Immunoradiometric assays (IRMA) are often processed in microplates. Typically, this involves a sandwich assay in which the analyte binds to a first antibody which is immobilized in the wells of a microplate or a microplate strip. After washing, the analyte captured in the wells is detected by incubation with a second 125I-labeled antibody. The 125I label in the wells after further washing may be quantitated by transferring individual microplate wells to tubes in a gamma counter. Counting immunoassays directly in the microplates substantially decreases the sample handling.
To facilitate high throughput analysis Packard Instrument Company has introduced the TopCount Microplate Scintillation and Luminescence Counter for counting organic or aqueous samples in either 96-well or 24-well microplates. Specially designed, solvent resistant microplates, PicoPlatesTM, are provided for efficient scintillation counting. Polystyrene microplates are dissolved by many common opaque solvents. Up to 12 wells of a 96-well microplate can be counted simultaneously. TopCount can count an entire microplate in a fraction of the time it takes for conventional liquid scintillation or gamma counting. The available microplate stackers hold 20 or 40 plates for unattended, high throughput processing. The on-line data reduction capability of TopCount can further speed the process of turning raw data into final answers.
In the experiments reported here, TopCount was used to count samples in organic solvents or on a solid support in the microplate format. The scintillation cocktail used was MicroScint-O, an environmentally safe formulation for counting samples in TopCount with most commonly used organic solvents such as heptane, ethyl acetate, methanol, and ethanol. Counting efficiencies were det