Potential errors in liquid scintillation counting of very dilute solutions of labeled tetracyclines

Determination of the specific activity of low concentrations of tritiated doxycycline or tritiated tetracycline by liquid scintillation counting in a toluene scintillator in glass vials gave misleadingly low values through adsorption to the wall of the vial. The phenomenon was more pronounced in nylon and polyethylene vials. Use of Cab-O-Sil or of siliconized glass vials gave better, but still unsatisfactory, results. The problem can be overcome by addition of unlabeled solute to bring the concentration of the solution being counted to above 5 μg/ml. Results obtained for doxycycline with Triton X-100 scintillator in glass or polyethylene vials were reliable, with little variation over a wide range of concentration, but nylon vials again proved to be unsatisfactory.