Pentobarbital quantification in the presence of phenobarbital by fluorescence polarization immunoassay

We developed a method for measuring pentobarbital in samples that also contain phenobarbital. The phenobarbital is destroyed by adding sodium hydroxide and then heating at 95 degrees C for 60 min. Pentobarbital is not affected by this pretreatment and can then be measured with an Abbott TDx barbiturates kit. Using this method, we obtained an average analytical recovery of 98% of added pentobarbital and a correlation of y = 0.953x + 3.4 vs HPLC. The intra-assay CV was 3.7% at 25.8 mg/L; the interassay CV was 6.2% at 16.1 mg/L. Other long-acting barbiturates, e.g., hexobarbital, are also effectively destroyed by this alkali pretreatment. Other short-acting barbiturates, e.g., secobarbital, are not removed and would produce an interference.