely thick"" films with a windowless, gas-flow, proportional counter

s imple, sensit ive, ~nd relia ble technique h as been devised for t he radioassay of nonvolatile, water-s oluble t ntmm compounds.The substan ce t o be analyzed is dissolved in a n aqueous solut ion of a t hickening agent , prefera bly sodium O-(carboxy methyl) cellulose or sodium algin ate.The solution is placed in a sha llow plan chet, a nd af ter evaporation of t he wat er , ~he result ing film, which is "~nfinitely t hick " t o trit ium beta par ticles, is counted with a 271", wmdowless, gas-flow, proportlOnal coun ter.By means of an empirical factor determine d with a. subst an ce of known rad ioactivity, t he counts are converted to m i cr~curies .I n a film h av~n & a t hi ckness of 0.7 mg/c:n 2 , t~e co unting e ffi ciency is a bout 4 per cen t ; t he standard devlatlOn from t he mean, ob tamed m a sen es of rout ine m eas ure ments, was less t h a n 2. pe r ee ~t.An ~ say.can re~dj]y be made ~th .tritiuIl!--containing material ha ving 0.01 m ICrocune of radlOactlvlty.'Ihe method, whICh IS a pplIcable to n on volatile watersotuble solids, solutions, or liquids, is sui table for rout ine a naly ses .'