Tritium-labeled compounds V. Radioassay of both carbon-14 and tritium in films, with a proportional counter

A con venien t p rocedure is described for the radioassay of both carbo n-14 and tr itium In water-soluble, nonvolatile co mpound s b y means of a windo wl ess, gas-flow, proporti o nal co un ter .The materia l a re co un tcd in uniform films of sodium O-(carboxy mct hy l)cellulose t hat are "infinite]y t h ick" to t he r adi ation of t ritium bu t n ot t o t hc radiation of car bon-l4.Films of unifor m t hic kn ess ar e ob tained by ne w tec hniques which are d escribed in detail.If only carbon-1 4 is presen t , its a bsolu te a ctivi ty can be calculated con veni ent ly by means of a n e mpiri call y establis hed c urve for t he coun ting-effi ciency.If both earbon-14 a nd tritium are presen t, t he films a re eoun tcd in t he prop or tiona l coun ter and a re t hen r eco un tcd i n t he prcse nce of a ~c r ec n t hat stop s a ll ra dia tio n fr o m tri tium bu t o nly a p ort ion of that fr o m car bo n-H.Fro m a film wi t h a t hi c kn css of 0.8 mg/cm2, approxim ately 43 perce n t o f t he r ad iation of carbon-14 is co un ted .Of t his e mergin g ra di atio n, appro xi matel.,-50percc n t passes t hrough a scree n o f 7~-mil do ublc-a luminized " M ylar ."B v usc of sui ta ble calibratio n cur vcs for co un t ing-e ffi ciency, earbon-14 and tri t ium in t hc a mc sa mpl e can be calculated from t hc co un ts wi t h, a nd witho ut, t he screc n.Satisfa ctory a na ly es can be mad e WIth samples co ntaining less t han 0.001 micro curi e of car bon-14 a nd 0.005 mi crocuri e o f t ri tiu m.The method is s ui ta ble for t he radi oassa ,' of a wid e va riety of la beled mate ria ls..'