New extraction method for radioimmunoassay of serum estradiol

A new extraction procedure is applied here to radioimmunoassay of serum estradiol. Small columns packed with porous, grainy kieselguhr are loaded with the samples. The kieselguhr material acts as support for the water phase, from which the lipophilic compound estradiol is eluted with diethyl ether. With use of a specially constructed rack, several columns can be handled simultaneously, which makes the procedure convenient for routine use: 40 columns can be processed in about 30 min, including application of samples, elution, and evaporation of solvent. The results correlate well (r = 0.98) with those by conventional liquid-liquid extraction, and the new method has lower interassay coefficients of variation. When tritiated estradiol in water solution was applied onto the columns, 97.7% was recovered in the extract. The current cost of materials used for the extraction is about $0.20 per column.