Advantage of Florisil over charcoal separation in a mechanized testosterone radioimmunoassay

We have facilitated radiommunoassay of testosterone by mechanizing every pipetting step and separating bound from free hormone by shaking assay tubes with activated magnesium silicate (Florisil) in a multi-tube Vortex mixer. Compared with dextran-charcoal separation, Florisil separation has the advantage that testosterone is only negligibly adsorbed except during active shaking, so that the interval for adding adsorbent does not produce discrepancies in bound hormone estimates between first and last assay tubes in a series. Our method gave CVs of 5% (within assay) and 6% (between assay) for samples containing 4.0 mug of testosterone per liter, considerably better than those obtained with charcoal (respectively 12 and 15%). The assay also had low blank values and demonstrated excellent accuracy.