Radioimmunoassay of benzoylecgonine in samples of forensic interest

A simple and economical radioimmunoassay for benzoylecgonine in blood or urine is described. Haemolysis, decomposition, common anticoagulants and sodium fluoride do not affect the results. A commercially available antiserum is used at a dilution of 1:600. The tracer is radioiodinated p-hydroxybenzoylecgonine. It is prepared by reacting p-acetoxybenzoic anhydride with methyl ecgonine followed by mild hydrolysis and radioiodination by the Iodo-gen method. The product is purified on a disposable silica cartridge and has a specific activity of about 30 TBq mmol-1. Polyethylene glycol is used to separate the bound and free fractions in the assay. The range of the dose-response curve is 0-400 ng ml-1 benzoylecgonine. The assay is largely specific for benzoylecgonine. Cocaine, ecgonine, methylecgonine and cinnamoyl cocaine have slight or negligible cross-reactivities. The inter-assay coefficient of variation is 7.5% and the recovery of benzoylecgonine from 'spiked' blood is 103%. The 'cut-off' is 20 ng ml-1 benzoylecgonine for both blood and urine. Radioimmunoassay and high-performance liquid chromatography results agree well.