The Intercomparison Studies Program (ISP) at the Oak Ridge National Laboratory (ORNL, Oak Ridge, TN, USA) provides natural-matrix
urine quality-assurance/quality-control (QA/QC) samples to radiobioassay analysis laboratories. In 2003, a single laboratory
(Los Alamos National Laboratory LANL, Los Alamos NM USA) requested a change in the test-samples provided previously by the
ISP. The change was requested to evaluate measurement performance for analyses conducted using thermal-ionization mass spectrometry
(TIMS). Radionuclides included 239Pu at two activity levels (75–150 μBq·sample−1 and 1200–1600 μBq·sample−1) and 238Pu (3700–7400 μBq·sample−1). In addition, 240Pu was added to the samples so that the 239+240Pu specific activity was 3700–7400 μBq·sample−1. In this paper, the results of testing during the period May, 2003 through September, 2005 are presented and discussed.