( . Dose curves for the enzyme inhibitor ...)Counting Non-Aqueous Samples in the TopCount Microplate Scintillation and Luminescence Counter

A sandwich immunoradiometric assay (IRMA) for TSH was performed in a solvent resistant PicoPlate or in microplate strips (Dynex Technologies) coated with anti-TSH MAb as the first antibody. An 125Ilabeled anti-TSH MAb was used to quantitate TSH bound to the solid support by the first antibody. The PicoPlate was counted in the TopCount with 250 μL MicroScint-O, and individual wells of the microplate strip were counted in a Packard Cobra gamma counter for comparison.

Results and Discussion

The average efficiencies and backgrounds obtained with TopCount with a small sample load are presented in Table 1. A small sample (10 μL) was used to provide the optimum cocktail performance. Counting efficiencies for ³H are comparable to those which can be obtained by conventional liquid scintillation counting.

The CPM results for equal volumes of ³H labeled samples in heptane counted with TopCount and a conventional LSC are compared in Figure 1. The data show that the TopCount correlates very well with the LSC (R²= 0.99). The slope of the line indicates that for this sample load TopCount CPM is approximately 40% of the CPM obtained by conventional LSC. This lower efficiency is a result of the much higher sample load; only 250 μL of cocktail was used in TopCount versus 5 mL in the LSC.

Table 2 shows a comparison of the % inhibition values for a control sample obtained from TopCount with the value obtained by LSC. Assays 1-4 were counted with TopCount using four different sample volumes, and assay five was counted with the Tri- Carb 2250CA. These control values are additional confirmation that TopCount provides consistent results regardless
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